MC-NRLF 


UNIVERSITY  OF  CALIFORNIA 

MEDICAL  CENTER  LIBRARY 

SAN  FRANCISCO 


A  TEXT-BOOK 


OF 


PHYSIOLOGY. 


BY 


M.  (FOSTER,  M.A.,  M.D.,  LL.D.,  F.R.S., 

PROFESSOR  OF  PHYSIOLOGY  IN  THE  UNIVERSITY  OF  CAMBRIDGE,  AND  FELLOW  OF  TRINITY 
COLLEGE,   CAMBRIDGE. 


SIXTH  AMERICAN  EDITION, 

THOROUGHLY  REVISED,   WITH  NOTES,  ADDITIONS,  AND  TWO   HUNDRED 
AND    FIFTY-SEVEN  ILLUSTRATIONS. 


PHILADELPHIA: 

LEA    BROTHERS    &   CO 

1895. 


Entered  according  to  Act  of  Congress  in  the  year  1895,  by 

LEA   BROTHERS  &  CO., 
in  the  Office  of  the  Librarian  of  Congress,  at  Washington.     All  rights  reserved. 


ELECTROTYPED  BY 
WESTCOTT  A  THOMSON,  PHILADA. 


AMERICAN  PUBLISHER'S  NOTICE 

TO  THE  SIXTH   EDITION. 


IN  the  preparation  of  the  Sixth  American  Edition  every  page  has  been 
subjected  to  careful  scrutiny  and  considerable  liberty  taken  with  the  text. 
Useless  verbiage  has  been  omitted,  obscure  sentences  have  been  revised  or 
entirely  rewritten,  a  large  number  of  typographical  errors  have  been  cor- 
rected, histological  details  (except  of  the  nervous  system)  have  been  mate- 
rially abridged,  much  that  was  too  theoretical  has  been  omitted,  and  such 
other  alterations  and  additions  have  been  made  as  to  bring  the  book  up  to 
date  and  render  it  in  a  more  advantageous  form  for  the  student.  The  his- 
tology of  the  nervous  system  has  been  retained  in  full,  as  has  also  the 
valuable  Chemical  Appendix — features  that  will  be  appreciated  by  both 
student  and  teacher.  The  more  important  additions  have  been  dis- 
tinguished by  enclosure  in  brackets  [  ]. 

PHILADELPHIA,  September,  1895.  r 


CONTENTS 


PAGE 

INTRODUCTION 17 


BOOK  I. 

BLOOD.    THE  TISSUES  OF  MOVEMENT.    THE  VASCULAR  MECHANISM. 

CHAPTER   I. 

BLOOD. 

The  Clotting  of  Blood      24 

The  Corpuscles  of  the  Blood  :  The  Ked  Corpuscles 37 

The  White  or  Colorless  Corpuscles        43 

Blood  Platelets 49 

The  Chemical  Composition  of  Blood        50 

The  Quantity  of  Blood,  and  its  Distribution  in  the  Body 53 

CHAPTER   II. 
THE  CONTRACTILE  TISSUES. 

The  Phenomena  of  Muscle  and  Nerve  :  Muscular  and  Nervous  Irritability    ...  55 

The  Phenomena  of  a  Simple  Muscular  Contraction : 65 

Tetanic  Contractions 73 

On  the  Changes  which    Take   Place  in  a  Muscle   during  a   Contraction  :    The 

Change  in  Form '.    .  77 

The  Chemistry  of  Muscle 83 

Thermal  Changes 90 

Electrical  Changes   ..'.* 92 

The  Changes  in  a  Nerve  during  the  Passage  of  a  Nervous  Impulse 97 

The   Nature  of  the   Changes   through  which    an    Electric   Current   is  Able   to 

Generate  a  Nervous  Impulse:  Action  of  the  Constant  Current 100 

The  Muscle-nerve  Preparation  as  a  Machine 107 

The  Circumstances  which  Determine  the  Degree  of  Irritability  of  Muscles  and 

Nerves * .  Ill 

The  Energy  of  Muscle  and  Nerve,  and  the  Nature  of  Muscular  and  Nervous 

Action 115 

On  Some  Other  Forms  of  Contractile  Tissue :  Plain,  Smooth  or  Unstriated  Mus- 
cular Tissue  .    .    . 116 

Ciliary  Movement 119 

Amoeboid  Movements 121 


10  CONTENTS. 

CHAPTER   III. 

PAGE 

GENEEAL  FEATUKES  OF  NERVOUS  TISSUES 122 

CHAPTER   IV. 
THE  VASCULAR  MECHANISM. 

The  Structure  and  Main  Features  of  the  Vascular  Apparatus    .    .        133 

The  Structure  of  Arteries,  Capillaries,  and  Veins 134 

The  Main  Features  of  the  Apparatus .    .    . 134 

The  Main  Facts  of  the  Circulation 136 

Hydraulic  Principles  of  the  Circulation 144 

Circumstances  Determining  the  Kate  of  the  Flow      150 

The  Heart 157 

The  Phenomena  of  the  Normal  Beat 157 

Endocardiac  Pressure 164 

Summary 174 

The  Work  Done 175 

The  Pulse ...  176 

The  Regulation  and  Adaptation  of  the  Vascular  Mechanism :  The  Regulation  of 

the  Beat  of  the  Heart 191 

The  Development  of  the  Normal  Beat 192 

The  Government  of  Heart-beat  by  the  Nervous  System 199 

Other  Influences  Regulating  or  Modifying  the  Beat  of  the  Heart 209 

Changes  in  the  Calibre  of  the  Minute  Arteries.     Vasomotor' Actions 211 

The  Course  of  Vaso-constrictor  and  Vaso-dilator  Fibres 219 

The  Effects  of  Vasomotor  Actions 221 

Vasomotor  Functions  of  the  Central  Nervous  System 222 

The  Capillary  Circulation 231 

Changes  in  the  Quantity  of  Blood 236 

A  Review  of  Some  of  the  Features  of  the  Circulation .  238 


BOOK   II. 

THE  TISSUES  OF   CHEMICAL  ACTION  WITH  THEIR  RESPECTIVE 
MECHANISMS.     NUTRITION. 

CHAPTER   I. 
THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

The  Characters  and  Properties  of  Saliva  and  Gastric  Juice  :  Saliva 246 

Gastric  Juice 251 

The  Act  of  Secretion  of  Saliva  and  Gastric  Juice  and  the  Nervous  Mechanisms 

which  Regulate  It • .  •    •                                   ...  260 

The  Changes  in  a  Gland  Constituting  the  Act  of  Secretion 

The  Properties  and  Characters  of  Bile,  Pancreatic  Juice,  and  Succus  Entericus  .    .  278 

Bile.                                                                              278 


CONTENTS.  11 

PAGE 

Pancreatic  Juice 281 

Succus  Entericus 285 

The  Secretion  of  Pancreatic  Juice  and  of  Bile .    .  286 

The  Structure  of  the  Intestines  :  The  Small  Intestine 292 

The  Muscular  Mechanisms  of  Digestion 293 

The  Changes  which  the  Food  Undergoes  in  the  Alimentary  Canal 306 

The  Changes  in  the  Stomach      306 

In  the  Small  Intestine      308 

In  the  Large  Intestine 312 

The  Feces 313 

The  Lacteals  and  the  Lymphatic  System 313 

The  Nature  and  Movements  of  Lymph  (including  Chyle) 314 

The  Characters  of  Lymph 315 

The  Movements  of  Lymph 317 

Absorption  from  the  Alimentary  Canal 325 

The  Course  Taken  by  the  Several  Products  of  Digestion 325 

The  Mechanism  of  Absorption .    .  329 

CHAPTER   II. 
RESPIRATION. 

The  Mechanics  of  Pulmonary  Respiration 333 

The  Respiratory  Movements 339 

Changes  of  the  Air  in  Respiration 344 

The  Respiratory  Changes  in  the  Blood 346 

The  Relations  of  Oxygen  in  the  Blood 350 

Products  of  the  Decomposition  of  Haemoglobin 358 

The  Relations  of  the  Carbonic  Acid  in  the  Blood 360 

The  Relations  of  the  Nitrogen  in  the  Blood 361 

The  Respiratory  Changes  in  the  Lungs  :  The  Entrance  of  Oxygen 361 

The  Exit  of  Carbonic  Acid 364 

The  Respiratory  Changes  in  the  Tissues 365 

The  Nervous  Mechanism  of  Respiration 369 

The  Effects  of  Changes  in  the  Composition  and  Pressure  of  the  Air  Breathed    .    .  386 

The  Relations  of  the  Respiratory  System  to  the  Vascular  and  Other  Systems     .    .  390 

Modified  Respiratory  Movements 403 

CHAPTER  III. 
THE  ELIMINATION  OF  WASTE  PRODUCTS. 

The  Composition  and  Characters  of  Urine 404 

Amounts  of  the  Several  Urinary   Constituents   Passed   in   Twenty-four   Hours. 

(After  Parkes.)      409 

The  Secretion  of  Urine    .    . 410 

Secretion  of  the  Renal  Epithelium 417 

The  Discharge  of  Urine 426 

Micturition 427 

The  Nature  and  Amount  of  Perspiration 431 

Cutaneous  Respiration 432 

The  Mechanism  of  the  Secretion  of  Sweat 434 


12  CONTENTS. 

CHAPTER   IV. 
THE  METABOLIC  PROCESSES  OF  THE  BODY. 

PAGE 

The  History  of  Glycogen 437 

Diabetes '. 448 

The  Formation  of  the  Constituents  of  Bile      453 

On  Urea  and  on  Nitrogenous  Metabolism  in  General 457 

On  Some  Structures  and  Processes  of  Obscure  Nature 465 

The  History  of  Fat.     Adipose  Tissue 470 

The  Mammary  Gland 477 

Average  Composition  of  Milk  in  Different  Animals 480 

CHAPTER   V. 

NUTRITION. 

Statistics  of  Nutrition 482 

Comparison  of  Income  and  Output  of  Material 485 

The  Energy  of  the  Body :  The  Income  of  Energy 492 

The  Expenditure 493 

Animal  Heat 496 

On  Nutrition  in  General     506 

On  Diet   .                                                                                                                             .  513 


BOOK  III. 

THE  CENTRAL  NERVOUS  SYSTEM  AND  ITS  INSTRUMENTS. 

CHAPTER   I. 

THE  SPINAL  CORD. 

On  Some  Features  of  the  Spinal  Nerves 525 

The  Structure  of  the  Spinal  Cord 529 

The  Reflex  Actions  of  the  Spinal  Cord 564 

The  Automatic  Actions  of  the  Spinal  Cord 578 

CHAPTER  II. 
THE  BRAIN. 

On  Some  General  Features  of  the  Structure  of  the  Brain 584 

The  Bulb 589 

The  Disposition  and  Connections  of  the  Gray  and  White  Matter  of  the  Brain  : 

The  Gray  Matter 601 

The  Central  Gray  Matter  and  the  Nuclei  of  the  Cranial  Nerves 601 


CONTENTS.  13 

PAGE 

The  Superficial  Gray  Matter 615 

The  Intermediate  Gray  Matter  of    the  Crural  System      615 

Other  Collections  of  Gray  Matter 625 

The  Arrangement  of  the  Fibres  of  the  Brain 627 

Longitudinal  Fibres  of  the  Pedal  System 628 

Longitudinal  Fibres  of  the  Tegmental  System 631 

Transverse  or  so-called  Commissural  Fibres    ...        634 

Summary    . 635 

On  the  Phenomena  Exhibited  by  an  Animal  Deprived  of  its  Cerebral  Hemispheres .  637 

The  Machinery  of  Coordinated  Movements .  643 

On  Some  Histological  Features  of  the  Brain 652 

The  Superficial  Gray  Matter  of  the  Cerebellum 653 

The  Cerebral  Cortex 655 

On  Voluntary  Movements  .......        661 

On  the  Development  within  the  Central  Nervous  System  of  Visual  and  of  some 

Other  Sensations :  Visual  Sensations 689 

Sensations  of  Smell 700 

Sensations  of  Taste 701 

Sensations  of  Hearing 701 

On  the  Development  of  Cutaneous  and  Some  Other  Sensations 703 

Some  Other  Aspects  of  the  Functions  of  the  Brain 716 

On  the  Time  Taken  up  by  Cerebral  Operations 724 

The  Lymphatic  Arrangements  of  the  Brain  and  Spinal  Cord 728 

The  Vascular  Arrangements  of  the  Brain  and  Spinal  Cord 732 


CHAPTER   III. 

SIGHT. 

The  Eye 738 

Dioptric  Mechanisms:  The  Formation  of  the  Image .    .  744 

Accommodation 745 

Imperfections  of  the  Dioptric  Apparatus 757 

Visual  Sensations 759 

The  Origin  of  Visual  Impulses 760 

Simple  Sensations 766 

Color  Sensations 769 

Visual  Perceptions 

Modified  Perceptions 

Binocular  Vision  :  Corresponding  or  Identical  Points 781 

Movements  of  the  Eyeballs 782 

The  Horopter 786 

Visual  Judgments 786 

The  Protected  Mechanisms  of  the  Eye 789 

CHAPTER  IV. 
HEARING,  SMELL,  AND  TASTE. 

Hearing  :  The  Ear 790 

The  Acoustic  Apparatus 798 


1 4  CONTENTS. 

PAGE 

Auditory  Sensations 800 

Auditory  Judgments •. 805 

Smell :  The  Nasal  Fossae 806 

Taste:  The  Gustatory  Mucous  Membrane 80S 

CHAPTEK  V. 
FEELING  AND  TOUCH. 

General  Sensibility  and  Tactile  Perceptions 812 

Tactile  Sensations  :  Sensations  of  Pressure 814 

Sensations  of  Temperature 814 

Tactile  Perceptions  and  Judgments 816 

The  Muscular  Sense 818 

CHAPTER  VI. 
SPECIAL  MUSCULAR  MECHANISMS. 

The  Voice:  The  Larynx 819 

Speech :  Vowels 826 

Consonants 827 

Locomotor  Mechanisms 829 


BOOK    IV. 

THE  TISSUES  AND  MECHANISMS  OF  REPRODUCTION. 

CHAPTER  I. 
ORGANS  OF  REPRODUCTION ,833 

CHAPTER   II. 

MENSTRUATION 038 

CHAPTER   III. 

IMPREGNATION      841 

CHAPTER  IV. 
THE  NUTRITION  OF  THE  EMBRYO 847 

CHAPTER   V. 

PARTURITION  .    853 


CONTENTS.  15 

CHAPTER   VI. 

PAGE 

THE  PHASES  OF  LIFE 855 

CHAPTER   VII. 

DEATH  .   863 


APPENDIX. 

THE  CHEMICAL  BASIS  OF  THE  ANIMAL  BODY    ,  .   865 


A  TEXT-BOOK  OF  PHYSIOLOGY. 


INTRODUCTION. 

§  1.  DISSECTION,  aided  by  microscopical  examination,  teaches  us  that 
the  body  of  man  is  made  up  of  certain  kinds  of  material,  so  differing  from 
each  other  in  optical  and  other  physical  characters  and  so  built  up  together 
as  to  give  the  body  certain  structural  features.  Chemical  examination 
further  teaches  us  that  these  kinds  of  material  are  composed  of  various 
chemical  substances,  a  large  number  of  which  have  this  characteristic,  that 
they  possess  a  considerable  amount  of  potential  energy  capable  of  being  set 
free,  rendered  actual,  by  oxidation  or  some  other  chemical  change.  Thus 
the  body,  as  a  whole,  may,  from  a  chemical  point  of  view,  be  considered  as 
a  mass  of  various  chemical  substances,  representing  altogether  a  considera- 
ble capital  of  potential  energy. 

§  2.  This  body  may  exist  either  as  a  living  body  or  (for  a  certain  time 
at  least)  as  a  dead  body,  and  the  living  body  may  at  any  time  become  a 
dead  body.  At  what  is  generally  called  the  moment  of  death  (but  arti- 
ficially so,  for  as  we  shall  see  the  processes  of  death  are  numerous  and 
gradual)  the  dead  body  so  far  as  structure  and  chemical  composition  are 
concerned  is  exceedingly  like  the  living  body ;  indeed  the  differences  be- 
tween the  two  are  such  as  can  be  determined  only  by  very  careful  examina- 
tion, and  are  still  to  a  large  extent  estimated  by  drawing  inferences  rather 
than  actually  observed.  At  any  rate  the  dead  body  at  the  moment  of  death 
resembles  the  living  body  in  so  far  as  it  represents  a  capital  of  potential 
energy.  From  that  moment  onward,  however,  the  capital  is  expended  ;  by 
processes  which  are  largely  those  of  oxidation,  the  energy  is  gradually  dis- 
sipated, leaving  the  body  chiefly  in  the  form  of  heat.  While  these  chemical 
processes  are  going  on  the  structural  features  disappear,  and  the  body,  with 
the  loss  of  nearly  all  its  energy,  is  at  last  resolved  into  "  dust  and  ashes." 

The  characteristic  of  the  dead  body  then  is  that,  being  a  mass  of  sub- 
stances of  considerable  potential  energy,  it  is  always  more  or  less  slowly 
losing  energy,  never  gaining  energy ;  the  capital  of  energy  present  at  the 
moment  of  death  is  more  or  less  slowly  diminished,  is  never  increased  or 
replaced. 

§  3.  When  on  the  other  hand  we  study  a  living  body  we  are  struck  with 
the  following  salient  facts  : 

1.  The  living  body  moves  of  itself,  either  moving  one  part  of  the  body 
on  another  or  moving  the  whole  body  from  place  to  place.     These  move- 
ments are  active ;    the  body  is  not  simply  pulled  or  pushed  by  external 
forces,  but  the  motive  power  is  in  the  body  itself,  the  energy  of  each  move- 
ment is  supplied  by  the  body  itself. 

2.  These  movements  are  determined  and  influenced,  indeed  often  seem  to 
be  started,  by  changes  in  the  surroundings  of  the  body.     Sudden  contact 
between  the  surface  of  the  body  and  some  foreign  object  will  often  call 


18  INTRODUCTION. 

forth  a  movement.  The  body  is  sensitive  to  changes  in  its  surroundings, 
and  this  sensitiveness  is  manifested  not  only  by  movements  but  by  other 
changes  in  the  body. 

3.  It  is  continually  generating  heat  and  giving  out  heat  to  surrounding 
things,  the  production  and  loss  of  heat,  in  the  case  of  man  and  certain 
other  animals,  being  so  adjusted  that  the  whole  body  is  warm,  that  is,  of  a 
temperature  higher  than  that  of  surrounding  things. 

4.  From  time  to  time  it  eats,  that  is  to  say  takes  into  itself  supplies  of 
certain  substances  known  as  food,  these  substances  being  in  the  main  similar 
to  those  which  compose  the  body,  and  being  like  them  chemical  bodies  of 
considerable  potential  energy,  capable  through  oxidation  or  other  chemical 
changes  of  setting  free  a  considerable  quantity  of  energy. 

5.  It  is  continually  breathing;  that  is,  taking  in  from  the  surrounding 
air  supplies  of  oxygen. 

6.  It  is  continually,  or  from  time  to  time,  discharging  from  itself  into  its 
surroundings  so-called  waste  matters,  which  waste  matters  may  be  broadly 
described  as  products  of  oxidation  of  the  substances  taken  in  as  food,  or  of 
the  substances  composing  the  body. 

Hence  the  living  body  may  be  said  to  be  distinguished  from  the  dead 
body  by  three  main  features. 

The  living  body  like  the  dead  is  continually  losing  energy  (and  losing  it 
more  rapidly  than  the  dead  body,  the  special  breathing  arrangements  per- 
mitting a  more  rapid  oxidation  of  its  substance),  but  unlike  the  dead  body 
is  by  means  of  food  continually  restoring  its  substance  and  replenishing  its 
store  of  energy. 

The  energy  set  free  in  the  dead  body  by  the  oxidation  and  other  chem- 
ical changes  of  its  substance  leaves  the  body  almost  exclusively  in  the  form 
of  heat,  whereas  a  great  deal  of  energy  leaves  the  living  body  as  mechanical 
work,  the  result  of  various  movements  of  the  body,  and  as  we  shall  see  a 
great  deal  of  the  energy  which  ultimately  leaves  the  body  as  heat,  exists  for 
a  while  within  the  living  body  in  other  forms  than  heat,  though  eventually 
transformed  into  heat. 

The  changes  in  the  surroundings  affect  the  dead  body  at  a  slow  rate  and 
in  a  general  way  only,  simply  lessening  or  increasing  the  amount  or  rate 
of  chemical  change  and  the  quantity  of  heat  thereby  set  free,  but  never 
diverting  the  energy  into  some  other  form  such  as  that  of  movement ; 
whereas  changes  in  the  surroundings  may  in  the  case  of  the  living  body 
rapidly,  profoundly,  and  in  special  ways  affect  not  only  the  amount  but  also 
the  kind  of  energy  set  free.  The  dead  body  left  to  itself  slowly  falls  to 
pieces,  slowly  dissipates  its  store  of  energy,  and  slowly  gives  out  heat ;  a 
higher  or  lower  temperature,  more  or  less  moisture,  a  free  or  scanty  supply 
of  oxygen,  the  advent  of  many  or  few  putrefactive  organisms,  these  may 
quicken  or  slacken  the  rate  at  which  energy  is  being  dissipated  but  do  not 
divert  that  energy  from  heat  into  motion  ;  whereas  in  the  living  body  so 
slight  a  change  of  surroundings  as  the  mere  touch  by  a  hair  of  some  par- 
ticular surface,  may  so  affect  the  setting  free  of  energy  as  to  lead  to  such  a 
discharge  of  energy  in  the  form  of  movement  that  the  previously  appar- 
ently quiescent  body  may  be  suddenly  thrown  into  the  most  violent  con- 
vulsions. 

The  differences,  therefore,  between  living  substance  and  dead  substance 
though  recondite  are  very  great,  and  the  ultimate  object  of  physiology  is 
to  ascertain  how  it  is  that  living  substance  can  do  what  dead  substance 
cannot — that  is,  can  renew  its  substance  and  replenish  the  energy  which  it 
is  continually  losing,  and  can,  according  to  the  nature  of  its  surroundings, 
vary  not  only  the  amount  but  also  the  kind  of  energy  which  it  sets  free. 


INTRODUCTION. 


19 


Thus  there  are  two  great  divisions  of  physiology  :  one  having  to  do  with  the 
renewal  of  substance  and  the  replenishment  of  energy,  the  other  having  to 
do  with  the  setting  free  of  energy. 

§  4.  Now  the  body  of  man  (or  one  of  the  higher  animals)  is  a  very  com- 
plicated structure,  consisting  of  different  kinds  of  material,  which  we  call 
tissues,  such  as  muscular,  nervous,  connective,  and  the  like,  variously 
arranged  in  organs  such  as  heart,  lungs,  muscles,  skin,  etc.,  all  built  up  to 
form  the  body  according  to  certain  morphological  laws.  But  all  this  com- 
plication, though  advantageous  and  indeed  necessary  for  the  fuller  life  of 
man,  is  not  essential  to  the  existence  of  life.  The  amoeba  [Fig.  1]  is  a 

[FIG.  1. 


Amoeba  princeps,  sho\Vn  in  different  forms  (A,  B,  C)  assumed  by  the  same  animal.] 

living  being ;  it  renews  its  substance,  replenishes  its  store  of  energy,  and 
sets  free  energy  now  in  one  form,  now  in  another ;  and  yet  the  amoeba  may 
be  said  to  have  no  tissues  and  no  organs ;  at  all  events  this  is  true  of  closely 
allied  but  not  so  well  known  simple  beings.  Using  the  more  familiar 
amoeba  as  a  type,  and,  therefore,  leaving  on  one  side  the  nucleus,  and  any 
distinction  between  endosarc  and  ectosarc,  we  may  say  that  its  body  is 
homogeneous  in  the  sense  that  if  we  divided  it  into  small  pieces,  each  piece 
would  be  like  all  the  others.  In  another  sense  it  is  not  homogeneous.  For 
we  know  that  the  amoeba  receives  into  its  substance  material  as  food,  and 
that  this  food  or  part  of  it  remains  lodged  in  the  body,  until  it  is  made  use 
of  and  built  up  into  the  living  substance  of  the  body,  and  each  piece  of  the 
living  substance  of  the  body  must  have  in  or  near  it  some  of  the  material 
which  it  is  about  to  build  up  into  itself.  Further,  we  know  that  the  amoeba 
gives  out  waste  matters  such  as  carbonic  acid  and  other  substances,  and  each 
piece  of  the  amoeba  must  contain  some  of  these  waste  matters  about  to  be, 
but  not  yet,  discharged  from  the  piece.  Each  piece  of  the  amoeba  will, 
therefore,  contain  these  three  things,  the  actual  living  substance,  the  food 
about  to  become  living  substance,  and  the  waste  matters  which  have  ceased 
to  be  living  substance. 

Moreover,  we  have  reasons  to  think  that  the  living  substance  does  not 
break  down  into  the  waste  matters  which  leave  the  body  at  a  single  bound, 
but  there  are  stages  in  the  downward  progress  between  the  one  and  the 
other.  Similarly,  though  our  knowledge  on  this  point  is  less  sure,  we  have 
reason  to  think  that  the  food  is  not  incorporated  into  the  living  substance  at 
a  single  step,  but  that  there  are  stages  in  the  upper  progress  from  the 
dead  food  to  the  living  substance.  Each  piece  of  the  body  of  the  amoeba 


20  INTRODUCTION. 

will,  therefore,  contain  substances  representing  various  stages  of  becoming 
living,  and  of  ceasing  to  be  living,  as  well  as  the  living  substance  itself. 
And  we  may  safely  make  this  statement  though  we  are  quite  unable  to 
draw  the  line  where  the  dead  food  on  its  way  up  becomes  living,  or  the 
living  substance  on  its  way  down  becomes  dead. 

§  5.  Nor  is  it  necessary  for  our  present  purpose  to  be  able  to  point  out 
under  the  microscope,  or  to  describe  from  a  histological  point  of  view,  the 
parts  which  are  living  and  the  parts  which  are  dead  food  or  dead  waste. 
The  body  of  the  amoeba  is  frequently  spoken  of  as  consisting  of  "  proto- 
plasm." The  name  was  originally  given  to  the  matter  forming  the  primor- 
dial utricle  of  the  vegetable  cell  as  distinguished  from  the  cell  wall  on  the 
one  hand,  and  from  the  fluid  contents  of  the  cell  or  cell  sap  on  the  other, 
and  also  we  may  add  from  the  nucleus.  It  has  since  been  applied  very 
generally  to  such  parts  of  animal  bodies  as  resemble,  in  their  general  fea- 
tures, the  primordial  utricle.  Thus  the  body  of  a  white  blood-corpuscle,  or 
of  a  gland  cell,  or  of  a  nerve  cell,  is  said  to  consist  of  protoplasm.  Such 
parts  of  animal  bodies  as  do  not  in  their  general  features  resemble  the 
matter  of  the  primordial  utricle  are  not  called  protoplasm,  or,  if  they  at 
some  earlier  stage  did  bear  such  resemblance,  but  no  longer  do  so,  are  some- 
times, as  in  the  case  of  the  substance  of  a  muscular  fibre,  called  "  differen- 
tiated protoplasm."  Protoplasm  in  this  sense  sometimes  appears,  as  in  the 
outer  part  of  most  amoebae,  as  a  mass  of  glassy-looking  material,  either  con- 
tinuous or  interrupted  by  more  or  less  spherical  spaces  or  vacuoles  filled 
with  fluid,  sometimes  as  in  a  gland  cell  as  a  more  refractive,  cloudy-looking, 
or  finely  granular  material  arranged  in  a  more  or  less  irregular  network,  or 
spongework,  the  interstices  of  which  are  occupied  either  by  fluid  or  by  some 
material  different  from  itself.  We  shall  return,  however,  to  the  features  of 
this  "  protoplasm  "  when  we  come  to  treat  of  white  blood-corpuscles  and 
other  "  protoplasmic"  structures.  Meanwhile  it  is  sufficient  for  our  present 
purpose  to  note  that  lodged  in  the  protoplasm,  discontinuous  with  it,  and 
forming  no  part  of  it,  are  in  the  first  place  collections  of  fluid,  of  watery 
solutions  of  various  substances,  occupying  the  more  regular  vacuoles  or 
the  more  irregular  spaces  of  the  network,  and  in  the  second  place  discrete 
granules  of  one  kind  or  another,  also  forming  no  part  of  the  protoplasm 
itself,  but  lodged  either  in  the  bars  or  substance  of  the  protoplasm  or  in  the 
vacuoles  or  meshes. 

Now,  there  can  be  little  doubt  that  the  fluids  and  the  discrete  granules  are 
dead  food  or  dead  waste,  but  the  present  state  of  our  knowledge  will  not 
permit  us  to  make  any  very  definite  statement  about  the  protoplasm  itself. 
We  may  probably  conclude,  indeed  we  may  be  almost  sure,  that  protoplasm 
in  the  above  sense  is  not  all  living  substance,  that  it  is  made  up  partly  of 
the  real  living  substance,  and  partly  of  material  which  is  becoming  living  or 
has  ceased  to  be  living;  and  in  the  case  where  protoplasm  is  described  as 
forming  a  network,  it  is  possible  that  some  of  the  material  occupying  the 
meshes  of  the  network  may  be,  like  part  of  the  network  itself,  really  alive. 
"  Protoplasm  "  in  fact,  as  in  the  sense  in  which  we  are  now  using  it,  and  shall 
continue  to  use  it,  is  a  morphological  term  ;  but  it  must  be  borne  in  mind  that 
the  same  word  "  protoplasm  "  is  also  frequently  used  to  denote  what  we  have 
just  now  called  "  the  real  living  substance."  The  word  then  embodies  &  phy- 
siological idea  ;  so  used  it  may  be  applied  to  the  living  substance  of  all  living 
structures,  whatever  the  microscopical  features  of  those  structures  ;  in  this 
sense  it  cannot  at  present,  and  possibly  never  will  be  recognized  by  the 
microscope,  and  our  knowledge  of  its  nature  must  be  based  on  inferences. 

Keeping  then  to  the  phrase  "  living  substance  "  we  may  say  that  each 
piece  of  the  body  of  the  amoeba  consists  of  living  subs-tan ce,  in  which  are 


INTRODUCTION.  21 

lodged,  or  with  which  are  built  up  in  some  way  or  other,  food  and  waste  in 
various  stages. 

Now,  an  amoeba  may  divide  itself  into  two,  each  half  exhibiting  all  the 
phenomena  of  the  whole ;  and  we  can  easily  imagine  the  process  to  be 
repeated,  until  the  amoeba  was  divided  into  a  multitude  of  exceedingly 
minute  amoebae,  each  having  all  the  properties  of  the  original.  But  it  is 
obvious,  as  in  the  like  division  of  a  mass  of  a  chemical  substance,  that  the 
division  could  not  be  repeated  indefinitely.  Just  as  in  division  of  the  chem- 
ical mass  we  come  to  the  chemical  molecule,  further  division  of  which 
changes  the  properties  of  the  substance,  so  in  the  continued  division  of  the 
amoeba  we  should  come  to  a  stage  in  which  further  division  interfered  with 
the  physiological  actions,  we  should  come  to  a  physiological  unit  correspond- 
ing to  but  greatly  more  complex  than  the  chemical  molecule.  This  unit  to 
remain  a  physiological  unit  and  to  continue  to  live  must  contain  not  only  a 
portion  of  the  living  substance  but  also  the  food  for  that  living  substance, 
in  several  at  least  of  the  stages,  from  the  initial  raw  food  up  to  the  final 
"  living  "  stages,  and  must  similarly  contain  various  stages  of  waste. 

§  6.  Now,  the  great  characteristic  of  the  typical  amoeba  (leaving  out  the 
nucleus)  is  that,  as  far  as  we  can  ascertain,  all  the  physiological  units  are 
alike;  they  all  do  the  same  things.  Each  and  every  part  of  the  body 
receives  food  more  or  less  raw  and  builds  it  up  into  its  own  living  substance ; 
each  and  every  part  of  the  body  may  be  at  one  time  quiescent  and  at  another 
in  motion ;  each  and  every  part  is  sensitive  and  responds  by  movement  or 
otherwise  to  various  changes  in  its  surroundings. 

The  body  of  man,  in  its  first  stage,  while  it  is  yet  an  ovum,  if  we  leave 
aside  the  nucleus  and  neglect  differences  caused  by  the  unequal  distribution 
of  food  material  or  yolk,  may  also  be  said  to  be  composed  of  like  parts  or 
like  physiological  units. 

By  the  act  of  segmentation,  however,  the  ovum  is  divided  into  parts  or 
cells  which  early  show  differences  from  each  other ;  and  these  differences 
rapidly  increase  as  development  proceeds.  Some  cells  put  on  certain  char- 
acters and  others  other  characters — that  is  to  say,  the  cells  undergo  histologi- 
cal  differentiation.  And  this  takes  place  in  such  a  way  that  a  number  of 
cells  lying  together  in  a  group  become  eventually  converted  into  a  tissue, 
and  the  whole  body  becomes  a  collection  of  such  tissues  arranged  together 
according  to  morphological  laws,  each  tissue  having  a  definite  structure,  its 
cellular  nature  being  sometimes  preserved,  sometimes  obscured  or  even  lost. 

This  histological  differentiation  is  accompanied  by  a  physiological  division 
of  labor.  Each  tissue  may  be  supposed  to  be  composed  of  physiological  units, 
the  units  of  the  same  tissue  being  alike  but  differing  from  the  units  of  other 
tissues  ;  and  corresponding  to  this  difference  of  structure,  the  units  of  different 
tissues  behave  or  act  differently.  Instead  of  all  the  units,  as  in  the  amoeba, 
doing  the  same  things  equally  well,  the  units  of  one  tissue  are  told  off,  as  it 
were,  to  do  one  thing  especially  well,  or  especially  fully,  and  thus  the  whole 
labor  of  the  body  is  divided  among  the  several  tissues. 

§  7.  The  several  tissues  may  thus  be  classified  according  to  the  work 
which  they  have  to  do ;  and  the  first  great  distinction  is  into  (1)  the  tissues 
which  are  concerned  in  the  setting  free  of  energy  in  special  ways,  and  (2) 
the  tissues  which  are  concerned  in  replenishing  the  substance  and  so  renew- 
ing the  energy  of  the  body. 

Each  physiological  unit  of  the  amoeba  while  it  is  engaged  in  setting  free 
energy  so  as  to  move  itself,  and  by  reason  of  its  sensitiveness  so  directing 
that  energy  as  to  produce  a  movement  suitable  to  the  conditions  of  its  sur- 
roundings, has  at  the  same  time  to  bear  the  labor  of  taking  in  raw  food,  of 
selecting  that  part  of  the  raw  food  which  is  useful  and  rejecting  that  which 


22  INTRODUCTION. 

is  useless,  and  of  working  up  the  accepted  part  through  a  variety  of  stages 
into  its  own  living  substance— that  is  to  say,  it  has  at  the  same  time  that  it 
is  feeling  and  moving  to  carry  on  the  work  of  digesting  and  assimilating. 
It  has,  moreover,  at  the  same  time  to  throw  out  the  waste  matters  arising 
from  the  changes  taking  place  in  its  own  substance,  having  first  brought 
these  waste  matters  into  a  condition  suitable  for  being  thrown  out. 

§  8.  In  the  body  of  man,  movements,  as  we  shall  see,  are,  broadly  speak- 
ing, carried  out  by  means  of  muscular  tissue,  and  the  changes  in  muscular 
tissue  which  lead  to  the  setting  free  of  energy  in  the  form  of  movement  are 
directed,  governed,  and  adapted  to  the  surroundings  of  man,  by  means  of 
nervous  tissue.  Rays  of  light  fall  on  the  nervous  substance  of  the  eye  called 
the  retina,  and  set  up  in  the  retina  changes  which  induce  in  the  optic  nerve 
other  changes,  which  in  turn  are  propagated  to  the  brain  as  nervous  impulses, 
both  the  excitation  and  the  propagation  involving  an  expenditure  of  energy. 
These  nervous  impulses  reaching  the  brain  may  induce  other  nervous  im- 
pulses which,  travelling  down  certain  nerves  to  certain  muscles,  may  lead  to 
changes  in  those  muscles  by  which  they  suddenly  grow  short  and  pull  upon 
the  bones  or  other  structures  to  which  they  are  attached,  in  which  case  we 
say  the  man  starts ;  or  the  nervous  impulses  reaching  the  brain  may  produce 
some  other  effects.  Similarly  sound  falling  on  the  ear,  or  contact  between 
the  skin  and  some  foreign  body,  or  some  change  in  the  air  or  other  surround- 
ings of  the  body,  or  some  change  within  the  body  itself  may  so  affect  the 
nervous  tissue  of  the  body  that  nervous  impulses  are  started  and  travel  to 
this  point  or  that,  to  the  brain  or  elsewhere,  and  eventually  may  either  reach 
some  muscular  tissue  and  so  give  rise  to  movements,  or  may  reach  other 
tissues  and  produce  some  other  effect. 

The  muscular  tissue  then  may  be  considered  as  given  up  to  the  produc- 
tion of  movement,  and  the  nervous  tissue  as  given  up  to  the  generation, 
transformation,  and  propagation  of  nervous  impulses.  In  each  case  there 
is  an  expenditure  of  energy,  which  in  the  case  of  the  muscle,  as  we  shall 
see,  leaves  the  body  partly  as  heat  and  partly  as  work  done,  but  in  the  case 
of  nervous  tissue  is  wholly  or  almost  wholly  transformed  into  heat  before  it 
leaves  the  body  ;  and  this  expenditure  necessitates  a  replenishment  of  energy 
and  a  renewal  of  substance. 

§  9.  In  order  that  these  master  tissues,  the  nervous  and  muscular  tissues, 
may  carry  on  their  important  works  to  the  best  advantage,  they  are  relieved 
of  much  of  the  labor  that  falls  upon  each  physiological  unit  of  the  amoeba. 
They  are  not  presented  with  raw  food,  they  are  not  required  to  carry  out  the 
necessary  transformations  of  their  immediate  waste  matters.  The  whole  of 
the  rest  of  the  body  is  engaged  (1)  in  so  preparing  the  raw  food,  and  so 
bringing  it  to  the  nervous  and  muscular  tissues  that  these  may  build  it  up 
into  their  own  substance  with  the  least  trouble,  and  (2)  in  receiving  the 
waste  matters  which  arise  in  muscular  and  nervous  tissues,  and  preparing 
them  for  rapid  and  easy  ejection  from  the  body. 

Thus  to  certain  tissues,  which  we  may  speak  of  broadly  as  "  tissues  of 
digestion,"  is  allotted  the  duty  of  acting  on  the  food  and  preparing  it  for  the 
use  of  the  muscular  and  nervous  tissues  ;  and  to  other  tissues,  which  we  may 
speak  of  as  "tissues  of  excretion,"  is  allotted  the  duty  of  clearing  the  body 
from  the  waste  matters  generated  by  the  muscular  and  nervous  tissues. 

§  10.  These  tissues  are  for  the  most  part  arranged  in  machines  or  mech- 
anisms called  organs,  and  the  workings  of  these  organs  involves  movement. 
The  movements  of  these  organs  are  carried  out,  like  the  other  movements  of 
the  body,  chiefly  by  means  of  muscular  tissue  governed  by  nervous  tissue. 
Hence  we  may  make  a  distinction  between  the  muscles  which  are  concerned 
in  producing  an  effect  on  the  world  outside  man's  body,  the  muscles  by  which 


INTRODUCTION.  23 

man  does  his  work  in  the  world,  and  the  muscles  which  are  concerned  in 
carrying  out  the  movements  of  the  internal  organs.  And  we  may  similarly 
make  a  distinction  between  the  nervous  tissue  concerned  in  carrying  out  the 
external  work  of  the  body  and  that  concerned  in  regulating  the  movements 
and,  as  we  shall  see,  the  general  conduct  of  the  internal  organs.  But  these 
two  classes  of  muscular  and  nervous  tissue,  though  distinct  in  work  and,  as 
we  shall  see,  often  different  in  structure,  are  not  separated  or  isolated.  On 
the  contrary,  while  it  is  the  main  duty  of  the  nervous  tissue  as  a  whole,  the 
nervous  system  as  we  may  call  it,  to  carry  out,  by  means  of  nervous  impulses 
passing  hither  and  thither,  what  may  be  spoken  of  as  the  work  of  man,  and 
in  this  sense  it  is  the  master  tissue,  it  also  serves  as  a  bond  of  union  between 
itself  and  the  muscles  doing  external  work  on  the  one  hand,  and  the  organs 
of  digestion  or  excretion  on  the  other,  so  that  the  activity  and  conduct  of 
the  latter  may  be  adequately  adapted  to  the  needs  of  the  former. 

§11.  Lastly,  the  food  prepared  and  elaborated  by  the  digestive  organs 
is  carried  and  presented  to  the  muscular  and  nervous  tissues  in  the  form  of 
a  complex  fluid  known  as  blood,  which,  driven  by  means  of  a  complicated 
mechanism  known  as  the  vascular  system,  circulates  all  over  the  body,  visit- 
ing in  turn  all  the  tissues  of  the  body,  and  by  a  special  arrangement  known 
as  the  respiratory  mechanism,  carrying  in  itself  to  the  several  tissues  a  sup- 
ply of  oxygen  as  well  as  of  food  more  properly  so  called. 

The  motive  power  of  this  vascular  system  is  supplied,  as  in  the  case  of 
the  digestive  system,  by  means  of  muscular  tissue,  the  activity  of  which  is 
similarly  governed  by  the  nervous  system,  and  hence  the  flow  of  blood  to 
this  part  or  that  part  is  regulated  according  to  the  needs  of  the  part. 

§  12.  The  above  slight  sketch  will  perhaps  suffice  to  show  not  only  how 
numerous  but  how  varied  are  the  problems  with  which  physiology  has  to 
deal. 

In  the  first  place,  there  are  what  may  be  called  general  problems,  such  as, 
How  the  food  after  its  preparation  and  elaboration  into  blood  is  built  up 
into  the  living  substance  of  the  several  tissues  ?  How  the  living  substance 
breaks  down  into  the  dead  waste  ?  How  the  building  up  and  breaking 
down  differ  in  the  different  tissues  in  such  a  way  that  energy  is  set  free  in 
different  modes,  the  muscular  tissue  contracting,  the  nervous  tissue  thrilling 
with  a  nervous  impulse,  the  secreting  tissue  doing  chemical  work,  and  the 
like?  To  these  general  questions  the  answers  which  we  can  at  present  give 
can  hardly  be  called  answers  at  all. 

In  the  second  place,  there  are  what  maybe  called  special  problems,  such 
as,  What  are  the  various  steps  by  which  the  blood  is  kept  replenished  with 
food  and  oxygen,  and  kept  free  from  an  accumulation  of  waste,  and  how  is 
the  activity  of  the  digestive,  respiratory,  and  excretory  organs,  which  effect 
this,  regulated  and  adapted  to  the  stress  of  circumstances?  What  are  the 
details  of  the  working  of  the  vascular  mechanism  by  which  each  and  every 
tissue  is  forever  bathed  with  fresh  blood,  and  how  is  that  working  delicately 
adapted  to  all  the  various  changes  of  the  body  ?  And,  compared  with  which 
all  other  special  problems  are  insignificant  and  preparatory  only,  How  do 
nervous  impulses  so  flit  to  and  fro  within  the  nervous  system  as  to  issue  in 
the  movements  which  make  up  what  we  sometimes  call  the  life  of  man  ?  It 
is  to  these  special  problems  that  we  must  chiefly  confine  our  attention,  and 
we  may  fitly  begin  with  a  study  of  the  blood. 


BOOK  I. 

BLOOD,    THE  TISSUES  OF  MOVEMENT,    THE  VASCULAR  MECHANISM, 


CHAPTER    I. 

BLOOD. 

§  13.  THE  several  tissues  are  traversed  by  minute  tubes,  the  capillary 
bloodvessels,  to  which  blood  is  brought  by  the  arteries,  and  from  which 
blood  is  carried  away  by  the  veins.  These  capillaries  form  networks  the 
meshes  of  which,  differing  in  form  and  size  in  the  different  tissues,  are  occu- 
pied by  the  elements  of  the  tissue,  which  consequently  lie  outside  the  capil- 
laries. 

The  blood  flowing  through  the  capillaries  consists,  under  normal  condi- 
tions, of  an  almost  colorless  fluid,  the  plasma,  in  which  are  carried  a  num- 
ber of  bodies,  the  red  and  the  white  corpuscles.  Outside  the  capillary  walls, 
filling  up  such  spaces  as  exist  between  the  capillary  walls  and  the  cells  or 
fibres  of  the  tissue,  or  between  the  elements  of  the  tissue  themselves,  is  found 
a  colorless  fluid  resembling  in  many  respects  the  plasma  of  blood,  and  called 
lymph.  Thus  all  the  elements  of  the  tissue  and  the  outsides  of  all  the  capil- 
laries are  bathed  with  lymph,  which,  as  we  shall  see  hereafter,  is  continually 
flowing  away  from  the  tissue  along  special  channels  to  pass  into  lymphatic 
vessels  and  thence  into  the  blood. 

As  the  blood  flows  through  the  capillaries  certain  constituents  of  the 
plasma  (together  with,  at  times,  white,  corpuscles,  and  under  exceptional 
circumstances  red  corpuscles)  pass  through  the  capillary  wall  into  the 
lymph,  and  certain  constituents  of  the  lymph  pass  through  the  capillary 
wall  into  the  blood  within  the  capillary.  There  is  thus  an  interchange  of 
material  between  the  blood  within  the  capillary  and  the  lymph  outside.  A 
similar  interchange  of  material  is  at  the  same  time  going  on  between  the 
lymph  and  the  tissue  itself.  Hence,  by  means  of  the  lymph  acting  as  mid- 
dleman, a  double  interchange  of  material  takes  place  between  the  blood 
within  the  capillary  and  the  tissue  outside  the  capillary.  In  every  tissue, 
so  long  as  life  lasts  and  the  blood  flows  through  the  bloodvessels,  a  double 
stream,  now  rapid,  now  slow,  is  passing  from  the  blood  to  the  tissue  and 
from  the  tissue  to  the  blood.  The  stream  from  the  blood  to  the  tissue  car- 
ries to  the  tissue  the  material  which  the  tissue  needs  for  building  itself  up 
and  for  doing  its  work,  including  the  all-important  oxygen.  The  stream 
from  the  tissue  to  the  blood  carries  into  the  blood  certain  of  the  products  of 
the  chemical  changes  which  have  been  taking  place  in  the  tissue,  products 
which  may  be  simple  waste,  to  be  cast  out  of  the  body  as  soon  as  possible, 
or  which  may  be  bodies  capable  of  being  made  use  of  by  some  other  tissue. 

A  third  stream,  that  from  the  lymph  lying  in  the  chinks  and  crannies 
of  the  tissue  along  the  lymph  channels  to  the  larger  lymph  vessels,  carries 
away  from  the  tissue  such  parts  of  the  material  coming  from  the  blood  as 

25 


26  BLOOD. 

are  not  taken  up  by  the  tissue  itself  and  such  parts  of  the  material  coming 
from  the  tissue  as  do  not  find  their  way  into  the  bloodvessel. 

In  most  tissues,  as  in  muscle  for  instance,  the  capillary  network  is  so 
close  set  and  the  muscular  fibre  lies  so  near  to  the  bloodvessel  that  the 
lymph  between  the  two  exists  only  as  a  very  thin  sheet;  but  in  some  tis- 
sues, as  in  cartilage,  the  bloodvessels  lie  on  the  outside  of  a  large  mass  of 
tissue,  the  interchange  between  the  central  parts  of  which  and  the  nearest 
capillary  bloodvessel  is  carried  on  through  a  long  stretch  of  lymph  passages. 
But  in  each  case  the  principle  is  the  same  ;  the  tissue,  by  the  help  of  lymph, 
lives  on  the  blood;  and  when  in  succeeding  pages  we  speak  of  changes 
between  the  blood  and  the  tissues,  it  will  be  understood,  whether  expressly 
stated  so  or  not,  that  the  changes  are  effected  by  means  of  the  lymph.  The 
blood  may  thus  be  regarded  as  an  internal  medium  bearing  the  same  rela- 
tions to  the  constituent  tissues  that  the  external  medium,  the  world,  does  to 
the  whole  individual.  Just  as  the  whole  organism  lives  on  the  things 
around  it,  its  air  and  its  food,  so  the  several  tissues  live  on  the  complex 
fluid  by  which  they  are  all  bathed,  and  which  is  to  them  their  immediate 
air  and  food. 

All  the  tissues  take  up  oxygen  from  the  blood  and  give  up  carbonic  acid 
to  the  blood,  but  not  always  at  the  same  rate  or  at  the  same  time.  More- 
over, the  several  tissues  take  up  from  the  blood  and  give  up  to  the  blood 
either  different  things  or  the  same  things  at  different  rates  or  at  different 
times. 

From  this  it  follows,  on  the  one  hand,  that  the  composition  and  charac- 
ters of  the  blood  must  be  forever  varying  in  different  parts  of  the  body  and 
at  different  times ;  and  on  the  other  hand,  that  the  united  action  of  all  the 
tissues  must  tend  to  establish  and  maintain  an  average  uniform  composition 
of  the  whole  mass  of  blood.  The  special  changes  which  blood  is  known  to 
undergo  while  it  passes  through  the  several  tissues  will  best  be  dealt  with 
when  the  individual  tissues  and  organs  come  under  our  consideration.  At 
present  it  will  be  sufficient  to  study  the  main  features  which  are  presented 
by  blood  brought,  so  to  speak,  into  a  state  of  equilibrium  by  the  common 
action  of  all  the  tissues. 

Of  all  these  main  features  of  blood  the  most  striking,  if  not  the  most 
important,  is  the  property  it  possesses  of  clotting  when  shed. 

THE  CLOTTING  OF  BLOOD. 

§  14.  Blood,  when  shed  from  the  bloodvessels  of  a  living  body,  is  per- 
fectly fluid.  In  a  short  time  it  becomes  viscid,  this  viscidity  increasing  rap- 
idly until  the  whole  mass  of  blood  under  observation  becomes  a  complete 
jelly.  The  vessel  into  which  it  has  been  shed  can  at  this  stage  be  inverted 
without  a  drop  of  the  blood  being  spilt.  The  jelly  is  of  the  same  bulk  as 
the  previously  fluid  blood,  and  if  carefully  shaken  out  will  present  a  com- 
plete mould  of  the  interior  of  the  vessel.  [Fig.  2.]  If  the  blood  in  this 
jelly  stage  be  left  untouched  in  a  glass  vessel,  a  few  drops  of  an  almost 
colorless  fluid  soon  make  their  appearance  on  the  surface  of  the  jelly. 
Increasing  in  number,  and  running  together,  the  drops  after  a  while  form 
a  superficial  layer  of  pale,  straw-colored  fluid.  Later  on,  similar  layers 
of  the  same  fluid  are  seen  at  the  sides  and  finally  at  the  bottom  of  the  jelly, 
which,  shrunk  to  a  smaller  size  and  of  firmer  consistency,  now  forms  a  clot 
or  crassamentum,  floating  in  a  perfectly  fluid  serum.  [Fig.  3.]  The  shrinking 
and  condensation  of  the  clot,  and  the  corresponding  increase  of  the  serum, 
continue  for  some  time.  The  upper  surface  of  the  clot  is  generally  slightly 
concave.  A  portion  of  the  clot  examined  under  the  microscope  is  seen  to 


THE  CLOTTING  OF  BLOOD. 


27 


consist  of  a  feltwork  of  fine  granular  fibrils,  in  the  meshes  of  which  are 
entangled  the  red  and  white  corpuscles  of  the  blood.    In  the  serum  nothing 


[FIG.  2. 


[FIG.  3. 


Bowl  of  recently  coagulated  blood, 
showing  the  whole  mass  uniformly 
solidified.  After  Daltou.] 


Bowl  of  coagulated  blood,  after  twelve 
hours,  showing  the  clot  contracted  and  float- 
ing in  the  fluid  serum.  After  Dalton.] 


[FIG.  4. 


can  be  seen  but  a  few  stray  corpuscles,  chiefly  white.     The  fibrils  are  com- 
posed of  a  substance  called  fibrin.     [Fig.  4.]     Hence  we  may  speak  of  the 
clot  as  consisting  of  fibrin  and  cor- 
puscles ;  and  the  act  of  clotting  is 
obviously    a    substitution     for    the 
plasma  of   fibrin    and    serum,    fol- 
lowed by  a  separation  of  the  fibrin 
and   corpuscles  from  the  serum. 

In  man,  blood  when  shed  becomes 
viscid  in  about  two  or  three  minutes, 
and  enters  the  jelly  stage  in  about 
five  or  ten  minutes.  After  the  lapse 
of  another  few  minutes  the  first 
drops  of  serum  are  seen,  and  clotting 
is  generally  complete  in  from  one  to 
several  hours.  The  time,  however, 
will  be  found  to  vary  according  to 
circumstances.  Among  animals  the 
rapidity  of  clotting  varies  exceed- 
ingly in  different  species.  The 
blood  of  the  horse  clots  with  re- 
markable slowness ;  so  slowly,  in- 
deed, that  many  of  the  red  and  also 

some  of  the  white  corpuscles  (both  these  being  specifically  heavier  than 
the  plasma)  have  time  to  sink  before  viscidity  sets  in.  In  consequence 
there  appears  on  the  surface  of  the  blood  an  upper  layer  of  colorless  plasma 
containing  in  its  deeper  portions  many  colorless  corpuscles  (which  are  lighter 
than  the  red).  This  layer  clots  like  the  other  parts  of  the  blood,  forming 
the  so-called  "  buffy  coat."  A  similar  buffy  coat  is  sometimes  seen  in  the 
blood  of  man  in  certain  abnormal  conditions  of  the  body. 

If  a  portion  of  horse's  blood  be  surrounded  by  a  cooling  mixture  of  ice 
and  salt,  and  thus  kept  about  0°  C.,  clotting  may  be  almost  indefinitely 
postponed.  Under  these  circumstances  a  more  complete  descent  of  the  cor- 
puscles takes  place,  and  a  considerable  quantity  of  colorless  transparent 
plasma  free  from  blood  corpuscles  may  be  obtained.  A  portion  of  this 
plasma  removed  from  the  freezing  mixture  clots  in  the  same  manner  as  does 
the  entire  blood.  It  first  becomes  vascid  and  then  forms  a  jelly,  which  sub- 
sequently separates  into  a  colorless  shrunken  clot  and  serum.  This  shows 
that  the  corpuscles  are  not  an  essential  part  of  the  clot. 


Coagulated  fibrin,  showing  its  fibrillated  con- 
dition.   After  Dalton.] 


28  BLOOD. 

If  a  few  cubic  centimetres  of  this  colorless  plasma,  or  of  a  similar  plasma 
which  may  be  obtained  from  almost  any  blood  by  means  which  we  will 
presently  describe,  be  diluted  with  many  times  its  bulk  of  a  0.6  per  cent, 
solution  of  sodium  chloride1  clotting  is  much  retarded,  and  the  various  stages 
may  be  more  easily  watched.  As  the  fluid  is  becoming  viscid,  fine  fibrils 
of  fibrin  will  be  seen  to  be  developed  in  it,  especially  at  the  sides  of  the  con- 
taining vessel.  As  these  fibrils  multiply  in  number,  the  fluid  becomes  more 
and  more  of  the  consistence  of  a  jelly  and  at  the  same  time  somewhat  opaque. 
Stirred  or  pulled  about  with  a  needle,  the  fibrils  shrink  up  into  a  small 
opaque  stringy  mass ;  and  a  very  considerable  bulk  of  the  jelly  may  by 
agitation  be  resolved  into  a  minute  fragment  of  shrunken  fibrin  floating  in  a 
quantity  of  what  is  really  diluted  serum.  If  a  specimen  of  such  diluted 
plasma  be  stirred  from  time  to  time,  as  soon  as  clotting  begins,  with  a  needle 
or  glass  rod,  the  fibrin  may  be  removed  piecemeal  as  it  forms,  and  the  jelly 
stage  may  be  altogether  done  away  with.  When  fresh  blood  which  has  not 
yet  had  time  to  clot  is  stirred  or  whipped  with  a  bundle  of  rods  (or  anything 
presenting  a  large  amount  of  rough  surface),  no  jelly-like  clotting  takes 
place,  but  the  rods  become  covered  with  a  mass  of  shrunken  fibrin.  Blood 
thus  whipped  until  fibrin  ceases  to  be  deposited,  is  found  to  have  entirely 
lost  its  power  of  clotting. 

Putting  these  facts  together,  it  is  very  clear  that  the  phenomena  of  the 
clotting  of  blood  are  caused  by  the  appearance  in  the  plasma  of  fine  fibrils 
of  fibrin.  So  long  as  these  are  scanty,  the  blood  is  simply  viscid.  When 
they  become  sufficiently  numerous,  they  give  the  blood  the  firmness  of  a 
jelly.  Soon  after  their  formation  they  begin  to  shrink,  and  while  shrinking 
enclose  in  their  meshes  the  corpuscles,  but  squeeze  out  the  fluid  parts  of 
the  blood.  Hence  the  appearance  of  the  shrunken  colored  clot  and  the 
colorless  serum. 

§  15.  Fibrin,  whether  obtained  by  whipping  freshly  shed  blood,  or  by 
washing  either  a  normal  clot,  or  a  clot  obtained  from  colorless  plasma, 
exhibits  the  same  general  characters.  It  belongs  to  that  class  of  complex 
unstable  nitrogenous  bodies  called  proteids,  which  form  a  large  portion  of 
all  living  bodies  and  an  essential  part  of  all  living  structures. 

Our  knowledge  of  proteids  is  at  present  too  imperfect,  and  probably  none 
of  them  have  yet  been  prepared  in  adequate  purity  to  justify  us  in  attempt- 
ing to  assign  to  them  any  definite  formula  ;  but  it  is  important  to  remember 
their  general  composition.  100  parts  of  a  proteid  contain  rather  more  than 
50  parts  of  carbon,  rather  more  than  15  of  nitrogen,  about  7  of  hydrogen, 
and  rather  more  than  20  of  oxygen ;  that  is  to  say,  they  contain  about  half 
their  weight  of  carbon,  and  only  about  i  their  weight  of  nitrogen  ;  and  yet, 
as  we  shall  see,  they  are  eminently  the  nitrogenous  substances  of  the  body. 
They  usually  contain  a  small  quantity  (1  or  2  per  cent.)  of  sulphur,  and 
many  also  have  some  phosphorus  attached  to  them  in  some  way  or  other. 
When  burnt  they  leave  a  variable  quantity  of  ash,  consisting  of  inorganic 
salts  of  which  the  bases  are  chiefly  sodium  and  potassium,  and  the  acids 
chiefly  hydrochloric,  sulphuric,  phosphoric,  and  carbonic. 

They  all  give  certain  reactions,  by  which  their  presence  may  be  recog- 
nized; of  these  the  most  characteristic  are  the  following:  Boiled  with  nitric 
acid  they  give  a  yellow  color,  which  deepens  into  orange  upon  the  addition  of 
ammonia.  This  is  called  the  xanthoproteie  test ;  the  color  is  due  to  a  product 
of  decomposition.  Boiled  with  the  mixture  of  mercuric  and  mercurous 
nitrates  known  as  Millon's  reagent,  they  give  a  pink  color.  Mixed  with  a 
strong  solution  of  sodic  hydrate  they  give,  on  the  addition  of  a  drop  or  two  of 

1  A  solution  of  sodium  chloride  of  this  strength  will  hereafter  be  spoken  of  as  "  normal 
saline  solution." 


THE  CLOTTING  OF  BLOOD.  29 

a  very  weak  solution  of  cupric  sulphate,  a  violet  or  pink  color  which  deepens 
on  heating.  These  are  artificial  reactions,  not  throwing  much  if  any  light 
on  the  constitution  of  proteids ;  but  they  are  useful  as  practical  tests  enabling 
us  to  detect  their  presence.  ' 

The  several  members  of  the  proteid  group  are  at  present  distinguished 
from  each  other  chiefly  by  their  respective  solubilities,  especially  in  various 
saline  solutions.  Fibrin  is  one  of  the  least  soluble ;  it  is  insoluble  in  water, 
almost  insoluble  in  dilute  neutral  saline  solutions,  and  very  sparingly  soluble 
in  more  concentrated  neutral  saline  solutions  and  in  dilute  acids  and  alkalies. 
In  strong  acids  and  alkalies  it  dissolves,  but  in  the  process  becomes  com- 
pletely changed  into  something  which  is  no  longer  fibrin.  In  dilute  acids  it 
swells  up  and  becomes  transparent,  but  when  the  acid  is  neutralized  returns 
to  its  previous  condition.  When  suspended  in  water  and  heated  to  100°  C., 
or  even  to  75°  C.,  it  becomes  changed,  and  still  less  soluble  than  before  ;  it  is 
said  in  this  case  to  be  coagulated  by  the  heat,  and,  as  we  shall  see,  nearly  all 
proteids  have  the  property  of  being  changed  in  nature,  of  undergoing 
coagulation  and  so  becoming  less  soluble  than  before,  by  being  exposed  to  a 
certain  high  temperature. 

Fibrin,  then,  is  a  proteid  distinguished  from  other  proteids  by  its  smaller 
solubility ;  it  is  further  distinguished  .by  its  peculiar  filamentous  structure, 
the  other  proteids  when  obtained  in  a  solid  form  appearing  either  in  amor- 
phous granules  or,  at  most,  in  viscid  masses. 

§  16.  We  may  now  return  to  the  serum. 

This  is  perfectly  fluid,  and  remains  fluid  until  it  decomposes.  It  is  of  a 
faint  straw-color,  due  to  the  presence  of  a  special  pigment  substance,  differing 
from  the  red  matter  which  gives  redness  to  the  red  corpuscles. 

Tested  by  the  xanthoproteic  and  other  tests  it  obviously  contains  a  large 
quantity  of  proteid  matter,  and  upon  examination  we  find  that  at  least  two 
distinct  proteid  substances  are  present  in  it. 

If  crystals  of  magnesium  sulphate  be  added  to  serum  and  gently  stirred 
until  they  dissolve,  it  will  be  seen  that  the  serum  as  it  approaches  saturation 
with  the  salt  becomes  turbid  instead  of  remaining  clear,  and  eventually  a 
white  amorphous  granular  or  flocculent  precipitate  makes  its  appearance. 
This  precipitate  may  be  separated  by  decantation  or  filtration,  washed  with 
saturated  solutions  of  magnesium  sulphate,  in  which  it  is  insoluble,  until  it 
is  freed  from  all  other  constituents  of  the  serum,  and  thus  obtained  fairly 
pure.  It  is  then  found  to  be  a  proteid  body,  distinguished  by  the  following 
characters  among  others : 

1.  It  is  (when  freed  from  any  adherent  magnesium  sulphate)  insoluble  in 
distilled  water ;  it  is  insoluble  in  concentrated  solutions  of  neutral  saline 
bodies,  such  as  magnesium  sulphate,  sodium  chloride,  etc.,  but  readily  soluble 
in  dilute  (e.  g.,  1  per  cent.)  solutions  of  the  same  neutral  saline  bodies. 
Hence  from  its  solutions  in  the  latter  it  maybe  precipitated  either  by  adding 
more  neutral  saline  substance  or  by  removing  by  dialysis  the  small  quantity 
of  saline  substance  present.     When  obtained  in  a  precipitated  form,  and 
suspended  in  distilled  water,  it  readily  dissolves  into  a  clear  solution  upon 
the  addition  of  a  small  quantity  of  some  neutral  saline  body.     By  these 
various  solutions  and  precipitations  it  is  not  really  changed  in  nature. 

2.  It  readily  dissolves  in  very  dilute  acids  (e.  g.,  in  hydrochloric  acid  even 
when  diluted  to  far  less  than  1  per  cent.),  and  it  is  similarly  soluble  in  dilute 
alkalies,  but  in  being  thus  dissolved  it  is  wholly  changed  in  nature,  and  the 
solutions  of  it  in  dilute  acid  and  dilute  alkalies  gives  reactions  quite  different 
from  those  of  the  solution  of  the  substance  in  dilute  neutral  saline  solutions. 
By  the  acid  it  is  converted  into  what  is  called  acid-albumin,  by  the  alkali 
into  alkali-albumin,  both  of  which  bodies  we  shall  have  to  study  later  on. 


30  BLOOD. 

3.  When  it  is  suspended  in  water  and  heated  it  becomes  altered  in  char- 
acter, coagulated,  and  all  its  reactions  are  changed.  It  is  no  longer  soluble 
in  dilute  neutral  saline  solutions,  not  even  in  dilute  acids  and  alkalies ;  it 
has  become  coagulated  proteid,  and  is  now  even  less  soluble  than  fresh  fibrin. 
When  a  solution  of  it  in  dilute  neutral  saline  solution  is  similarly  heated,  a 
similar  change  takes  place,  a  precipitate  falls  down  which  on  examination 
is  found  to  be  coagulated  proteid.  The  temperature  at  which  this  change 
takes  place  is  somewhere  about  75°  C.,  though  shifting  slightly  according  to 
the  quantity  of  saline  substance  present  in  the  solution. 

One  of  the  proteids  present  in  blood-serum  is  paraglobulin,  characterized 
by  its  solubility  in  dilute  neutral  saline  solutions,  its  insolubility  in  distilled 
water  and  concentrated  saline  solutions,  its  ready  solubility,  and  at  the  same 
time  conversion  into  other  bodies,  in  dilute  acids  and  alkalies,  and  in  its 
becoming  converted  into  coagulated  proteid,  and  so  being  precipitated  from 
its  solutions  at  75°  C. 

These  reactions  are  given  by  a  number  of  proteid  bodies  forming  a  group 
called  globulins,  the  particular  globulin  present  in  blood-serum  being  para- 
globulin. 

The  amount  of  it  present  in  blood-serum  varies  in  various  animals,  and 
apparently  in  the  same  animal  at  different  times.  In  100  parts  by  weight  of 
serum  there  are  generally  present  about  8  or  9  parts  of  proteids  altogether, 
and  of  these  some  3  our  4,  more  or  less,  may  be  taken  as  paraglobulin. 

§  17.  If  the  serum  from  which  the  paraglobulin  has  been  precipitated  by 
the  addition  of  neutral  salt,  and  removed  by  filtration,  be  subjected  to  di- 
alysis, the  salt  added  may  be  removed,  and  a  clear,  somewhat  diluted  serum 
free  from  paraglobulin  may  be  obtained. 

This  still  gives  abundant  proteid  reactions,  so  that  the  serum  still  contains 
a  proteid,  or  some  proteids  still  more  soluble  than  the  globulins,  since  they 
will  remain  in  solution,  and  are  not  precipitated,  even  when  dialysis  is  con- 
tinued until  the  serum  is  practically  freed  from  both  the  neutral  salt  added 
to  it  and  the  diffusible  salts  previously  present  in  the  natural  serum. 

When  this  serum  is  heated  to  75°  C.  a  precipitate  makes  its  appearance  ; 
the  proteids  still  present  are  coagulated  at  this  temperature. 

We  have  some  reasons  for  thinking  that  more  than  one  proteid  is  present, 
but  they  are  all  closely  allied  to  each  other,  and  we  may  for  the  present 
speak  of  them  as  if  they  were  one,  and  call  the  proteid  left  in  serum,  after 
removal  of  the  paraglobulin,  by  the  name  of  albumin,  or,  to  distinguish  it 
from  other  albumins  found  elsewhere,  serum-albumin.  Serum-albumin  is 
distinguished  by  being  more  soluble  than  the  globulins,  since  it  is  soluble  in 
distilled  water,  even  in  the  absence  of  all  neutral  salts.  Like  the  glubulins, 
though  with  much  less  ease,  it  is  converted  by  dilute  acids  and  dilute  alkalies 
into  acid-  or  into  alkali-albumin.  The  percentage  amount  of  serum-albumin 
in  serum  may  be  put  down  as  4  or  5,  more  or  less,  but  it  varies  and  some- 
times is  less  abundant  than  paraglobulin.  In  some  animals  (snakes)  it  is 
said  to  disappear  during  starvation. 

The  more  important  characters  of  the  three  proteids  which  we  have  just 
studied  may  be  stated  as  follows : 

Soluble  in  distilled  water  and  in  saline  solutions  of  all  strengths  .    .     serum-albumin. 

Insoluble  in  distilled  water,  readily  soluble  in  dilute  saline  solutions, 

insoluble  in  concentrated  saline  solutions paraglobulin. 

Insoluble  in  distilled  water,  bardly  soluble  at  all  in  dilute  saline 
solutions,  and  very  little  soluble  in  more  concentrated  saline  solu- 
tions  fibrin. 

Besides  paraglobulin  and  serum-albumin,  serum  contains  a  very  large 
number  of  substances,  generally  in  small  quantity,  which,  since  they  have 


THE  CLOTTING   OF  BLOOD.  31 

to  be  extracted  by  special  methods,  are  called  extractives ;  of  these  some  are 
nitrogenous,  some  non-nitrogenous.  Serum  contains  besides  important  inor- 
ganic saline  substances ;  but  to  these  we  shall  return. 

§  18.  With  the  knowledge  which  we  have  gained  of  the  proteids  of  clotted 
blood  we  may  go  back  to  the  question  :  Clotting  being  due  to  the  appearance 
in  blood  plasma  of  a  proteid  substance,  fibrin,  which  previously  did  not  exist 
in  it  as  such,  what  are  the  causes  which  led  to  the  appearance  of  fibrin  ? 

We  learn  something  by  studying  circumstances  which  affect  the  rapidity 
with  which  the  blood  of  the  same  individual  clots  when  shed.  These  are  as 
follows : 

A  temperature  of  40°  C.,  which  is  about  or  slightly  above  the  tempera- 
ture of  the  blood  of  warm-blooded  animals,  is  perhaps  the  most  favorable  to 
clotting.  A  further  rise  of  a  few  degrees  is  apparently  also  beneficial,  or  at 
least  not  injurious  ;  but  upon  a  still  further  rise  the  effect  changes,  and  when 
blood  is  rapidly  heated  to  56°  C.  no  clotting  at  all  may  take  place.  At  this 
temperature  certain  proteids  of  the  blood  are  coagulated  and  precipitated 
before  clotting  can  take  place,  and  with  this  change  the  power  of  the  blood 
to  clot  is  wholly  lost.  If,  however,  the  heating  be  not  very  rapid,  the  blood 
may  clot  before  this  change  has  time  to  come  on.  When  the  temperature 
instead  of  being  raised  is  lowered  below  40°  C.  the  clotting  becomes  delayed 
and  prolonged  ;  and  at  the  temperature  of  0°  or  1°  C.  the  blood  will  remain 
fluid,  and  yet  capable  of  clotting  when  withdrawn  from  the  adverse  circum- 
stances, for  a  very  long,  it  might  almost  be  said,  for  an  indefinite  time. 

A  small  quantity  of  blood  shed  into  a  small  vessel  clots  sooner  than  a 
large  quantity  shed  into  a  larger  one  ;  and  in  general  the  greater  the  amount 
of  foreign  surface  with  which  the  blood  comes  in  contact  the  more  rapid  the 
clotting.  When  shed  blood  is  stirred  or  "  whipped  "  the  fibrin  makes  its 
appearance  sooner  than  when  the  blood  is  left  to  clot  in  the  ordinary  way ; 
so  that  here,  too,  the  accelerating  influence  of  contact  with  foreign  bodies 
makes  itself  felt.  Similarly,  movement  of  shed  blood  hastens  clotting,  since 
it  increases  the  amount  of  contact  with  foreign  bodies.  So  also  the  addition 
of  spongy  platinum  or  of  powdered  charcoal,  or  of  other  inert  powders,  to 
tardily  clotting  blood  will,  by  influence  of  surface,  hasten  clotting.  Con- 
versely, blood  brought  into  contact  with  pure  oil  does  not  clot  so  rapidly  as 
when  in  contact  with  glass  or  metal ;  and  blood  will  continue  to  flow  for  a 
longer  time  without  clotting  through  a  tube  smeared  inside  with  oil  than 
through  a  tube  not  so  smeared.  The  influence  of  the  oil  in  such  cases  is  a 
physical  not  a  chemical  one ;  any  pure  neutral  inert  oil  will  do.  As  far  as 
we  know  these  influences  affect  only  the  rapidity  with  which  the  clotting 
takes  place — that  is,  the  rapidity  with  which  the  fibrin  makes  its  appear- 
ance, not  the  amount  of  clot,  not  the  quantity  of  fibrin  formed,  though 
when  clotting  is  very  much  retarded  by  cold  changes  may  ensue  whereby 
the  amount  of  clotting  which  eventually  takes  place  is  indirectly  affected. 

Mere  exposure  to  air  exerts  apparently  little  influence  on  the  process  of 
clotting.  Blood  collected  direct  from  a  bloodvessel  over  mercury  so  as 
wholly  to  exclude  the  air,  clots  as  readily  as  blood  freely  exposed  to  the  air. 
It  is  only  when  blood  is  much  laden  with  carbonic  acid,  the  presence  of 
which  is  antagonistic  to  clotting,  that  exclusion  of  air,  by  hindering  the 
escape  of  the  excess  of  carbonic  acid,  delays  clotting. 

These  facts  teach  us  that  fibrin  does  not,  as  was  once  thought,  make  its 
appearance  in  shed  blood  because  the  blood  when  shed  ceases  to  share  in  the 
movement  of  the  circulation,  or  because  the  blood  is  cooled  on  leaving  the 
warm  body,  or  because  the  blood  is  then  more  freely  exposed  to  the  air  ;  they 
further  suggest  the  view  that  the  fibrin  is  the  result  of  some  chemical  change, 
the  conversion  into  fibrin  of  something  which  is  not  fibrin,  the  change  like 


32  BLOOD. 

other  chemical  changes  being  most  active  at  an  optimum  temperature,  and 
like  so  many  other  chemical  changes,  being  assisted  by  the  influences  exerted 
by  the  presence  of  inert  bodies. 

And  we  have  direct  experimental  evidence  that  plasma  does  contain  an 
antecedent  of  fibrin  which,  by  chemical  change,  is  converted  into  fibrin. 

§  19.  If  blood  be  received  direct  from  the  bloodvessels  into  one-third  its 
bulk  of  a  saturated  solution  of  some  neutral  salt  such  as  magnesium  sulphate, 
and  the  two  gently  but  thoroughly  mixed,  clotting,  especially  at  a  moder- 
ately low  temperature,  will  be  deferred  for  a  very  long  time.  If  the  mixture 
be  allowed  to  stand,  the  corpuscles  will  sink,  and  a  colorless  plasma  will  be 
obtained  similar  to  the  plasma  gained  from  horse's  blood  by  cold,  except  that 
it  contains  an  excess  of  the  neutral  salt.  The  presence  of  the  neutral  salt 
has  acted  in  the  same  direction  as  cold ;  it  has  prevented  the  occurrence  of 
clotting.  It  has  not  destroyed  the  fibrin  ;  for  if  some  of  the  plasma  be  diluted 
with  from  five  to  ten  times  its  bulk  of  water,  it  will  clot  speedily  in  quite  a 
normal  fashion,  with  the  production  of  quite  normal  fibrin. 

The  separation  of  the  fluid  plasma  from  the  corpuscles  and  from  other  bodies 
heavier  than  the  plasma  is  much  facilitated  by  the  use  of  the  centrifugal  machine. 
This  consists  essentially  of  a  tireless  \yheel  with  several  spokes,  placed  in  a  hori- 
zontal position  and  made  to  revolve  with  great  velocity  (1000  revolutions  per  min- 
ute for  instance)  around  its  axis.  Tubes  of  metal  or  of  very  strong  glass  are  sus- 
pended at  the  ends  of  the  spokes  by  carefully  adjusted  joints.  As  the  wheel 
rotates  with  increasing  velocity,  each  tube  gradually  assumes  a  horizontal  posi- 
tion, bottom  outward,  without  spilling  any  of  its  contents.  As  the  rapid  rotation 
continues  the  corpuscles  and  heavier  particles  are  driven  to  the  bottom  of  the  tube, 
and  if  a  very  rapid  movement  he  continued  for  a  long  time  will  form  a  compact 
cake  at  the  bottom  of  the  tube.  When  the  rotation  is  stopped  the  tubes  gradually 
return  to  their  upright  position  again  without  anything  being  spilt,  and  the  clear 
plasma  in  each  tube  can  then  be  decanted  off. 

If  some  of  the  colorless  transparent  plasma,  obtained  either  by  the  action 
of  neutral  salts  from  any  blood,  or  by  the  help  of  cold  from  horse's  blood,  be 
treated  with  some  solid  neutral  salt,  such  as  sodium  chloride,  to  saturation, 
a  white  flaky,  somewhat  sticky  precipitate  will  make  its  appearance.  If  this 
precipitate  be  removed,  the  fluid  no  longer  possesses  the  power  of  clotting 
(or  very  slightly  so),  even  though  the  neutral  salt  present  be  removed  by 
dialysis,  or  its  influence  lessened  by  dilution.  With  the  removal  of  the  sub- 
stance precipitated,  the  plasma  has  lost  its  power  of  clotting. 

If  the  precipitate  itself,  after  being  washed  with  a  saturated  solution  of 
the  neutral  salt  (in  which  it  is  insoluble)  so  as  to  get  rid  of  all  serum  and 
other  constituents  of  the  plasma,  be  treated  with  a  small  quantity  of  water, 
it  readily  dissolves,1  and  the  solution  rapidly  filtered  gives  a  clear,  colorless 
filtrate,  which  is  at  first  perfectly  fluid.  Soon,  however,  the  fluidity  gives 
way  to  viscidity,  and  this  in  turn  to  a  jelly  condition,  and  finally  the  jelly 
shrinks  into  a  clot  floating  in  a  clear  field  ;  in  other  words,  the  filtrate  clots 
like  plasma.  Thus  there  is  present  in  cooled  plasma,  and  in  plasma  kept 
from  clotting  by  the  presence  of  neutral  salts,  a  something  precipitable  by 
saturation  with  neutral  salts — a  something  which,  since  it  is  soluble  in  very 
dilute  saline  solutions,  cannot  be  fibrin  itself,  but  which  in  solution  speedily 
gives  rise  to  the  appearance  of  fibrin.  To  this  substance  its  discoverer, 
Denis,  gave  the  name  of  plasmine. 

The  substance  thus  precipitated  is  not  however  a  single  body,  but  a  mix- 
turo  of  at  least  two  bodies.  If  sodium  chloride  be  carefully  added  to  plasma 
to  an  extent  of  about  13  per  cent,  a  white  flaky  viscid  precipitate  is  thrown 

1  The  substance  itself  is  not  soluble  in  distilled  water,  but  a  quantity  of  the  neutral 
•salt  always  clings  to  the  precipitate,  and  thus  the  addition  of  water  virtually  gives  rise 
to  dilute  saline  solution,  in  which  the  substance  is  readily  soluble. 


THE  CLOTTING  OF  THE  BLOOD.  33 

down  very  much  like  plasmine.  If  after  the  removal  of  the  first  precipitate 
more  sodium  chloride,  and  especially  if  magnesium  sulphate  be  added,  a 
second  precipitate  is  thrown  down,  less  viscid  and  more  granular  than  the 
first. 

The  second  precipitate  when  examined  is  found  to  be  identical  with  the 
paraglobulin,  coagulating  at  75°  C.,  which  we  have  already  seen  to  be  a 
constituent  of  serum. 

The  first  precipitate  is  also  a  proteid  belonging  to  the  globulin  group, 
but  differs  from  paraglobulin,  not  only  in  being  more  readily  precipitated 
by  sodium  chloride,  and  in  being  when  precipitated  more  viscid,  but  also  in 
other  respects,  and  especially  in  being  coagulated  at  a  far  lower  temperature 
than  paraglobulin,  viz.,  at  56°  C.  Now,  while  isolated  paraglobulin  cannot 
by  any  means  known  to  us  be  converted  into  fibrin,  and  as  its  presence  in 
the  so-called  plasmine  does  not  seem  to  be  essential  to  the  formation  of 
fibrin  out  of  plasmine,  the  presence  in  plasmine  of  the  body  coagulating  at 
56°  C.  does  seem  essential  to  the  conversion  of  plasmine  into  fibrin,  and  we 
have  reason  for  thinking  that  it  is  itself  converted,  in  part  at  least,  into 
fibrin.  Hence  it  has  received  the  name  of  fibrinogen. 

§  20.  The  reasons  for  this  view  are  as  follows  : 

Besides  blood,  which  clots  naturally  when  shed,  there  are  certain  fluids 
in  the  body  which  do  not  clot  naturally,  either  in  the  body  or  when  shed, 
but  which  by  certain  artificial  means  may  be  made  to  clot,  and  in  clotting 
to  yield  quite  normal  fibrin. 

Thus  the  so-called  serous  fluid  taken  some  hours  after  death1  from  the 
pericardial,  pleural,  or  peritoneal  cavities,  the  fluid  found  in  the  enlarged 
serous  sac  of  the  testis,  known  as  hydrocele  fluid,  and  other  similar  fluids, 
will  in  the  majority  of  cases,  when  obtained  free  from  blood  or  other  admix- 
tures, remain  fluid  almost  indefinitely,  showing  no  disposition  whatever  to 
clot.2  Yet,  in  most  cases  at  all  events,  these  fluids,  when  a  little  blood,  or  a 
piece  of  blood  clot,  or  a  little  serum  is  added  to  them,  will  clot  rapidly  and 
firmly,3  giving  rise  to  an  unmistakable  clot  of  normal  fibrin,  differing  only 
from  the  clot  of  blood  in  that,  when  serum  is  used,  it  is  colorless,  being  free 
from  red  corpuscles. 

Now  blood  (or  blood  clot,  or  serum)  contains  many  things,  to  any  one  of 
which  the  clotting  power  thus  seen  might  be  attributed.  But  it  is  found 
that  in  many  cases  clotting  may  be  induced  in  the  fluids  of  which  we  are 
speaking  by  the  mere  addition,  and  that  even  in  exceedingly  small  quantity, 
of  a  substance  which  can  be  extracted  from  blood,  or  from  serum,  or  from 
blood  clot,  or  even  from  washed  fibrin,  or  indeed  from  other  sources,  a  sub- 
stance whose  exact  nature  is  uncertain,  it  being  doubtful  whether  it  is  a 
proteid  at  all,  and  whose  action  is  peculiar. 

If  serum,  or  whipped  blood  or  a  broken-up  clot  be  mixed  with  a  large 
quantity  of  alcohol  and  allowed  to  stand  some  days,  the  proteids  present  are 
in  time  so  changed  by  the  alcohol  as  to  become  insoluble  in  water.  Hence 
if  the  copious  precipitate  caused  by  the  alcohol,  after  longstanding,  be  sepa- 
rated by  filtration  from  the  alcohol,  dried  at  a  low  temperature,  not  exceed- 
ing 40°  C.,  and  extracted  with  distilled  water,  the  aqueous  extract  contains 
very  little  proteid  matter,  indeed  very  little  organic  matter  at  all.  Never- 
theless, even  a  small  quantity  of  this  aqueous  extract  added  alone  to  certain 
specimens  of  hydrocele  fluid  or  other  of  the  fluids  spoken  of  above,  will  bring 
about  a  speedy  clotting.  The  same  aqueous  extract  has  also  a  remarkable 

1  If  it  be  removed  immediately  after  death  it  generally  clots  readily  and  firmly,  giving 
a  colorless  clot  consisting  of  fibrin  and  white  corpuscles. 

2  In  some  specimens,  however,  a  spontaneous  coagulation,  generally  slight,  but  in  ex- 
ceptional cases  massive,  may  be  observed. 

3  In  a  few  cases  no  coagulation  can  thus  be  induced. 

3 


34  BLOOD. 

effect  in  hastening  the  clotting  of  fluids  which,  though  they  will  eventually 
clot,  do  so  very  slowly.  Thus  plasma  may,  by  the  careful  addition  of  a  cer- 
tain quantity  of  neutral  salt  and  water,  be  reduced  to  such  a  condition  that 
it  clots  very  slowly  indeed,  taking  perhaps  days  to  complete  the  process.  The 
addition  of  a  small  quantity  of  the  aqueous  extract  we  are  describing  will, 
however,  bring  about  a  clotting  which  is  at  once  rapid  and  complete. 

The  active  substance,  whatever  it  be,  in  this  aqueous  extract  exists  in 
small  quantity  only,  and  its  clotting  virtues  are  at  once  and  forever  lost 
when  the  solution  is  boiled.  Further,  there  is  no  reason  to  think  that  the 
active  substance  actually  enters  into  the  formation  of  the  fibrin  to  which  it. 
gives  rise.  It  appears  to  belong  to  a  class  of  bodies  playing  an  important 
part  in  physiological  processes,  and  called  ferments,  of  which  we  shall  have 
more  to  say  hereafter.  We  may,  therefore,  speak  of  it  as  the  fibrin  ferment, 
the  name  given  to  it  by  its  discoverer,  Alexander  Schmidt. 

This  fibrin  ferment  is  present  in  and  may  be  extracted  from  clotted  or 
whipped  blood,  and  from  both  the  clot1  and  the  serum  of  clotted  blood ;  and 
since  in  most,  if  not  all,  cases  where  blood  or  blood  clot  or  serum  produces 
clotting  in  hydrocele  or  pericardial  fluid,  an  exactly  similar  clotting  may  be 
induced  by  the  mere  addition  of  fibrin  ferment,  we  seem  justified  in  con- 
cluding that  the  clotting  virtues  of  the  former  are  due  to  the  ferment  which 
they  contain. 

Now,  when  fibrinogen  is  precipitated  from  plasma,  as  above  described,  by 
sodium  chloride,  redissolved,  and  reprecipitated,  more  than  once,  it  may  be 
obtained  in  solution,  by  help  of  a  dilute  neutral  saline  solution,  in  an  ap- 
proximately pure  condition,  at  all  events  free  from  other  proteids.  Such  a 
solution  will  not  clot  spontaneously ;  it  may  remain  fluid  indefinitely ;  and 
yet  on  the  addition  of  a  little  fibrin  ferment  it  will  clot  readily  and  firmly, 
yielding  quite  normal  fibrin. 

This  body  fibrinogen  is  also  present  and  may  be  separated  out  from  the 
specimens  of  hydrocele,  pericardial,  and  other  fluids  which  clot  on  the 
addition  of  fibrin  ferment,  and  when  the  fibrinogen  has  been  wholly  removed 
from  these  fluids  they  refuse  to  clot  on  the  addition  of  fibrin  ferment. 

Paraglobulin,  on  the  other  hand,  whether  prepared  from  plasmine  by 
separation  of  the  fibrinogen,  or  from  serum,  or  from  other  fluids  in  which  it 
is  found,  cannot  be  converted  by  fibrin  ferment,  or  indeed  by  any  other 
means,  into  fibrin.  And  fibrinogen  isolated,  as  described  above,  or  serous 
fluids  which  contain  fibrinogen,  can  be  made,  by  means  of  fibrin  ferment,  to 
yield  quite  normal  fibrin  in  the  complete  absence  of  paraglobulin.  A  solu- 
tion of  paraglobulin  obtained  from  serum  or  blood  clot  will,  it  is  true,  clot 
pericardial  or  hydrocele  fluids  containing  fibrinogen,  or  indeed  a  solution  of 
fibrinogen,  but  this  is  apparently  due  to  the  fact  that  the  paraglobulin  has 
in  these  cases  some  fibrin  ferment  mixed  with  it ;  it  is  also  possible  that, 
under  certain  conditions,  the  presence  of  paraglobulin  may  be  favorable  to 
the  action  of  the  ferment. 

When  the  so-called  plasmine  is  precipitated,  as  directed  in  §  19,  fibrin 
ferment  is  carried  down  with  the  fibrinogen  and  paraglobulin,  and  when  the 
plasmine  is  re-dissolved  the  ferment  is  present  in  the  solution  and  ready  to 
act  on  the  fibrinogen.  Hence  the  re-dissolved  plasmine  clots  spontaneously. 
When  fibrinogeu  is  isolated  from  plasma  by  repeated  precipitation  and 
solution,  the  ferment  is  washed  away  from  it,  and  the  pure  ferment-free 
fibrinogen,  ultimately  obtained,  does  not  clot  spontaneously. 

So  far  it  seems  clear  that  there  does  exist  a  proteid  body,  fibrinogen, 
which  may  by  the  action  of  fibrin  ferment  be  directly,  without  the  interven- 

1  A  powerful  solution  of  fibrin  ferment  may  be  readily  prepared  by  simply  extracting 
a  washed  blood  clot  with  a  10  per  cent,  solution  of  sodium  chloride. 


THE  CLOTTING  OF  BLOOD.  35 

tion  of  other  proteids,  converted  into  the  less  soluble  fibrin.  Our  knowledge 
of  the  constitution  of  proteid  bodies  is  too  imperfect  to  enable  us  to  make 
any  very  definite  statement  as  to  the  exact  nature  of  the  change  thus 
effected  ;  but  we  may  say  this  much.  Fibrinogen  and  fibrin  have  about  the 
same  elementary  composition,  fibrin  containing  a  trifle  more  nitrogen. 
When  fibrinogen  is  converted  into  fibrin  by  means  of  fibrin  ferment,  the 
weight  of  the  fibrin  produced  is  always  less  than  that  of  the  fibrinogen 
which  is  consumed,  and  there  is  always  produced  at  the  same  time  a  certain 
quantity  of  another  proteid,  belonging  to  the  globulin  family.  There  are 
reasons,  however,  why  we  cannot  speak  of  the  ferment  as  splitting  up 
fibrinogen  into  fibrin  and  a  globulin ;  it  seems  more  probable  that  the  fer- 
ment converts  the  fibrinogen  first  into  a  body  which  we  might  call  soluble 
fibrin,  and  then  turns  this  body  into  a  veritable  fibrin  ;  but  further  inquiries 
on  the  subject  are  needed. 

It  may  be  added  that  among  the  conditions  necessary  for  the  due  action 
of  fibrin  ferment  on  fibrinogen,  the  presence  of  a  certain  quantity  of  some 
neutral  salt  seems  to  be  one.  In  the  total  absence  of  all  neutral  salts  the 
ferment  cannot  convert  the  fibrinogen  into  fibrin.  There  are  some  reasons 
also  for  thinking  that  the  presence  of  a  lime  salt,  such  as  calcium  sulphate, 
though  it  may  be  in  minute  quantity  only,  is  essential.  If  the  calcium 
salts  are  taken  out  of  fibrinogen,  the  fibrinogen  no  longer  coagulates  upon 
the  addition  of  fibrin  ferment  free  from  lime  salts.  Oxalate  of  potassium 
added  to  freshly  drawn  blood  also  prevents  coagulation,  apparently  because 
of  the  precipitation  of  the  lime  salts ;  the  addition  now  of  calcium  salts  is 
promptly  followed  by  coagulation. 

§  21.  We  may  conclude,  then,  that  the  plasma  of  blood  when  shed,  or, 
at  all  events,  soon  after  it  has  been  shed,  contains  fibrinogen ;  and  it  also 
seems  probable  that  the  clotting  comes  about  because  the  fibrinogen  is  con- 
verted into  fibrin  by  the  action  of  fibrin  ferment ;  but  we  are  still  far  from 
a  definite  answer  to  the  question,  why  blood  remains  fluid  in  the  body  and 
yet  clots  when  shed  ? 

We  have  already  said  that  blood,  or  blood  plasma,  brought  up  to  a  tem- 
perature of  56°  C.  as  soon  as  possible  after  its  removal  from  the  living 
bloodvessels,  gives  a  proteid  precipitate  and  loses  its  power  of  clotting. 
This  may  be  taken  to  show  that  blood,  as  it  circulates  in  the  living  blood- 
vessels, contains  fibrinogen  as  such,  and  that  when  the  blood  is  heated  to 
56°  C.,  which  is  the  coagulating  point  of  fibrinogen,  the  fibrinogen  present 
is  coagulated  and  precipitated,  and  consequently  no  fibrin  can  be  formed. 

Further,  while  clotted  blood  undoubtedly  contains  an  abundance  of 
fibrin  ferment,  no  ferment,  or  a  minimal  quantity  only,  is  present  in  blood 
as  it  leaves  the  bloodvessels.  If  the  blood  be  received  directly  from  the 
bloodvessels  into  alcohol,  the  aqueous  extract  prepared  as  directed  above 
contains  no  ferment,  or  merely  a  trace.  Apparently  the  ferment  makes  its 
appearance  in  the  blood  as  the  result  of  changes  taking  place  in  the  blood 
after  it  has  been  shed. 

We  might  from  this  be  inclined  to  conclude  that  blood  clots  when  shed, 
but  not  before,  because,  fibrinogen  being  always  present,  the  shedding  brings 
about  changes  which  produce  fibrin  ferment,  not  previously  existing,  and 
this  acting  on  the  fibrinogen  gives  rise  to  fibrin.  But  we  meet  with  the  fol- 
lowing difficulty  :  A  very  considerable  quantity  of  very  active  ferment  may 
be  injected  into  the  blood-current  of  a  living  animal  without  necessarily 
producing  any  clotting  at  all.  Obviously  either  blood  within  the  blood- 
vessels does  not  contain  fibrinogen  as  such,  and  the  fibrinogen  detected  by 
heating  the  blood  to  56°  C.  is  the  result  of  changes  which  have  already 
ensued  before  the  temperature  is  reached ;  or  in  the  living  circulation  there 


36  BLOOD. 

are  agencies  at  work  which  prevent  any  ferment  which  may  be  introduced 
into  the'  circulation  from  producing  its  usual  effect  on  fibrinogeii ;  or  there 
are  agencies  at  work  which  destroy,  or  do  away  with  the  fibrin,  little  by 
little,  as  it  is  formed. 

§  22.  And  indeed,  when  we  reflect  how  complex  blood  is  and  the  many 
and  great  changes  of  which  it  is  susceptible,  we  shall  not  wonder  that  the 
question  we  are  putting  cannot  be  offered  off  hand. 

The  corpuscles  with  which  blood  is  crowded  are  living  structures,  and 
consequently  are  continually  acting  upon  and  being  acted  upon  by  the 
plasma.  The  red  corpuscles  it  is  true  are,  as  we  shall  see,  peculiar  bodies, 
with  a  restricted  life  and  a  very  specialized  work,  and  possibly  their  influ- 
ence on  the  plasma  is  not  very  great ;  but  we  have  reason  to  think  that  the 
relations  between  the  white  corpuscles  and  the  plasma  are  close  and  im- 
portant. 

Then  again  the  blood  is  not  only  acting  upon  and  being  acted  upon  by 
the  several  tissues  as  it  flows  through  the  various  capillaries,  but  along 
the  whole  of  its  course  through  the  heart,  arteries,  capillaries,  and  veins,  is 
acting  upon  and  being  acted  upon  by  the  vascular  walls,  which  like  the  rest 
of  the  body  are  alive,  and  being  alive  are  continually  undergoing  and  pro- 
moting change. 

That  relations  of  some  kind,  having  a  direct  influence  on  the  clotting  of 
blood,  do  exist  between  the  blood  and  the  vascular  walls  is  shown  by  the 
following  facts : 

After  death,  when  all  motion  of  the  blood  has  ceased,  the  blood  remains 
for  a  long  time  fluid.  It  is  not  until  some  time  afterward,  at  an  epoch 
when  post-mortem  changes  in  the  blood  and  in  the  bloodvessels  have  had 
time  to  develop  themselves,  that  clotting  begins.  Thus  some  hours  after 
death  the  blood  in  the  great  veins  may  be  found  still  perfectly  fluid.  Yet 
such  blood  has  not  lost  its  power  of  clotting ;  it  still  clots  when  removed 
from  the  body,  and  clots  too  when  received  over  mercury  without  exposure 
to  air,  showing  that,  though  the  blood,  being  highly  venous,  is  rich  in  car- 
bonic acid  and  contains  little  or  no  oxygen,  its  fluidity  is  not  due  to  any 
excess  of  carbonic  acid  or  absence  of  oxygen.  Eventually  it  does  clot  even 
within  the  vessels,  but  perhaps  never  so  firmly  and  completely  as  when 
shed.  It  clots  first  in  the  larger  vessels,  but  remains  fluid  in  the  smaller 
vessels  for  a  very  long  time,  for  many  hours  in  fact,  since  in  these  the  same 
bulk  of  blood  is  exposed  to  the  influence  of,  and  reciprocally  exerts  an 
influence  on,  a  larger  surface  of  the  vascular  walls  than  in  the  larger  ves- 
sels. And  if  it  be  urged  that  the  result  is  here  due  to  influences  exerted 
by  the  body  at  large,  by  the  tissues  as  well  as  by  the  vascular  walls,  this 
objection  will  not  hold  good  against  the  following  experiment. 

If  the  jugular  vein  of  a  large  animal,  such  as  an  ox  or  horse,  be  carefully 
ligatured  when  full  of  blood,  and  the  ligatured  portion  excised,  the  blood 
in  many  cases  remains  perfectly  fluid,  along  the  greater  part  of  the  length 
of  the  piece,  for  twenty-four  or  even  forty-eight  hours.  The  piece  so  liga- 
tured may  be  suspended  in  a  framework  and  opened  at  the  top  so  as  to  imi- 
tate a  living  test-tube,  and  yet  the  blood  will  often  remain  long  fluid,  though 
a  portion  removed  at  any  time  into  a  glass  or  other  vessel  will  clot  in  a  few 
minutes.  If  two  such  living  test-tubes  be  prepared,  the  blood  may  be 
poured  from  one  to  the  other  without  clotting  taking  place. 

A  similar  relation  of  the  fluid  to  its  containing  living  wall  is  seen  in  the 
case  of  those  serous  fluids  which  clot  spontaneously.  If,  as  soon  after  death 
as  the  body  is  cold  and  the  fat  is  solidified,  the  pericardium  be  carefully  re- 
moved from  a  sheep  by  an  incision  round  the  base  of  the  heart,  the  peri- 
cardia! fluid  (which,  as  we  have  already  seen,  during  life  and  some  little 


THE  CLOTTING  OF  BLOOD.  37 

time  after  death,  possesses  the  power  of  clotting)  may  be  kept  in  the  peri- 
cardial  bag  as  in  a  living  cup  for  many  hours  without  clotting,  and  yet  a 
small  portion  removed  with  a  pipette  clots  at  once. 

This  relation  between  the  blood  and  the  vascular  wall  may  be  disturbed 
or  overridden  :  clotting  may  take  place  or  may  be  induced  within  the  living 
bloodvessel.  When  the  living  membrane  is  injured,  as  when  an  artery  or 
vein  is  sharply  ligatured,  or  when  it  is  diseased,  as  for  instance  in  aneurism, 
a  clot  is  apt  to  be  formed  at  the  injured  or  diseased  spot ;  and  in  certain 
morbid  conditions  of  the  body  clots  are  formed  in  various  vascular  tracts. 
Absence  of  motion,  which  in  shed  blood  as  we  have  seen  is  unfavorable  to 
clotting,  is  apt  within  the  body  to  lead  to  clotting.  Thus,  when  an  artery 
is  ligatured,  the  blood  in  the  tract  of  the  artery  on  the  cardiac  side  of  the 
ligature,  between  the  ligature  and  the  branch  last  given  off  by  the  artery, 
ceasing  to  share  in  the  circulation,  remains  motionless  or  nearly  so,  and 
along  this  tract  a  clot  forms,  firmest  next  to  the  ligature  and  ending  near 
where  the  branch  is  given  off;  this  perhaps  may  be  explained  by  the  fact 
that  the  walls  of  the  tract  suffer  in  their  nutrition  by  the  stagnation  of  the 
blood,  and  that  consequently  the  normal  relation  between  them  and  the 
contained  blood  is  disturbed. 

That  the  blood  within  the  living  bloodvessels,  though  not  actually  clot- 
ting under  normal  circumstances,  may  easily  be  made  to  clot,  that  the  blood 
is  always  on  the  point  of  clotting,  is  shown  by  the  fact  that  a  foreign  body 
such  as  a  needle  thrust  into  the  interior  of  a  bloodvessel  or  a  thread  drawn 
through  and  left  in  a  bloodvessel,  is  apt  to  become  covered  with  fibrin. 
Some  influence  exerted  by  the  needle  or  thread,  whatever  may  be  the  cha- 
racter of  that  influence,  is  sufficient  to  determine  a  clotting,  which  other- 
wise would  not  have  taken  place. 

The  same  instability  of  the  blood,  as  regards  clotting,  is  strikingly  shown, 
in  the  case  of  the  rabbit,  by  the  result  of  injecting  into  the  bloodvessels  a 
small  quantity  of  a  solution  of  a  peculiar  proteid,  prepared  from  certain 
structures  such  as  the  thymus  body.  Massive  clotting  of  the  blood  in 
almost  all  the  bloodvessels,  small  and  large,  takes  place  with  great  rapid- 
ity, leading  to  the  sudden  death  of  the  animal.  In  contrast  to  this  effect 
may  be  mentioned  the  result  of  injecting  into  the  bloodvessels  of  a  dog  a 
quantity  of  a  solution  of  a  body  called  albumose,  of  which  we  shall  here- 
after have  to  treat  as  a  product  of  the  digestion  of  proteid  substances,  to 
the  extent  of  0.3  gramme  per  kilo  of  body  weight.  So  far  from  producing 
clotting,  the  injected  albumose  has  such  an  effect  on  the  blood  that  for  several 
hours  after  the  injection  shed  blood  will  refuse  to  clot  of  itself  and  remain 
quite  fluid,  though  it  can  be  made  to  clot  by  special  treatment. 

§  23.  All  the  foregoing  facts  tend  to  show  that  the  blood  as  it  is  flow- 
ing through  the  healthy  bloodvessels  is,  as  far  as  clotting  is  concerned,  in  a 
state  of  unstable  equilibrium,  which  may  at  any  moment  be  upset,  even 
within  the  bloodvessels,  and  which  is  upset  directly  the  blood  is  shed, 
with  clotting  as  a  result.  Our  present  knowledge  does  not  permit  us  to 
make  an  authoritative  statement  as  to  the  exact  nature  of  this  equilib- 
rium. There  are  reasons,  however,  for  thinking  that  the  white  corpuscles 
play  an  important  part  in  the  matter.  Wherever  clotting  occurs  natu- 
rally, white  corpuscles  are  present;  and  this  is  true  not  only  of  blood  but 
also  of  such  specimens  of  pericardial  or  other  serous  fluids  as  clot  natu- 
rally. And  many  arguments  which  we  cannot  enter  upon  here,  may  be 
adduced  all  pointing  to  the  same  conclusion,  that  the  white  corpuscles  play 
nn  important  part  in  the  process  of  clotting.  But  it  would  lead  us  too  far 
into  controversial  matters  to  attempt  to  define  what  that  part  is,  or  to  explain 
the  exact  nature  of  the  equilibrium  of  which  we  have  spoken. 


38 


BLOOD. 


What  we  do  know  is  that  in  blood  soon  after  it  has  been  shed  the  body 
which  we  have  called  fibrinogen  is  present,  as  also  the  body  which  we  have 
called  fibrin  ferment,  that  the  latter  acting  on  the  former  will  produce 
fibrin,  and  that  the  appearance  of  fibrin  is  undoubtedly  the  cause  of  what 
is  called  clotting.  We  seem  justified  in  concluding  that  the  clotting  of  shed 
blood  is  due  to  the  conversion  by  ferment  of  fibrinogen  into  fibrin.  The 
further  inference  that  clotting  within  the  body  is  the  same  thing  as  clotting 
outside  the  body,  and  similarly  due  to  the  transformation  of  fibrinogen  by 
ferment  into  fibrin,  though  probable,  is  not  proved.  We  do  not  yet  know 
the  exact  nature  and  condition  of  the  blood  within  the  living  bloodvessels, 
and  until  we  know  that  we  cannot  satisfactorily  explain  why  blood  in  the 
living  bloodvessels  is  usually  fluid  but  can  at  times  clot. 


THE  CORPUSCLES  OF  THE  BLOOD. 

The  Red   Corpuscles. 

§  24.  The  redness  of  blood  is  due  exclusively  to  the  red  corpuscles. 
The  plasma  as  seen  in  thin  layers  within  the  living  bloodvessels  appears 
colorless,  as  does  also  a  thin  layer  of  serum  ;  but  a  thick  layer  of  serum 
(and  probably  of  plasma)  has  a  faint  yellowish  tinge  due,  as  we  have 
said,  to  the  presence  of  a  small  quantity  of  a  special  pigment. 

The  corpuscles  appear  under  the  microscope  as  fairly  homogeneous,  im- 
perfectly translucent  biconcave  discs  with  a  diameter  of  7  to  8  (i.  and  a 


[FIG.  5. 


FIG.  6. 


FIG.  5.— Human  Blood  as  seen  on  the  Warm  Stage  (magnified  about  1200  diameters) :  r,  r, 
single  red  corpuscles  seen  lying  flat;  r',  r',  red  corpuscles  on  their  edge  and  viewed  in  profile  ;  r", 
red  corpuscles  arranged  in  rouleaux  ;  c,  c,  crenate  red  corpuscles;  p,  a  finely  granular  pale  cor- 
puscle; <7,  a  coarsely  granular  pale  corpuscle.  Both  have  two  or  three  distinct  vacuoles,  and 
were  undergoing  changes  of  shape  at  the  moment  of  observation  ;  in  g  a  nucleus  also  was  visible. 

FIG.  6.— Human  Red  Corpuscles  Lying  Singly  and  Collected  into  Rolls.  (As  seen  under  an 
ordinary  high  power  of  the  microscope.)] 

thickness  of  1  to  2  //.  Being  discs  they  are  circular  in  outline  when  seen 
on  the  flat,  but  rod-shaped  when  seen  in  profile.  [Fig.  5.]  Being  bicon- 
cave, with  a  thicker  rounded  rim  surrounding  a  thinner  centre,  the  rays  of 


THE  COKPUSCLES  OF  THE  BLOOD.  39 

light  in  passing  through  them,  when  they  are  examined  by  transmitted  light, 
are  more  refracted  at  the  rim  than  in  the  centre.  The  effect  of  this  is  that, 
when  viewed  at  what  may  be  considered  the  proper  focus,  the  centre  of  a 
corpuscle  appears  clear,  while  a  slight  opacity  marks  out  indistinctly  the 
inner  margin  of  the  thicker  rim,  whereas,  when  the  focus  is  shifted  either 
up  or  down,  the  centre  becomes  dark  and  the  rest  of  the  corpuscle  clear. 
Any  body  of  the  same  shape,  and  composed  of  substances  of  the  same 
refractive  power,  would  produce  the  same  optical  effects.  Otherwise  the 
corpuscle  appears  homogeneous,  without  distinction  of  parts  and  without 
a  nucleus.  A  single  corpuscle  seen  by  itself  has  a  very  faint  color,  look- 
ing yellow  rather  than  red,  but  when  several  corpuscles  lie  one  upon  the 
top  of  the  other  the  mass  is  distinctly  red. 

The  red  corpuscle  is  elastic,  in  the  sense  that  it  may  be  deformed  by 
pressure  or  traction,  but  when  the  pressure  or  traction  is  removed  regains  its 
previous  form.  Its  shape  is  also  much  influenced  by  the  physical  conditions 
of  the  plasma,  serum,  or  fluid  in  which  for  the  time  being  it  is.  If  the 
plasma  or  serum  be  diluted  with  water,  the  disc,  absorbing  water,  swells  up 
into  a  sphere  [Fig.  7],  becoming  a  disc  again  on  the  removal  of  the  dilution. 
If  the  serum  be  concentrated,  the  disc,  giving 
out  water,  shrinks  irregularly  and  assumes  [FIG.  7. 

various  forms  ;  one  of  these  forms  is  that  of  a      a        6         c         d       e 
number  of  blunted  protuberances  projecting  all      •       ©k      tfk      JF?|    f^\ 
over  the  surface  of  the  corpuscle,  which  is  then      J       |p      Jp      *"*    ^^ 
said  to  be  crenate  ;  in  a  drop  of  blood  examined 
under  the  microscope,  crenate  corpuscles  are      ^  ^W 
often  seen  at  the  edge  of  the  cover-slip  where        "%if  9  C& 

evaporation  is^  leading  to  concentration  of  the       a_e>  successive  effects  of  water 
plasma,  or,  as  it  should  then  perhaps  rather  be     upon  a  red  corpuscle;  /,  effect  of 
called,  serum.     In  blood  just  shed  the  red  cor-     solution  of  salt,  crenated;  g,  effect 
puscles  are  apt  to  adhere  to  each  other  by  their     of  tannic  acid.] 
flat  surfaces,  much  more  than  to  the  glass  or 

other  surface  with  which  the  blood  is  in  contact,  and  hence  arrange  them- 
selves in  rolls.  This  tendency,  however,  to  form  rolls  very  soon  diminishes 
after  the  blood  is  shed. 

Though  a  single  corpuscle  is  somewhat  translucent,  a  comparatively  thin 
layer  of  blood  is  opaque ;  type,  for  instance,  cannot  be  read  through  even  a 
thin  layer  of  blood. 

When  a  quantity  of  whipped  blood  (or  blood  otherwise  deprived  of  fibrin) 
is  frozen  and  thawed  several  times  it  changes  color,  becoming -of  a  darker 
hue,  and  is  then  found  to  be  much  more  transparent,  so  that  type  can  now 
be  easily  read  through  a  moderately  thin  layer.  It  is  then  spoken  of  as 
laky  blood.  The  same  change  may  be  effected  by  shaking  the  blood  with 
ether,  or  by  adding  a  small  quantity  of  bile  salts,  and  in  other  ways.  Upon 
examination  of  laky  blood  it  is  found  that  the  red  corpuscles  are  "  broken 
up"  or  at  least  altered,  and  that  the  redness  which  previously  was  confined 
to  them  is  now  diffused  through  the  serum.  Normal  blood  is  opaque  because 
each  corpuscle,  while  permitting  some  rays  of  light  (chiefly  red)  to  pass 
through,  reflects  many  others,  and  the  brightness  of  the  hue  of  normal  blood 
is  due  to  this  reflection  of  light  from  the  surfaces  of  the  several  corpuscles. 
Laky  blood  is  transparent  because  there  are  no  longer  intact  corpuscles  to 
present  surfaces  for  the  reflection  of  light,  and  the  darker  hue  of  laky  blood 
is  similarly  due  to  the  absence  of  reflection  from  the  several  corpuscles. 

When  laky  blood  is  allowed  to  stand  a  sediment  is  formed  (and  may  be 
separated  by  the  centrifugal  machine)  which  on  examination  is  found^  to 
consist  of  discs,  or  fragments  of  discs,  of  a  colorless  substance  exhibiting 


40  BLOOD. 

under  high  powers  an  obscurely  spongy  or  reticular  structure.  These  color- 
less, thin  discs  seen  flatwise  often  appear  as  mere  rings.  The  substance 
composing  them  stains  with  various  reagents  and  may  thus  be  made  more 
evident. 

The  red  corpuscle,  then,  consists  obviously  of  a  colorless  framework,  with 
which  in  normal  conditions  a  red  coloring  matter  is  associated ;  but  by 
various  means  the  coloring  matter  may  be  driven  from  the  framework  and 
dissolved  in  the  serum. 

The  framework  is  spoken  of  as  stroma;  it  is  a  modified  or  differentiated 
protoplasm,  and  upon  chemical  analysis  yields  proteid  substances,  some  of 
them  at  least  belonging  to  the  globulin  group,  and  other  matters,  among 
which  is  a  peculiar  complex  fat  called  lecithin,  of  which  we  shall  have  to 
speak  in  treating  of  nervous  tissue. 

The  red  coloring  matter  which  in  normal  conditions  is  associated  with 
this  stroma  is  called  haemoglobin,  and  may  by  proper  methods  be  split  up 
into  a  proteid  belonging  to  the  globulin  group,  and  into  a  colored  pigment, 
containing  iron,  called  hcematin.  Haemoglobin  is,  therefore,  a  very  complex 
body.  It  is  found  to  have  remarkable  relations  to  oxygen,  and  indeed,  as 
we  shall  see,  the  red  corpuscles  by  virtue  of  their  haemoglobin  have  a  special 
work  in  respiration  ;  they  carry  oxygen  from  the  lungs  to  the  several  tissues. 
We  shall  therefore  defer  the  further  study  of  haemoglobin  until  we  have  to 
deal  with  respiration. 

The  red  corpuscle,  then,  consists  of  a  disc  of  colorless  stroma  with  which 
is  associated  in  a  peculiar  way  the  complex  colored  body  haemoglobin. 
Though  the  haemoglobin,  as  is  seen  in  laky  blood,  is  readily  soluble  in  serum 
(and  it  is  also  soluble  in  plasma),  in  the  intact  normal  blood  it  remains  con- 
fined to  the  corpuscle;  obviously  there  is  some  special  connection  between 
the  stroma  and  the  haemoglobin  ;  it  is  not  until  the  stroma  is  altered,  we  may 
perhaps  say  killed  (as  by  repeated  freezing  and  thawing),  that  it  loses  its  hold 
on  the  haemoglobin,  which  thus  set  free  passes  into  solution  in  the  serum. 
The  disc  of  stroma  when  separated  from  the  haemoglobin  has,  as  we  have  just 
said,  an  obscurely  spongy  texture  ;  but  we  do  not  know  accurately  the  exact 
condition  of  the  stroma  in  the  intact  corpuscle  or  how  it  holds  the  haemo- 
globin. There  is  certainly  no  definite  membrane  or  envelope  to  the  corpuscle, 
for  by  exposing  blood  to  a  high  temperature,  60°  C.,  the  corpuscle  will  break 
up  into  more  or  less  spherical  pieces,  each  still  consisting  of  stroma  and 
haemoglobin. 

The  quantity  of  stroma  necessary  to  hold  a  quantity  of  haemoglobin  is 
exceedingly  small.  Of  the  total  solid  matter  of  a  corpuscle  more  than  90 
per  cent,  is  haemoglobin.  A  red  corpuscle  in  fact  is  a  quantity  of  haemo- 
globin held  together  in  the  form  of  a  disc  by  a  minimal  amount  of  stroma. 
Hence  whatever  effect  the  stroma  per  se  may  have  upon  the  plasma,  this,  in 
the  case  of  mammals  at  all  events,  must  be  insignificant ;  the  red  corpuscle 
is  practically  simply  a  carrier  of  haemoglobin. 

§  25.  The  average  number  of  red  corpuscles  in  human  blood  may  be 
probably  put  down  at  about  5  millions  in  a  cubic  millimetre  (the  range  in 
different  mammals  is  said  to  be  from  3  to  18  millions),  but  the  relation  of 
corpuscle  to  plasma  varies  a  great  deal  even  in  health,  and  very  much  in 
disease.  Obviously  the  relation  may  be  affected  (1)  by  an  increase  or  de- 
crease of  the  plasma,  (2)  by  an  actual  decrease  or  increase  of  red  corpus- 
cles. Now,  the  former  must  frequently  take  place.  The  blood,  as  we  have 
already  urged,  is  always  being  acted  upon  by  changes  in  the  tissues  and, 
indeed,  is  an  index  of  those  changes ;  hence  the  plasma  must  be  continu- 
ally changing,  though  always  striving  to  return  to  the  normal  condition. 
Thus  when  a  large  quantity  of  water  is  discharged  by  the  kidney,  the 


THE  CORPUSCLES  OF  THE  BLOOD. 


41 


skin  or  the  bowels,  that  water  comes  really  from  the  blood,  and  the  drain 
of  water  must  tend  to  diminish  the  bulk  of  the  plasma,  and  so  to  increase 
the  relative  number  of  red  corpuscles,  though  the  effect  is  probably  soon 
remedied  by  the  passage  of  water  from  the  tissues  into  the  blood.  So 
again  when  a  large  quantity  of  water  is  drunk,  this  passes  into  the  blood 
and  tends  temporarily  to  dilute  the  plasma  (and  so  to  diminish  the  relative 
number  of  red  corpuscles),  though  this  condition  is  in  turn  soon  remedied 
by  the  passage  of  the  superfluous  fluid  to  the  tissues  and  excretory  organs. 
The  greater  or  less  number  of  red  corpuscles,  then,  in  a  given  bulk  of  blood 
may  be  simply  due  to  less  or  more  plasma,  but  we  have  reason  to  think  that 
the  actual  number  of  the  corpuscles  in  the  blood  does  vary  from  time  to 
time.  This  is  especially  seen  in  certain  forms  of  disease  which  may  be 
spoken  of  under  the  general  term  of  anaemia  (there  being  several  kinds  of 
anaemia),  in  which  the  number  of  red  corpuscles  is  distinctly  diminished. 

The  redness  of  blood  may,  however,  be  influenced  not  only  by  the  number 
of  red  corpuscles  in  each  cubic  millimetre  of  blood,  but  also  by  the  amount 
of  haemoglobin  in  each  corpuscle,  and  to  a  less  degree  by  the  size  of  the 
corpuscles.  If  we  compare,  with  a  common  standard,  the  redness  of  two 
specimens  of  blood  unequally  red,  and  then  determine  the  relative  number 
of  corpuscles  in  each,  we  may  find  that  the  less  red  specimen  has  as  many 
corpuscles  as  the  redder  one,  or  at  least  the  deficiency  in  redness  is  greater 
than  can  be  accounted  for  by  the  paucity  of  red  corpuscles.  Obviously,  in 
such  a  case,  the  red  corpuscles  have  too  little  haemoglobin.  In  some  cases 
of  anaemia  the  deficiency  of  haemoglobin  in  each  corpuscle  is  more  striking 
than  the  scantiness  of  red  corpuscles. 

The  number  of  corpuscles  in  a  specimen  of  blood  is  determined  by  mixing  a 
small  but  carefully  measured  quantity  of  the  blood  with  a  large  quantity  of  some 

[FIG.  8. 


Hsemacytometer  of  Gowers :  A,  pipette  for  measuring  the  diluting  solution  ;  E,  capillary  tube 
for  measuring  blood  ;  C,  cell  with  divisions  on  slide,  cover-glass  and  springs ;  D,  vessel  to  mix 
solutions ;  E,  mixer ;  F,  guarded  spear-pointed  needle  for  sticking  finger.] 

indifferent  fluid,  e.  g.,  a  5  per  cent,  solution  of  sodium  sulphate,  and  then  actually 
counting  the  corpuscles  in  a  known  minimal  bulk  of  the  mixture. 


42  BLOOD. 

This,  perhaps,  may  be  most  conveniently  done  by  the  method  generally  known 
as  that  of  Growers  (Hsemacytometer)  [Fig.  8 j,  improved  by  Malassez.  A  glass 
slide,  in  a  metal  frame,  is  ruled  into  minute  rectangles,  e.  g.  \  mm.  by  i  mm.,  so 
as  to  give  a  convenient  area  of  ^V  of  a  square  mm.  Three  small  screws  in  the  frame 
permit  a  coverslip  to  be  brought  to  a  fixed  distance,  e.  g.  \  mm.,  from  the  surface 
of  the  slide.  The  blood  having  been  diluted,  e.  g.  to  100  times  its  volume,  a  small 
quantity  of  the  diluted  (and  thoroughly  mixed)  blood,  sufficient  to  occupy  fully  the 
space  between  the  coverslip  and  the  glass  slide  when  the  former  is  brought  to  its 
proper  position,  is  placed  on  the  slide,  and  the  coverslip  brought  down.  The 
volume  of  diluted  blood  now  lying  over  each  of  the  rectangles  will  be  T^o  (?\yX£) 
of  a  cubic  mm.;  and  if,  when  the  corpuscles  have  subsided,  the  number  of  cor- 
puscles lying  within  a  rectangle  be  counted,  the  result  will  give  the  number  of 
corpuscles  previously  distributed  through  y^  of  a  cubic  mm.  of  the  diluted 
blood.  This  multiplied  by  100  will  give  the  number  of  corpuscles  in  1  cubic 
mm.  of  the  diluted  blood,  and  again  multiplied  by  100  the  number  in  1  cubic 
mm.  of  the  entire  blood.  It  is  advisable  to  count  the  number  of  corpuscles  in 
several  of  the  rectangles,  and  to  take  the  average.  For  the  convenience  of  count- 
ing, each  rectangle  is  subdivided  into  a  number  of  very  small  squares,  e.  g.  into  20, 
each  with  a  side  of  -£$  mm.,  and  so  an  area  of  ^^  of  a  square  mm. 

Since  the  actual  number  of  red  corpuscles  in  a  specimen  of  blood  (which 
may  be  taken  as  a  sample  of  the  whole  blood)  is  sometimes  more,  sometimes 
less,  it  is  obvious  that  either  red  corpuscles  may  be  temporarily  withdrawn 
from  and  returned  to  the  general  blood  current,  or  that  certain  red  corpuscles 
are,  after  a  while,  made  away  with,  and  that  new  ones  take  their  place.  We 
have  no  satisfactory  evidence  of  the  former  being  the  case  in  normal  condi- 
tions, whereas  we  have  evidence  that  old  corpuscles  do  die  and  that  new 
ones  are  born. 

§  26.  The  red  corpuscles,  we  have  already  said,  are  continually  engaged 
in  carrying  oxygen,  by  means  of  their  haemoglobin,  from  the  lungs  to  the 
tissues ;  they  load  themselves  with  oxygen  at  the  lungs  and  unload  at  the 
tissues.  It  is  extremely  unlikely  that  this  act  should  be  repeated  indefinitely 
without  leading  to  changes  which  may  be  familiarly  described  as  wear  and 
tear,  and  which  would  ultimately  lead  to  the  death  of  the  corpuscles. 

We  shall  have  to  state  later  on  that  the  liver  discharges  into  the  alimen- 
tary canal,  as  a  constituent  of  bile,  a  considerable  quantity  of  a  pigment 
known  as  bilirubin,  and  that  this  substance  has  remarkable  relations  with, 
and,  indeed,  may  be  regarded  as  a  derivative  of  hcematin,  which,  as  we  have 
seen  (§  24),  is  a  product  of  the  decomposition  of  haemoglobin.  It  appears 
probable,  in  fact,  that  the  bilirubin  of  bile  (and  this  as  we  shall  see  is  the 
chief  biliary  pigment,  and  the  source  of  the  other  biliary  pigments)  is  not 
formed  wholly  anew  in  the  body,  but  is  manufactured  in  some  way  or  other 
out  of  haematin  derived  from  haemoglobin.  This  must  entail  a  daily  con- 
sumption of  a  considerable  quantity  of  haemoglobin,  and  since  we  know  no 
other  source  of  haemoglobin  besides  the  red  corpuscles,  and  have  no  evidence 
of  red  corpuscles  continuing  to  exist  after  having  lost  their  haemoglobin, 
must,  therefore,  entail  a  daily  destruction  of  many  red  corpuscles. 

Even  in  health,  then,  a  number  of  red  corpuscles  must  be  continually 
disappearing ;  and  in  disease  the  rapid  and  great  diminution  which  may 
take  place  in  the  number  of  red  corpuscles  shows  that  large  destruction 
may  occur. 

We  cannot  at  present  accurately  trace  out  the  steps  of  this  disappearance 
of  red  corpuscles.  In  the  spleen  pulp,  red  corpuscles  have  been  seen  in 
various  stages  of  disorganization,  some  of  them  lying  within  the  substance 
of  large  colorless  corpuscles,  and  as  it  were  being  eaten  by  them.  There  is 
also  evidence  that  destruction  takes  place  in  the  liver  itself,  and,  indeed, 
elsewhere.  But  the  subject  has  not  yet  been  adequately  worked  out. 

§  27.  This  destruction  of  red  corpuscles  necessitates  the  birth  of  new 


THE  CORPUSCLES  OF  THE  BLOOD.  43 

corpuscles,  to  keep  up  the  normal  supply  of  haemoglobin  ;  and,  indeed,  the 
cases  in  which  after  even  great  loss  of  blood  by  hemorrhage  a  healthy 
ruddiness  returns,  and  that  often  rapidly,  showing  that  the  lost  corpuscles 
have  been  replaced,  as  well  as  the  cases  of  recovery  from  the  disease  anaemia, 
prove  that  red  corpuscles  are,  even  in  adult  life,  born  somewhere  in  the 
body. 

In  the  adult,  as  in  the  embryo,  the  red  corpuscles  appear  to  be  formed 
out  of  preceding  colored  nucleated  cells. 

In  the  interior  of  bones  is  a  peculiar  tissue  called  marrow,  which,  in  most 
parts  being  very  full  of  bloodvessels,  is  called  red  marrow.  In  this  red 
marrow  the  capillaries  and  minute  veins  form  an  intricate  labyrinth  of  rel- 
atively wide  passages  with  very  thin  walls,  and  through  this  labyrinth  the 
flow  of  blood  is  comparatively  slow.  In  the  passages  of  this  labyrinth  are 
found  colored  nucleated  cells,  that  is  to  say,  cells  the  cell  substance  of  which 
has  undergone  more  or  less  differentiation  into  haemoglobin  and  stroma. 
And  there  seems  to  be  going  on  in  red  marrow  a  multiplication  of  such 
colored  nucleated  cells,  which  become  transformed  into  red  non-nucleated 
discs,  that  is,  into  ordinary  red  corpuscles,  and  pass  into  the  general  blood 
current.  In  other  words,  a  formation  of  red  corpuscles,  not  wholly  unlike 
that  which  takes  place  in  the  embryo,  is  in  the  adult  continually  going  on 
in  the  red  marrow  of  the  bones. 

A  similar  formation  of  red  corpuscles  has  also  been  described,  though 
with  less  evidence,  as  taking  place  in  the  spleen,  especially  under  particular 
circumstances,  such  as  after  great  loss  of  blood. 

The  formation  of  red  corpuscles  is,  therefore,  a  special  process,  taking 
place  in  special  regions ;  we  have  no  satisfactory  evidence  that  the  ordinary 
white  corpuscles  of  the  blood  are,  as  they  travel  in  the  current  of  the  circu- 
lation, transformed  into  red  corpuscles. 

The  red  corpuscles,  then,  to  sum  up,  are  useful  to  the  body  on  account  of 
the  haemoglobin,  which  constitutes  so  nearly  the  whole  of  their  solid  matter. 
What  functions  the  stroma  may  have  besides  the  mere,  so  to  speak,  mechan- 
ical one  of  holding  the  haemoglobin  in  the  form  of  a  corpuscle  we  do  not 
know.  The  primary  use  of  the  haemoglobin  is  to  carry  oxygen  from  the 
lungs  to  the  tissues,  and  it  would  appear  that  it  is  advantageous  to  the  econ- 
omy that  the  haemoglobin  should  be  as  it  were  bottled  up  in  corpuscles  rather 
than  simply  diffused  through  the  plasma.  How  long  a  corpuscle  may  live 
carrying  oxygen  we  do  not  exactly  know ;  the  red  corpuscles  of  one  animal, 
e.  g.,  a  bird,  injected  into  the  vessels  of  another,  e.  g.,  a  mammal,  disappear 
within  a  few  days ;  but  this  affords  no  measure  of  the  life  of  a  corpuscle  in 
its  own  home.  Eventually,  however,  the  red  corpuscle  dies,  its  place  being 
supplied  by  a  new  one.  The  haemoglobin  set  free  from  the  dead  corpuscles 
appears  to  have  a  secondary  use  in  forming  the  pigment  of  the  bile  and 
possibly  other  pigments. 

The  White  or  Colorless  Corpuscles. 

§  28.  The  white  corpuscles  are  far  less  numerous  than  the  red  ;  a  speci- 
men of  ordinary  healthy  blood  will  contain  several  hundred  red  corpuscles 
to  each  white  corpuscle,  though  the  proportion,  even  in  health,  varies  consid- 
erably under  different  circumstances,  ranging  from  1  in  300  to  1  in  700. 
But  though  less  numerous,  the  white  corpuscles  are  probably  of  greater 
importance  to  the  blood  itself  than  are  the  red  corpuscles ;  the  latter  are 
chiefly  limited  to  the  special  work  of  carrying  oxygen  from  the  lungs  to  the 
tissues,  while  the  former  probably  exert  a  considerable  influence  on  the  blood 
plasma  itself,  and  help  to  maintain  it  in  a  proper  condition. 


44  BLOOD. 


When  seen  in  a  normal  condition,  and  "  at  rest,"  the  white  corpuscle  is  a 
small,  spherical,  colorless  mass,  varying  in  size,  but  with  an  average  diameter 
of  about  10  v,  and  presenting  generally  a  finely  but  sometimes  a  coarsely 
granular  appearance.  [Fig.  9.]  The  surface,  even  when  the  corpuscle  is 


[FIG.  9. 


a,  white  corpuscles  of  human  blood;  d,  red  corpuscles  (high  power).] 

perfectly  at  rest,  is  not  absolutely  smooth  and  even,  but  somewhat  irregular, 
thereby  contributing  to  the  granular  appearance ;  and  at  times  these  irreg- 
ularities are  exaggerated  into  protuberances  or  "pseudopodia"  of  varying 
size  or  form,  the  corpuscle  in  this  way  assuming  various  forms  without 
changing  its  bulk,  and  by  the  assumption  of  a  series  of  forms  shifting  its 
place.  Of  these  "  amoeboid  movements,"  as  they  are  called,  we  shall  have 
to  speak  later  on. 

In  carrying  on  these  amoeboid  movements  the  corpuscle  may  transform 
itself  from  a  spherical  mass  into  a  thin,  flat,  irregular  plate ;  and  when  this 
occurs  there  may  be  seen  at  times  in  the  midst  of  the  extended  finely  gran- 
ular mass  or  cell  body,  a  smaller  body  of  different  aspect  and  refractive  power, 
the  nucleus.  The  normal  presence  of  a  nucleus  in  the  white  corpuscles  may 
also  be  shown  by  treating  the  corpuscle  with  dilute  acetic  acid,,  which  swells 
up  and  renders  more  transparent  the  cell  body  but  makes  the  nucleus  more 
refractive  and  more  sharply  defined,  and  so  more  conspicuous,  or  by  the  use 
of  staining  reagents,  the  majority  of  which  stain  the  nucleus  more  readily 
and  more  deeply  than  the  cell  body.  In  what  perhaps  may  be  considered 
a  typical  white  corpuscle,  the  nucleus  is  a  spherical  mass  about  2-3  //  in 
diameter,  but  it  varies  in  size  in  different  corpuscles,  and  not  unfrequently 
is  irregular  in  form,  at  least  after  the  action  of  reagents.  It  occasionally 
appears  as  if  about  to  divide  into  fragments,  and  sometimes  a  corpuscle  may 
contain  two  or  even  more  (then  generally  small)  nuclei.  Though  staining 
readily  with  staining  reagents,  the  nucleus  of  an  ordinary  white  corpuscle 
does  not  show  the  nuclear  network  which  is  so  characteristic,  as  we  shall  see, 
of  the  nuclei  of  many  cells,  and  which  in  these  is  the  part  of  the  nucleus 
which  especially  stains;  in  the  closely  allied  lymph  corpuscles,  to  which  we 
shall  have  immediately  to  refer,  a  nuclear  network  is  present. 

The  cell  body  of  the  white  corpuscle  may  be  taken  as  a  good  example  of 
what  we  have  called  undiffereritiated  protoplasm.  Optically,  it  consists  of  a 
uniformly  transparent  but  somewhat  refractive  material  or  basis,  in  which 
are  imbedded  minute  particles,  generally  spherical  in  form,  and  in  which 
sometimes  occur  minute  vacuoles  filled  with  fluid;  it  is  rarely,  if  ever,  that 
any  distinct  network,  like  that  which  is  sometimes  observed  in  other  cells, 
can  be  seen  in  the  cell  body  of  a  white  corpuscle  whether  stained  or  not.  The 
imbedded  particles  are  generally  very  small,  and  for  the  most  part  distributed 
uniformly  over  the  cell  body,  giving  it  a  finely  granular  aspect ;  sometimes, 
however,  the  particles  are  relatively  large,  making  the  corpuscles  coarsely 
granular,  the  coarse  granules  being  frequently  confined  to  one  or  another 


THE  CORPUSCLES  OF  THE  BLOOD.  45 

part  of  the  cell  body.  These  particles  or  granules,  whether  coarse  or  fine, 
vary  in  nature;  some  of  them,  as  shown  by  their  greater  refractive  power, 
their  staining  with  osmic  acid,  and  their  solution  by  solvents  of  fat,  are  fatty 
in  nature;  others  may  similarly  be  shown  by  their  reactions  to  be  proteid  in 
nature. 

The  material  in  which  these  granules  are  imbedded,  and  which  forms  the 
greater  part  of  the  cell  body,  has  no  special  optical  features ;  so  far  as  can 
be  ascertained,  it  appears  under  the  microscope  to  be  homogeneous ;  no 
definite  structure  can  be  detected  in  it.  It  must  be  borne  in  mind  that  the 
whole  corpuscle  consists  largely  of  water,  the  total  solid  matter  amounting 
to  not  much  more  than  10  per  cent.  The  transparent  material  of  the  cell 
body  must,  therefore,  be  in  a  condition  which  we  may  call  semifluid,  or  semi- 
solid,  without  being  called  upon  to  define  what  we  exactly  mean  by  these 
terms.  This  approach  to  fluidity  appears  to  be  connected  with  the  great 
mobility  of  the  cell  body,  as  shown  in  its  amoeboid  movements. 

§  29.  When  we  submit  to  chemical  examination  a  sufficient  mass  of 
white  corpuscles,  separated  out  from  the  blood  by  special  means  and  obtained 
tolerably  free  from  red  corpuscles  and  plasma  (or  apply  to  the  white  blood- 
corpuscles  the  chemical  results  obtained  from  the  more  easily  procured  lymph- 
corpuscles,  which,  as  we  shall  see,  are  very  similar  to,  and,  indeed,  in  many 
ways  related  to  the  white  corpuscles  of  the  blood),  we  find  that  this  small 
solid  matter  of  the  corpuscle  consists  largely  of  certain  proteids. 

One  of  these  proteids  is  a  body  either  identical  with,  or  closely  allied  to, 
the  proteid  called  myosin,  which  we  shall  have  to  study  more  fully  in  con- 
nection with  muscular  tissue.  At  present  we  may  simply  say  that  myosin  is 
a  body  intermediate  between  fibrin  and  globulin,  being  less  soluble  than  the 
latter  and  more  soluble  than  the  former ;  thus  while  it  is  hardly  at  all  soluble 
in  a  1  per  cent,  solution  of  sodium  chloride  or  other  neutral  salt,  it  is,  unlike 
fibrin,  speedily  and  wholly  dissolved  by  a  10  per  cent,  solution.  Myosin  is 
further  interesting  because,  as  we  shall  see,  just  as  fibrin  is  formed  in  the 
clotting  of  blood  from  fibrinogen,  so  myosin  is  formed  out  of  a  preceding 
myosinogen,  during  a  kind  of  clotting  which  takes  place  in  muscular  fibre 
and  which  is  spoken  of  as  rigor  mortis.  And  we  have  reasons  for  thinking 
that  in  the  living  white  blood-corpuscle  there  does  exist  a  body  identical 
with  or  allied  to  myosinogen,  which  we  may  speak  of  as  being  in  a  fluid  con- 
dition; and  which  on  the  death  of  the  corpuscle  is  converted,  by  a  kind  of 
clotting,  into  myosin,  or  into  an  allied  body  which,  being  solid,  gives  the 
body  of  the  corpuscle  a  stiffness  and  rigidity  which  it  did  not  possess  during 
life. 

Besides  this  myosin  or  myosin-like  proteid,  the  white  corpuscles  also 
contain  either  paraglobulin  itself  or  some  other  member  of  the  globulin 
group,  as  well  as  a  body  or  bodies  like  or  identical  with  serum-albumin. 

In  addition,  there  is  present,  in  somewhat  considerable  quantity,  a  sub- 
stance of  a  peculiar  nature,  which  since  it  is  confined  to  the  nuclei  of  the 
corpuscles,  and  further  seems  to  be  present  in  all  nuclei,  has  been  called 
nudein.  This  nuclein,  which  though  a  complex  nitrogenous  body  is  very 
different  in  composition  and  nature  from  proteids,  is  remarkable  on  the  one 
hand  for  being  a  very  stable  inert  body,  and  on  the  other  for  containing  a 
large  quantity  (according  to  some  observers  nearly  10  per  cent.)  of  phos- 
phorus, which  appears  to  enter  more  closely  into  the  structure  of  the  molecule 
than  it  does  in  the  case  of  proteids. 

Next  in  importance  to  the  proteids,  as  constant  constituents  of  the  white 
corpuscles,  come  certain  fats.  Among  these  the  most  conspicuous  is  the 
complex  fatty  body  lecithin. 

In  the  case  of  many  corpuscles  at  all  events  we  have  evidence  of  the 


46  BLOOD. 

presence  of  a  member  of  the  large  group  of  carbohydrates,  comprising 
starches  and  sugar,  viz.,  the  starch-like  body  glycogen,  which  we  shall  have 
to  study  more  fully  hereafter.  This  glycogen  may  exist  in  the  living  cor- 
puscle as  glycogen,  but  it  is  very  apt,  after  the  death  of  the  corpuscles,  to 
become  changed  by  hydration  into  some  form  of  sugar,  such  as  maltose  or 
dextrose. 

Lastly,  the  ash  of  the  white  corpuscles  is  characterized  by  containing  a 
relatively  large  quantity  of  potassium  and  of  phosphates  and  by  being  rela- 
tive poor  in  chlorides  and  in  sodium.  But  in  this  respect  the  "corpuscle  is 
merely  an  example  of  what  seems  to  be  a  general  rule  (to  which,  however, 
there  may  be  exceptions)  that  while  the  elements  of  the  tissues  themselves 
are  rich  in  potassium  and  phosphates,  the  blood  plasma  or  lymph  on  which 
they  live  abounds  in  chlorides  and  sodium  salts. 

§  30.  In  the  broad  features  above  mentioned,  the  white  blood-corpuscle 
may  be  taken  as  a  picture  and  example  of  all  living  tissues.  If  we  examine 
the  histological  elements  of  any  tissue,  whether  we  take  an  epithelium  cell, 
or  a  nerve  cell,  or  a  cartilage  cell,  or  a  muscular  fibre,  we  meet  with  very 
similar  features.  Studying  the  element  morphologically,  we  find  a  nucleus1 
and  a  cell  body,  the  nucleus  having  the  general  characters  described  above 
with  frequently  other  characters  introduced,  and  the  cell  body  consisting  of 
at  least  more  than  one  kind  of  material,  the  materials  being  sometimes  so 
disposed  as  to  produce  the  optical  effect  simply  of  a  transparent  mass  in 
which  granules  are  imbedded,  in  which  case  we  speak  of  the  cell  body  as 
protoplasmic,  but  at  other  times  so  arranged  that  the  cell  body  possesses  dif- 
ferentiated structure.  Studying  the  element  from  a  chemical  point  of  view, 
we  find  proteids  always  present,  and  among  these  bodies  identical  with  or 
more  or  less  closely  allied  to  myosin,  we  generally  have  evidence  of  the 
presence  also  of  fat  of  some  kind  and  of  some  member  or  members  of  the 
carbohydrate  group,  and  the  ash  always  contains  potassium  and  phosphates, 
with  sulphates,  chlorides,  sodium,  and  calcium,  to  which  may  be  added  mag- 
nesium and  iron. 

We  stated  in  the  Introduction  that  living  matter  is  always  undergoing 
chemical  change ;  this  continued  chemical  change  we  may  denote  by  the 
term  metabolism.  We  further  urged  that  as  long  as  living  matter  is  alive, 
the  chemical  change  or  metabolism  is  of  a  double  kind.  On  the  one  hand, 
the  living  substance  is  continually  breaking  down  into  simpler  bodies,  with 
a  setting  free  of  energy ;  this  part  of  the  metabolism  we  may  speak  of  as 
made  up  of  katabolic  changes.  On  the  other  hand,  the  living  substance  is 
continually  building  itself  up,  embodying  energy  into  itself  and  so  replen- 
ishing its  store  of  energy  ;  this  part  of  the  metabolism  we  may  speak  of  as 
made  up  of  anabolic  changes.  We  also  urged  that  in  every  piece  of  living 
tissue  there  might  be  (1)  the  actual  living  substance  itself,  (2)  material  which 
is  present  for  the  purpose  of  becoming,  and  is  on  the  way  to  become  living 
substance — that  is  to  say,  food  undergoing  or  about  to  undergo  anabolic 
changes,  and  (3)  material  which  has  resulted  from,  or  is  resulting  from,  the 
breaking  down  of  the  living  substance — that  is  to  say,  material  which  has 
undergone  or  is  undergoing  katabolic  changes,  and  which  we  speak  of  as 
waste.  In  using  the  word  "  living  substance,''  however,  we  must  remember 
that  in  reality  it  is  not  a  substance  in  the  chemical  sense  of  the  word,  but 
material  undergoing  a  series  of  changes. 

If,  now,  we  ask  the  question,  which  part  of  the  body  of  the  white  cor- 
puscle (or  of  a  similar  element  of  another  tissue)  is  the  real  living  substance, 
and  which  part  is  food  or  waste,  we  ask  a  question  which  we  cannot  as  yet 
definitely  answer.  We  have  at  present  no  adequate  morphological  criteria 

1  The  existence  of  multinuclear  structures  does  not  affect  the  present  argument. 


THE  CORPUSCLES  OF  THE  BLOOD.  47 

to  enable  us  to  judge,  by  optical  characters,  what  is  really  living  and  what 
is  not. 

The  material  which  appears  in  the  cell  body  in  the  form  of  distinct 
granules,  merely  lodged  in  the  more  transparent  material,  cannot  be  part 
of  the  real  living  substance ;  it  must  be  either  food  or  waste.  Many  of 
these  granules  are  fat,  and  we  have  at  times  an  opportunity  of  observing 
that  they  have  been  introduced  into  the  corpuscle  from  the  surrounding 
plasma.  The  white  corpuscle,  as  we  have  said,  has  the  power  of  executing 
amoeboid  movements ;  it  can  creep  around  objects,  envelope  them  with  its 
own  substance,  and  so  put  them  inside  itself.  The  granules  of  fat  thus  in- 
troduced may  be  subsequently  extruded  or  may  disappear  within  the  cor- 
puscle ;  in  the  latter  case  they  are  obviously  changed,  and  apparently  made 
use  of  by  the  corpuscle.  In  other  words,  these  fatty  granules  are  apparently 
food  material  on  their  way  to  be  worked  up  in  the  living  substance  of  the 
corpuscle. 

But  we  have  also  evidence  that  similar  granules  of  fat  may  make  their 
appearance  wholly  within  the  corpuscle  ;  they  are  products  of  the  activity  of 
the  corpuscle.  We  have  further  reason  to  think  that  in  some  cases,  at  all 
events,  they  arise  from  the  breaking  down  of  the  living  substance  of  the 
corpuscle,  that  they  are  what  we  have  called  waste  products. 

But  all  the  granules  visible  in  a  corpuscle  are  not  necessarily  fatty  in 
nature  ;  some  of  them  may  undoubtedly  be  proteid  granules,  and  it  is  possi- 
ble that  some  of  them  may  at  times  be  of  carbohydrate  or  other  nature.  In 
all  cases,  however,  they  are  either  food  material  or  waste  products.  And 
what  is  true  of  the  easily  distinguished  granules  is  also  true  of  other  sub- 
stances, in  solution  or  in  a  solid  form,  but  so  disposed  as  not  to  be  optically 
recognized. 

Hence  a  part,  and  it  may  be  no  inconsiderable  part,  of  the  white  corpus- 
cle, may  be  not  living  substance  at  all,  but  either  food  or  waste.  Further,  it 
does  not  necessarily  follow  that  the  whole  of  any  quantity  of  material,  fatty 
or  otherwise,  introduced  into  the  corpuscle  from  without,  should  actually 
be  built  up  into  and  so  become  part  of  the  living  substance;  the  changes 
from  raw  food  to  living  substance  are,  as  we  have  already  said,  probably 
many,  and  it  may  be  that  after  a  certain  number  of  changes,  few  or  many, 
part  only  of  the  material  is  accepted  as  worthy  of  being  made  alive,  and 
the  rest,  being  rejected,  becomes  at  once  waste  matter  ;  or  the  material  may, 
even  after  it  has  undergone  this  or  that  change,  never  actually  enter  into 
the  living  substance,  but  all  become  waste  matter.  We  say  waste  matter, 
but  this  does  not  mean  useless  matter.  The  matter  so  formed  may  without 
entering  into  the  living  substance  be  of  some  subsidiary  use  to  the  corpuscle, 
or,  as  probably  more  often  happens,  being  discharged  from  the  corpuscle, 
may  be  of  use  to  some  other  part  of  the  body.  We  do  not  know  how  the 
living  substance  builds  itself  up,  but  we  seem  compelled  to  admit  that, 
in  certain  cases  at  all  events,  it  is  able  in  some  way  or  other  to  produce 
changes  on  material  while  that  material  is  still  outside  the  living  substance 
as  it  were,  before  it  enters  into  and  indeed  without  its  ever  actually  entering 
into  the  composition  of  the  living  substance.  On  the  other  hand,  we  must 
equally  admit  that  some  of  the  waste  substances  are  the  direct  products  of 
the  katabolic  changes  of  the  living  substance  itself — were  actually  once  part 
of  the  living  substance.  Hence  we  ought,  perhaps,  to  distinguish  the  products 
of  the  activity  of  living  matter  into  waste  products  proper,  the  direct  results 
of  katabolic  changes,  and  into  by-products  which  are  the  results  of  changes 
effected  by  the  living  matter  outside  itself,  and  which  cannot,  therefore,  be 
considered  as  necessarily  either  anabolic  or  katabolic. 

Concerning  the  chemical  characters  of  the  living  matter  itself  we  cannot 


48  BLOOD. 

at  present  make  any  very  definite  statement.  We  may  say  that  the  proteid 
myosin,  or  rather  the  proteid  antecedent  or  antecedents  of  myosin,  enter 
in  some  way  into  its  structure,  but  we  are  not  justified  in  saying  that  the 
living  substance  consists  only  of  proteid  matter  in  a  peculiar  condition. 
And,  indeed,  the  persistency  with  which  some  representative  of  fatty  bodies 
and  some  representative  of  carbohydrates  always  appear  in  living  tissue 
would,  perhaps,  rather  lead  us  to  suppose  that  these  equally  with  proteid 
material  were  essential  to  its  structure.  Again,  though  the  behavior  of  the 
nucleus,  as  contrasted  with  that  of  the  cell  body,  leads  us  to  suppose  that 
the  living  substance  of  the  former  is  a  different  kind  from  that  of  the  latter, 
we  do  not  know  exactly  in  what  the  difference  consists.  The  nucleus,  as  we 
have  seen,  contains  nuclein  which,  perhaps,  we  may  regard  as  a  largely 
modified  proteid  ;  but  being  a  body  which  is  remarkable  for  its  stability,  for 
the  difficulty  with  which  it  is  changed  by  chemical  reagents,  cannot  be 
regarded  as  an  integral  part  of  the  essentially  mobile  living  substance  of 
the  nucleus. 

In  this  connection  it  may  be  worth  while  to  again  call  attention  to  the 
fact  that  the  corpuscle  contains  a  very  large  quantity  of  water,  viz.,  about 
90  per  cent.  Part  of  this,  we  do  not  know  how  much,  probably  exists  in  a 
more  or  less  definite  combination  with  the  protoplasm,  somewhat  after  the 
manner  of,  to  use  what  is  a  mere  illustration,  the  water  of  crystallization  of 
salts.  If  we  imagine  a  whole  group  of  different  complex  salts  continually 
occupied  in  turn  in  being  crystallized  and  being  decrystallized,  the  water 
thus  engaged  by  the  salts  will  give  us  a  rough  image  of  the  water  which 
passes  in  and  out  of  the  substance  of  the  corpuscle  as  the  result  of  its  meta- 
bolic activity.  We  might  call  this  "  water  of  metabolism."  Another  part 
of  the  water,  carrying  in  this  case  substances  in  solution,  probably  exists  in 
spaces  or  interstices  too  small  to  be  seen  with  even  the  highest  powers  of  the 
microscope.  Still  another  part  of  the  water  similarly  holding  substances  in 
solution  exists  at  times  in  definite  spaces  visible  under  the  microscope,  more 
or  less  regularly  spherical,  and  called  vacuoles. 

We  have  dwelt  thus  at  length  on  the  white  corpuscle  in  the  first  place, 
because,  as  we  have  already  said,  what  takes  place  in  it  is  in  a  sense  a  picture 
of  what  takes  place  in  all  living  structures,  and  in  the  second  place  because 
the  facts  which  we  have  mentioned  help  us  to  understand  how  the  white 
corpuscle  may  carry  on  in  the  blood  a  work  of  no  important  kind  ;  for  from 
what  has  been  said  it  is  obvious  that  the  white  corpuscle  is  continually 
acting  upon  and  being  acted  upon  by  the  plasma. 

§  31.  To  understand,  however,  the  work  of  these  white  corpuscles,  we 
must  learn  what  is  known  of  their  history. 

In  successive  drops  of  blood  taken  at  different  times  from  the  same 
individual,  the  number  of  colorless  corpuscles  will  be  found  to  vary  very 
much,  not  only  relatively  to  the  red  corpuscles,  but  also  absolutely.  They 
must,  therefore,  "  come  and  go." 

In  treating  of  the  lymphatic  system  we  shall  have  to  point  out  that  a 
very  large  quantity  of  fluid  called  lymph,  containing  a  very  considerable 
number  of  bodies,  very  similar  in  their  general  characters  to  the  white  cor- 
puscles of  the  blood,  is  being  continually  poured  into  the  vascular  system  at 
the  point  where  the  thoracic  duct  joins  the  great  veins  on  the  left  side  of 
the  neck,  and  to  a  less  extent  where  the  other  large  lymphatics  join  the 
venous  system  on  the  right  side  of  the  neck.  These  corpuscles  of  lymph, 
which,  as  we  have  just  said,  closely  resemble,  and,  indeed,  are  with  difficulty 
distinguished  from  the  white  corpuscles  of  the  blood,  but  of  which,  when 
they  exist  outside  the  vascular  system,  it  will  be  convenient  to  speak  of  as 
leucocytes,  are  found  along  the  whole  length  of  the  lymphatic  system,  but 


THE  CORPUSCLES  OF  THE  BLOOD.  49 

are  more  numerous  in  the  lymphatic  vessels  after  these  have  passed  through 
the  lymphatic  glands.  These  lymphatic  glands  are  partly  composed  of 
what  is  known  as  adenoid  tissue,  a  special  kind  of  connective  tissue  arranged 
as  a  delicate  network.  The  meshes  of  this  are  crowded  with  colorless  nucle- 
ated cells,  which,  though  varying  in  size,  are,  for  the  most  part,  small,  the 
nucleus  being  surrounded  by  a  relatively  small  quantity  of  cell  substance. 
Many  of  these  cells  show  signs  that  they  are  undergoing  cell-division,  and 
we  have  reason  to  think  that  cells  so  formed,  acquiring  a  larger  amount  of 
cell  substance,  become  veritable  leucocytes.  In  other  words,  leucocytes 
multiply  in  the  lymphatic  glands,  and  leaving  the  glands  by  the  lymphatic 
vessels,  make  their  way  to  the  blood.  Patches  and  tracts  of  similar  adenoid 
tissue,  not  arranged,  however,  as  distinct  glands,  but  similarly  occupied  by 
developing  leucocytes  and  similarly  connected  with  lymphatic  vessels,  are 
found  in  various  parts  of  the  body,  especially  in  the  mucous  membranes. 
Hence  we  may  conclude  that  from  various  parts  of  the  body,  the  lymph- 
atics are  continually  bringing  to  the  blood  an  abundant  supply  of  leuco- 
cytes, and  that  these  in  the  blood  become  ordinary  white  corpuscles.  This 
is  probably  the  chief  source  of  the  white  corpuscles,  for  though  the  white 
corpuscles  have  been  seen  dividing  in  the  blood  itself,  no  large  increase 
takes  place  in  that  way. 

§  32.  It  follows  that  since  white  corpuscles  are  thus  continually  being 
added  to  the  blood,  white  corpuscles  must  as  continually  either  be  destroyed, 
or  be  transformed,  or  escape  from  the  interior  of  the  bloodvessels ;  otherwise 
the  blood  would  soon  be  blocked  with  white  corpuscles. 

Some  do  leave  the  bloodvessels.  In  treating  of  the  circulation  we  shall 
have  to  point  out  that  white  corpuscles  are  able  to  pierce  the  walls  of  the 
capillaries  and  minute  veins,  and  thus  to  make  their  way  from  the  interior 
of  the  bloodvessels  into  spaces  filled  with  lymph,  the  "  lymph  spaces,"  as- 
they  are  called,  of  the  tissue  lying  outside  the  bloodvessels.  This  is  spoken 
of  as  the  "  migration  of  the  white  corpuscles."  In  an  "inflamed  area" 
large  numbers  of  white  corpuscles  are  thus  drained  away  from  the  blood 
into  the  lymph  spaces  of  the  tissue ;  and  it  is  probable  that  a  similar  loss 
takes  place,  more  or  less,  under  normal  conditions.  These  migrating  cor- 
puscles may,  by  following  the  devious  tracks  of  the  lymph,  find  their  way 
back  into  the  blood ;  some  of  them,  however,  may  remain,  and  undergo 
various  changes.  Thus,  in  inflamed  areas,  when  suppuration  follows  in- 
flammation, the  white  corpuscles  which  have  migrated  may  become  "  pus 
corpuscles,"  or,  where  thickening  and  growth  follow  upon  inflammation, 
may,  according  to  many  authorities,  become  transformed  into  temporary  or 
permanent  tissue,  especially  connective  tissue ;  but  this  transformation  into 
tissue  is  disputed.  When  an  inflammation  subsides  without  leaving  any 
effect  a  few  corpuscles  only  will  be  found  in  the  tissue;  those  which  had 
previously  migrated  must,  therefore,  have  been  disposed  of  in  some  way  or 
another. 

In  speaking  of  the  formation  of  red  corpuscles  (§  27)  we  saw  that  not 
only  it  is  not  proved  that  the  nucleated  corpuscles  which  give  rise  to  red 
corpuscles  are  ordinary  white  corpuscles,  but  that  in  all  probability  the  real 
hsematoblasts,  the  parents  of  red  corpuscles,  are  special  corpuscles  developed 
in  the  situations  where  the  manufacture  of  red  corpuscles  takes  place.  So 
far,  therefore,  from  assuming,  as  is  sometimes  done,  that  the  white  corpuscles 
of  the  blood  are  all  of  them  on  their  way  to  become  red  corpuscles,  it  may 
be  doubted  whether  any  of  them  are.  In  any  case,  however,  even  making 
allowance  for  those  which  migrate,  a  very  considerable  number  of  the  white 
corpuscles  must  "  disappear  "  in  some  way  or  other  from  the  blood  stream, 
and  we  may,  perhaps,  speak  of  their  disappearance  as  being  a  "  destruction  " 


50  BLOOD. 

or  "  dissolution."  We  have,  as  yet,  no  exact  knowledge  to  guide  us  in  the 
matter,  but  we  can  readily  imagine,  that  upon  the  death  of  the  corpuscle, 
the  substances  composing  it,  after  undergoing  changes,  are  dissolved  by  and 
become  part  of  the  plasma.  If  so,  the  corpuscles,  as  they  die,  must  repeat- 
edly influence  the  composition  and  nature  of  the  plasma. 

But  if  they  thus  affect  the  plasma  in  their  death,  it  is  even  more  probable 
that  they  influence  it  during  their  life.  Being  alive,  they  must  be  continually 
taking  in  and  giving  out.  As  we  have  already  said,  they  are  known  to  ingest, 
after  the  fashion  of  an  amoeba,  solid  particles  of  various  kinds,  such  as  fat 
or  carmine,  present  in  the  plasma,  and  probably  digest  such  of  these  particles 
as  are  nutritious.  But  if  they  ingest  these  solid  matters  they  probably  also 
carry  out  the  easier  task  of  ingesting  dissolved  matters.  If,  however,  they 
thus  take  in,  they  must  also  give  out;  and  thus  by  the  removal  on  the  one 
hand  of  various  substances  from  the  plasma,  and  by  the  addition  on  the  other 
hand  of  others,  they  must  be  .continually  influencing  the  plasma.  We  have 
already  said  that  the  white  corpuscles  in  shed  blood  as  they  die  are  supposed 
to  play  an  important  part  in  the  clotting  of  blood  ;  similarly  they  may  dur- 
ing their  whole  life  be  engaged  in  carrying  out  changes  in  the  proteids  of 
the  plasma  which  do  not  lead  to  clotting,  but  which  prepare  them  for  their 
various  uses  in  the  body. 

Pathological  facts  aflbrd  support  to  this  view.  The  disease  called  leuco- 
cythsemia  (or  leukaemia)  is  characterized  by  an  increase  of  the  white  cor- 
puscles, both  absolute  and  relative  to  the  red  corpuscles,  the  increase,  due 
to  an  augmented  production  or  possibly  to  a  retarded  destruction,  being  at 
times  so  great  as  to  give  the  blood  a  pinkish-gray  appearance,  like  that  of 
blood  mixed  with  pus.  We  accordingly  find  that  in  this  disease  the  plasma 
is  in  many  ways  profoundly  affected  and  fails  to  nourish  the  tissues.  As 
a  further  illustration  of  the  possible  action  of  the  white  corpuscles  we  may 
state  that,  according  to  some  observers  in  certain  diseases  in  which  minute 
organisms,  such  as  bacteria,  make  their  appearance  in  the  blood,  the  white 
corpuscles  "  take  up  "  these  bacteria  into  their  substance,  and  thus  probably, 
by  exerting  an  influence  on  them,  modify  the  course  of  the  disease  of  which 
these  organisms  are  the  essential  cause. 

If  the  white  corpuscles  are  thus  engaged  during  their  life  in  carrying  on 
important  labors,  we  may  expect  them  to  differ  in  appearance  according  to 
their  condition.  Some  of  the  corpuscles  are  spoken  of  as  "  faintly  "  or 
"  finely  "  granular.  Other  corpuscles  are  spoken  of  as  "  coarsely  "  granular, 
their  cell  substance  being  loaded  with  conspicuously  discrete  granules.  It 
may  be,  of  course,  that  there  are  two  distinct  kinds  of  corpuscles,  having 
different  functions  and  possibly  different  origins  and  histories ;  but  since 
intermediate  forms  are  met  with  containing  a  few  coarse  granules  only,  it 
is  more  probable  that  the  one  form  is  a  phase  of  the  other  ;  that  a  faintly 
granular  corpuscle,  by  taking  in  granules  from  without  or  by  producing 
granules  within  itself  as  products  of  its  metabolism,  may  become  a  coarsely 
granular  corpuscle. 

Whether,  however,  the  white  corpuscles  are  really  all  of  one  kind,  or 
whether  they  are  different  kinds  performing  different  functions,  must  at 
present  be  left  an  open  question. 

Blood  Platelets. 

§  33.  In  a  drop  of  blood  examined  with  care  immediately  after  removal 
may  be  seen  a  number  of  exceedingly  small  bodies  (2//  to  3/Jt  in  diameter), 
frequently  disc-shaped,  but  sometimes  of  a  rounded  or  irregular  form,  homo- 
geneous in  appearance  when  quite  fresh,  but  apt  to  assume  a  faintly  gran- 


THE  CHEMICAL  COMPOSITION  OF   BLOOD. 


51 


[FiG.  10. 


ular  aspect.     They  are  called  blood  platelets,  or  blood  plaques.     They  have 

been  supposed  by  some  to  become  developed  into,  and,  indeed,  to  be  early 

stages  of,  the  red  corpuscles,  arid  hence  have 

been  called  ha3inatoblasts ;   but  this  view  has 

not  been  confirmed  ;  indeed,  as  we  have  seen 

(§  27),  the  real  hsematoblasts,  or  developing  red 

corpuscles,  are  of  quite  a  different  nature. 

They  speedily  undergo  change  after  removal 
from  the  body,  apparently  dissolving  in  the 
plasma ;  they  break  up,  part  of  their  substance 
disappearing,  while  the  rest  becomes  granular. 
Their  granular  remains  are  apt  to  run  together, 
forming  in  the  plasma  the  shapeless  masses 
which  have  long  been  known  and  described  as 
<4  lumps  of  protoplasm."  By  appropriate  re- 
agents, however,  these  platelets  may  be  fixed 
and  stained  in  the  condition  in  which  they 
appear  after  leaving  the  body. 

The  substance  composing  them  is  peculiar, 
and,  though  we  may  perhaps  speak  of  them  as 
consisting  of  living  material,  their  nature  is  at 
present  obscure.  They  may  be  seen  within  the 
living  bloodvessels  [Fig.  10],  and  therefore 
must  be  regarded  as  real  parts  of  the  blood, 
and  not  as  products  of  the  changes  taking  vem-3 
place  in  blood  after  it  has  been  shed. 

When  a  needle  or  thread  or  other  foreign  body  is  introduced  into  the 
interior  of  a  bloodvessel,  they  are  apt  to  collect  upon,  and,  indeed,  are  the 
precursors  of  the  clot  which  in  most  cases  forms  around  the  needle  or  thread. 
They  are  also  found  in  the  thrombi  or  plugs  which  sometimes  form  in  the 
bloodvessels  as  the  result  of  disease  or  injury.  Indeed,  it  has  been  main- 
tained that  what  are  called  white  thrombi  (to  distinguish  them  from  red 
thrombi,  which  are  plugs  of  corpuscles  and  fibrin)  are  in  reality  aggrega- 
tions of  blood  platelets ;  and  for  various  reasons  blood  platelets  have  been 
supposed  to  play  an  important  part  in  the  clotting  of  blood,  carrying  out 
the  work  which,  in  this  respect,  is  by  others  attributed  to  the  white  corpus- 
cles. But  no  very  definite  statement  can  at  present  be  made  about  this ; 
and,  indeed,  the  origin  and  whole  nature  of  these  blood  platelets  is  at  pre- 
sent obscure. 


Fibrin  Filaments  and  Blood 
Platelets:  A,  network  of  fibrin, 
shown  after  washing  away  the 
corpuscles  from  a  preparation  of 
blood  that  has  been  allowed  to 
clot ;  many  of  the  filaments  radiate 
from  small  clumps  of  blood  plate- 
lets. B  (from  Osier),  blood  cor- 
puscles and  elementary  particles 
or  blood  platelets  within  a  small 


THE  CHEMICAL  COMPOSITION  OF  BLOOD. 

§  34.  We  may  now  pass  briefly  in  review  the  chief  chemical  characters 
of  blood,  remembering  always  that,  as  we  have  already  urged,  the  chief 
chemical  interests  of  blood  are  attached  to  the  changes  which  it  undergoes 
in  the  several  tissues;  these  will  be  considered  in  connection  with  each 
tissue  at  the  appropriate  place. 

The  average  specific  gravity  of  human  blood  is  1055,  varying  from  1045 
to  1075  within  the  limits  of  health. 

The  reaction  of  blood  as  it  flows  from  the  bloodvessels  is  found  to  be 
distinctly  though  feebly  alkaline.  If  a  drop  be  placed  on  a  piece  of  faintly 
red,  highly  glazed  litmus  paper,  and  then  wiped  off",  a  blue  stain  will  be  left. 

The  whole  blood  contains,  in  a  certain  quantity,  gases — viz.,  oxygen,  car- 
bonic acid,  and  nitrogen,  which  are  held  in  the  blood  in  a  peculiar  way — 
which  vary  in  different  kinds  of  blood,  and  so  serve  especially  to  distin- 


52  BLOOD. 

guish  arterial  from  venous  blood,  and  which  may  be  given  off  from  blood 
when  exposed  to  an  atmosphere,  according  to  the  composition  of  that 
atmosphere.  These  gases  of  blood  we  shall  study  in  connection  with  res- 
piration. 

The  normal  blood  consists  of  corpuscles  and  plasma. 

If  the  corpuscles  be  supposed  to  retain  the  amount  of  water  proper  to 
them,  blood  may,  in  general  terms,  be  considered  as  consisting  by  weight 
of  from  about  one-third  to  somewhat  less  than  one-half  of  corpuscles,  the 
rest  being  plasma.  As  we  have  already  seen,  the  number  of  corpuscles  in 
a  specimen  of  blood  is  found  to  vary  considerably,  not  only  in  different  ani- 
mals and  in  different  individuals,  but  in  the  same  individual  at  different 
times. 

The  plasma  is  resolved  by  the  clotting  of  the  blood  into  serum  and  fibrin. 

§  35.  The  serum  contains  in  100  parts  : 

Proteid  substances ,    .  about  8  or  9  parts. 

Fats,  various  extractives,  and  saline  matters "    2  or  1  part. 

Water  ...,,..  90    parts. 

The  proteids  are  paraglobulin  and  serum-albumin  (there  being  probably 
more  than  one  kind  of  serum-albumin)  in  varying  proportion.  We  may, 
perhaps,  roughly  speaking,  say  that  they  occur  in  about  equal  quantities. 

Conspicuous  and  striking  as  are  the  results  of  clotting,  massive  as  appears 
to  be  the  clot  which  is  formed,  it  must  be  remembered  that  by  far  the  greater 
part  of  the  clot  consists  of  corpuscles.  The  amount  by  weight  of  fibrin  re- 
quired to  bind  together  a  number  of  corpuscles,  in  order  to  form  even  a 
large,  firm  clot,  is  exceedingly  small.  Thus,  the  average  quantity  by  weight 
of  fibrin  in  human  blood  is  said  to  be  0.2  per  cent. ;  the  amount,  however, 
which  can  be  obtained  from  a  given  quantity  of  plasma  varies  extremely, 
the  variation  being  due  not  only  to  circumstances  affecting  the  blood,  but  to 
the  method  employed. 

The  fats,  which  are  scanty,  except  after  a  meal  or  in  certain  pathological 
conditions,  consist  of  the  neutral  fats — stearin,  palmitin,  and  olein — with  a 
certain  quantity  of  their  respective  alkaline  soaps.  The  peculiar  complex 
fat  lecithin  occurs  in  very  small  quantities  only ;  the  amount  present  of  the 
peculiar  alcohol  cholesterin,  which  had  so  fatty  an  appearance  is  also  small. 
Among  the  extractives  present  in  serum  may  be  put  down  nearly  all  the 
nitrogenous  and  other  substances  which  form  the  extractives  of  the  body 
and  of  food,  such  as  urea,  kreatin,  sugar,  lactic  acid,  etc.  A  very  large 
number  of  these  have  been  discovered  in  the  blood  under  various  circum- 
stances, the  consideration  of  which  must  be  left  for  the  present.  The  pecu- 
liar odor  of  blood  or  of  serum  is  probably  due  to  the  presence  of  volatile 
bodies  of  the  fatty  acid  series.  The  faint  yellow  color  of  serum  is  due  to  a 
special  yellow  pigment.  The  most  characteristic  and  important  chemical 
feature  of  the  saline  constitution  of  the  serum  is  the  preponderance,  at 
least  in  man  and  most  animals,  of  sodium  salts  over  those  of  potassium. 
In  this  respect  the  serum  offers  a  marked  contrast  to  the  corpuscles.  Less 
marked,  but  still  striking,  is  the  abundance  of  chlorides  and  the  poverty 
of  phosphates  in  the  serum,  as  compared  with  the  corpuscles.  The  salts 
may  in  fact  briefly  be  described  as  consisting  chiefly  of  sodium  chloride, 
with  some  amount  of  sodium  carbonate,  or  more  correctly  sodium  bicar- 
bonate, and  potassium  chloride  with  small  quantities  of  sodium  sulphate, 
sodium  phosphate,,  calcium  phosphate,  and  magnesium  phosphate.  And 
of  even  the  small  quantity  of  phosphates  found  in  the  ash,  part  of  the 
phosphorus  exists  in  the  serum  itself,  not  as  a  phosphate,  but  as  phos- 
phorus in  some  organic  body. 


QUANTITY  OF  BLOOD,  AND  ITS  DISTKIBUTION  IN  THE  BODY.     53 

§  36.  The  red  corpuscles  contain  lees  water  than  the  serum,  the  amount 
of  solid  matter  being  variously  estimated  at  from  30  to  40  or  more  per  cent. 
The  solids  are  almost  entirely  organic  matter,  the  inorganic  salts  amounting 
to  less  than  1  per  cent.  Of  the  organic  matter  again  by  far  the  larger  part 
consists  of  haemoglobin.  In  100  parts  of  the  dried  organic  matter  of  the 
corpuscles  of  human  blood,  about  90  parts  are  haemoglobin,  about  8  parts 
are  proteid  substances,  and  about  2  parts  are  other  substances.  Of  the 
last,  one  of  the  most  important,  forming  about  a  quarter  of  them  and 
apparently  being  always  present,  is  lecithin.  Cholesterin  appears  also  to 
be  normally  present.  The  proteids  which  form  the  stroma  of  the  red  cor- 
puscles appear  to  belong  chiefly  to  th^  globulin  family.  As  regards  the 
inorganic  constituents,  the  corpuscles  are  distinguished  by  the  relative 
abundance  of  the  salts  of  potassium  and  of  phosphates.  This  at  least  is 
the  case  in  man ;  the  relative  quantities  of  sodium  and  potassium  in  the 
corpuscles  and  serum  respectively  appear,  however,  to  vary  in  different 
animals ;  in  some  the  sodium  salts  are  in  excess  even  in  the  corpuscles. 

§  37.  The  proteid  matrix  of  the  white  corpuscles  we  have  stated  to  be 
composed  of  myosin  (or  an  allied  body),  paraglobulin  and  possibly  other 
proteids.  The  nuclei  contain  nuclein.  The  white  corpuscles  are  found  to 
contain,  in  addition  to  proteid  material,  lecithin  and  other  fats,  glycogen, 
extractives  and  inorganic  salts,  there  being  in  the  ash,  as  in  that  of  the  red 
corpuscles,  a  preponderance  of  potassium  salts  and  of  phosphates. 

The  main  facts  of  interest  then  in  the  chemical  composition  of  the  blood 
are  as  follows  :  The  red  corpuscles  consist  chiefly  of  haemoglobin.  The  or- 
ganic solids  of  serum  consist  partly  of  serum-albumin,  and  partly  of  para- 
globulin.  The  serum  or  plasma  contrasts,  in  man  at  least,  with  the  corpus- 
cles, inasmuch  as  the  former  contains  chiefly  chlorides  and  sodium  salts, 
while  the  latter  are  richer  in  phosphates  and  potassium  salts.  The  extrac- 
tives of  the  blood  are  remarkable  rather  for  their  number  and  variability 
than  for  their  abundance,  the  most  constant  and  important  being  perhaps 
urea,  kreatin,  sugar,  and  lactic  acid. 

THE  QUANTITY  OF  BLOOD,  AND  ITS  DISTRIBUTION  IN  THE  BODY. 

§  38.  The  quantity  of  blood  contained  in  the  whole  vascular  system  is  a 
balance  struck  between  the  tissues  which  give  to,  and  those  which  take  away 
from,  the  blood.  Thus  the  tissues  of  the  alimentary  canal  largely  add  to  the 
blood  water  and  the  material  derived  from  food,  while  the  excretory  organs 
largely  take  away  water  and  the  other  substances  constituting  the  excretions. 
Other  tissues  both  give  and  take,  and  the  considerable  drain  from  the  blood 
to  the  lymph  spaces  which  takes  place  in  the  capillaries  is  met  by  the  flow 
of  lymph  into  the  great  veins. 

From  the  result  of  a  few  observations  on  executed  criminals  it  has  been 
concluded  that  the  total  quantity  of  blood  in  the  human  body  is  about  T^th 
of  the  body  weight.  But  in  various  animals,  the  proportion  of  the  weight 
of  the  blood  to  that  of  the  body  has  been  found  to  vary  very  considerably 
in  different  individuals ;  and  probably  this  holds  good  for  nian  also,  at  all 
events  within  certain  limits. 

In  the  same  individual  the  quantity  probably  does  not  vary  largely.  A 
sudden  drain  upon  the  water  of  the  blood  by  great  activity  of  the  excretory 
organs,  as  by  profuse  sweating,  or  a  sudden  addition  to  the  water  of  the  blood, 
as  by  drinking  large  quantities  of  water  or  by  injecting  fluid  into  the  blood- 
vessels, is  rapidly  compensated  for  by  the  passage  of  water  from  the  tissues 
to  the  blood,  or  from  the  blood  to  the  tissues.  As  we  have  already  said,  the 
tissues  are  continually  striving  to  keep  up  an  average  composition  of  the 


54  THE  CONTRACTILE  TISSUES. 

blood,  and  in  so  doing  keep  up  an  average  quantity.  In  starvation  the 
quantity  (and  -quality)  of  the  blood  is  maintained  for  a  long  time  at  the 
expense  of  the  tissues,  so  that  after  some  days'  privation  of  food  and  drink, 
while  the  fat,  the  muscles,  and  other  tissues  have  been  largely  diminished, 
the  quantity  of  blood  remains  nearly  the  same. 

The  total  quantity  of  blood  present  in  an  animal  body  is  estimated  in  the  fol- 
lowing way :  As  much  blood  as  possible  is  allowed  to  escape  from  the  vessels ; 
this  is  measured  directly.  The  vessels  are  then  washed  out  with  water  or  normal 
saline  solution,  and  the  washings  carefully  collected,  mixed,  and  measured.  A 
known  quantity  of  blood  is  diluted  with  water  or  normal  saline  solution  until  it 
possesses  the  same  tint  as  a  measured  specimen  of  the  washings.  This  gives  the 
amount  of  blood  (or  rather  of  haemoglobin)  in  the  measured  specimen,  from  which 
the  total  quantity  in  the  whole  washings  is  calculated.  Lastly,  the  whole  body  is 
carefully  minced  and  washed  free  from  blood.  The  washings  are  collected  and 
filtered,  and  the  amount  of  blood  in  them  is  estimated,  as  before,  by  comparison 
with  a  specimen  of  diluted  blood.  The  quantity  of  blood,  as  calculated  from  the 
two  washings,  together  with  the  escaped  and  directly  measured  blood,  gives  the 
total  quantity  of  blood  in  the  body. 

The  method  is  not  free  from  objections,  but  other  methods  are  even  more 
imperfect. 

The  blood  is  in  round  numbers  distributed  as  follows : 

About  one-fourth  in  the  heart,  lungs,  large  arteries  and  veins. 

About  one-fourth  in  the  liver. 

About  one-fourth  in  the  skeletal  muscles. 

About  one-fourth  in  the  other  organs. 

Since  in  the  heart  and  great  bloodvessels  the  blood  is  simply  in  transit, 
without  undergoing  any  great  changes  (and  in  the  lungs,  as  far  as  we  know, 
being  limited  to  respiratory  changes),  it  follows  that  the  alterations  which 
take  place  in  the  blood  passing  through  the  liver  and  skeletal  muscles  far 
exceed  those  which  occur  in  the  rest  of  the  body. 


CHAPTER    II. 

THE  CONTRACTILE   TISSUES. 

§  39.  IN  order  that  the  blood  may  nourish  the  several  tissues  it  is  carried 
to  and  from  them  by  the  vascular  mechanism  ;  and  this  carriage  entails 
active  movements.  In  order  that  the  blood  may  adequately  nourish  the  tis- 
sues, it  must  be  replenished  by  food  from  the  alimentary  canal,  and  purified 
from  waste  by  the  excretory  organs ;  and  both  these  processes  entail  move- 
ments. Hence  before  we  proceed  further  we  must  study  some  of  the  general 
characters  of  the  movements  of  the  body. 

Most  of  the  movements  of  the  body  are  carried  out  by  means  of  the  mus- 
cles of  the  trunk  and  limbs,  which  being  connected  with  the  skeleton  are 
frequently  called  skeletal  muscles.  A  skeletal  muscle  when  subjected  to 
certain  influences  suddenly  shortens,  bringing  its  two  ends  nearer  together; 
and  it  is  the  shortening,  acting  upon  various  bony  levers  or  by  help  of  other 
mechanical  arrangements,  which  produces  the  movement.  Such  a  temporary 
shortening,  called  forth  by  certain  influences,  and  due  as  we  shall  see  to 
changes  taking  place  in  the  muscular  tissue  forming  the  chief  part  of  the 


THE  PHENOMENA   OF  MUSCLE  AND  NERVE.  55 

muscle,  is  technically  called  a  contraction  of  the  muscle ;  and  the  muscular 
tissue  is  spoken  of  as  a  contractile  tissue.  The  heart  is  chiefly  composed  of 
muscular  tissue,  differing  in  certain  minor  features  from  the  muscular  tissue 
of  the  skeletal  muscles,  and  the  beat  of  the  heart  is  essentially  a  contraction 
of  the  muscular  tissue  composing  it,  a  shortening  of  the  peculiar  muscular 
fibres  of  which  the  heart  is  chiefly  made  up.  The  movements  of  the  ali- 
mentary canal  and  of  many  other  organs  are  similarly  the  results  of  the 
contraction  of  the  muscular  tissue  entering  into  the  composition  of  those 
organs,  of  the  shortening  of  certain  muscular  fibres  built  up  into  those 
organs.  In  fact,  almost  all  the  movements  of  the  body  are  the  result  of 
the  contraction  of  muscular  fibres,  of  various  nature  and  variously  disposed. 

Some  few  movements,  however,  are  carried  out  by  structures  which  can- 
not be  called  muscular.  Thus,  in  the  pulmonary  passages  and  elsewhere, 
movement  is  effected  by  means  of  cilia  attached  to  epithelium  cells ;  and 
elsewhere,  as  in  the  case  of  the  migrating  white  corpuscles  of  the  blood, 
transference  from  place  to  place  in  the  body  is  brought  about  by  amoeboid 
movements.  But  as  we  shall  see  the  changes  in  the  epithelium  cell  or  white 
corpuscle  which  are  at  the  bottom  of  ciliary  or  amoeboid  movements  are,  in 
all  probability,  fundamentally  the  same  as  those  which  take  place  in  a  mus- 
cular fibre  when  it  contracts :  they  are  of  the  nature  of  a  contraction,  and 
hence  we  may  speak  of  all  these  as  different  forms  of  contractile  tissue. 

Of  all  these  various  forms  of  contractile  tissue,  the  skeletal  muscles,  on 
account  of  the  more  complete  development  of  their  functions,  will  be  better 
studied  first ;  the  others,  on  account  of  their  very  simplicity,  are  in  many 
respects  less  satisfactorily  understood. 

All  the  ordinary  skeletal  muscles  are  connected  with  nerves.  We  have 
no  reason  for  thinking  that  they  are  thrown  into  contraction,  under  normal 
conditions,  otherwise  than  by  the  agency  of  nerves. 

Muscles  and  nerves  being  thus  so  closely  allied,  and  having  besides  so 
many  properties  in  common,  it  will  conduce*  to  clearness  and  brevity  if  we 
treat  them  together. 

THE  PHENOMENA  OF  MUSCLE  AND  NERVE. 

Muscular  and  Nervous  Irritability. 

§  40.  The  skeletal  muscles  of  a  frog,  the  brain  and  spinal  cord  of  which 
have  been  destroyed,  do  not  exhibit  any  spontaneous  movements  or  contrac- 
tions, even  though  the  nerves  be  otherwise  quite  intact.  Left  undisturbed, 
the  whole  body  may  decompose  without  any  contraction  of  any  of  the  skele- 
tal muscles  having  been  witnessed.  Neither  the  skeletal  muscles  nor  the 
nerves  distributed  to  them  possess  any  power  of  automatic  action. 

If,  however,  a  muscle  be  laid  bare  and  be  more  or  less  violently  disturbed 
— if,  for  instance,  it  be  pinched,  or  touched  with  a  hot  wire,  or  brought  into 
contact  with  certain  chemical  substances,  or  subjected  to  the  action  of  gal- 
vanic currents — it  will  move,  that  is,  contract,  whenever  it  is  thus  disturbed. 
Though  not  exhibiting  any  spontaneous  activity,  the  muscle  is  (and  con- 
tinues for  some  time  after  the  general  death  of  the  animal  to  be)  irritable. 
Though  it  remains  quite  quiescent  when  left  untouched,  its  powers  are  then 
dormant  only — not  absent.  These  require  to  be  roused  or  "  stimulated  "  by 
some  change  or  disturbance  in  order  that  they  may  manifest  themselves. 
The  substances  or  agents  which  are  thus  able  to  evoke  the  activity  of  an 
irritable  muscle  are  spoken  of  as  stimuli. 

But  to  produce  a  contraction  in  a  muscle  the  stimulus  need  not  be  ap- 
plied directly  to  the  muscle ;  it  may  be  applied  indirectly  by  means  of  the 


56  THE  CONTRACTILE  TISSUES. 

nerve.  Thus,  if  the  trunk  of  a  nerve  be  pinched,  or  subjected  to  sudden 
heat,  or  dipped  in  certain  chemical  substances,  or  acted  upon  by  various 
galvanic  currents,  contractions  are  seen  in  the  muscles  to  which  branches  of 
the  nerves  are  distributed. 

The  nerve,  like  the  muscle,  is  irritable;  it  is  thrown  into  a  state  of 
activity  by  a  stimulus :  but,  unlike  the  muscle,  it  does  not  itself  contract. 
The  stimulus  does  not  give  rise  in  the  nerve  to  any  visible  change  of  form ; 
but  that  changes  of  some  kind  or  other  are  set  up  and  propagated  along  the 
nerve  down  to  the  muscle  is  shown  by  the  fact  that  the  muscle  contracts 
when  a  part  of  the  nerve  at  some  distance  from  itself  is  stimulated.  Both 
nerve  and  muscle  are  irritable,  but  only  the  muscle  is  contractile — L  e.,  mani- 
fests its  irritability  by  contraction.  The  nerve  manifests  its  irritability  by 
transmitting  along  itself,  without  any  visible  alteration  of  form,  certain 
molecular  changes  set  up  by  the  stimulus.  We  shall  call  these  changes 
thus  propagated  along  a  nerve  "  nervous  impulses." 

§  41.  We  have  stated  above  that  the  muscle  may  be  thrown  into  con- 
traction by  stimuli  applied  directly  to  itself.  But  it  might  fairly  be  urged 
that  the  contractions  so  produced  are  in  reality  due  to  the  fact  that  the 
stimulus,  although  apparently  applied  directly  to  the  muscle,  is,  after 
all,  brought  to  bear  on  some  of  the  many  fine  nerve-branches,  which,  as 
we  shall  see,  are  abundant  in  the  muscle,  passing  along  and  between  the 
muscular  fibres,  in  which  they  finally  end.  The  following  facts,  however, 
go  far  to  prove  that  the  muscular  fibres  themselves  are  capable  of  being 
directly  stimulated  without  the  intervention  of  any  nerves :  When  a  frog 
(or  other  animal)  is  poisoned  with  urari,  the  nerves  may  be  subjected  to  the 
strongest  stimuli  without  causing  any  contractions  in  the  muscles  to  which 
they  are  distributed ;  yet  even  ordinary  stimuli,  applied  directly  to  the 
muscle,  readily  cause  contractions.  If,  before  introducing  the  urari  into  the 
system,  a  ligature  be  passed  underneath  the  sciatic  nerve  in  one  leg — for 
instance,  the  right — and  drawn  tightly  round  the  whole  leg  to  the  exclusion 
of  the  nerve,  it  is  evident  that  the  urari,  when  injected  into  the  back  of  the 
animal,  will  gain  access  to  the  right  sciatic  nerve  above  the  ligature,  but 
not  below,  while  it  will  have  free  access  to  the  rest  of  the  body,  including 
the  whole  left  sciatic.  If,  as  soon  as  the  urari  has  taken  effect,  the  two 
sciatic  nerves  be  stimulated,  no  movement  of  the  left  leg  will  be  produced 
by  stimulating  the  left  sciatic,  whereas  strong  contractions  of  the  muscles 
of  the  right  leg  below  the  ligature  will  follow  stimulation  of  the  right 
sciatic,  whether  the  nerve  be  stimulated  above  or  below  the  ligature.  Now, 
since  the  upper  parts  of  both  sciatics  are  equally  exposed  to  the  action  of 
the  poison,  it  is  clear  that  the  failure  of  the  left  nerve  to  cause  contraction 
is  not  attributable  to  any  change  having  taken  place  in  the  upper  portion 
of  the  nerve,  else  why  should  not  the  right,  which  has  in  its  upper  portion 
been  equally  exposed  to  the  action  of  the  poison,  also  fail?  Evidently  the 
poison  acts  on  some  parts  of  the  nerve  lower  down.  If  a  single  muscle  be 
removed  from  the  circulation  (by  ligaturing  its  bloodvessels),  previous  to 
the  poisoning  with  urari,  that  muscle  will  contract  when  any  part  of  the 
nerve  going  to  it  is  stimulated,  though  no  other  muscle  in  the  body  will 
contract  when  its  nerve  is  stimulated.  Here  the  whole  nerve  right  down  to 
the  muscle  has  been  exposed  to  the  action  of  the  poison,  and  yet  it  has  lost 
none  of  its  power  over  the  muscle.  On  the  other  hand,  if  the  muscle  be 
allowed  to  remain  in  the  body,  and  so  be  exposed  to  the  action  of  the  poison, 
but  the  nerve  be  divided  high  up  and  the  part  connected  with  the  muscle 
gently  lifted  up  before  the  urari  is  introduced  into  the  system,  so  that  no 
blood  flows  to  it.  and  so  that  it  is  protected  from  tl  &  influence  of  the 
poison,  stimulation  of  the  nerve  will  be  found  to  produce  no  contractions  in 


THE  PHENOMENA  OF  MUSCLE  AND  NERVE.        57 

the  muscle,  though  stimuli  applied  directly  to  the  muscle  at  once  causes  it 
to  contract.  From  these  facts  it  is  clear  that  urari  poisons  the  ends  of  the 
nerve  within  the  muscle  long  before  it  affects  the  trunk  ;  and  it  is  exceedingly 
probable  that  it  is  the  very  extreme  ends  of  the  nerves  (possibly  the  end- 
plates  or  peculiar  structures  in  which  the  nerve  fibres  end  in  the  muscular 
fibres,  for  urari  poisoning,  at  least  when  profound,  causes  a  slight,  but  yet 
distinctly  recognizable  effect  in  the  microscopic  appearance  of  these  struc- 
tures) which  are  affected.  The  phenomena  of  urari  poisoning  go  far  to 
prove  that  muscles  are  capable  of  being  made  to  contract  by  stimuli  ap- 
plied directly  to  the  muscular  fibres  themselves ;  and  there  are  other  facts 
which  support  this  view. 

§  42.  When,  in  a  recently  killed  frog,  we  stimulate  by  various  means 
and  in  various  ways  the  muscles  and  nerves,  it  will  be  observed  that  the 
movements  thus  produced,  though  very  various,  may  be  distinguished  to  be 
of  two  kinds.  On  the  one  hand,  the  result  may  be  a  mere  twitch,  as  it 
were,  of  this  or  that  muscle  ;  on  the  other  hand,  one  or  more  muscles  may 
remain  shortened  or  contracted  for  a  considerable  time — a  limb,  for  instance, 
being  raised  up  or  stretched  out,  and  kept  raised  up  or  stretched  out  for 
many  seconds.  And  we  find,  upon  examination,  that  a  stimulus  may  be 
applied  either  in  such  a  way  as  to  produce  a  mere  twitch,  a  passing  rapid 
contraction  which  is  over  and  gone  in  a  fraction  of  a  second,  or  in  such  a 
way  as  to  keep  the  muscle  shortened  or  contracted  for  as  long  a  time  as,  up 
to  certain  limits,  we  may  choose.  The  mere  twitch  is  called  a  single  or 
simple  muscular  contraction;  the  sustained  contraction,  which,  as  we  shall 
see,  is  really  the  result  of  rapidly  repeated  simple  contractions,  is  called  a 
tetanic  contraction. 

§43.  In  order  to  study  these  contractions  adequately,  we  must  have 
recourse  to  the  "  graphic  method,"  as  it  is  called,  and  obtains  a  tracing  or 
other  record  of  the  change  of  form  of  the  muscle.  To  do  this  conveniently, 
it  is  best  to  operate  with  a  muscle  isolated  from  the  rest  of  the  body  of  a 
recently  killed  animal,  and  carefully  prepared  in  such  a  way  as  to  remain 
irritable  for  some  time.  The  muscles  of  cold-blooded  animals  remain  irri- 
table after  removal  from  the  body  far  longer  than  those  of  warm-blooded 
animals,  and  hence  those  of  the  frog  are  generally  made  use  of.  We  shall 
study  presently  the  conditions  which  determine  this  maintenance  of  the  irri- 
tability of  muscles  arid  nerves  after  removed  from  the  body. 

A  muscle  thus  isolated,  with  its  nerve  left  attached  to  it,  is  called  a 
muscle-nerve  preparation.  The  most  convenient  muscle  for  this  purpose  in 
the  frog  is,  perhaps,  the  gastrocnemius,  which  should  be  dissected  out  so  as 
to  leave  carefully  preserved  the  attachment  to  the  femur  above,  some  portion 
of  the  tendon  (tendo  Achillis)  below,  and  a  considerable  length  of  the  sciatic 
nerve  with  its  branches  going  to  the  muscle.  (Fig.  11.) 

§  44.  We  may  apply  to  such  a  muscle-nerve  preparation  the  various 
kinds  of  stimuli  (mechanical,  such  as  pricking  or  pinching ;  thermal,  such 
as  sudden  heating ;  chemical,  such  as  acids  or  other  active  chemical  sub- 
stances ;  or  electrical)  and  these  we  may  apply  either  to  the  muscle  directly 
or  to  the  nerve,  thus  affecting  the  muscle  indirectly.  Of  all  these  stimuli 
by  far  the  most  convenient  for  general  purposes  are  electrical  stimuli  of 
various  kinds  ;  and  these,  except  for  special  purposes,  are  best  applied  to  the 
nerve,  and  not  directly  to  the  muscle. 

Of  electrical  stimuli,  again,  the  currents,  as  they  are  called,  generated  by 
a  voltaic  cell,  are  most  convenient,  though  the  electricity  generated  by  a 
rotating  magnet,  or  that  produced  by  friction  may  be  employed.  Making 
use  of  a  cell  or  battery  of  cells — Daniell's,  Grove's,  Leclanche,  or  any  other 
— we  must  distinguish  between  the  current  produced  by  the  cell  itself,  the 


58 


THE  CONTRACTILE  TISSUES. 


constant  current,  as  we  shall  call  it,  and  the  induced  current  obtained  from 
the  constant  current  by  means  of  an  induction  coil,  as  it  is  called ;  for 

FIG.  11. 


A  Muscle-nerve  Preparation  :  TO,  the  muscle,  gastrocnemius  of  frog;  n,  the  sciatic  nerve,  all 
the  branches  being  cut  away  except  that  supplying  the  muscle  ;  /,  femur;  cl.,  clamp  ;  t.  a.,  tendo 
Achillis ;  sp.  c.,  end  of  spinal  canal. 

the  physiological  effects  of  the  two  kinds  of  current  are  in  many  ways 
different. 

It  may,  perhaps,  be  worth  while  to  remind  the  reader  of  the  following  facts : 
In  a  galvanic  battery,  the  substance  (plate  of  zinc,  for  instance)  which  is  acted 
upon  and  used  by  the  liquid  is  called  the  positive  element,  and  the  substance 
which  is  not  so  acted  upon  and  used  up  (plate,  etc.,  of  copper,  platinum,  or  carbon, 
etc. )  is  called  the  negative  element.  A  galvanic  action  is  set  up  when  the  positive 
(zinc)  and  the  negative  (copper)  elements  are  connected  outside  the  battery  by 
some  conducting  material,  such  as  a  wire,  and  the  current  is  said  to  flow  in  a  cir- 
cuit or  circle  from  the  zinc  or  positive  element  to  the  copper  or  negative  element 
inside  the  battery,  and  then  from  the  copper  or  negative  element  back  to  the  zinc 
or  positive  element  through  the  wire  outside  the  battery.  If  the  conducting  wire 
be  cut  through,  the  current  ceases  to  flow ;  but  if  the  cut  ends  be  brought  into 
contact,  the  current  is  re-established  and  continues  to  flow  so  long  as  the  contact 
is  good.  The  ends  of  the  wires  are  called  "  poles,"  or  when  used  for  physiological 
purposes,  in  which  case  they  may  be  fashioned  in  various  ways,  are  spoken  of  as 
electrodes.  When  the  poles  are  brought  into  contact  or  are  connected  by  some 
conducting  material,  galvanic  action  is  set  up,  and  the  current  flows  through  the 
battery  and  wires;  this  is  spoken  of  as  "making  the  current"  or  "completing  or 
closing  the  circuit."  When  the. poles  are  drawn  apart  from  each  other,  or  when 
some  non-conducting  material  is  interposed  between  them,  the  galvanic  action  is 
arrested;  this  is  spoken  of  as  "breaking  the  current"  or  "opening  the  circuit." 
The  current  passes  from  the  wire  connected  with  the  negative  (copper)  element  in 
the  battery  to  the  wire  connected  with  the  positive  (zinc)  element  in  the  battery ; 
hence,  the  pole  connected  with  the  copper  (negative)  element  is  called  the  positive 
pole,  and  that  connected  with  the  zinc  (positive)  element  is  called  the  negative  pole. 
When  used  for  physiological  purposes  the  positive  pole  becomes  the  positive  elec- 


THE  PHENOMENA   OF  MUSCLE  AND  NERVE. 


59 


trode,  and  the  negative  pole  the  negative  electrode.  The  positive  electrode  is  often 
spoken  of  as  the  anode  (ana,  up),  and  the  negative  electrode  as  the  kathode  (kata, 
down). 

A  piece  of  nerve  of  ordinary  length,  though  not  a  good  conductor,  is  still  a 
conductor,  and  when  placed  on  the  electrodes  completes  the  circuit,  permitting 
the  current  to  pass  through  it ;  in  order  to  remove  the  nerve  from  the  influence 
of  the  current  it  must  be  lifted  off  from  the  electrodes.  This  is  obviously  incon- 
venient ;  and  hence  it  is  usual  to  arrange  a  means  of  opening  or  closing  the  cir- 
cuit at  some  point  along  one  of  the  two  wires.  This  may  be  done  in  various  ways 
— by  fastening  one  part  of  the  wire  into  a  cup  of  mercury,  and  so  by  dipping  the 
other  part  of  the  wire  into  the  cup  to  close  the  circuit  and  make  the  current,  and 
by  lifting  it  out  of  the  mercury  to  open  the  circuit  and  break  the  current ;  or  by 
arranging  between  the  two  parts  of  the  wires  a  movable  bridge  of  good  con- 
ducting material,  such  as  brass,  which  can  be  put  down  to  close  the  circuit  or 
raised  up  to  open  the  circuit ;  or  in  other  ways.  Such  a  means  of  closing  and 
opening  a  circuit,  and  so  of  making  or  breaking  a  current,  is  called  a  key. 

A  key  which  is  frequently  used  by  physiologists  goes  by  the  name  of  Du  Bois- 
Reymond's  key.  Though  undesirable  in  many  respects,  it  has  the  advantage  that  it 
can  be  used  in  two  different  ways ;  when  arranged  as  in  A,  Fig.  12,  the  brass  bridge 


FIG.  12. 


Kat 


Diagram  of  Du  Bois-Reymond  Key  used,  A,  for  making  and  breaking  ;  B,  for  short-circuiting. 

of  K,  the  key,  being  down,  and  forming  a  means  of  good  conduction  between  the 
brass  plates  to  which  the  wires  are  screwed,  the  circuit  is  closed  and  the  current 
passes  from  the  positive  pole  (end  of  the  negative  (copper)  element)  to  the  posi- 
tive electrode,  or  anode,  An.,  through  the  nerve,  to  the  negative  electrode,  or 
kathode,  Kat.,  and  thenc^  back  to  the  negative  pole  (end  of  the  positive  (zinc) 
element)  in  the  battery ;  on  raising  the  brass  bridge,  the  circuit  is  opened,  the 
current  is  broken,  and  no  current  passes  through  the  electrodes.  When  arranged  as 
in  B,  if  the  brass  bridge  be  "  down,"  the  resistance  offered  by  it  is  so  small,  compared 
with  the  resistance  offered  by  the  nerve  between  the  electrodes,  that  the  whole 
current  from  the  battery  passes  through  the  bridge  back  to  the  battery,  and  none, 
or  only  an  infinitesimal  portion,  passes  into  the  nerve.  When,  on  the  other  hand, 
the  bridge  is  raised,  and  so  the  conduction  between  the  two  sides  suspended,  the 
current  is  not  able  to  pass  directly  from  one  side  to  the  other,  but  can  and  does 
pass  along  the  wire  through  the  nerve  back  to  the  battery.  Hence,  in  arrange- 
ment A,  "putting  down  the  key,"  as  it  is  called,  makes  a  current  in  the  nerve, 
and  "raising"  or  "opening  the  key"  breaks  the  current.  In  arrangement  B. 
however,  putting  down  the  key  diverts  the  current  from  the  nerve  by  sending  it 
through  the  bridge,  and  so  back  to  the  battery ;  the  current,  instead  of  making 
the  longer  circuit  through  the  electrodes,  makes  the  shorter  circuit  through  the 


60  THE  CONTRACTILE  TISSUES. 

key  ;  hence,  this  is  called  "short-circuiting."  When  the  bridge  is  raised  the  cur- 
rent passes  through  the  nerve  on  the  electrodes.  Thus,  " '  putting  down ' '  and 
"raising  "  or  "  opening  "  the  key  have  contrary  effects  in  A  and  B.  In  B,  it  will 
be  observed,  the  battery  is  always  at  work,  the  current  is  always  flowing  either 
through  the  electrodes  (key  up)  or  through  the  key  (key  down) ;  in  A,  the  battery 
is  not  at  work  until  the  circuit  is  made  by  putting  down  the  key.  And  in  many 
cases  it  is  desirable  to  take,  so  to  speak,  a  sample  of  the  current  while  the  battery 
is  in  full  swing,  rather  than  just  as  it  begins  to  work.  Moreover,  in  B  the  elec- 
trodes are,  when  the  key  is  down,  wholly  shut  off  from  the  current ;  whereas,  in 
A,  when  the  key  is  up,  one  electrode  is  still  in  direct  connection  with  the  battery, 
and  this  connection  leading  to  what  is  known  as  unipolar  action,  may  give  rise  to 
stimulation  of  the  nerve.  Hence  the  use  of  the  key  in  the  form  B. 

Other  forms  of  key  may  be  used.  Thus,  in  the  Morse  key  (F,  Fig.  1 3)  contact 
is  made  by  pressing  down  a  lever  handle  (ha)  ;  when  the  pressure  is  removed,  the 
handle,  driven  up  by  a  spring,  breaks  contact.  In  the  arrangement  shown  in  the 
figure,  one  wire  from  the  battery  being  brought  to  the  binding  screw  6,  while  the 
binding  screw  a  is  connected  with  the  other  wire,  putting  down  the  handle  makes 
connection  between  a  and  &,  and  thus  makes  a  current.  By  arranging  the  wires 
in  the  several  binding  screws  in  a  different  way,  the  making  contact  by  depressing 
the  handle  may  be  used  to  short  circuit. 

In  an  "  induction  coil,"  Figs.  13  and  14,  the  wire  connecting  the  two  elements 
of  a  battery  is  twisted  at  some  part  of  its  course  into  a  close  spiral,  called  the 
jtriiiHinj  coil.  Thus,  in  Fig.  13,  the  wire  cc//x,  connected  with  the  copper  or  nega- 
tive plate  c.  p.  of  the  battery,  E,  joins  the  primary  coil,  pr.  c. ,  and  then  passes  on 
as  ?/x/,  through  the  "key"  F,  to  the  positive  (zinc)  plate,  z.p.,  of  the  battery. 
Over  this  primary  coil,  but  quite  unconnected  with  it,  slides  another  coil,  the 
secoiidart/  coil  ,s\  c.  ;  the  ends  of  the  wire  forming  this  coil,  T/X/  and  xx/,  are  con- 
tinued on  in  the  arrangement  illustrated  in  the  figure  as  y'  and  ?/,  and  as  x/  and 
.-c,  and  terminate  in  electrodes.  If  these  electrodes  are  in  contact  or  connected 
with  conducting  material,  the  circuit  of  the  secondary  coil  is  said  to  be  closed ; 
otherwise  it  is  open. 

In  such  an  arrangement  it  is  found  that  at  the  moment  when  the  primary  cir- 
cuit is  closed,  i.  e.,  when  the  primary  current  is  "  made,"  a  secondary  "induced" 
current  is,  for  an  exceedingly  brief  period  of  time,  set  up  in  the  secondary  coil. 
Thus,  in  Fig.  13,  when,  by  moving  the  "key"  f]  y///  and  xx//,  previously  not  in 
connection  with  each  other,  are  put  into  connection,  and  the  primary  current  thus 
made,  at  that  instant  a  current  appears  in  the  wires,  ?/",  x",  etc.,  but  almost  imme- 
diately disappears.  A  similar  almost  instantaneous  current  is  also  developed  when 
the  primary  current  is  "broken,"  but  not  till  then.  So  long  as  the  primary  cur- 
rent flows  with  uniform  intensity,  no  current  is  induced  in  the  secondary  coil.  It 
is  only  when  the  primary  current  is  either  made  or  broken,  or  suddenly  varies 
in  intensity,  that  a  current  appears  in  the  secondary  coil.  In  each  case  the  cur- 
rent is  of  very  brief  duration,  gone  in  an  instant  almost,  and  may  therefore  be 
spoken  of  as  "a  shock,"  an  induction  shock;  being  called  a  "making  shock," 
when  it  is  caused  by  the  making,  and  a  "breaking  shock,"  when  it  is  caused  by 
the  breaking  of  the  primary  circuit.  The  direction  of  the  current  in  the  making 
shock  is  opposed  to  that  of  the  primary  current ;  thus,  in  the  figure,  while,  the 
primary  current  flows  from  x///  to  y'" ,  the  induced  making  shock  flows  from  y  to 
x.  The  current  of  the  breaking  shock,  on  the  other  hand,  flows  in  the  same 
direction  as  the  primary  current  from  x  to  ?/,  and  is  therefore  in  direction  the 
reverse  of  the  making  shock.  Compare  Fig.  14,  where  arrangement  is  shown  in 
a  diagrammatic  manner. 

The  current  from  the  battery,  upon  its  first  entrance  into  the  primary  coil,  as  it 
passes  along  each  twist  of  that  coil,  gives  rise  in  the  neighboring  twists  of  the 
same  coil  to  a  momentary  induced  current  having  a  direction  opposite  to  its  own, 
and  therefore  tending  to  weaken  itself.  It  is  not  until  this  "self-induction"  has 
passed  off  that  the  current  in  the  primary  coil  is  established  in  its  full  strength. 
Owing  to  this  delay  in  the  full  establishment  of  the  current  in  the  primary  coil, 
the  induced  current  in  the  secondary  coil  is  developed  more  slowly  than  it  would 
be  were  no  such  "  self-induction  "  present.  On  the  other  hand,  when  the  current 
from  the  battery  is  "broken  "  or  "shut  off"  from  the  primary  coil,  no  such  delay 
is  offered  to  its  disappearance,  and  consequently  the  induced  current  in  the  second- 


THE  PHENOMENA  OF  MUSCLE  AND  NEKVE. 
FIG.  13. 


61 


Diagram  illustrating  Apparatus  arranged  for  Experiments  with  Muscle  and  Nerve:  A.  The 
moist  chamber  containing  the  muscle-nerve  preparation.  The  muscle  m,  supported  by  the  clamp 
cl.,  which  firmly  grasps  the  end  of  the  femur/,  is  connected  by  means  of  the  S-hooks  and  a  thread 
with  the  lever  /,  placed  below  the  moist  chamber.  The  nerve  n,  with  a  portion  of  the  spinal  col- 
umn ri  still  attached  to  it,  is  placed  on  the  electrode-holder  el,  in  contact  with  the  wires  x,  y.  The 
whole  of  the  interior  of  the  glass  case  gl,  is  kept  saturated  with  moisture,  and  the  electrode-holder 
is  so  constructed  that  a  piece  of  moistened  blotting-paper  may  be  placed  on  it  without  coming 
into  contact  with  the  nerve.  - 

B.  The  revolving  cylinder  bearing  the  smoked  paper  on  which  the  lever  writes. 

C.  Du  Bois-Reymond's  key  arranged  for  short-circuiting.    The  wires  x  and  y  of  the  electrode- 


62 


THE  CONTRACTILE  TISSUES. 


ary  coil  is  developed  with  unimpeded  rapidity.      We  shall  see  later  on  that  a 
rapidly  developed  current  is  more  effective  as  a  stimulus  than  is  a  more  slowly. 


FIG.  14. 


Diagram  of  an  Induction  Coil :  +  positive  pole,  end  of  negative  element ;  —  negative  pole, 
end  of  positive  element  of  battery ;  K,  Du  Bois-Reymond's  key ;  pr.  c.  primary  coil,  current 
shown  by  feathered  arrow ;  sc.  c.  secondary  coil,  current  shown  by  unfeathered  arrow. 

developed  current.  Hence  the  making  shock,  where  rapidity  of  production  is 
interfered  with  by  the  self-induction  of  the  primary  coil,  is  less  effective  as  a 
stimulus  than  the  breaking  shock  whose  development  is  not  thus  interfered  with. 

The  strength  of  the  induced  current  depends,  on  the  one  hand,  on  the  strength 
of  the  current  passing  through  the  primary  coil — that  is,  on  the  strength  of  the 
battery.  It  also  depends  on  the  relative  position  of  the  two  coils.  Thus  a  second- 
ary coil  is  brought  nearer  and  nearer  to  the  primary  coil  and  made  to  overlap  it 
more  and  more ;  the  induced  current  becomes  stronger  and  stronger,  though  the 
current  from  the  battery  remains  the  same.  With  an  ordinary  battery,  the  sec- 
ondary coil  may  be  pushed  to  some  distance  away  from  the  primary  coil,  and  yet 
shocks  sufficient  to  stimulate  a  muscle  will  be  obtained.  For  this  purpose,  how- 
ever, the  two  coils  should  be  in  the  same  line  ;  when  the  secondary  coil  is  placed 
crosswise,  at  right  angles  to  the  primary,  no  induced  current  is  developed,  and  at 
intermediate  angles  the  induced  current  has  intermediate  strengths. 

When  the  primary  current  is  repeatedly  and  rapidly  made  and  broken,  the 
secondary  current  being  developed  with  each  make  and  with  each  break,  a  rapidly 
recurring  series  of  alternating  currents  is  developed  in  the  secondary  coil  and 

holder  are  connected  through  binding  screws  in  the  floor  of  the  moist  chamber  with  the  wires 
x',  y',  and  these  are  secured  in  the  key,  one  on  either  side.  To  the  same  key  are  attached  the 
wires  x",  y",  coming  from  the  secondary  coils  s.  c.  of  the  induction  coil  D.  This  secondary  coil 
can  be  made  to  slide  up  and  down  over  the  primary  coilpr.  c.,  with  which  are  connected  the  two 
wires  x'"  and  y'";  x'"is  connected  directly  with  one  pole— for  instance,  the  copper  pole  c.  p.  of  the 
battery  E;  y'"  is  carried  to  a  binding  screw,  a,  of  the  Morse  key  F,  and  is  continued  as  y'^  from 
another  binding  screw,  b,  of  the  key  to  the  zinc  pole  z.  p.  of  the  battery. 

Supposing  everything  to  be  arranged,  and  the  battery  charged ;  on  depressing  the  handle  ha, 
of  the  Morse  key  F,  a  current  will  be  made  in  the  primary  coil  pr.  c.,  passing  from  c.  p.  through 
x"'  to  pr.  c.,  and  thence  through  y"  to  a,  thence  to  6,  and  so  through  y"~  to  z.  p.  On  removing  the 
finger  from  the  handle  of  F,  a  spring  thrusts  up  the  handle,  and  the  primary  circuit  is  in  conse- 
quence immediately  broken. 

At  the  instant  that  the  primary  current  is  either  made  or  broken,  an  induced  current  is  for  the 
instant  developed  in  the  secondary  coil  s.  c.  If  the  cross-bar  h  in  the  Du  Bois-Reymond's  key  be 
raised  (as  shown  in  the  thick  line  in  the  figure),  the  wires  x",  x',  x,  the  nerve  between  the  elec- 
trodes and  the  wires  y,  y',  y",  form  the  complete  secondary  circuit,  and  the  nerve  consequently 
experiences  a  making  or  breaking  induction-shock  whenever  the  primary  current  is  made  or 
broken.  If  the  cross-bar  of  the  Du  Bois-Reymond  key  be  shut  down,  as  in  the  dotted  line  h'  in 
the  figure,  the  resistance  of  the  cross-bar  is  so  slight  compared  with  that  of  the  nerve  and  of  the 
wires  going  from  the  key  to  the  nerve,  that  the  whole  secondary  (induced)  current  passes  from 
x"  to  y"  (or  from  y"io  x")  along  the  cross-bar,  and  practically  none  passes  into  the  nerve.  The 
nerve,  being  thus  "  short-circuited,"  is  not  affected  by  any  changes  in  the  current. 

The  figure  is  intended  merely  to  illustrate  the  general  method  of  studying  muscular  contraction  ; 
it  is  not  to  be  supposed  that  the  details  here  given  are  universally  adopted,  or,  indeed,  the  best 
for  all  purposes. 


THP:  PHENOMENA  OF  MUSCLE  AND  NERVE. 


63 


passes  through  its  electrodes.  We  shall  frequently  speak  of  this  as  the  interrupted 
induction  current,  or  more  briefly  the  interrupted  current ;  it  is  sometimes  spoken 
of  as  thefaradic  current,  and  the  application  of  it  to  any  tissue  is  spoken  of  as 
faradization. 

Such  a  repeated  breaking  and  making  of  the  primary  current  may  be  effected 
in  many  various  ways.  In  the  instruments  commonly  used  for  the  purpose,  the 
primary  current  is  made  and  broken  by  means  of  a  vibrating  steel  slip  working 
against  a  magnet :  hence  the  instrument  is  called  a  magnetic  interrupter.  See 
Fig.  15. 

The  two  wires  x  and  y  from  the  battery  are  connected  with  the  two  brass  pillars 
a  and  d  bv  means  of  screws.  Directly  contact  is  thus  made  current,  indicated 


FIG 


The  Magnetic  Interrupter. 


in  the  figure  by  the  thick  interrupted  line,  passes  in  the  direction  of  the  arrows, 
up  the  pillar  a,  along  the  steel  spring  ft,  as  far  as  the  screw  c,  the  point  of  which, 
armed  with  platinum,  is  in  contact  with  a  small  platinum  plate  on  b.  The  cur- 
rent passes  from  b  through  c  and  a  connecting  wire  into  the  primary  coil  p.  Upon 
its  entering  into  the  primary  coil,  an  induced  (making)  current  is  for  the  instant 
developed  in  the  secondary  coil  (not  shown  in  the  figure).  From  the  primary  coil 
p  the  current  passes,  by  a  connecting  wire,  through  the  double  spiral  w,  and.  did 
nothing  happen,  would  continue  to  pass  from  m  by  a  connecting  wire  to  the  pillar 
c?,  and  so  by  the  wire  y  to  the  battery.  The  whole  of  this  course  is  indicated  by 
the  thick  interrupted  line  with  its  arrows. 

As  the  current,  however,  passes  through  the  spirals  m,  the  iron  cores  of  these 
are  made  magnetic.  They,  in  consequence,  draw  down  the  iron  bar  e,  fixed  at  the 
end  of  the  spring  ft,  the  flexibility  of  the  spring  allowing  this.  But  when  e  is 
drawn  down,  the  platinum  plate  on  the  upper  surface  of  b  is  also  drawn  away  from 
the  screw  c,  and  thus  the  current  is  "  broken  "  at  b.  (Sometimes  the.  screw  /  is 
so  arranged  that  when  e  is  drawn  down  a  platinum  plate  on  the  wider  surface  of  b 
is  brought  into  contact  with  the  platinum-armed  point  of  the  screw  /  The  cur- 
rent then  passes  from  b.  not  to  c,  but  to  /,  and  so  down  the  pillar  d,  in  the  direction 
indicated  by  the  thin  interrupted  line,  and  out  of  the  battery  by  the  wire  y,  and  is 
thus  cut  off  from  the  primary  coil.  But  this  arrangement  is  unnecessary.)  At  the 
instant  that  the  current  is  thus  broken  and  so  cut  off  from  the  primary  coil,  an 
induced  (breaking)  current  is  for  the  moment  developed  in  the  secondary  coil.  But 
the  current  is  cut  off  not  only  from  the  primary  coil,  but  also  from  the  spirals  m  ; 
in  consequence,  their  cores  cease  to  be  magnetized,  the  bar  e  ceases  to  be  attracted 
by  them,  and  the  spring  ft,  by  virtue  of  its  elasticity,  resumes  its  former  position 


64 


THE  CONTRACTILE  TISSUES. 


in  contact  with  the  screw  c.  This  return  of  the  spring,  however,  re-establishes 
the  current  in  the  primary  coil  and  in  the  spirals,  and  the  spring  is  drawn  down, 
to  be  released  once  more  in  the  same  manner  as  before.  Thus,  as  long  as  the 
current  is  passing  along  jc,  the  contact  of  b  with  c  is  alternately  being  made  and 
broken,  and  the  current  is  constantly  passing  into  and  being  shut  off  from  p, 
the  periods  of  alternation  being  determined  by  the  periods  of  vibration  of  the 
spring  b.  With  each  passage  of  the  current  into  or  withdrawal  from  the  pri- 
mary coil,  an  induced  (making  and,  respectively,  breaking)  current  is  developed 
in"  a  secondary  coil. 

As  thus  used,  each  "  making  shock,"  as  explained  above,  is  less  powerful  than 
the  corresponding  "breaking  shock;"  and,  indeed,  it  sometimes  happens  that 
instead  of  each  make,  as  well  as  each  break,  acting  as  a  stimulus,  giving  rise 
to  a  contraction,  the  "breaks"  only  are  effective,  the  several  "makes"  giving 
rise  to  no  contraction. 

But  what  is  known  as  Helmholtz's  arrangement  (Fig.  16),  however,  the  making 
and  breaking  shocks  may  be  equalized.  For  this  purpose  the  screw  c  is  raised  out 


The  Magnetic  Interrupter  with  Helmholtz's  Arrangement  for  Equalizing 
the  Make  and  Break  Shock. 

of  reach  of  the  excursions  of  the  spring  6,  and  a  moderately  thick  wire  w,  offering 
a  certain  amount  only  of  resistance,  is  interposed  between  the  upper  binding  screw 
of  on  the  pillar  rr,  and  the  binding  screw  c'  leading  to  the  primary  coil.  Under 
these  arrangements  the  current  from  the  battery  passes  through  a',  along  the  inter- 
posed wire  to  c',  through  the  primary  coil,  and  thus,  as  before,  to  m.  As  before, 
by  the  magnetism  of  m,  e  is  drawn  down  and  b  brought  in  contact  with  /.  As  the 
result  of  this  contact,  the  current  from  the  battery  can  now  pass  by  a,  /,  and  d 
(shown  by  the  thin  interrupted  line),  back  to  the  battery;  but  not  the  whole  of 
the  current,  some  of  it  can  still  pass  along  the  wire  w  to  the  primary  coil,  the  rela- 
tive amount  being  determined  by  the  relative  resistance  offered  by  the  two  courses. 
Hence  at  each  successive  magnetization  of  m,  the  current  in  the  primary  coil  does 
not  entirely  disappear  when  b  is  brought  in  contact  with  /;  it  is  only  so  far  dimin- 
ished that  m  ceases  to  attract  e,  and  hence  by  the  release  of  b  from  /  the  whole 
current  once  more  passes  along  w.  Since,  at  what  corresponds  to  the  "  break  "  the 
current  in  the  primary  coil  is  diminished  only,  not  absolutely  done  away  with, 
self-induction  makes  its  appearance  at  the  "break"  as  well  as  at  the  "make;" 
thus  the  "  breaking  "  and  "making  "  induced  currents  or  shocks  in  the  secondary 
coil  are  equalized.  They  are  both  reduced  to  the  lower  efficiency  of  the  "mak- 
ing ' '  shock  in  the  old  arrangement ;  hence  to  produce  the  same  strength  of 


THE  PHENOMENA  OF  MUSCLE  AND  NERVE.        65 

stimulus  with  this  arrangement  a  stronger  current  must  be  applied  or  the  sec- 
ondary coil  pushed  over  the  primary  coil  to  a  greater  extent  than  with  the  other 
arrangement. 

The  Phenomena  of  a  Simple  Muscular   Contraction. 

§  45.  If  the  far  end  of  the  nerve  of  a  muscle-nerve  preparation  (Figs. 
11  and  13)  be  laid  on  electrodes  connected  with   the  secondary  coil  of  an 
induction-machine,  the  passage  of  a  single  induction-shock,  which  may  be 
taken  as  a  convenient  form  of  an  almost  momentary  stimulus,  will  produce 
no  visible  change  in  the  nerve,  but  the  muscle  will  give  a  twitch,  a  short, 
sharp  contraction,  i.  e.,  will  for  an  instant  shorten  itself,  becoming  thicker 
the  while,  and  then  return  to  its  previous  condition.    If  one  end  of  the  mus- 
cle be  attached  to  a  lever,  while  the  other  is  fixed,  the  lever  will  by  its  move- 
ments indicate  the  extent  and  duration  of  the  shortening.     If  the  point  of 
the  lever  be  brought  to  bear  on  some  rapidly  travelling  surface,  on  which  it 
leaves  a  mark  (being  for  this  purpose  armed  with  a  pen  and  ink  if  the  sur- 
face be  plain  paper,  or  with  a  bristle  or  finely  pointed  piece  of  platinum  foil 
if  the  surface  be  smoked  glass  or  paper),  so  long  as  the  muscle  remains  at 
rest  the  lever  will  describe  an  even  line,  which  we  may  call  the  base  line. 
If,  however,  the  muscle  shortens  the  lever  will  rise  above  the  base  line  and 
thus  describe  some  sort  of  curve  above  the  base  line.     Now  it  is  found  that 
when  a  single  induction-shock  is  sent  through  the  nerve  the  twitch  which  the 
muscle  gives  causes  the  lever  to  describe  some  such  curve  as  that  shown  in 
Fig.  17  ;  the  lever  (after  a  brief  interval  immediately  succeeding  the  open- 
ing or  shutting  the  key,  of  which  we  shall  speak  presently)  rises  at  first  rap- 
idly but  afterward  more  slowly,  showing  that  the  muscle  is  correspondingly 
shortening ;  then  ceases  to  rise,  showing  that  the  muscle  is  ceasing  to  grow 
shorter ;  then  descends,  showing  that  the  muscle  is  lengthening  again,  and 
finally,  sooner  or  later,  reaches  and  joins  the  base  line,  showing  that  the 
muscle  after  the  shortening  has  regained  its  previous  natural  growth.    Such 
a  curve  described  by  a  muscle  during  a  twitch  or  simple  muscular  contrac- 
tion, caused  by  a  single  induction-shock  or  by  any  other  stimulus  producing 
the  same  effect,  is  called  a  curve  of  a  simple  muscular  contraction,  or  more 
shortly,  a  "  muscle-curve."     It  is  obvious  that  the  exact  form  of  the  curve 
described  by  identical  contractions  of  a  muscle  will  depend  on  the  rapidity 
with  which  the  recording  surface  is  travelling.  Thus  if  the  surface  be  travel- 
ling slowly  the  up-stroke  corresponding  to  the  shortening  will  be  very  abrupt 

FIG.  17.  FIG.  18. 


A  Muscle-curve  from  the  Gastrocnemius  of  the  Frog :  This  curve,  like  all  succeeding  ones, 
unless  otherwise  indicated,  is  to  be  read  from  left  to  right— that  is  to  say,  while  the  lever  and 
tuning-fork  were  stationary  the  recording  surface  was  travelling  from  right  to  left. 

a  indicates  the  moment  at  which  the  induction-shock  is  sent  into  the  nerve,  b  the  com- 
mencement, c  the  maximum,  and  d  the  close  of  the  contraction. 

Below  the  muscle-curve  is  the  curve  drawn  by  a  tuning-fork  making  100  double  vibrations 
a  second,  each  complete  curve  representing  therefore  one-hundredth  of  a  second. 

and  the  down  stroke  also  very  steep,  as  in  Fig.  18,  which  is  a  curve  from  a 
gastrocnemius    muscle  of  a  frog,  taken  with  a  slowly  moving  drum,  the 


66 


THE  CONTRACTILE  TISSUES. 


FIG.  19. 


"tuning-fork  being  the  same  as  that  used  in  Fig.  17;  indeed,  with  a  very 
slow  movement,  the  two  may  be  hardly  separable  from  each  other.  On 
the  other  hand,  if  the  surface  travel  very  rapidly  the 
curve  may  be  immediately  long  drawn  out,  as  in*  Fig. 
19,  which  is  a  curve  from  a  gastrocnemius  muscle  of 
a  frog,  taken  with  a  very  rapidly  moving  pendulum 
myograph,  the  tuning-fork  making  about  500  vibra- 
tions a  second.  On  examination,  however,  it  will  be 
found  that  both  these  extreme  curves  are  fundamen- 
tally the  same  as  the  medium  one,  when  account  is 
taken  of  the  different  rapidities  of  the  travelling  sur- 
face in  the  several  cases. 

In  order  to  make  the  "  muscle-curve "  complete, 
it  is  necessary  to  mark  on  the  recording  surface  the 
exact  time  at  which  the  induction  shock  is  sent  into 
the  nerve,  and  also  to  note  the  speed  at  which  the 
recording  surface  is  travelling. 

In  the  pendulum  myograph  the  rate  of  movement 
can  be  calculated  from  the  length  of  the  pendulum ; 
but  even  in  this  it  is  convenient,  and  in  the  case  of 
the  spring  myograph  and  revolving  cylinder  is  neces- 
sary, to  measure  the  rate  of  movement  directly  by 
means  of  a  vibrating  tuning-fork,  or  of  some  body 
vibrating  regularly.  Indeed  it  is  best  to  make  such 
a  direct  measurement  with  each  curve  that  is  taken. 

A  tuning-fork,  as  is  known,  vibrates  so  many  times 
a  second  according  to  its  pitch.  If  a  tuning-fork, 
armed  with  a  light  marker  on  one  of  its  prongs  and 
vibrating  say  100  a  second — i.  e.,  executing  a  double 
vibration,  moving  forward  and  backward,  100  times 
a  second — be  brought  while  vibrating  to  make  a  trac- 
ing on  the  recording  surface  immediately  below  the 
lever  belonging  to  the  muscle,  we  can  use  the  curve 
or  rather  curves  described  by  the  tuning-fork  to 
measure  the  duration  of  any  part  or  of  the  whole  of 
the  muscle-curve.  It  is  essential  that  at  starting  the 
point  of  the  marker  of  the  tuning-fork  should  be  ex- 
actly underneath  the  marker  of  the  lever,  or  rather, 
since  the  point  of  the  lever  as  it  moves  up  and  down 
describes  not  a  straight  line  but  an  arc  of  a  circle  of 
which  its  fulcrum  is  the  centre  and  itself  (from  the 
fulcrum  to  the  tip  of  the  marker)  the  radius,  that 
the  point  of  the  marker  of  the  tuning-fork  should 
be  exactly  on  the  arc  described  by  the  marker  of  the 
lever,  either  above  or  below  it,  as  may  prove  most 
convenient.  If,  then,  at  starting  the  tuning-fork 
marker  be  thus  on  the  arc  of  the  lever  marker,  and 
we  note  on  the  curve  of  the  tuning  fork  the  place 
where  the  arc  of  the  lever  cuts  it  at  the  beginning 
and  at  the  end  of  the  muscle-curve,  as  at  Fig.  17, 
we  can  count  the  number  of  vibrations  of  the  tuning- 
fork  which  have  taken  place  between  the  two  marks, 
and  so  ascertain  the  whole  time  of  the  muscle-curve ; 
if,  for  instance,  there  have  been  10  double  vibrations,  each  occupying  T^Q- 
second,  the  whole  curve  has  taken  y1^  second  to  make.  In  the  same  way  we 


THE  PHENOMENA  OF  MUSCLE  AND  NERVE.        67 

can  measure  the  duration  of  the  rise  of  the  curve  or  of  the  fall,  or  of  any 
part  of  it. 

Though  the  tuning-fork  may,  by  simply  striking  it,  be  set  going  long 
enough  for  the  purposes  of  an  observation,  it  is  convenient  to  keep  it  going 
by  means  of  an  electric  current  and  a  magnet,  very  much  as  the  spring  in 
the  magnetic  interrupter  (Fig.  15)  is  kept  going. 

It  is  not  necessary  to  use  an  actual  tuning-fork ;  any  rod,  armed  with  a 
marker,  which  can  be  made  to  vibrate  regularly,  and  whose  time  of  vibra- 
tion is  known,  may  be  used  for  the  purpose :  thus  a  reed,  made  to  vibrate 
by  a  blast  of  air,  is  sometimes  employed. 

The  exact  moment  at  which  the  induction  shock  is  thrown  into  the 
nerve  may  be  recorded  on  the  muscle-curve  by  means  of  a  "  signal,"  which 
may  be  applied  in  various  ways. 

A  large  steel  lever  armed  with  a  marker  is  arranged  over  a  small  coil  by  means 
of  a  light  spring  in  such  a  way  that  when  the  coil  by  the  passage  of  a  current 
through  it  becomes  a  magnet  it  pulls  the  lever  down  to  itself;  on  the  current  being 
broken,  and  the  magnetization  of  the  coil  ceasing,  the  lever  by  help  of  the  spring 
Hies  up.  The  marker  of  such  a  lever  is  placed  immediately  under — i.  c. ,  at  some 
point  on  the  arc  described  by — the  marker  of  the  muscle  (or  other)  lever.  Hence 
by  making  a  current  in  the  coil  and  putting  the  signal  lever  down,  or  by  breaking 
an  already  existing  current,  and  letting  the  signal  lever  fly  up,  we  can  make  at 
pleasure  a  mark  corresponding  to  any  part  we  please  of  the  muscle  (or  other)  curve. 

If,  in  order  to  magnetize  the  coil  of  the  signal,  we  use,  as  we  may  do,  the  pri- 
mary current  which  generates  the  induction-shock,  the  breaking  or  making  of  the 
primary  current,  whichever  we  use  to  produce  the  induction-shock,  will  make  the 
signal  lever  fly  up  or  come  down.  Hence  we  shall  have  on  the  recording  surface, 
under  the  muscle,  a  mark  indicating  the  exact  moment  at  which  the  primary  cur- 
rent was  broken  or  made.  Now  the  time  taken  up  by  the  generation  of  the  induced 
current  and  its  passage  into  the  nerve  between  the  electrodes  is  so  infinitesimally 
small,  that  we  may,  without  appreciable  error,  take  the  moment  of  the  breaking  or 
making  of  the  primary  current  as  the  moment  of  the  entrance  of  the  induction- 
shock  into  the  nerve.  Thus  we  can  mark  below  the  muscle-curve,  or  by  describing 
the  arc  of  the  muscle  lever,  on  the  muscle-curve  itself,  the  exact  moment  at  which 
the  induction-shock  falls  into  the  nerve  between  the  electrodes,  as  is  done  at  a  in 
Figs.  17,  18,  19. 

In  the  pendulum  myograph  a  separate  signal  is  not  needed.  If,  having  placed 
the  muscle  lever  in  the  position  in  which  we  intend  to  make  it  record,  we  allow 
the  glass  plate  to  descend  until  the  tooth  a'  just  touches  the  rod  c  (so  that  the  rod 
is  just  about  to  be  knocked  down,  and  so  break  the  primary  circuit)  and  make  on 
the  base  line,  which  is  meanwhile  being  described  by  the  lever  marker,  a  mark  to 
indicate  where  the  point  of  the  marker  is  under  these  circumstances,  and  then 
bring  back  the  plate  to  its  proper  position,  the  mark  which  we  have  made  will 
mark  the  moment  of  the  breaking  of  the  primary  circuit,  and  so  of  the  entrance 
of  the  induction-shock  into  the  nerve.  For  it  is  just  when,  as  the  glass  plate 
swings  down,  the  marker  of  the  lever  comes  to  the  mark  which  we  have  made 
that  the  rod  c  is  knocked  back  and  the  primary  current  is  broken. 

A  ''signal"  like  the  above,  in  an  improved  form  known  as  Desprez's,  may  be 
used  also  to  record  time,  and  thus  the  awkwardness  of  bringing  a  large  tuning- 
fork  up  to  the  recording  surface  obviated.  For  this  purpose  the  signal  is  intro- 
duced into  a  circuit  the  current  of  which  is  continually  being  made  and  broken  by 
a  tuning-fork  (Fig.  21 ).  The  tuning-fork  once  set  vibrating  continues  to  make  and 
break  the  current  at  each  of  its  vibrations,  and  as  stated  above  is  kept  vibrating  by 
the  current.  But  each  make  or  break  caused  by  the  tuning-fork  affects  also  the 
small  coil  of  the  signal,  causing  the  lever  of  the  signal  to  fall  down  or  fly  up.  Thus 
the  signal  describes  vibration-curves  synchronous  with  those  of  the  tuning-fork 
driving  it.  The  signal  may  similarly  be  worked  by  means  of  vibrating  agents  other 
than  a  tuning-fork. 

Various  recording  surfaces  may  be  used.  The  form  most  generally  useful  is  a 
cylinder  covered  with  smoked  paper  and  made  to  revolve  by  clockwork  or  other- 
wise; such  a  cylinder  driven  by  clockwork  is  shown  in  Fig.  13,  B.  By  using  a 


68 


THE  CONTRACTILE  TISSUES. 
FIG.  20. 


The  Pendulum  Myograph  :  The  figure  is  diagrammatic,  the  essentials  only  of  the  instrument 
being  shown.  The  smoked  glass  plate  A  swings  with  the  pendulum  B  on  carefully  adjusted 
bearings  at  C.  The  contrivances  by  which  the  glass  plate  can  be  removed  and  replaced  at 
pleasure  are  not  shown.  A  second  glass  plate  so  arranged  that  the  first  glass  plate  may  be 
moved  up  and  down  without  altering  the  swing  of  the  pendulum  is  also  omitted.  Before  com- 
mencing an  experiment  the  pendulum  is  raised  up  (in  the  figure  to  the  right),  and  is  kept  in  that 
position  by  the  tooth  a  catching  on  the  spring-catch  b.  On  depressing  the  catch  b  the  glass  plate 
is  set  free,  swings  into  the  new  position  indicated  by  the  dotted  lines,  and  is  held  in  that  posi- 
tion by  the  tooth  a'  catching  on  the  catch  b'.  In  the  course  of  its  swing  the  tooth  a'  coming  in 
contact  with  the  projecting  steel  rod  c,  knocks  it  on  one  side  into  the  position  indicated  by  the 


THE  PHENOMENA  OF  MUSCLE  AND  NERVE.  69 

cylinder  of  large  radius  with  adequate  gear,  a  high  speed  for  instance,  in  a  second, 
can  be  obtained.     In  the  spring  myograph  a  smoked  glass  plate  is  thrust  rapidly 


FIG.  21. 

! 
p 


Diagram  of  an  Arrangement  of  a  Vibrating  Tuning-fork  with  a  Desprez  Signal :  The  current 
flows  along  the  wire  /  connected  with  the  positive  (  +  )  pole  or  end  of  the  negative  plate  (N)  of 
the  battery,  through  the  tuning-fork,  down  the  pin  connected  with  the  end  of  the  lower  prong, 
to  the  mercury  in  the  cup  Hg,  and  so  by  a  wire  (shown  in  figure)  to  the  binding  screw  e.  From 
this  binding  screw  part  of  the  current  flows  through  the  coil  d  between  the  prongs  of  the  tuning- 
fork,  and  thence  by  the  wire  c  to  the  binding  screw  a,  while  another  part  flows  through  the  wire 
g,  through  the  coil  of  the  Desprez  signal  back  by  the  wire  b,  to  the  binding  screw  a.  From  the 
binding  screw  a  the  current  passes  back  to  the  negative  (— )  pole  or  end  of  the  positive  element 
(P)  of  the  battery.  As  the  current  flows  through  the  coil  of  the  Desprez  signal  from  g  to  b,  the 
core  of  coil  becoming  magnetized  draws  down  the  marker  of  the  signal.  As  the  current  flows 
through  the  coil  d.the  core  of  that  coil,  also  becoming  magnetized,  draws  up  the  lower  prong  of 
the  fork.  But  the  pin  is  so  adjusted  that  the  drawing  up  of  the  prong  lifts  the  point  of  the  pin 
out  of  the  mercury.  In  consequence,  the  current,  being  thus  broken  at  Hg,  flows  neither 
through  d  nor  through  the  Desprez  signal.  The  core  of  the  Desprez  thus  ceasing  to  be  magnetized, 
the  marker  flies  back,  being  usually  assisted  by  a  spring  (not  shown  in  the  figure).  But  since 
the  current  ceases  to  flow  through  d,  the  core  of  d  ceases  to  lift  up  the  prong,  and  the  pin,  in  the 
descent  of  the  prong,  makes  contact  once  more  with  the  mercury.  The  re-establishment  of  the 
current,  however,  once  more  acting  on  the  two  coils,  again  pulls  down  the  marker  of  the  signal, 
and  again  by  magnetizing  the  core  of  d  pulls  up  the  prong  and  once  more  breaks  the  current. 
Thus  the  current  is  continually  made  and  broken,  the  rapidity  of  the  interruptions  being 
determined  by  the  vibration  periods  of  the  tuning-fork,  and  the  lever  of  the  signal  rising  and 
falling  synchronously  with  the  movements  of  the  tuning-fork. 

s 

forward  along  a  groove  by  means  of  a  spring  suddenly  thrown  into  action.  In  the 
pendulum  myograpli,  Fig.  20,  a  smoked  glass  plate  attached  to  the  lower  end  of  a 
long  frame  swinging  like  a  pendulum,  is  suddenly  let  go  at  a  certain  height,  and 
so  swings  rapidly  through  an  arc  of  a  circle.  The  disadvantage  of  the  last  two 
methods  is  that  the  surface  travels  at  a  continually  changing  rate,  whereas,  in  the 
revolving  cylinder,  careful  construction  and  adjustment  will  secure  a  very  uniform 
rate. 

§  46.  Having  thus  obtained  a  time  record,  and  an  indication  of  the  exact 
moment  at  which  the  induction-shock  falls  into  the  nerve,  we  may  for  present 
purposes  consider  the  muscle-curve  complete.  The  study  of  such  a  curve, 
as,  for  instance,  that  shown  in  Fig.  17,  taken  from  the  gastrocnemius  of  a 
frog,  teaches  us  the  following  facts : 

1.  That  although  the  passage  of  the  induced  current  from  electrode  to 
electrode  is  practically  instantaneous,  its  effect,  measured  from  the  entrance 
of  the  shock  into  the 'nerve  to  the  return  of  the  muscle  to  its  natural  length 


dotted  line  c'.  The  rod  c  is  in  electric  continuity  with  the  wire  x  of  the  primary  coil  of  an 
induction-machine.  The  screw  d  is  similarly  in  electric  continuity  with  the  wire  y  of  the  same 
primary  coil.  The  screw  d  and  the  rod  c  are  armed  with  platinum  at  the  points  at  which  they 
are  in  contact,  and  both  are  insulated  by  means  of  the  ebonite  block  e.  As  long  as  c  and  d  are 
in  contact  the  circuit  of  the  primary  coil  to  which  x  and  y  belong  is  closed.  When  in  its  swing 
the  tooth  a'  knocks  c  away  from  d,  at  that  instant  the  circuit  is  broken,  and  a  "  breaking  "  shock 
is  sent  through  the  electrodes  connected  with  the  secondary  coil  of  the  machine,  and  so  through 
the  nerve.  The  lever  I,  the  end  only  of  which  is  shown  in  the  figure,  is  brought  to  bear  on  the 
glass  plate,  and  when  at  rest  describes  a  straight  line,  or  more  exactly  an  arc  of  a  circle  of  large 
radius.  The  tuning-fork  /,  the  ends  only  of  the  two  limbs  of  which  are  shown  in  the  figure 
placed  immediately  below  the  lever,  serves  to  mark  the  time. 


70  THE  CONTRACTILE  TISSUES. 

after  the  shortening,  takes  an  appreciable  time.  In  the  figure,  the  whole 
curve  from  a  to  d  takes  up  about  the  same  time  as  eleven  double  vibrations 
of  the  tuning-fork.  Since  each  double  vibration  here  represents  T^7  second, 
the  duration  of  the  whole  curve  is  rather  more  than  y1^  second. 

2.  In  the  first  portion  of  this  period,  from  a  to  6,  there  is  no  visible 
change,  no  raising  of  the  lever,  no  shortening  of  the  muscle. 

3.  It  is  not  until  b,  that  is  to  say,  after  the  lapse  of  about  T^  second, 
that  the  shortening  begins.     The  shortening,  as  shown  by  the  curve  is  at 
first  slow,  but  soon  becomes   more  rapid,  and  then  slackens  again  until  it 
reaches  a  maximum  at  c;  the  whole  shortening  occupying  rather  more  than 
•rfo  second. 

4.  Arrived  at  the  maximum  of  shortening,  the  muscle  at  once  begins  to 
relax,  the  lever  descending  at  first  slowly,  then  more  rapidly,  and  at  last 
more  slowly  again,  until  at  d  the  muscle  has  regained  its  natural  length  ; 
the  whole  return  from  the  maximum  of  contraction  to  the  natural  length 
occupying  rather  more  than  T-§Q  second. 

Thus  a  simple  muscular  contraction,  a  simple  spasm  or  twitch,  produced 
by  a  momentary  stimulus,  such  as  a  single  induction-shock,  consists  of  three 
main  phases : 

1.  A  phase  antecedent  to  any  visible  alteration  in  the  muscle.     This 
phase,  during  which  invisible  preparatory  changes  are  taking  place  in  the 
nerve  and  muscle,  is  called  the  "  latent  period." 

2.  A  phase  of  shortening  or,  in  the  more  strict  meaning  of  the  word, 
contraction. 

3.  A  phase  of  relaxation  or  return  to  the  original  length. 

In  the  case  we  are  considering,  the  electrodes  are  supposed  to  be  applied 
to  the  nerve  at  some  distance  from  the  muscle.  Consequently  the  latent 
period  of  the  curve  comprises  not  only  the  preparatory  actions  going  on  in 
the  muscle  itself,  but  also  the  changes  necessary  to  conduct  the  immediate 
effect  of  the  induction-shock  from  the  part  of  the  nerve  between  the  elec- 
trodes along  a  considerable  length  of  nerve  down  to  the  muscle.  It  is  obvi- 
ous that  these  latter  changes  might  be  eliminated  by  placing  the  electrodes 
on  the  muscle  itself  or  on  the  nerve  close  to  the  muscle.  If  this  were  done, 
the  muscle  and  lever  being  exactly  as  before,  and  care  were  taken  that  the 
induction-shock  entered  into  the  nerve  at  the  new  spot,  at  the  moment  when 
the  point  of  the  lever  had  reached  exactly  the  same  point  of  the  travelling 
surface  as  before,  two  curves  would  be  gained  having  the  relations  shown  in 
Fig.  22.  The  two  curves  resemble  each  other  in  almost  all  points,  except 
that  in  the  curve  taken  with  the  shorter  piece  of  nerve,  the  latent  period, 
the  distance  a  to  b  as  compared  with  the  distance  a  to  bf  is  shortened ;  the 
contraction  begins  rather  earlier.  A  study  of  the  two  curves  teaches  us  the 
following  two  facts : 

1.  Shifting  the  electrodes  from  a  point  of  the  nerve  at  some  distance 
from  the  muscle  to  a  point  of  the  nerve  close  to  the  muscle  has  only  short- 
ened the  latent  period  a  very  little.  Even  when  a  very  long  piece  of  nerve 
is  taken  the  difference  in  the  two  curves  is  very  small,  and,  indeed,  in  order 
that  it  may  be  clearly  recognized  or  measured,  the  travelling  surface  must 
be  made  to  travel  very  rapidly.  It  is  obvious,  therefore,  that  by  far  the 
greater  part  of  the  latent  period  is  taken  up  by  changes  in  the  muscle  itself, 
changes  preparatory  to  the  actual  visible  shortening.  Of  course,  even  when 
the  electrodes  are  placed  close  to  the  muscle,  the  latent  period  includes  the 
changes  going  on  in  the  short  piece  of  nerve  still  lying  between  the  elec- 
trodes and  the  muscular  fibres.  To  eliminate  this  with  a  view  of  determin- 
ing the  latent  period  in  the  muscle  itself,  the  electrodes  might  be  placed 
directly  on  the  muscle  poisoned  with  urari.  If  this  were  done,  it  would  be 


THE  PHENOMENA  OF  MUSCLE  AND  NEKVE.  71 

found  that  the  latent  period  remained  about  the  same,  that  is  to  say,  that  in 
all  cases  the  latent  period  is  chiefly  taken  up  by  changes  in  the  muscular  as 
distinguished  from  the  nervous  elements. 

2.  Such  difference  as  does  exist  between  the  two  curves  in  the  figure 
indicates  the  time  taken  up  by  the  propagation,  along  the  piece  of  nerve,  of 


FIG.  22. 


Curves  Illustrating  the  Measurement  of  the  Velocity  of  a  Nervous  Impulse :  The  same  muscle- 
nerve  preparation  is  stimulated  (1)  as" far  as  possible  from  the  muscle,  (2)  as  near  as  possible  to 
the  muscle ;  both  contractions  are  registered  in  exactly  the  same  way. 

In  (1)  the  stimulus  enters  the  nerve  at  the  time  indicated  by  the  line  a,  the  contraction  begins 
at  b';  the  whole  latent  period,  therefore,  is  indicated  by  the  distance  from  a  to  b'. 

In  (2)  the  stimulus  enters  the  nerve  at  exactly  the  same  time  a ;  the  contraction  begins  at  b  ; 
the  latent  period,  therefore,  is  indicated  by  the  distance  between  a  and  b. 

The  time  taken  up  by  the  nervous  impulse  in  passing  along  the  length  of  nerve  between  1  and 
2  is,  therefore,  indicated  by  the  distance  between  6  and  b',  which  may  be  measured  by  the  tuning- 
fork  curve  below ;  each  double  vibration  of  the  tuning-fork  corresponds  to  1-120  or  0.0083  second. 

the  changes  set  up  at  the  far  end  of  the  nerve  by  the  induction-shock. 
These  changes  we  have  already  spoken  of  as  constituting  a  nervous  impulse  ; 
and  the  above  experiment  shows  that  it  takes  a  small  but  yet  distinctly 
appreciable  time  for  a  nervous  impulse  to  travel  along  a  nerve.  In  the 
figure  the  difference  between  the  two  latent  periods,  the  distance  between  b 
and  6',  seems  almost  too  small  to  measure  accurately ;  but  if  a  long  piece 
of  nerve  be  used  for  the  experiment,  and  the  recording  surface  be  made  to 
travel  very  fast,  the  difference  between  the  duration  of  the  latent  period 
when  the  induction-shock  is  sent  in  at  a  point  close  to  the  muscle,  and  that 
when  it  is  sent  in  at  a  point  as  far  away  as  possible  from  the  muscle,  may 
be  satisfactorily  measured  in  fractions  of  a  second.  If  the  length  of  nerve 
between  the  two  points  be  accurately  measured,  the  rate  at  which  a  nervous 
impulse  travels  along  the  nerve  to  a  muscle  can  thus  be  easily  calculated. 
This  has  been  found  to  be  in  the  frog  about  28,  and  in  man  about  33 
metres  per  second,  but  varies  considerably,  especially  in  warm-blooded 
animals. 

Thus  when  a  momentary  stimulus,  such  as  a  single  induction-shock,  is  sent 
into  a  nerve  connected  with  a  muscle,  the  following  events  take  place :  a 
nervous  impulse  is  started  in  the  nerve  and  this  travelling  down  to  the  muscle 
produces  in  the  muscle,  first  the  invisible  changes  which  constitute  the  latent 
period,  secondly  the  changes  which  bring  about  the  shortening  or  contraction 
proper,  and  thirdly  the  changes  which  bring  about  the  relaxation  and  return 
to  the  original  length.  The  changes  taking  place  in  each  of  these  three  phases 
are  changes  of  living  matter  ;  they  vary  with  the  condition  of  the  living  sub- 
stance of  the  muscle,  and  only  take  place  so  long  as  the  muscle  is  alive. 
Though  the  relaxation  which  brings  back  the  muscle  to  its  original  length 
is  assisted  by  the  muscle  being  loaded  with  a  weight  or  otherwise  stretched, 
this  is  not  essential  to  the  actual  relaxation,  and  with  the  same  load  the 
return  will  vary  according  to  the  condition  of  the  muscle ;  the  relaxation 
must  be  considered  as  an  essential  part  of  the  whole  contraction  no  less  than 
the  shortening  itself. 

§  47.  Not  only,  as  we  shall  see  later  on,  does  the  whole  contraction  vary 


72  THE  CONTRACTILE  TISSUES. 

in  extent  and  character  according  to  the  condition  of  the  muscle,  the  strength 
of  the  induction-shock,  the  load  which  the  muscle  is  bearing,  and  various 
attendant  circumstances,  but  the  three  phases  may  vary  independently.  The 
latent  period  may  be  longer  or  shorter,  the  shortening  may  take  a  longer  or 
shorter  time  to  reach  the  same  height,  and  especially  the  relaxation  may  be 
slow  or  rapid,  complete  or  imperfect.  Even  when  the  same  strength  of 
induction-shock  is  used  the  contraction  may  be  short  and  sharp  or  very  long 
drawn  out,  so  that  the  curves  described  on  a  recording  surface  travelling  at 
the  same  rate  in  the  two  cases  appear  very  different ;  and  under  certain  cir- 
cumstances, as  when  a  muscle  is  fatigued,  the  relaxation,  more  particularly 
the  last  part  of  it,  may  be  so  slow,  that  it  may  be  several  seconds  before  the 
muscle  really  regains  its  original  length. 

Hence,  if  we  say  that  the  duration  of  a  simple  muscular  contraction  of  the 
gastrocnemius  of  a  frog  under  ordinary  circumstances  is  about  y1-^  second,  of 
which  y^  is  taken  up  by  the  latent  period,  Tf  ¥  by  the  contraction,  and  y^ 
by  the  relaxation,  these  must  be  taken  as  "  round  numbers,"  stated  so  as  to 
be  easily  remembered.  The  duration  of  each  phase  as  well  as  of  the  whole 
contraction  varies  in  different  animals,  in  different  muscles  of  the  same 
animal,  and  in  the  same  muscle  under  different  conditions. 

The  muscle-curve  which  we  have  been  discussing  is  a  curve  of  changes  in 
the  length  only  of  the  muscle  ;  but  if  the  muscle,  instead  of  being  suspended, 
were  laid  flat  on  a  glass  plate  and  a  lever  laid  over  its  belly,  we  should  find, 
upon  sending  an  induction-shock  into  the  nerve,  that  the  lever  was  raised, 
showing  that  the  muscle  during  the  contraction  became  thicker.  And,  if  we 
took  a  graphic  record  of  the  movements  of  the  lever,  we  should  obtain  a 
curve  very  similar  to  the  one  just  discussed  ;  after  a  latent  period  the  lever 
would  rise,  showing  that  the  muscle  was  getting  thicker,  and  afterward  would 
fall,  showing  that  the  muscle  was  becoming  thin  again.  In  other  words,  in 
contraction  the  lessening  of  the  muscle  lengthwise  is  accompanied  by  an 
increase  crosswise  ;  indeed,  as  we  shall  see  later  on,  the  muscle  in  contracting 
is  not  diminished  in  bulk  at  all  (or  only  to  an  exceedingly  small  extent, 
about  y^TTir  °f  its  total  bulk),  but  makes  up  for  its  diminution  in  length  by 
increasing  in  its  other  diameters. 

§  48.  A  single  induction  shock  is,  as  we  have  said,  the  most  convenient 
form  of  stimulus  for  producing  a  simple  muscular  contraction,  but  this  may 
also  be  obtained  by  other  stimuli,  provided  that  these  are  sufficiently  sudden 
and  short  in  their  action,  as,  for  instance,  by  a  prick  of,  or  a  sharp  blow  on, 
the  nerve  or  muscle.  For  the  production  of  a  single  simple  muscular  con- 
traction the  changes  in  the  nerve  leading  to  the  muscle  must  be  of  such  a 
kind  as  to  constitute  what  may  be  called  a  single  nervous  impulse,  and  any 
stimulus  which  will  evoke  a  single  nervous  impulse  only  may  be  used  to 
produce  a  simple  muscular  contraction. 

As  a  rule,  however,  most  stimuli,  other  than  single  induction-shocks,  tend 
to  produce  in  a  nerve  several  nervous  impulses,  and,  as  we  shall  see,  the 
nervous  impulses  which  issue  from  the  central  nervous  system,  and  so  pass 
along  nerves  to  muscles,  are,  as  a  rule,  not  single  and  simple,  but  complex. 
Hence,  as  a  matter  of  fact,  a  simple  muscular  contraction  is  within  the  living 
body  a  comparatively  rare  event  (at  least  as  far  -as  the  skeletal  muscles  are 
concerned),  and  cannot  easily  be  produced  outside  the  body  otherwise  than 
by  a  single  induction-shock. "  The  ordinary  form  of  muscular  contraction  is 
not  a  simple  muscular  contraction,  but  the  more  complex  form  known  as  a 
tetanic  contraction,  to  the  study  of  which  we  must  now  turn. 


THE  PHENOMENA  OF  MUSCLE  AND  NERVE. 


73 


Tetanic  Contraction. 

§  49.  If  a  single  induction-shock  be  followed  at  a  certain  interval  by  a 
second  shock  of  the  same  strength,  the  first  simple  contraction  will  be  fol- 
lowed by  a  second  simple  contraction,  both  contractions  being  separate  and 
distinct ;  and  if  the  shocks  be  repeated  a  series  of  rhythmically  recurring 
separate  simple  contractions  may  be  obtained.  If,  however,  the  interval 
between  two  shocks  be  made  short,  for  instance,  only  just  long  enough  to 
allow  the  first  contraction  to  have  passed  its  maximum  before  the  latent 
period  of  the  second  is  over,  the  curves  of  the  two  contractions  will  bear 
some  such  relation  to  each  other  as  that  shown  in  Fig.  23.  It  will  be 


FIG.  23. 


Tracing  of  a  Double  Muscle-curve.  While  the  muscle  (gastrocnemius  of  frog)  was  engaged  in 
the  first  contraction  (whose  complete  course,  had  nothing  intervened,  is  indicated  by  the  dotted 
line),  a  second  induction-shock  was  thrown  in,  at  such  a  time  that  the  second  contraction  began 
just  as  the  first  was  beginning  to  decline.  The  second  curve  is  seen  to  start  from  the  first,  as 
does  the  first  from  the  base-line. 

observed  that  the  second  curve  is  almost  in  all  respects  like  the  first  except 
that  it  starts,  so  to  speak,  from  the  first  curve  instead  of  from  the  base-line. 

The  second  nervous  impulse  has  acted  on  the  already  contracted  muscle, 
and  made  it  contract  again  just  as  it  would  have  done  if  there  had  been  no 
first  impulse  and  the  muscle  had  been  at  rest.  The  two  contractions  are 
added  together  and  the  lever  is  raised  nearly  double  the  height  it  would 
have  been  by  either  alone.  If  in  the  same  way  a  third  shock  follows  the 
second  at  a  sufficiently  short  interval,  a  third  curve  is  piled  on  top  of  the 
second  ;  the  same  with  a  fourth,  and  so  on.  A  more  or  less  similar  result 
would  occur  if  the  second  contraction  began  at  another  phase  of  the  first. 
The  combined  effect  is,  of  course,  greatest  when  the  second  contraction  begins 
at  the  maximum  of  the  first,  being  less  both  before  and  afterward. 

Hence,  the  result  of  a  repetition  of  shocks  will  depend  largely  on  the 
rate  of  repetition.  If,  as  in  Fig.  24,  the  shocks  follow  each  other  so  slowly 

FIG.  24. 


Muscle-curve.    Single  Induction-shock  Repeated  Slowly. 

that  one  contraction  is  over,  or  almost  over,  before  the  next  begins,  each 

t contraction  will  be  distinct,  or  nearly  distinct,  and  there  will  be  little  or  no 
combined  effect. 
If,  however,  the  shocks  be  repeated  more  rapidly,  as  in  Fig.  25,  each  sue- 


74  THE  CONTRACTILE  TISSUES. 

ceeding  contraction  will  start  from  some  part  of  the  preceding  one,  and  the 
lever  will  be  raised  to  a  greater  height  at  each  contraction. 


FIG.  25. 


Muscle-curve.    Single  Induction-shock  Repeated  More  Rapidly. 

If  the  frequency  of  the  shocks  be  still  further  increased,  as  in  Fig.  26, 
the  rise  due  to  the  combination  of  contraction  will  be  still  more  rapid,  and 
a  smaller  part  of  each  contraction  will  be  visible  on  the  curve. 

In  each  of  these  three  curves  it  will  be  noticed  that  the  character  of  the 
curve  changes  somewhat  during  its  development.  The  change  is  the  result 

FIG.  26. 


Muscle-curve.    Single  Induction-shock  Still  More  Rapidly. 

of  commencing  fatigue,  caused  by  the  repetition  of  the  contractions,  the 
fatigue  manifesting  itself  by  an  increasing  prolongation  of  each  contrac- 
tion, shown  especially  in  a  delay  of  relaxation,  and  by  an  increasing  dimi- 
nution in  the  height  of  the  contraction.  Thus,  in  Fig.  24,  the  contractions, 
quite  distinct  at  first,  become  fused  later ;  the  fifth  contraction,  for  instance, 
is  prolonged  so  that  the  sixth  begins  before  the  lever  has  reached  the  base 
line ;  yet  the  summit  of  the  sixth  is  hardly  higher  than  the  summit  of  the 
fifth,  since  the  sixth,  though  starting  at  a  higher  level,  is  a  somewhat 
weaker  contraction.  See  also,  in  Fig.  25,  the  lever  rises  rapidly  at  first  but 
more  slowly  afterward,  owing  to  an  increasing  diminution  in  the  height  of 
the  single  contractions.  In  Fig.  26  the  increment  of  rise  of  the  curve  due 
to  each  contraction  diminishes  very  rapidly,  and  though  the  lever  does  con- 
tinue to  rise  during  the  whole  series,  the  ascent  after  about  the  sixth  con- 
traction is  very  gradual  indeed,  and  the  indications  of  the  individual  con- 
tractions are  much  less  marked  than  at  first. 

Hence,  when  shocks  are  repeated  with  sufficient  rapidity,  it  results  that 
after  a  certain  number  of  shocks,  the  succeeding  impulses  do  not  cause  any 
further  shortening  of  the  muscle,  any  further  raising  of  the  lever,  but 
merely  keep  up  the  contraction  already  existing.  The  curve  thus  reaches 
a  maximum,  which  it  maintains,  subject  to  the  depressing  effects  of  exhaus- 
tion, so  long  as  the  shocks  are  repeated.  When  these  cease  to  be  given,  the 
muscle  returns  to  its  natural  length. 


THE  PHENOMENA  OF  MUSCLE  AND  NERVE.  75 

When  the  shocks  succeed  each  other  still  more  rapidly  than  in  Fig.  26, 
the  individual  contractions,  visible  at  first,  may  become  fused  together  and 
wholly  lost  to  view  in  the  latter  part  of  the  curve.  When  the  shocks  succeed 
each  other  still  more  rapidly  (the  second  contraction  beginning  in  the 
ascending  portion  of  the  first)  it  becomes  difficult  or  impossible  to  trace 
out  any  of  the  single  contractions.1  The  curve  then  described  by  the  lever 
is  of  the  kind  shown  in  Fig.  27,  where  the  primary  current  of  an  induction- 


Tetanus  Produced  with  the  Ordinary  Magnetic  Interrupter  of  an  Induction-machine.  (Recording 
surface  travelling  slowly.)    The  interrupted  current  is  thrown  in  at  a. 

machine  was  rapidly  made  and  broken  by  the  magnetic  interruptor,  Fig.  15. 
The  lever,  it  will  be  observed,  rises  at  a  (the  recording  surface  is  travelling 
too  slowly  to  allow  the  latent  period  to  be  distinguished),  at  first  very 
rapidly,  in  fact  in  an  unbroken  and  almost  a  vertical  line,  and  so  very 
speedily  reaches  the  maximum,  which  is  maintained  so  long  as  the  shocks 
continue  to  be  given  ;  when  these  cease  to  be  given,  the  curve  descends  at 
first  very  rapidly  and  then  more  and  more  gradually  toward  the  base-line, 
which  it  reaches  just  at  the  end  of  the  figure. 

This  condition  of  muscle,  brought  about  by  rapidly  repeated  shocks,  this 
fusion  of  a  number  of  simple  twitches  into  an  apparently  smooth  continuous 
effort,  is  known  as  tetanus  or  tetanic  contraction.  The  above  facts  are  most 
clearly  shown  when  induction-shocks,  or  at  least  galvanic  currents  in  some 
form  or  other,  are  employed.  They  are  seen,  however,  whatever  be  the  form 
of  stimulus  employed.  Thus,  in  the  case  of  mechanical  stimuli,  while  a 
single  quick  blow  may  cause  a  single  twitch,  a  pronounced  tetanus  may  be 
obtained  by  rapidly  striking  successively  fresh  portions  of  a  nerve.  With 
chemical  stimulation,  as  when  a  nerve  is  dipped  in  acid,  it  is  impossible  to 
secure  a  momentary  application  ;  hence  tetanus,  generally  irregular  in  cha- 
racter, is  the  normal  result  of  this  mode  of  stimulation.  In  the  living  body, 
the  contractions  of  the  skeletal  muscles,  brought  about  either  by  the  will  or 
otherwise,  are  generally  tetanic  in  character.  Even  very  short  sharp  move- 
ments, such  as  a  sudden  jerk  of  a  limb  or  a  wink  of  the  eyelid,  are  in  reality 
examples  of  tetanus  of  short  duration. 

If  the  lever,  instead  of  being  fastened  to  the  tendon  of  a  muscle  hung 
vertically,  be  laid  across  the  belly  of  a  muscle  placed  in  a  horizontal  position 
and  the  muscle  be  thrown  into  tetanus  by  a  repetition  of  induction-shocks,  it 
will  be  seen  that  each  shortening  of  the  muscle  is  accompanied  by  a  corre- 
sponding thickening,  and  that  the  total  shortening  of  the  tetanus  is  accom- 
panied by  a  corresponding  total  thickening.  And,  indeed,  in  tetanus  we  can 
observe  more  easily  than  in  a  single  contraction  that  the  muscle  in  contract- 

1  The  ease  with  which  the  individual  contractions  can  be  made  out  depends  in  part,  it 
need  hardly  be  said,  on  the  rapidity  with  which  the  recording  surface  travels. 


76  THE  CONTRACTILE  TISSUES. 

ing  changes  in  form  only — not  in  bulk.  If  a  living  muscle  or  group  of 
muscles  be  placed  in  a  glass  jar  or  chamber,  the  closed  top  of  which  is  pro- 
longed into  a  narrow  glass  tube,  and  the  chamber  be  filled  with  water  (or 
preferably  with  a  solution  of  sodium  chloride,  0.6  per  cent,  in  strength, 
usually  called  "  normal  saline  solution,"  which  is  less  injurious  to  the  tissue 
than  simple  water)  until  the  water  rises  into  the  narrow  tube,  it  is  obvious 
that  any  change  in  the  bulk  of  the  muscle  will  be  easily  shown  by  a  rising 
or  falling  of  the  column  of  fluid  in  the  narrow  tube.  It  is  found  that  when 
the  muscle  is  made  to  contract,  even  in  the  most  forcible  manner,  the  change 
of  level  in  the  height  of  the  column  which  can  be  observed  is  practically 
insignificant ;  there  appears  to  be  a  fall  indicating  a  diminution  of  bulk  to 
the  extent  of  about  one  ten-thousandth  of  the  total  bulk  of  the  muscle.  So  that 
we  may  fairly  say  that  in  a  tetanus,  and  hence  in  a  simple  "contraction,  the 
lessening  of  the  length  of  the  muscle  causes  a  corresponding  increase  in  the 
other  directions ;  the  substance  of  the  muscle  is  displaced,  not  diminished. 

§  50.  So  far  we  have  spoken  simply  of  an  induction-shock  or  of  induction- 
shocks  without  any  reference  to  their  strength,  and  of  a  living  or  irritable 
muscle  without  any  reference  to  the  degree  or  extent  of  its  irritability.  But 
induction-shocks  may  vary  in  strength,  and  the  irritability  of  the  muscle 
may  vary. 

If  we  slide  the  secondary  coil  a  long  way  from  the  primary  coil,  and  thus 
make  use  of  extremely  feeble  induction-shocks,  we  shall  probably  find  that 
these  shocks,  applied  even  to  a  quite  fresh  muscle-nerve  preparation,  produce 
no  contraction.  If  we  then  gradually  slide  the  secondary  coil  nearer  and 
nearer  the  primary  coil,  and  keep  on  trying  the  effects  of  the  shocks,  we  shall 
find  that  after  a  while,  in  a  certain  position  of  the  coils,  a  very  feeble  con- 
traction makes  its  appearance.  As  the  secondary  coil  comes  still  nearer  to 
the  primary  coil,  the  contractions  grow  greater  and  greater.  After  a  while, 
however — and  that,  indeed,  in  ordinary  circumstances,  very  speedily  increas- 
ing— the  strength  of  the  shock  no  longer  increases  the  height  of  the  contrac- 
tion ;  the  maximum  contraction  of  which  the  muscle  is  capable  with  such 
shocks,  however  strong,  has  been  reached. 

If  we  use  a  tetanizing  or  interrupted  current,  we  shall  obtain  the  same 
general  results ;  we  may,  according  to  the  strength  of  the  current,  get  no 
contraction  at  all,  or  contractions  of  various  extent  up  to  a  maximum,  which 
cannot  be  exceeded.  Under  favorable  conditions  the  maximum  contraction 
may  be  very  considerable  ;  the  shortening  in  tetanus  may  amount  to  three- 
fifths  of  the  total  length  of  the  muscle. 

The  amount  of  contraction,  then,  depends  on  the  strength  of  the  stimulus, 
whatever  be  the  stimulus ;  but  this  holds  good  within  certain  limits  only ; 
to  this  point,  however,  we  shall  return  later  on. 

§  51.  If,  having  ascertained  in  a  perfectly  fresh  muscle-nerve  prepara- 
tion the  amount  of  contraction  produced  by  this  and  that  strength  of  stim- 
ulus, we  leave  the  preparation  by  itself  for  some  time — say  for  a  few  hours 
— and  then  repeat  the  observations,  we  shall  find  that  stronger  stimuli- 
stronger  shocks,  for  instance — are  required  to  produce  the  same  amount  of 
contraction  as  before;  that  is  to  say.  the  irritability  of  the  preparation,  the 
power  to  respond  to  stimuli,  has  in  the  meanwhile  diminished.  After  a 
further  interval  we  should  find  the  irritability  still  further  diminished  ;  even 
very  strong  shocks  would  be  unable  to  evoke  contractions  as  large  as  those 
previously  caused  by  weak  shocks.  At  last  we  should  find  that  no  shocks, 
no  stimuli,  however  strong,  were  able  to  produce  any  visible  contraction  what- 
ever. The  amount  of  contraction,  in  fact,  evoked  by  a  stimulus  depends  not 
only  on  the  strength  of  the  stimulus,  but  also  on  the  degree  of  irritability  of 
the  muscle-nerve  preparation. 


CHANGES  IN  A  MUSCLE  DURING  CONTRACTION.  77 

Immediately  upon  removal  from  the  body,  the  preparation  possesses  a 
certain  amount  of  irritability,  not  differing  very  materially  from  that  which 
the  muscle  and  nerve  possess  while  within  and  forming  an  integral  part  of 
the  body  ;  but  after  removal  from  the  body  the  preparation  loses  irritability, 
the  rate  of  loss  being  dependent  on  a  variety  of  circumstances  ;  and  this  goes 
on  until,  since  no  stimulus  which  we  can  apply  will  give  rise  to  a  contrac- 
tion, we  say  the  irritability  has  wholly  disappeared. 

We  might  take  this  disappearance  of  irritability  as  marking  the  death  of 
the  preparation,  but  it  is  followed  sooner  or  later  by  a  curious  change  in  the 
muscle,  which  is  called  rigor  mortis,  and  which  we  shall  study  presently  ;  and 
it  is  convenient  to  regard  this  rigor  mortis  as  marking  the  death  of  the  muscle. 

The  irritable  muscle,  then,  when  stimulated  either  directly,  the  stimulus 
being  applied  to  itself,  or  indirectly,  the  stimulus  being  applied  to  its  nerve, 
responds  to  the  stimulus  by  a  change  of  form  which  is  essentially  a  shorten- 
ing and  thickening.  By  the  shortening  (and  thickening)  the  muscle  in  con- 
tracting is  able  to  do  work,  to  move  the  parts  to  which  it  is  attached  ;  it  thus 
sets  free  energy.  We  have  now  to  study  more  in  detail  how  this  energy  is 
set  free,  and  the  laws  which  regulate  its  expenditure. 

ON  THE  CHANGES  WHICH  TAKE  PLACE  IN  A  MUSCLE  DURING  A 

CONTRACTION. 

The  Change  in  Form. 

§  52.  An  ordinary  skeletal  muscle  consists  of  elementary  muscle  fibres, 
bound  together  in  variously  arranged  bundles  by  connective  tissue  which 
carries  bloodvessels,  nerves,  and  lymphatics. 

The  contraction  of  a  muscle  is  the  contraction  of  all  or  some  of  its  ele- 
mentary fibres,  the  connective  tissue  being  passive ;  hence  while  those  fibres 
of  muscle  which  end  directly  in  the  tendon,  in  contracting  pull  directly  on 
the  tendon,  those  which  do  not  so  end  pull  indirectly  on  the  tendon  by 
means  of  the  connective  tissue  between  the  bundles,  which  connective 
tissue  is  continuous  with  the  tendon. 

Each  muscle  is  supplied  by  one  or  more  branches  of  nerves  composed  of 
medullated  fibres,  with  a  certain  proportion  of  non-medullated  fibres.  These 
branches  running  in  the  connective  tissue  divide  into  smaller  branches  and 
t\vigs  between  the  bundles  and  fibres.  Some  of  the  nerve  fibres  are  dis- 
tributed to  the  bloodvessels,  and  others  end  in  a  manner  of  which  we 
shall  speak  later  on  in  treating  of  muscular  sensations ;  but  by  far  the 
greater  part  of  the  medullated  fibres  and  in  the  muscular  fibres,  the 
arrangement  being  such  that  every  muscular  fibre  is  supplied  with  at  least 
one  medullated  nerve  fibre,  which  joins  the  muscular  fibre  somewhere  about 
the  middle  between  its  two  ends  or  sometimes  nearer  one  end,  in  a  special 
nerve  ending,  of  which  we  shall  presently  have  to  speak,  called  an  end- 
plate.  The  nerve  fibres  thus  destined  to  end  in  the  muscular  fibres  divide 
as  they  enter  the  muscle,  so  that  what,  as  it  enters  the  muscle,  is  a  single 
nerve 'fibre,  may,  by  dividing,  end  as  several  nerve  fibres  in  several  muscu- 
lar fibres.  Sometimes  two  nerve  fibres  join  one  muscular  fibre,  but  in  this 
case  the  end-plate  of  each  nerve  fibre  is  still  at  some  distance  from  the  end 
of  the  muscular  fibre.  It  follows  that  when  a  muscular  fibre  is  stimulated 
by  means  of  a  nerve  fibre,  the  nervous  impulse  travelling  down  the  nerve 
fibre  falls  into  the  muscular  fibre  not  at  one  end  but  at  about  its  middle  ;  it  is 
the  middle  of  the  fibre  which  is  affected  first  by  the  nervous  impulse,  and 
the  changes  in  the  muscular  substance  started  in  the  middle  of  the  muscu- 
lar fibre  travel  thence  to  the  two  ends  of  the  fibre.  In  an  ordinary  skele- 


78  THE  CONTRACTILE  TISSUES. 

tal  muscle,  however,  as  we  have  said,  the  fibres  and  bundles  of  fibres  begin 
and  end  at  different  distances  from  the  ends  of  the  muscle,  and  the  nerve  or 
nerves  going  to  the  muscle  divide  and  spread  out  in  the  muscle  in  such  a 
way  that  the  end-plates,  in  which  the  individual  fibres  of  the  nerve  end,  are 
distributed  widely  over  the  muscle  at  very  different  distances  from  the  ends 
of  the  muscle.  Hence,  if  we  suppose  a  single  nervous  impulse,  such  as  that 
generated  by  a  single  induction-shock,  or  a  series  of  such  impulses  to  be 
started  at  the  same  time  at  some  part  of  the  trunk  of  the  nerve  in  each  of 
the  fibres  of  the  nerve  going  to  the  muscle,  these  impulses  will  reach  very 
different  parts  of  the  muscle  at  about  the  same  time  and  the  contractions 
which  they  set  going  will  begin,  so  to  speak,  nearly  all  over  the  whole  mus- 
cle at  the  same  time,  and  will  not  all  start  in  any  particular  zone  or  area  of 
the  muscle. 

§  53.  The  wave  of  contraction.  We  have  seen,  however,  that  under  the 
influence  of  urari  the  nerve  fibre  is  unable  to  excite  contractions  in  a  mus- 
cular fibre,  although  the  irritability  of  the  muscular  fibre  itself  is  retained. 
Hence  in  a  muscle  poisoned  by  urari  the  contraction  begins  at  that  part  of 
the  muscular  substance  which  is  first  affected  by  the  stimulus,  and  we  may 
start  a  contraction  in  what  part  of  the  muscle  we  please  by  properly  placing 
the  electrodes. 

Some  muscles,  such,  for  instance,  as  the  sartorius  of  the  frog,  though  of 
some  length,  are  composed  of  fibres  which  run  parallel  to  each  other  from 
one  end  of  the  muscle  to  the  other.  If  such  a  muscle  be  poisoned  with 
urari  so  as  to  eliminate  the  action  of  the  nerves  and  stimulated  at  one  end 
(an  induction-shock  sent  through  a  pair  of  electrodes  placed  at  some  little 
distance  apart  from  each  other  at  the  end  of  the  muscle  may  be  employed, 
but  better  results  are  obtained  if  a  mode  of  stimulation,  of  which  we  shall 
have  to  speak  presently,  viz.  the  application  of  the  "  constant  current,"  be 
adopted),  the  contraction  which  ensues  starts  from  the  end  stimulated,  and 
travels  thence  along  the  muscle.  If  two  levers  be  made  to  rest  on,  or  be 
suspended  from,  two  parts  of  such  a  muscle  placed  horizontally,  the  parts 
being  at  a  known  distance  from  each  other  and  from  the  part  stimulated, 
the  progress  of  the  contraction  may  be  studied. 

The  movements  of  the  levers  indicate  in  this  case  the  thickening  of  the 
fibres  which  is  taking  place  at  the  parts  on  which  the  levers  rest  or  to  which 
they  are  attached ;  and  if  we  take  a  graphic  record  of  these  movements, 
bringing  the  two  levers  to  mark  one  immediately  below  the  other,  we  shall 
find  that  the  lever  nearer  the  part  stimulated  begins  to  move  earlier,  reaches 
its  maximum  earlier,  and  returns  to  rest  earlier  than  does  the  further  lever. 
The  contraction,  started  by  the  stimulus,  in  travelling  along  the  muscle  from 
the  part  stimulated  reaches  the  nearer  lever  some  little  time  before  it  reaches 
the  further  lever,  and  has  passed  by  the  nearer  lever  some  little  time  before 
it  has  passed  by  the  further  lever ;  and  the  further  apart  the  two  levers  are 
the  greater  will  be  the  difference  in  time  between  their  movements.  In  other 
words,  the  contraction  travels  along  the  muscle  in  the  form  of  a  wave,  each 
part  of  the  muscle  in  succession  from  the  end  stimulated  swelling  out  and 
shortening  as  the  contraction  reaches  it,  and  then  returning  to  its  original 
state.  And  what  is  true  of  the  collection  of  parallel  fibres  which  we  call 
the  muscle  is  also  true  of  each  fibre,  for  the  swelling  at  any  part  of  the 
muscle  is  only  the  sum  of  the  swelling  of  the  individual  fibres ;  and  if 
we  were  able  to  take  a  single  long  fibre  and  stimulate  it  at  one  end,  we 
should  be  able,  under  the  microscope,  to  see  a  swelling  or  bulging  accom- 
panied by  a  corresponding  shortening,  i.  e.,  to  see  a  contraction,  sweep 
along  the  fibre  from  end  to  end. 

If,  in  the  graphic  record  of  the  two  levers  just  mentioned,  we  count  the 


CHANGES  IN   A  MUSCLE  DURING   CONTRACTION.  79 

number  of  vibrations  of  the  tuning-fork  which  intervene  between  the  mark 
on  the  record  which  indicates  the  beginning  of  the  rise  of  the  near  lever 
(that  is,  the  arrival  of  the  contraction  wave  at  this  lever)  and  the  mark 
which  indicates  the  beginning  of  the  rise  of  the  far  lever,  this  will  give  us 
the  time  which  it  has  taken  the  contraction  wave  to  travel  from  the  near  to 
the  far  lever.  Let  us  suppose  this  to  be  0.005  second.  Let  us  suppose  the 
distance  between  the  two  levers  to  be  15  mm.  The  contraction  wave,  then, 
has  taken  0.005  second  to  travel  15  mm.,  that  is  to  say  it  has  travelled  at  the 
rate  of  3  metres  per  second.  And,  indeed,  we  find  by  this,  or  by  other 
methods,  that  in  the  frog's  muscles  the  contraction  wave  does  travel  at  a 
rate  which  may  be  put  down  as  from  3  to  4  metres  a  second,  though  it 
varies  under  different  conditions.  In  the  warm-blooded  mammal  the  rate 
is  somewhat  greater,  and  may  probably  be  put  down  at  5  metres  a  second 
in  the  excised  muscle,  rising  possibly  to  10  metres  in  a  muscle  within  the 
living  body. 

If,  again,  in  the  graphic  record  of  the  two  levers  we  count,  in  the  case  of 
either  lever,  the  number  of  vibrations  of  the  tuning-fork  which  intervene 
between  the  mark  where  the  lever  begins  to  rise  and  the  mark  where  it  has 
finished  its  fall  and  returned  to  the  base-line,  we  can  measure  the  time  inter- 
vening between  the  contraction  wave  reaching  the  lever  and  leaving  the  lever 
on  its  way  on  ward,  that  is  to  say  we  can  measure  the  time  which  it  has  taken 
the  contraction  wave  to  pass  over  the  part  of  the  muscle  on  which  the  lever 
is  resting.  Let  us  suppose  this  time  to  be,  say,  0.1  second.  But  a  wave  which 
is  travelling  at  the  rate  of  3  metres  a  second,  and  takes  0.1  second  to  pass 
over  any  point  must  be  300  mm.  long.  And,  indeed,  we  find  that  in  the 
frog  the  length  of  the  contraction  wave  may  be  put  down  as  varying  from 
200  to  400  mm.,  and  in  the  mammal  it  is  not  very  different. 

Now,  as  we  have  said,  the  very  longest  muscular  fibre  is  stated  to  be  at 
most  only  about  40  mm.  in  length  ;  hence,  in  an  ordinary  contraction,  dur- 
ing the  greater  part  of  the  duration  of  the  contraction  the  whole  length  of 
the  fibre  will  be  occupied  by  the  contraction  wave.  Just  at  the  beginning 
of  the  contraction  there  will  be  a  time  when  the  front  of  the  contraction 
wave  has  reached,  for  instance,  only  half  way  down  the  fibre  (supposing  the 
stimulus  to  be  applied,  as  in  the  case  we  have  been  discussing,  at  one  end 
only),  and  just  at  the  end  of  contraction  there  will  be  a  time,  for  instance, 
when  the  contraction  has  left  the  half  of  the  fibre  next  to  the  stimulus,  but 
has  not  yet  cleared  away  from  the  other  half.  But  nearly  all  the  rest  of  the 
time  every  part  of  the  fibre  will  be  in  some  phase  or  other  of  contraction, 
though  the  parts  nearer  the  stimulus  will  be  in  more  advanced  phases  than 
the  parts  further  from  the  stimulus. 

This  is  true  when  a  muscle  of  parallel  fibres  is  stimulated  artificially  at 
one  end  of  the  muscles,  and  when,  therefore,  each  fibre  is  stimulated  at  one 
end.  It  is,  of  course,  all  the  more  true  when  a  muscle  of  ordinary  construc- 
tion is  stimulated  by  means  of  its  nerve.  The  stimulus  of  the  nervous 
impulse  impinges  in  this  case  on  the  muscle  fibre  at  the  end-plate  which, 
as  we  have  said,  is  placed  toward  the  middle  of  the  fibre,  and  the  contrac- 
tion wave  travels  from  the  end-plate  in  opposite  directions  toward  each  end, 
and  has  accordingly  only  about  half  the  length  of  the  fibre  to  run  in.  All 
the  more,  therefore,  must  the  whole  fibre  be  in  a  state  of  contraction  at  the 
same  time. 

It  will  be  observed  that  the  contraction  wave  includes  not  only  the  con- 
traction proper  and  the  thickening  and  shortening,  but  the  relaxation  and 
return  to  the  natural  form  ;  the  first  part  of  the  wave  up  to  the  summit  of 
the  crest  corresponds  to  the  shortening  and  thickening ;  the  decline  from 
the  summit  onward  corresponds  to  the  relaxation.  But  we  have  already 


80  THE  CONTRACTILE  TISSUES. 

insisted  that  the  relaxation  is  an  essential  part  of  the  whole  act ;  indeed, 
in  a  certain  sense,  as  essential  as  the  shortening  itself. 

§  54.  Optical  changes  in  a  muscular  fibre  during  contraction.  So  far  we 
have  been  dealing  with  the  muscle  as  a  whole  and  as  observed  with  the 
naked  eye,  though  we  have  incidentally  spoken  of  fibres.  We  have  now, 
confining  our  attention  exclusively  to  skeletal  muscles,  to  consider  what  mi- 
croscopic changes  take  place  during  a  contraction,  what  are  the  relations  of 
the  histological  features  of  the  muscle  fibre  to  the  act  of  contraction.  Un- 
fortunately, our  knowledge  of  the  minute  structure  of  the  fibre  is  as  yet  so 
limited  that  any  statements  must  of  necessity  be  but  speculative.  When 
muscle  contracts  there  is  a  translocatiou  of  molecules  whereby  there  occurs 
not  only  a  change  of  form,  but  other  optical  (polariscopical  and  microscopi- 
cal) alterations  which  are  due  to  the  movement  of  refractive  particles. 

The  long  cylindrical  sheath  of  sarcolemma  is  occupied  by  muscle  sub- 
stance. After  death  the  muscle  substance  may  separate  from  the  sarco- 
lemma, leaving  the  latter  as  a  distinct  sheath,  but  during  life  the  muscle 
substance  is  adherent  to  the  sarcolemma,  so  that  no  line  of  separation  between 
the  two  can  be  made  out ;  the  movements  of  the  one  follow  exactly  all  the 
movements  of  the  other. 

Scattered  in  the  muscle  substance,  but,  in  the  mammal,  lying  for  the  most 
part  close  under  the  sarcolemma,  are  a  number  of  nuclei,  oval  in  shape,  with 
their  long  axes  parallel  to  the  length  of  the  fibre.  Around  each  nucleus  is 
a  thin  layer  of  granular-looking  substance  very  similar  in  appearance  to 
that  forming  the  body  of  a  white  blood-corpuscle,  and  like  that  often  spoken 
of  as  undifferentiated  protoplasm.  A  small  quantity  of  the  same  granular 
substance  is  prolonged  for  some  distance,  as  a  narrow  conical  streak  from 
each  end  of  the  nucleus,  along  the  length  of  the  fibre. 

With  the  exception  of  these  nuclei  with  their  granular-looking  bed  and 
the  end-plate  or  end-plates,  to  be  presently  described,  all  the  rest  of  the 
space  enclosed  by  the  sarcolemma  from  one  end  of  the  fibre  to  the  other 
appears  to  be  occupied  by  a  peculiar  material,  striated  muscle  substance. 

It  is  called  striated  because  it  is  marked  out,  and  that  along  the  whole 
length  of  the  fibre,  by  transverse  bands  [Fig.  28],  stretching  right  across 
the  fibre,  of  substance  which  is  very  transparent,  bright 
[FIG.  28.  substance,  alternating  with  similar  bands  of  substance 

which  has  a  dim  cloudy  appearance,  dim  substance ; 
that  is  to  say,  the  fibre  is  marked  out  along  its  whole 
length  by  alternate  bright  bands  and  dim  bands.  The 

^ t  bright  bands  are  on  an  average  about  1  p.  or  1.5  >j.  and 

Diagrammatic  Repre-  the  dim  bands  about  2.5  //  or  3  /JL  thick.  By  careful 
sentation  of  a  Muscle-  focusing,  both  bright  bands  and  dim  bands  may  be 
^Tri-ht'bSsr^85  traced  through  the  whole  thickness  of  the  fibre,  so' that 
the  whole  fibre  appears  to  be  composed  of  bright  discs 
and  dim  discs  placed  alternately  one  upon  the  other  along  the  whole  length 
of  the  fibre,  the  arrangement  being  broken  by  the  end-plate  and  here  and 
there  by  the  nuclei. 

§  55.  We  may  now  return  to  the  question,  What  happens  when  a  con- 
traction wave  sweeps  over  the  fibre  ? 

Muscular  fibres  may  be  examined  even  under  high  powers  of  the  micro- 
scope while  they  are  yet  living  and  contractile  ;  the  contraction  itself  may 
be  seen,  but  the  rate  at  which  the  wave  travels  is  too  rapid  to  permit  satis- 
factory observations  to  be  made  as  to  the  minute  changes  which  accompany 
the  contraction.  It  frequently  happens  however  that  when  living  muscle 
has  been  treated  with  certain  reagents,  as  for  instance  with  osmic  acid  vapor, 
and  subsequently  prepared  for  examination,  fibres  are  found  in  which  a 


CHANGES  IN  A   MUSCLE  DURING   CONTRACTION.  81 

bulging,  a  thickening  and  shortening,  over  a  greater  or  less  part  of  the 
length  of  the  fibre,  has  been  fixed  by  the  osmic  acid  or  other  reagent.  Such 
a  bulging  obviously  differs  from  a  normal  contraction  in  being  confined  to  a 
part  of  the  length  of  the  fibre,  whereas,  as  we  have  said,  a  normal  wave  of 
contraction,  being  very  much  longer  than  any  fibre,  occupies  the  whole 
length  of  the  fibre  at  once.  We  may  however  regard  this  bulging  as  a  very 
short,  a  very  abbreviated  wave  of  contraction,  and  assume  that  the  changes 
visible  in  such  a  short  bulging  also  take  place  in  a  normal  contraction. 

Admitting  this  assumption,  we  learn  from  such  preparations  that  in  the 
contracting  region  of  the  fibre,  while  both  dim  and  bright  bands  become 
broader  across  the  fibre,  and  correspondingly  thinner  along  the  length  of  the 
fibre,  a  remarkable  change  takes  place  between  the  dim  bands,  bright  bands, 
and  granular  lines.  We  have  seen  that  in  the  fibre  at  rest  the  intermediate 
line  in  the  bright  band  is  in  most  cases  inconspicuous ;  in  the  contracting 
fibre,  on  the  contrary,  a  dark  line  in  the  middle  of  the  bright  band  in  the 
position  of  the  intermediate  line  becomes  very  distinct.  As  we  pass  along 
the  fibre  from  the  beginning  of  the  contraction  wave  to  the  summit  of  the 
wave,  where  the  thickening  is  greatest,  this  line  becomes  more  and  more 
striking,  until  at  the  height  of  the  contraction  it  becomes  a  very  marked 
dark  line  or  thin  dark  band.  Pari  passu  with  this  change,  the  distinction 
between  the  dim  and  the  bright  bands  become  less  and  less  marked  ;  these 
appear  to  become  confused  together,  until  at  the  height  of  the  contraction, 
the  whole  space  between  each  two  now  conspicuous  dark  lines  is  occupied  by 
a  substance  which  can  be  called  neither  dim  nor  bright,  but  which  in  con- 
trast to  the  dark  line  appears  more  or  less  bright  and  transparent.  So  that 
in  the  contracting  part  there  is,  at  the  height  of  the  contraction,  a  reversal 
of  the  state  of  things  proper  to  the  part  at  rest.  The  place  occupied  by  the 
bright  band,  in  the  state  of  rest,  is  now  largely  filled  by  a  conspicuous 'dark 
line  which  previously  was  represented  by  the  inconspicuous  intermediate 
line,  and  the  place  occupied  by  the  conspicuous  dim  band  of  the  fibre  at  rest 
now  seems  by  comparison  with  the  dark  line  the  brighter  part  of  the  fibre. 
The  contracting  fibre  is,  like  the  fibre  at  rest,  striated,  but  its  striation  is  dif- 
ferent in  its  nature  from  the  natural  striation  of  the  resting  fibre ;  and  it  is 
held  by  some  that  in  the  earlier  phases  of  the  contraction,  while  the  old  nat- 
ural striation  is  being  replaced  by  the  new1  striation,  there  is  a  stage  in  which 
all  striation  is  lost. 

We  may  add  that  the  outline  of  the  sarcolemma,  which  in  the  fibre  at 
rest  is  quite  even,  becomes  during  the  contraction  indented  opposite  the 
intermediate  line,  and  bulges  out  in  the  interval  between  each  two  interme- 
diate lines,  the  bulging  and  indentation  becoming  more  marked  the  greater 
the  contraction. 

§  56.  We  can  learn  something  further  about  this  remarkable  change  by 
examining  the  fibre  under  polarized  light. 

When  ordinary  light  is  sent  through  a  Nicol  prism  (which  is  a  rhomb  of  Ice- 
land spar  divided  into  two  in  a  certain  direction,  the  halves  being  subsequently 
cemented  together  in  a  special  way)  it  undergoes  a  change  in  passing  through  the 
prism  arid  is  said  to  be  polarized.  One  effect  of  this  polarization  is  that  a  ray 
of  light  which  has  passed  through  one  Nicol  prism  will  or  will  not  pass  through  a 
second  Nicol  according  to  the  relative  position  of  the  two  prisms.  Thus,  if  the 
second  Nicol  be  so  placed  that  what  is  called  its  "optic  axis  "  be  in  a  line  with  or 
parallel  to  the  optic  axis  of  the  first  Nicol  the  light  passing  through  the  first  Nicol 
will  also^pass  through  the  second.  But  if  the  second  Nicol  be  rotated  until  its 
optic  axis  is  at  right  angles  with  the  optic  axis  of  the  first  Nicol  none  of  the  light- 
passing  through  the  former  will  pass  through  the  latter;  the  prisms  in  this  position 
are  said  to  be  "crossed."  In  intermediate  positions  more  or  less  light  passes 
through  the  second  Nicol  according  to  the  angle  between  the  two  optic  axes. 

6 


82  THE  CONTRACTILE  TISSUES. 

Hence  when  one  Nicol  is  placed  beneath  the  stage  of  a  microscope  so  that  the 
light  from  the  mirror  is  sent  through  it,  and  another  Nicol  is  placed  in  the  eye- 
piece, the  field  of  the  microscope  will  appear  dark  when  the  eye-piece  Nicol  is 
rotated  so  that  its  optic  axis  is  at  right  angles  to  the  optic  axis  of  the  lower  Nicol, 
and  consequently  the  light  passing  through  the  lower  Nicol  is  stopped  by  it.  If, 
however,  the  optic  axis  of  the  eye-piece  Nicol  is  parallel  to  that  of  the  lower  Nicol, 
the  light  from  the  latter  will  pass  through  the  former  and  the  field  will  be  bright ; 
and  as  the  eye-piece  is  gradually  rotated  from  one  position  to  the  other  the  bright- 
ness of  the  field  will  diminish  or  increase. 

Both  the  Nicols  are  composed  of  doubly  refractive  material.  If  now  a  third 
doubly  refractive  material  be  placed  on  the  stage,  and  therefore  between  the  two 
Nicols,  the  light  passing  through  the  lower  Nicol  will  (in  a  certain  position  of  the 
doubly  refractive  material  on  the  stage,  that  is  to  say,  when  its  optic  axes  have  a 
certain  position)  pass  through  it.  and  also  through  the  crossed  Nicol  in  the  eye- 
piece. Hence  the  doubly  refractive  material  on  the  stage  (or  such  parts  of  it  as 
are  in  the  proper  position  in  respect  to  their  optic  axes)  will,  when  the  eye-piece 
Nicol  is  crossed,  appear  illuminated  and  bright  on  a  dark  field.  In  this  way  the 
existence  of  doubly  refractive  material  in  a  preparation  may  be  detected. 

When  muscle  prepared  arid  mounted  in  Canada  balsam  is  examined  in 
the  microscope  between  Nicol  prisms,  one  on  the  stage  below  the  object,  and 
the  other  in  the  eye-piece,  the  fibres  stand  out  as  bright  objects  on  the  dark 
ground  of  the  field  when  the  axes  of  the  prisms  are  crossed.  On  closer 
examination  it  is  seen  that  the  parts  which  are  bright  are  chiefly  the  dim 
bands.  This  indicates  that  it  is  the  dim  bands  which  are  doubly  refractive, 
anisotropic,  or  are  chiefly  made  up  of  anisotropic  substance;  there  seems, 
however,  to  be  some  slight  amount  of  anisotropic  substance  in  the  bright 
bands,  though  these  as  a  whole  appear  singly  refractive  or  isotropic.  The 
fibre  accordingly  appears  banded  or  striated  with  alternate  bands  of  aniso- 
tropic and  isotropic  material.  According  to  most  authors  such  an  .alterna- 
tion of  anisotropic  and  (chiefly)  isotropic  bands  which  is  obvious  in  a  dead 
and  prepared  fibre.exists  also  in  the  living  fibre  ;  but  some  maintain  that  the 
living  fibre  is  uniformly  anisotropic. 

Now,  when  a  fibre  contracts,  in  spite  of  the  confusion  previously  men- 
tioned between  dim  and  bright  bands,  there  is  no  confusion  between  the 
anisotropic  and  isotropic  material.  The  anisotropic,  doubly  refractive  bands, 
bright  under  crossed  Nicols,  occupying  the  position  of  the  dim  bands  in  the 
resting  fibre,  remain  doubly  refractive,  bright  under  crossed  Nicols,  even  at 
the  very  height  of  the  contraction.  The  isotropic,  singly  refractive  bands, 
dark  under  crossed  Nicols,  occupying  the  position  of  the  bright  bands  in  the 
fibre  at  rest,  remain  isotropic  and  dark  under  crossed  Nicols  at  the  very 
height  of  the  contraction.  All  that  can  be  seen  is  that  the  singly  refractive 
isotropic  bands  become  very  thin  indeed  during  the  contraction,  while  the 
anistropic  bands,  though  of  course  becoming  thinner  and  broader  in  the 
contraction,  do  not  become  so  thin  as  do  the  isotropic  bands  ;  in  other  words, 
while  both  bands  become  thinner  and  broader,  the  doubly  refractive  aniso- 
tropic band  seems  to  increase  at  the  expense  of  the  singly  refractive  isotropic 
band. 

§  57.  The  mere  broadening  and  shortening  of  each  section  of  the  fibre 
is  at  bottom  a  translocation  of  the  molecules  of  the  muscle  substance.  If 
we  imagine  a  company  of  100  soldiers,  ten  ranks  deep,  with  ten  men  in  each 
rank,  rapidly,  but  by  a  series  of  gradations,  to  extend  out  into  a  double  line 
with  50  men  in  each  line,  we  shall  have  a  rough  image  of  the  movement  of 
the  molecules  during  a  muscular  contraction.  But,  from  what  has  been  said, 
it  is  obvious  that  the  movement,  in  striated  muscle  at  least,  is  a  very  com- 
plicated one ;  in  other  forms  of  contractile  tissue  it  may  be,  as  we  shall  see, 
more  simple.  Why  the  movement  is  so  complicated  in  striated  muscle,  what 
purposes  it  serves,  why  the  skeletal  muscles  are  striated,  we  do  not  at  present 


CHANGES  IN  A   MUSCLE  DURING   CONTRACTION.  83 

know.  Apparently  where  swift  and  rapid  contraction  is  required  the  con- 
tractile tissue  is  striated  muscle  ;  but  how  the  striation  helps,  so  to  speak,  the 
contraction  we  do  not  know.  We  cannot  say  what  share  in  the  act  of  con- 
traction is  to  be  allotted  to  the  several  parts.  Since,  during  a  contraction, 
the  fibre  bulges  out  more  opposite  to  each  dim  disc,  and  is  indented  opposite 
to  each  bright  disc,  since  the  dim  disc  is  more  largely  composed  of  aniso- 
tropic  material  than  the  rest  of  the  fibre,  and  since  the  anisotropic  material 
in  the  position  of  the  dim  disc  increases  during  a  contraction,  we  might  per- 
haps infer  that  the  dim  disc  rather  than  the  bright  disc  is  the  essentially 
active  part.  Assuming  that  the  fibrillar  substance  is  more  abundant  in  the 
dim  discs,  while  the  intern"  brillar  substance  is  more  abundant  in  the  bright 
discs,  and  that  the  fibrillar  substance  is  anisotropic  (and  hence  the  dim  discs 
largely  anisotropic),  while  the  interfibrillar  substance  is  isotropic,  we  might 
also  be  inclined  to  infer  it  is  the  fibrillar  and  not  the  interfibrillar  substance 
which  really  carries  out  the  contraction ;  but  even  this  much  is  not  yet 
definitely  proved. 

One  thing  must  be  remembered.  The  muscle  substance,  though  it  pos- 
sesses the  complicated  structure,  and  goes  through  the  remarkable  changes 
which  we  have  described,  is  while  it  is  living  and  intact  in  a  condition  which 
we  are  driven  to  speak  of  as  semi-fluid.  The  whole  of  it  is  essentially  mobile. 
The  very  act  of  contraction  indeed  shows  this ;  but  it  is  mobile  in  the  sense 
that  no  part  of  it,  except  of  course  the  nuclei  and  sarcolemma,  neither  dim 
nor  bright  substance,  neither  fibrillar  nor  interfibrillar  substance,  can  be 
regarded  as  a  hard  and  fast  structure.  A  minute  nematoid  worm  has  been 
seen  wandering  in  the  midst  of  the  substance  of  a  living  contractile  fibre ; 
as  it  moved  along,  the  muscle  substance  gave  way  before  it,  and  closed  up 
again  behind  it,  dim  bands  and  bright  bands  all  falling  back  into  their  proper 
places.  We  may  suppose  that  in  this  case  the  worm  threaded  its  way  in  a 
fluid  interfibrillar  substance  between  and  among  highly  extensible  and  elastic 
fibrilke.  But  even  on  such  a  view,  and  still  more  on  the  view  that  the 
fibrillar  substance  also  was  broken  and  closed  up  again,  the  maintenance  of 
such  definite  histological  features  as  those  which  we  have  described  in 
material  so  mobile  can  only  be  effected,  even  in  the  fibre  at  rest,  at  some  con- 
siderable expenditure  of  energy,  which  energy  it  may  be  expected  has  a 
chemical  source.  During  the  contraction  there  is  a  still  further  expenditure 
of  energy,  some  of  which,  as  we  have  seen,  may  leave  the  muscle  as  "  work 
done  ; "  this  energy,  likewise,  may  be  expected  to  have  a  chemical  source. 
We  must,  therefore,  now  turn  to  the  chemistry  of  muscle. 

The  Chemistry  of  Muscle. 

§  58.  We  said  in  the  Introduction  that  it  was  difficult  to  make  out  with 
certainty  the  exact  chemical  differences  between  dead  and  living  substance. 
Muscle,  however,  in  dying  undergoes  a  remarkable  chemical  change,  which 
may  be  studied  with  comparative  ease.  All  muscles,  within  a  certain  time 
after  "general  "  death  of  the  body,  lose  their  irritability,  which  is  succeeded 
by  an  event  somewhat  more  sudden,  viz.,  the  entrance  into  the  condition 
known  as  rigor  mortis.  The  occurrence  of  rigor  mortis,  or  cadaveric  rigidity, 
as  it  is  sometimes  called,  which  may  be  considered  as  a  token  of  the  death 
of  the  muscle,  is  marked  by  the  following  features:  The  living  muscle 
possesses  a  certain  translucency,  the  rigid  muscle  is  distinctly  more  opaque. 
The  living  muscle  is  very  extensible  and  elastic,  it  stretches  readily  and  to  a 
considerable  extent  when  a  weight  is  hung  upon  it,  or  when  any  traction  is 
applied  to  it,  but  speedily  and,  under  normal  circumstances,  completely 
returns  to  its  original  length  when  the  weight  of  traction  is  removed  ;  as  we 


84  THE  CONTRACTILE  TISSUES. 

shall  see,  however,  the  rapidity  and  completeness  of  the  return  depends  on 
the  condition  of  the  muscle,  a  well-nourished  active  muscle  regaining  its 
normal  length  much  more  rapidly  and  completely  than  a  tired  and  exhausted 
muscle.  A  dead  rigid  muscle  is  much  less  extensible,  and  at  the  same  time 
much  less  elastic ;  the  muscle  now  requires  considerable  force  to  stretch  it, 
and  when  the  force  is  removed,  does  riot,  as  before,  return  to  its  former 
length.  To  the  touch  the  rigid  muscle  has  lost  much  of  its  former  softness, 
and  has  become  firmer  and  more  resistant.  The  entrance  into  rigor  mortis 
is,  moreover,  accompanied  by  a  shortening  or  contraction,  which  may,  under 
certain  circumstances,  be  considerable.  The  energy  of  this  contraction  is 
not  great,  so  that  any  actual  shortening  is  easily  prevented  by  the  presence 
of  even  a  slight  opposing  force. 

Now  the  chemical  features  of  the  dead  rigid  muscle  are  also  strikingly 
different  from  those  of  the  living  muscle. 

§  59.  If  a  dead  muscle,  from  which  all  fat,  tendon,  fascia,  and  connec- 
tive tissue  have  been  as  much  as  possible  removed,  and  which  has  been 
freed  from  blood  by  the  injection  of  "normal  "  saline  solution,  be  minced 
and  repeatedly  washed  with  water,  the  washings  will  contain  certain  forms 
of  albumin  and  certain  extractive  bodies  of  which  we  shall  speak  directly. 
When  the  washing  has  been  continued  until  the  wash-water  gives  no  proteid 
reaction,  a  large  portion  of  muscle  will  still  remain  undissolved.  If  this- 
be  treated  with  a  10  per  cent,  solution  of  a  neutral  salt,  ammonium 
chloride  being  the  best,  a  large  portion  of  it  will  become  dissolved  ;  the 
solution,  however,  is  more  or  less  imperfect  and  filters  with  difficulty.  If 
the  filtrate  be  allowed  to  fall  drop  by  drop  into  a  large  quantity  of  dis- 
tilled water,  a  white  flocculent  matter  will  be  precipitated.  This  flocculent 
precipitate  is  myosin.  Myosin  is  a  proteid,  giving  the  ordinary  proteid 
reactions,  and  having  the  same  general  elementary  composition  as  other 
proteids.  It  is  soluble  in  dilute  saline  solutions,  especially  those  of  ammo- 
nium chloride,  and  may  be  classed  in  the  globulin  family,  though  it  is 
not  so  soluble  as  paraglobulin,  requiring  a  stronger  solutio"n  of  a  neutral 
salt  to  dissolve  it ;  thus,  while  soluble  in  a  5  or  10  per  cent,  solution  of  such 
a  salt,  it  is  far  less  soluble  in  a  1  per  cent,  solution,  which,  as  we  have  seen, 
readily  dissolves  paraglobulin.  From  its  solutions  in  neutral  saline  solu- 
tion it  is  precipitated  by  saturation  with  a  neutral  salt,  preferably 
sodium  chloride,  and  may  be  purified  by  being  washed  with  a  saturated 
solution,  dissolved  again  in  a  weaker  solution,  and  reprecipitated  by  satu- 
ration. Dissolved  in  saline  solutions  it  readily  coagulates  when  heated — i.  e.y 
is  converted  into  coagulated  proteid — and  it  is  worthy  of  notice  that  it 
coagulates  at  a  comparatively  low  temperature,  viz.,  about  56°  C. ;  this, 
it  will  be  remembered,  is  the  temperature  at  which  fibrinogen  is  coagulated, 
whereas  paraglobulin,  serum-albumin,  and  many  other  proteids,  do  not 
coagulate  until  a  higher  temperature  (75°  C.)  is  reached.  Solutions  of 
myosin  are  precipitated  by  alcohol,  and  the  precipitate,  as  in  the  case  of 
other  proteids,  becomes,  by  continued  action  of  the  alcohol,  altered  into 
coagulated  insoluble  proteid. 

We  have  seen  that  paraglobulin,  and,  indeed,  any  member  of  the 
globulin  group,  is  very  readily  changed  by  the  action  of  dilute  acids  into  a 
body  called  acid-albumin,  characterized  by  not  being  soluble  either  in  water 
or  in  dilute  saline  solutions,  but  readily  soluble  in  dilute  acids  and  alkalies, 
from  its  solutions  in  either  of  which  it  is  precipitated  by  neutralization, 
and  by  the  fact  that  the  solutions  in  dilute  acids  and  alkalies  are  not  coagu- 
lated by  heat.  When,  therefore,  a  globulin  is  dissolved  in  dilute  acid, 
what  takes  place  is  not  a  mere  solution,  but  a  chemical  change  ;  the  globulin 
cannot  be  got  back  from  the  solution,  it  has  been  changed  into  acid-albu- 


CHANGES  IN  A  MUSCLE  DURING  CONTRACTION.  85 

min.  Similarly  when  globulin  is  dissolved  in  dilute  alkalies  it  is  changed 
into  alkali-albumin ;  and,  broadly  speaking,  alkali-albumin  precipitated  by 
neutralization  can  be  changed  by  solution  with  dilute  acids  into  acid-albu- 
min, and  acid-albumin  by  dilute  alkalies  into  alkali-albumin. 

Now  myosin  is  similarly,  and  even  more  readily  than  is  globulin,  con- 
verted into  acid-albumin,  and  by  treating  a  muscle,  either  washed  or  not, 
directly  with  dilute  hydrochloric  acid,  the  myosin  may  be  converted  into 
acid-albumin  and  dissolved  out.  Acid-albumin  obtained  by  dissolving 
muscle  in  dilute  acid  used  to  be  called  syntonin,  and  it  used  to  be  said  that 
a  muscle  contained  syntonin  ;  the  muscle,  however,  contains  myosin,  not 
syntonin,  but  it  may  be  useful  to  retain  the  word  syntonin  to  denote  acid- 
albumin  obtained  by  the  action  of  dilute  acid  on  myosin.  By  the  action  of 
dilute  alkalies,  myosin  may  similarly  be  converted  into  alkali-albumin. 

From  what  has  been  above  stated  it  is  obvious  that  myosin  has  many 
analogies  with  fibrin,  and  we  have  yet  to  mention  some  striking  analogies ; 
it  is,  however,  much  more  soluble  than  fibrin,  and,  speaking  generally,  it 
may  be  said  to  be  intermediate  in  its  character  between  fibrin  and  globulin. 
On  keeping,  and  especially  on  drying,  its  solubility  is  much  diminished. 

Of  the  substances  which  are  left  in  washed  muscle,  from  which  all  the 
myosin  has  been  extracted  by  ammonium  chloride  solution,  little  is 
known.  If  washed  muscle  be  treated  directly  with  dilute  hydrochloric 
acid,  a  large  part  of  the  material  of  the  muscle  passes,  as  we  have  said, 
at  once  into  syntonin.  The  quantity  of  syntonin  thus  obtained  may  be 
taken  as  roughly  representing  the  quantity  of  myosin  previously  existing 
in  the  muscle.  A  more  prolonged  action  of  the  acid  may  dissolve  out  other 
proteids,  besides  myosin,  left  after  the  washing.  The  portion  insoluble  in 
dilute  hydrochloric  acid  consists  in  part  of  the  gelatin-yielding  and  other 
substances  of  the  sarcolemma  and  of  the  connective  and  other  tissues 
between  the  bundles,  of  the  nuclei  of  these  tissues  and  of  the  fibres  them- 
selves, and  in  part,  possibly,  of  some  portions  of  the  muscle  substance  itself. 
We  are  not,  however,  at  present  in  a  position  to  make  any  very  definite 
statement  as  to  the  relation  of  the  myosin  to  the  structural  features  of 
muscle.  Since  the  dim  bands  are  rendered  very  indistinct  by  the  action  of 
a  10  per  cent,  sodium  chloride  solution,  we  may,  perhaps,  infer  that  myosin 
enters  largely  into  the  composition  of  the  dim  bands,  and,  therefore,  of  the 
fibrillse ;  but  it  would  be  hazardous  to  say  much  more  than  this. 

§  60.  Living  muscle  may  be  frozen,  and  yet,  after  certain  precautions, 
will,  on  being  thawed,  regain  its  irritability,  or,  at  all  events,  will  for  a  time 
be  found  to  be  still  living  in  the  sense  that  it  has  not  yet  passed  into  rigor 
mortis.  We  may,  therefore,  take  living  muscle  which  has  been  frozen  as 
still  living. 

If  living  contractile  muscle,  freed  as  much  as  possible  from  blood,  be  frozen, 
and  while  frozen  minced  and  rubbed  up  in  a  mortar  with  four  times  its 
weight  of  snow  containing  1  per  cent,  of  sodium  chloride,  a  mixture  is 
obtained  which,  at  a  temperature  just  below  0°  C.,  is  sufficiently  fluid  to  be 
filtered,  though  with  difficulty.  The  slightly  opalescent  filtrate,  or  muscle- 
plasma,  as  it  is  called,  is  at  first  quite  fluid,  but  will,  when  exposed  to  the 
ordinary  temperature,  become  a  solid  jelly,  and  afterward  separate  into  a 
clot  and  serum.  It  will,  in  fact,  coagulate  like  blood-plasma,  with  this  dif- 
ference, that  the  clot  is  not  firm  and  fibrillar,  but  loose,  granular,  and  floc- 
culent.  During  the  coagulation  the  fluid,  which  before  was  neutral  or 
slightly  alkaline,  becomes  distinctly  acid. 

The  clot  is  myosin.  It  gives  all  the  reactions  of  myosin  obtained  from 
dead  muscle. 

The  serum  contains  an  albumin  very  similar  to,  if  not  identical  with 


86  THE  CONTRACTILE  TISSUES. 

serum-albumin,  a  globulin  differing  somewhat  from  and  coagulating  at  a 
lower  temperature  than  paraglobulin,  and  whic,h  to  distinguish  it  from  the 
globulin  of  blood  has  been  called  myoglobulm,  some  other  proteids  which 
need  not  be  described  here,  and  various  "  extractives  "  of  which  we  shall 
speak  directly.  Such  muscles  as  are  red  also  contain  a  small  quantity  of 
haemoglobin,  and  of  another  allied  pigment  called  kistohcematin,  to  which 
pigments,  indeed,  their  redness  is  due. 

Thus,  while  dead  muscle  contains  myosin,  albumin,  and  other  proteids, 
extractives,  and  certain  insoluble  matters,  together  with  gelatinous  and  other 
substances  not  referable  to  the  muscle  substance  itself,  living  muscle  contains 
no  myosin,  but  some  substance  or  substances  which  bear  somewhat  the  same 
relation  to  myosin  that  the  antecedents  of  fibrin  do  to  fibrin,  and  which  give 
rise  to  myosin  upon  the  death  of  the  muscle.  There  are,  indeed,  reasons  for 
thinking  that  the  myosin  arises  from  the  conversion  of  a  previously  existing 
body  which  may  be  called  myosinogen,  and  that  the  conversion  takes  place, 
or  may  take  place,  by  the  action  of  a  special  ferment,  the  conversion  of 
myosinogen  into  myosin  being  very  analogous  to  the  conversion  of  fibrinogen 
into  fibrin. 

We  may,  in  fact,  speak  of  rigor  mortis  as  characterized  by  a  coagulation 
of  the  muscle-plasma,  comparable  to  the  coagulation  of  blood-plasma,  but 
differing  from  it  inasmuch  as  the  product  is  not  fibrin,  but  myosin.  The 
rigidity,  the  loss  of  suppleness,  and  the  diminished  translucency  appear  to 
be  at  all  events  largely,  though  probably  not  wholly,  due  to  the  change  from 
the  fluid  plasma  to  the  solid  myosin.  We  might  compare  a  living  muscle 
to  a  number  of  fine  transparent  membranous  tubes  containing  blood-plasma. 
When  this  blood-plasma  entered  into  the  "jelly"  stage  of  coagulation,  the 
system  of  tubes  would  present  many  of  the  phenomena  of  rigor  mortis. 
They  would  lose  much  of  their  suppleness  and  translucency,  and  acquire 
a  certain  amount  of  rigidity. 

§  61.  There  is,  however,  one  very  marked  and  important  difference  be- 
tween the  rigor  mortis  of  muscle  and  the  coagulation  of  blood.  Blood 
during  its  coagulation  undergoes  a  slight  change  only  in  its  reaction  ;  but 
muscle  during  the  onset  of  rigor  mortis  becomes  distinctly  acid. 

A  living  muscle  at  rest  is  in  reaction  neutral,  or,  possibly  from  some 
remains  of  lymph  adhering  to  it,  faintly  alkaline.  If,  on  the  other  hand, 
the  reaction  of  a  thoroughly  rigid  muscle  be  tested,  it  will  be  found  to  be 
most  distinctly  acid.  This  development  of  an  acid  reaction  is  witnessed  not 
only  in  the  solid  untouched  fibre  but  also  in  expressed  muscle-plasma ;  it 
seems  to  be  associated  in  some  way  with  the  appearance  of  the  myosin. 

The  exact  causation  of  this  acid  reaction  has  not  at  present  been  clearly 
worked  out.  Since  the  coloration  of  the  litmus  produced  is  permanent, 
carbonic  acid,  which,  as  we  shall  immediately  state,  is  set  free  at  the  same 
time,  cannot  be  regarded  as  the  active  acid,  for  the  reddening  of  litmus 
produced  by  carbonic  acid  speedily  disappears  on  exposure.  On  the  other 
hand,  it  is  possible  to  extract  from  rigid  muscle  a  certain  quantity  of  lactic 
acid,  or  rather  of  a  variety  of  lactic  acid  known  as  sarcolactic  acid;1  and 
it  has  been  thought  that  the  appearance  of  the  acid  reaction  of  rigid  muscle 
is  due  to  a  new  formation  or  to  an  increased  formation  of  this  sarcolactic 
acid.  There  is  much  to  be  said  in  favor  of  this  view,  but  it  cannot  at 
present  be  regarded  as  established  beyond  dispute. 

Coincident  with  the  appearance  of  this  acid  reaction,  though,  as  we  have 
said,  not  the  direct  cause  of  it,  a  large  development  of  carbonic  acid  takes 

1  There  are  many  varieties  of  lactic  acid,  which  are  isomeric,  having  the  same  compo- 
sition, CsHeOs,  but  differ  in  their  reactions  and  especially  in  the  solubility  of  their  zine 
salts.  The  variety  present  in  muscle  is  distinguished  as  sarcolactic  acid. 


CHANGES  IN  A   MUSCLE  DURING  CONTRACTION.  87 

place  when  muscle  becomes  rigid.  Irritable  living  muscular  substance,  like 
nil  living  substance,  is  continually  respiring,  that  is  to  say,  is  continually 
consuming  oxygen  and  giving  out  carbonic  acid.  In  the  body,  the  arterial 
blood  going  to  the  muscle  gives  up  some  of  its  oxygen,  and  gains  a  quantity 
of  carbonic  acid,  thus  becoming  venous  as  it  passes  through  the  muscle 
capillaries.  Even  after  removal  from  the  body,  the  living  muscle  continues 
to  take  up  from  the  surrounding  atmosphere  a  certain  quantity  of  .oxygen 
and  to  give  out  a  certain  quantity  of  carbonic  acid. 

At  the  onset  of  rigor  mortis  there  is  a  very  large  and  sudden  increase  in 
this  production  of  carbonic  acid,  in  fact  an  outburst,  as  it  were,  of  that  gas. 
This  is  a  phenomena  deserving  special  attention.  Knowing  that  the  car- 
bonic acid  which  is  the  outcome  of  the  respiration  of  the  whole  body  is  the 
result  of  the  oxidation  of  carbon-holding  substances,  we  might  very  natu- 
rally suppose  that  the  increased  production  of  carbonic  acid  attendant  on 
the  development  of  rigor  mortis  is  due  to  the  fact  that  during  that  event  a 
certain  quantity  of  the  carbon-holding  constituents  of  the  muscle  are  sud- 
denly oxidized.  But  such  a  view  is  negatived  by  the  following  facts :  In 
the  first  place,  the  increased  production  of  carbonic  acid  during  rigor  mortis 
is  not  accompanied  by  a  corresponding  increase  in  the  consumption  of 
oxygen.  In  the  second  place,  a  muscle  (of  a  frog,  for  instance)  contains  in 
itself  no  free  or  loosely  attached  oxygen  ;  when  subjected  to  the  action  of  a 
mercurial  air-pump  it  gives  off  no  oxygen  to  a  vacuum,  offering  in  this 
respect  a  marked  contrast  to  blood  ;  and  yet,  when  placed  in  an  atmosphere 
free  from  oxygen,  it  will  not  only  continue  to  give  off  carbonic  acid  while  it 
remains  alive,  but  will  also  exhibit  at  the  onset  of  rigor  mortis  the  same 
increased  production  of  carbonic  acid  that  is  shown  by  a  muscle  placed  in 
an  atmosphere  containing  oxygen.  It  is  obvious  that  in  such  a  case  the 
carbonic  acid  does  not  arise  from  the  direct  oxidation  of  the  muscle  sub- 
stance, for  there  is  no  oxygen  present  at  the  time  to  carry  on  that  oxidation. 
We  are  driven  to  suppose  that  during  rigor  mortis,  some  complex  body,  con- 
taining in  itself  ready-formed  carbonic  acid,  so  to  speak,  is  split  up,  and 
thus  carbonic  acid  is  set  free,  the  process  of  oxidation  by  which  that  car- 
bonic acid  was  formed  out  of  the  carbon-holding  constituents  of  the  muscle 
having  taken  place  at  some  anterior  date. 

Living  resting  muscle,  then,  is  alkaline  or  neutral  in  reaction,  and  the 
substance  of  its  fibres  contains  a  coagulable  plasma.  Dead  rigid  muscle,  on 
the  other  hand,  is  acid  in  reaction,  and  no  longer  contains  a  coagulable 
plasma,  but  is  laden  with  the  solid  myosin.  Further,  the  change  from  the 
living  irritable  condition  to  that  of  rigor  mortis  is  accompanied  by  a  large 
and  sudden  development  of  carbonic  acid. 

It  is  found,  moreover,  that  there  is  a  certain  amount  or  parallelism 
between  the  intensity  of  the  rigor  mortis,  the  degree  of  acid  reaction  and 
the  quantity  of  carbonic  acid  given  ont.  If  we  suppose,  as  we  fairly 
may  do,  that  the  intensity  of  the  rigidity  is  dependent  on  the  quantity  of 
myosin  deposited  in  the  fibres,  and  the  acid  reaction  to  the  development,  if 
not  of  lactic  acid,  at  least  of  some  other  substance,  the  parallelism  between 
the  three  products,  myosin,  acid-producing  substance,  and  carbonic  acid, 
would  suggest  the  idea  that  all  three  are  the  results  of  the  splitting-up  of  the 
same  highly  complex  substance.  No  one  has  at  present,  however,  succeeded 
in  isolating  or  in  otherwise  definitely  proving  the  existence  of  such  a  body,  and 
though  the  idea  seems  tempting,  it' may  in  the  end  prove  totally  erroneous. 

§  62.  As  to  the  other  proteids  of  muscle,  such  as  the  albumin  and  the 
globulin,  we  know  as  yet  nothing  concerning  the  parts  which  they  play  and 
the  changes  which  they  undergo  in  the  living  muscle  or  in  rigor  mortis. 

Besides  the  fat  which  is  found,  and  that  riot  infrequently  in  abundance, 


88  THE  CONTRACTILE  TISSUES. 

in  the  connective  tissue  between  the  fibres,  there  is  also  present  in  the 
muscular  substance  within  the  sarcolemma,  always  some  and  at  times 
a  great  deal  of  fat,  chiefly  ordinary  fat,  viz.,  stearin,  palmitiri,  and  olein 
in  variable  proportion,  but  also  the  more  complex  fat  lecithin.  As  to 
the  function  of  these  several  fats  in  the  life  of  the  muscle  we  know  little 
or  nothing 

Carbohydrates,  the  third  of  the  three  great  classes  in  which  we  may  group 
the  energy-holding  substances  of  which  the  animal  body  and  its  food  are 
alike  composed,  viz.,  proteids,  fat,  and  carbohydrates,  are  represented  in 
muscle  by  a  peculiar  body,  glycogen,  which  we  shall  have  to  study  in  detail 
later  on.  We  must  here  merely  say  that  glycogen  is  a  body  closely  allied 
to  starch,  having  a  formula,  which  may  be  included  under  the  general 
formula  for  starches,  x  (C6H,0O5),  and  may  like  it  be  converted  by  the  action 
of  acids,  or  by  the  action  of  particular  ferments  known  as  amylolytic  fer- 
ments, into  some  form  of  sugar,  dextrose  (C6H,2O6),  or  some  allied  sugar. 
Many,  if  not  all,  living  muscles  contain  a  certain  amount,  and  some,  under 
certain  circumstances,  a  considerable  amount,  of  glycogen.  During  or  after 
rigor  mortis  this  glycogen  is  very  apt  to  be  converted  into  dextrose,  or  an 
allied  sugar.  The  muscles  of  the  embryo  at  an  early  stage  contain  a  rela- 
tively enormous  quantity  of  glycogen,  a  fact  which  suggests  that  the  glyco- 
gen of  muscle  is  carbohydrate  food  of  the  muscle  about  to  be  wrought  up 
into  the  living  muscular  substance. 

The  bodies  which  we  have  called  extractives  are  numerous  and  varied. 
They  are  especially  interesting,  since  it  seems  probable  that  they  are  waste 
products  of  the  metabolism  of  the  muscular  substance,  and  the  study  of 
them  may  be  expected  to  throw  light  on  the  chemical  change  which  mus- 
cular substance  undergoes  during  life.  Since,  as  we  shall  see,  muscular 
substance  forms  by  far  the  greater  part  of  the  nitrogenous,  that  is,  proteid, 
portion  of  the  body,  the  nitrogenous  extractives  of  muscle  demand  peculiar 
attention.  Now  the  body  urea,  which  we  shall  have  to  study  in  detail  later 
on,  far  exceeds  in  importance  all  the  other  nitrogenous  extractives  of  the 
body  as  a  whole,  since  it  is  practically  the  one  form  in  which  nitrogenous 
wastes  leave  the  body ;  if  we  include  with  urea  the  closely  allied  uric  acid 
(which  for  present  purposes  may  simply  be  regarded  as  a  variety  of  urea), 
we  may  say  that  all  the  nitrogen  taken  in  as  food  sooner  or  later  leaves  the 
body  as  urea ;  compared  with  this  all  other  nitrogenous  waste  thrown  out 
from  the  body  is  insignificant.  Of  the  urea  which  thus  leaves  the  body,  a 
considerable  portion  must  at  some  time  or  other  have  existed,  or  to  speak 
more  exactly,  its  nitrogen  must  have  existed  as  the  nitrogen  of  the  proteids 
of  muscular  substance.  Nevertheless,  no  urea  at  all  is,  in  normal  condi- 
tions, present  in  muscular  substance  either  living  and  irritable  or  dead  and 
rigid ;  urea  does  not  arise  in  muscular  substance  itself  as  one  of  the  imme- 
diate waste  products  of  muscular  substance. 

There  is,  however,  always  present  in  relatively  considerable  amount,  on 
an  average  about  0.25  per  cent,  of  wet  muscle,  a  remarkable  body,  kreatin. 
This  is,  in  one  sense,  a  compound  of  urea ;  it  may  be  split  up  into  urea  and 
sarcosin.  This  latter  body  is  a  methyl  glycin,  that  is  to  say,  a  glycin  in 
which  methyl  has  been  substituted  for  hydrogen,  and  glycin  itself  is  amido- 
acetic  acid,  a  compound  of  amidogen,  that  is  a  representative  of  ammonia 
and  acetic  acid.  Hence  kreatin  contains  urea,  which  has  close  relations 
with  ammonia,  together  with  another  representative  of  ammonia,  and  a  sur- 
plus of  carbon  and  hydrogen  arranged  as  a  body  belonging  to  the  fatty  acid 
series.  We  shall  have  to  return  to  this  kreatin  and  consider  its  relation  to 
urea  and  to  muscle  when  we  come  to  deal  with  urine. 

The  other  nitrogenous  extractives,  such    as    karnin,   hypoxanthin   (or 


CHANGES  IN   A   MUSCLE   DURING   CONTRACTION,  89 

sarkin),  xanthiu,  taurin,  etc.,  occur  in  small  quantity,  and  need  not  be 
dwelt  on  here. 

Among  non-nitrogenous  extractives  the  most  important  is  the  sarcolactic 
acid,  of  which  we  have  already  spoken  ;  to  this  may  be  added  sugar  in  some 
form  or  other,  either  coming  from  glycogen  or  from  some  other  source. 

The  ash  of  muscle,  like  the  ash  of  the  blood  corpuscles,  and,  indeed  the 
ash  of  the  tissues  in  general,  as  distinguished  from  the  blood,  or  plasma,  or 
lymph  on  which  the  tissues  live,  is  characterized  by  the  preponderance  of 
potassium  salts  and  of  phosphates  ;  these  form,  in  fact,  nearly  80  per  cent,  of 
the  whole  ash. 

§  63.  We  may  now  pass  on  to  the  question,  What  are  the  chemical 
changes  which  take  place  when  a  living  resting  muscle  enters  into  a  con- 
traction ?  These  changes  are  most  evident  after  the  muscle  has  been  sub- 
jected to  a  prolonged  tetanus ;  but  there  can  be  no  doubt  that  the  chemical 
events  of  a  tetanus  are,  like  the  physical  events,  simply  the  sum  of  the  results 
of  the  constituent  single  contractions. 

In  the  first  place  the  muscle  becomes  acid,  not  so  acid  as  in  rigor  mortis, 
but  still  sufficiently  so  after  a  vigorous  tetanus  to  turn  blue  litmus  distinctly 
red.  The  cause  of  the  acid  reaction,  like  that  of  rigor  mortis,  is  doubtful, 
but  is  in  all  probability  the  same  in  both  cases. 

In  the  second  place,  a  considerable  quantity  of  carbonic  acid  is  set  free ; 
and  the  production  of  carbonic  acid  in  muscular  contraction  is  altogether 
similar  to  the  production  of  carbonic  acid  during  rigor  mortis ;  it  is  not 
accompanied  by  any  corresponding  increase  in  the  consumption  of  oxygen. 
This  is  evident  even  in  a  muscle  through  which  the  circulation  of  blood  is 
still  going  on  ;  for  though  the  blood  passing  through  a  contracting  muscle 
gives  up  more  oxygen  than  the  blood  passing  through  a  resting  muscle,  the 
increase  in  the  amount  of  oxygen  taken  up  falls  below  the  increase  in  the 
carbonic  acid  given  out.  But  it  is  still  more  markedly  shown  in  a  muscle 
removed  from  the  body ;  for  in  such  a  muscle  both  the  contraction  and  the 
increase  in  the  production  of  carbonic  acid  will  go  on  in  the  absence  of  oxy- 
gen. A  frog's  muscle  suspended  in  an  atmosphere  of  nitrogen  will  remain 
irritable  for  some  considerable  time,  and  at  each  vigorous  tetanus  an  increase 
in  the  production  of  carbonic  acid  may  be  readily  ascertained. 

Moreover,  there  seems  to  be  a  correspondence  between  the  energy  of  the 
contraction  and  the  amount  of  carbonic  acid  and  the  degree  of  acid  reaction 
produced,  so  that,  though  we  are  now  treading  on  somewhat  uncertain 
ground,  we  are  naturally  led  to  the  view  that  the  essential  chemical  process 
lying  at  the  bottom  of  a  muscular  contraction  as  of  rigor  mortis  is  the  split- 
ting up  of  some  highly  complex  substance.  But  here  the  resemblance 
between  rigor  mortis  and  contraction  ends.  We  have  no  satisfactory  evi- 
dence of  the  formation  during  a  contraction  of  any  body  like  myosin.  And 
this  difference  in  chemical  results  tallies  with  an  important  difference  between 
rigid  muscle  and  contracting  muscle.  The  rigid  muscle,  as  we  have  seen, 
becomes  less  extensible,  less  elastic,  less  translucent ;  the  contracting  muscle 
remains  no  less  translucent,  elastic,  and  extensible  than  the  resting  muscle, 
indeed,  there  are  reasons  for  thinking  that  the  muscle  in  contracting  becomes 
actually  more  extensible  for  the  time  being. 

But  if  during  a  contraction  myosin  is  not  formed,  what  changes  of  proteid 
or  nitrogenous  matter  do  take  place?  We  do  not  know.  We  have  no  evi- 
dence that  kreatin,  or  any  other  nitrogenous  extractive,  is  increased  by  the 
contraction  of  muscle ;  we  have  no  evidence  of  any  nitrogen  waste  at  all  as 
the  result  of  a  contraction  ;  and,  indeed,  as  we  shall  see  later  on,  the  study 
of  the  waste  products  of  the  body  as  a  whole  lead  us  to  believe  that  the 
energy  of  the  work  done  by  the  muscles  of  the  body  comes  from  the  poten- 


90  THE  CONTRACTILE  TISSUES. 

tial  energy  of  carbon  compounds,  and  not  of  nitrogen  compounds  at  all. 
But  to  this  point  we  shall  have  to  return. 

§  64.  We  may  sum  up  the  chemistry  of  muscle  somewhat  as  follows : 

During  life  the  muscular  substance  is  continually  taking  up  from  the 
blood,  that  is,  from  the  lymph,  proteid,  fatty  and  carbohydrate  material, 
saline  matters  and  oxygen  ;  these  it  builds  up  into  itself,  how  we  do  not 
know,  and  so  forms  the  peculiar  complex  living  muscular  substance.  The 
exact  nature  of  this  living  substance  is  unknown  to  us.  What  we  do  know 
is  that  it  is  largely  composed  of  proteid  material,  and  that  such  bodies  as 
myosinogen,  myoglobulin,  and  albumin  have  something  to  do  with  the  build- 
ing of  it  up. 

During  rest  this  muscular  substance,  while  taking  in  and  building  itself 
up  out  of  or  by  means  of  the  above-mentioned  materials  is  continually  giving 
off  carbonic  acid  and  continually  forming  nitrogenous  waste  such  as  kreatin. 
It  also  probably  gives  off  some  amount  of  sarcolactic  acid,  and  possibly 
other  non-nitrogenous  waste  matters. 

During  a  contraction  there  is  a  great  increase  in  the  quantity  of 
carbonic  acid  given  off,  of  lactic  acid  and  some  other  substance  formed 
giving  an  acid  reaction,  a  greater  consumption  of  oxygen,  though  the 
increase  is  not  equal  to  the  increase  of  carbonic  acid,  but  as  far  as  we 
can  learn,  no  increase  of  nitrogenous  waste. 

During  rigor  mortis  there  is  a  similar  increased  production  of  carbonic 
acid  and  of  some  other  acid-producing  substance,  accompanied  by  remark- 
able conversion  of  myosinogen  into  myosin,  by  which  the  rigidity  of  the 
dead  fibre  is  brought  about. 

Thermal  Changes. 

§  65.  The  chemical  changes  during  a  contraction  set  free  a  quantity  of 
energy,  but  only  a  portion  of  this  energy  appears  in  the  "  work  done,"  a  con- 
siderable portion  takes  on  the  form  of  heat.  Though  we  shall  have  hereafter 
to  treat  this  subject  more  fully,  the  leading  facts  may  be  given  here. 

Whenever  a  muscle  contracts  its  temperature  rises,  indicating  that  heat 
is  given  out.  When  a  mercury  thermometer  is  plunged  into  a  mass  of  mus- 
cles, such  as  those  of  the  thigh  of  the  dog,  a  rise  of  the  mercury  is  observed 
upon  the  muscles  being  thrown  into  a  prolonged  contraction.  More  exact 
results,  however,  are  obtained  by  means  of  a  thermopile,  by  the  help  of 
which  the  rise  of  temperature  caused  by  a  few  repeated  single  contractions, 
or  indeed  by  a  single  contraction,  may  be  observed,  and  the  amount  of  heat 
given  out  approximately  measured. 

The  thermopile  may  consist  either  of  a  single  junction  in  the  form  of  a  needle 
plunged  into  the  substance  of  the  muscle,  or  of  several  junctions,  either  in  the 
shape  of  a  flat  surface  carefully  opposed  to  the  surface  of  muscle  (the  pile  being 
balanced  so  as  to  move  with  the  contracting  muscle,  and  thus  to  keep  the  contact 
exact),  or  in  the  shape  of  a  thin  wedge,  the  edge  of  which  comprising  the  actual 
junctions,  is  thrust  into  a  mass  of  muscles  and  held  in  position  by  them.  In  all 
cases  the  fellow-junction  or  junctions  must  be  kept  at  a  constant  temperature. 

Another  delicate  method  of  determining'  the  changes  of  temperature  of  a  tissue 
is  based  upon  the  measurement  of  alterations  in  electric  resistance  which  a  fine 
wire,  in  contact  with  or  plunged  into  the  tissue,  undergoes  as  the  temperature  of 
the  tissue  changes. 

It  has  been  calculated  that  the  heat  given  out  by  the  muscles  of  the 

thigh  of  a  frog  in  a  single  contraction  amounts  to  3.1  micro-units  of  heat1 

for  each  gramme  of  muscle,  the  result  being  obtained  by  dividing  by  five 

the  total  amount  of  heat  given  out  in  five  successive  single  contractions.    It 

1  The  micro-unit  being  a  milligramme  of  water  raised  one  degree  Centigrade. 


CHANGES  IN   A   MUSCLE  DURING   CONTRACTION.  91 

will,  however,  be  safer  to  regard  these  figures  as  illustrative  of  the  fact  that 
the  heat  given  out  is  considerable,  rather  than  as  data  for  elaborate  calcula- 
tions. Moreover,  we  have  no  satisfactory  quantitative  determinations  of  the 
heat  given  out  by  the  muscles  of  warm-blooded  animals,  though  there  can 
be  no  doubt  that  it  is  much  greater,  than  that  given  out  by  the  muscles  of 
the  frog. 

There  can  hardly  be  any  doubt  that  the  heat  thus  set  free  is  the  product 
of  chemical  changes  within  the  muscle — changes  which,  though  they  cannot 
for  the  reasons  given  above  (§  63)  be  regarded  as  simple  and  direct  oxida- 
tions, yet,  since  they  are  processes  dependent  on  the  antecedent  entrance  of 
oxygen  into  the  muscle,  may  be  spoken  of  in  general  terms  as  a  combustion  ; 
so  that  the  muscle  may  be  likened  to  a  steam-engine,  in  which  the  combustion 
of  a  certain  amount  of  material  gives  rise  to  the  development  of  energy  in 
two  forms,  as  heat  and  as  movement,  there  being  certain  quantitative  rela- 
tions between  the  amount  of  energy  set  free  as  heat  and  that  giving  rise  to 
movement.  We  must,  however,  carefully  guard  ourselves  against  pressing 
this  analogy  too  closely.  In  the  steam-engine  we  can  distinguish  clearly 
between  the  fuel  which,  through  its  combustion,  is  the  sole  source  of  energy, 
and  the  machinery,  which  is  not  consumed  to  provide  energy,  and  only  suffers 
wear  and  tear.  In  the  muscle  we  cannot  with  certainty  at  present  make  such 
a  distinction.  It  may  be  that  the  chemical  changes  at  the  bottom  of  a  con- 
traction do  not  involve  the  real  living  material  of  the  fibre,  but  only  some 
substance  manufactured  by  the  living  material  and  lodged  in  some  way,  we 
do  not  know  how,  in  the  living  material.  It  may  be  that  when  a  fibre  con- 
tracts it  is  this  substance  within  the  fibre  which  explodes,  and  not  the  fibre 
itself.  If  we  further  suppose  that  this  substance  is  some  complex  compound 
of  carbon  and  hydrogen,  into  which  no  nitrogen  enters,  we  shall  have  an 
explanation  of  the  difficulty  referred  to  above  (§  63),  namely,  that  nitro- 
genous waste  is  not  increased  by  a  contraction.  The  special  contractile 
carbon-hydrogen  substance,  may  then  be  compared  to  the  charge  of  a  gun, 
the  products  of  its  explosion  being  carbonic  and  sarcolactic  acids,  while  the 
real  living  material  of  the  fibre  may  be  compared  to  the  gun  itself,  but  to  a 
gun  which  itself  is  continually  undergoing  change  far  beyond  mere  wear  and 
tear,  among  the  products  of  which  change  nitrogenous  bodies  like  kreatin 
are  conspicuous.  This  view  will  certainly  explain  why  kreatin  is  not 
increased  during  the  contraction,  while  the  carbonic  and  lactic  acids  are. 
But  it  must  be  remembered  that  such  a  view  is  not  yet  proved  ;  it  may  be 
the  living  material  of  the  fibre,  as  a  whole,  which  is  continually  breaking 
down  in  an  explosive  decomposition,  and  as  continually  building  itself  up 
again  out  of  the  material  supplied  by  the  blood. 

In  a  steam-engine  only  a  certain  amount  of  the  total  potential  energy  of 
the  fuel  issues  as  work,  the  rest  being  lost  as  heat,  the  proportion  varying, 
but  the  work  rarely,  if  ever,  exceeding  one-tenth  of  the  total  energy,  and 
generally  being  less.  In  the  case  of  the  muscle  we  are  not  at  present  in  a 
position  to  draw  up  an  exact  equation  between  the  latent  energy  on  the  one 
hand,  and  the  two  forms  of  actual  energy  on  the  other.  We  have  reason  to 
think  that  the  proportion  between  heat  and  work  varies  considerably  under 
different  circumstances,  the  work  sometimes  rising  as  high  as  one-fifth,  some- 
times possibly  sinking  as  low  as  one-twenty-fourth  of  the  total  energy ;  and 
observations  seem  to  show  that  the  greater  the  resistance  which  the  muscle 
has  to  overcome,  the  larger  the  proportion  of  the  total  energy  expended 
which  goes  out  as  work  done.  The  muscle,  in  fact,  seems  to  be  so  far  self- 
regulating  that  the  more  work  it  has  to  do  the  greater,  within  certain 
limits,  is  the  economy  with  which  it  works. 

Lastly,  it  must  be  remembered  that  the  giving  out  of  heat  by  the  muscle 


92  THE  CONTRACTILE  TISSUES. 

is  not  confined  to  the  occasions  when  it  is  actually  contracting.  When,  at  a 
later  period,  we  treat  of  the  heat  of  the  body  generally,  evidence  will  be 
brought  forward  that  the  muscles,  even  when  at  rest,  are  giving  rise  to 
heat,  so  that  the  heat  given  out  at  a  contraction  is  not  some  wholly  new 
phenomenon,  but  a  temporary  exaggeration  of  what  is  continually  going 
on  at  a  more  feeble  rate. 

Electrical    Changes. 

§  66.  Besides  chemical  and  thermal  changes,  a  remarkable  electric 
change  takes  place  whenever  a  muscle  contracts. 

Muscle-currents.— If  a  muscle  be  removed  in  an  ordinary  manner  from 
the  body,  and  two  non-polarizable  electrodes,1  connected  with  a  delicate 
galvanometer  and  many  convolutions  and  high  resistance,  be  placed  on  two 

FIG.  29. 


ch.c 


Non-polarizable  Electrodes :  a,  the  glass  tube  ;  z,  the  amalgamated  zinc  slips  connected  with 
their  respective  wires ;  2.  s.,  the  zinc  sulphate  solution  ;  ch.  c. ,  the  plug  of  china-clay  ;  c',  the  por- 
tion of  the  china-clay  plug  projecting  from  the  end  of  the  tube  ;  this  can  be  moulded  into  any 
required  form. 

points  of  the  surface  of  the  muscle,  a  deflection  of  the  galvanometer  will 
take  place,  indicating  the  existence  of  a  current  passing  through  the  gal- 
vanometer from  the  one  point  of  the  muscle  to  the  other,  the  direction  and 
amount  of  the  deflection  varying  according  to  the  position  of  the  points. 
The  "muscle-currents"  thus  revealed  are  seen  to  the  best  advantage  when 
the  muscle  chosen  is  a  cylindrical  or  prismatic  one  with  parallel  fibres,  and 
when  the  two  tendinous  ends  are  cut  off  by  clean  incisions  at  right  angles  to 
the  long  axis  of  the  muscle.  The  muscle  then  presents  a  transverse  section 
(artificial)  at  each  end  and  a  longitudinal  surface.  We  may  speak  of  the 
latter  as  being  divided  into  two  equal  parts  by  an  imaginary  transverse  line 
on  its  surface  called  the  "  equator,"  containing  all  the  points  of  the  surface 
midway  between  the  two  ends.  Fig.  30  is  a  diagrammatic  representation  of 
such  a  muscle,  the  line  ab  being  the  equator.  In  such  a  muscle  the  develop- 
ment of  the  muscle-currents  is  found  to  be  as  follows : 

The  greatest  deflection  is  observed  when  one  electrode  is  placed  at  the 

1  These  (Fig.  29)  consist  essentially  of  a  slip  of  thoroughly  amalgamated  zinc,  dipping 
into  a  saturated  solution  of  zinc  sulphate,  which  in  turn  is  brought  into  connection  with 
the  nerve  or  muscle  by  means  of  a  plug  or  bridge  of  china-clay  moistened  with  normal 
sodium  chloride  solution  ;  it  is  important  that  the  zinc  should  be  thoroughly  amalga- 
mated. This  form  of  electrode  gives  rise  to  less  polarization  than  do  simple  platinum  or 
copper  electrodes.  The  clay  affords  a  connection  between  the  zinc  and  the  tissue,  which 
neither  acts  on  the  tissue  nor  is  acted  on  by  the  tissue.  Contact  of  any  tissue  with  copper 
or  platinum  is  in  itself  sufficient  to  develop  a  current. 


CHANGES  IN  A   MUSCLE  DURING  CONTRACTION. 


93 


mid-point  or  equator  of  the  muscle,  and  the  other  at  either  cut  end ;  and 
the  deflection  is  of  such  a  kind  as  to  show  that  positive  currents  are  con- 
tinually passing  from  the  equator  through  the  galvanometer  to  the  cut  end  ; 
that  is  to  say,  the  cut  end  is  negative,  relatively  to  the  equator.  The  cur- 
rents outside  the  muscle  may  be  considered  as  completed  by  currents  in  the 
muscle  from  the  cut  end  to  the  equator.  In  the  diagram,  Fig.  30,  the  arrows 


FIG.  30. 


Diagram  illustrating  the  Electric  Currents  of  Nerve  and  Muscle:  Being  purely  diagrammatic, 
it  may  serve  for  a  piece  either  of  nerve  or  of  muscle,  except  that  the  currents  at  the  transverse 
section  cannot  be  shown  in  a  nerve.  The  arrows  show  the  direction  of  the  current  through  the 
galvanometer. 

ab,  the  equator.  The  strongest  currents  are  those  shown  by  the  dark  lines,  as  from  a,  at  equator, 
to  x  or  to  y  at  the  cut  ends.  The  current  from  a  to  c  is  weaker  than  from  a  to  y,  though  both,  as 
shown  by  the  arrows,  have  the  same  direction.  A  current  is  shown  from  e,  which  is  near  the 
equator,  to  /,  which  is  further  from  the  equator.  The  current  (in  muscle)  from  a  point  in  the  cir- 
cumference to  a  point  nearer  the  centre  of  the  transverse  section  is  shown  at  gh.  From  a  to  b,  or 
from  x  to  y,  there  is  no  current,  as  indicated  by  the  dotted  lines. 

indicate  the  direction  of  the  currents.  If  one  electrode  be  placed  at  the 
equator  ab,  the  effect  is  the  same  at  whichever  of  the  two  cut  ends,  x  or  y, 
the  other  is  placed.  If,  one  electrode  remaining  at  the  equator,  the  other  be 
shifted  from  the  cut  end  to  a  spot  (c)  nearer  to  the  equator,  the  current  con- 
tinues to  have  the  same  direction,  but  is  of  less  intensity  in  proportion  to  the 
nearness  of  the  electrodes  to  each  other.  If  the  two  electrodes  be  placed  at 
unequal  distances  (c  and  /),  one  on  either  side  of  the  equator,  there  will  be 
a  feeble  current  from  the  one  nearer  the  equator  to  the  one  further  off,  and 
the  current  will  be  the  feebler  the  more  nearly  they  are  equidistant  from  the 
equator.  If  they  are  quite  equidistant — as,  for  instance,  when  one  is  placed 
on  one  cut  end  (x)  and  the  other  on  the  other  cut  end  (y) — there  will  be  no 
current  at  all. 

If  one  electrode  be  placed  at  the  circumference  of  the  transverse  section 
and  the  other  at  the  centre  of  the  transverse  section,  there  will  be  a  current 
through  the  galvanometer  from  the  former  to  the  latter  ;  there  will  be  a  cur- 
rent of  similar  direction,  but  of  less  intensity,  when  one  electrode  is  at  the 
circumference  (<?)  of  the  transverse  section  and  the  other  at  some  point  (7i) 
nearer  the  centre  of  the  transverse  section.  In  fact,  the  points  which  are 
relatively  most  positive  and  most  negative  to  each  other  are  points  on  the 
equator  and  the  two  centres  of  the  transverse  sections  ;  and  the  intensity  of 
the  current  between  any  two  points  will  depend  on  the  respective  distances 
of  those  points  from  the  equator  and  from  the  centre  of  the  transverse 
section. 


94  THE  CONTRACTILE  TISSUES. 

Similar  currents  may  be  observed  when  the  longitudinal  surface  is  not 
the  natural  but  an  artificial  one ;  indeed,  they  may  be  witnessed  in  even  a 
piece  of  muscle,  provided  it  be  of  cylindrical  shape  and  composed  of 
parallel  fibres. 

These  "  muscle-currents"  are  not  mere  transitory  currents,  disappearing 
as  soon  as  the  circuit  is  closed  ;  on  the  contrary,  they  last  a  very  consider- 
able time.  They  must,  therefore,  be  maintained  by  some  changes  going  on 
in  the  muscle — by  continual  chemical  action,  in  fact.  They  disappear  as 
the  irritability  of  the  muscle  vanishes,  and  are  connected  with  those  nutri- 
tive, so-called  vital,  changes  which  maintain  the  irritability  of  the  muscle. 

Muscle-currents,  such  as  have  just  been  described,  may,  we  repeat,  be 
observed  in  any  cylindrical  muscle  suitably  prepared,  and  similar  currents, 
with  variations  which  need  not  be  discussed  here,  may  be  seen  in  muscles 
of  irregular  shape  with  obliquely  or  otherwise  arranged  fibres.  And  Du 
Bois-Reymond,  to  whom  chiefly  we  are  indebted  for  our  knowledge  of 
these  currents,  has  been  led  to  regard  them  as  essential  and  important  prop- 
erties of  living  muscle.  He  has,  moreover,  advanced  the  theory  that  muscle 
may  be  considered  as  composed  of  electro-motive  particles  or  molecules, 
each  of  which,  like  the  muscle  at  large,  has  a  positive  equator  and  neg- 
ative ends,  the  whole  muscle  being  made  up  of  these  molecules  in  some- 
what the  same  way  (to  use  an  illustration,  which  must  not,  however,  be 
strained  or  considered  as  an  exact  one)  as  a  magnet  may  be  supposed  to 
be  made  up  of  magnetic  particles,  each  with  its  north  and  south  pole. 

There  are  reasons,  however,  for  thinking  that  these  muscle-currents  have 
no  such  fundamental  origin,  that  they  are,  in  fact,  of  surface,  and,  indeed, 
of  artificial  origin.  Without  entering  into  the  controversy  on  this  question, 
the  following  important  facts  may  be  mentioned : 

1.  When  a  muscle  is  examined  while  it  still  retains  uninjured  its  natural 
tendinous  terminations,  the  currents  are  much  weaker  than  when  artificial 
transverse  sections  have  been  made;  the  natural  tendinous  end  is  less  nega- 
tive than  the  cut  surface.     But  the  tendinous  end  becomes  at  once  negative 
when  it  is  dipped  in  water  or  acid — indeed,  when  it  is  in   any  way  injured. 
The    less    roughly,    in    fact,   a  muscle  is  treated  the  less  evident  are  the 
muscle-currents ;  and  it  is  maintained  that  if  adequate   care  be  taken  to 
maintain  a  muscle  in  an  absolutely  natural   condition,  no  such  currents  as 
those  we  have  been  describing  exist  at  all — that  natural  living  muscle  is 
isoelectric,  as  it  is  called. 

2.  The  surface  of  the  uninjured  inactive1   ventricle  of  the  frog's  heart, 
which  is  practically  a  mass  of  muscle,  is  isoelectric ;  no  current  is  obtained 
when  the  electrodes  are  placed  on  any  two  points  of  the  surface.     If,  how- 
ever, any  part  of  the  surface  be  injured,  or  if  the  ventricle  be  cut  across 
so  as  to  expose  a  cut  surface,  the  injured  spot  or  the  cut  surface  becomes 
at  once  more  powerfully  negative  toward  the  uninjured  surface,  a  strong 
current  being  developed  which  passes  through  the  galvanometer  from  the 
uninjured  surface  to  the  cut  surface  or  to  the  injured  spot.     The  negativity 
thus  developed  in  a  cut  surface  passes  off  in  the  course  of  some  hours,  but 
may  be  restored  by  making  a  fresh  cut  and  exposing  a  fresh  surface. 

The  temporary  duration  of  the  negativity  after  injury,  and  its  renewal 
upon  fresh  injury,  in  the  case  of  the  ventricle,  in  contrast  to  the  more  per- 
manent negativity  of  injured  skeletal  muscle,  is  explained  by  the  different 
structure  of  the  two  kinds  of  muscle.  The  cardiac  muscle,  as  we  shall 
hereafter  see,  is  composed  of  short  fibre-cells ;  when  a  cut  is  made  a  cer- 
tain number  of  these  fibre-cells  are  injured,  giving  rise  to  negativity,  but 
the  injury  done  to  them  stops  with  them,  and  is  not  propagated  to  the  cells 
1  The  necessity  of  its  being  inactive  will  be  seen  subsequently. 


CHANGES  IN   A   MUSCLE  DURING   CONTRACTION.  95 

with  which  they  are  in  contact ;  hence,  upon  their  death,  the  negativity 
and  the  current  disappear.  A  fresh  cut,  involving  new  cells,  produces 
fresh  negativity  and  a  new  current.  In  the  long  fibres  of  the  skeletal 
muscle,  on  the  other  hand,  the  effects  of  the  injury  are  slowly  propagated 
along  the  fibre  from  the  spot  injured. 

Now,  when  a  muscle  is  cut  or  injured,  the  substance  of  the  fibres  dies 
at  the  cut  or  injured  surface.  And  many  physiologists,  among  whom  the 
most  prominent  is  Hermann,  have  been  led  by  the  above  and  other  facts 
to  the  conclusion  that  muscle-currents  do  not  exist  naturally  in  un- 
touched, uninjured  muscles,  that  the  muscular  substance  is  naturally, 
when  living,  isoelectric,  but  that  whenever  a  portion  of  the  muscular  sub- 
stance dies,  it  becomes,  while  dying,  negative  to  the  living  substance,  and 
thus  gives  rise  to  currents.  They  explain  the  typical  currents  (as  they 
might  be  called)  manifested  by  a  muscle  with  a  natural  longitudinal  surface 
and  artificial  transverse  sections,  by  the  fact  that  the  dying  cut  ends  are  neg- 
ative relatively  to  the  rest  of  the  muscle. 

Du  Bois-Reymond  and  those  with  him  offer  special  explanations  of 
the  above  facts  and  of  other  objections  which  have  been  urged  against 
the  theory  of  naturally  existing  electro-motive  molecules.  Into  these  we 
cannot  enter  here.  We  must  rest  content  with  the  statement  that  in 
an  ordinary  muscle  currents  such  as  have  been  described  may  be  witnessed, 
but  that  strong  arguments  may  be  adduced  in  favor  of  the  view  that  these 
currents  are  not  "  natural  "  phenomena,  but  essentially  of  artificial  origin. 
It  will,  therefore,  be  best  to  speak  of  them  as  currents  of  rest. 

§  67.  Current  of  action.  Negative  variation  of  the  muscle-current. — The 
controversy  whether  the  "currents  of  rest"  observable  in  a  muscle  be  of 
natural  origin  or  not,  does  not  affect  the  truth  or  the  importance  of  the  fact 
that  an  electrical  change  takes  place  and  a  current  is  developed  in  a  muscle 
whenever  it  enters  into  a  contraction.  When  currents  of  rest  are  observ- 
able in  a  muscle,  these  are  found  to  undergo  a  diminution  upon  the  occur- 
rence of  a  contraction,  and  this  diminution  is  spoken  of  as  "  the  negative 
variation  "  of  the  currents  of  rest.  The  negative  variation  may  be  seen 
when  a  muscle  is  thrown  into  a  single  contraction,  but  is  most  readily 
shown  when  the  muscle  is  tetanized.  Thus,  if  a  pair  of  electrodes  be 
placed  on  a  muscle,  one  at  the  equator  and  the  other  at  or  near  the  trans- 
verse section,  so  that  a  considerable  deflection  of  the  galvanometer  needle, 
indicating  a  considerable  current  of  rest,  be  gained,  the  needle  of  the  gal- 
vanometer will,  when  the  muscle  is  tetanized  by  an  interrupted  current 
sent  through  its  nerve  (at  a  point  too  far  from  the  muscle  to  allow  of 
any  escape  of  the  current  into  the  electrodes  connected  with  the  galvano- 
meter), swing  back  toward  zero ;  it  returns  to  its  original  deflection  when 
the  tetanizing  current  is  shut  off. 

Not  only  may  this  negative  variation  be  shown  by  the  galvanometer,  but 
it,  as  well  as  the  current  of  rest,  may  be  used  as  a  galvanic  shock,  and  so 
employed  to  stimulate  a  muscle,  as  in  the  experiment  known  as  "  the  rheo- 
scopic  frog."  For  this  purpose  the  muscles  and  nerves  need  to  be  very 
irritable  and  in  thoroughly  good  condition.  Two  muscle-nerve  preparations, 
A  and  B,  having  been  made,  and  each  placed  on  a  glass  plate  for  the  sake 
of  insulation,  the  nerve  of  the  one,  B,  is  allowed  to  fall  on  the  muscle  of  the 
other,  A,  in  such  a  way  that  one  point  of  the  nerve  comes  in  contact  with 
the  equator  of  the  muscle,  and  another  point  with  one  end  of  the  muscle  or 
with  a  point  at  some  distance  from  the  equator.  At  the  moment  the  nerve 
is  let  fall  and  contact  made,  a  current — viz.,  the  "current  of  rest"  of  the 
muscle  A— passes  through  the  nerve ;  this  acts  as  a  stimulus  to  the  nerve, 
and  so  causes  a  contraction  in  the  muscle  connected  with  the  nerve.  Thus, 


96  THE  CONTRACTILE  TISSUES. 

the  muscle  A  acts  as  a  battery,  the  completion  of  the  circuit  of  which  by 
means  of  the  nerve  of  B  serves  as  a  stimulus,  causing  the  muscle  B  to 
contract. 

If,  while  the  nerve  of  B  is  still  in  contact  with  the  muscle  of  A,  the 
nerve  of  the  latter  is  tetanized  with  an  interrupted  current,  not  only  is  the 
muscle  of  A  thrown  into  tetanus,  but  also  that  of  B,  the  reason  being  as 
follows :  At  each  spasm  of  which  the  tetanus  of  A  is  made  up,  there  is  a 
negative  variation  of  the  muscle-current  of  A.  Each  negative  variation  of 
the  muscle-current  of  A  serves  as  a  stimulus  to  the  nerve  of  B,  and  is  hence 
the  cause  of  a  spasm  in  the  muscle  of  B ;  and  the  stimuli  following  each 
other  rapidly,  as  being  produced  by  the  tetanus  of  A  they  must  do,  the 
spasms  in  B  to  which  they  give  rise  are  also  fused  into  a  tetanus  in  B.  B, 
in  fact,  contracts  in  harmony  with  A.  This  experiment  shows  that  the 
negative  variation  accompanying  the  tetanus  of  a  muscle,  though  it  causes 
only  a  single  swing  of  the  galvanometer,  is  really  made  up  of  a  series  of 
negative  variations,  each  single  negative  variation  corresponding  to  the 
single  spasms  of  which  the  tetanus  is  made  up. 

But  an  electrical  change  may  be  manifested  even  in  cases  when  no  cur- 
rents of  rest  exist.  We  have  stated  (§  66)  that  the  surface  of  the  uninjured 
inactive  ventricle  of  the  frog's  heart  is  isoelectric,  no  currents  being  observed 
when  the  electrodes  of  a  galvanometer  are  placed  on  two  points  of  the  sur- 
face. Nevertheless,  a  most  distinct  current  is  developed  whenever  the 
ventricle  contracts.  This  may  be  shown  either  by  the  galvanometer  or  by 
the  rheoscopic  frog.  If  the  nerve  of  an  irritable  muscle-nerve  preparation 
be  laid  over  a  pulsating  ventricle,  each  beat  is  responded  to  by  a  twitch  of 
the  muscle  of  the  preparation.  In  the  case  of  ordinary  muscles,  too, 
instances  occur  in  which  it  seems  impossible  to  regard  the  electrical  change 
manifested  during  the  contraction  as  the  mere  diminution  of  a  pre-existing 
current. 

Accordingly,  those  who  deny  the  existence  of  "  natural "  muscle-currents 
speak  of  a  muscle  as  developing  during  a  contraction  a  "current  of  action," 
occasioned,  as  they  believe,  by  the  muscular  substance  as  it  is  entering  into 
the  state  of  contraction  becoming  negative  toward  the  muscular  substance 

which  is  still  at  rest,  or  has  returned  to  a 

FJG  31  state  of   rest.      In   fact,   they    regard   the 

negativity  of  muscular  substance  as  cha- 
racteristic alike  of  beginning  death  and  of  a 
beginning  contraction.  So  that,  in  muscu- 
lar contraction  a  wave  of  negativity,  start- 
ing from  the  end-plate  when  indirect,  or 
from  the  point  stimulated  when  direct 
stimulation  is  used,  passes  along  the  mus- 
cular substance  to  the  ends  or  end  of  the 
fibre. 

For  instance,  we  will  suppose  two  elec- 
trodes placed  on  two  points  (Fig.  31),  A 
and  B,  of  a  fibre  about  to  be  stimulated  by 
a  single  induction-shock  at  one  end.  Before 
the  stimulation  the  fibre  is  isoelectric,  and 
the  needle  of  the  galvanometer  stands  at  zero.  At  a  certain  time  after  the 
shock  has  been  sent  through  the  stimulating  electrodes  O),  as  the  wave  of 
contraction  is  travelling  down  the  fibre,  the  section  of  the  fibre  beneath  A 
will  become  negative  toward  the  rest  of  the  fibre,  and  so  negative  toward 
the  portion  of  the  fibre  under  £—i.e.,  A  will  be  negative  relatively  to  B, 
and  this  will  be  shown  by  a  deflection  of  the  needle.  A  little  later  B  will 


CHANGES  IN  A  MUSCLE  DURING  CONTRACTION.  97 

be  entering  into  contraction,  and  will  be  becoming  negative  toward  the  rest 
of  the  fibre,  including  the  part  under  A,  whose  negativity  by  this  time  is 
passing  off — that  is  to  say,  B  will  now  be  negative  toward  A ,  and  this  will 
be  shown  by  a  deflection  of  the  needle  in  a  direction  opposite  to  that  of  the 
deflection  which  has  just  previously  taken  place.  Hence,  between  two  elec- 
trodes placed  along  a  fibre  a  single  wave  of  contraction  will  give  rise  to  two 
currents  of  different  phases,  to  a  diphasic  change  ;  and  this,  indeed,  is  found 
to  be  the  case. 

This  being  so,  it  is  obvious  that  the  electrical  result  of  tetanizing  a 
muscle  when  wave  after  wave  follows  along  each  fibre  is  a  complex  matter ; 
but  it  is  maintained  that  the  apparent  negative  variation  of  tetanus  can 
be  explained  as  the  net  result  of  a  series  of  currents  of  action  due  to  the 
individual  contractions,  the  second  phase  of  the  current  in  each  contrac- 
tion being  less  marked  than  the  first  phase.  We  cannot,  however,  enter 
more  fully  here  into  a  discussion  of  this  difficult  subject. 

Whichever  view  be  taken  of  the  nature  of  these  phenomena,  it  is  im- 
portant to  remember  that  the  electrical  changes  are  closely  allied  to  the 
chemical  events  involved  in  the  contraction  of  the  muscle  and  to  the  change 
of  form  of  the  muscle.  Their  exact  relations  to  each  other  await  research. 

The  Changes  in  a  Nerve  during  the  Passage  of  a  Nervous  Impulse. 

§  68.  The  change  in  the  form  of  a  muscle  during  its  contraction  is  a 
thing  which  can  be  seen  and  felt ;  but  the  changes  in  a  nerve  during  its 
activity  are  invisible  and  impalpable.  We  stimulate  one  end  of  a  nerve 
going  to  a  muscle,  and  we  see  this  followed  by  a  contraction  of  the  muscle 
attached  to  the  other  end  ;  or  we  stimulate  a  nerve  still  connected  with  the 
central  nervous  system,  and  we  see  this  followed  by  certain  movements,  or 
by  other  tokens  which  show  that  disturbances  have  been  set  up  in  the  cen- 
tral nervous  system.  We  know,  therefore,  that  some  changes  or  other,  con- 
stituting what  we  have  called  a  nervous  impulse,  have  been  propagated 
along  the  nerve  ;  but  the  changes  are  such  as  we  cannot  see.  It  is  possible, 
however,  to  learn  something  about  them. 

§  69.  The  chemistry  of  a  nerve.  We  have  spoken  of  the  medulla  as  fatty, 
and  yet  it  is  in  reality  very  largely  composed  of  a  substance  which  is  not  (in 
the  strict  sense  of  the  word)  a  fat.  When  we  examine  chemically  a  quantity 
of  nerve  (or  what  is  practically  the  same  thing,  a  quantity  of  that  part  of  the 
central  nervous  system  which  is  called  white  matter,  and  which  as  we  shall 
see  is  chiefly  composed,  like  a  nerve,  of  medullated  nerves,  and  is  to  be  pre- 
ferred for  chemical  examination  because  it  contains  a  relatively  small  quantity 
of  connective  tissue),  we  find  that  a  very  large  proportion,  according  to  some 
observers  about  half,  of  the  dried  matter  consists  of  a  peculiar  body, 
cholesterin.  Now,  cholesterin  is  not  a  fat  but  an  alcohol ;  like  glycerin,  how- 
ever, which  is  also  an  alcohol,  it  forms  compounds  with  fatty  acids ;  and 
though  we  do  not  know  definitely  the  chemical  condition  in  which  cholesterin 
exists  during  life  in  the  medulla,  it  is  more  than  probable  that  it  exists  in 
some  combination  w7ith  some  of  the  really  fatty  bodies  also  present  in  the 
medulla,  and  not  in  a  free  isolated  state.  It  is  singular  that  besides  being 
present  in  such  large  quantities  in  nervous  tissue,  and  to  a  small  extent  in 
other  tissues  and  in  blood,  cholesterin  is  a  normal  constituent  of  bile,  and 
forms  the  greater  part  of  gall-stones  when  these  are  present ;  in  gall-stones  it 
is  undoubtedly  present  in  a  free  state.  Besides  cholesterin,  "  white"  nervous 
matter  contains  a  less  but  still  considerable  quantity  of  a  complex  fat,  whose 
nature  is  disputed.  According  to  some  authorities  rather  less  than  half  this 
complex  fat  consists  of  the  peculiar  body  lecithin,  which  we  have  already 
7 


98  THE  CONTRACTILE  TISSUES. 

seen  to  be  present  also  in  blood  corpuscles  and  in  muscle.  Lecithin  contains 
the  radicle  of  stearic  acid  (or  of  oleic,  or  of  palmitic  acid)  associated  not,  as 
in  ordinary  fats,  with  simple  glycerin,  but  with  the  more  complex  glycerin- 
phosphoric  acid,  and  further  combined  with  a  nitrogenous  body,  neurin, 
an  ammonia  compound  of  some  considerable  complexity ;  it  is  therefore  of 
remarkable  nature,  since,  though  a  fat,  it  contains  both  nitrogen  and  phos- 
phorus. According  to  the  same  authorities  the  remainder  of  the  complex 
fat  consists  of  another  fatty  body,  also  apparently  containing  nitrogen  but 
no  phosphorus,  called  cerebrin.  Other  authorities  regard  both  these  bodies, 
lecithin  and  cerebrin,  as  products  of  decomposition  of  a  still  more  complex 
fat,  called  protagon.  Obviously  the  fat  of  the  white  matter  of  the  central 
nervous  system  and  of  spinal  nerves  (of  which  fat  by  far  the  greater  part 
must  exist  in  the  medulla,  and  form  nearly  the  whole  of  the  medulla)  is  a 
very  complex  body  indeed,  especially  so  if  the  cholesterin  exists  in  combina- 
tion with  the  lecithin,  or  cerebrin  (or  protagon).  Being  so  complex  it  is 
naturally  very  unstable,  and  indeed,  in  its  stability  resembles  proteid  matter. 
Hence,  probably,  the  reason  why  the  medulla  changes  so  rapidly  and  so  pro- 
foundly after  the  death  of  the  nerve. 

The  presence  in  such  large  quantity  of  this  complex  fatty  medulla  renders 
the  chemical  examination  of  the  other  constituents  of  a  nerve  very  difficult, 
and  our  knovyledge  of  the  chemical  nature  of,  and  of  the  chemical  changes 
going  on  in  the  axis-cylinder,  is  very  limited.  Examined  under  the  micro- 
scope the  axis-cylinder  gives  the  xanthoproteic  reaction  and  other  indications 
that  it  is  proteid  in  nature  ;  beyond  this  we  are  largely  confined  to  inferences. 

After  the  fats  of  the  medulla  (and  the  much  smaller  quantity  of  fat 
present  in  the  axis-cylinder),  the  proteids  of  the  axis-cylinder,  and  the  other 
soluble  substances  present  in  one  or  the  other,  or  gathered  round  the  nuclei 
of  the  neurilemma,  have  by  various  means  been  dissolved  out  of  a  nerve 
fibre,  certain  substances  still  remain.  One  of  these  in  small  quantity  is  the 
nuclein  of  the  nuclei ;  another  in  larger  quantity  is  the  substance  neuro- 
keratin  which  forms,  as  we  have  seen,  a  supporting  framework  for  the 
medulla,  and  whose  most  marked  characteristic  is,  perhaps,  its  resistance 
to  solution. 

In  the  ash  of  nerves  there  is  a  preponderance  of  potassium  salts  and 
phosphates,  but  not  so  marked  as  in  the  case  of  muscle. 

§  70.  The  nervous  impulse. — The  chemical  analogy  between  the  substance 
of  the  muscle  and  that  of  the  axis-cylinder  would  naturally  lead  us  to  sup- 
pose that  the  progress  of  a  nervous  impulse  along  a  nerve  fibre  was  accom- 
panied by  chemical  changes  similar  to  those  taking  place  in  a  muscle  fibre. 
Whatever  changes,  however,  do  or  may  take  place  are  too  slight  to  be 
recognized  by  the  means  at  our  disposal.  We  have  no  satisfactory  evidence 
that  in  a  nerve  even  repeated  nervous  impulses  can  give  rise  to  an  acid 
reaction,  or  that  the  death  of  a  nerve  fibre  leads  to  such  reaction.  The  gray 
matter  of  the  central  nervous  system,  it  is  true,  is  said  to  be  slightly  acid 
during  life  and  to  become  more  acid  after  death  ;  but  in  this  gray  matter, 
nerve  cells  are  relatively  abundant ;  the  white  matter,  composed  chiefly  of 
nerve  fibre,  is  and  remains,  during  action  as  well  as  rest,  and  even  after 
death,  neutral  or  slightly  alkaline. 

Nor  have  we  satisfactory  evidence  that  the  progress  of  a  nervous  impulse 
is  accompanied  by  any  setting  free  of  energy  in  the  form  of  heat. 

In  fact,  beyond  the  terminal  results,  such  as  a  muscular  contraction  in  the 
case  of  a  nerve  going  to  a  muscle,  or  some  affection  of  the  central  nervous 
system  in  the  case  of  a  nerve  still  in  connection  with  its  nervous  centre, 
there  is  one  event  and  one  event  only  which  we  are  able  to  recognize  as  the 
objective  token  of  a  nervous  impulse,  and  that  is  an  electric  change.  For  a 


CHANGES  IN  A  MUSCLE  DUKING  CONTRACTION.  99 

piece  of  nerve  removed  from  the  body  exhibits  nearly  the  same  electric 
phenomena  as  a  piece  of  muscle.  It  has  an  equator  which  is  electrically 
positive  relatively  to  the  two  cut  ends.  In  fact,  the  diagram  Fig.  30,  and 
the  description  which  was  given  in  §  66  of  the  electric  changes  in  muscle 
may  be  applied  almost  as  well  to  a  nerve,  except  that  the  currents  are  in 
all  cases  much  more  feeble  in  the  case  of  nerves  than  of  muscles,  and  the 
special  currents  from  the  circumference  to  the  centre  of  the  transverse  sec- 
tions cannot  well  be  shown  in  a  slender  nerve ;  indeed,  it  is  doubtful  if  they 
exist  at  all. 

During  the  passage  of  a  nervous  impulse  the  "natural  nerve  current" 
undergoes  a  negative  variation,  just  as  the  "  natural  muscle  current "  under- 
goes a  negative  variation  during  a  contraction.  There  are,  moreover,  reasons 
in  the  case  of  the  nerve,  as  in  the  case  of  the  muscle,  which  lead  us  to  doubt 
the  pre-existence  of  any  such  "  natural  "  currents.  A  nerve  in  an  absolutely 
natural  condition  appears  to  be,  like  a  muscle,  isoelectric ;  hence  we  may 
say  that  in  a  nerve  during  the  passage  of  a  nervous  impulse,  as  in  a  mus- 
cle during  a  muscular  contraction,  a  "  current  of  action  "  is  developed. 

This  "  current  of  action  "  or  "  negative  variation  "  may  be  shown  either 
by  a  galvanometer  or  by  the  rheoscopic  frog.  If  the  nerve  of  the  "  muscle 
nerve  preparation,"  B  (see  §  67)  be  placed  in  an  appropriate  manner  on  a 
thoroughly  irritable  nerve,  A  (to  which,  of  course,  no  muscle  need  be  attached), 
touching  for  instance  the  equator  and  one  end  of  the  nerve,  then  single 
induction-shocks  sent  into  the  far  end  of  A  will  cause  single  spasms  in  the 
muscle  of  J5,  while  tetanization  of  A,  i.  e.}  rapidly  repeated  shocks  sent  into 
A,  will  cause  tetanus  of  the  muscle  of  B. 

That  this  current,  whether  it  be  regarded  as  an  independent  "  current  of 
action  "  or  as  a  negative  variation  of  a  "  pre-existing  "  current,  is  an  essential 
feature  of  a  nervous  impulse  is  shown  by  the  fact  that  the  degree  of  intensity 
of  the  one  varies  with  that  of  the  other.  They  both  travel,  too,  at  the  same 
rate.  In  describing  the  muscle-curve,  and  the  method  of  measuring  the 
muscular  latent  period,  we  have  incidentally  shown  (§  46)  how  at  the  same 
time  the  velocity  of  the  nervous  impulse  may  be  measured,  and  stated  that 
the  rate  in  the  nerves  of  a  frog  is  about  28  metres  per  second.  By  means  of 
a  special  and  somewhat  complicated  apparatus  it  is  ascertained  that  the  cur- 
rent of  action  travels  along  an  isolated  piece  of  nerve  at  the  same  rate.  It 
also,  like  the  molecular  change  in  a  muscle  preceding  the  contraction,  and 
indeed  like  the  contraction  itself,  travels  in  the  form  of  a  wave,  rising  rapidly 
to  a  maximum  at  each  point  of  the  nerve  and  then  more  gradually  declin- 
ing again.  The  length  of  the  wave  may  by  special  means  be  measured,  and 
is  found  to  be  about  18  mm. 

When  an  isolated  piece  of  nerve  is  stimulated  in  the  middle,  the  current 
of  action  is  propagated  equally  well  in  both  directions,  and  that  whether  the 
nerve  be  a  chiefly  sensory  or  a  chiefly  motor  nerve,  or  indeed  if  it  be  a  nerve- 
root  composed  exclusively  of  motor  or  of  sensory  fibres.  Taking  the  current 
of  action  as  the  token  of  a  nervous  impulse,  we  infer  from  this  that  when  a 
nerve  fibre  is  stimulated  artificially  at  any  part  of  its  course,  the  nervous 
impulse  set  going  travels  in  both  directions. 

We  used  just  now  the  phrase  "  tetanization  of  a  nerve,"  meaning  the 
application  to  a  nerve  of  rapidly  repeated  shocks  such  as  would  produce 
tetanus  in  the  muscle  to  which  the  nerve  was  attached,  and  we  shall  have 
frequent  occasion  to  employ  the  phrase.  It  must,  however,  be  understood 
that  there  is  in  the  nerve,  in  an  ordinary  way,  no  summation  of  nervous 
impulses  comparable  to  the  summation  of  muscular  contractions.  Putting 
aside  certain  cases  which  we  cannot  discuss  here,  we  may  say  that  the  series 
of  shocks  sent  in  at  the  far  end  of  the  nerve  start  a  series  of  impulses ;  these 


100  THE   CONTRACTILE  TISSUES. 

travel  down  the  nerve  and  reach  the  muscle  as  a  series  of  distinct  impulses  ; 
and  the  first  changes  in  the  muscle,  the  molecular  latent-period  changes, 
also  form  a  series  the  members  of  which  are  distinct.  It  is  not  until  these 
molecular  changes  become  transformed  into  visible  changes  of  form  that  any 
fusion  or  summation  takes  place. 

§  71.  Putting  together  the  facts  contained  in  this  and  the  preceding  sec- 
tions, the  following  may  be  taken  as  a  brief  approximate  history  of  what 
takes  place  in  a  muscle  and  nerve  when  the  latter  is  subjected  to  a  single 
induction-shock.  At  the  instant  that  the  induced  current  passes  into  the 
nerve,  changes  occur,  of  whose  nature  we  know  nothing  certain,  except  that 
they  cause  a  "  current  of  action  "  or  "  negative  variation  "  of  the  "  natural  " 
nerve-current.  These  changes  propagate  themselves  along  the  nerve  in  both 
directions  as  a  nervous  impulse  in  the  form  of  a  wave,  having  a  wave-length 
of  about  18  mm.,  and  a  velocity  (in  frog's  nerve)  of  about  28  m.  per  second. 
Passing  down  the  nerve  fibres  to  the  muscle,  flowing  along  the  branching 
and  narrowing  tracts,  the  wave  at  last  breaks  on  the  end-plates  of  the  fibres 
of  the  muscle.  Here  it  is  transmitted  into  what  we  may  call  a  muscle 
impulse,  with  a  shorter,  steeper  wave,  and  a  greatly  diminished  velocity 
(about  3  m.  per  second).  This  muscle  impulse,  of  which  we  know  hardly 
more  than  that  it  is  marked  by  a  current  of  action,  travels  from  each  end- 
plate  in  both  directions  to  the  end  of  the  fibre,  where  it  appears  to  be  lost ; 
at  all  events,  we  do  not  know  what  becomes  of  it.  As  this  impulse  wave, 
whose  development  takes  place  entirely  within  the  latent  period,  leaves  the 
end-plate,  it  is  followed  by  an  explosive  decomposition  of  material,  leading 
to  a  discharge  of  carbonic  acid,  to  the  appearance  of  some  substance  or 
substances  with  an  acid  reaction,  and  probably  of  other  unknown  things, 
with  a  considerable  development  of  heat.  This  explosive  decomposition  gives 
rise  to  the  visible  contraction  wave,  which  travels  behind  the  invisible  mus- 
cle impulse  at  about  the  same  rate,  but  with  a  vastly  increased  wave-length. 
The  fibre,  as  the  wave  passes  over  it,  swells  and  shortens,  and  thus  brings  its 
two  ends  nearer  together. 

When  repeated  shocks  are  given,  wave  follows  wave  of  nervous  impulse, 
muscle  impulse,  and  visible  contraction  ;  but  the  last  do  not  keep  distinct ; 
they  are  fused  into  the  continued  shortening  which  we  call  tetanus. 

THE  NATURE  OF  THE  CHANGES  THROUGH  WHICH  AN  ELECTRIC  CUR- 
RENT is  ABLE  TO  GENERATE  A  NERVOUS  IMPULSE. 

Action  of  the   Constant   Current. 

§  72.  In  the  preceding  account,  the  stimulus  applied  in  order  to  give 
rise  to  a  nervous  impulse  has  always  been  supposed  to  be  an  induction-shock, 
single  or  repeated.  This  choice  of  stimulus  has  been  made  on  account  of 
the  almost  momentary  duration  of  the  induced  current.  Had  we  used  a 
current  lasting  for  some  considerable  time,  the  problems  before  us  would 
have  become  more  complex,  in  consequence  of  our  having  to  distinguish 
between  the  events  taking  place  while  the  current  was  passing  through  the 
nerve  from  those  which  occurred  at  the  moment  when  the  current  was 
thrown  into  the  nerve  or  at  the  moment  when  it  was  shut  off  from  the  nerve. 
These  complications  do  arise  when,  instead  of  employing  the  induced  current 
as  a  stimulus,  we  use  a  constant  current,  i.  e.,  when  we  pass  through  the  nerve 
(or  muscle)  a  current  direct  from  the  battery  without  the  intervention  of 
any  induction-coil. 

'Before  making  the  actual  experiment,  we  might,  perhaps,  naturally  sup- 
pose that  the  constant  current  would  act  as  a  stimulus  throughout  the  whole 


STIMULUS   BY   ELECTRIC  CURRENT.  101 

time  during  which  it  was  applied ;  that,  so  long  as  the  current  passed  along 
the  nerve,  nervous  impulses  would  be  generated  ;  and  that  these  would  throw 
the  muscle  into  something,  at  all  events,  like  tetanus.  And,  under  certain 
conditions,  this  does  take  place ;  occasionally  it  does  happen  that  at  the 
moment  the  current  is  thrown  into  the  nerve  the  muscle  of  the  muscle-nerve 
preparation  falls  into  a  tetanus,  which  is  continued  until  the  current  is  shut 
off ;  but  such  a  result  is  exceptional.  In  the  vast  majority  of  cases  what 
happens  is  as  follows :  At  the  moment  that  the  circuit  is  made,  the  mo- 
ment that  the  current  is  thrown  into  the  nerve,  a  single  twitch,  a  simple 
contraction,  the  so-called  making  contraction,  is  witnessed  ;  but  after  this  has 
passed  away  the  muscle  remains  absolutely  quiescent,  in  spite  of  the  current 
continuing  to  pass  through  the  nerve,  and  this  quiescence  is  maintained  until 
the  circuit  is  broken,  until  the  current  is  shut  off  from  the  nerve,  when 
another  simple  contraction,  the  so-called  breaking  contraction,  is  observed. 
The  mere  passage  of  a  constant  current  of  uniform  intensity  through  a  nerve 
does  not,  under  ordinary  circumstances,  act  as  a  stimulus  generating  a  nerv- 
ous impulse  ;  such  an  impulse  is  only  set  up  when  the  current  either  falls  into 
or  is  shut  off  from  the  nerve.  It  is  the  entrance  or  the  exit  of  the  current, 
and  not  the  continuance  of  the  current,  which  is  the  stimulus.  The  quies- 
cence of  the  nerve  and  muscle  during  the  passage  of  the  current  is,  however, 
dependent  on  the  current  remaining  uniform  in  intensity,  or,  at  least,  not 
being  suddenly  increased  or  diminished.  Any  sufficiently  sudden  and  large 
increase  or  diminution  of  the  intensity  of  the  current  will  act  like  the 
entrance  or  exit  of  a  current,  and  by  generating  a  nervous  impulse  give  rise 
to  a  contraction.  If  the  intensity  of  the  current,  however,  be  very  slowly 
and  gradually  increased  or  diminished,  a  very  wide  range  of  intensity  may 
be  passed  through  without  any  contraction  being  seen.  It  is  the  sudden 
change  from  one  condition  to  another,  and  not  the  condition  itself,  which 
causes  the  nervous  impulse. 

In  many  cases,  both  a  "  making  "  and  a  "  breaking  "  contraction,  each  a 
simple  twitch,  are  observed,  and  this  is,  perhaps,  the  commonest  event ;  but 
when  the  current  is  very  weak,  and  again  when  the  current  is  very  strong, 
either  the  breaking  or  the  making  contraction  may  be  absent ;  i.  e.,  there 
may  be  a  contraction  only  when  the  current  is  thrown  into  the  nerve,  or 
only  when  it  is  shut  off  from  the  nerve. 

Under  ordinary  circumstances  the  contractions  witnessed  with  the  con- 
stant current  either  at  the  make  or  at  the  break,  are  of  the  nature  of  a 
"  simple  "  contraction  ;  but,  as  has  already  been  said,  the  application  of  the 
current  may  give  rise  to  very  pronounced  tetanus.  Such  a  tetanus  is  seen 
sometimes  when  the  current  Is  made,  lasting  during  the  application  of  the 
current,  sometimes  when  the  current  is  broken,  lasting  some  time  after  the 
current  has  been  wholly  removed  from  the  nerve.  The  former  is  spoken  of 
as  a  "  making,"  the  latter  as  a  "  breaking  "  tetanus.  But  these  exceptional 
results  of  the  application  of  the  constant  current  need  not  detain  us  now. 

The  great  interest  attached  to  the  action  of  the  constant  current  lies  in 
the  fact  that,  during  the  passage  of  the  current,  in  spite  of  the  absence  of 
all  nervous  impulses,  and  therefore  of  all  muscular  contractions,  the  nerve 
is  for  the  time  both  between  and  on  each  side  of  the  electrodes  profoundly 
modified  in  a  most  peculiar  manner.  This  modification,  important  both  for 
the  light  it  throws  on  the  generation  of  nervous  impulses  and  for  its  practi- 
cal applications,  is  known  under  the  name  of  electrotonus. 

§  73.  Electrotonus. — The  marked  feature  of  the  electrotonic  condition  is 
that  the  nerve,  though  apparently  quiescent,  is  changed  in  respect  to  its  irri- 
tability ;  and  that  in  a  different  way  in  the  neighborhood  of  the  two  elec- 
trodes respectively. 


102  THE  CONTRACTILE  TISSUES. 

Suppose  that  on  the  nerve  of  a  muscle-nerve  preparation  are  placed  two 
(non-polarizable)  electrodes  (Fig.  32,  a,  &),  connected  with  a  battery  and 
arranged  with  a  key,  so  that  a  constant  current  can  at  pleasure  be  thrown 
into  or  shut  off  from  the  nerve.  This  constant  current,  whose  effects  we  are 
about  to  study,  may  be  called  the  "  polarizing  current."  Let  a  be  the  posi- 
tive electrode  or  anode,  arid  k  the  negative  electrode  or  kathode,  both  placed 
at  some  distance  from  the  muscle,  and  also  with  a  certain  interval  between 
each  other.  At  the  point  x  let  there  be  applied  a  pair  of  electrodes  con- 
nected with  an  induction-coil.  Let  the  muscle  further  be  connected  with  a 
lever,  so  that  its  contractions  can  be  recorded  and  their  amount  measured. 
Before  the  polarizing  current  is  thrown  into  the  nerve,  let  a  single  induction- 
shock  of  known  intensity  (a  weak  one  being  chosen,  or,  at  least,  not  one 
which  would  cause  in  the  muscle  a  maximum  contraction)  be  thrown  in  at 
x.  A  contraction  of  a  certain  amount  will  follow.  The  contraction  may 

FIG.  32. 


Muscle-nerve  Preparations :  with  the  nerve  exposed  in  A  to  a  descending  and  in  B  to  an  ascend- 
ing constant  current.  In  each,  a  is  the  anode,  k  the  kathode  of  the  constant  current ;  x  represents 
the  spot  where  the  induction-shocks,  used  to  test  the  irritability  of  the  nerve,  are  sent  in. 

be  taken  as  a  measure  of  the  irritability  of  the  nerve  at  the  point  x.  Now 
let  the  polarizing  current  be  thrown  in  and  let  the  kathode  or  negative  pole 
be  nearest  the  muscle,  as  in  Fig.  32,  A,  so  that  the  current  passes  along  the 
nerve  in  a  direction  from  the  central  nervous  system  toward  the  muscle ; 
such  a  current  is  spoken  of  as  a  descending  one.  The  entrance  of  the  polar- 
izing current  into  the  nerve  will  produce  a  "  making"  contraction  ;  this  we 
may  neglect.  If  while  the  current  is  passing  the  same  induction-shock  as 
before  be  sent  through  x,  the  contraction  which  results  will  be  found  to  be 
greater  than  on  the  former  occasion.  If  the  polarizing  current  be  now  shut 
off,  a  "breaking"  contraction  will  probably  be  produced;  this  we  also  may 
neglect.  If,  now,  the  point  x,  after  a  short  interval,  be  again  tested  with  the 
same  induction-shock  as  before,  the  contraction  will  be  no  longer  greater, 
but  of  the  same  amount,  or  perhaps  not  so  great  as  at  first.  During  the 
passage  of  the  polarizing  current,  therefore,  the  irritability  of  the  nerve  at 
the  point  x  has  been  temporarily  increased,  since  the  same  shock  applied  to 
it  causes  a  greater  contraction  during  the  presence  than  in  the  absence  of  the 
current.  But  this  is  only  true  so  long  as  the  polarizing  current  is  a  descend- 
ing one — so  long  as  the  point  x  lies  on  the  side  of  the  kathode.  On  the  other 
hand,  if  the  polarizing  current  had  been  an  ascending  one,  with  the  anode  or 


STIMULUS  BY   ELECTRIC  CURRENT.  103 

positive  pole  nearest  the  muscle,  as  in  Fig.  32,  B,  the  irritability  of  the  nerve 
at  x  would  have  been  found  to  be  diminished,  instead  of  increased,  by  the  polar- 
izing current ;  the  contraction  obtained  during  the  passage  of  the  constant 
current  would  be  less  than  before  the  passage  of  the  current,  or  might  be 
absent  altogether,  and  the  contraction  after  the  current  had  been  shut  off 
would  be  as  great,  or  perhaps  greater,  than  before.  That  is  to  say,  when  a 
constant  current  is  applied  to  a  nerve,  the  irritability  of  the  nerve  between 
the  polarizing  electrodes  and  the  muscle  is,  during  the  passage  of  the 
current,  increased  when  the  kathode  is  nearest  the  muscle  (and  the  polariz- 
ing current  descending)  and  diminished  when  the  anode  is  nearest  the 
muscle  (and  the  polarizing  current  ascending).  The  same  result,  mutatis 
mutandis,  and  with  some  qualifications  which  we  need  not  discuss,  would  be 
gained  if  x  were  placed,  not  between  the  muscle  and  the  polarizing  current, 
but  on  the  far  side  of  the  latter.  Hence,  it  may  be  stated  generally  that 
during  the  passage  of  a  constant  current  through  a  nerve  the  irritability  of 
the  nerve  is  increased  in  the  region  of  the  kathode,  and  diminished  in  the 
region  of  the  anode.  The  changes  in  the  nerve  which  give  rise  to  this 
increase  of  irritability  in  the  region  of  the  kathode  are  spoken  of  as  kat- 
electrotonus,  and  the  nerve  is  said  to  be  in  a  katelectrotonic  condition.  Simi- 
larly the  changes  in  the  region  of  the  anode  are  spoken  of  as  anelectrotonus, 
and  the  nerve  is  said  to  be  in  an  anelectrotonic  condition.  It  is  also  often 
usual  to  speak  of  the  katelectrotonic  increase,  and  anelectrotonic  decrease  of 
irritability. 

This  law  remains  true  whatever  be  the  mode  adopted  for  determining 
the  irritability.  The  result  holds  good  not  only  with  a  single  induction- 
shock,  but  also  with  a  tetanizing  interrupted  current,  with  chemical  and 
mechanical  stimuli.  It  further  appears  to  hold  good  not  only  in  a  dissected 
nerve-muscle  preparation  but  also  in  the  intact  nerves  of  the  living  body. 
The  increase  and  decrease  of  irritability  are  most  marked  in  the  immediate 
neighborhood  of  the  electrodes,  but  spread  for  a  considerable  distance  in 
each  direction  in  the  extrapolar  regions.  The  same  modification  is  not  con- 
fined to  the  extrapolar  region,  but  exists  also  in  the  intrapolar  region.  In 
the  intrapolar  region  there  must  be,  of  course,  a  neutral  or  indifferent  point, 
where  the  katelectrotonic  increase  merges  into  the  anelectrotonic  decrease, 


Diagram  illustrating  the  Variations  of  Irritability  during  Electrotonus,  with  Polarizing  Cur- 
rents of  Increasing  Intensity.  (From  Pfluger )  The  anode  is  supposed  to  be  placed  at  A,  the 
kathode  at  B  ;  AB  is  consequently  the  intrapolar  district.  In  each  of  the  three  curves,  the  por- 
tion of  the  curve  below  the  base  line  represents  diminished  irritability,  that  above,  increased  ir- 
ritability. y\  represents  the  effect  of  a  weak  current;  the  indifferent  point  x\  is  near  the  anode  A. 
In  2/2,  a  stronger  current,  the  indifferent  point  x*  is  nearer  the  kathode  B,  the  diminution  of  irri- 
tability in.anelectrotonusandthe  increase  in  katelectrotonus  being  greater  than  In  y\ ;  the  effect 
also  spreads  for  a  greater  distance  along  the  extrapolar  regions  in  both  directions.  In  ys  the  same 
events  are  seen  to  be  still  more  marked, 

and  where,  therefore,  the  irritability  is  unchanged.     When  the  polarizing 
current  is  a  weak  one,  this  indifferent  point  is  nearer  the  anode  than  the 


104 


THE  CONTRACTILE  TISSUES. 


kathode,  but  as  the  polarizing  current  increases  in  intensity,  draws  nearer 
and  nearer  the  kathode  (see  Fig.  33). 

The  amount  of  increase  and  decrease  is  dependent :  (1 )  On  the  strength 
of  the  current,  the  stronger  current  up  to  a  certain  limit  producing  the 
greater  effect.  (2)  On  the  irritability  of  the  nerve,  the  more  irritable, 
better  conditioned  nerve  being  the  more  affected  by  a  current  of  the  same 
intensity. 

In  the  experiments  just  described  the  increase  or  decrease  of  irritability 
is  taken  to  mean  that  the  same  stimulus  starts  in  the  one  case  a  larger  or 
more  powerful,  and  in  the  other  case  a  smaller  or  less  energetic  impulse ; 
but  we  have  reason  to  think  that  the  mere  propagation  or  condition  of  im- 
pulses started  elsewhere  is  also  affected  by  the  electrotonic  condition.  At  all 
events  anelectrotonus  appears  to  offer  an  obstacle  to  the  passage  of  a  nervous 
impulse. 

$  74.  Electrotonic  currents.  During  the  passage  of  a  constant  current  through 
IL  nerve,  variations  in  the  electric  currents  belonging  to  the  nerve  itself  may  be 


G  H 

Diagram  illustrating  Electrotonic  Currents :  P  the  polarizing  battery,  with  k  a  key,  p  the 
anode,  andp'  the  kathode.  At  the  left  end  of  the  piece  of  nerve  the  natural  current  flows  through 
the  galvanometer  G  from  g  to  g',  in  the  direction  of  the  arrows ;  its  direction,  therefore,  is  the 
same  as  that  of  the  polarizing  current ;  consequently  it  appears  increased,  as  indicated  by  the 
sign  -K  The  current  at  the  other  end  of  the  piece  of  nerve,  from  h  to  h'  through  the  galvan- 
ometer H,  flows  in  a  contrary  direction  to  the  polarizing  current;  it  consequently  appears  to  be 
diminished,  as  indicated  by  the  sign  — . 

N.  B.— For  simplicity's  sake,  the  polarizing  current  is  here  supposed  to  be  thrown  in  at  the 
middle  of  a  piece  of  nerve,  and  the  galvanometer  placed  at  the  two  ends.  Of  course  it  will  be 
understood  that  the  former  may  be  thrown  in  anywhere,  and  the  latter  connected  with  any  two 
pairs  of  points  which  will  give  currents. 

observed  ;  and  these  variations  have  certain  relations  to  the  variations  of  the  irri- 
tability of  the  nerve.  Thus  if  a  constant  current  supplied  by  the  battery  P 
(Fig.  34)  be  applied  to  a  piece  of  nerve  by  means  of  two  non-polarizable  electrodes 


STIMULUS  BY   ELECTRIC  CURRENT.  105 

p.  p',  the  "  currents  of  rest  "  obtainable  from  the  various  points  of  the  nerve  will 
be  'different  during  the  passage  of  the  polarizing  current  from  those  which  were 
manifest  before  or  after  the  current  was  applied  ;  and,  moreover,  the  changes  in  the 
nerve-currents  produced  by  the  polarizing  current  will  not  be  the  same  in  the 
neighborhood  of  the  anode  (p)  as  those  in  the  neighborhood  of  the  kathode  (p/). 
Thus,  let  G  and  H  be  two  galvanometers  so  connected  with  the  two  ends  of  the 
nerve  as  to  afford  good  and  clear  evidence  of  the  ''currents  of  rest."  Before  the 
polarizing  current  is  thrown  into  the  nerve,  the  needle  of  H  will  occupy  a  posi- 
tion indicating  the  passage  of  a  current  of  a  certain  intensity  from  h  to  h' 
though  the  galvanometer  (from  the  positive  longitudinal  surface  to  the  negative 
cut  end  of  the  nerve),  the  circuit  being  completed  by  a  current  in  the  nerve  from 
Ax  to /i,  i.  e.,  the  current  will  flow  in  the  direction  of  the  arrow.  Similarly  the 
needle  of  G  will,  by  its  deflection,  indicate  the  existence  of  a  current  flowing  from 
g  to  //through  the  galvanometer,  and  from  g/  to  g  through  the  nerve,  in  the 
direction  of  the  arrow. 

At  the  instant  that  the  polarizing  current  is  thrown  into  the  nerve  at  p  p',  the 
currents  at  gg',  hh'  will  undergo  a  "negative  variation,"  that  is,  the  nerve  at 
each  point  will  exhibit  a  "current  of  action"  corresponding  to  the  nervous  im- 
pulse, which,  at  the  making  of  the  polarizing  current,  passes  in  both  directions 
along  the  nerve,  and  may  cause  a  contraction  in  the  attached  muscle.  The  cur- 
rent of  action  is,  as  we  have  seen,  of  extremely  short  duration,  it  is  over  and 
gone  in  a  small  fraction  of  a  second.  It,  therefore,  must  not  be  confounded  with 
a  permanent  effect  which,  in  the  case  we  are  dealing  with,  is  observed  in  both 
galvanometers.  This  effect,  which  is  dependent  on  the  direction  of  the  polariz- 
ing current,  is  as  follows :  Supposing  that  the  polarizing  current  is  flowing  in  the 
direction  of  the  arrow  in  the  figure,  that  is,  passes  in  the  nerve  from  the  positive 
electrode  or  anode  p  to  the  negative  electrode  or  kathode  p',  it  is  found  that  the 
current  through  the  galvanometer  G  is  increased,  while  that  through  His  dimin- 
ished. The  polarizing  current  has  caused  the  appearance,  in  the  nerve  outside 
the  electrodes,  of  a  current  having  the  same  direction  as  itself,  called  the  "elec- 
trotonic"  current;  and  this  electrotonic  current  adds  to,  or  takes  away  from,  the 
natural  nerve-current  or  "current  of  rest"  according  as  it  is  flowing  in  the  same 
direction  as  that  or  in  an  opposite  direction. 

The  strength  of  the  electrotonic  current  is  dependent  on  the  strength  of  the 
polarizing  current,  and  on  the  length  of  the  intrapolar  region  which  is  exposed 
to  the  polarizing  current.  When  a  strong  polarizing  current  is  used,  the  electro- 
motive force  of  the  electrotonic  current  may  be  much  greater  than  that  of  the 
natural  nerve-current. 

The  strength  of  the  electrotonic  current  varies  with  the  irritability,  or  vital 
condition  of  the  nerve,  being-  greater  with  the  more  irritable  nerve ;  and  a  dead 
nerve  will  not  manifest  electrotonic  currents.  Moreover,  the  propagation  of  the 
current  is  stopped  by  a  ligature,  or  by  crushing  the  nerve. 

We  may  speak  of  the  conditions  which  give  rise  to  this  electrotonic  current  as 
a  physical  electrotonus  analogous  to  that  physiological  electrotonus  which  is  made 
known  by  variations  in  irritability.  The  physical  electrotonic  current  is  probably 
due  to  the  escape  of  the  polarizing  current  along  the  nerve  under  the  peculiar 
conditions  of  the  living  nerve  ;  but  we  must  not  attempt  to  enter  here  into  this 
difficult  subject  or  into  the  allied  question  as  to  the  exact  connection  between  the 
physical  and  the  physiological  electrotonus,  though  there  can  be  little  doubt  that 
the  latter  is  dependent  on  the  former. 

§  75.  These  variations  of  irritability  at  the  kathode  and  anode  respect- 
ively, thus  brought  about  by  the  action  of  the  constant  current,  are  inter- 
esting theoretically,  because  we  may  trace  a  connection  between  them  and 
the  nervous  impulse  which  is  the  result  of  the  making  or  breaking  of  a 
constant  current. 

For  we  have  evidence  that  a  nervous  impulse  is  generated  when  a  portion 
of  the  nerve  passes  suddenly  from  a  normal  condition  to  a  state  of  katelec- 
trotonus  or  from  a  state  of  anelectrotonus  back  to  a  normal  condition,  but 
that  the  passage  from  a  normal  condition  to  anelectrotonus  or  from  katelec- 
trotonus  back  to  a  normal  condition  is  unable  to  generate  an  impulse. 
Hence  when  a  constant  current  is  "  made  "  the  impulse  is  generated  only  at 


106  THE  CONTRACTILE  TISSUES. 

the  kathode  where  the  nerve  passes  suddenly  into  katelectrotonus ;  when 
the  current  on  the  other  hand  is  "  broken  "  the  impulse  is  generated  only  at 
the  anode  where  the  nerve  passes  suddenly  back  from  anelectrotonus  into 
a  normal  condition.  We  have  an  indirect  proof  of  this  in  the  facts  to 
which  we  drew  attention  a  little  while  back,  viz.,  that  a  contraction  some- 
times occurs  at  the  "  breaking"  only,  sometimes  at  the  "  making"  only  of 
the  constant  current,  sometimes  at  both.  For  it  is  found  that  this  depends 
partly  on  the  strength  of  the  current  in  relation  to  the  irritability  of  the 
nerve,  partly  on  the  direction  of  the  current,  whether  ascending  or  descend- 
ing ;  and  the  results  obtained  with  strong,  medium  and  weak  descending 
and  ascending  currents  have  been  stated  in  the  form  of  a  "  law  of  contrac- 
tion." We  need  not  enter  into  the  details  of  this  "  law,"  but  will  merely 
say  that  the  results  which  it  formulates  are  best  explained  by  the  hypothe- 
sis just  stated.  We  may  add  that  when  the  constant  current  is  applied  to 
certain  structures  composed  of  plain  muscular  fibres,  whose  rate  of  contrac- 
tion we  have  seen  to  be  slow,  the  making  contraction  may  be  actually  seen 
to  begin  at  the  kathode  and  travel  toward  the  anode,  and  the  breaking  con- 
traction to  begin  at  the  anode  and  travel  thence  toward  the  kathode. 

Since  in  katelectrotonus  the  irritability  is  increased,  and  in  anelectrotonus 
decreased,  both  the  entrance  from  the  normal  condition  into  katelectrotonus 
and  the  return  from  anelectrotonus  to  the  normal  condition  are  instances  of 
a  passage  from  a  lower  stage  of  irritability  to  a  higher  stage  of  irritability. 
Hence,  the  phenomena  of  electrotonus  would  lead  us  to  the  conception  that 
a  stimulus  in  provoking  a  nervous  impulse  produces  its  effect  by,  in  some 
way  or  other,  suddenly  raising  the  irritability  to  a  higher  pitch.  But  what 
we  are  exactly  to  understand  by  raising  the  irritability,  what  molecular 
change  is  the  cause  of  the  rise,  and  how  either  electric  or  other  stimuli  can 
produce  this  change,  are  matters  we  cannot  discuss  here. 

Besides  their  theoretical  importance,  the  phenomena  of  electrotonus  have 
also  a  practical  interest.  When  an  ascending  current  is  passed  along  a  nerve 
going  to  a  muscle  or  group  of  muscles,  the  region  between  the  electrodes  and 
the  muscle  is  thrown  into  anelectrotonus  and  its  irritability  is  diminished. 
If  the  current  be  of  adequate  strength,  the  irritability  may  be  so  much 
lessened  that  nervous  impulses  cannot  be  generated  in  that  part  of  the  nerve 
or  cannot  pass  along  it.  Hence,  by  this  means  the  irregular  contractions  of 
muscles  known  as  "cramp"  may  be  abolished.  Similarly,  by  bringing  into 
a  condition  of  anelectrotonus  a  portion  of  a  sensory  nerve  in. which  violent 
impulses  are  being  generated,  giving  rise  in  the  central  nervous  system  to 
sensations  of  pain,  the  impulses  are  toned  down  or  wholly  abolished,  and  the 
pain  ceases.  So,  on  the  other  hand,  we  may  at  pleasure  heighten  the  irrita- 
bility of  a  part  by  throwing  it  into  katelectrotonus.  In  this  way  the  con- 
stant current,  properly  applied,  becomes  a  powerful  remedial  means. 

Lastly,  though  we  are  dealing  now  with  nerves  going  to  muscles — that 
is  to  say,  with  motor  nerves  only — we  may  add  that  what  we  have  said  about 
electrotonus  and  the  development  of  nervous  impulses  by  it  appears  to  apply 
equally  well  to  sensory  nerves. 

§  76.  In  a  general  way  muscular  fibres  behave  toward  an  electric  cur- 
rent very  much  as  do  nerve  fibres ;  but  there  are  certain  important  dif- 
ferences. 

In  the  first  place,  muscular  fibres,  devoid  of  nerve  fibres,  are  much  more 
readily  thrown  into  contractions  by  the  breaking  and  making  of  a  constant 
current  than  by  the  more  transient  induction-shock;  the  muscular  substance 
seems  to  be  more  sluggish  than  the  nervous  substance,  and  requires  to  be 
acted  upon  for  a  longer  time.  This  fact  may  be  made  use  of,  and,  indeed, 
is  in  medical  practice  made  use  of,  to  determine  the  condition  of  the  nerves 


THE  MUSCLE-NERVE  PREPARATION   AS  A   MACHINE.         107 

supplying  a  muscle.  If  the  intra-muscular  nerves  be  still  in  good  condition, 
the  muscle,  as  a  whole,  responds  readily  to  single  induction-shocks,  because 
these  can  act  upon  the  intra-muscular  nerves.  If  these  nerves,  on  the  other 
hand,  have  lost  their  irritability,  the  muscle  does  not  respond  readily  to 
single  induction-shocks,,  or  to  the  interrupted  current,  but  can  still  easily  be 
thrown  into  contraction  by  the  constant  current. 

In  the  second  place,  while  in  a  nerve  no  impulse  is,  as  a  rule,  generated 
during  the  passage  of  a  constant  current,  between  the  break  and  the  make, 
provided  that  it  is  not  too  strong,  and  that  it  remains  uniform  in  strength, 
in  a  urarized  muscle,  on  the  other  hand,  even  with  moderate  and  perfectly 
uniform  currents,  a  kind  of  tetanus  or  apparently  a  series  of  rhythmically 
repeated  contractions  is  very  frequently  witnessed  during  the  passage  of  the 
current.  The  exact  nature  and  cause  of  these  phenomena  in  muscle,  we 
must  not,  however,  discuss  here. 

THE  MUSCLE-NERVE  PREPARATION  AS  A  MACHINE. 

§  77.  The  facts  described  in  the  foregoing  sections  show  that  a  muscle 
with  its  nerve  may  be  justly  regarded  as  a  machine  which,  when  stimulated, 
will  do  a  certain  amount  of  work.  But  the  actual  amount  of  work  which  a 
muscle-nerve  preparation  will  do  is  found  to  depend  on  a  large  number  of 
circumstances,  and  consequently  to  vary  within  very  wide  limits.  These 
variations  will  be  largely  determined  by  the  condition  of  the  muscle  and 
nerve  in  respect  to  their  nutrition  ;  in  other  words,  by  the  degree  of  irrita- 
bility manifested  by  the  muscle  or  by  the  nerve,  or  by  both.  But  quite 
apart  from  the  general  influences  affecting  its  nutrition  and  thus  its  irrita- 
bility, a  muscle-nerve  preparation  is  affected  as  regards  the  amount  of  its 
work  by  a  variety  of  other  circumstances,  which  we  may  briefly  consider 
here,  reserving  to  a  succeeding  section  the  study  of  variations  in  irritability. 

The  influence  of  the  nature  and  mode  of  application  of  the  stimulus.  When 
we  apply  a  weak  stimulus — a  weak  induction-shock — to  a  nerve  we  get  a 
small  contraction,  a  slight  shortening  of  the  muscle ;  when  we  apply  a 
stronger  stimulus — a  stronger  induction-shock — we  get  a  larger  contraction, 
a  greater  shortening  of  the  muscle.  We  take,  other  things  being  equal,  the 
amount  of  contraction  of  the  muscle  as  a  measure  of  the  nervous  impulse, 
and  say  that  in  the  former  case  a  weak  or  slight,  in  the  latter  case  a  stronger 
or  larger,  nervous  impulse  has  been  generated.  Now,  the  muscle  of  the 
muscle-nerve  preparation  consists  of  many  muscular  fibres,  and  the  nerve  of 
many  nerve  fibres ;  and  we  may  fairly  suppose  that  in  two  experiments  we 
may  in  the  one  experiment  bring  the  induction-shock  or  other  stimulus  to 
bear  on  a  few  fibres  only,  and  in  the  other  experiment  on  many  or  even  all 
the  fibres  of  the  nerve.  In  the  former  case,  only  those  muscular  fibres  in 
which  the  few  nerve  fibres  stimulated  end  will  be  thrown  into  contraction, 
the  others  remaining  quiet,  and  the  shortening  of  the  muscle,  as  a  whole, 
since  only  a  few  fibres  take  part  in  it,  will  necessarily  be  less  than  when  all 
the  fibres  of  the  nerve  are  stimulated  and  all  the  fibres  of  the  muscles  con- 
tract. That  is  to  say,  the  amount  of  contraction  will  depend  on  the  number 
of  fibres  stimulated.  For  simplicity's  sake,  however,  we  will  in  what  fol- 
lows, except  when  otherwise  indicated,  suppose  that  when  a  nerve  is  stimu- 
lated, all  the  fibres  are  stimulated  and  all  the  muscular  fibres  contract. 

This  being  premised,  we  may  say  that,  other  things  being  equal,  the  mag- 
nitude of  a  nervous  impulse,  and  so  the  magnitude  of  the  ensuing  contraction, 
is  directly  dependent  on  what  we  may  call  the  strength  of  the  stimulus. 
Tim*  taking  a  single  induction-shock  as  the  most  manageable  stimulus,  we 
find  that  if,  before  we  begin,  we  place  the  secondary  coil  (Fig.  14,  sec.  c.)  a 


108  THE  CONTRACTILE  TISSUES. 

long  way  off  the  primary  coil  pr.  c.,  no  visible  effect  at  all  follows  upon  the 
discharge  of  the  induction-shock.  The  passage  of  the  momentary  weak  cur- 
rent is  either  unable  to  produce  any  nervous  impulse  at  all,  or  the  weak 
nervous  impulse  to  which  it  gives  rise  is  unable  to  stir  the  sluggish  muscular 
substance  to  a  visible  contraction.  As  we  slide  the  secondary  coil  toward 
the  primary,  sending  in  an  induction-shock  at  each  new  position,  we  find 
that  at  a  certain  distance  between  the  secondary  and  primary  coils,  the  mus- 
cle responds  to  each  induction-shock1  with  a  contraction  which  makes  itself 
visible  by  the  slightest  possible  rise  of  the  attached  lever.  This  position  of 
the  coils,  the  battery  remaining  the  same  and  other  things  being  equal, 
marks  the  minimal  stimulus  giving  rise  to  the  minimal  contraction.  As  the 
secondary  coil  is  brought  nearer  to  the  primary,  the  contractions  increase  in 
height  corresponding  to  the  increase  in  the  intensity  of  the  stimulus.  Very 
soon,  however,  an  increase  in  the  stimulus  caused  by  further  sliding  the 
secondary  coil  over  the  primary  fails  to  cause  any  increase  in  the  contrac- 
tion. This  indicates  that  the  maximal  stimulus  giving  rise  to  the  maximal 
contraction  has  been  reached ;  though  the  shocks  increase  in  intensity  as 
the  secondary  coil  is  pushed  further  and  further  over  the  primary,  the  con- 
tractions remain  of  the  same  height,  until  fatigue  lowers  them. 

With  single  induction-shocks  then  the  muscular  contraction,  and  by 
inference  the  nervous  impulse,  increases  with  an  increase  in  the  intensity  of 
the  stimulus,  between  the  limits  of  the  minimal  and  maximal  stimuli ;  and 
this  dependence  of  the  nervous  impulse,  and  so  of  the  contraction,  on  the 
strength  of  the  stimulus  may  be  observed  not  only  in  electric  but  in  all 
kinds  of  stimuli. 

It  may  here  be  remarked  that  in  order  for  a  stimulus  to  be  effective,  a 
certain  abruptness  in  its  action  is  necessary.  Thus  as  we  have  seen  the  con- 
stant current  when  it  is  passing  through  a  nerve  with  uniform  intensity  does 
not  give  rise  to  a  nervous  impulse,  and  indeed  it  may  be  increased  or 
diminished  to  almost  any  extent  without  generating  nervous  impulses,  pro- 
vided that  the  change  be  made  gradually  enough  ;  it  is  only  when  there  is 
a  sudden  change  that  the  current  becomes  effective  as  a  stimulus.  And  the 
reason  why  the  breaking  induction-shock  is  more  potent  as  a  stimulus  than 
the  making  shock  is  because  as  we  have  seen  (§  44)  the  current  which  is 
induced  in  the  secondary  coil  of  an  induction-machine  at  the  breaking  of  the 
primary  circuit  is  more  rapidly  developed,  and  has  a  sharper  rise,  than  the 
current  which  appears  when  the  primary  circuit  is  made.  Similarly  a  sharp 
tap  on  a  nerve  will  produce  a  contraction,  when  a  gradually  increasing 
pressure  will  fail  to  do  so  ;  and  in  general  the  efficiency  of  a  stimulus  of  any 
kind  will  depend  in  part  on  the  suddenness  or  abruptness  of  its  action. 

A  stimulus  in  order  that  it  may  be  effective  must  have  an  action  of  a 
certain  duration,  the  time  necessary  to  produce  an  effect  varying  according 
to  the  strength  of  the  stimulus  and  being  different  in  the  case  of  a  nerve 
from  what  it  is  in  the  case  of  a  muscle.  It  would  appear  that  an  electric  cur- 
rent applied  to  a  nerve  must  have  a  duration  of  at  least  about  0.0015  second 
to  cause  any  contraction  at  all,  and  needs  a  longer  time  than»this  to  produce 
its  full  effect.  A  muscle  fibre  apart  from  its  nerve  fibre  requires  a  still 
longer  duration  of  the  stimulus,  and  hence,  as  we  have  already  stated,  a 
muscle  poisoned  by  urari,  or  which  has  otherwise  lost  the  action  of  its 
nerves,  will  not  respond  as  readily  to  induction-shocks  as  to  the  more  slowly 
acting,  breaking  and  making  of  a  constant  current. 

In  the  case  of  electric  stimuli,  the  same  current  will  produce  a  stronger 

1  In  these  experiments  either  the  breaking  or  making  shock  must  be  used,  not  some- 
times one  and  sometimes  the  other,  for,  as  we  have  stated,  the  two  kinds  of  shock  differ 
in  efficiency,  the  breaking  being  the  most  potent. 


THE  MUSCLE-NERVE  PREPARATION  AS  A   MACHINE.         109 

contraction  when  it  is  sent  along  the  nerve  than  when  it  is  sent  across  the 
nerve  ;  indeed  it  is  maintained  that  a  current  which  passes  through  a  nerve 
in  an  absolutely  transverse  direction  is  powerless  to  generate  impulses. 

It  would  also  appear,  at  all  events  up  to  certain  limits,  that  the  longer 
the  piece  of  nerve  through  which  the  current  passes,  the  greater  is  the  effect 
of  the  stimulus.  • 

When  two  pairs  of  electrodes  are  placed  on  the  nerve  of  a  long  and  per- 
fectly fresh  and  successful  nerve-preparation,  one  near  to  the  cut  end,  and 
the  other  nearer  the  muscle,  it  is  found  that  the  same  stimulus  produces  a 
greater  contraction  when  applied  through  the  former  pair  of  electrodes  than 
through  the  latter.  This  has  been  interpreted  as  meaning  that  the  impulse 
started  at  the  further  electrodes  gathers  strength,  like  an  avalanche,  in  its 
progress  to  the  muscle.  It  is  more  probable,  however,  that  the  larger  con- 
traction produced  by  stimulation  of  the  part  of  the  nerve  near  the  cut  end 
is  due  to  the  stimulus  setting  free  a  larger  impulse,  i.  e.,  to  this  part  of  the 
nerve  being  more  irritable.  The  mere  section,  possibly  by  developing  nerve 
currents,  increases  for  a  time  the  irritability  at  the  cut  end.  A  similar 
greater  irritability  may,  however,  also  be  observed  in  the  part  of  the  nerve 
nearer  the  spinal  cord  while  it  is  still  in  connection  with  the  spinal  cord ; 
and  it  is  possible  that  the  irritability  of  a  nerve  may  vary  considerably  at 
different  points  of  its  course. 

§  78.  We  have  seen  that  when  single  stimuli  are  repeated  with  sufficient 
frequency,  the  individual  contractions  are  fused  into  tetanus ;  as  the  fre- 
quency of  the  repetition  is  increased,  the  individual  contractions  are  less 
obvious  on  the  curve,  until  at  last  we  get  a  curve  on  which  they  seem  to  be 
entirely  lost  and  which  we  may  speak  of  as  a  complete  tetanus.  By  such  a 
tetanus  a  much  greater  contraction,  a  much  greater  shortening  of  the  muscle, 
is  of  course  obtained  than  by  single  contractions. 

The  exact  frequency  of  repetition  required  to  produce  complete  tetanus 
will  depend  chiefly  on  the  length  of  the  individual  contractions,  and  this 
varies  in  different  animals,  in  different  muscles  of  the  same  animal,  and  in 
the  same  muscle  under  different  conditions.  Jn  a  cold-blooded  animal,  a 
single  contraction  is  as  a  rule  more  prolonged  than  in  a  warm-blooded  ani- 
mal, and  tetanus  is  consequently  produced  in  the  former  by  a  less  frequent 
repetition  of  the  stimulus.  A  tired  muscle  has  a  longer  contraction  than  a 
fresh  muscle,  and  hence  in  many  tetanus  curves  the  individual  contractions, 
easily  recognized  at  first,  disappear  later  on,  owing  to  the  individual  contrac- 
tions being  lengthened  out  by  the  exhaustion  caused  by  the  tetanus  itself. 
In  many  animals,  e.  g.,  the  rabbit,  some  muscles  (such  as  the  adductor  mag- 
nus  femoris)  are  pale,  while  others  (such  as  the  semitendinosus)  are  red. 
The  red  muscles  are  not  only  more  richly  supplied  with  bloodvessels,  but  the 
muscle  substance  of  the  fibres  contains  more  haemoglobin  than  the  pale,  and 
there  are  other  structural  differences.  Now  the  single  contraction  of  one  of 
these  red  muscles  is  more  prolonged  than  a  single  contraction  of  one  of  the 
pale  muscles  produced  by  the  same  stimulus.  Hence  the  red  muscles  are 
thrown  into  complete  tetanus  with  a  repetition  of  much  less  frequency  than 
that  required  for  the  pale  muscles.  Thus,  ten  stimuli  in  a  second  are  quite 
sufficient  to  throw  the  red  muscles  of  the  rabbit  into  complete  tetanus,  while 
the  pale  muscles  require  at  least  twenty  stimuli  in  a  second. 

So  long  as  signs  of  the  individual  contractions  are  visible  on  the  curve  of 
tetanus  it  is  easy  to  recognize  that  each  stimulation  produces  one  of  the  con- 
stituent single  contractions,  and  that  the  number  so  to  speak  of  the  vibra- 
tions of  the  muscle  making  up  the  tetanus  corresponds  to  the  number  of 
stimulations ;  but  the  question  whether,  when  we  increase  the  number  of 
stimulations  beyond  that  necessary  to  produce  a  complete  tetanus,  we  still 


110  THE  CONTRACTILE  TISSUES. 

increase  the  number  of  constituent  single  contractions  is  one  not  so  easy  to 
answer.  And  connected  with  this  question  is  another  difficult  one.  What  is 
the  rate  of  repetition  of  single  contractions  making  up  those  tetanic  contrac- 
tions which  as  we  have  said  are  the  kind  of  contractions  by  which  the  vol- 
untary, and  indeed  other  natural,  movements  of  the  body  are  carried  out? 
What  is  the  evidence  that  these  ar£  really  tetanic  in  character  ? 

When  a  muscle  is  thrown  into  tetanus,  a  more  or  less  musical  sound  is 
produced.  This  may  be  heard  by  applying  a  stethoscope  directly  over  a 
contracting  muscle,  and  a  similar  sound  but  of  a  more  mixed  origin  and 
less  trustworthy  may  be  heard  when  the  masseter  muscles  are  forcibly  con- 
tracted or  when  a  finger  is  placed  in  the  ear,  and  the  muscles  of  the  same 
arm  are  contracted. 

When  the  stethoscope  is  placed  over  a  muscle,  the  nerve  of  which  is 
stimulated  by  induction-shocks  repeated  with  varying  frequency,  the  note 
heard  will  vary  with  the  frequency  of  the  shocks,  being  of  higher  pitch  with 
the  more  frequent  shocks.  Now  it  has  been  thought  that  the  vibrations  of 
the  muscle  giving  rise  to  the  "  muscle  sound  "  are  identical  with  the  single 
contractions  making  up  the  tetanus  of  the  muscle.  And  since,  in  the 
human  body,  when  a  muscle  is  thrown  into  contraction  in  a  voluntary 
effort,  or  indeed  in  any  of  the  ordinary  natural  movements  of  the  body, 
the  fundamental  tone  of  the  sound  corresponds  to  about  19  or  20  vibrations 
a  second,  it  has  been  concluded  that  the  contraction  taking  place  in  such 
cases  is  a  tetanus  of  which  the  individual  contractions  follow  each  other 
about  19  or  20  times  a  second.  But  investigations  seem  to  show  that  the 
vibrations  giving  rise  to  the  muscle  sound  do  not  really  correspond  to  the 
shortenings  and  relaxations  of  the  individual  contractions,  and  that  the 
pitch  of  the  note  cannot  therefore  be  taken  as  an  indication  of  the  number 
of  single  contractions  making  up  the  tetanus ;  indeed,  as  we  shall  see  in 
speaking  of  the  sounds  of  the  heart,  a  single  muscular  contraction  may 
produce  a  sound  which  though  differing  from  the  sound  given  out  during 
tetanus  has  to  a  certain  extent  musical  characters.  Nevertheless  the  special 
characters  of  the  muscle  sound  given  out  by  muscles  in  the  natural  move- 
ments of  the  body  may  be  taken  as  showing  at  least  that  the  contractions 
of  the  muscle  in  these  movements  are  tetanic  in  nature,  and  the  similarity 
of  the  note  in  all  the  voluntary  efforts  of  the  body  and  indeed  in  all  move- 
ments carried  out  by  the  central  nervous  system  is  at  least  consonant  with 
the  view  that  the  repetition  of  single  contractions  is  of  about  the  same  fre- 
quency in  all  these  movements.  What  that  frequency  is,  and  whether  it  is 
exactly  identical  in  all  these  movements,  is  not  at  present  perhaps  abso- 
lutely determined ;  but  certain  markings  on  the  myographic  tracings  of 
these  movements  and  other  facts  seem  to  indicate  that  it  is  about  12  a 
second. 

§  79.  The  influence  of  the  load.  It  might  be  imagined  that  a  muscle 
which,  when  loaded  with  a  given  weight  and  stimulated  by  a  current  of  a 
given  intensity,  had  contracted  to  a  certain  extent,  would  only  contract  to 
half  that  extent  when  loaded  with  twice  the  weight  and  stimulated  with  the 
same  stimulus.  Such,  however,  is  not  necessarily  the  case ;  the  height  to 
which  the  weight  is  raised  may  be  in  the  second  instance  as  great,  or  even 
greater  than  in  the  first.  That  is  to  say,  the  resistance  offered  to  the  con- 
traction actually  augments  the  contraction  ;  the  tension  of  the  muscular 
fibre  increases  the  facility  with  which  the  explosive  changes  resulting  in  a 
contraction  take  place.  And  we  have  other  evidence  that  anything  which 
tends  to  stretch  the  muscular  fibres — that  any  tension  of  the  muscular 
fibres,  whether  during  rest  or  during  contraction — increases  the  metabolism 
of  the  muscle.  There  is,  of  course,  a  limit,  to  this  favorable  action  of  the 


DEGREE  OF   IRRITABILITY   OF   MUSCLES   AND   NERVES.       Ill 

resistance.  As  the  load  continues  to  be  increased,  the  height  of  the  con- 
traction is  diminished,  and  at  last  a  point  is  reached  at  which  the  muscle  is 
unable  (even  when  the  stimulus  chosen  is  the  strongest  possible)  to  lift  the 
load  at  all. 

In  a  muscle  viewed  as  a  machine  we  have  to  deal,  not  merely  with  the 
height  of  the  contraction — that  is,  with  the  amount  of  shortening — but  with 
the  work  done.  And  this  is  measured  by  multiplying  the  number  of  units 
of  height  to  which  the  load  is  raised  into  the  number  of  units  of  weight  of 
the  load.  Hence,  it  is  obvious  from  the  foregoing  observations  that  the 
work  done  must  be  largely  dependent  on  the  weight  itself.  Thus,  there  is  a 
certain  weight  of  load  with  which,  in  any  given  muscle  stimulated  by  a 
given  stimulus,  the  most  work  will  be  done,  as  may  be  seen  from  the  follow- 
ing example : 

Load,  in  grammes        ....          0          50        100        150        200         250 
Height  of  contractions,  in  millimetres      14  9  7  5  2  0 

Work  done,  in  gram-millimetres        .          0        450        700        750        400  0 

§  80.  The  influence  of  the  size  and  form  of  the  muscle.  Since  all  known 
muscular  fibres  are  much  shorter  than  the  wave-length  of  a  contraction,  it 
is  obvious  that  the  longer  the  fibre  the  greater  will  be  the  shortening  caused 
by  the  same  contraction  wave ;  the  greater  will  be  the  height  of  the  con- 
traction with  the  same  stimulus.  Hence,  in  a  muscle  of  parallel  fibres,  the 
height  to  which  the  load  is  raised  as  the  result  of  a  given  stimulus  applied 
to  its  nerve,  will  depend  on  the  length  of  the  fibres,  while  the  maximum 
weight  of  load  capable  of  being  lifted  will  depend  on  the  number  of  the 
fibres,  since  the  load  is  distributed  among  them.  Of  two  muscles,  therefore, 
of  equal  length  (and  of  the  same  quality)  the  most  work'  will  be  done  by 
that  which  has  the  larger  number  of  fibres — that  is  to  say,  the  fibres  being 
of  equal  width,  which  has  the  greatest  sectional  area ;  and  of  two  muscles 
with  equal  sectional  areas,  the  most  work  will  be  done  by  that  which  is  the 
longer.  If  the  two  muscles  are  unequal  both  in  length  and  sectional  area, 
the  work  done  will  be  the  greater  in  the  one  which  has  the  larger  bulk, 
which  contains  the  greater  number  of  cubic  units.  In  speaking^  therefore, 
of  the  work  which  can  be  done  by  a  muscle,  we  may  use  as  a  standard  a 
cubic  unit  of  bulk ;  or,  the  specific  gravity  of  the  muscle  being  the  same,  a 
unit  of  weight. 

We  learn,  then,  from  the  foregoing  paragraphs  that  the  work  done  by  a 
muscle-nerve  preparation  will  depend,  not  only  on  the  activity  of  the  nerve 
and  muscle  as  determined  by  their  own  irritability,  but  also  on  the  character 
and  mode  of  application  of  the  stimulus ;  on  the  kind  of  contraction 
(whether  a  single  spasm,  or  a  slowly  repeated  or  a  rapidly  repeated  tetanus) 
on  the  load  itself,  and  on  the  size  and  form  of  the  muscle.  Taking  the  most 
favorable  circumstances — viz.,  a  well-nourished,  lively  preparation,  a  maxi- 
mum stimulus  causing  a  rapid  tetanus,  and  an  appropriate  load— we  may 
determine  the  maximum  work  done  by  a  given  weight  of  muscle,  say  one 
gramme.  This  in  the  case  of  the  muscles  of  the  frog  has  been  estimated  at 
about  four  gram-metres  for  one  gramme  of  muscle. 

THE  CIRCUMSTANCES  WHICH  DETERMINE  THE  DEGREE  OF  IRRITABILITY 
OF  MUSCLES  AND  NERVES. 

§  81.  A  muscle-nerve  preparation  at  the  time  that  it  is  removed  from 
the  body  possesses  a  certain  degree  of  irritability ;  it  responds  by  a  contrac- 
tion of  a  certain  amount  to  a  stimulus  of  a  certain  strength  applied  to  the 
nerve  or  to  the  muscle.  After  a  while,  the  exact  period  depending  on  a 


112  THE  CONTRACTILE  TISSUES. 

variety  of  circumstances,  the  same  stimulus  produces  a  smaller  contraction, 
i.  e.y  the  irritability  of  the  preparation  has  diminished.  In  other  words,  the 
muscle  or  nerve  or  both  have  become  partially  "exhausted,"  and  the 
exhaustion  subsequently  increases,  the  same  stimulus  producing  smaller  con- 
tractions, until  at  last  all  irritability  is  lost,  no  stimulus,  however  strong, 
producing  any  contraction,  whether  applied  to  the  nerve  or  directly  to  the 
muscle ;  and  eventually  the  muscle,  as  we  have  seen,  becomes  rigid.  The 
progress  of  this  exhaustion  is  more  rapid  in  the  nerves  than  in  the  muscles ; 
for  some  time  after  the  nerve-trunk  has  ceased  to  respond  to  even  the 
strongest  stimulus,  contractions  may  be  obtained  by  applying  the  stimulus 
directly  to  the  muscle.  It  is  much  more  rapid  in  the  warm-blooded  than  in 
the  cold-blooded  animals.  The  muscles  and  nerves  of  the  former  lose  their 
irritability  when  removed  from  the  body,  after  a  period  varying  according 
to  circumstances,  from  a  few  minutes  to  two  or  three  hours ;  those  of  cold- 
blooded animals  (or  at  least  of  an  amphibian  or  a  reptile)  may,  under 
favorable  conditions,  remain  irritable  for  two,  three,  or  even  more  days. 
The  duration  of  irritability  in  warm-blooded  animals  may,  however,  be 
considerably  prolonged  by  reducing  the  temperature  of  the  body  before 
death. 

If  with  some  thin  body  a  sharp  blow  be  struck  across  a  muscle  which  has 
entered  into  the  later  stages  of  exhaustion  a  wheal  lasting  for  several  seconds  is 
developed.  This  wheal  appears  to  be  a  contraction  wave  limited  to  the  part  struck, 
and  disappearing  very  slowly  without  extending  to  the  neighboring  muscular  sub- 
stance. It  has  been  called  an  "  uii.o-muscular "  contraction,  because  it  may  be 
brought  out  even  when  ordinary  stimuli  have  ceased  to  produce  any  effect.  It 
may,  however,  be  accompanied  at  its  beginning  by  an  ordinary  contraction.  It  is 
readily  produced  in  the  living  body  on  the  pectoral  and  other  muscles  of  persons 
suffering  from  phthisis  and  other  exhausting  diseases. 

This  natural  exhaustion  and  diminution  of  irritability  in  muscles  and 
nerves  removed  from  the  body  may  be  modified  both  in  the  case  of  the 
muscle  and  of  the  nerve  by  a  variety  of  circumstances.  Similarly,  while 
the  nerve  and  muscle  still  remain  in  the  body,  the  irritability  of  the  one  or 
of  the  other  may  be  modified  either  in  the  way  of  increase  or  of  decrease 
by  certain  general  influences,  of  which  the  most  important  are  severance 
from  the  central  nervous  system  and  variations  in  temperature,  in  blood- 
supply,  and  in  functional  activity. 

The  effects  of  severance  from  the  central  nervous  system.  When  a  nerve, 
such,  for  instance,  as  the  sciatic,  is  divided  in  situ,  in  the  living  body,  there 
is,  first  of  all,  observed  a  light  increase  of  irritability,  noticeable  especially 
near  the  cut  end,  but  after  a  while  the  irritability  diminishes  and  gradually 
disappears.  Both  the  slight  initial  increase  and  the  subsequent  decrease 
begin  at  the  cut  end  and  advance  centrifugally  toward  the  peripheral  ter- 
minations. This  centrifugal  feature  of  the  loss  of  irritability  is  often 
spoken  of  as  the  Hitter- Valli  law..  In  a  mammal  it  may  be  two  or  three 
days  ;  in  a  frog,  as  many,  or  even  more  weeks,  before  irritability  has  dis- 
appeared from  the  nerve-trunk.  Itjis  maintained  in  the  small  (and  espe- 
cially in  the  intramuscular)  branches  for  still  longer  periods. 

In  the  central  portion  of  the  divided  nerve  similar  changes  may  be 
traced  as  far  only  as  the  next  node  of  Ranvier.  Beyond  this  the  nerve 
usually  remains  in  a  normal  condition. 

If  the  muscle  thus  deprived  of  its  nervous  elements  be  left  to  itself  its 
irritability,  however  tested,  sooner  or  later  diminishes ;  but  if  the  muscle  be 
periodically  thrown  into  contractions  by  artificial  stimulation  with  the  con- 
stant current,  the  decline  of  irritability  and  attendant  loss  of  nutritive 
power  may  be  postponed  for  some  considerable  time.  But  as  far  as  our  ex- 


DEGREE  OF  IRRITABILITY   OF  MUSCLES  AND  NERVES.       113 

perience  goes  at  present  the  artificial  stimulation  cannot  fully  replace  the 
natural  one,  and  sooner  or  later  the  muscle  like  the  nerve  suffers  degenera- 
tion, loses  all  irritability,  and  ultimately  its  place  is  taken  by  connective 
tissue. 

§  82.  The  influence  of  temperature.  We  have  already  seen  that  sudden 
heat  (and  the  same  might  be  said  of  cold  when  sufficiently  intense),  applied 
to  a  limited  part  of  a  nerve  or  muscle,  as  when  the  nerve  or  muscle  is 
touched  with  a  hot  wire,  will  act  as  a  stimulus.  It  is,  however,  much 
more  difficult  to  generate  nervous  or  muscular  impulses  by  exposing  a  whole 
nerve  or  muscle  to  a  gradual  rise  of  temperature. 

A  muscle  may  be  gradually  cooled  to  0°  C.  or  below  without  any  con- 
traction being  caused  ;  but  when  it  is  heated  to  a  limit,  which  in  the  case  of 
frog's  muscles  is  about  45°  C.,  of  mammalian  muscles  about  50°  C.,  a  sud- 
den change  takes  place  :  the  muscle  falls  at  the  limiting  temperature  into  a 
rigor  mortis,  which  is  initiated  by  a  forcible  contraction  or  at  least  short- 
ening. 

Moderate  warmth,  e.  g.,  in  the  frog  an  increase  of  temperature  up  to 
somewhat  below  45°  C.,  favors  both  muscular  and  nervous  irritability.  All 
the  molecular  processes  are  hastened  and  facilitated :  the  contraction  is  for 
a  given  stimulus  greater  and  more  rapid,  i.  e.,  of  shorter  duration,  and  ner- 
vous impulses  are  generated  more  readily  by  slight  stimuli.  Owing  to  the 
quickening  of  the  chemical  changes,  the  supply  of  new  material  may  prove 
insufficient ;  hence  muscles  and  nerves  removed  from  the  body  lose  their 
irritability  more  rapidly  at  a  high  than  at  a  low  temperature. 

The  gradual  application  of  cold  to  a  nerve,  especially  when  the  tempera- 
ture is  thus  brought  near  to  0°  C.,  slackens  all  the  molecular  processes,  so 
that  the  wave  of  nervous  impulse  is  lessened  and  prolonged,  the  velocity  of 
its  passage  being  much  diminished,  e.  g.,  from  28  metres  to  1  metre  per  second. 
At  about  0°  C.,  the  irritability  of  the  nerve  disappears  altogether. 

When  a  muscle  is  exposed  to  similar  cold,  e.  g.,  to  a  temperature  very 
little  above  zero,  the  contractions  are  remarkably  prolonged ;  they  are 
diminished  in  height  at  the  same  time,  but  not  in  proportion  to  the  increase 
of  their  duration.  Exposed  to  a  temperature  of  zero  or  below,  muscles  soon 
lose  their  irritability,  without,  however,  undergoing  rigor  mortis. 

§  83.  The  influence  of  blood-supply.  When  a  muscle  still  within  the 
body  is  deprived  by  any  means  of  its  proper  blood-supply,  as  when  the 
blood-vessels  going  to  it  are  ligatured,  the  same  gradual  loss  of  irritability 
and  final  appearance  of  rigor  mortis  are  observed  as  in  muscles  removed 
from  the  body.  Thus,  if  the  abdominal  aorta  be  ligatured,  the  muscles  of 
the  lower  limbs  lose  their  irritability  and  finally  become  rigid.  So  also  in 
systemic  death,  when  the  blood-supply  to  the  muscles  is  cut  off  by  the  cessa- 
tion of  the  circulation,  loss  of  irritability  ensues,  and  rigor  mortis  eventually 
follows.  In  a  human  corpse  the  muscles  of  the  body  enter  into  rigor  mortis 
in  a  fixed  order;  first,  those  of  the  jaw  and  neck,  then  those  of  the  trunk, 
next  those  of  the  arms,  and  lastly  those  of  the  legs.  The  rapidity  with 
which  rigor  mortis  comes  on  after  death  varies  considerably,  being  deter- 
mined both  by  external  circumstances  and  by  the  internal  conditions  of  the 
body.  Thus,  external  warmth  hastens  and  cold  retards  the  onset.  After 
great  muscular  exertion,  as  in  hunted  animals,  and  when  death  closes 
wasting  diseases,  rigor  mortis  in  most  cases  comes  on  rapidly.  As  a  general 
rule,  it  may  be  said  that  the  later  it  is  in  making  its  appearance  the  more 
pronounced  it  is,  and  the  longer  it  lasts ;  but  there  are  many  exceptions, 
and  when  the  state  is  recognized  as  being  fundamentally  due  to  a  clotting  of 
myosin,  it  is  easy  to  understand  that  the  amount  of  rigidity,  i.  e.,the  amount 
of  the  clot,  and  the  rapidity  of  the  onset,  i.  e.,  the  quickness  with  which 

8 


114  THE  CONTRACTILE  TISSUES. 

coagulation  takes  place,  may  vary  independently.  The  rapidity  of  onset 
after  muscular  exercise  and  wasting  disease  may,  perhaps,  be  in  part  depend- 
ent on  an  increase  of  acid  reaction,  which  is  produced  under  those  circum- 
stances in  the  muscle,  for  this  seems  to  be  favorable  to  the  coagulation  of 
the  muscle  plasma.  When  rigor  mortis  has  once  become  thoroughly  estab- 
lished in  a  muscle  through  deprivation  of  blood,  it  cannot  be  removed  by 
any  subsequent  supply  of  blood.  Thus,  when  the  abdominal  aorta  has 
remained  ligatured  until  the  lower  limbs  have  become  completely  rigid, 
untying  the  ligature  will  not  restore  the  muscles  to  an  irritable  condition ; 
it  simply  hastens  the  decomposition  of  the  dead  tissues  by  supplying  them 
with  oxygen  and,  in  the  case  of  the  mammal,  with  warmth  also.  A  muscle, 
however,  may  acquire  as  a  whole  a  certain  amount  of  rigidity  on  account  of 
some  of  the  fibres  becoming  rigid,  while  the  remainder,  though  they  have 
lost  their  irritability,  have  not  yet  advanced  into  rigor  mortis.  At  such  a 
juncture  a  renewal  of  the  blood-stream  may  restore  the  irritability  of  those 
fibres  which  are  not  yet  rigid,  and  thus  appear  to  do  away  with  rigor  mortis  ; 
yet,  it  appears  that  in  such  cases  the  fibres  which  have  actually  become 
rigid  never  regain  their  irritability,  but  undergo  degeneration. 

Mere  loss  of  irritability,  even  though  complete,  if  stopping  short  of  the 
actual  coagulation  of  the  muscle  substance,  may  be  with  care  removed. 

The  influence  of  blood-supply  cannot  be  so  satisfactorily  studied  in  the 
case  of  nerves  as  in  the  case  of  muscles  ;  there  can,  however,  be  little  doubt 
that  the  effects  are  analogous. 

§  84.  The  influence  of  functional  activity.  This,  too,  is  more  easily  studied 
in  the  case  of  muscles  than  of  nerves. 

When  a  muscle  within  the  body  is  unused,  it  wastes ;  when  used  it  (within 
certain  limits)  grows.  Both  these  facts  show  that  the  nutrition  of  a  muscle 
is  favorably  affected  by  its  functional  activity.  Part  of  this  may  be  an 
indirect  effect  of  the  increased  blood-supply  which  occurs  when  a  muscle 
contracts.  When  a  nerve  going  to  a  muscle  is  stimulated,  the  bloodvessels 
of  the  muscle  dilate.  Hence  at  the  time  of  the  contraction  more  blood 
flows  through  the  muscle,  and  this  increased  flow  continues  for  some  little 
while  after  the  contraction  of  the  muscle  has  ceased.  But,  apart  from  the 
blood-supply,  it  is  probable  that  the  exhaustion  caused  by  a  contraction  is 
immediately  followed  by  a  reaction  favorable  to  the  nutrition  of  the  muscle  ; 
and  this  is  a  reason,  possibly  the  chief  reason,  why  a  muscle  is  increased  by 
use,  that  is  to  say,  the  loss  of  substance  and  energy  caused  by  the  contrac- 
tion is  subsequently  more  than  made  up  for  by  increased  metabolism  during 
the  following  period  of  rest. 

A.  muscle,  even  within  the  body,  after  prolonged  action  is  fatigued,  i.  e., 
a  stronger  stimulus  is  required  to  produce  the  same  contraction  ;  in  other 
words,  its  irritability  may  be  lessened  by  functional  activity.  Whether 
functional  activity,  therefore,  is  injurious  or  beneficial  depends  on  its  amount 
in  relation  to  the  condition  of  the  muscle.  The  muscle  is  sooner  fatigued 
and  exhausted  than  the  motor  nerve. 

The  sense  of  fatigue  of  which,  after  prolonged  or  unusual  exertion,  we  are 
conscious  in  our  own  bodies,  is  probably  of  complex  origin,  and  its  nature, 
like  that  of  the  normal  muscular  sense  of  which  we  shall  have  to  speak 
hereafter,  is  at  present  not  thoroughly  understood.  It  seems  to  be  in  the  first 
place  the  result  of  changes  in  the  muscles  themselves,  but  it  is  possibly  also 
caused  by  changes  in  the  nervous  apparatus  concerned  in  muscular  action, 
and  especially  in  those  parts  of  the  central  nervous  system  which  are  con- 
cerned in  the  production  of  voluntary  impulses.  lu  any  case  it  cannot  be 
taken  as  an  adequate  measmre  of  the  actual  fatigue  of  the  muscles ;  for  a 
man  who  says  he  is  absolutely  exhausted  may,  under  excitement,  perform  a 


THE  ENERGY  OF  MUSCLE  AND  NERVE.  115 

very  large  amount  of  work  with  his  already  weary  muscles.  The  will,  in 
fact,  rarely  if  ever  calls  forth  the  greatest  contractions  of  which  the  muscles 
are  capable. 

Absolute  (temporary)  exhaustion  of  the  muscles,  so  that  the  strongest 
stimuli  produce  no  contraction,  may  be  produced  even  within  the  body  by 
artificial  stimulation  ;  recovery  takes  place  on  rest.  Out  of  the  body  absolute 
exhaustion  takes  place  readily.  Here,  also,  recovery  may  take  place. 
Whether  in  any  given  case  it  does  occur  or  not,  is  determined  by  the  amount 
of  contraction  causing  the  exhaustion,  and  by  the  previous  condition  of  the 
muscle.  In  all  cases  recovery  is  hastened  by  renewal  (natural  or  artificial) 
of  the  blood-stream. 

The  more  rapidly  the  contractions  follow  each  other,  the  less  the  interval 
between  any  two  contractions,  the  more  rapid  the  exhaustion.  A  certain 
number  of  single  induction-shocks  repeated  rapidly,  say  every  second  or 
oftener,  bring  about  exhaustive  loss  of  irritability  more  rapidly  than  the 
same  number  of  shocks  repeated  less  rapidly,  for  instance  every  5  or  10 
seconds.  Hence  tetanus  is  a  ready  means  of  producing  exhaustion. 

In  exhausted  muscles  the  elasticity  is  much  diminished  ;  the  tired  muscle 
returns  less  readily  to  its  natural  length  than  does  the  fresh  one. 

The  exhaustion  due  to  contraction  may  be  the  result  either  of  the  con- 
sumption of  the  store  of  really  contractile  material  present  in  the  muscle ; 
or  of  the  accumulation  in  the  tissue  of  the  products  of  the  act  of  contrac- 
tion ;  or  of  both  of  these  causes. 

The  restorative  influence  of  rest,  in  the  case  of  a  muscle  removed  from 
the  circulation,  may  be  explained  by  supposing  that  during  the  repose,  either 
the  internal  changes  of  the  tissue  manufacture  new  explosive  material  out  of 
the  comparatively  raw  material  already  present  in  the  fibres,  or  the  directly 
hurtful  products *of  the  act  of  contraction  undergo  changes  by  which  they 
are  converted  into  comparatively  inert  bodies.  A  stream  of  fresh  blood 
may  exert  its  restorative  influence  not  only  by  quickening  the  above  two 
events,  but  also  by  carrying  off  the  immediate  waste  products  while  at  the 
same  time  it  brings  new  raw  material.  It  is  not  known  to  what  extent  each 
of  these  parts  is  played.  That  the  products  of  contraction  are  exhausting  in 
their  effect,  is  shown  by  the  facts  that  the  injection  of  a  solution  of  the 
muscle-extractives  into  the  vessels  of  a  muscle  produces  exhaustion,  and  that 
exhausted  muscles  are  recovered  by  the  simple  injection  of  inert  saline 
solutions  into  their  bloodvessels.  But  the  matter  has  not  yet  been  fully 
worked  out. 

One  important  element  brought  by  fresh  blood  is  oxygen.  This,  as  we 
have  seen,  is  not  necessary  for  the  carrying  out  of  the  actual  contraction,  and 
yet  is  essential  to  the  maintenance  of  irritability.  The  oxygen  absorbed  by 
the  muscle  apparently  enters  in  some  peculiar  way  into  the  formation  of  that 
complex  explosive  material  the  decomposition  of  which  in  the  act  of  contrac- 
tion, though  it  gives  rise  to  carbonic  acid  and  other  products  of  oxidation, 
is  not  in  itself  a  process  of  direct  oxidation. 

THE  ENERGY  OF  MUSCLE  AND  NERVE,  AND  THE  NATURE  OF 
MUSCULAR  AND  NERVOUS  ACTION. 

§  85.  We  may  briefly  recapitulate  some  of  the  chief  results  arrived  at 
in  the  preceding  pages  as  follows : 

A  muscular  contraction  itself  is  essentially  a  translocation  of  molecules, 
a  change  of  form,  not  of  bulk.  We  cannot  say,  however,  anything  definite 
as  to  the  nature  of  this  translocation  or  as  to  the  way  in  which  it  is  brought 
about.  For  instance  we  cannot  satisfactorily  explain  the  contraction  between 


116  THE  CONTRACTILE  TISSUES. 

the  striation  of  a  muscular  fibre  and  a  muscular  contraction.  Nearly  all 
rapidly  contracting  muscles  are  striated,  and  we  must  suppose  that  the 
striation  is  of  some  use ;  but  it  is  not  essential  to  the  carrying  out  of  a  con- 
traction, for,  as  we  shall  see,  the  contraction  of  a  non-striated  muscle  is 
fundamentally  the  same  as  that  of  a  striated  muscle.  But  whatever  be  the 
exact  way  in  which  the  translocation  is  effected,  it  is  in  some  way  or  other 
the  result  of  a  chemical  change,  of  an  explosive  decomposition  of  certain 
parts  of  the  muscle  substance.  The  energy  which  is  expended  in  the 
mechanical  work  done  by  the  muscle  has  its  source  in  the  energy  latent  in 
the  muscle  substance  and  set  free  by  that  explosion.  Concerning  the  nature 
of  that  explosion  we  only  know  at  present  that  it  results  in  the  production 
of  carbonic  acid  and  in  an  increase  of  the  acid  reaction,  and  that  heat  is  set 
free  as  well  as  the  specific  muscular  energy.  There  is  a  general  parallelism 
between  the  extent  of  metabolism  taking  place  and  the  amount  of  energy  set 
free  ;  the  greater  the  development  of  carbonic  acid,  the  larger  is  the  con- 
traction and  the  higher  the  temperature. 

It  is  important  to  remember  that,  as  we  have  already  urged,  relaxation, 
the  return  to  the  original  length,  is  an  essential  part  of  the  whole  contraction 
no  less  than  shortening  itself.  It  is  true  that  the  return  to  the  original 
length  is  assisted  by  the  stretching  exerted  by  the  load,  and  in  the  case  of 
muscles  within  the  living  body  is  secured  by  the  action  of  antagonistic 
muscles  or  by  various  anatomical  relations ;  but  the  fact  that  the  complete- 
ness and  rapidity  of  the  return  are  dependent  on  the  condition  of  the  muscle, 
that  is,  on  the  complex  changes  within  the  muscle  making  up  what  we  call 
its  nutrition,  the  tired  muscle  relaxing  much  more  slowly  than  the  untired 
muscle,  shows  that  the  relaxation  is  due  in  the  main  to  intrinsic  processes 
going  on  in  the  muscle  itself,  processes  which  we  might  characterize  as  the 
reverse  of  those  of  contraction.  In  fact,  to  put  the  matter  forcibly,  adopting 
the  illustration  used  in  §  57,  and  regarding  relaxation  as  a  change  of  molecules 
from  a  "  formation  "  of  one  hundred  in  two  lines  of  fifty  each  to  a  formation 
of  ten  columns  each  ten  deep,  it  would  be  possible  to  support  the  theory  that 
the  really  active  forces  in  muscle  are  those  striving  to  maintain  the  latter 
formation  in  columns,  and  that  the  falling  into  double  lines,  that  is  to  say 
the  contraction,  is  the  result  of  these  forces  ceasing  to  act;  in- other  words, 
that  the  contracted  state  of  the  muscular  fibre  is  what  may  be  called  the 
natural  state,  that  the  relaxed  condition  is  only  brought  about  at  the  expense 
of  changes  counteracting  the  natural  tendencies  of  the  fibre.  Without  going 
so  far  as  this,  however,  we  may  still  recognize  that  both  contraction  and  relax- 
ation are  the  result  of  changes  which,  since  they  seem  to  be  of  a  chemical 
nature  in  the  one  case,  are  probably  so  in  the  other  also. 

ON  SOME  OTHER  FORMS  OF  CONTRACTILE  TISSUE. 

Plain,  Smooth  or  Unstriated  Muscular  Tissue. 

§  86.  This,  in  vertebrates  at  all  events,  rarely  occurs  in  isolated  masses 
or  muscles,  as  does  striated  muscular  tissue,  but  is  usually  found  taking  part 
in  the  structure  of  complex  organs, such  for  instance  as  the  intestines;  hence 
the  investigation  of  its  properties  is  beset  with  many  difficulties. 

§  87.  As  far  as  we  know,  plain  muscular  tissue  in  its  chemical  features 
resembles  striated  muscular  tissue.  It  contains  albumin,  some  forms  of 
globulin,  and  antecedents  of  mvosin  which  upon  the  death  of  the  fibres 
become  myosin  ;  for  plain  muscular  tissue  after  death  becomes  rigid,  losing 
its  extensibility  and  probably  becoming  acid,  though  the  acidity  is  not  so 
marked  as  in  striated  muscle.  Kreatin  has  also  been  found,  as  well  as- 


ON  SOME  OTHER  FORMS  OF  CONTRACTILE  TISSUE.  117 

glycogen,  and,  indeed,  it  seems  probable  that  the  whole  metabolism  of 
plain  muscular  tissue  is  fundamentally  the  same  as  that  of  the  striated 
muscles. 

§  88.  In  their  general  physical  features  plain  muscular  fibres  also  resem- 
ble striated  fibres,  and  like  them  they  are  irritable  and  contractile ;  when 
stimulated  they  contract.  The  fibres  vary  in  natural  length  in  different 
situations,  those  of  the  bloodvessels,  for  instance,  being  shorter  and  stouter 
than  those  of  the  intestine ;  but  in  the  same  situation  the  fibres  may  also  be 
found  in  one  of  two  different  conditions.  In  the  one  case  the  fibres  are 
long  and  thin,  in  the  other  case  they  are  reduced  in  length,  it  may  be  to 
one-half  or  even  to  one-third,  and  are  correspondingly  thicker,  broader, 
and  less  pointed  at  the  ends,  their  total  bulk  remaining  unaltered.  In 
the  former  case  they  are  relaxed  or  elongated,  in  the  latter  case  they  are 
contracted. 

The  facts  of  the  contraction  of  plain  muscular  tissue  may  be  studied  in 
the  intestine,  the  muscular  coat  of  which  consists  of  an  outer  thin  sheet 
composed  of  fibres  and  bundles  of  fibres  disposed  longitudinally,  and  of  an 
inner  much  thicker  sheet  of  fibres  disposed  circularly  ;  in  the  ureter  a  similar 
arrangement  of  two  coats  obtains. 

If  a  mechanical  or  electrical  (or,  indeed,  any  other)  stimulus  be  brought 
to  bear  on  a  part  of  a  fresh,  living,  still  warm  intestine  (the  small  intestine 
is  the  best  to  work  with)  a  circular  contraction  is  seen  to  take  place  at  the 
spot  stimulated  ;  the  intestine  seems  nipped  in  ringwise,  as  if  tied  round  with 
an  invisible  cord,  and  the  part  so  constricted,  previously  vascular  and  red, 
becomes  pale  and  bloodless.  The  individual  fibres  of  the  circular  coat  in 
the  region  stimulated  have  each  become  shorter,  and  the  total  effect  of  the 
shortening  of  the  multitude  of  fibres  all  having  the  same  circular  disposition 
is  to  constrict  or  narrow  the  lumen  or  tube  of  the  intestine.  The  longitudi- 
nally disposed  fibres  of  the  outer  longitudinal  coat  will  at  the  same  time 
similarly  contract  or  shorten  in  a  longitudinal  direction,  but  this  coat  being 
relatively  much  thinner  than  the  circular  coat,  the  longitudinal  contraction 
is  altogether  overshadowed  by  the  circular  contraction.  A  similar  mode  of 
contraction  is  also  seen  when  the  ureter  is  similarly  stimulated. 

The  contraction  thus  induced  is  preceded  by  a  very  long  latent  period  and 
lasts  a  very  considerable  time,  in  fact,  several  seconds,  after  which  relaxa- 
tion slowly  takes  place.  We  may  say  then  that  over  the  circularly  disposed 
fibres  of  the  intestine  (or  ureter)  at  the  spot  in  question  there  has  passed  a 
contraction-wave  remarkable  for  its  long  latent  period  and  for  the  slowness 
of  its  development,  the  wave  being  propagated  from  fibre  to  fibre.  From 
the  spot  so  directly  stimulated  the  contraction  may  pass  also  as  a  wave  (with 
a  length  of  1  cm.  and  a  velocity  of  from  20  to  30  millimetres  a  second  in 
the  ureter),  along  the  circular  coat  both  upwards  and  downwards.  The 
longitudinal  fibres  at  the  spot  stimulated  are,  as  we  have  said,  also  thrown 
into  contractions  of  altogether  similar  character,  and  a  wave  of  contraction 
may  thus  also  travel  longitudinally  along  the  longitudinal  coat  both  upwards 
and  downwards.  It  is  evident,  however,  that  the  wave  of  Contraction  of 
which  we  are  now  speaking  is,  in  one  respect,  different  from  the  wave  of 
contraction  treated  of  in  dealing  with  striated  muscle.  In  the  latter  case 
the  contraction-wave  is  a  simple  wave  propagated  along  the  individual  fibre, 
and  starting  from  the  end-plate  or,  in  the  case  of  direct  stimulation,  from 
the  part  of  the  fibre  first  affected  by  the  stimulus  ;  we  have  no  evidence  that 
the  contraction  of  one  fibre  can  communicate  contraction  to  neighboring 
fibres,  or,  indeed,  in  any  way  influence  neighboring  fibres.  In  the  case  of 
the  intestine  or  ureter  the  wave  is  complex,  being  the  sum  of  the  contraction- 
waves  of  several  fibres  engaged  in  different  phases,  and  is  propagated  from 


118  THE  CONTRACTILE  TISSUES. 

fibre  to  fibre,  both  in  the  direction  of  the  fibres,  as  when  the  whole  circum- 
ference of  the  intestine  is  engaged  in  the  contraction,  or  when  the  wave 
travels  longitudinally  along  the  longitudinal  coat,  and  also  in  a  direction  at 
right  angles  to  the  axes  of  the  fibres,  as  when  the  contraction-wave  travels 
lengthways  along  the  circular  coat  of  the  intestine,  or  when  it  passes  across 
a  breadth  of  the  longitudinal  coat ;  that  is  to  say,  the  changes  leading  to 
contraction  are  communicated  not  only  in  a  direct  manner  across  the  cement 
substance  uniting  the  fibres  of  a  bundle,  but  also  in  an  indirect  manner, 
probably  by  means  of  nerve  fibres  from  bundle  to  bundle  across  the  connective 
tissue  between  them.  Moreover,  it  is  obvious  that  even  the  contraction-wave 
which  passes  along  a  single  unstriated  fibre  differs  from  that  passing  along  a 
striated  fibre,  in  the  very  great  length  both  of  its  latent  period  and  of  the 
duration  of  its  contraction.  Hence,  much  more  even  than  in  the  case  of  a 
striated  muscle,  the  whole  of  each  fibre  must  be  occupied  by  the  contraction- 
wave,  and,  indeed,  be  in  nearly  the  same  phase  of  the  contraction  at  the 
same  time. 

Waves  of  contraction  thus  passing  along  the  circular  and  longitudinal 
coats  of  the  intestine  constitute  what  is  called  peristaltic  action. 

Like  the  contractions  of  striated  muscle  the  contractions  of  plain  mus- 
cles may  be  started  by  stimulation  of  nerves  going  to  the  part,  the  nerves 
supplying  plain  muscular  tissue  running  for  the  most  part  in  the  so-called 
sympathetic  system,  but  being,  as  we  shall  see,  ultimately  connected  with 
the  spinal  cord  or  brain.  Here,  however,  we  come  upon  an  important  dis- 
tinction between  the  striated  skeletal  muscles  and  the  plain  muscles  of  the 
viscera.  As  a  general  rule,  the  skeletal  muscles  are  thrown  into  contrac- 
tion only  by  nervous  impulses  reaching  them  along  their  nerves ;  spontane- 
ous movements  of  the  skeletal  muscles,  that  is,  contractions  arising  out  of 
changes  in  the  muscles  themselves,  are  extremely  rare  and  when  they  occur 
are  abnormal ;  so-called  "  cramps  "  for  instance,  which  are  prolonged  tetanic 
contractions  of  skeletal  muscles  independent  of  the  will,  though  their  occur- 
rence is  largely  due  to  the  condition  of  the  muscle  itself,  generally  the  re- 
sult of  overwork,  are  probably  actually  started  by  nervous  impulses  reaching 
them  from  without.  On  the  other  hand,  the  plain  muscles  of  the  viscera, 
of  the  intestine,  uterus,  and  ureter,  for  instance,  and  of  the  bloodvessels  very 
frequently  fall  into  contractions  and  so  carry  out  movements  of  the  organs 
to  which  they  belong  quite  independently  of  the  central  nervous  system. 
These  organs  exhibit  "  spontaneous  "  movements  quite  apart  from  the  will, 
quite  apart  from  the  central  nervous  system,  and  under  favorable  circum- 
stances continue  to  do  this  for  some  time  after  they  have  been  entirely  isolated 
and  removed  from  the  body.  So  slight,  indeed,  is  the  connection  between 
the  movements  of  organs  and  parts  supplied  with  plain  muscular  fibres  and 
the  will,  that  these  muscular  fibres  have  sometimes  been  called  involuntary 
muscles  ;  but  this  name  is  undesirable,  since  some  muscles  consisting  entirely 
of  plain  muscular  fibres  (e.  g.,  the  ciliary  muscles  by  which  the  eye  is  accom- 
modated for  viewing  objects  at  different  distances)  are  directly  under  the 
influence  of  the  will,  and  some  muscles  composed  of  striated  fibres  (e.  g., 
those  of  the  heart)  are  wholly  removed  from  the  influence  of  the  will. 

We  shall  best  study,  however,  the  facts  relating  to  the  movements  of 
parts  provided  with  plain  muscular  fibres  when  we  come  to  consider  the 
parts  themselves. 

Like  the  skeletal  muscles,  whose  nervous  elements  have  been  rendered 
functionally  incapable  (§  76),  plain  muscles  are  much  more  sensitive  to  the 
making  and  breaking  of  a  constant  current  than  to  induction-shocks ;  a 
current,  when  very  brief,  like  that  of  an  induction-shock,  produces  little  or 
no  effect. 


ON  SOME  OTHER  FORMS   OF  CONTRACTILE  TISSUE.  119 

The  contraction  of  plain  muscular  fibres  is,  as  we  said,  very  slow  in  its 
development  and  very  long  in  its  duration,"even  when  started  by  a  momen- 
tary stimulus,  such  as  a  single  induction-shock.  The  contraction  after  a 
stimulation  often  lasts  so  long  as  to  raise  the  question,  whether  what  has  been 
produced  is  not  a  single  contraction  but  a  tetanus.  Tetanus,  however,  that 
is,  the  fusion  of  a  series  of  contractions,  seems  to  be  of  rare  occurrence, 
though  probably  it  may  be  induced  in  plain  muscular  tissue ;  but  the  ends 
of  tetanus  are  gained  by  a  kind  of  contraction  which,  rare  or  at  least  not 
prominent  in  skeletal  muscle,  becomes  of  great  importance  in  plain  muscular 
tissue  by  a  kind  of  contraction  called  a  tonic  contraction.  The  subject  is 
one  not  without  difficulties,  but  it  would  appear  that  a  plain  muscular  fibre 
may  remain  for  a  very  considerable  time  in  a  state  of  contraction,  the 
amount  of  shortening  thus  maintained  being  either  small  or  great;  it  is  then 
said  to  be  in  a  state  of  tonic  contraction.  This  is  especially  seen  in  the  case 
of  the  plain  muscular  tissue  of  the  arteries,  and  we  shall  have  to  return  to 
this  matter  in  dealing  with  the  circulation. 

The  muscular  tissue  which  enters  into  the  construction  of  the  heart  is  of 
a  peculiar  nature,  being  on  the  one  hand  striated  and  on  the  other  in  some 
respects  similar  to  plain  muscular  tissue,  but  this  we  shall  consider  in  deal- 
ing with  the  heart  itself. 

Ciliary  Movement. 

§  89.  Nearly  all  the  movements  of  the  body  which  are  not  due  to  physi- 
cal causes,  such  as  gravity,  the  diffusion  of  liquids,  etc.,  are  carried  out  by 
muscles,  either  striated  or  plain  ;  but  some  small  and  important  effects  in  the 
way  of  movement  are  produced  by  the  action  of  cilia,  and  by  those  changes 
of  protoplasm  which  are  called  amoeboid. 

§  90.  Ciliary  action,  in  the  form  in  which  it  is  most  common,  in  mam- 
mals and,  indeed,  vertebrates,  consists  in  the  cilium  being  at  one  moment 
straight  or  vertical,  at  the  next  moment  being  bent  down  suddenly  into 
a  hook  or  sickle  form,  and  then  more  slowly  returning  to  the  straight  erect 
position.  When  the  cilia  are  vigorous  this  double  movement  is  repeated 
with  very  great  rapidity — so  rapidly  that  the  individual  movements  cannot 
be  seen  ;  it  is  only  when,  by  reason  of  fatigue,  the  action  becomes  slow  that 
the  movement  itself  can  be  seen  ;  what  is  seen  otherwise  is  simply  the  effect 
of  the  movement.  The  movements  when  slow  have  been  counted  at  about 
eight  (double  movements)  in  a  second  ;  probably  when  vigorous  they  are 
repeated  from  twelve  to  twenty  times  a  second. 

The  flexion  takes  place  in  one  direction  only,  and  all  the  cilia  of  each 
cell  and,  indeed,  of  all  the  cells  of  the  same  epithelium  move  in  the  same 
direction.  Moreover,  the  same  direction  is  maintained  during  the  whole 
life  of  the  epithelium ;  thus  the  cilia  of  the  epithelium  of  the  trachea  and 
bronchial  passages  move  during  the  whole  of  life  in  such  a  way  as  to  drive 
the  fluid  lying  upon  them  upward  toward  the  mouth  ;  as  far  as  we  know,  in 
vertebrates,  or  at  least  in  mammals,  the  direction  is  not  and  cannot  by  any 
means  be  reversed. 

The  flexion  is  very  rapid,  but  the  return  to  the  erect  position  is  much 
slower ;  hence  the  total  effect  of  the  blow,  supposing  the  cilium  and  the  cell 
to  be  fixed,  is  to  drive  the  thin  layer  of  fluid  in  which  the  cilium  is 
working,  and  which  always  exists  over  the  epithelium,  and  any  particles 
which  may  be  floating  in  that  fluid,  in  the  same  direction  as  that  in  which 
the  blow  is  given.  If  the  cell  be  not  attached*  but  floating  free,  the  effect 
of  the  blow  may  be  to  drive  the  cell  itself  backward  ;  and  when  perfectly 
fresh  ciliated  epithelium  is  teased  out  and  examined  in  an  inert  fluid,  such 


120  THE  CONTRACTILE  TISSUES. 

as  normal  saline  solution,  isolated  cells  or  small  groups  of  cells  may  be  seen 
rowing  themselves  about,  as  it  were,  by  the  action  of  their  cilia. 

All  the  cilia  of  a  cell  move,  as  we  have  just  said,  in  the  same  direction, 
but  not  quite  at  the  same  time.  If  we  call  the  side  of  the  cell  toward  which 
the  cilia  bend  the  front  of  the  cell  and  the  opposite  side  the  back,  the  cilia 
at  the  back  move  a  trifle  before  those  at  the  front,  so  that  the  move- 
ment runs  over  the  cell  in  the  direction  of  the  movement  itself.  Sim- 
ilarly, taking  any  one  cell,  the  cilia  of  the  cells  behind  it  move  slightly 
before  and  the  cilia  of  the  cells  in  front  of  it  slightly  after  its  own  cilia 
move.  Hence,  in  this  way,  along  a  whole  stretch  of  epithelium,  the 
movement  or  bending  of  the  cilia  sweeps  over  the  surface  in  ripples  or 
waves,  very  much  as,  when  the  wind  blows,  similar  waves  of  bending  sweep 
over  a  field  of  corn  or  tall  grass.  By  this  arrangement  the  efficacy  of  the 
movement  is  secured,  and  a  steady  stream  of  fluid  carrying  particles  is 
driven  over  the  surface  in  a  uniform  continued  direction  ;  if  the  cilia  of 
separate  cells,  and  still  more  if  the  separate  cilia  of  each  cell,  moved 
independently  of  the  others,  all  that  would  be  produced  would  be  a  series 
of  minute  "  wabbles,"  of  as  little  use  for  driving  the  fluid  definitely  on- 
ward as  the  efforts  of  a  boat's  crew  all  rowing  out  of  time  are  for  propelling 
the  boat. 

Swift  bending  and  slower  straightening  is  the  form  of  ciliary  movement 
generally  met  with  in  the  ciliated  epithelium  of  mammals  and,  indeed,  of 
vertebrates ;  but  among  the  invertebrates  we  find  other  kinds  of  movement, 
such  as  a  to-and-fro  movement,  equally  rapid  in  both  directions,  a  cork- 
screw movement,  a  simple  undulatory  movement,  and  many  others.  In 
each  case  the  kind  of  movement  seems  adapted  to  secure  a  special  end. 
Thus  even  in  the  mammal  while  the  one-sided  blow  of  the  cilia  of  the  epi- 
thelial cells  secures  a  flow  of  fluid  over  the  epithelium,  the  tail  of  the  sper- 
matozoon, which  is  practically  a  single  cilium,  by  moving  to  and  fro  in  an 
undulatory  fashion  drives  the  head  of  the  spermatozoon  onward  in  a  straight 
line,  like  a  boat  driven  by  a  single  oar  worked  at  the  stern. 

Why  and  exactly  how  the  cilium  of  the  epithelial  cells  bends  swiftly  and 
straightens  slowly,  always  acting  in  the  same  direction,  is  a  problem  dif- 
ficult at  present  to  answer  fully.  Some  have  thought  that  the  body  of 
the  cell  is  contractile,  or  contains  contractile  mechanisms  pulling  upon  the 
cilia,  which  are  thus  simple  passive  puppets  in  the  hands  of  the  cells.  But 
there  is  no  satisfactory  evidence  for  such  a  view.  On  the  whole  the  evi- 
dence is  in  favor  of  the  view  that  the  action  is  carried  out  by  the  cilium 
itself,  that  the  bending  is  a  contraction  of  the  cilium,  and  that  the  straight- 
ening corresponds  to  the  relaxation  of  a  muscular  fibre.  But  even  then 
the  exact  manner  in  which  the  contraction  bends  and  the  relaxation 
straightens  the  filament  is  not  fully  explained.  We  have  no  positive  evi- 
dence that  a  longitudinal  half,  the  inside  we  might  say,  of  the  filament  is 
contractile,  and  the  other  half,  the  outside,  elastic,  a  supposition  which  has 
been  made  to  explain  the  bending  and  straightening.  In  fact,  no  adequate 
explanation  of  the  matter  has  as  yet  been  given,  and  it  is  really  only  on 
general  grounds  we  conclude  that  the  action  is  an  effect  of  contractility. 

In  the  vertebrate  animal  cilia  are,  as  far  as  we  know,  wholly  independent 
of  the  nervous  system,  and  their  movement  is  probably  ceaseless.  In  such 
animals,  however,  as  infusoria,  hydrozoa,  etc.,  the  movements  in  a  ciliary 
tract  may  often  be  seen  to  stop  and  to  go  on  again,  to  be  now  fast,  now  slow, 
according  to  the  needs  of  the  economy,  and,  as  it  almost  seems,  according  to 
the  will  of  the  creature  ;  indeed,  in  some  of  these  animals  the  ciliary  move- 
ments are  clearly  under  the  influence  of  the  nervous  system. 

Observations  with  galvanic  currents,  constant  and  interrupted,  have  not 


ON  SOME  OTHER  FORMS  OF  CONTRACTILE  TISSUE.  121 

led  to  any  satisfactory  results,  and,  as  far  as  we  know  at  present,  ciliary 
action  is  most  affected  by  changes  of  temperature  and  chemical  media. 
Moderate  heat  quickens  the  movements,  but  a  rise  of  temperature  beyond  a 
certain  limit  (about  40°  C.,  in  the  case  of  the  pharyngeal  membrane  of  the 
frog)  becomes  injurious ;  cold  retards.  Very  dilute  alkalies  are  favorable, 
acids  are  injurious.  An  excess  of  carbonic  acid  or  an  absence  of  oxygen 
diminishes  or  arrests  the  movements,  either  temporarily  or  permanently, 
according  to  the  length  of  the  exposure.  Chloroform  or  ether  in  slight 
doses  diminishes  or  suspends  the  action  temporarily,  in  excess  kills  and  dis- 
organizes the  cells. 

Amoeboid  Movements. 

§  91.  The  white  blood-corpuscles,  as  we  have  said  (§  28),  are  able  of 
themselves  to  change  their  form  and  by  repeated  changes  of  form  to  move 
from  place  to  place.  Such  movements  of  the  substance  of  the  corpuscles  are 
called  amoeboid,  since  they  closely  resemble  and  appear  to  be  identical  in 
nature  with  the  movements  executed  by  the  amceba  and  similar  organisms. 
The  movements  of  the  endoplasm  of  the  vegetable  cell  seems  also  to  be  of 
the  same  kind. 

The  amoeba  changes  its  form  (and  shifts  its  place)  by  throwing  out  pro- 
jections of  its  substance,  called  pseudopodia  which  may  be  blunt  and  short, 
broad  bulgings  as  it  were,  or  may  be  so  long  and  thin  as  to  be  mere  filaments, 
or  may  be  of  an  intermediate  character.  As  we  watch  the  outline  of  the 
hyaline  ectosarc  we  may  see  a  pseudopodium  beginning  by  a  slight  bulging 
of  the  outline ;  the  bulging  increases  by  the  neighboring  portions  of  the 
ectosarc  moving  into  it,  the  movement  under  the  microscope  reminding  one 
of  the  flowing  of  melted  glass.  As  the  pseudopodium  grows  larger  and 
engages  the  whole  thickness  of  the  ectosarc  at  the  spot,  the  granules  of  the 
endosarc  may  be  seen  streaming  into  it,  forming  a  core  of  endosarc  in  the 
middle  of  the  bulging  of  ectosarc.  The  pseudopodium  may  continue  to 
grow  larger  and  larger  at  the  expense  of  the  rest  of  the  body,  and  eventually 
the  whole  of  the  amoeba  including  the  nucleus  may,  as  it  were,  have 
passed  into  the  pseudopodium;  the  body  of  the  amceba  will  now  occupy 
the  place  of  the  pseudopodium  instead  of  its  old  place ;  in  other  words,  it 
will  in  changing  its  form  have  also  changed  its  place. 

During  all  these  movements,  and  during  all  similar  amoeboid  movements, 
the  bulk  of  the  organism  will,  as  far  as  can  be  ascertained,  have  remained 
unchanged ;  the  throwing  out  a  pseudopodium  in  one  direction  is  accom- 
panied by  a  corresponding  retraction  of  the  body  in  other  directions.  If,  as 
sometimes  happens,  the  organism  throws  out  pseudopodia  in  various  direc- 
tions at  the  same  time,  the  main  body  from  which  the  pseudopodia  project  is 
reduced  in  thickness  ;  from  being  a  spherical  lump,  for  instance,  it  becomes  a 
branched  film.  The  movement  is  brought  about  not  by  increase  or  decrease 
of  substance  but  by  mere  translocation  of  particles  ;  a  particle  which  at  one 
moment  was  in  one  position  moves  into  a  new  position,  several  particles  thus 
moving  toward  the  same  point  cause  a  bulging  at  that  point,  and  several 
particles  moving  away  from  the  same  point  cause  a  retraction  at  that  point ; 
but  no  two  particles  get  nearer  to  each  other  so  as  to  occupy  together  less 
space  and  thus  lead  to  condensation  of  substance,  or  get  further  from  each 
other  so  as  to  occupy  more  space  and  thus  lead  to  increase  of  bulk. 

In  this  respect,  in  that  there  is  no  change  of  bulk  but  only  a  shifting  of 
particles  in  their  relative  position  to  each  other,  the  amoeboid  movement 
resembles  a  muscular  contraction ;  but  in  other  respects  the  two  kinds  of 
movement  seem  different,  and  the  question  arises,  have  we  the  right  to  speak 


122  GENERAL  FEATURES  OF  NERVOUS  TISSUES. 

of  the  substance  which  can  only  execute  amoeboid  movements  as  being  con- 
tractile ? 

We  may,  if  we  admit  that  contractility  is  at  bottom  simply  the  power  of 
shifting  the  relative  position  of  particles,  admit  that  muscular  contraction  is 
a  specialized  form  of  contraction.  In  a  plain  muscular  fibre  (which  we  may 
take  as  simpler  than  the  striated  muscle)  the  shifting  of  particles  is  special- 
ized in  the  sense  that  it  has  always  a  definite  relation  to  the  long  axis  of  the 
fibre;  when  the  fibre  contracts  a  certain  number  of  particles  assume  a  new 
position  by  moving  at  right  angles  to  the  long  axis  of  the  fibre,  and  the  fibre 
in  consequence  becomes  shorter  and  broader.  In  a  white  blood-corpuscle, 
amoeba,  or  other  organism  executing  amoeboid  movements,  the  shifting  of  the 
particles  is  not  limited  to  any  axis  of  the  body  of  the  organism  ;  at  the  same 
moment  one  particle  or  one  set  of  particles  may  be  moving  in  one  direction, 
and  another  particle  or  another  set  of  particles  in  another  direction.  A 
pseudopodium  short  and  broad,  or  long,  thin,  and  filamentous,  may  be  thrust 
out  from  any  part  of  the  surface  of  the  body  and  in  any  direction  ;  and  a 
previously  existing  pseudopodium  may  be  shortened,  or  be  wholly  drawn 
back  into  the  substance  of  the  body. 


CHAPTER  III. 

ON  THE  MORE  GENERAL  FEATURES  OF  NERVOUS  TISSUES. 

§  92.  In  the  preceding  chapter  we  have  dealt  with  the  properties  of  nerves 
going  to  muscles,  the  nerves  which  we  called  motor,  and  have  incidentally 
spoken  of  other  nerves  which  we  called  sensory.  Both  these  kinds  of  nerves 
are  connected  with  the  brain  and  spinal  cord,  and  form  part  of  the  general 
nervous  system.  We  shall  have  to  study  hereafter  in  detail  the  brain  and 
spinal  cord  ;  but  the  nervous  system  intervenes  so  repeatedly  in  the  processes 
carried  out  by  other  tissues  that  it  will  be  desirable,  before  proceeding 
further,  to  discuss  some  of  its  more  general  features. 

The  nervous  system  consists  (1)  of  the  brain  and  spinal  cord  forming 
together  the  cerebrospinal  axis  or  central  nervous  system,  (2)  of  the  nerves 
passing  from  that  axis  to  nearly  all  parts  of  the  body,  those  which  are  con- 
nected with  the  spinal  cord  being  called  spinal  and  those  which  are  connected 
with  the  brain,  within  the  cranium,  being  called  cranial,  and  (3)  of  ganglia 
distributed  along  the  nerves  in  various  parts  of  the  body. 

The  spinal  cord  obviously  consists  of  a  number  of  segments  or  metameres, 
following  in  succession  along  its  axis,  each  metamere  giving  off  on  each  side 
a  pair  of  spinal  nerves  ;  and  a  similar  division  into  metameres  may  be  traced 
in  the  brain,  though  less  distinctly,  since  the  cranial  nerves  are  arranged  in 
manner  somewhat  different  from  that  of  the  spinal  nerves.  We  may  take  a 
single  spinal  metamere,  represented  diagrammatically  in  Fig.  35,  as  illus- 
trating the  general  features  of  the  nervous  system  ;  and  since  the  half  on 
one  side  of  the  median  line  resembles  the  half  on  the  other  side  we  may 
deal  with  one  lateral  half  only. 

Each  spinal  nerve  arises  by  two  roots.  The  metamere  of  the  central  ner- 
vous system  C  consists,  as  we  shall  hereafter  see,  of  gray  matter  Gr  in  the 
interior  and  white  matter  W  on  the  outside.  From  the  anterior  part  of 
gray  matter  is  given  off  the  anterior  nerve  root  A  and  from  the  posterior 
part  the  posterior  nerve  root  P.  The  latter  passes  into  a  swelling  or  gan- 
glion G,  "  the  ganglion  of  the  posterior  root,"  or  more  shortly  "  the  spinal 


GENERAL  FEATURES  OF  NERVOUS  TISSUES. 


123 


FIG.  35. 


Scheme  of  the  Nerves  of  a  Segment  of  the  Spinal  Cord :  Gr  gray,  W  white  matter  of  spinal 
cord.  A  anterior,  P  posterior  root  G  ganglion  on  the  posterior  root  N  whole  nerve,  N'  spinal 
nerve  proper,  ending  in  M  skeletal  or  somatic  muscle,  S  somatic  sensory  cell  or  surface,  Xin 
other  ways.  V  visceral  nerve  (white  raimis  communicans)  passing  to  a  ganglion  of  the  sympa- 
thetic chain  2,  and  passing  on  as  V  to  supply  the  more  distant  ganglion  <r,  then  as  V"  to  the  peri- 
pheral ganglion  <r'  and  ending  in  m  splanchnic  muscle,  s  splanchnic  sensory  cell  or  surface,  x 
other  possible  splanchnic  endings.  From  2  is  given  off  the  revehent  nerve  r.  v  (gray  ramus 
communicans)  which  partly  passes  backward  toward  the  spinal  cord,  and  partly  runs  as  v.  m,  in 
connection  with  the  spinal  nerve,  to  supply  vasomotor  (constrictor)  fibres  to  the  muscles  (TO')  of 
bloodvessels  in  certain  parts,  for  example,  in  the  limbs.  Sy,  the  sympathetic  chain  uniting  the 
ganglia  of  the  series  2.  The  terminations  of  the  other  nerves  arising  from  2,  a,  a'  are  not 
shown. 


124  GENERAL  FEATURES  OF  NERVOUS  TISSUES. 

ganglion  ";  the  anterior  root  does  not  pass  into  this  ganglion.  Beyond  the 
ganglion  the  roots  join  to  form  the  nerve  trunk  N.  We  shall  later  on  give 
the  evidence  that  the  nerve  fibres  composing  the  posterior  root  P  are,  as  far 
as  we  know  at  present,  exclusively  occupied  in  carrying  nervous  impulses 
from  the  tissues  of  the  body  to  the  central  nervous  system,  and  that  the  fibres 
composing  the  anterior  root  A  are  similarly  occupied  in  carrying  impulses 
from  the  central  nervous  system  to  the  several  tissues :  that  is  to  say,  the 
former  is  made  up  of  sensory  fibres,  or  (since  the  impulses  passing  along 
them  to  the  central  system  may  give  rise  to  effects  other  than  sensations) 
afferent  fibres,  while  the  latter  is  made  up  of  motor,  or  (since  the  impulses 
passing  along  them  from  the  central  nervous  system  may  produce  effects 
other  than  movements),  efferent  fibres.  The  nerve  trunk  N  is  consequently 
a  mixed  nerve  composed  of  afferent  and  efferent  fibres. 

By  far  the  greater  part  of  this  mixed  nerve,  dividing  into  various  branches, 
is  distributed  (Nf)  to  the  skin  and  the  skeletal  muscles,  some  of  the  fibres 
(motor)  ending  in  muscular  fibres  (M\  others  (sensory)  ending  in  epithelial 
cells  ($)  connected  with  the  skin,  which  we  shall  consider  hereafter  under 
the  name  of  sensory  epithelial  cells,  while  others,  X,  after  dividing  into 
minute  branches  and  forming  plexuses  end,  in  ways  not  yet  definitely  deter- 
mined, in  tissues  associated  with  the  skin  and  skeletal  muscles.  Morpholo- 
gists  distinguish  the  parts  which  go  to  form  the  skin,  skeletal  muscles,  etc., 
as  somatic,  from  the  splanchnic  parts  which  go  to  form  the  viscera.  We  may, 
accordingly,  call  this  main  part  of  the  spinal  nerve  the  somatic  division  of 
the  nerve. 

Soon  after  the  mixed  nerve  jV  leaves  the  spinal  canal,  it  gives  off  a  small 
branch  V,  which  under  the  name  of  (white)  ramus  communicans,  joins  one 
of  a  longitudinal  series  of  ganglia  (S)  conspicuous  in  the  thorax  as  the  main 
sympathetic  chain.  This  branch  is  destined  to  supply  the  viscera,  and  might, 
therefore,  be  called  the  splanchnic  division  of  the  spinal  nerve.  We  may  say 
at  once,  without  entering  into  details,  that  the  whole  of  the  sympathetic 
system,  with  its  ganglia,  plexuses,  and  nerves,  is  to  be  regarded  as'  a  develop- 
ment or  expansion  of  the  visceral  or  splanchnic  divisions  of  certain  spinal 
nerves.  By  means  of  this  system  splanchnic  fibres  from  the  central  nervous 
system  are  distributed  to  the  tissues  of  the  viscera,  some  of  them  on  their 
way  passing  through  secondary  ganglia  <r,  and,  it  may  be,  tertiary  ganglia. 
There  are,  however,  as  we  shall  see,  certain  nerves  or  fibres  which  do  not  run 
in  the  sympathetic  system,  and  yet  are  distributed  to  the  viscera  and  are 
"splanchnic"  in  nature.  We  cannot,  therefore,  use  the  word  sympathetic 
to  denote  all  the  fibres  which  are  splanchnic  in  nature.  On  the  other  hand, 
the  "  splanchnic  nerves  "  of  the  anatomist  form  a  part  only  of  the  splanchnic 
system  in  the  above  sense,  the  term  thus  used  is  limited  to  particular  nerves 
of  the  splanchnic  system  distributed  to  the  abdomen  ;  and  the  double  use  of 
the  term  splanchnic  might  lead  to  confusion.  The  difficulty,  may,  perhaps, 
be  avoided  by  calling  the  splanchnic  nerves  of  the  anatomist  "  abdominal 
splanchnic."  The  majority  of  these  splanchnic  fibres  seem  to  be  efferent  in 
nature,  carrying  impulses  from  the  central  nervous  system  to  the  tissues, 
some  ending  in  plain  muscular  fibres  (m),  others  in  other  ways  (#)  ;  but  some 
of  the  fibres  are  afferent  and  convey  impulses  from  the  viscera  to  the  central 
nervous  system,  and  it  is  probable  that  some  of  these  begin  or  end  in  epithe- 
lial cells  of  the  viscera  (s). 

We  shall  have  occasion  in  the  next  chapter  to  speak  of  nerves  which 
govern  the  bloodvessels  of  the  body,  the  so-called  vasomotor  nerves.  A  cer- 
tain class  of  these,  namely,  the  vaso-constrictor  nerves  or  fibres  are  branches 
of  the  splanchnic  divisions  of  the  cerebro-spinal  nerves,  and,  as  we  shall  see, 
the  vaso-constrictor  nerves  of  the  skeletal  muscles,  skin,  and  other  parts  sup- 


GENERAL  FEATURES  OF  NERVOUS  TISSUES.  125 

plied  by  somatic  nerves,  after  running  for  some  distance  in  the  splanchnic 
division  (  F),  turn  aside  (r.  v  and  v.  m)  and  join  the  somatic  division,  the 
fibres  of  which  they  accompany  on  their  way  to  the  tissues  whose  bloodvessels 
(in'~)  they  supply. 

We  have  seen  (§  68)  that  a  nerve  going  to  a  muscle  is  composed  of  nerve 
fibres,  chiefly  medullated,  some,  however,  being  non-medullated,  bound 
together  by  connective  tissues.  The  same  description  holds  good  for  the 
whole  somatic  division  of  each  of  the  spinal  nerves.  The  splanchnic  division 
also  consists  of  medullated  and  non-medullated  fibres  bound  together  by  con- 
nective tissue,  but  in  it  the  nou-medullated  fibres  preponderate,  some  branches 
appearing  to  contain  hardly  any  medullated  fibres  at  all.  The  non-medul- 
lated fibres  which  are  found  in  the  somatic  division  appear  to  be  fibres  which 
have  joined  that  division  from  the  splanchnic  division.  So  prominent  are 
non-medullated  fibres  in  splanchnic  nerves  and  hence  in  the  sympathetic 
system  that  they  are  sometimes  called  sympathetic  fibres. 

We  have  said  that  the  axis-cylinder,  whether  of  a  medullated  or  non- 
medullated  fibre,  is  to  be  considered  as  a  long-drawn-out  process  of  a  nerve 
cell.  Nerve  cells  are  found  in  three  main  situations.  1.  In  the  central  ner- 
vous system,  the  brain,  and  spinal  cord.  2.  In  the  several  ganglia  placed 
along  the  course  of  the  nerves,  both  the  spinal  ganglia,  and  the  ganglia  of 
the  splanchnic  or  sympathetic  system.  3.  At  the  terminations  of  nerves  in 
certain  tissues.  Some  of  these  latter  are  to  be  regarded  as  small,  more  or 
less  terminal,  ganglia,  and  similar  minute  ganglia  consisting  of  two  or  three 
cells  only  are  found  frequently  along  the  course  of  splanchnic  nerves  and 
occasionally  along  the  course  of  spinal  nerves;  such  cells  really,  therefore, 
belong  to  the  second  group.  But  besides  this,  in  certain  situations,  as,  for 
instance,  in  certain  organs  of  the  skin,  and  in  the  organs  of  special  sense, 
nerves,  generally  afferent  or  sensory  in  nature,  either  actually  end  in,  or  at 
their  termination  are  connected  with,  cells  which  appear  to  be  of  a  nervous 
nature ;  such  cells  form  a  distinct  category  by  themselves. 

Hence  along  its  whole  course  a  nerve  consists  exclusively  of  nerve  fibres 
(and  the  connective  tissue  supporting  them),  except  in  the  central  nervous 
system  from  which  it  springs  in  the  ganglia,  great  and  small,  through  which 
it  passes,  or  which  are  attached  to  it  at  one  part  or  another  of  its  course,  in 
both  of  which  situations  nerve  cells  are  found,  and  at  its  termination  where 
its  fibres  may  end  in  nerve  cells. 

The  features  of  these  nerve  cells  differ  in  these  several  situations.  The 
characters  of  the  terminal  cells  which,  as  we  have  said,  are  chiefly  sensory, 
and  the  structure  of  the  brain  and  spinal  cord,  we  shall  study  in  detail  later 
on.  We  may  here  confine  our  attention  to  the  nerve  cells  of  the  ganglia 
and  to  some  of  the  broad  features  of  the  nerve  cells  of  the  spinal  cord. 

§  93.  Spinal  ganglia.  When  a  longitudinal  section  of  a  spinal  ganglion 
is  examined  under  a  low  power,  the  fibres  of  the  posterior  root  as  they  enter 
the  ganglion  are  observed  to  spread  out  and  pass  between  relatively  large 
and  conspicuously  nucleated  cells,  which  are  to  a  large  extent  arranged  in 
groups,  somewhat  after  the  fashion  of  a  bunch  of  grapes.  These  are  the 
nerve  cells ;  they  have  frequently  a  diameter  of  about  100//.,  but  may  be  still 
larger  or  may  be  much  smaller.  In  a  transverse  section  it  will  be  observed 
that  a  large  compact  mass  of  these  cells  lies  on  the  outer  side  of  the  gang- 
lion, and  that  the  racemose  groups  on  the  inner  side  are  smaller.  A  quantity 
)f  connective  tissue  carrying  bloodvessels  and  lymphatics  runs  between  the 
roups,  and  passing  into  each  group  runs  between  the  cells  and  fibres ;  and 
thick  wrapping  of  connective  tissue  continuous  with  the  sheath  of  the 
icrves  surrounds  and  forms  a  sheath  for  the  whole  ganglion. 

Each  of  the  nerve  cells — ganglionic  cells,  as  they  are  called — examined 


126  GENERAL  FEATURES  OF  NERVOUS  TISSUES. 

under  a  higher  power,  either  after  having  been  isolated  or  in  an  adequately 
thin  and  prepared  section,  will  present  the  following  features : 

The  cell  consists  of  a  cell  body  which  is,  normally,  pear-shaped,  having  a 
broad  end  in  which  is  placed  the  nucleus,  and  a  narrow  end  which  thins  out 
into  a  stalk,  and  is  eventually  continued  on  as  a  nerve  fibre.  The  substance 
of  the  cell  body  is  of  the  kind  which  we  call  finely  granular  protoplasm ; 
sometimes  there  is  an  appearance  of  fibrillation,  the  fibrillse  passing  in 
various  directions  in  the  body  of  the  cell  and  being  gathered  together  in  a 
longitudinal  direction  in  the  stalk.  Sometimes  the  cell  body  immediately 
around  the  nucleus  appears  of  a  different  grain  from  that  nearer  the  stalk, 
and  not  unfrequently  near  the  nucleus  is  an  aggregation  of  discrete  pigment 
granules  imbedded  in  the  protoplasm. 

The  nucleus,  like  the  nuclei  of  nearly  all  nerve  cells,  is  large  and  con- 
spicuous, and  when  in  a  normal  condition  is  remarkably  clear  and  refractive, 
though  it  appears  to  consist  like  other  nuclei  of  a  nuclear  membrane  and 
network  and  nuclear  interstitial  material.  Even  more  conspicuous,  perhaps, 
is  a  very  large  spherical,  highly  refractive  nucleolus;  occasionally  more  than 
one  nucleolus  is  present. 

Surrounding  the  cell  body  is  a  distinct  sheath  or  capsule  consisting  of  a 
transparent,  hyaline,  or  faintly  fibrillated  membrane,  lined  on  the  inside  by 
one  layer  or  by  two  layers  of  flat,  polygonal,  nucleated  epithelioid  cells  or 
plates ;  that  is  to  say,  cells  which  resemble  epithelium  cells,  but  differ  not 
only  in  being  extremely  flattened,  but  also  in  the  cell  body  being  transformed 
from  ordinary  granular  protoplasm  into  a  more  transparent  differentiated 
material.  In  stained  specimens  the  nuclei  of  these  plates  are  very  conspicu- 
ous. Under  normal  conditions  this  sheath  is  in  close  contact  with  the  whole 
body  of  the  cell,  but  in  hardened  and  prepared  specimens  the  cell  body  is 
sometimes  seen  shrunk  away  from  the  sheath,  leaving  a  space  between  them. 
Occasionally  the  cell  body  while  remaining  attached  to  the  sheath  at  three 
or  four  or  more  points  is  retracted  elsewhere,  and  accordingly  assumes  a 
more  or  less  stellate  form  ;  but  this  artificial  condition  must  not  be  con- 
founded with  the  natural  branched  form  which,  as  we  shall  see,  other  kinds 
of  nerve  cells  possess. 

When  a  section  is  made  through  a  hardened  ganglion,  the  plane  of  the 
section  passes  through  the  stalks  of  few  only  of  the  cells,  and  that  rarely  for 
any  great  distance  along  the  stalk,  since  in  the  case  of  many  of  the  cells  the 
stalk  is  more  or  less  curved  and  consequently  runs  out  of  the  plane  of  sec- 
tion ;  but  in  properly  isolated  cells  we  can  see  that  in  many  cases,  and  we 
have  reasons  to  believe  that  in  all  cases,  the  stalk  of  the  cell  is,  as  we  have 
said,  continued  on  into  a  nerve  fibre.  As  the  cell  body  narrows  into  the 
stalk  several  nuclei  make  their  appearance,  lodged  on  it ;  these  are  small 
granular  nuclei,  wholly  unlike  the  nucleus  of  the  cell  body  itself,  and  more 
like,  though  not  quite  like,  the  nuclei  of  the  neurilemma  of  a  nerve.  They 
are  probably  of  the  same  nature  as  the  latter ;  and,  indeed,  as  we  trace  the 
narrowing  stalk  downward  a  fine  delicate  sheath,  which,  if  present,  is  at 
least  not  obvious  over  the  cell  body,  makes  its  appearance,  and  a  little  further 
on,  between  this  sheath,  which  is  now  clearly  a  neurilemma,  and  the  stalk  of 
the  cell  body,  which  has  by  this  time  become  a  cylinder  of  uniform  width 
and  is  now  obviously  an  axis-cylinder,  a  layer  of  medulla,  very  fine  at  first, 
but  rapidly  thickening,  is  established.  The  stalk  of  the  nerve  cell  thus 
becomes  an  ordinary  medullated  nerve  fibre.  The  sheath  of  the  cell  is  con- 
tinued also  on  to  the  nerve  fibre,  not  as  was  once  thought  as  the  neurilemma, 
but  as  that  special  sheath  of  connective  tissue  of  which  we  have  already 
spoken  as  Henle's  sheath,  and  which  ultimately  becomes  fused  with  the  con- 
nective tissue  of  the  nerve. 


GENERAL  FEATURES  OF   NERVOUS  TISSUES.  127 

At  some  variable  distance  from  the  cell  the  nerve  fibre  bears  the  first 
node,  and  either  at  this  or  some  early  succeeding  node  the  fibre  divides  into 
two ;  as  we  have  seen,  division  of  a  medullated  nerve  fibre  always  takes 
place  at  a  node.  The  two  divisions  thus  arising  run  in  opposite  directions, 
forming  in  this  way  a  T-piece ;  and  while  one  division  runs  in  one  direction 
toward  the  posterior  root,  the  other  runs  in  an  opposite  direction  toward  the 
nerve  trunk.  The  nerve  cell  is  thus,  as  it  were,  a  side  piece  attached  to  a 
fibre  passing  through  the  ganglion  on  its  way  from  the  posterior  root  to  the 
nerve  trunk.  It  cannot  be  said  that  in  any  one  ganglion  this  connection  has 
been  traced  in  the  case  of  every  nerve  cell  of  the  ganglion ;  but  the  more 
care  is  taken,  and  the  more  successful  the  preparation,  the  greater  is  the 
number  of  cells  which  may  be  isolated  with  their  respective  T-pieces ;  so 
that  we  may  conclude  that,  normally,  every  cell  of  a  ganglion  is  connected 
on  the  one  hand  with  a  fibre  of  the  posterior  root,  and  on  the  other  hand 
with  a  fibre  of  the  nerve  trunk.  We  have  reasons  further  to  believe  that 
every  fibre  of  the  posterior  root  in  passing  through  the  ganglion  on  its  way 
to  the  mixed  nerve  trunk  is  thus  connected  with  a  nerve  cell ;  but  this  has 
been  called  in  question.  In  certain  animals — for  instance,  certain  fishes — 
the  cells  of  the  spinal  ganglia  are  not  pear-shaped,  but  oval  or  fusiform,  and 
each  narrow  end  is  prolonged  into  a  nerve  fibre,  one  end  thus  being  con- 
nected with  the  posterior  root  and  the  other  with  the  nerve  trunk.  In  such 
a  case  the  nerve  cell  is  simply  a  direct  enlargement  of  the  axis-cylinder,  with 
a  nucleus  placed  in  the  enlargement.  The  nerve  cells  above  described  are 
similar  enlargements,  also  bearing  nuclei,  placed  not  directly  in  the  course 
of  the  axis-cylinder,  but  on  one  side,  and  connected  with  the  axis-cylinder 
by  the  cross-piece  of  the  T-piece.  Hence  the  ordinary  ganglion  cell  is 
spoken  of  as  being  unipolar,  those  of  fishes  being  called  bipolar. 

In  examining  spinal  ganglia  cells  are  sometimes  found  which  bear  no 
trace  of  any  process  connecting  them  with  a  nerve  fibre.  Such  cells  are 
spoken  of  as  apolar.  It  is  possible  that  such  a  cell  may  be  a  young  cell 
which  has  not  yet  developed  its  nerve  process,  or  an  old  cell  which  has  by 
degeneration  lost  its  nerve  process. 

§  94.  The  ganglia  of  the  splanchnic  system,  like  the  spinal  ganglia,  con- 
sist of  nerve  cells  and  fibres  imbedded  in  connective  tissue,  which,  however, 
is  of  a  looser  and  less  compact  nature  in  them  than  in  the  spinal  ganglia. 
As  far  as  the  characters  of  their  nuclei,  the  nature  of  their  cell  substance, 
and  the  possession  of  a  sheath,  are  concerned,  what  has  been  said  concerning 
the  nerve  cells  of  spinal  ganglia  holds,  in  general,  good  for  those  of  splanch- 
nic ganglia;  and,  indeed,  in  certain  ganglia  of  the  splanchnic  system  con- 
nected with  the  cranial  nerves  the  nerve  cells  appear  to  be  wholly  like  those 
of  spinal  ganglia.  In  most  splanchnic  ganglia,  however,  in  those  which  are 
generally  called  sympathetic  ganglia,  two  important  differences  may  be  ob- 
served between  what  we  may  call  the  characteristic  nerve  cell  of  the  splanch- 
nic ganglion  and  the  cell  of  the  spinal  ganglion. 

In  the  first  place,  while  the  nerve  cell  of  the  spinal  ganglia  has  one 
process  only,  the  nerve  cell  of  the  splanchnic  ganglia  may  have,  and  fre- 
quently has,  two,  three,  or  even  four  or  five  processes ;  it  is  a  multipolar 
cell. 

In  the  second  place,  while  these  processes  of  the  splanchnic  ganglion 
cell  are  continued  on  as  nerve  fibres,  as  is  the  single  process  of  the  spinal 
ganglion  cell,  the  nerve  fibres  so  formed  are,  in  the  case  of  most  of  the  pro- 
cesses of  a  cell,  and  sometimes  in  the  case  of  all  the  processes,  non-medul- 
lated  fibres,  and  remain  non-medullated  as  far  as  they  can  be  traced.  In 
some  instances  one  process  becomes  at  a  little  distance  from  the  cell  a 
medullated  fibre,  while  the  other  processes  become  non-medullated  fibres; 


128  GENERAL  FEATURES  OF  NERVOUS  TISSUES. 

and  we  are  led  to  believe  that  in  this  case  the  medullated  fibre  is  proceeding 
to  the  cell  on  its  way  from  the  central  nervous  system,  and  that  the  non- 
medullated  fibres  are  proceeding  from  the  cell  on  their  way  to  more  peri- 
pherally placed  parts ;  the  nerve  cell  seems  to  serve  as  a  centre  for  the 
division  of  nerve  fibres,  and  also  for  the  change  from  medullated  to  non- 
medullated  fibres. 

In  consequence  of  its  thus  possessing  several  processes,  the  splanchnic 
ganglion  cell  is  more  or  less  irregular  and  often  star-like  in  form,  in  con- 
trast to  the  pear  shape  of  the  spinal  ganglion  cell.  But  in  certain  situations 
in  certain  animals — for  instance,  in  the  frog — in  many  of  the  ganglia  of  the 
abdomen,  and  in  the  small  ganglia  in  the  heart,  pear-shaped  splanchnic 
ganglion  cells  are  met  with.  In  such  cases  the  nucleated  sheath  is  distinctly 
pear-shaped  or  balloon-shaped,  and  the  large  conspicuous  nucleus  is  placed, 
as  in  the  spinal  ganglion  cell,  near  the  broad  end,  but  the  cell  substance  of 
the  cell  is  gathered  at  the  stalk,  not  into  a  single  fibre,  but  into  two  fibres, 
one  of  which  is  straight  and  the  other  twisted  spirally  round  the  straight 
one.  The  two  fibres  run  for  some  distance  togther  in  the  same  funnel-shaped 
prolongation  of  the  nucleated  sheath  of  the  cell,  but  eventually  separate, 
each  fibre  acquiring  a  sheath  (sheath  of  Henle)  of  its  own.  Generally,  if  not 
always,  one  fibre,  usually  the  straight  one,  becomes  a  medullated  fibre,  while 
the  other,  usually  the  twisted  or  spiral  one,  is  continued  as  a  non-medullated 
fibre,  while  within  the  common  nucleated  sheath  both  fibres,  especially  the 
spiral  one,  bear  nuclei  of  the  same  character  as  those  seen  in  a  corresponding 
situation  in  the  spinal  ganglion  cell.  It  has  been  maintained  that  the  straight 
and  spiral  fibres  take  origin  from  different  parts  of  the  nerve  cell,  but  this 
has  not  been  definitely  proved. 

In  the  walls  of  the  intestine,  in  connection  with  splanchnic  nerves,  are 
found  peculiar  nerve  cells  forming  what  are  known  as  the  plexuses  of  Meiss- 
ner  and  Auerbach,  but  we  shall  postpone  for  the  present  any  description  of 
these  or  of  other  peculiar  splanchnic  cells. 

§  95.  In  the  central  nervous  system  nerve  cells  are  found  in  the  so-called 
gray  matter  only  ;  they  are  absent  from  the  white  matter.  In  the  gray  matter 
of  the  spinal  cord,  in  the  parts  spoken  of  as  the  anterior  cornua,  we  meet 
with  remarkable  nerve  cells  of  the  following  characters.  The  cells  are  large, 
varying  in  diameter  from  50,a  to  140/jt,  and  each  consists  of  a  cell  body  sur- 
rounding a  large  conspicuous  refractive  nucleus,  in  which  is  placed  an  even 
still  more  conspicuous  nucleolus.  The  nucleus  resembles  the  nuclei  of  the 
ganglion  cells  already  described,  and  the  cell  body,  like  the  cell  body  of  the 
ganglion  cells,  is  composed  of  finely  granular  protoplasm,  often  fibrillated, 
though  generally  obscurely  so ;  frequently  a  yellowish-brown  pigment  is 
deposited  in  a  part  of  the  cell  body  not  far  from  the  nucleus.  The  cell  body 
is  prolonged  sometimes  into  two  or  three  only,  but  generally  into  several 
processes,  which  appear  more  distinctly  fibrillated  than  the  more  central 
parts  of  the  cell  body.  These  processes  are  of  two  kinds.  One  process,  and, 
apparently,  one  only,  but  in  the  case  of  the  cells  of  the  anterior  cornu, 
always  one,  is  prolonged  as  a  thin  unbranched  band,  which  retains  a  fairly 
uniform  diameter  for  a  considerable  distance  from  the  cell,  and  when  suc- 
cessfully traced  is  found  sooner  or  later  to  acquire  a  medulla  and  to  become 
the  axis-cylinder  of  a  nerve  fibre ;  the  processes  which  thus  pass  out  from 
the  gray  matter  of  the  anterior  cornu  through  the  white  matter  form  the 
anterior  roots  of  the  spinal  nerve.  Such  a  process  is  accordingly  called  the 
axis-cylinder  process.  'The  other  processes  of  the  cell  rapidly  branch,  and  so 
divide  into  very  delicate  filaments  which  are  soon  lost  to  view  in  the  sub- 
stance of  the  gray  matter.  Indeed,  the  gray  matter  is  partly  made  up  of  a 
plexus  of  delicate  filaments  arising,  on  the  one  hand,  from  the  divisions  of 


GENERAL  FEATURES  OF  NERVOUS  TISSUES.  129 

processes  of  the  nerve  cells,  and  on  the  other,  from  the  division  of  the  axis- 
cylinders  of  fibres  running  in  the  gray  matter. 

The  cell  is  not  surrounded  like  the  ganglion  cell  by  a  distinct  sheath.  As 
we  shall  see  later  on,  while  treating  in  detail  of  the  central  nervous  system, 
all  the  nervous  elements  of  the  spinal  cord  are  supported  by  a  network  or 
spongework  of  delicate  peculiar  tissue  called  neuroglia,  analogous  to  and 
serving  much  the  same  function  as,  but  different  in  origin  and  nature  from, 
connective  tissue,  This  neuroglia  forms  a  sheath  to  the  nerve  cell  and  to  its 
processes,  as  well  as  to  the  nerve  fibres  running  both  in  the  white  and  the 
gray  matter ;  hence  within  the  central  nervous  system  the  fibres,  whether 
medullated  or  not  possess  no  separate  neurilemma ;  tubular  sheaths  of  the 
neuroglia  give  the  axis-cylinder  and  medulla  all  the  support  they  need. 

All  the  nerve  cells  of  the  anterior  cornu  probably  possess  an  axis-cylinder 
process,  and  other  cells  similarly  provided  with  an  axis-cylinder  process  are 
found  in  other  parts  of  the  gray  matter.  But  in  certain  parts,  as,  for  instance, 
in  the  posterior  cornu,  many  of  the  cells  appear  to  possess  no  axis-cylinder 
process  ;  in  such  cases  all  the  processes  appear  to  branch  out  rapidly  into  fine 
filaments.  Except  for  this  absence,  apparent  or  real,  of  an  axis-cylinder 
process,  such  cells  resemble  in  their  general  features  the  cells  of  the  anterior 
cornu,  though  they  are  generally  somewhat  smaller.  Speaking  generally, 
the  great  feature  of  the  nerve  cells  of  the  central  nervous  system  as  distin- 
guished from  the  ganglion  cells  is  the  remarkable  way  in  which  their  pro- 
cesses branch  off  into  a  number  of  delicate  fi.laments,  corresponding  to  the 
delicate  filaments  or  fibrillse  in  which  at  its  termination  in  the  tissues  the 
axis-cylinder  of  a  nerve  often  ends. 

§  96.  From  the  above  description  it  is  obvious  that  in  the  spinal  cord 
(to  which  as  representing  the  central  nervous  system  we  may  at  present  con- 
fine ourselves,  leaving  the  brain  for  later  study)  afferent  fibres  (fibres  of  the 
posterior  root)  are  in  some  way  by  means  of  the  gray  matter  brought  into 
connection  with  efferent  fibres  (fibres  of  the  anterior  root)  ;  in  other  words, 
the  spinal  cord  is  a  centre  uniting  afferent  and  efferent  fibres.  The  spinal 
ganglia  are  not  centres  in  this  sense  ;  the  nerve  cells  composing  the  ganglia 
are  simply  relays  on  the  afferent  fibres  of  the  posterior  root,  they  have  no 
connection  whatever  with  efferent  fibres,  they  are  connected  with  fibres  of 
one  kind  only.  Concerning  the  ganglia  of  the  splanchnic  system  we  cannot 
in  all  cases  make  at  present  a  positive  statement,  but  the  evidence  so  far  at 
our  disposal  points  to  the  conclusion  that  in  them  as  in  the  spinal  ganglia 
each  nerve  cell  belongs  to  fibres  of  one  function  only,  that  where  several 
processes  of  a  cell  are  prolonged  into  nerve  fibres  these  fibres  have  all  the 
same  function,  the  nerve  cell  being,  as  in  the  spinal  ganglia,  a  mere  relay. 
We  have  no  satisfactory  evidence  that  in  a  ganglion  the  fibres  springing 
from  or  connected  with  one  cell  join  another  cell  so  as  to  convert  the 
ganglion  into  a  centre  joining  together  cells  whose  nerve  fibres  have  dif- 
ferent functions. 

We  shall  have  later  on  to  bring  forward  evidence  that  the  nucleated  cell 
body  of  a  nerve  cell  in  a  ganglion  or  elsewhere  is  in  some  way  or  other  con- 
nected with  the  nutrition,  the  growth  and  repair  of  the  nerve  fibres  springing 
from  it.  Besides  this  nutritive  function  the  multipolar  cells  of  the  splanchnic 
ganglia  appear  to  serve  the  purpose  of  multiplying  the  tracts  along  which 
nervous  impulses  may  pass.  An  impulse,  for  instance,  reaching  a  multipolar 
cell  in  one  of  the  proximal  sympathetic  ganglia  along  one  fibre  or  process 
(the  fibre  in  very  many  cases  being  a  medullated  fibre)  can  pass  out  of  the 
cell  in  various  directions  along  several  processes  or  fibres,  which,  in  the 
majority  of  cases,  if  not  always,  are  non-medullated  fibres.  Thus  these  nerve 
cells  are  organs  of  distribution  for  impulses  of  the  same  kind.  What  further 

9 


130  GENERAL  FEATURES  OF  NERVOUS  TISSUES. 

modifications  of  the  impulses  thus  passing  through  them  these  ganglia  may 
bring  about  we  do  not  know. 

It  is  only  in  some  few  instances  that  we  have  any  indications,  and  those 
of  a  very  doubtful  character,  that  the  ganglia  of  the  splanchnic  system  can 
carry  out  either  of  the  two  great  functions  belonging  to  what  is  physiologi- 
cally called  a  nerve  centre,  namely  the  function  of  starting  nervous  impulses 
anew  from  within  itself,  the  function  of  an  automatic  centre  so  called,  and  the 
function  of  being  so  affected  by  the  advent  of  afferent  impulses  as  to  send 
forth  in  response  efferent  impulses,  of  converting,  as  it  were,  afferent  into 
efferent  impulses,  the  function  of  a  reflex  centre  so  called. 

In  the  central  nervous  system,  the  brain  with  the  spinal  cord,  which 
supplies  the  nervous  centres  for  automatic  actions  and  for  reflex  actions — 
indeed  all  the  processes  taking  place  in  the  central  nervous  system  (at 
least  all  such  as  come  within  the  province  of  physiology) — fall  into  or 
may  be  considered  as  forming  part  of  one  or  the  other  of  these  two 
categories. 

§'  97.  Reflex  actions.  In  a  reflex  action  afferent  impulses  reaching  the 
nervous  centre  give  rise  to  the  discharge  of  efferent  impulses,  the  discharge 
following  so  rapidly  and  in  such  away  as  to  leave  no  doubt  that  it  is  caused 
by  the  advent  at  the  centre  of  the  afferent  impulses.  Thus  a  frog  from 
which  the  brain  has  been  removed  while  the  rest  of  the  body  has  been  left 
intact  will  frequently  remain  quite  motionless  (as  far  at  least  as  the  skeletal 
muscles  are  concerned)  for  an  almost  indefinite  time  ;  but  if  its  skin  be 
pricked,  or  if  in  other  ways  afferent  impulses  be  generated  in  afferent 
fibres  by  adequate  stimulation,  movement  of  the  limbs  or  body  will  imme- 
diately follow.  Obviously  in  this  instance  the  stimulation  of  afferent  fibres 
has  been  the  cause  of  the  discharge  of  impulses  along  efferent  fibres. 

The  machinery  involved  in  a  such  a  reflex  act  consists  of  three  parts :  (1) 
the  afferent  fibres,  (2)  the  nerve  centre,  in  this  case  the  spinal  cord,  and  (3) 
the  efferent  fibres.    [Fig.  36.]    If  any  one  of  these  three 
parts  be  missing  the  reflex  act  cannot  take  place ;  if,  for 
instance,  the    afferent  nerves  or  the  efferent  nerves  be 
cut  across  in  their  course,  or  if  the  centre,  the  spinal 
cord,  be  destroyed,  the  reflex  action  cannot  take  place. 
Reflex  actions  can  be  carried  out  by  means  of  the 
brain,  as  we  shall  see  while  studying  that  organ  in  de- 
tail, but  the  best  and  clearest  examples  of  reflex  action 
are  manifested  by  the  spinal  cord  ;  in  fact,  reflex  action 
is  one  of   the  most  important    functions  of   the  spinal 
cord.     We  shall  have  to  study  the  various  reflex  actions 
Diagram  illustrating  of   the  spinal    cord    in  detail   hereafter,  but  it  will  be 
Simplest  Form  of  Re-  desirable  to  point  out  here  some  of   their  general  fea- 

flex  Apparatus.] 

tures. 

When  we  stimulate  the  nerve  of  a  muscle-nerve  preparation  the 
result,  though  modified  in  part  by  the  condition  of  the  muscle  and 
nerve,  whether  fresh  and  irritable  or  exhausted,  for  instance,  is  directly 
dependent  on  the  nature  and  strength  of  the  stimulus.  If  we  use  a  single 
induction-shock  we  get  a  simple  contraction,  if  the  interrupted  current 
we  get  a  tetanus,  if  we  use  a  weak  shock  we  get  a  slight  contraction,  if 
a  strong  shock  a  large  contraction,  and  so  on  ;  and  throughout  our  study 
of  muscular  contractions  we  assumed  that  the  amount  of  contraction  might 
be  taken  as  a  measure  of  the  magnitude  of  the  nervous  impulses  generated 
by  the  stimulus.  And  it  need  hardly  be  said  that  when  we  stimulate  cer- 
tain fibres  only  of  a  motor  nerve,  it  is  only  the  muscular  fibres  in  which 
those  nerve  fibres  end  which  are  thrown  into  contraction. 


GENERAL  FEATURES  OF  NERVOUS  TISSUES.  131 

In  a  reflex  action,  on  the  other  hand,  the  movements  called  forth  by  the 
same  stimulus  may  be  in  one  case  insignificant  and  in  another  violent  and 
excessive,  the  result  depending  on  the  arrangements  and  condition  of  the 
reflex  mechanism.  Thus  the  mere  contact  of  a  hair  with  the  mucous  mem- 
brane lining  the  larynx,  a  contact  which  can  originate  only  the  very  slightest 
afferent  impulses,  may  call  forth  a  convulsive  fit  of  coughing,  in  which  a  very 
large  number  of  muscles  are  thrown  into  violent  contractions;  whereas  the 
.same  contact  of  the  hair  with  other  surfaces  of  the  body  may  produce  no 
obvious  effect  at  all.  Similarly,  while  in  the  brainless  but  otherwise  normal 
frocr,  a  slight  touch  on  the  skin  of  the  flank  will  produce  nothing  but  a  faint 
flicker  of  the  underlying  muscles,  the  same  touch  on  the  same  part  of  a  frog 
poisoned  with  strychnine  will  produce  violent  lasting  tetanic  contractions  of 
nearly  all  the  muscles  of  the  body.  Motor  impulses,  as  we  have  seen,  travel 
jilong  motor  nerves  without  any  great  expenditure  of  energy,  and  probably 
without  increasing  that  expenditure  as  they  proceed ;  and  the  same  is  appa- 
rently the  case  with  afferent  impulses  passing  along  afferent  nerves.  When, 
however,  in  a  reflex  action  afferent  impulses  reach  the  nerve  centre,  a  change 
in  the  nature  and  magnitude  of  the  impulses  takes  place.  It  is  not  that  in 
the  nerve  centre  the  afferent  impulses  are  simply  turned  aside  or  reflected 
into  efferent  impulses  ;  and  hence  the  name  "reflex"  action  is  a  bad  one. 
It  is  rather  that  the  afferent  impulses  act  afresh,  as  it  were,  as  a  stimulus  to 
the  nerve  centre,  producing  according  to  circumstances  and  conditions  either 
a  few  weak  efferent  impulses  or  a  multitude  of  strong  ones.  The  nerve  centre 
may  be  regarded  as  a  collection  of  explosive  charges  ready  to  be  discharged 
i\nd  so  to  start  efferent  impulses  along  certain  efferent  nerves,  and  these 
charges  are  so  arranged  and  so  related  to  certain  afferent  nerves,  that  afferent 
impulses  reaching  the  centre  along  those  nerves  may  in  one  case  discharge  a 
few  only  of  the  charges,  and  so  give  rise  to  feeble  movements,  and  in  another 
<^ase  discharge  a  very  large  number,  and  so  give  rise  to  large  and  violent 
movements.  In  a  reflex  action  then  the  number,  intensity,  character,  and 
distribution  of  the  efferent  impulses,  and  so  the  kind  and  amount  of  move- 
ment, will  depend  chiefly  on  what  takes  place  in  the  centre,  and  this  will,  in 
turn,  depend,  on  the  one  hand,  on  the  condition  of  the  centre,  and,  on  the 
other,  on  the  special  relations  of  the  centre  of  the  afferent  impulses.  At 
the  same  time  we  are  able  to  recognize  in  most  reflex  actions  a  certain  rela- 
tion between  the  strength  of  the  stimulus,  or  the  magnitude  of  the  afferent 
impulses  and  the  extent  of  the  movement  or  the  magnitude  of  the  efferent 
impulses. 

We  may  add,  without  going  more  fully  into  the  subject  here,  that  in 
most  reflex  actions  a  special  relation  may  be  observed  between  the  part 
stimulated  and  the  resulting  movement.  In  the  simplest  cases  of  reflex 
action  this  relation  is  merely  of  such  a  kind  that  the  muscles  thrown  into 
action  are  those  governed  by  a  motor  nerve  which  is  the  fellow  of  the  sen- 
sory nerve,  the  stimulation  of  which  calls  forth  the  movement.  In  the  more 
complex  reflex  action  of  the  brainless  frog,  and  in  other  cases,  the  relation 
is  of  such  a  kind  that  the  resulting  movement  bears  an  adaptation  to  the 
stimulus;  the  foot  is  withdrawn  from  the  stimulus,  or  the  movement  is  cal- 
culated to  push  or  wipe  away  the  stimulus.  In  other  words,  a  certain 
purpose  is  evident  in  the  reflex  action. 

Thus  in  all  cases,  except  perhaps  the  very  simplest,  the  movements  called 
forth  by  a  reflex  action  are  exceedingly  complex  compared  with  those  which 
result  from  the  direct  stimulation  of  a  motor  trunk. 

§  98.  Automatic  actions.  Efferent  impulses  frequently  issue  from  the 
brain  and  spinal  cord  and  so  give  rise  to  movements  without  being  obviously 
preceded  by  any  stimulation.  Such  movements  are  spoken  of  as  automatic 


132  GENERAL  FEATURES  OF  NERVOUS  TISSUES. 

or  spontaneous.  The  efferent  impulses  in  such  cases  are  started  by  changes 
in  the  nerve  centre  which  are  not  the  immediate  result  of  the  arrival  at  the 
nerve  centre  of  afferent  impulses  from  without,  but  which  appear  to  arise  in 
the  nerve  centre  itself.  Changes  of  this  kind  may  recur  rhythmically  ;  thus, 
as  we  shall  see,  we  have  reason  to  think  that  in  a  certain  part  of  the  medulla 
oblongata  changes  of  the  nervous  material,  recurring  rhythmically,  lead  to 
the  rhythmic  discharge  along  certain  nerves  of  efferent  impulses  whereby 
muscles  connected  with  the  chest  are  rhythmically  thrown  into  action  and  a 
rhythmically  repeated  breathing  is  brought  about.  And  other  similar  rhyth- 
mic automatic  movements  may  be  carried  out  by  other  parts  of  the  spinal 
cord. 

From  the  brain  itself  a  much  more  varied  and  apparently  irregular  dis- 
charge of  efferent  impulses,  not  the  obvious  result  of  any  immediately  fore- 
going afferent  impulses,  and  therefore  not  forming  part  of  reflex  actions,  is 
very  common,  constituting  what  we  speak  of  as  volition,  efferent  impulses 
thus  arising  being  called  volitional  or  voluntary  impulses.  The  spinal  cord 
apart  from  the  brain  does  not  appear  capable  of  executing  these  voluntary 
movements ;  but  to  this  subject  we  shall  return  when  we  come  to  speak  of 
the  central  nervous  system  in  detail. 

We  said  just  now  that  there  is  no  satisfactory  evidence  that  the  ganglia 
of  the  splanchnic  system  ever  act  as  centres  of  reflex  action.  The  evidence, 
however,  that  these  ganglia  may  serve  as  centres  of  rhythmic  automatic 
action  seems  at  first  sight  of  some  strength.  Several  organs  of  the  body 
containing  muscular  tissue,  the  most  notable  being  the  heart,  are  during  life 
engaged  in  rhythmic  automatic  movements,  and  in  many  cases  continue  these 
movements  after  removal  from  the  body.  In  nearly  all  these  cases  ganglia 
are  present  in  connection  with  the  muscular  tissue';  and  the  presence  and 
intact  condition  of  these  ganglia  seem,  at  all  events  in  many  cases,  in  some 
way  essential  to  the  due  performance  of  the  rhythmic  automatic  movements. 
Indeed,  it  has  been  thought  that  the  movements  in  question  are  really  due 
to  the  rhythmic  automatic  generation  in  the  cells  of  these  ganglia  of  efferent 
impulses  which  passing  down  to  the  appropriate  muscular  fibres  call  forth 
the  rhythmic  movement.  When  we  come  to  study  these  movements  in  detail, 
we  shall  find  reasons  for  coming  to  the  conclusion  that  this  view  is  not  sup- 
ported by  adequate  evidence ;  and,  indeed,  though  it  is  perhaps  immature  to 
make  a  dogmatic  statement,  all  the  evidence  goes,  as  we  have  already  said, 
to  show  that  the  great  use  of  the  ganglia  of  the  splanchnic  system,  like  that 
of  the  spinal  ganglia,  is  connected  with  the  nutrition  of  the  nerves,  and  that 
these  structures  do  not,  like  the  central  nervous  system,  act  as  centres  either 
automatic  or  reflex. 

§  99.  Inhibitory  nerves.  We  have  said  that  the  fibres  of  the  anterior 
root  should  be  called  efferent  rather  than  motor,  because  though  they  all 
carry  impulses  outward  from  the  central  nervous  system  to  the  tissues,  the 
impulses  which  they  carry  do  not  in  all  cases  lead  to  the  contraction  of  mus- 
cular fibres.  Some  of  these  efferent  fibres  are  distributed  to  glandular  struc- 
tures, for  instance  to  the  salivary  glands,  and  impulses  passing  along  these 
lead  to  changes  in  epithelial  cells  and  their  surroundings  whereby,  without 
any  muscular  contraction  necessarily  intervening,  secretion  is  brought  about ; 
the  action  of  these  fibres  of  secretion  we  shall  study  in  connection  with 
digestion. 

Besides  this  there  are  efferent  fibres  going  to  muscular  tissue  or  at  all 
events  to  muscular  organs,  the  impulses  passing  along  which,  so  far  from 
bringing  about  muscular  contraction,  diminish,  hinder,  or  stop  movements 
already  in  progress.  Thus,  if  when  the  heart  is  beating  regularly,  that  is 
to  say,  when  the  muscular  fibres  which  make  up  the  greater  part  of  the 


FEATURES  OF  VASCULAR  APPARATUS.  133 

heart  are  rhythmically  contracting,  the  branches  of  the  pneumogastric  nerve 
going  to  the  heart  be  adequately  stimulated,  for  instance  with  the  interrupted 
current,  the  heart  will  stop  beating ;  and  that  not  because  the  muscles  of 
the  heart  are  thrown  into  a  continued  tetanus,  the  rhythmic  alternation  of 
contraction  and  relaxation  being  replaced  by  sustained  contraction,  but  be- 
cause contraction  disappears  altogether,  all  the  muscular  fibres  of  the  heart 
remaining  for  a  considerable  time  in  complete  relaxation  and  the  whole  heart 
being  quite  flaccid.  If  a  weaker  stimulus  be  employed  the  beat  may  not  be 
actually  stopped,  but  slowed  or  weakened.  And,  as  we  shall  see,  there  are 
many  other  cases  where  the  stimulation  of  efferent  fibres  hinders,  weakens, 
or  altogether  stops  a  movement  already  in  progress.  Such  an  effect  is  called 
an  inhibition,  and  the  fibres  stimulation  of  which  produces  the  effect  are 
called  "  inhibitory  "  fibres. 

The  phenomena  of  inhibition  are  not,  however,  confined  to  such  cases  as 
the  heart,  where  the  efferent  nerves  are  connected  with  muscular  tissues. 
Thus  the  activity  of  a  secreting  gland  may  be  inhibited,  as  for  instance  when 
emotion  stops  the  secretion  of  saliva,  and  the  mouth  becomes  dry  from  fear. 
In  this  instance,  however,  it  is  probable  that  inhibition  is  brought  about  not 
by  inhibitory  impulses  passing  to  the  gland  and  arresting  secretion  in  the 
gland  itself,  but  rather  by  an  arrest,  in  the  central  nervous  system,  of  the 
nervous  impulses  which,  normally,  passing  down  to  the  gland,  excite  it 
to  action.  And,  indeed,  as  we  shall  see  later  on,  there  are  many  illus- 
trations of  the  fact  that  afferent  impulses  reaching  a  nervous  centre, 
instead  of  stimulating  it  to  activity,  may  stop  or  inhibit  an  activity  previ- 
ously going  on.  In  fact  it  is  probable,  though  not  actually  proved  in  every 
case,  that  wherever  in  any  tissue,  energy  is  being  set  free,  nervous  impulses 
brought  to  bear  on  the  tissue  may  affect  the  rate  or  amount  of  the  energy 
set  free  in  two  different  ways  ;  on  the  one  hand,  they  may  increase  or  quicken 
the  setting  free  of  energy,  and  on  the  other  hand,  they  may  slacken,  hinder, 
or  inhibit  the  setting  free  of  energy.  And  in  at  all  events  a  large  number 
of  cases  it  is  possible  to  produce  the  one  effect  by  means  of  one  set  of  nerve 
fibres  and  the  other  effect  by  another  set  of  nerve  fibres.  We  shall  have 
occasion,  however,  to  study  the  several  instances  of  this  double  action  in  the 
appropriate  places. 


CHAPTER    IV. 

THE  VASCULAR  MECHANISM. 
THE  STRUCTURE  AND  MAIN  FEATURES  OF  THE  VASCULAR  APPARATUS. 

§  100.  THE  blood  is  the  internal  medium  on  which  the  tissues  live ; 
from  it  the  tissues  draw  their  food  and  oxygen,  to  it  they  give  up  the  pro- 
ducts of  waste  matters  which  they  form.  The  tissues,  with  some  few  excep- 
tions, are  traversed  by,  and  thus  the  elements  of  the  tissues  surrounded  by, 
networks  of  minute  thin-walled  tubes,  the  capillary  bloodvessels.  The  ele- 
mentary striated  muscle  fibre,  for  instance,  is  surrounded  by  capillaries, 
running  in  the  connective  tissue  outside  but  close  to  the  sarcolemma,  arranged 
in  a  network  with  more  or  less  rectangular  meshes.  These  capillaries  are 
closed  tubes  with  continuous  walls,  and  the  blood,  which,  as  we  shall  see,  is 
continually  streaming  through  them,  is  as  a  whole  confined  to  their  channels 
and  does  not  escape  from  them.  The  elements  of  the  tissues  lie  outside  the 
capillaries  and  form  extra-vascular  islets,  of  different  form  and  size  in  the 


134 


THE  VASCULAR  MECHANISM. 


different  tissues,  surrounded  by  capillary  networks.  But  the  walls  of  the 
capillaries  are  so  thin  and  of  such  a  nature  that  certain  of  the  constituents 
of  the  blood  pass  from  the  interior  of  the  capillary,  through  the  capillary 
wall  to  the  elements  of  the  tissue  outside  the  capillary,  and  similarly  certain 
of  the  constituents  of  the  tissue,  to  wit,  certain  substances  the  result  of  the 
metabolism  continually  going  on  in  the  tissue,  pass  from  the  tissue  outside 
the  capillary  through  the  capillary  wall  into  the  blood  flowing  through  the 
capillary.  Thus  as  we  have  already  said,  §  13,  there  is  a  continual  inter- 
change of  material  between  the  blood  in  the  capillary  and  the  elements  of 
the  tissue  outside  the  capillary,  the  lymph  acting  as  middle-man.  By  this 
interchange  the  tissue  lives  on  the  blood,  and  the  blood  is  affected  by  its 
passage  through  the  tissue.  In  the  small  arteries  which  end  in,  and  in  the 
small  veins  which  begin  in  the  capillaries,  a  similar  interchange  takes  place  ; 
but  the  amount  of  interchange  diminishes  as,  passing  in  each  direction  from 
the  capillaries,  the  walls  of  the  arteries  and  veins  become  thicker ;  and 
indeed;  in  all  but  the  minute  veins  and  arteries,  the  interchange  is  so  small 
that  it  may  practically  be  neglected.  It  is  in  the  capillaries  (and  minute 
arteries  and  veins)  that  the  business  of  the  blood  is  done ;  it  is  in  these  that 
the  interchange  takes  place  ;  and  the  object  of  the  vascular  mechanism  is  to 
cause  the  blood  to  flow  through  these  in  a  manner  best  adapted  for  carryino- 
on  this  interchange  under  varying  circumstances.  The  use  of  the  arteries  is 
in  the  main  simply  to  carry  the  blood  in  a  suitable  manner  from  the  heart 
to  the  capillaries,  the  use  of  the  veins  is  in  the  main  simply  to  carry  the 
blood  from  the  capillaries  back  to  the  heart,  and  the  use  of  the  heart  is  in 
the  main  simply  to  drive  the  blood  in  a  suitable  manner  through  the 
arteries  into  the  capillaries  and  from  the  capillaries  back  along  the  veins 
to  itself  again.  The  structure  of  these  several  parts  is  adapted  to  these 
several  uses. 

Main  Features  of  the  Apparatus. 

§  101.  We  may  now  pass  briefly  in  review  some  of  the  main  features  of 
the  several  parts  of  the  vascular  apparatus — heart,  arteries,  veins,  and 
capilliaries. 

The  heart  is  a  muscular  pump — that  is,  a  pump  the  force  of  whose  strokes 
is  supplied  by  the  contraction  of  muscular  fibres  working  intermittently,  the 
strokes  being  repeated  so  many  times  (in  man  about  72  times)  a  minute. 
It  is  so  constructed  and  furnished  with  valves  in  such  a  way  "that  at  each 
stroke  it  drives  a  certain  quantity  of  blood  with  a  certain  force  and  a  certain 
rapidity  from  the  left  ventricle  into  the  aorta,  and  so  into  the  arteries,  receiv- 
ing during  the  stroke  and  the  interval  between  that  stroke  and  the  next  the 
same  quantity  of  blood  from  the  veins  into  the  right  auricle.  We  omit  for 
simplicity's  sake  the  pulmonary  circulation  by  which  the  same  quantity  of 
blood  is  driven  at  each  stroke  from  the  right  ventricle  into  the  lungs  and 
received  into  the  left  auricle.  The  rhythm  of  the  beat,  that  is  the  frequency 
of  repetition  of  the  strokes,  and  the  characters  of  each  beat  or  stroke,  are 
determined  by  changes  taking  place  in  the  tissues  of  the  heart  itself,  though 
they  are  also  influenced  by  causes  working  from  without. 

The  arteries  are  tubes,  with  relatively  stout  walls,  branching  from  the 
aorta  all  over  the  body.  The  constitution  of  their  walls,  especially  of  their 
middle  coat,  gives  the  arteries  two  salient  properties.  In  the  first  place 
they  are  very  elastic,  in  the  sense  that  they  will  stretch  readily,  both  length- 
wise and  crosswise,  when  pulled,  and  return  readily  to  their  former  size  and 
shape  when  the  pull  is  taken  off.  If  fluid  be  driven  into  one  end  of  a  piece 
of  artery,  the  other  end  of  which  is  tied,  the  artery  will  swell  out  to  a  very 
great  extent,  but  return  immediately  to  its  former  calibre  when  the  fluid  is 


FEATURES  OF  VASCULAR  APPARATUS.  135 

let  out.  This  elasticity  is  chiefly  due  to  the  elastic  elements  in  the  coats, 
elastic  membranes,  and  feltworks,  but  the  muscular  fibres,  being  themselves 
also  elastic,  contribute  to  the  result.  By  reason  of  their  possessing  such 
stout,  elastic  walls,  the  arteries  when  empty  do  not  collapse,  but  remain  as 
open  tubes.  In  the  second  place  the  arteries,  by  virtue  of  their  muscular 
elements,  are  contractile;  when  stimulated  either  directly,  as  by  applying  an 
electric  or  mechanical  stimulus  to  the  arterial  walls,  or  indirectly,  by  means 
of  the  so-called  vasomotor  nerves,  which  we  shall  have  to  study  presently, 
the  arteries  shrink  in  calibre,  the  circularly  disposed  muscular  fibres 
contracting,  and  so,  in  proportion  to  the  amount  of  their  contraction, 
narrowing  the  lumen  or  bore  of  the  vessel.  The  contraction  of  these 
arterial  muscular  fibres,  like  that  of  all  plain,  non-striated  muscular  fibres, 
is  slow  and  long-continued,  with  a  long  latent  period,  as  compared  with 
the  contraction  of  skeletal  striated  muscular  fibres.  Owing  to  this  mus- 
cular element  in  the  arterial  walls,  the  calibre  of  an  artery  may  be  very 
narrow  or  very  wide,  or  in  an  intermediate  condition  between  the  two, 
neither  very  narrow  nor  very  wide,  according  as  the  muscular  fibres  are  very 
much  contracted  or  not  contracted  at  all,  or  only  moderately  contracted. 
We  have  further  seen  that,  while  the  relative  proportion  of  elastic  and 
muscular  elements  differs  in  different  arteries,  as  a  general  rule  the  elastic 
elements  predominate  in  the  larger  arteries  and  the  muscular  elements  in 
the  smaller  arteries,  so  that  the  larger  arteries  may  be  spoken  of  as  eminently 
elastic,  or  as  especially  useful  on  account  of  their  elastic  properties,  and  the 
smaller  arteries  as  eminently  muscular,  or  as  especially  useful  on  account  of 
their  muscular  properties.  Thus,  in  the  minute  arteries  which  are  just  pass- 
ing into  capillaries  the  muscular  coat,  though  composed  often  of  a  single 
layer,  and  that  sometimes  an  imperfect  one,  of  muscular  fibres,  is  a  much 
more  conspicuous  and  important  part  of  the  arterial  wall  than  that  furnished 
by  the  elastic  elements. 

The  arteries  branching  out  from  a  single  aorta  down  to  multitudinous 
capillaries  in  nearly  every  part  of  the  body  diminish  in  bore  as  they  divide. 
Where  an  artery  divides  into  two  or  gives  off  a  branch,  though  the  bore  of 
each  division  is  less  than  that  of  the  artery  before  the  division  or  branching, 
the  two  together  are  greater ;  that  is  to  say,  the  united  sectional  area  of  the 
branches  is  greater  than  the  sectional  area  of  the  trunk.  Hence,  the  sec- 
tional area  of  the  arterial  bed  through  which  the  blood  flows  goes  on  increas- 
ing from  the  aorta  to  the  capillaries.  If  all  the  arterial  branches  were  thrown 
together  into  one  channel,  this  would  form  a  hollow  cone  with  its  apex  at 
the  aorta  and  its  base  at  the  capillaries.  The  united  sectional  area  of  the 
capillaries  may  be  taken  as  several  hundred  times  that  of  the  sectional  area 
of  the  aorta,  so  greatly  does  the  arterial  bed  widen  out. 

The  capillaries  are  channels  of  variable  but  exceedingly  small  size. 
The  thin  sheet  of  cemented  epithelioid  plates  which  forms  the  only  wall  of 
a  capillary  is  elastic,  permitting  the  channel  offered  by  the  same  capillary 
to  differ  much  in  width  at  different  times,  to  widen  when  blood  and  blood- 
corpuscles  are  being  pressed  through  it  and  to  narrow  again  when  the 
pressure  is  lessened  or  cut  off.  The  same  thin  sheet  permits  water  and  sub- 
stances, including  gases,  in  solution  to  pass  through  itself  from  the  blood  to 
the  tissue  outside  the  capillary,  and  from  the  tissue  to  the  blood,  and  thus 
carries  on  the  interchange  of  material  between  the  blood  and  the  tissue.  In 
certain  circumstances,  at  all  events,  white  or  even  red  corpuscles  may  also 
pass  through  the  wall  to  the  tissue  outside. 

The  minute  arteries  and  veins  with  which  the  capillaries  are  continuous 
allow  of  a  similar  interchange  of  material,  the  more  so  the  smaller  they  are. 

The  walls  of  the  veins  are  thinner,  weaker  and  less  elastic  than  those  of 


136  THE  VASCULAR  MECHANISM. 

the  arteries,  and  possess  a  very  variable  amount  of  muscular  tissue ;  they 
collapse  when  the  veins  are  empty.  Though  all  veins  are  more  or  less  elastic 
and  some  veins  are  distinctly  muscular,  the  veins  as  a  whole  cannot,  like  the 
arteries,  be  characterized  as  eminently  elastic  and  contractile  tubes ;  they 
are  rather  to  be  regarded  as  simple  channels  for  conveying  the  blood  from 
the  capillaries  to  the  heart,  having  just  so  much  elasticity  as  will  enable 
them  to  accommodate  themselves  to  the  quantity  of  blood  passing  through 
them,  the  same  vein  being  at  one  time  full  and  distended,  and  at  another 
time  empty  and  shrunk,  and  only  gifted  with  any  great  amount  of  muscular 
contractility  in  special  cases  for  special  reasons.  The  united  sectional  area 
of  the  veins,  like  that  of  the  arteries,  diminishes  from  the  capillaries  to  the 
heart ;  but  the  united  sectional  area  of  the  venss  cavse  at  their  junction  with 
the  right  auricle  is  greater  than,  nearly  twice  as  great  as,  that  of  the  aorta 
at  its  origin.  The  total  capacity  also  of  the  veins  is  much  greater  than  that 
of  the  arteries.  The  veins  alone  can  hold  the  total  mass  of  blood  which  in 
life  is  distributed  over  both  arteries  and  veins.  Indeed,  nearly  the  whole 
blood  is  capable  of  being  received  by  what  is  merely  a  part  of  the  venous 
system,  viz.,  the  vena  porta  and  its  branches. 

THE  MAIN  FACTS  OF  THE  CIRCULATION. 

§  102.  Before  we  attempt  to  study  in  detail  the  working  of  these  several 
parts  of  the  mechanism,  it  will  be  well,  even  at  the  risk  of  some  future  repe- 
tition, to  take  a  very  brief  survey  of  some  of  the  salient  points. 

At  each  beat  of  the  heart,  which  in  man  is  repeated  about  72  times  a 
minute,  the  contraction  or  systole  of  the  ventricles  drives  a  certain  quantity 
of  blood,  probably  amounting  to  about  180  c.c.  (4  to  6  oz.),  with  very  great 
force  into  the  aorta  (and  the  same  quantity  of  blood  with  less  force  into  the 
pulmonary  artery).  The  discharge  of  blood  from  the  ventricle  into  the 
aorta  is  very  rapid,  and  the  time  taken  up  by  it  is,  as  we  shall  see,  much 
less  than  the  time  which  intervenes  between  it  and  the  next  discharge 
of  the  next  beat.  So  that  the  flow  from  the  heart  into  the  arteries  is 
most  distinctly  intermittent,  sudden  rapid  discharges  alternating  with 
relatively  long  intervals  during  which  the  arteries  receive  no  blood  from 
the  heart. 

At  each  beat  of  the  heart  just  as  much  blood  flows,  as  we  shall  see,  from 
the  veins  into  the  right  auricle  as  escapes  from  the  left  ventricle  into  the 
aorta ;  but,  as  we  shall  also  see,  this  inflow  is  much  slower,  takes  a  longer 
time,  than  the  discharge  from  the  ventricle. 

When  the  finger  is  placed  on  an  artery  in  the  living  body  a  sense  of  resist- 
ance is  felt,  and  this  resistance  seems  to  be  increased  at  intervals,  correspond- 
ing to  the  heart-beats,  the  artery  at  each  heart-beat  being  felt  to  rise  up  or 
expand  under  the  finger,  constituting  what  we  shall  study  hereafter  as  the 
pulse.  In  certain  arteries  this  pulse  may  be  seen  by  the  eye.  When  the 
finger  is  similarly  placed  on  a  corresponding  vein  very  little  resistance  is  felt, 
and,  under  ordinary  circumstances,  no  pulse  can  be  perceived  by  the  touch 
or  by  the  eye. 

When  an  artery  is  severed,  the  flow  of  blood  from  the  proximal  cut  end, 
that  on  the  heart  side,  is  not  equable,  but  comes  in  jets,  corresponding  to  the 
heart-beats,  though  the  flow  does  not  cease  between  the  jets.  The  blood  is 
ejected  with  considerable  force,  and  may  in  a  large  artery  of  a  large  animal 
be  spurted  out  to  the  distance  of  some  feet.  The  larger  the  artery  and  the 
nearer  to  the  heart,  the  greater  the  force  with  which  the  blood  issues,  and 
the  more  marked  the  intermittence  of  the  flow.  The  flow  from  the  distal  cut 
end,  that  away  from  the  heart,  may  be  very  slight  or  may  take  place  with 


THE  MAIN   FACTS  OF  THE  CIRCULATION,  137 

considerable  force  and  marked  intermittence,  according  to  the  amount  of 
collateral  communication. 

When  a  corresponding  vein  is  severed,  the  flow  of  blood,  which  is  chiefly 
from  the  distal  cut  end,  that  in  connection  with  the  capillaries,  is  not  jerked 
but  continuous ;  the  blood  comes  out  with  comparatively  little  force,  and 
"  wells  up  "  rather  than  "  spurts  out."  The  flow  from  the'proximal  cut  end, 
that  on  the  heart  side,  may  amount  to  nothing  at  all  or  may  be  slight  or 
may  be  considerable,  depending  on  the  presence  or  absence  of  valves  and  the 
amount  of  collateral  communication. 

When  an  artery  is  ligatured  the  vessel  swells  on  the  proximal  side,  toward 
the  heart,  and  the  throbbing  of  the  pulse  may  be  felt  right  up  to  the  liga- 
ture. On  the  distal  side  the  vessel  is  empty  and  shrunk,  and  no  pulse  can 
be  felt  in  it  unless  there  be  free  collateral  communication. 

When  a  vein  is  ligatured  the  vessel  swells  on  the  distal  side,  away  from 
the  heart,  but  no  pulse  is  felt ;  while  on  the  proximal  side,  toward  the  heart, 
it  is  empty  and  collapsed  unless  there  be  too  free  collateral  communication. 

§  103.  When  the  interior  of  an  artery — for  instance,  the  carotid — is 
placed  in  communication  with  a  long  glass  tube  of  not  too  great  a  bore,  held 
vertically,  the  blood,  immediately  upon  the  communication  being  effected, 
may  be  seen  to  rush  into  and  to  fill  the  tube  for  a  certain  distance,  forming 
in  it  a  column  of  blood  of  a  certain  height.  The  column  rises  not  steadily, 
but  by  leaps,  each  leap  corresponding  to  a  heart-beat,  and  each  leap  being 
less  than  its  predecessor ;  and  this  goes  on,  the  increase  in  the  height  of  the 
column  at  each  heart-beat  each  time  diminishing,  until  at  last  the  column 
ceases  to  rise  and  remains  for  a  while  at  a  mean  level,  above  and  below 
which  it  oscillates  with  slight  excursions  at  each  heart-beat. 

To  introduce  such  a  tube  an  artery— say  the  carotid  of  a  rabbit— is  laid  bare, 
ligatured  at  a  convenient  spot,  V  Fig.  37,  and  further  temporarily  closed  a  little 
distance  lower  down  nearer  the  heart  by  a  small  pair  of  u bull-dog"  forceps,  bd, 
or  by  a  ligature  which  can  be  easily  slipped.  A  longitudinal  incision  is  now  made 
in  the  artery  between  the  forceps,  bd,  and  the  ligature  /'  (only  the  drop  or  two  of 
blood  which  happens  to  remain  inclosed  between  the  two  being  lost) ;  the  end  of 
the  tube,  represented  by  c  in  the  figure,  is  introduced  into  the  artery  and  secured 
by  the  ligature  /.  The  interior  of  the  tube  is  now  in  free  communication  with  the 
interior  of  the  artery,  but  the  latter  is  by  means  of  the  forceps  at  present  shut  off 
from  the  heart.  On  removing  the  forceps  a  direct  communication  is  at  once  estab- 
lished between  the  tube  and  the  artery  below ;  in  consequence  the  blood  from  the 
heart  flows  through  the  artery  into  the  tube. 

This  experiment  shows  that  the  blood  as  it  is  flowing  into  the  carotid  is 
exerting  a  considerable  pressure  on  the  walls  of  the  artery.  At  the  moment 
when  the  forceps  are  removed  there  is  nothing  but  the  ordinary  pressure  of 
the  atmosphere  to  counterbalance  this  pressure  within  the  artery,  and  con- 
sequently a  quantity  of  blood  is  pressed  out  into  the  tube ;  and  this  goes  on 
until  the  column  of  blood  in  the  tube  reaches  such  a  height  that  its  weight  is 
equal  to  the  pressure  within  the  artery,  whereupon  no  more  blood  escapes. 
The  whole  column  continues  to  be  raised  a  little  at  each  heart-beat,  but  sinks 
as  much  during  the  interval  between  each  two  beats,  and  thus  oscillates,  as 
we  have  said,  above  and  below  a  mean  level.  In  a  rabbit  this  column  of 
blood  will  generally  have  the  height  of  about  90  cm.  (3  feet)  ;  that  is  to  say, 
the  pressure  which  the  blood  exerts  on  the  walls  of  the  carotid  of  a  rabbit  is 
equal  to  the  pressure  exerted  by  a  column  of  rabbit's  blood  90  cm.  high. 
This  is  equal  to  the  pressure  of  a  column  of  water  about  95  cm.  high,  and  to 
the  pressure  of  a  column  of  mercury  about  70  mm.  high. 

If  a  like  tube  be  similarly  introduced  into  a  corresponding  vein — say 
the  jugular  vein — it  will  be  found  that  the  column  of  blood,  similarly  formed 


138 


THE  VASCULAR  MECHANISM. 


in  the  tube,  will  be  a  very  low  one,  not  more  than  a  very  few  centimetres 
high,  and  that  while  the  level  of  the  column  may  vary  a  good  deal,  owing, 
as  we  shall  see  later,  to  the  influence  of  the  respiratory  movement,  there  will 
not,  as  in  the  artery,  be  oscillations  corresponding  to  the  heart-beats. 

"We  learn,  then,  from  this  simple  experiment,  that  in  the  carotid  of  the 
rabbit  the  blood  while  it  flows  through  that  vessel  is  exerting  a  considerable 
mean  pressure  on  the  arterial  walls,  equivalent  to  that  of  a  column  of  mer- 
cury about  70  mm.  high,  but  that  in  the  jugular  vein  the  blood  exerts  on 
the  venous  walls  a  very  slight  mean  pressure,  equivalent  to  that  of  a  column 
of  mercury  3  or  4  mm.  high.  We  speak  of  this  mean  pressure  exerted  by 
the  blood  on  the  walls  of  the  bloodvessels  as  blood-pressure,  and  we  say  that 
the  blood- pressure  in  the  carotid  of  the  rabbit  is  very  high  (70  mm.  Hg), 
while  that  in  the  jugular  vein  is  very  low  (only  3  or  4  mm.  Hg.). 

In  the  normal  state  of  things  the  blood  flows  through  the  carotid  to  the 
arterial  branches  beyond,  and  through  the  jugular  vein  toward  the  heart; 
the  pressure  exerted  by  the  blood  on  the  artery  or  on  the  vein  is  a  lateral 
pressure  on  the  walls  of  the  artery  and  vein,  respectively.  In  the  above  ex- 
periment the  pressure  measured  is  not  exactly  this,  but  the  pressure  exerted 
at  the  end  of  the  artery  (or  of  the  vein)  where  the  tube  is  attached.  We 
might  directly  measure  the  lateral  pressure  in  the  carotid  by  somewhat 
modifying  the  procedure  described  above.  We  might  connect  the  carotid 
with  a  tube  the  end  of  which  was  not  straight,  but  made  in  the  form  of  a 
T-piece,  and  might  introduce  the  T-piece  in  such  a  way  that  the  blood 
should  flow  along  one  limb  (the  vertical  limb)  of  the  T-piece  from  the 
proximal  to  the  distal  part  of  the  carotid,  and  at  the  same  time  by  the  other 
(horizontal)  limb  of  the  T-piece  into  the  main  upright  part  of  the  glass 
tube  The  column  of  blood  in  the  tube  would  then  be  a  measure  of  the 
pressure  which  the  blood  as  it  is  flowing  along  the  carotid  is  exerting  on  a 
portion  of  its  walls  corresponding  to  the  mouth  of  the  horizontal  limb  of  the 
T-piece.  If  we  were  to  introduce  into  the  aorta,  at  the  place  of  origin  of  the 
carotid,  a  similar  (larger)  T-piece,  and  to  connect  the  glass  tube  with  the 
horizontal  limb  of  the  T-piece  by  a  piece  of  elastic  tubing  of  the  same  length 
and  bore  as  the  carotid,  the  column  of  blood  rising  up  in  the  tube  would  be 
the  measure  of  the  lateral  pressure  exerted  by  the  blood  on  the  walls  of  the 
aorta  at  the  origin  of  the  carotid  artery  and  transmitted  to  the  rigid  glass 
tube  through  a  certain  length  of  elastic  tubing.  And,  indeed,  what  is 
measured  in  the  experiment  previously  described  is  not  the  lateral  pressure 
in  the  carotid  itself  at  the  spot  where  the  glass  tube  is  introduced,  but  the 
lateral  pressure  of  the  aorta  at  the  origin  of  the  carotid  modified  by  the 
influences  exerted  by  the  length  of  the  carotid  between  its  origin  and  the 
spot  where  the  tube  is  introduced. 

§  104.  Such  an  experiment  as  the  one  described  has  the  disadvantages 
that  the  animal  is  weakened  by  the  loss  of  the  blood  which  goes  to  form  the 
column  in  the  tube,  and  that  the  blood  in  the  tube  soon  clots,  and  so  brings 
the  experiment  to  an  end.  Blood-pressure  may  be  more  conveniently  studied 
by  connecting  the  interior  of  the  artery  (or  vein)  with  a  mercury  gauge  or 
manometer  (Fig.  37)  the  proximal  descending  limb  of  which,  m,  is  filled 
above  the  mercury  with  some  innocuous  fluid,  as  is  also  the  tube  connecting 
the  manometer  with  the  artery.  Using  such  an  instrument,  we  should 
observe  very  much  the  same  facts  as  in  the  more  simple  experiment. 

Immediately  that  communication  is  established  between  the  interior  of 
the  artery  and  the  manometer,  blood  rushes  from  the  former  into  the  latter, 
driving  some  of  the  mercury  from  the  descending  limb,  m,  into  the  ascending 
limb,  m',  and  thus  causing  the  level  of  the  mercury  in  the  ascending  limb  to 
rise  rapidly.  This  rise  is  marked  by  jerks  corresponding  with  the  heart- 


THE  MAIN  FACTS  OF  THE  CIRCULATION.  139 

FIG.  37. 


Apparatus  for  Investigating  Blood-pressure,  At  the  upper  right-hand  corner  is  seen,  on  an 
enlarged  scale,  the  carotid  artery,  clamped  by  the  forceps  bd,  with  the  vagus  nerve  v  lying  by  its 
side.  The  artery  has  been  ligatured  at  I'  and  the  glass  canula  c  has  been  introduced  into  the 
artery  between  the  ligature  I'  and  the  forceps  bd,  and  secured  in  position  by  the  ligature  I.  The 
shrunken  artery  on  the  distal  side  of  the  canula  is  seen  at  ca'. 

p.b  is  a  box  containing  a  bottle  holding  a  saturated  solution  of  sodium  carbonate  or  a  solution 
of  sodium  bicarbonate  of  sp.  gr.  1083,  and  capable  of  being  raised  or  lowered  at  pleasure.  The 
solution  flows  by  the  tube  p.t  regulated  by  the  clamp  c"  into  the  tube  t.  A  syringe,  with  a  stop- 
cock, may  be  substituted  for  the  bottle,  and  attached  at  c".  This,  indeed,  is  in  many  respects  a 


140 


THE  VASCULAE  MECHANISM. 


beats.  Having  reached  a  certain  level,  the  mercury  ceases  to  rise  any  more. 
It  does  not,  however,  remain  absolutely  at  rest,  but  undergoes  oscillations ; 
it  keeps  rising  and  falling.  Each  rise,  which  is  very  slight  compared  with 
the  total  height  to  which  the  mercury  has  risen,  has  the  same  rhythm  as  the 
systole  of  the  ventricle.  Similarly,  each  fall  corresponds  with  the  diastole. 
If  a  float,  swimming  on  the  top  of  the  mercury  in  the  ascending  limb  of 
the  manometer,  and  bearing  a  brush  or  other  marker,  be  brought  to  bear  on 
a  travelling  surface,  some  such  tracing  as  that  represented  in  Fig.  38  will  be 


FIG.  38. 


A/A 


FIG.  39. 


Tracing  of  Arterial  Pressure  with  a  Mercury  Manometer :  The  smaller  curves  p  p  are  the  pulse 
curves.  The  space  from  r  to  r  embraces  a  respiratory  undulation.  The  tracing  is  taken  from  a 
dog,  and  the  irregularities  visible  in  it  are  those  frequently  met  with  in  this  animal. 

described.  Each  of  the  smaller  curves  {p  p}  corresponds  to  a  heart-beat, 
the  rise  corresponding  to  the  systole  and  the  fall  to  the  diastole  of  the  ven- 
tricle. The  larger  undulations  (r  r)  in  the  tracing,  which  are  respiratory 

in  origin,  will  be  discussed  hereafter. 
In  Fig.  39  are  given  two  tracings 
taken  from  the  carotid  of  a  rabbit ;  in 
the  lower  curve  the  recording  surface 
is  travelling  more  rapidly  than  in  the 
upper  curve ;  otherwise  the  curves  are 
alike  and  repeat  the  general  features 
of  the  curve  from  the  dog. 

Description  of  experiment.  In  a  carotid, 
or  other  bloodvessel,  prepared  as  explained, 
a  small  glass  tube,  of  suitable  bore,  called 
a  cannla  is  introduced  by  the  method 
described  above,  and  is  subsequently  con- 
nected, by  means  of  a  short  piece  of  India- 
rubber  tubing  (Fig.  37  /,)  and  a  leaden  or 
other  tube  t  which  is  at  once  flexible  and 
yet  not  extensible,  with  the  descending 
limb,  w,  of  the  manometer  or  mercury 
gauge.  The  canula,  tube,  and  descending 
limb  of  the  manometer  are  all  filled  with  some  fluid,  which  tends  to  prevent  clot- 
ting of  the  blood,  the  one  chosen  being  generally  a  strong  solution  (sp.  gr.  1083) 
of  sodium  bicarbonate,  but  other  fluids  may  be  chosen.  In  order  to  avoid  loss 
of  blood,  a  quantity  of  fluid  is  injected  into-  the  flexible  tube  sufficient  to  raise  the 

more  convenient  plan.  The  tube  t  is  connected  with  the  leaden  tube  t,  and  the  stopcock  c  with 
the  manometer,  of  which  m  is  the  descending  and  m'  the  ascending  limb,  and  s  the  support.  The 
mercury  in  the  ascending  limb  bears  on  its  surface  the  float  fl.,  a  long  rod  attached  to  which  is 
fitted  with  the  pen  p,  writing  on  the  recording  surface  r.  The  clamp  d.  at  the  end  of  the  tube  t 
has  an  arrangement  shown  on  a  larger  scale  at  the  right  hand  upper  corner. 

The  descending  tube  m  of  the  manometer  and  the  tube  t  being  completely  filled  along  its  whole 
length  with  fluid  to  the  exclusion  of  all  air,  the  canula  c  is  filled  with  fluid,  slipped  into  the 
open  end  of  the  thick-walled  India-rubber  tube  i,  until  it  meets  the  tube  t  (whose  position  within 
the  India-rubber  tube  is  shown  by  the  dotted  lines),  and  is  then  securely  fixed  in  this  position  by 
the  clamp  cl. 

The  stopcocks  cand  c"  are  now  opened,  and  the  pressure  bottle  raised  or  fluid  driven  in  by  the 
syringe  until  the  mercury  in  the  manometer  is  raised  to  the  required  height.  The  clamp  c"  is 
then  closed  and  the  forceps  bd  removed  from  the  artery.  The  pressure  of  the  blood  in  the  carotid 
ca  is  in  consequence  brought  to  bear  through  t  upon  the  mercury  in  the  manometer. 


Blood-pressure  Curves  from  the  Carotid 
of  Rabbit,  the  time  marker  in  each  case 
marking  seconds. 


THE  MAIN   FACTS  OF  THE  CIRCULATION. 


141 


mercury  in  the  ascending  limb  of  the  manometer  to  a  level  a  very  little  below 
what  may  be  beforehand  guessed  at  as  the  probable  mean  pressure.  When  the 
forceps  bd  are  removed,  the  pressure  of  the  blood  in  the  carotid  is  transmit- 
ted through  the  flexible  tube  to  the  manometer,  the  level  of  the  mercury  in  the 
ascending  limb  of  which  falls  a  little,  or  sinks  a  little  at  first,  or  may  do  neither, 
according  to  the  success  with  which  the  probable  mean  pressure  has  been  guessed, 
and  continues  to  exhibit  the  characteristic  oscillations  until  the  experiment  is 
brought  to  an  end  by  the  blood  clotting  or  otherwise. 

Tracings  of  the  movements  of  the  column  of  mercury  in  the  manometer  may 
be  taken  either  on  a  smoked  surface  of  a  revolving  cylinder  (  Fig.  11  ),  or  by  means 
of  a  brush  and  ink  on  a  continuous  roll  of  paper,  as  in  the  more  complex  kymo- 
graph (Fig.  40). 

§  105.  By  the  help  of  the  manometer  applied  to  various  arteries  and 
veins  we  learn  the  following  facts : 

1.  The  mean  blood-pressure  is  high  in  all  the  arteries,  but  is  greater  in 
the  larger  arteries  nearer  the  heart  than  in  the  smaller  arteries  further  from 

FIG.  40. 


Ludwig's  Kymograph  for  Recording  on  a  Continuous  Roll  of  Paper. 


the  heart ;  it  diminishes,  in  fact,  along  the  arterial  tract  from  the  heart 
toward  the  capillaries. 

2.  The  mean  blood-pressure  is  low  in  the  veins,  but  is  greater  in  the 
smaller  veins  nearer  the  capillaries  than  in  the  larger  veins  nearer  the  heart, 
diminishing,  in  fact,  from  the  capillaries  toward  the  heart.  In  the  large 
veins  near  the  heart  it  may  be  negative,  that  is  to  say,  the  pressure  of  blood 
in  the  vein  bearing  on  the  proximal  descending  limb  of  the  manometer  may 
be  less  than  the  pressure  of  the  atmosphere  on  the  ascending  distal  limb,  so 


142 


THE  VASCULAR   MECHANISM. 


that  when  communication  is  made  between  the  interior  of  the  vein  and  the 
manometer,  the  mercury  sinks  in  the  distal  and  rises  in  the  proximal  limb, 
being  sucked  up  toward  the  vein. 

The  manometer  cannot  well  be  applied  to  the  capillaries,  but  we  may 
measure  the  blood-pressure  in  the  capillaries  in  an  indirect  way.  It  is  well 
known  that  when  any  portion  of  the  skin  is  pressed  upon,  it  becomes  pale 
and  bloodless  ;  this  is  due  to  the  pressure  driving  the  blood  out  of  the  capil- 
laries and  minute  vessels  and  preventing  any  fresh  blood  entering  into  them. 
By  carefully  investigating  the  amount  of  pressure  necessary  to  prevent  the 
blood  entering  the  capillaries  and  minute  arteries  of  the  web  of  the  frog's 
foot,  or  of  the  skin  beneath  the  nail  or  elsewhere  in  man,  the  internal  pres- 
sure which  the  blood  is  exercising  on  the  walls  of  the  capillaries  and  minute 
arteries  and  veins  may  be  approximately  determined.  In  the  frog's  web 
this  has  been  found  to  be  equal  to  about  7  to  11  mm.  of  mercury.  In  the 
mammal  the  capillary  blood-pressure  is  naturally  higher  than  this  and  may 
be  put  down  at  from  20  to  30  mm.  It  is,  therefore,  considerable,  being 
greater  than  that  in  the  veins,  though  less  than  that  in  the  arteries. 

3.  There  is  thus  a  continued  decline  of  blood-pressure  from  the  root  of 
the  aorta,  through  the  arteries,  capillaries,  and  veins  to  the  right  auricle. 
We  find,  however,  on  examination  that  the  most  marked  fall  of  pressure 

takes  place  between  the  small  arteries 

FlG-  41-  on  the  one  side  of  the  capillaries  and 

the  small  veins  on  the  other,  the 
curve  of  pressure  being  somewhat 
of  the  form  given  in  Fig.  41,  which 
is  simply  intended  to  show  this  fact 
graphically  and  has  not  been  con- 
structed by  exact  measurements. 

4.  In  the  arteries  this  mean  pres- 
sure is  marked  by  oscillations  corre- 
sponding to   the   heart  beats,  each 
oscillation   consisting  of  a  rise  (in- 
crease of  pressure  above  the  mean) 
corresponding  to  the  systole  of  the 
ventricle,   followed    by 'a   fall    (de- 
Diagram  of  Blood-pressure:  A, arteries ;  p,  pe-  crease  of  pressure  below  the  mean) 
>  caplllaries>  and  corresponding  to  the  diastole  of  the 

ventricle. 

5.  These  oscillations,  which  we  may  speak  of  as  the  pulse,  are  largest 
and  most  conspicuous  in  the  large  arteries  near  the  heart,  diminish  from  the 
heart  toward  the  capillaries,  and  are,  under  ordinary  circumstances,  wholly 
absent  from  the  veins  along  the  whole  extent  from  the  capillaries  to  the 
heart. 

Obviously  a  great  change  takes  place  in  that  portion  of  the  circulation 
which  comprises  the  capillaries,  the  minute  arteries  leading  to  and  the 
minute  veins  leading  away  from  the  capillaries,  and  which  we  may  speak 
of  as  the  "  peripheral  region."  It  is  here  that  a  great  drop  of  pressure 
takes  place ;  it  is  here  also  that  the  pulse  disappears. 

§  106.  If  the  web  of  a  frog's  foot  be  examined  with  a  microscope,  the 
blood,  as  judged  of  by  the  movements  of  the  corpuscles,  is  seen  to  be  passing 
in  a  continuous  stream  from  the  small  arteries  through  the  capillaries  to  the 
veins.  The  velocity  is  greater  in  the  arteries  than  in  the  veins,  and  greater 
in  both  than  in  the  capillaries.  In  the  arteries  faint  pulsations,  synchronous 
with  the  heart's  beat,  are  frequently  visible ;  but  these  disappear  in  the 
capillaries,  in  which  the  flow  is  even, 'that  is,  not  broken  by  pulsations,  and 


THE  MAIN  FACTS  OF  THE  CIRCULATION.  143 

this  evenness  of  flow  is  continued  on  along  the  veins  as  far  as  we  can  trace 
them.  Not  infrequently  variations  in  velocity  and  in  the  distribution  of 
the  blood,  due  to  causes  which  will  be  hereafter  discussed,  are  witnessed 
from  time  to  time. 

The  character  of  the  flow  through  the  smaller  capillaries  is  very 
variable.  Sometimes  the  corpuscles  are  seen  passing  through  the  chan- 
nel in  single  file  with  great  regularity ;  •  at  other  times  they  may  be 
few  and  far  between.  Some  of  the  capillaries  are  wide  enough  to  permit 
two  or  more  corpuscles  abreast.  In  all  cases  the  blood  as  it  passes  through 
the  capillary  stretches  and  expands  the  walls.  Sometimes  a  corpuscle  may 
remain  stationary  at  the  entrance  into  a  capillary,  the  channel  itself  being 
for  some  little  distance  entirely  free  from  corpuscles.  Sometimes  many 
corpuscles  will  appear  to  remain  stationary  in  one  or  more  capillaries  for  a 
brief  period  and  then  to  move  on  again.  Any  one  of  these  conditions 
readily  passes  into  another ;  and,  especially  with  a  somewhat  feeble  circu- 
lation, instances  of  all  of  them  may  be  seen  in  the  same  field  of  the  micro- 
scope. It  is  only  when  the  vessels  of  the  web  are  unusually  full  of  blood 
that  all  the  capillaries  can  be  seen  equally  filled  with  corpuscles.  The 
long,  oval  red  corpuscle  moves  with  its  long  axis  parallel  to  the  stream, 
occasionally  rotating  on  its  long  axis,  and  sometimes,  in  the  larger  chan- 
nels, on  its  short  axis.  The  flexibility  and  elasticity  of  a  corpuscle  are 
well  seen  when  it  is  being  driven  into  a  capillary  narrower  than  itself,  or 
when  it  becomes  temporarily  lodged  at  the  angle  between  two  diverging 
channels. 

These  and  other  phenomena,  on  which  we  shall  dwell  later  on,  may  be 
readily  seen  in  the  web  of  the  frog's  foot  or  in  the  stretched-out  tongue  or  in 
the  mesentery  of  the  frog  ;  and  essentially  similar  phenomena  may  be 
observed  in  the  mesentery  or  other  transparent  tissues  of  a  mammal.  All 
over  the  body,  wherever  capillaries  are  present,  the  corpuscles  and  the 
plasma  are  being  driven  in  a  continuous  and  though  somewhat  irregular 
yet  on  the  whole  steady  flow  through  channels  so  minute  that  the  passage 
is  manifestly  attended  with  considerable  difficulties. 

It  is  obvious  that  the  peculiar  characters  of  the  flow  through  the  minute 
arteries,  capillaries,  and  veins  affords  an  explanation  of  the  great  change 
taking  place  in  the  peripheral  region  between  the  arterial  flow  and  the 
venous  flow.  The  united  sectional  area  of  the  capillaries  is,  as  we  have  seen, 
some  hundreds  of  times  greater  than  the  sectional  area  of  the  aorta  ;  but 
this  united  sectional  area  is  made  up  of  thousands  of  minute  passages, 
varying  in  man  from  5  to  20  //,  some  of  them,  therefore,  being  in  an  undis- 
tended  condition,  smaller  than  the  diameter  of  a  red  corpuscle.  Even  were 
the  blood  a  simple  liquid  free  from  all  corpuscles,  these  extremely  minute 
passages  would  occasion  an  enormous  amount  of  friction,  and  thus  present  a 
considerable  obstacle  or  resistance  to  the  flow  of  blood  through  them.  Still 
greater  must  be  the  friction  and  resistance  occasioned  by  the  actual  blood 
with  its  red  and  white  corpuscles.  The  blood  in  fact  meets  with  great  dif- 
ficulties in  its  passage  through  the  peripheral  region,  and  sometimes,  as  we 
shall  see,  the  friction  and  resistance  are  so  great  in  the  peripheral  vessels  of 
this  or  that  area  that  no  blood  passes  through  them  at  all,  and  an  arrest  of 
the  flow  takes  place  in  the  area. 

The  resistance  to  the  flow  of  blood  thus  caused  by  the  friction  generated 
in  so  many  minute  passages  is  one  of  the  most  important  physical  facts  in  the 
circulation.  In  the  large  arteries  the  friction  is  small ;  it  increases  gradually 
as  they  divide,  but  receives  its  chief  and  most  important  addition  in  the 
minute  arteries  and  capillaries;  it  is  relatively  greater  in  the  minute  arteries 
than  in  the  capillaries  on  account  of  the  flow  being  more  rapid  in  the  former, 


144  THE  VASCULAR  MECHANISM. 

for  friction  diminishes  rapidly  with  a  diminution  in  the  rate  of  flow.  We 
may  speak  of  it  as  the  "  peripheral  friction,"  and  the  resistance  which  it 
offers  as  the  "  peripheral  resistance."  It  need  perhaps  hardly  be  said  that 
this  peripheral  resistance  not  only  opposes  the  flow  of  blood  through  the 
capillaries  and  minute  arteries  themselves  where  it  is  generated,  but,  working 
backward  along  the  whole  arterial  system,  has  to  be  overcome  by  the  heart 
at  each  systole  of  the  ventricle. 

Hydraulic  Principles  of  the  Circulation. 

§  107.  In  the  circulation,  then,  the  following  three  facts  of  fundamental 
importance  are  met  with  : 

1.  The  systole  of  the  ventricle,  driving  at  intervals  a  certain  quantity 
of  blood,  with  a  certain  force,  into  the  aorta. 

2.  The  peripheral  resistance  just  described. 

3.  A  long  stretch   of  elastic  tubing  (the  arteries),  reaching  from  the 
ventricle  to  the  region  of  peripheral  resistance. 

From  these  facts  we  may  explain  the  main  phenomena  of  the  circula- 
tion, which  we  have  previously  sketched,  on  purely  physical  principles 
without  any  appeal  to  the  special  properties  of  living  tissues,  beyond  the 
provision  that  the  ventricle  remains  capable  of  good  rhythmical  contrac- 
tions, that  the  arterial  walls  retain  their  elasticity,  and  that  the  friction  be- 
tween the  blood  and  the  lining  of  the  peripheral  vessels  remains  the  same  ; 
we  may  thus  explain  the  high  pressure  and  pulsatile  flow  in  the  arteries, 
the  steady  stream  through  the  capillaries,  the  low  pressure  and  the  uniform 
pulseless  flow  in  the  veins,  and  finally  the  continued  flow  of  the  blood  from 
the  aorta  to  the  mouths  of  the  venae  cavse. 

All  the  above  phenomena  in  fact  are  the  simple  results  of  an  intermit- 
tent force  (like  that  of  the  systole  of  the  ventricle)  working  in  a  closed 
circuit  of  branching  tubes,  so  arranged  that  while  the  individual  tubes  first 
diminish  in  calibre  (from  the  heart  to  the  capillaries)  and  then  increase 
(from  the  capillaries  to  the  heart),  the  area  of  the  bed  first  increases  and 
then  diminishes,  the  tubes  together  thus  forming  two  cones  placed  base  to 
base  at  the  capillaries,  with  their  apices  converging  at  the  heart,  and  pre- 
senting at  their  conjoined  bases  a  conspicuous  peripheral  resistance,  the 
tubing  on  one  side,  the  arterial,  being  eminently  elastic,  and,  on  the  other, 
the  venous,  affording  a  free  and  easy  passage  for  the  blood.  It  is  the 
peripheral  resistance  (for  the  resistance  offered  by  the  friction  in  the  larger 
vessels  may,  when  compared  with  this,  be  practically  neglected),  reacting 
through  the  elastic  walls  of  the  arteries  upon  the  intermittent  force  of  the 
heart,  which  gives  the  circulation  of  the  blood  its  peculiar  features. 

§  108.  Circumstances  determining  the  character  of  the  flow.  When  fluid 
is  driven  by  an  intermittent  force,  as  by  a  pump,  through  a  perfectly  rigid 
tube,  such  as  a  glass  one  (or  a  system  of  such  tubes),  there  escapes  at  each 
stroke  of  the  pump  from  the  distal  end  of  the  tube  (or  system  of  tubes)  just 
as  much  fluid  as  enters  it  at  the  proximal  end.  What  happens  is  very  like 
what  would  happen  if,  with  a  wide  glass  tube  completely  filled  with  billiard 
balls  lying  in  a  row,  an  additional  ball  were  pushed  in  at  one  end :  each 
ball  would  be  pushed  on  in  turn  a  stage  further  and  the  last  ball  at  the 
further  end  would  tumble  out.  The  escape,  moreover,  takes  place  at  the 
same  time  as  the  entrance. 

This  result  remains  the  same  when  any  resistance  to  the  flow  is  intro- 
duced into  the  tube,  as  for  instance  when  the  end  of  the  tube  is  narrowed. 
The  force  of  the  pump  remaining  the  same,  the  introduction  of  the  resist- 
ance undoubtedly  lessens  the  quantity  of  fluid  issuing  at  the  distal  end  at 


THE  MAIN   FACTS  OF  THE  CIRCULATION.  145 

each  stroke,  but  it  at  the  same  time  lessens  the  quantity  entering  at  the 
proximal  end ;  the  inflow  and  outflow  remain  equal  to  each  other,  and  still 
occur  at  the  same  time. 

In  an  elastic  tube,  such  as  an  India-rubber  one  (or  in  a  system  of  such 
tubes),  whose  sectional  area  is  sufficiently  great  to  offer  but  little  resistance 
to  the  progress  of  the  fluid,  the  flow  caused  by  an  intermittent  force  is  also 
intermittent.  The  outflow  being  nearly  as  easy  as  the  inflow,  the  elasticity 
of  the  walls  of  the  tube  is  scarcely  at  all  called  into  play.  The  tube  be- 
haves practically  like  a  rigid  tube.  When,  however,  sufficient  resistance  is 
introduced  into  any  part  of  the  course,  the  fluid  being  unable  to  pass  by 
the  resistance  as  rapidly  as  it  enters  the  tube  from  the  pump,  tends  to  accu- 
mulate on  the  proximal  side  of  the  resistance.  This  it  is  able  to  do  by  ex- 
panding the  elastic  walls  of  the  tube.  At  each  stroke  of  the  pump  a  cer- 
tain quantity  of  fluid  enters  the  tube  at  the  proximal  end.  Of  this  only  a 
fraction  can  pass  through  the  resistance  during  the  stroke.  At  the  moment 
when  the  stroke  ceases,  the  rest  still  remains  on  the  proximal  side  of  the 
resistance,  the  elastic  tube  having  expanded  to  receive  it.  During  the 
interval  between  this  and  the  next  stroke,  the  distended  elastic  tube,  striving 
to  return  to  its  natural  undistended  condition,  presses  on  this  extra  quantity 
of  fluid  which  it  contains  and  tends  to  drive  it  past  the  resistance. 

Thus,  in  the  rigid  tube  (and  in  the  elastic  tube  without  the  resistance) 
there  issues,  from  the  distal  end  of  the  tube  at  each  stroke,  just  as  much 
fluid  as  enters  it  at  the  proximal  end,  while  between  the  strokes  there  is 
perfect  quiet.  In  the  elastic  tube  with  resistance,  on  the  contrary,  the  quan- 
tity which  passes  the  resistance  is  only  a  fraction  of  that  which  enters  the 
tube  from  the  pump  at  any  one  stroke,  the  remainder  or  a  portion  of  the- 
remainder  continuing  to  pass  during  the  interval  between  the  strokes.  In 
the  former  case  the  tube  is  no  fuller  at  the  end  of  the  stroke  than  at  the 
beginning ;  in  the  latter  case  there  is  an  accumulation  of  fluid  between  the 
pump  and  the  resistance,  and  a  corresponding  distention  of  that  part  of  the 
tube  at  the  close  of  each  stroke — an  accumulation  and  distention,  however, 
which  go  on  diminishing  during  the  interval  between  that  stroke  and  the 
next.  The  amount  of  fluid  thus  remaining  after  the  stroke  will  depend  on 
the  amount  of  resistance  in  relation  to  the  force  of  the  stroke  and  on  the 
distensibility  of  the  tube ;  and  the  amount  which  passes  the  resistance  before 
the  next  stroke  will  depend  on  the  degree  of  elastic  reaction  of  which  the 
tube  is  capable.  Thus,  if  the  resistance  be  very  considerable  in  relation  to 
the  force  of  the  stroke,  and  the  tube  very  distensible,  only  a  small  portion 
of  the  fluid  will  pass  the  resistance,  the  greater  part  remaining  lodged  be- 
tween the  pump  and  the  resistance.  If  the  elastic  reaction  be  great,  a  large 
portion  of  this  will  be  passed  on  through  the  resistance  before  the  next 
stroke  comes.  In  other  words,  the  greater  the  resistance  (in  relation  to  the 
force  of  the  stroke),  and  the  more  the  elastic  force  is  brought  into  play,  the 
less  intermittent,  the  more  nearly  continuous,  will  be  the  flow  on  the  far 
side  of  the  resistance. 

If  the  first  stroke  be  succeeded  by  a  second  stroke  before  its  quantity  of 
fluid  has  all  passed  by  the  resistance,  there  will  be  an  additional  accumula- 
tion of  fluid  on  the  near  side  of  the  resistance,  an  additional  distention  of 
the  tube,  an  additional  strain  on  its  elastic  powers,  and,  in  consequence,  the 
flow  between  this  second  stroke  and  the  third  will  be  even  more  marked  than 
that  between  the  first  and  second,  though  all  three  strokes  were  of  the  same 
force,  the  addition  being  due  to  the  extra  amount  of  elastic  force  called  into 
play.  In  fact,  it  is  evident  that,  if  there  be  a  sufficient  store  of  elastic  power 
to  fall  back  upon,  by  continually  repeating  the  stroke  a  state  of  things  will 
be  at  last  arrived  at  in  which  the  elastic  force,  called  into  play  by  the  con- 
10 


146  THE  VASCULAK  MECHANISM. 

tinually  increasing  distention  of  the  tube  on  the  near  side  of  the  resistance, 
will  be  sufficient  to  drive  through  the  resistance,  between  each  two  strokes, 
just  as  much  fluid  as  enters  the  near  end  of  the  system  as  each  stroke.  In 
other  words,  the  elastic  reaction  of  the  walls  of  the  tube  will  have  converted 
the  intermittent  into  a  continuous  flow.  The  flow  on  the  far  side  of  the 
resistance  is  in  this  case  not  the  direct  result  of  the  strokes  of  the  pump.  All 
the  force  of  the  pump  is  spent,  first  in  getting  up,  and  afterward  in  keeping 
up,  the  distention  of  the  tube  on  the  near  side  of  the  resistance ;  the  imme- 
diate cause  of  the  continuous  flow  lies  in  the  distention  of  the  tube  which 
leads  it  to  empty  itself  into  the  far  side  of  the  resistance  at  such  a  rate  that 
it  discharges  through  the  resistance  during  a  stroke  and  in  the  succeeding 
interval  just  as  much  as  it  receives  from  the  pump  by  the  stroke  itself. 

This  is  exactly  what  takes  place  in  the  vascular  system.  The  friction  in 
the  minute  arteries  and  capillaries  presents  a  considerable  resistance  to  the 
flow  of  blood  through  them  into  the  small  veins.  In  consequence  of  this 
resistance  the  force  of  the  heart's  beat  is  spent  in  maintaining  the  whole  of 
the  arterial  system  in  a  state  of  great  distention  ;  the  arterial  walls  are  put 
greatly  on  the  stretch  by  the  pressure  of  the  blood  thrust  into  them  by  the 
repeated  strokes  of  the  heart ;  this  is  the  pressure  which  we  spoke  of  above 
as  blood-pressure.  The  greatly  distended  arterial  system  is,  by  the  elastic 
reaction  of  its  elastic  walls,  continually  tending  to  empty  itself  by  overflow- 
ing through  the  capillaries  into  the  venous  system ;  and  it  overflows  at  such 
a  rate  that  just  as  much  blood  passes  from  the  arteries  to  the  veins  during 
each  systole  and  its  succeeding  diastole  as  enters  the  aorta  at  each  systole. 

§  109.  Indeed,  the  important  facts  of  the  circulation  which  we  have  not 
as  yet  studied  may  be  roughly  but  successfully  imitated  on  an  artificial  model, 
Fig.  42,  in  which  an  elastic  syringe  represents  the  heart,  a  long  piece  of 
elastic  India-rubber  tubing  the  arteries,  another  piece  of  tubing  the  veins, 
and  a  number  of  smaller  connecting  pieces  the  minute  arteries  and  capil- 
laries. If  these  connecting  pieces  be  made  at  first  somewhat  wide,  so  as  to 
offer  no  great  resistance  to  the  flow  from  the  artificial  arteries  to  the  artificial 
veins,  but  be  so  arranged  that  they  may  be  made  narrow  by  the  screwing-up 
of  clamps  or  otherwise,  it  is  possible  to  illustrate  the  behavior  of  the  vascular 
mechanism  when  the  peripheral  resistance  is  less  than  usual  (and  as  we  shall 
see  later  on  it  is  possible  in  the  living  organism  either  to  reduce  or  to  increase 
what  may  be  considered  as  the  normal  peripheral  resistance),  and  to  compare 
that  behavior  with  the  behavior  of  the  mechanism  when  the  peripheral 
resistance  is  increased. 

The  whole  apparatus  being  placed  flat  on  a  table,  so  as  to  avoid  differences 
in  level  in  different  parts  of  it,  and  filled  with  water,  but  so  as  not  to  distend 
the  tubing,  the  two  manometers  attached,  one  (A)  to  the  arterial  side  of  the 
tubing  and  the  other  (V)  to  the  venous  side,  ought  to  show  the  mercury 
standing  at  equal  heights  in  both  limbs  of  both  instruments,  since  nothing 
but  the  pressure  of  the  atmosphere  is  bearing  on  the  fluid  in  the  tubes,  and 
that  equally  all  over. 

If,  now,  the  connecting  pieces  being  freely  open,  that  is  to  say,  the  periph- 
eral resistance  being  very  little,  we  imitate  a  ventricular  beat  by  the  stroke  of 
the  pump,  we  shall  observe  the  following :  Almost  immediately  after  the  stroke 
the  mercury  in  the  arterial  manometer  will  rise,  but  will  at  once  fall  again, 
and  very  shortly  afterward  the  mercury  in  the  venous  tube  will  in  a  similar 
manner  rise  and  fall.  If  we  repeat  the  strokes  with  a  not  too  rapid  rhythm, 
each  stroke  having  the  same  force,  and  make,  as  may  by  a  simple  contrivance 
be  effected,  the  two  manometers  write  on  the  same  recording  surface,  We  shall 
obtain  curves  like  those  of  Fig.  43,  A  and  V.  At  each  stroke  of  the  pump 
the  mercury  in  the  arterial  manometers  rises,  but  forthwith  falls  again  to  or 


THE  MAIN  FACTS  OF  THE  CIRCULATION. 


147 


nearly  to  the  base  line  ;  no  mean  arterial  pressure,  or  very  little,  is  established. 
The  contents  of  the  ventricle  (syringe)  thrown  into  the  arterial  system  dis- 


FIG.  42. 


Arterial  Scheme :  P,  unshaded,  is  an  elastic  tube  to  represent  the  arterial  system,  branching 
at  .AT  and  Y,  and  ending  in  the  region  of  peripheral  resistance,  including  the  capillaries,  which 
are  imitated  by  filling  loosely  with  small  pieces  of  sponge  the  parts  shown  as  dilated  in  the  figure. 
The  capillaries  are  gathered  up  into  the  venous  system,  shaded,  which  terminates  at  0.  Water  is 
d  riven  into  the  arterial  system  at  P  by  means  of  an  elastic  bag  syringe  or  any  other  form  of  pump. 
Clamps  are  placed  on  the  undilated  tubes,  c,  c',  c".  When  these  clamps  are  tightened,  the  only 
access  for  the  water  from  the  arterial  to  the  venous  side  is  through  the  dilated  parts  filled  with 
sponge,  which  offers  a  considerable  resistance  to  the  flow  of  fluid  through  them.  When  the 
clamps  are  unloosed  the  fluid  passes,  with  much  less  resistance,  through  the  undilated  tubes. 
Thus,  by  tightening  or  loosening  the  clamps  the  "peripheral"  resistance  may  be  increased  or 
diminished  at  pleasure. 

At  A,  on  the  arterial  side,  and  at  V,  on  the  venous  side,  manometers  can  be  attached.  At  a 
and  v  (and  also  at  x  and  y),  by  means  of  clamps,  the  flow  of  fluid  from  an  artery  and  from  a  vein, 
under  various  conditions,  may  be  observed.  At  Sa,  S'a,  and  Sv,  sphygmographs  may  be  applied. 

tend  it,  but  the  passage  through  the  peripheral  region  is  so  free  that  an  equal 
quantity  of  fluid  passes  through  to  the  veins  immediately,  and  hence  the 

FIG.  43. 


Tracings  taken  from  an  Artificial  Scheme,  with  the  Peripheral  Resistance  Slight :  A,  arterial ; 
V,  venous  manometer.  This  figure,  to  save  space,  is  on  a  smaller  scale  than  the  corresponding 
Fig.  44. 

mercury  at  once  falls.     But  the  fluid  thus  passing  easily  into  the  veins  dis- 
tends these  too,  and  the  mercury  in  their  manometer  rises  too,  but  only  to 


148 


THE  VASCULAR  MECHANISM. 


fall  again,  as  a  corresponding  quantity  issues  from  the  ends  of  the  veins  into 
the  basin,  which  serves  as  an  artificial  auricle.  Now  introduce  "  peripheral 
resistance  "  by  screwing  up  the  clamps  on  the  connecting  tubes,  and  set  the 
pump  to  work  again  as  before.  With  the  first  stroke  the  mercury  in  the 
arterial  manometer  (Fig.  44,  A1)  rises  as  before,  but  instead  of  falling  rapidly 
it  falls  slowly,  because  it  now  takes  a  longer  time  for  a  quantity  of  fluid 
equal  to  that  which  has  been  thrust  into  the  arterial  system  by  the  ventricu- 
lar stroke  to  pass  through  the  narrowed  peripheral  region.  Before  the  curve 
has  fallen  to  the  base  line,  before  the  arterial  system  has  had  time  to  dis- 
charge through  the  narrowed  peripheral  region  as  much  fluid  as  it  received 
from  the  ventricle,  a  second  stroke  drives  more  fluid  into  the  arteries,  dis- 
tending them  this  time  more  than  it  did  before,  and  raising  the  mercury  to 

FIG.  44. 


Tracings  taken  from  an  Artificial  Scheme,  with  the  Peripheral  Resistance  Considerable:  Alt 
arterial ;  V1,  venous  manometer. 

a  still  higher  level.  A  third,  a  fourth,  and  succeeding  strokes  produce  the 
same  effect,  except  that  the  additional  height  to  which  the  mercury  is  raised 
at  each  stroke  becomes  at  each  stroke  less  and  less,  until  a  state  of  things  is 
reached  in  which  the  mercury,  being  on  the  fall  when  the  stroke  takes  place, 
is  by  the  stroke  raised  just  as  high  as  it  was  before,  and  then  beginning  to 
fall  again  is  again  raised  just  as  high,  and  so  on.  With  each  succeeding 
stroke  the  arterial  system  has  become  more  and  more  distended  ;  but  the 
more  distended  it  is  the  greater  is  the  elastic  reaction  brought  into  play  ;  this 
greater  elastic  reaction  more  and  more  overcomes  the  obstacle  presented  by 
the  peripheral  resistance  and  drives  the  fluid  more  and  more  rapidly  through 
the  peripheral  region.  At  last  the  arterial  system  is  so  distended,  and  the 
force  of  the  elastic  reaction  so  great,  that  during  the  stroke  and  the  succeed- 
ing interval  just  as  much  fluid  passes  through  the  peripheral  region  as  enters 
the  arteries  at  the  stroke.  In  other  words,  the  repeated  strokes  have  estab- 
lished a  mean  arterial  pressure  which,  at  the  point  where  the  manometer 


THE  MAIN  FACTS  OF  THE  CIRCULATION.  149 

is  affixed,  is  raised  slightly  at  each  ventricular  stroke,  and  falls  slightly 
between  the  strokes. 

Turning  now  to  the  venous  manometer,  Fig.  44,  V1,  we  observe  that  each 
stroke  of  the  pump  produces  on  this  much  less  effect  than  it  did  before  the 
introduction  of  the  increased  peripheral  resistance.  The  mercury,  instead  of 
distinctly  rising  and  falling  at  each  stroke,  now  shows  nothing  more  than 
very  gentle  undulations ;  it  feels  to  a  very  slight  degree  only  the  direct 
effect  of  the  ventricular  stroke ;  it  is  simply  raised  slightly  above  the  base 
line,  and  remains  fairly  steady  at  this  level.  The  slight  rise  marks  the 
mean  pressure  exerted  by  the  fluid  at  the  place  of  attachment  of  the  manom- 
eter. This  mean  "  venous  "  pressure  is  a  continuation  of  the  mean  arterial 
pressure  so  obvious  in  the  arterial  manometer,  but  is  much  less  than  that  be- 
cause a  large  part  of  the  arterial  mean  pressure  has  been  expended  in  driv- 
ing the  fluid  past  the  peripheral  resistance.  What  remains  is,  however, 
sufficient  to  drive  the  fluid  along  the  wide  venous  tubing  right  to  the  open 
end. 

Thus  this  artificial  model  may  be  made  to  illustrate  how  it  comes  about 
that  the  blood  flows  in  the  arteries  at  a  relatively  high  pressure,  which  at 
each  ventricular  systole  is  raised  slightly  above  and  at  each  diastole  falls 
slightly  below  a  certain  mean  level,  and  flows  in  the  veins  at  a  much  lower 
pressure,  which  does  not  show  the  immediate  effects  of  each  heart-beat. 

If  two  manometers,  instead  of  one,  were  attached  to  the  arterial  system, 
one  near  the  pump  and  the  other  further  off,  close  to  the  peripheral  resist- 
ance, the  pressure  shown  by  the  near  manometer  would  be  found  to  be  greater 
than  that  shown  by  the  far  one.  The  pressure  at  the  far  point  is  less 
because  some  of  the  pressure  exerted  at  the  near  point  has  been  used  to  drive 
the  fluid  from  the  near  point  to  the  far  one.  Similarly  on  the  venous  side,  a 
manometer  placed  close  to  the  peripheral  region  would  show  a  higher  pres- 
sure than  that  shown  by  one  further  off,  because  it  is  the  pressure  still  remain- 
ing in  the  veins  near  the  capillaries  which,  assisted,  as  we  shall  see,  by  other 
events,  drives  the  blood  onward  to  the  larger  veins.  The  blood-pressure  is 
at  its  highest  at  the  root  of  the  aorta  and  at  its  lowest  at  the  mouths  of  the 
vena3  cavse,  and  is  falling  all  the  way  from  one  point  to  the  other,  because 
all  the  way  it  is  being  used  up  to  move  the  blood  from  one  point  to  the 
other.  The  great  drop  of  pressure  is,  as  we  have  said,  in  the  peripheral 
region,  because  more  work  has  to  be  done  in  driving  the  blood  through  this 
region  than  in  driving  the  blood  from  the  heart  to  this  region,  or  from  this 
region  to  the  heart. 

The 'manometer  on  the  arterial  side  of  the  model  shows,  as  we  have  seen, 
an  oscillation  of  pressure,  a  pulse  due  to  each  heart-beat,  and  the  same  pulse 
may  be  felt  by  placing  a  finger,  or  rendered  visible  by  placing  a  light  lever, 
on  the  arterial  tube.  It  may  further  be  seen  that  this  pulse  is  most  marked 
nearest  the  pump,  and  becomes  fainter  as  we  pass  to  the  periphery ;  but  we 
must  reserve  the  features  of  the  pulse  for  a  special  study.  On  the  venous 
side  of  the  model  no  pulse  can  be  detected  by  the  manometer  or  by  the 
finger,  provided  that  the  peripheral  resistance  be  adequate.  If  the  periph- 
eral resistance  be  diminished,  as  by  unscrewing  the  clamps,  then,  as  neces- 
sarily follows  from  what  has  gone  before,  the  pulse  passes  over  on  to  the 
venous  side ;  and,  as  we  shall  have  occasion  to  point  out  later  on,  in  the 
living  organism  the  peripheral  resistance  in  particular  areas  may  be  at 
times  so  much  lessened  that  a  distinct  pulsation  appears  in  the  veins. 

If  in  the  model,  when  the  pump  is  in  full  swing,  and  arterial  pressure  well 
established,  the  arterial  tube  be  pricked  or  cut,  or  the  small  side  tube  a 
be  opened,  the  water  will  gush  out  in  jets,  as  does  blood  from  a  cut  artery  in 
the  living  body ;  whereas,  if  the  venous  tube  be  similarly  pricked  or  cut,  or 


150  THE  VASCULAR  MECHANISM. 

the  small  tube  v  be  opened,  the  water  will  simply  ooze  out  or  well  up,  as 
does  blood  from  a  vein  in  the  living  body.  If  the  arterial  tube  be  ligatured, 
it  will  swell  on  the  pump  side  and  shrink  on  the  peripheral  side;  if  the 
venous  tube  be  ligatured,  it  will  swell  on  the  side  nearest  the  capillaries  and 
shrink  on  the  other  side.  In  short,  the  dead  model  will  show  all  the  main 
facts  of  the  circulation  which  we  have  as  yet  described. 

§  110.  In  the  living  body,  however,  there  are  certain  helps  to  the  circu- 
lation which  cannot  be  imitated  by  such  a  model  without  introducing  great 
and  undesirable  complications ;  but  these  chiefly  affect  the  flow  along  the 
veins. 

The  veins  are  in  many  places  provided  with  valves  so  constructed  as  to 
offer  little  or  no  resistance  to  the  flow  from  the  capillaries  to  the  heart,  but 
effectually  to  block  a  return  toward  the  capillaries.  Hence  any  external 
pressure  brought  to  bear  upon  a  vein  tends  to  help  the  blood  to  move  for- 
ward toward  the  heart.  In  the  various  movements  carried  out  by  the 
skeletal  muscles,  such  an  external  pressure  is  brought  to  bear  on  many  of 
the  veins,  and  hence  these  movements  assist  the  circulation.  Even  passive 
movements  of  the  limbs  have  a  similar  effect.  So,  also,  the  movements  of 
the  alimentary  canal,  carried  out  by  means  of  plain  muscular  tissue,  promote 
the  flow  along  the  veins  coming  from  that  canal,  and  when  we  come  to  deal 
with  the  spleen  we  shall  see  that  the  plain  muscular  fibres  which  are  so 
abundant  in  that  organ  in  some  animals,  serve  by  rhythmical  contractions 
to  pump  the  blood  regularly  away  from  the  spleen  along  the  splenic  veins. 

When  we  come  to  deal  with  respiration,  we  shall  see  that  each  enlarge- 
ment of  the  chest  constituting  an  inspiration  tends  to  draw  the  blood  toward 
the  chest,  and  each  return  or  retraction  of  the  chest  walls  in  expiration  tends 
to  drive  the  blood  away  from  the  chest.  The  arrangement  of  the  valves  of 
the  heart  causes  this  action  of  the  respiratory  pump  to  promote  the  flow  of 
blood  in  the  direction  of  the  normal  circulation  ;  and,  indeed,  were  the  heart 
perfectly  motionless,  the  working  of  this  respiratory  pump  alone  would  tend 
to  drive  the  blood  from  the  vense  cavse  through  the  heart  into  the  aorta,  and 
so  to  keep  up  the  circulation  ;  the  force  so  exerted,  however,  would,  without 
the  aid  of  the  heart,  be  able  to  overcome  a  very  small  part  only  of  the 
resistance  in  the  capillaries  and  small  vessels  of  the  lungs,  and  so  would 
prove  actually  ineffectual. 

There  are,  then,  several  helps  to  the  flow  along  the  veins,  but  it  must  be 
remembered  that,  however  useful,  they  are  helps  only,  and  not  the  real  cause 
of  the  circulation.  The  real  cause  of  the  flow  is  the  ventricular  str.oke,  and 
this  is  sufficient  to  drive  the  blood  from  the  left  ventricle  to  the  right  auricle, 
even  when  every  muscle  of  the  body  is  at  rest  and  breathing  is  for  a  while 
stopped,  when,  therefore,  all  the  helps  we  are  speaking  of  are  wanting. 

Circumstances  Determining  the  Rate  of  the  Flow. 

§  111.  We  may  now  pass  on  to  consider  briefly  the  rate  at  which  the 
blood  flows  through  the  vessels,  and  first  the  rate  of  flow  in  the  arteries. 

When  even  a  small  artery  is  severed,  a  considerable  quantity  of  blood 
escapes  from  the  proximal  cut  end  in  a  very  short  space  of  time.  That  is  to 
say,  the  blood  moves  in  the  arteries  from  the  heart  to  the  capillaries  with  a 
very  considerable  velocity.  By  various  methods,  this  velocity  of  the  blood- 
current  has  been  measured  at  different  parts  of  the  arterial  system ;  the 
results,  owing  to  imperfections  in  the  methods  employed,  cannot  be  regarded 
as  satisfactorily  exact,  but  may  be  accepted  as  approximately  true.  They 
show  that  the  velocity  of  the  arterial  stream  is  greatest  in  the  largest  arteries 
near  the  heart,  and  diminishes  from  the  heart  toward  the  capillaries.  Thus, 


THE  MAIN   FACTS  OF  THE  CIRCULATION. 


151 


in  a  large  artery  of  a  large  animal,  such  as  the  carotid  of  a  dog  or  horse,  and 
probably  in  the  carotid  of  a  man,  the  blood  flows  at  the  rate  of  300  or  500 
mm.  a  second.  In  the  very  small  arteries  the  rate  is  probably  only  a  few 
mm.  a  second. 

Methods.     The  haemadromometer  of  Volkmann.  [Fig.  45.]     An  artery— e.g.,  a 
carotid— is  clamped  in  two  places,  and  divided  between  the  clamps.     Two  canulse, 

[FiG.  45. 


Volkmann's  Hsemadromometer:  The  conical  portions  of  the  instrument  are  inserted  in  the 
cut  ends  of  a  vein  or  artery.  By  a  simple  arrangement  of  a  double  stopcock  the  blood-current 
can  be  made  to  pass  immediately  through  the  transverse  arm,  as  in  A,  or  to  pass  through  the 
graduated  U-shaped  tube,  as  in  B.] 

of  a  bore  as  nearly  equal  as  possible  to  that  of  the  artery,  or  of  a  known  bore,  are 
inserted  in  the  two  ends.  The  two  canulse  are  connected  by  means  of  two  stop- 
cocks, which  work  together,  with  the  two  ends  of  a  long  glass  tube,  bent  in  the 
shape  of  a  U,  and  filled  with  normal  saline  solution,  or  with  a  colored  innocuous 
fluid.  The  clamps  on  the  artery  being  released,  a  turn  of  the  stopcocks  permits 
the  blood  to  enter  the  proximal  end  of  the  long  U-tube,  along  which  it  courses, 
driving  the  fluid  out  into  the  artery  through  the  distal  end.  Attached  to  the  tube 
is  a  graduated  scale,  by  means  of  which  the  velocity  with  which  the  blood  flows 
along  the  tube  may  be  read  off.  Even  supposing  the  canulse  to  be  of  the  same 
bore  as  the  artery,  it  is  evident  that  the  conditions  of  the  flow  through  the  tube  are 
such  as  will  only  admit  of  the  resu]t  thus  gained  being  considered  as  an  approxi- 
mative estimation  of  the  real  velocity  in  the  artery  itself. 

The  rheometer  (StronnihrT  of  Ludwig.  This  consists  of  two  glass  bulbs,  A  and 
B,  Fig.  46,  communicating  above  with  each  other  and  with  the  common  tube  C, 
by  which  they  can  be  filled.  Their  lower  ends  are  fixed  in  the  metal  disc  />, 
which  can  be  made  to  rotate,  through  two  right  angles,  round  the  lower  disc  E. 
In  the  upper  disc  are  two  holes,  a  and  b,  continuous  with  A  and  B  respectively, 
and  in  the  lower  disc  are  two  similar  holes  a'  and  &',  similarly  continuous  with 
the  tubes  G  and  H.  Hence,  in  the  position  of  the  discs  shown  in  the  figure,  the 
tube  G  is  continuous  through  the  two  discs  with  the  bulb  A,  and  the  tube  H 
with  the  bulb  B.  On  turning  the  disc  D  through  two  right  angles,  the  tube  G 


152 


THE  VASCULAK  MECHANISM. 


becomes  continuous  with  B  instead  of  A.  and  the  tube  H  with  A  instead  of  B. 
There  is  a  further  arrangement,  omitted  from  the  figure  for  the  sake  of  simplicity, 
by  which  when  the  disc  D  is  turned  through  one  instead  of  two  right  angles  from 
either  of  the  above  positions,  G  becomes  directly  continuous  with  H,  both  being 
complete!}7  shut  off  from  the  bulbs. 

The  ends  of  the  tubes  Hand  G  are  made  to  fit  exactly  into  two  canulse  inserted 
into  the  two  cut  ends  of  the  artery  about  to  be  experimented  upon,  and  having  a 
bore  as  nearly  equal  as  possible  to  that  of  the  artery. 

The  method  of  experimenting  is  as  follows :  The  disc  /),  being  placed  in  the 
intermediate  position,  so  that  a  and  b  are  both  cut  off  from  a'  and  b',  the  bulb  A 
is  filled  with  pure  olive  oil  up  to  the  mark  x,  and  the  bulb  B,  the  rest  of  A,  and 
the  junction  C,  with  defibrinated  blood ;  and  C  is  then  clamped.  The  tubes  H 
and  G  are  also  filled  with  defibrinated  blood,  and  G  is  inserted  into  the  canula 
of  the  central,  H  into  that  of  the  peripheral,  end  of  the  artery.  On  removing 

FIG.  46. 


Ludwig's  Stromuhr  and  a  Diagrammatic  Representation  of  the  Same. 


the  clamps  from  the  artery  the  blood  flows  through  G  to  H,  and  so  back  into  the 
artery.  The  observation  now  begins  by  turning  the  disc  D  into  the  position 
shown  in  the  figure ,  the  blood  then  flows  into  A,  driving  the  oil  there  contained 
out  before  it  into  the  bulb  B,  in  the  direction  of  the  arrow,  the  defibrinated  blood 
previously  present  in  B  passing  by  H  into  the  artery,  and  so  into  the  system.  At 
the  moment  that  the  blood  is  seen  to  rise  to  the  mark  x,  the  disc  D  is  with  all 
possible  rapidity  turned  through  two  right  angles  ;  and  thus  the  bulb  J5,  now 
largely  filled  with  oil,  placed  in  communication  with  G.  The  blood-stream  now 
drives  the  oil  back  into  A,  and  the  new  blood  in  A  through  H  into  the  artery. 
As  soon  as  the  oil  has  wholly  returned  to  its  original  position,  the  disc  is  again 
turned  round,  and  A  once  more  placed  in  communication  with  £,  and  the  oil  once 
more  driven  from  A  to  B.  And  this  is  repeated  several  times,  indeed  generally 
until  the  clotting  of  the  blood  or  the  admixture  of  the  oil  with  the  blood  puts  an 
end  to  the  experiment.  Thus  the  flow  of  blood  is  used  to  fill  alternately  with 
blood  or  oil  the  space  of  the  bulb  A,  whose  cavity  as  far  as  the  mark  x  has  been 
exactly  measured ;  hence  if  the  number  of  times  in  any  given  time  the  disc  D 
has  to  be  turned  round  be  known,  the  number  of  times  A  has  been  filled  is  also 
known,  and  thus  the  quantity  of  blood  which  has  passed  in  that  time  through 
the  canula  connected  with  the  tube  G  is  directly  measured.  For  instance,  sup- 


THE   MAIN   FACTS  OF  THE  CIRCULATION. 


153 


[FiG.  47. 


posing  that  the  quantity  held  by  the  bulb  A  when  filled  up  to  the  mark  x  is  5  c.c., 
and  supposing  that  from  the  moment  of  allowing  the  first  5  c.c.  of  blood  to  begin 
to  enter  the  tube  to  the  moment  when  the  escape  of  the  last  5  c.c.  from  the  artery 
into  the  tube  was  complete,  100  seconds  had  elapsed,  during  which  time  5  c.c. 
has  been  received  ten  times  into  the  tube  from  the  artery  (all  but  the  last  5  c.c. 
being  returned  into  the  distal  portion  of  the  artery),  obviously  0.5  c.c.  of  blood 
had  flowed  from  the  proximal  section  of  the  artery  in  one  second.  Hence,  sup- 
posing that  the  diameter  of  the  canula  (arid  of  the  artery,  they  being  the  same) 
were  2  mm.,  with  area  therefore  of  3.14  square  mm.,  an  outflow  through  the  sec- 
tion of  0.5  c.c.  or  500  mm.  in  a  second  would  give  (-|~)  a  velocity  of  about  159 
mm.  in  a  second. 

The  hgematachometer  of  Vierordt  [Fig.  47]  is  constructed  on  the  principle  of 
measuring  the  velocity  of  the  current  by  observing  the  amount  of  deviation  under- 
gone by  a  pendulum,  the  free  end  of  which  hangs  loosely  in  the  stream.  A  square 
or  rectangular  chamber,  one  side  of  which  is  of  glass 
and  marked  with  a  graduated  scale  in  the  form  of  an 
arc  of  a  circle,  is  connected  by  means  of  two  short 
tubes  with  the  two  cut  ends  of  an  artery ;  the  blood 
consequently  flows  from  the  proximal  (central)  por- 
tion of  the  artery  through  the  chamber  into  the  distal 
portion  of  the  artery.  Within  the  chamber  and  sus- 
pended from  its  roof  is  a  short  pendulum,  which  when 
the  blood-stream  is  cut  off  from  the  chamber  hangs 
motionless  in  a  vertical  position,  but  when  the  blood 
is  allowed  to  flow  through  the  chamber,  is  driven  by 
the  force  of  the  current  out  of  its  position  of  rest.  The 
pendulum  is  so  placed  that  a  marker  attached  to  its  free  end  travels  close  to  the 
inner  surface  of  the  glass  side  along  the  arc  of  the  graduated  side.  Hence  the 
amount  of  deviation  from  a  vertical  position  may  easily  be  read  off  on  the  scale 
from  the  outside.  The  graduation  of  the  scale  having  been  carried  out  by  experi- 
menting with  streams  of  known  velocity,  the  velocity  can  at  once  be  calculated  from 
the  amount  of  deviation. 

An  instrument  based  on  the  same  principle  has  been  invented  by  Chauveau  and 
improved  by  Lortet,  Fig.  48.     In  this  the  part  which  corresponds  to  the  pendulum 

FIG.  48. 


Hsematachometer  of  Vierordt  • 
a,  b,  mouthpieces.] 


Hsematachometer  of  Chauveau  and  Lortet. 


in  Vierordt' s  instrument  is  prolonged  outside  the  chamber,  and  thus  the  portion 
within  the  chamber  is  made  to  form  the  short  arm  of  a  lever,  the  fulcrum  of  which 
is  at  the  point  where  the  wall  of  the  chamber  is  traversed,  and  the  long  arm  of 
which  projects  outside.  A  somewhat  wide  tube,  the  wall  of  which  is  at  one  point 


154  THE  VASCULAR  MECHANISM. 

composed  of  an  India-rubber  membrane,  is  introduced  between  the  two  cut  ends 
of  an  artery.  A  long  light  lever  pierces  the  India-rubber  membrane.  The  short 
expanded  arm  of  this  lever  projecting  within  the  tube  is  moved  on  its  fulcrum  in 
the  India-rubber  ring  by  tne  current  of  blood  passing  through  the  tube,  the 
greater  the  velocity  of  the  current  the  larger  being  the  excursion  of  the  lever. 
The  movements  of  the  short  arm  give  rise  to  corresponding  movements  in  the  oppo- 
site direction  of  the  long  arm  outside  the  tube,  and  these,  by  means  of  a  marker 
attached  to  the  end  of  the  long  arm,  may  be  directly  inscribed  on  a  recording  sur- 
face. This  instrument  is  very  well  adapted  for  observing  changes  in  the  velocity 
of  the  flow.  In  determining  actual  velocities,  for  which  purpose  it  has  to  be 
experimentally  graduated,  it  is  not  so  useful. 

In  the  capillaries,  the  rate  is  slowest  of  all.  In  the  web  of  the  frog  the 
flow  as  judged  by  the  movement  of  the  red  corpuscles  may  be  directly 
measured  under  the  microscope  by  means  of  a  micrometer,  and  is  found  to 
be  about  half  a  millimeter  in  a  second  ;  but  this  is  probably  a  low  estimate, 
since  it  is  only  when  the  circulation  is  somewhat  slow,  slower  perhaps  than 
what  ought  to  be  considered  the  normal  rate,  that  the  red  corpuscles  can  be 
distinctly  seen.  In  the  mammal  the  rate  has  been  estimated  at  about  0.75 
millimetres  a  second,  but  is  probably  quicker  than  even  this. 

As  regards  the  veins,  the  flow  is  very  slow  in  the  small  veins  emerging 
from  the  capillaries,  but  increases  as  these  join  into  larger  trunks,  until  in  a 
large  vein,  such  as  the  jugular  of  the  dog,  the  rate  is  about  200  mm.  a 
second. 

§  112.  It  will  be  seen,  then,  that  the  velocity  of  the  flow  is  in  inverse 
proportion  to  the  width  of  the  bed,  to  the  united  sectional  areas  of  the 
vessels.  It  is  greatest  at  the  aorta,  it  diminishes  along  the  arterial  system  to 
the  capillaries,  to  the  united  bases  of  the  cones  spoken  of  in  §  101,  where  it 
is  least,  and  from  thence  increases  again  along  the  venous  system. 

And,  indeed,  it  is  this  width  of  the  bed,  and  this  alone,  which  determines 
the  general  velocity  of  the  flow  at  various  parts  of  the  system.  The  slowness 
of  the  flow  in  the  capillaries  is  not  due  to  there  being  so  much  more  friction 
in  their  narrow  channels  than  in  the  wider  canals  of  the  larger  arteries. 
For  the  peripheral  resistance  caused  by  the  friction  in  the  capillaries  and 
small  arteries  is  an  obstacle  not  only  to  the  flow  of  blood  through  these 
small  vessels  where  the  resistance  is  actually  generated,  but  also  to  the 
escape  of  the  blood  from  the  large  into  the  small  arteries,  and  indeed  from 
the  heart  into  the  large  arteries.  It  exerts  its  influence  along  the  whole 
arterial  tract.  And  it  is  obvious  that  if  it  were  this  peripheral  resistance 
which  checked  the  flow  in  the  capillaries,  there  could  be  no  recovery  of 
velocity  along  the  venous  tract. 

The  blood  is  flowing  through  a  closed  system  of  tubes,  the  bloodvessels, 
under  the  influence  of  one  propelling  force,  the  systole  of  the  ventricle,  for 
this  is  the  force  which  drives  the  blood  from  ventricle  to  auricle,  though,  as 
we  have  seen,  its  action  is  modified  in  the  several  parts  of  the  system.  In 
such  a  system  the  same  quantity  of  fluid  must  pass  each  section  of  the  sys- 
tem at  the  same  time,  otherwise  there  would  be  a  block  at  one  place  and  a 

deficiency  at  another.    If,  for  instance,  a  fluid 
FIG.  49.  is  made  to  flow  by  some  one  force,  pressure  or 

gravity,  through  a  tube  A  (Fig.  49)  with  an 
enlargement  B,  it  is  obvious  that  the  same 
quantity  of  fluid  must  pass  through  the  sec- 
tion b  as  passes  through  the  section  a  in  the 
same  time — for  instance,  a  second.  Other- 
a  I  c  wise,  if  less  passes  through  b  than  a,  the 

fluid  would  accumulate  in  B,  or  if  more, 
B  would  be  emptied.     In  the  same  way  just  as  much  must  pass  in  the 


THE  MAIN   FACTS  OF  THE  CIRCULATION.  155 

same  time  through  the  section  c  as  passes  through  a  or  b.  But  if  just  as 
many  particles  of  water  have  to  get  through  the  narrow  section  a  in  the 
same  time  as  they  have  to  get  through  the  broader  section  c,  they  must 
move  quicker  through  a  than  through  c,  or  more  slowly  through  c  than 
through  a.  For  the  same  reason  water  flowing  along  a  river  impelled  bv 
one  force — viz.,  that  of  gravity — rushes  rapidly  through  a  "  narrow  "  and 
flows  sluggishly  when  the  river  widens  out  into  a  "  broad."  The  flow 
through  B  will  be  similarly  slackened  if  B,  instead  of  being  simply  a  single  en- 
largement of  the  tube  A,  consists  of  a  number  of  small  tubes  branching  out 
from  A,  with  a  united  sectional  area  greater  than  the  sectional  area  of  A. 
In  each  of  such  small  tubes,  at  the  line  c,  for  instance,  the  flow  will  be 
slower  than  at  a,  where  the  small  tubes  branch  out  from  A,  or  at  b,  where 
they  join  again  to  form  a  single  tube.  Hence  it  is  that  the  blood  rushes 
swiftly  through  the  arteries,  tarries  slowly  through  the  capillaries,  but 
quickens  its  pace  again  in  the  veins. 

An  apparent  contradiction  to  this  principle  that  the  rate  of  flow  is  de- 
pendent on  the  width  of  the  bed  is  seen  in  the  case  where,  the  fluid  having 
alternative  routes,  one  of  the  routes  is  temporarily  widened.  Suppose  a  tube 
A  dividing  into  two  branches  of  equal  length  x  and  y  which  unite  again  to 
form  the  tube  V.  Suppose,  to  start  with,  x  and  y  are  of  equal  diameter  ; 
then  the  resistance  offered  by  each  being  equal,  the  flow  will  be  equally 
rapid  through  the  two,  being  just  so  rapid  that  as  much  fluid  passes  in  a 
given  time  through  x  and  y  together  as  passes  through  A  or  through  V- 
But  now  suppose  y  to  be  widened  ;  the  widening  will  diminish  the  resistance 
offered  by  y,  and  in  consequence,  supposing  that  no  material  change  takes 
place  in  the  pressure  or  force  which  is  driving  the  fluid  along,  more  fluid 
will  now  pass  along  y  in  a  given  time  than  did  before  ;  that  is  to  say,  the 
rapidity  of  the  flow  in  y  will  be  increased.  It  will  be  increased  at  the  ex- 
pense of  the  flow  through  x,  since  it  will  still  hold  good  that  the  flow  through 
x  and  y  together  is  equal  to  the  flow  through  A  and  through  V.  We  shall 
have  occasion  later  on  to  point  out  that  a  small  artery,  or  a  set  of  small 
arteries,  may  be  more  or  less  suddenly  widened  without  materially  affecting 
the  general  blood-pressure  which  is  driving  the  blood  through  the  artery  or 
set  of  arteries.  In  such  cases  the  flow  of  blood  through  the  widened  artery 
or  arteries  is  for  the  time  being  increased  in  rapidity,  not  only  in  spite  of, 
but  actually  in  consequence  of,  the  artery  being  widened. 

It  must  be  understood  in  fact  that  this  dependence  of  the  rapidity  of  the 
flow  on  the  width  of  the  bed  applies  to  the  general  rate  of  flow  of  the  whole 
circulation,  and  that,  besides  the  above  instance,  other  special  and  temporary 
variations  occur  due  to  particular  circumstances.  Thus  changes  of  pressure 
may  alter  the  rapidity  of  flow.  The  cause  of  the  flow  through  the  whole 
system  is  the  pressure  of  the  ventricular  systole  manifested  as  what  we  have 
called  blood-pressure.  At  each  point  along  the  system  nearer  the  left  ven- 
;ricle,  and  therefore  further  from  the  right  auricle,  the  pressure  is  greater 
lan  at  a  point  further  from  the  left  ventricle  and  so  nearer  the  right  auricle  ; 
it  is  this  difference  of  pressure  which  is  the  real  cause  of  the  flow  from  the 
one  point  to  the  other ;  and  other  things  being  equal  the  rapidity  of  the  flow 
will  depend  on  the  amount  of  the  difference  of  pressure.  Hence,  temporary 
or  local  variations  in  rapidity  of  flow  may  be  caused  by  the  establishment 
of  temporary  or  local  differences  of  pressure.  For  example,  at  any  point 
ilong  the  arterial  system  the  flow  is  increased  in  rapidity  during  the  tem- 
porary increase  of  pressure  due  to  the  ventricular  systole,  i.  e.,  the  pulse, 
and  diminished  during  the  subsequent  temporary  decrease,  the  increase  and 
decrease  being  the  more  marked  the  nearer  the  point  to  the  heart.  And  we 
shall  probably  meet  later  on  with  other  instances. 


156  THE  VASCULAR  MECHANISM. 

§  113.  Time  of  the  entire  circuit.  It  is  obvious  from  the  foregoing  that  a 
red  corpuscle  in  performing  the  whole  circuit,  iii  travelling  from  the  left 
ventricle  back  to  the  left  ventricle,  would  spend  a  large  portion  of  its  time 
in  the  capillaries,  minute  arteries,  and  veins.  The  entire  time  taken  up  in 
the  whole  circuit  has  been  approximately  estimated  by  measuring  the  time 
it  takes  for  an  easily  recognized  chemical  substance  after  injection  into  the 
jugular  vein  of  one  side  to  appear  in  the  blood  of  the  jugular  vein  of  the 
other  side. 

While  small  quantities  of  blood  are  being  drawn  at  frequently  repeated  intervals 
from  the  jugular  vein  of  one  side,  or  while  the  blood  from  the  vein  is  being  allowed 
to  fall  in  a  minute  stream  on  an  absorbent  paper  covering  some  travelling  surface, 
an  iron  salt  such  as  potassium  ferrocyanide  (or  preferably  sodium  ferrocyanide  as 
being  more  innocuous)  is  injected  into  the  jugular  vein  of  the  other  side.  If  the 
time  of  the  injection  be  noted,  and  the  time  after  the  injection  into  one  side  at 
which  evidence  of  the  presence  of  the  iron  salt  can  be  detected  in  the  sample  of 
blood  from  the  vein  of  the  other  side  be  noted,  this  gives  the  time  it  has  taken  the 
salt  to  perform  the  circuit ;  and  on  the  supposition  that  mere  diffusion  does  not 
materially  affect  the  result,  the  time  which  it  takes  the  blood  to  perform  the  same 
circuit  is  thereby  given. 

In  the  horse  this  time  has  been  experimentally  determined  at  about  30 
seconds  and  in  the  dog  at  about  15  seconds.  In  man  it  is  probably  from  20 
to  25  seconds. 

Taking  the  rate  of  flow  through  the  capillaries  at  about  1  mm.  a  second  it 
would  take  a  corpuscle  as  long  a  time  to  get  through  about  20  mm.  of  capil- 
laries as  to  perform  the  whole  circuit.  Hence,  if  any  corpuscle  had  in  its 
circuit  to  pass  through  10  mm.  of  capillaries,  half  the  whole  time  of  its 
journey  would  be  spent  in  the  narrow  channels  of  the  capillaries.  Inasmuch 
as  the  purposes  served  by  the  blood  are  chiefly  carried  out  in  the  capillaries, 
it  is  obviously  of  advantage  that  its  stay  in  them  should  be  prolonged. 
Since,  however,  the  average  length  of  a  capillary  is  about  0.5  mm.,  about 
half  a  second  is  spent  in  the  capillaries  of  the  tissues  and  another  half  sec- 
ond in  the  capillaries  of  the  lungs. 

§  114.  We  may  now  briefly  summarize  the  broad  features  of  the  circu- 
lation, which  we  have  seen  may  be  explained  on  purely  physical  principles, 
it  being  assumed  that  the  ventricle  delivers  a  certain  quantity  of  blood  with 
a  certain  force  into  the  aorta  at  regular  intervals,  and  that  the  physical 
properties  of  the  bloodvessels  remain  the  same. 

We  have  seen  that  owing  to  the  peripheral  resistance  offered  by  the 
capillaries  and  small  vessels  the  direct  effect  of  the  ventricular  stroke  is  to 
establish  in  the  arteries  a  mean  arterial  pressure  which  is  greatest  at  the  root 
of  the  aorta  and  diminishes  toward  the  small  arteries,  some  of  it  being  used 
up  to  drive  the  blood  from  the  aorta  to  the  small  arteries,  but  which  retains 
at  the  region  of  the  small  arteries  sufficient  power  to  drive  through  the 
small  arteries,  capillaries,  and  veins  just  as  much  blood  as  is  being  thrown 
into  the  aorta  by  the  ventricular  stroke.  We  have  seen,  further,  that  in  the 
large  arteries  at  each  stroke  the  pressure  rises  and  falls  a  little  above  and 
below  the  mean,  thus  constituting  the  pulse,  but  that  this  extra  distention 
with  its  subsequent  recoil  diminishes  along  the  arterial  tract  and  finally 
vanishes ;  it  diminishes  and  vanishes  because  it  too,  like  the  whole  force  of 
the  ventricular  stroke,  of  a  fraction  of  which  it  is  the  expression,  is  used  up 
in  establishing  the  mean  pressure ;  we  shall,  however,  consider  again  later 
on  the  special  features  of  this  pulse.  We  have  seen,  further,  that  the  task 
of  driving  the  blood  through  the  peripheral  resistance  of  the  small  arteries 
and  capillaries  consumes  much  of  this  mean  pressure,  which  consequently  is 
much  less  in  the  small  veins  than  in  the  corresponding  small  arteries,  but 


THE  HEART.  157 

that  sufficient  remains  to  drive  the  blood,  even  without  the  help  of  the  aux- 
iliary agents  which  are  generally  in  action,  from  the  small  veins  right  back 
to  the  auricle.  Lastly  we  have  seen  that  while  the  above  is  the  cause  of  the 
flow  from  ventricle  to  auricle,  the  changing  rate  of  the  flow,  the  diminish- 
ing swiftness  in  the  arteries,  the  sluggish  crawl  through  the  capillaries,  the 
increasing  quickness  through  the  veins  are  determined  by  the  changing  width 
of  the  vascular  "  bed." 

Before  we  proceed  to  consider  any  further  details  as  to  the  phenomena 
of  the  flow  through  the  vessels,  we  must  turn  aside  to  study  the  heart. 

THE  HEART. 

§  115,  The  heart  is  a  valvular  pump  which  works  on  mechanical  princi- 
ples, but  the  motive  power  of  which  is  supplied  by  the  contraction  of  its 
muscular  fibres.  Its  action  consequently  presents  problems  which  are  partly 
mechanical  and  partly  vital.  Regarded  as  a  pump,  its  effects  are  determined 
by  the  frequency  of  the  beats,  by  the  force  of  each  beat,  by  the  character  of 
each  beat — whether,  for  instance,  slow  and  lingering,  or  sudden  and  sharp — 
and  by  the  quantity  of  fluid  ejected  at  each  beat.  Hence,  with  a  given 
frequency,  force,  and  character  of  beat,  and  a  given  quantity  ejected  at  each 
beat,  the  problems  which  have  to  be  dealt  with  are  for  the  most  part 
mechanical.  The  vital  problems  are  chiefly  connected  with  the  causes  which 
determine  the  frequency,  force,  and  character  of  the  beat.  The  quantity 
ejected  at  each  beat  is  governed  more  by  the  state  of  the  rest  of  the  body 
than  by  that  of  the  heart  itself. 

The  Phenomena  of  the  Normal  Beat. 

The  visible  movements.  When  the  chest  of  a  mammal  is  opened  and  arti- 
ficial respiration  kept  up  the  heart  may  be  watched  beating.  Owing  to  the 
removal  of  the  chest-wall,  what  is  seen  is  not  absolutely  identical  with  what 
takes  place  within  the  intact  chest,  but  the  main  events  are  the  same  in  both 
cases.  A  complete  beat  of  the  whole  heart  or  cardiac  cycle  may  be  observed 
to  take  place  as  follows : 

The  great  veins,  inferior  and  superior  vense  cavse,  and  pulmonary  veins 
are  seen,  while  full  of  blood,  to  contract  in  the  neighborhood  of  the  heart ; 
the  contraction  runs  in  a  peristaltic  wave  toward  the  auricles,  increasing  in 
intensity  as  it  goes.  Arrived  at  the  auricles,  which  are  then  full  of  blood, 
the  wave  suddenly  spreads,  at  a  rate  too  rapid  to  be  fairly  judged  by  the  eye, 
over  the  whole  of  those  organs,  which  accordingly  contract  with  a  sudden 
sharp  systole.  In  the  systole,  the  walls  of  the  auricles  press  toward  the 
auriculo-ventricular  orifices,  and  the  auricular  appendages  are  drawn  inward, 
becoming  smaller  and  paler.  During  the  auricular  systole,  the  ventricles 
may  be  seen  to  become  turgid.  Then  follows,  as  it  were  immediately,  the 
ventricular  systole,  during  which  the  ventricles  become  more  conical.  Held 
between  the  fingers  they  are  felt  to  become  tense  and  hard.  As  the  systole 
progresses,  the  aorta  and  pulmonary  arteries  expand  and  elongate,  the  apex 
is  tilted  slightly  upward,  and  the  heart  twists  somewhat  on  its  long  axis, 
moving  from  the  left  and  behind  toward  the  front  and  right  so  that  more  of 
the  left  ventricle  becomes  displayed.  As  the  systole  gives  way  to  the  suc- 
ceeding diastole,  the  ventricles  resume  their  previous  form  and  position,  the 
aorta  and  pulmonary  artery  shrink  and  shorten,  the  heart  turns  back  toward 
the  left,  and  thus  the  cycle  is  completed. 

In  the  normal  beat,  the  two  ventricles  are  perfectly  synchronous  in  ac- 
tion, they  contract  at  the  same  time  and  relax  at  the  same  time,  and  the 


158 


THE   VASCULAK   MECHANISM. 


two  auricles  are  similarly  synchronous  in  action.  It  has  been  maintained, 
however,  that  the  synchronism  may  at  times  not  be  perfect. 

Before  we  attempt  to  study  in  detail  the  several  parts  of  this  compli- 
cated series  of  events,  it  will  be  convenient  to  take  a  rapid  survey  of  what 
is  taking  place  within  the  heart  during  such  a  cycle. 

§  116.  The  cardiac  cycle.  We  may  take  as  the  end  of  the  cycle  the 
moment  at  which  the  ventricles  having  emptied  their  contents  have  relaxed 
and  returned  to  the  diastolic  or  resting  position  and  form.  At  this  moment 
the  blood  is  flowing  freely  with  a  fair  rapidity,  but  as  we  have  seen  at  a  very 
low  pressure,  through  the  venae  cavse  into  the  right  auricle  (we  may  confine 
ourselves  at  first  to  the  right  side),  and  since  there  is  now  nothing  to  keep 
the  tricuspid  valve  shut,  some  of  this  blood  probably  finds  its  way  into  the 
ventricle  also.  This  goes  on  for  some  little  time,  and  then  comes  the  sharp, 
short  systole  of  the  auricle,  which,  since  it  begins,  as  we  have  seen,  as  a  wave 
of  contraction  running  forward  along  the  ends  of  the  venae  cavse,  drives  the 
blood  not  backward  into  the  veins  but  forward  into  the  ventricle ;  this  end 
is  further  secured  by  the  fact  that  the  systole  has  behind  it  on  the  venous 
side  the  pressure  of  the  blood  in  the  veins,  increasing,  as  we  have  seen,  back- 
ward toward  the  capillaries,  and  before  it  the  relatively  empty  cavity  of  the 
ventricle,  in  which  the  pressure  is  at  first  very  low.  By  the  complete  con- 
traction of  the  auricular  walls  the  complete  or  nearly  complete  emptying  of 
the  cavity  is  insured.  No  valves  are  present  in  the  mouth  of  the  superior 
vena  cava,  for  they  are  not  needed ;  and  the  imperfect  Eustachian  valve  at 
the  mouth  of  the  inferior  vena  cava  cannot  be  of  any  great  use  in  the  adult, 
though  in  its  more  developed  state  in  the  foetus  it  had  an  important  function 
in  directing  the  blood  of  the  inferior  vena  cava  through  the  foramen  ovale 
into  the  left  auricle.  The  valves  in  the  coronary  vein  are,  however,  probably 
of  some  use  in  preventing  a  reflux  into  that  vessel. 

As  the  blood  is  being  driven  by  the  auricular  systole  into  the  ventricle,  a 
reflux  current  is  probably  set  up,  by  which  the  blood,  passing  along  the 
sides  of  the  ventricle,  gets  between  them  and  the  flaps  of  the  tricuspid  valve 
and  so  tends  to  float  these  up.  [Figs.  50,  51.]  It  is  further  probable  that 


[Fio.  50. 


FIG.  51. 


Diagrams  of  Valves  of  the  Heart.    After  Dalton.] 

the  same  reflux  current,  continuing  somewhat  later  than  the  flow  into  the 
ventricle,  is  sufficient  to  bring  the  flaps  into  apposition,  without  any  regurgi- 
tation  into  the  auricle,  at  the  close  of  the  auricular  systole,  before  the  ven- 


THE   HEART.  159 

tricular  systole  has  begun.  According  to  some  authors,  however,  the  closure 
of  the  valve  is  effected,  at  the  very  beginning  of  the  ventricular  systole,  by 
the  contraction  of  the  papillary  muscles  ;  the  chordae  tendineae  of  a  papillary 
muscle  are  attached  to  the  adjacent  edges  of  two  flaps,  so  that  the  shorten- 
ing of  the  muscle  tends  to  bring  these  edges  into  apposition. 

The  auricular  systole  is,  as  we  have  said,  immediately  followed  by  that  of 
the  ventricle.  Whether  the  contraction  of  the  ventricular  walls  (which  as 
we  shall  see  is  a  simple  though  prolonged  contraction  and  not  a  tetanus) 
begins  at  one  point  and  swiftly  travels  over  the  rest  of  the  fibres,  or  begins 
all  over  the  ventricle  at  once,  is  a  question  not  at  present  definitely  settled ; 
but  in  any  case  the  walls  exert  on  the  contents  a  pressure  which  is  soon 
brought  to  bear  on  the  whole  contents  and  very  rapidly  rises  to  a  maximum. 
The  only  effect  upon  the  valve  of  this  increasing  intra-ventricular  pressure 
is  to  render  the  valve  more  and  more  tense,  and  in  consequence  more  secure, 
the  chordae  tendinese  (the  slackening  of  which  through  the  change  of  form  of 
the  ventricle  is  probably  obviated  by  a  regulative  contraction  of  the  papillary 
muscles)  at  the  same  time  preventing  the  valve  from  being  inverted  or  even 
bulging  largely  into  the  auricle,  and  indeed,  according  to  some  observers, 
keeping  the  valvular  sheet  actually  convex  to  the  ventricular  cavity,  by 
which  means  the  complete  emptying  of  the  ventricle  is  more  fully  effected. 
[Figs.  50,  51.]  The  connection,  to  which  we  have  just  referred,  of  the  chordae 
of  the  same  papillary  muscle  with  the  adjacent  edges  of  two  flaps,  also  assists 
in  keeping  the  flaps  in  more  complete  apposition.  Morever  the  extreme 
borders  of  the  valves,  outside  the  attachments  of  the  chordae,  are  excessively 
thin,  so  that  when  the  valve  is  closed,  these  thin  portions  are  pressed  flat 
together  back  to  back ;  hence,  while  the  tougher  central  parts  of  the  valves 
bear  the  force  of  the  ventricular  systole,  the  opposed  thin  membranous 
edges,  pressed  together  by  the  blood,  more  completely  secure  the  closure  of 
the  orifice. 

At  the  commencement  of  the  ventricular  systole  the  semilunar  valves  of 
the  pulmonary  artery  are  closed,  and  are  kept  closed  by  the  high  pressure  of 
the  blood  in  the  artery.  As,  however,  the  ventricle  continues  to  press  with 
greater  and  greater  force  on  its  contents,  making  the  ventricle  hard  and 
tense  to  the  touch,  the  pressure  within  the  ventricle  becomes  greater  than 
that  in  the  pulmonary  artery,  and  this  greater  pressure  forces  open  the  semi- 
lunar  valves  and  allows  the  escape  of  the  contents  into  the  artery.  The 
ventricular  systole  may  be  seen  and  felt  in  the  exposed  heart  to  be  of  some 
duration  ;  it  is  strong  enough  and  long  enough  to  empty  the  ventricle  com- 
pletely ;  indeed,  as  we  shall  see,  it  probably  lasts  longer  than  the  discharge 
of  blood,  so  that  there  is  a  brief  period  during  which  the  ventricle  is  empty 
but  yet  contracted. 

During  the  ventricular  systole  the  semilunar  valves  are  pressed  outward 
toward  but  not  close  to  the  arterial  walls,  reflux  currents  probably  keeping 
them  in  an  intermediate  position,  so  that  their  orifice  forms  an  equilateral 
triangle  with  curved  sides ;  thus  they  offer  little  obstacle  to  the  escape  of 
blood  from  the  cavity  of  the  ventricle.  The  ventricle,  as  we  have  seen,  pro- 
pels the  blood  with  great  force  and  rapidity  into  the  pulmonary  artery,  and 
the  whole  contents  are  speedily  ejected.  Now,  when  a  force  which  is  driving 
a  fluid  with  great  rapidity  along  a  closed  channel  suddenly  ceases  to  act, 
the  fluid,  by  its  momentum,  continues  to  move  onward  after  the  force  has 
ceased  ;  in  consequence  of  this  a  negative  pressure  makes  its  appearance  in 
the  rear  of  the  fluid,  and,  sucking  the  fluid  back  again,  sets  up  a  reflux  cur- 
rent. So  when  the  last  portions  of  blood  leave  the  ventricle  a  negative 
pressure  makes  its  appearance  behind  them,  and  leads  to  a  reflux  current 
from  the  artery  toward  the  ventricle.  This  alone  would  be  sufficient  to  bring 


160  THE  VASCULAR  MECHANISM. 

the  valves  together ;  and,  in  the  opinion  of  some,  is  the  real  cause  of  the 
closure  of  the  valves ;  others,  however,  as  we  shall  see  later  on,  maintain 
that  subsequent  to  this  reflux  due  to  mere  negative  pressure  a  somewhat  later 
reflux,  in  which  the  elastic  reaction  of  the  arterial  walls  is  concerned,  more 
completely  fills  and  renders  tense  the  pockets,  causing  their  free  margins  to 
come  into  close  and  firm  contact,  and  thus  entirely  blocks  the  way.  The 
corpora  Arantii  meet  in  the  centre,  and  the  thin  membranous  festoons  or 
lunulse  are  brought  into  exact  apposition.  As  in  the  tricuspid  valves,  so 
here,  while  the  pressure  of  the  blood  is  borne  by  the  tougher  bodies  of  the 
several  valves,  each  two  thin  adjacent  lunulse,  pressed  together  by  the  blood 
acting  on  both  sides  of  them,  are  kept  in  complete  contact,  without  any 
strain  being  put  upon  fhem ;  in  this  way  the  orifice  is  closed  in  a  most 
efficient  manner. 

The  ventricular  systole  now  passes  off,  the  muscular  walls  relax,  the 
ventricle  returns  to  its  previous  form  and  position,  and  the  cycle  is  once 
more  ended. 

What  thus  takes  place  in  the  right  side  takes  place  in  the  left  side  also. 
There  is  the  same  sudden  sharp  auricular  systole  beginning  at  the  roots  of 
the  pulmonary  veins,  the  same  systole  of  the  ventricle,  but,  as  we  shall  see, 
one  much  more  powerful  and  exerting  much  more  force  ;  the  mitral  valve 
with  its  two  flaps  acts  exactly  like  the  tricuspid  valve,  and  the  action  of  the 
semilunar  valves  of  the  aorta  simply  repeats  that  of  the  valves  of  the 
pulmonary  artery. 

We  may  now  proceed  to  study  some  of  the  cardiac  events  in  detail. 

§  117.  The  change  of  form.  The  exact  determination  of  the  changes  in 
form  and  position  of  the  heart,  especially  of  the  ventricles,  during  a  cardiac 
cycle  is  attended  with  difficulties 

The  ventricles,  for  instance,  are  continually  changing  their  form  :  they 
change  while  their  cavities  are  being  filled  from  the  auricles,  they  change 
while  the  contraction  of  their  walls  is  getting  up  the  pressure  on  their  con- 
tents, they  change  while  under  the  influence  of  that  pressure,  their  contents 
being  discharged  into  the  arteries,  and  they  change  when,  their  cavities 
having  been  emptied,  their  muscular  walls  relax. 

We  may  take  it  for  granted  that  the  internal  cavities  are  obliterated  by 
the  systole,  for  it  is  probable  that  practically  the  whole  contents  are  driven 
out  at.  each  stroke,  and  probably  also  each  cavity  is  emptied  from  its  apex 
toward  the  mouth  of  the  artery. 

With  regard  to  changes  in  external  form,  there  seems  no  doubt  that  the 
side-to-side  diameter  is  much  lessened.  It  seems  also  clear  that  the  front-to 
back  diameter  is  greater  during  the  whole  time  of  the  systole  than  during 
the  diastole,  the  increase  taking  place  during  the  first  part  of  the  systole.  If 
a  light  lever  be  placed  on  the  surface  of  the  heart  of  a  mammal,  the  chest 
having  been  opened  and  artificial  respiration  being  kept  up,  some  such  curve 
as  that  represented  in  Fig.  52  is  obtained.  The  rise  of  the  lever  in  describing 
such  a  curve  is  due  to  the  elevation  of  the  part  of  the  front  surface  of  the 
heart  on  which  the  lever  is  resting.  Such  an  elevation  might  be  caused, 
especially  if  the  lever  were  placed  near  the  apex,  by  the  heart  being  "  tilted  " 
upward  during  the  systole,  but  only  a  small  portion  at  most  of  the  rise  can 
be  attributed  to  this 'cause;  the  rise  is  perhaps  best  seen  when  the  lever  is 
placed  in  the  middle  portion  of  the  ventricle,  and  must  be  chiefly  due  to  an 
increase  in  the  front-to-back  diameter  of  the  ventricle  during  the  beat.  We 
shall  discuss  this  curve  later  on  in  connection  with  other  curves  and  may 
here  simply  say  that  the  part  of  the  curve  from  b'  to  d  probably  corresponds 
to  the  actual  systole  of  the  ventricle,  that  is,  to  the  time  during  which  the 
fibres  of  the  ventricle  are  undergoing  contraction,  the  sudden  fall  from  d 


THE  HEAET. 


161 


onward  representing  the  relaxation  which  forms  the  first  part  of  the  diastole. 
If  this  interpretation  of  the  curve  he  correct,  it  is  obvious  that  the  front-to- 


FIG.  52. 


Tracing  from  Heart  of  Cat,  obtained  by  placing  a  Light  Lever  on  the  Ventricle,  the  Chest 
having  been  Opened.    The  tuning-fork  curve  marks  50  vibrations  per  second. 

back  diameter  is  greater  during  the  whole  of  the  systole  than  it  is  during 
diastole,  since  the  lever  is  raised  up  all  this  time. 

This  increase  of  the  front-to-back  diameter  combined  with  a  decrease  of 
the  side-to-side  diameter  has  for  a  result  a  change  in  the  form  of  the  section 
of  the  base  of  the  ventricles.  During  the  diastole  this  has  somewhat  the 
form  of  an  ellipse  with  the  long  axis  from  side  to  side,  but  with  the  front 
part  of  the  ellipse  much  more  convex  than  the  back,  since  the  back  surface 
of  the  ventricle  is  somewhat  flattened.  During  the  systole  this  ellipse  is  by 
the  shortening  of  the  side-to-side  diameter  and  the  increase  of  the  front-to- 
back  diameter  converted  into  a  figure  much  more  nearly  resembling  a  circle. 
It  is  urged,  moreover,  that  the  whole  of  the  base  is  constricted,  and  that  the 
greater  efficiency  of  the  auriculo-ventricular  valves  is  thereby  secured. 

As  to  the  behavior  of  the  long  diameter  from  base  to  apex  observers  are 
not  agreed.  Some  maintain  that  it  is  shortened,  and  others  that  it  is  prac- 
tically unchanged.  If  any  shortening  does  take  place,  it  must  be  largely 
compensated  by  the  elongation  of  the  great  vessels,  which,  as  stated  above, 

1  The  vertical  or  rather  curved  lines  (segments  of  circles)  introduced  into  this  and 
many  other  curves  are  of  use  for  the  purpose  of  measuring  parts  of  the  curve.  A  complete 
curve  should  exhibit  an  "abscissa"  line.  This  may  be  drawn  by  allowing  the  lever,  ar- 
ranged for  the  experiment  but  remaining  at  rest,  to  mark  with  its  point  on  the  recording 
surface  set  in  motion ;  a  straight  line,  the  abscissa  line,  is  thus  described,  and  may  be 
drawn  before  or  after  the  curve  itself  is  made,  and  may  be  placed  above  or  preferably  be- 
low the  curve.  When  a  tuning-fork  or  other  time-marker  is  used,  the  line  of  the  time- 
marker  or  a  line  drawn  through  the  curves  of  the  tuning-fork  will  serve  as  an  abscissa 
line.  After  a  tracing  has  been  made,  the  recording  surface  should  be  brought  back  to 
such  a  position  that  the  point  of  the  lever  coincides  with  some  point  of  the  curve  which 
it  is  desired  to  mark  ;  if  the  lever  be  then  gently  moved  up  and  down,  the  point  of  the 
lever  will  describe  a  segment  of  a  circle  (the  centre  of  which  lies  at  the  axis  of  the  lever), 
which  segment  should  be  made  long  enough  to  cut  both  the  curve  and  the  abscissa  line 
(the  tuning-fork  curves  or  other  time-marking  line)  where  this  is  drawn.  By  moving 
the  recording  surface  backward  and  forward  similar  segments  of  circles  may  be  drawn 
through  other  points  of  the  curve.  The  lines  a,  b,  c  in  Fig.  52  were  thus  drawn.  The 
distance  between  any  two  of  these  points  may  thus  be  measured  on  the  tuning-fork  curve  or 
other  time  curve,  or  on  the  abscissa  line.  Similar  lines  may  be  drawn  on  the  tracing 
after  its  removal  from  the  recording  instrument  in  the  following  way:  Take  a  pair  of 
compasses,  the  two  points  of  which  are  fixed  just  as  far  apart  as  the  length  of  the  lever 
used  in  the  experiment,  ^measured  from  its  axis  to  its  writing  point.  By  means  of  the 
compasses  find  the  position  on  the  tracing  of  the  centre  of  the  circle  of  which  any  one 
of  the  previously  drawn  curved  lines  forms  a  segment.  Through  this  centre  draw  a  line 
parallel  to  the  abscissa.  By  keeping  one  point  of  the  compass  on  this  line  but  moving  it 
along  the  line  backward  or  forward  a  segment  of  a  circle  may  be  drawn  so  as  to  cut  any 
point  of  the  curve  that  may  be  desired,  and  also  the  abscissa  line  or  the  time  line.  Such 
a  segment  of  a  circle  may  be  used  for  the  same  purposes  as  the  original  one,  and  any 
number  of  such  segments  may  be  drawn. 

' 


162  THE  VASCULAR  MECHANISM. 

may  be  seen  in  an  inspection  of  the  beating  heart.  For  there  is  evidence 
that  the  apex,  though,  as  we  have  seen,  it  is  during  the  systole  somewhat 
twisted  round  and  at  the  same  time  brought  closer  to  the  chest-wall,  does 
not  change  its  position  up  or  down — i.  e.,  in  the  long  axis  of  the  body.  If 
in  a  rabbit  or  dog  a  needle  be  thrust  through  the  chest-wall  so  that  its  point 
plunges  into  the  apex  of  the  heart,  though  the  needle  quivers  its  head 
moves  neither  up  nor  down,  as  it  would  do  if  its  point  in  the  apex  moved 
down  or  up. 

§  118.  Cardiac  impulse.  If  the  hand  be  placed  on  the  chest,  a  shock  or 
impulse  will  be  felt  at  each  beat,  and  on  examination  this  impulse,  "  cardiac 
impulse,"  will  be  found  to  be  synchronous  with  the  systole  of  the  ventricle. 
In  man,  the  cardiac  impulse  may  be  most  distinctly  felt  in  the  fifth  costal 
interspace,  about  an  inch  below  and  a  little  to  the  median  side  of  the  left 
nipple.  In  an  animal  the  same  impulse  may  also  be  felt  in  another  way — 
viz.,  by  making  an  incision  through  the  diaphragm  from  the  abdomen,  and 
placing  the  finger  between  the  chest-wall  and  the  apex.  It  then  can  be  dis- 
tinctly recognized  as  the  result  of  the  hardening  of  the  ventricle  during  the 
systole.  And  the  impulse  which  is  felt  on  the  outside  of  the  chest  is  chiefly 
the  effect  of  the  same  hardening  of  the  stationary  portion  of  the  ventricle  in 
contact  with  the  chest-wall,  transmitted  through  the  chest-wall  to  the  finger. 
In  its  flaccid  state,  during  diastole,  the  apex  is  (in  a  standing  position,  at 
least)  at  this  point  in  contact  with  the  chest-wall,  lying  between  it  and  the 
tolerably  resistant  diaphragm.  During  the  systole,  while  being  brought 
even  closer  to  the  chest-wall  by  the  tilting  of  the  ventricle  and  by  the  move- 
ment to  the  front  and  to  the  right,  of  which  we  have  already  spoken,  it 
suddenly  grows  tense  and  hard.  The  ventricles,  in  executing  their  systole, 
have  to  contract  against  resistance.  They  have  to  produce  within  their 
cavities  pressures  greater  than  those  in  the  aorta  and  pulmonary  arteries, 
respectively.  This  is,  in  fact,  the  object  of  the  systole.  Hence,  during  the 
swift  systole,  the  ventricular  portion  of  the  heart  becomes  suddenly  tense, 
somewhat  in  the  same  way  as  a  bladder  full  of  fluid  would  become  tense  and 
hard  when  forcibly  squeezed.  The  sudden  pressure  exerted  by  the  ventricle, 
thus  become  suddenly  tense  and  hard,  aided  by  the  closer  contact  of  the 
apex  with  the  chest-wall  (which,  however,  by  itself  without  the  hardening 
of  contraction  would  be  insufficient  to  produce  the  effect),  gives  an  impulse 
of  shock  both  to  the  chest-wall  and  to  the  diaphragm,  which  may  be  felt 
readily  both  on  the  chest-wall,  and  also  through  the  diaphragm  when  the 
abdomen  is  opened  and  the  finger  inserted.  If  the  modification  of  the 
sphygmograph  (of  which  we  shall  speak  in  dealing,  later  on,  with  the  pulse), 
called  the  cardiograph,  be  placed  on  the  spot  where  the  impulse  is  felt  most 
strongly,  the  lever  is  seen  to  be  raised  during  the  systole  of  the  ventricles, 
and  to  fall  again  as  the  systole  passes  away,  very  much  as  if  it  were  placed 
on  the  heart  directly.  A  tracing  may  thus  be  obtained  (see  Fig.  58),  of 
which  we  shall  have  to  speak  more  fully  immediately  (see  §  122).  If  the 
button  of  the  lever  be  placed,  not  on  the  exact  spot  of  the  impulse,  but 
at  a  little  distance  from  it,  the  lever  will  be  depressed  during  the  sys- 
tole. While  at  the  spot  of  impulse  itself  the  contact  of  the  ventricle  is 
increased  during  systole,  away  from  the  spot  the  ventricle  retires  from 
the  chest-wall  (by  the  diminution  of  its  right-to-left  diameter),  and  hence, 
by  the  mediastinal  attachments  of  the  pericardium,  draws  the  chest-wall 
after  it. 

§  119.  The  sounds  of  the  heart.  When  the  ear  is  applied  to  the  chest, 
either  directly  or  by  means  of  a  stethoscope,  two  sounds  are  heard,  the  first 
a  comparatively  long,  dull,  booming  sound,  the  second  a  short,  sharp,  sudden 
one.  Between  the  first  and  second  sounds  the  interval  of  time  'is  very 


THE  HEART.  163 

short — too  short  to  be  measurable — but  between  the  second  and  the  suc- 
ceeding first  sound  there  is  a  distinct  pause.  The  sounds  have  been 
likened  to  the  pronunciation  of  the  syllables  lubb,  dup,  so  that  the  cardiac 
cycle,  as  far  as  the  sounds  are  concerned,  might  be  represented  by :  liibb, 
dup,  pause. 

The  second  sound,  which  is  short  and  sharp,  presents  no  difficulties.  It  is 
coincident  in  point  of  time  with  the  closure  of  the  semilunar  valves,  and  is 
heard  to  the  best  advantage  over  the  second  right  costal  cartilage  close  to  its 
junction  with  the  sternum — i.  e.,  at  the  point  where  the  aortic  arch  comes 
nearest  to  the  surface,  and  to  which  sounds  generated  at  the  aortic  orifice 
would  be  best  conducted.  Its  characters  are  such  as  would  belong  to  a 
sound  generated  by  membranes  like  the  semilunar  valves  being  suddenly 
made  tense  and  so  thrown  into  vibrations.  It  is  obscured  and  altered  or 
replaced  by  a  "  murmur  "  when  the  semilunar  valves  are  affected  by  disease, 
and  may  be  artificially  obliterated,  a  murmur  taking  its  place,  by  passing  a 
wire  down  the  arteries  and  hooking  up  the  aortic  valves.  There  can  be  no 
doubt,  in  fact,  that  the  second  sound  is  due  to  the  semilunar  valves  being 
thrown  into  vibrations  at  their  sudden  closure.  The  sound  heard  at  the 
second  right  costal  cartilage  is  chiefly  that  generated  by  the  aortic  valves, 
and  murmurs  or  other  alterations  in  the  sound  caused  by  changes  in  the 
aortic  valves  are  heard  most  clearly  at  this  spot.  But  even  here  the  sound 
is  not  exclusively  of  aortic  origin,  for  in  certain  cases  in  which  the  semi- 
lunar  valves  on  the  two  sides  of  the  heart  are  not  wholly  synchronous 
in  action  the  sound  heard  here  is  double  ("  reduplicated  second  sound  "), 
one  being  due  to  the  aorta  and  one  to  the  pulmonary  artery.  While 
the  sound  is  listened  to  on  the  left  side  of  the  sternum  at  the  same 
level,  the  pulmonary  artery  is  supposed  to  have  the  chief  share  in  pro- 
ducing what  is  heard,  and  changes  in  the  sound  heard  more  clearly  here 
than  on  the  right  side  are  taken  as  indications  of  mischief  in  the  pulmo- 
nary valves. 

The  first  sound,  longer,  duller,  and  of  a  more  "  booming  "  character  than 
the  second,  heard  with  greatest  distinctness  at  the  spot  where  the  cardiac 
impulse  is  felt,  presents  many  difficulties  in  the  way  of  a  complete  explana- 
tion. It  is  heard  distinctly  when  the  chest-walls  are  removed.  The  cardiac 
impulse,  therefore,  can  have  little  or  nothing  to  do  with  it.  In  point  of  time 
it  is  coincident  with  the  systole  of  the  ventricles,  and  may  be  heard  to  the 
greatest  advantage  at  the  spot  of  the  cardiac  impulse — that  is  to  say,  at  the 
place  where  the  ventricles  corne  nearest  to  the  surface,  and  to  which  sounds 
generated  in  the  ventricle  would  be  best  conducted. 

It  is  more  closely  coincident  with  the  closure  and  consequent  vibrations  of 
the  auriculo-ventricular  valves  than  with  the  entire  systole  ;  for,  on  the  one 
hand,  it  dies  away  before  the  second  sound  begins,  whereas,  as  we  shall  see,  the 
actual  systole  lasts  up  to,  if  not  beyond,  the  closure  of  the  semilunar  valves  ; 
and,  on  the  other  hand,  the  auriculo-ventricular  valve  ceases  to  be  tense  and  to 
vibrate  as  soon  as  the  contents  of  the  ventricle  are  driven  out.  This  suggests 
that  the  sound  is  caused  by  the  sudden  tension  of  the  auriculo-ventricular 
valves,  and  this  view  is  supported  by  the  facts  that  the  sound  is  obscured, 
altered,  or  replaced  by  murmurs  when  the  tricuspid  or  mitral  valves  are  dis- 
eased, and  that  the  sound  is  also  altered,  or,  according  to  some  observers, 
wholly  done  away  with,  when  blood  is  prevented  from  entering  the  ven- 
tricles by  ligature  of  the  venae  cavse.  On  the  other  hand,  the  sound  has  not 
the  sharp  character  which  one  would  expect  in  a  sound  generated  by  the 
vibration  of  membranes  such  as  the  valves  in  question,  but  in  its  booming 
qualities  rather  suggests  a  muscular  sound.  Further,  according  to  some 
observers,  the  sound,  though  somewhat  modified,  may  still  be  heard  when 


164  THE  VASCULAR   MECHANISM. 

the  large  veins  are  clamped  so  that  no  blood  enters  the  ventricle,  and,  indeed, 
may  be  recognized  in  the  few  beats  given  by  a  mammalian  ventricle  rapidly 
cut  out  of  the  living  body  by  an  incision  carried  below  the  auriculo-ven- 
tricular  ring.  Hence  the  view  has  been  adopted  that  this  first  sound  is  a 
muscular  sound.  In  discussing  the  muscular  sound  of  skeletal  muscle  (see 
§  78),  we  saw  reasons  to  distrust  the  view  that  this  sound  was  generated  by 
the  repeated  individual  simple  contractions  which  made  up  the  tetanus,  and 
hence  correspond  in  tone  to  the  number  of  those  simple  contractions  re- 
peated in  a  second,  and  to  adopt  the  view  that  the  sound  was  really  due  to  a 
repetition  of  unequal  tensions  occurring  in  a  muscle  during  the  contraction. 
Now,  the  ventricular  systole  is  undoubtedly  a  simple  contraction,  a  prolonged 
simple  contraction,  not  a  tetanus,  and  therefore  under  the  old  view  of  the 
nature  of  a  muscular  sound,  could  not  produce  such  a  sound  ;  but,  accepting 
the  other  view,  and  reflecting  how  complex  must  be  the  course  of  the  systolic 
wave  of  contraction  over  the  twisted  fibres  of  the  ventricle,  we  shall  not  find 
great  difficulty  in  supposing  that  that  wave  is  capable  in  its  progress  of  pro- 
ducing such  repetitions  of  unequal  tensions  as  might  give  rise  to  a  "  muscular 
sound,"  and  consequently  in  regarding  the  first  sound  as  mainly  so  caused. 
Accepting  such  a  view  of  the  origin  of  the  sound,  we  should  expect  to  find 
the  tension  of  the  muscular  fibres,  and  so  the  nature  of  sound  dependent  on 
the  quantity  of  fluid  present  in  the  ventricular  cavities,  and  hence  modified 
by  ligature  of  the  great  veins,  and  still  more  by  the  total  removal  of  the 
auricles  with  the  auriculo-ventricular  valves.  We  may  add  that  we  should 
expect  to  find  it  modified  by  the  escape  of  blood  from  the  ventricles  into  the 
arteries  during  the  systole  itself,  and  might  regard  this  as  explaining  why  it 
dies  away  before  the  ventricle  has  ceased  to  contract. 

Moreover,  seeing  that  the  auriculo-ventricular  valves  must  be  thrown  into 
sudden  tension  at  the  onset  of  the  ventricular  systole,  which,  as  we  have  seen, 
is  developed  with  considerable  rapidity,  not  far  removed  at  all  events  from 
the  rapidity  with  which  the  semilunar  valves  are  closed,  a  rapidity,  there- 
fore, capable  of  giving  rise  to  vibrations  of  the  valves  adequate  to  produce 
a  sound,  it  is  difficult  to  escape  the  conclusion  that  the  closure  of  these  valves 
must  also  generate  a  sound  which  in  a  normally  beating  heart  is  mingled 
in  some  way  with  the  sound  of  muscular  origin,  although  the  ear  cannot 
detect  the  mixture. 

If  we  accept  this  view,  that  the  sound  is  of  double  origin,  partly  "  muscu- 
lar," partly  "  valvular,"  both  causes  being  dependent  on  the  tension  of  the 
ventricular  cavities,  we  can  perhaps  more  easily  understand  how  it  is  that 
the  normal  first  sound  is  at  times  so  largely,  indeed  we  may  say  so  com- 
pletely, altered  and  obscured  in  diseases  of  the  auriculo-ventricular  valves. 

Since  the  left  ventricle  forms  the  entire  left  apex  of  the  heart,  the  mur- 
murs or  other  changes  of  the  first  sound  heard  most  distinctly  at  the  spot  of 
cardiac  impulse  belong  to  the  mitral  valve  of  the  left  ventricle.  Murmurs 
generated  in  the  tricuspid  valve  of  the  right  ventricle  are  heard  more  dis- 
tinctly in  the  median  line  below  the  end  of  the  sternum. 

Endocardiae  Pressure. 

§  120.  Since  the  heart  exists  for  the  purpose  of  exerting  pressure  on  the 
blood  within  its  cavities,  by  which  pressure  the  circulation  of  the  blood  is 
effected,  the  study  of  the  characters  of  this  endocardiac  pressure  possesses 
great  interest.  ITn fortunately,  the  observation  of  this  pressure  is  attended 
with  great  difficulties.  The  ordinary  mercury  manometer  which  is  so  useful 
in  studying  the  pressure  in  the  arteries  fails  us  when  applied  to  the  heart. 
It  is  true  that  a  long  canula,  or  tube  open  at  the  end,  filled  with  sodium 


THE   HEART. 


165 


carbonate  solution,  may  be  introduced  into  the  jugular  vein  and  so  slipped 
down  into  either  the  right  auricle  or  the  right  ventricle,  or  may  be  similarly 
introduced  into  the  carotid  artery  and  with  care  slipped  down  through  the 
aorta,  past  the  semilunar  valves,  into  the  left  ventricle,  and  having  been 
thus  introduced  may,  like  the  ordinary  canula  used  in  studying  arterial 
pressure  (§  104),  be  brought  into  connection  with  a  mercury  manometer. 
In  this  way,  as  in  the  case  of  an  artery,  a  graphic  record  may  be  obtained  of 
the  changes  of  pressure  taking  place  in  either  of  the  above  three  cavities. 
But  the  changes  in  the  ventricular  cavities  are  so  great  and  rapid,  that  the 
inertia  of  the  mercury,  an  evil  in  the  case  of  an  artery,  comes  so  largely  into 
play  that  the  curve  described  by  the  float  on  the  mercury  is  far  from  being 
an  accurate  record  of  the  changes  of  pressure  in  the  cavity. 

The  mercury  manometer  may,  however,  be  made  to  yield  valuable  results 
by  adopting  the  ingenious  contrivance  of  converting  the  ordinary  manometer 
into  a  maximum  or  a  minimum  instrument. 

The  principle  of  the  maximum  manometer,  Fig.  53,  consists  in  the  introduction 
into  the  tube  leading  from  the  heart  to  the  mercury  column  of  a  (modified  cup- 

[FiG.  53. 


The  Maximum  Manometer  of  Goltz  and  Gaule.  At  e  a  connection  is  made  with  the  tube  lead- 
ing to  the  heart.  When  the  screw-clamp  k  is  closed,  the  valve  v  comes  into  action,  and  the  in- 
strument, in  the  position  of  the  valve  shown  in  the  figure,  is  a  maximum  manometer.  By  revers- 
ing the  direction  of  v  it  is  converted  into  a  minimum  manometer.  When  k  is  opened,  the  varia- 
tions of  pressure  are  conveyed  along  a,  and  the  instrument  then  acts  like  an  ordinary  manometer. 

and-ball)  valve,  opening,  like  the  aortic  semilunar  valves,  easily  from  the  heart, 
but  closing  firmly  when  fluid  attempts  to  return  to  the  heart.  The  highest  pres- 
sure is  that  which  drives  the  longest  column  of  fluid  past  the  valve,  raising  the 
mercury  column  to  a  corresponding  height.  Since  this  column,  once  past  the 
valve,  cannot  return,  the  mercury  remains  at  the  height  to  which  it  was  raised 
by  it  and  thus  records  the  maximum  pressure.  By  reversing  the  direction  of  the 
valve,  the  manometer  is  converted  from  a  maximum  into  a  minimum  instrument. 

The  maximum  manometer  applied  to  the  cavity  of  either  ventricle  or  of 
the  right  auricle,  gives  a  record  of  the  highest  pressure  reached  within  that 
cavity,  and  the  minimum  manometer  similarly  shows  the  lowest  pressure 
reached,  during  the  time  that  the  instrument  is  applied. 

The  maximum  manometer  thus  employed  shows  that  the  maximum  pres- 


166  THE  VASCULAR  MECHANISM. 

sure  in  the  left  ventricle  is  distinctly  greater  than  the  mean  pressure  in  the 
aorta  (the  ordinary  mercury  manometer  having  previously  given  the  para- 
doxical result,  due"  to  the  inertia  of  the  mercury,  that  the  mean  pressure  in 
the  left  ventricle  might  be  less  than  in  the  aorta),  that  the  maximum  pres- 
sure in  the  right  ventricle  is  less  than  in  the  left,  and  in  the  right  auricle  is 
still  less.  In  the  dog,  for  example,  the  pressure  in  the  left  ventricle  reaches 
a  maximum  of  about  140  mm.  (mercury),  in  the  right  ventricle  of  about  60 
mm.,  and  in  the  right  auricle  of  about  20  mm. 

But  the  chief  interest  attaches  to  the  minimum  pressure  observed  ;  for  the 
minimum  manometer  records  a  negative  pressure  in  the  cavities  of  the  heart— 
i.  <?.,  shows  that  the  pressure  in  them  may  fall  below  that  of  the  atmosphere. 
Thus  in  the  left  ventricle  (of  the  dog)  a  minimum  pressure  varying  from 
— 52  to  — 20  mm.  may  be  reached,  the  minimum  of  the  right  ventricle 
being  from  — 17  to  — 16  mm.,  and  of  the  right  auricle  from  — 12  to  — 17  mm.1 
Part  of  this  diminution  of  pressure  in  the  cardiac  cavities  may  be  due,  as 
will  be  explained  in  a  later  part  of  this  work,  to  the  aspiration  of  the  thorax 
in  the  respiratory  movements.  But  even  when  the  thorax  is  opened,  and 
artificial  respiration  kept  up,  under  which  circumstances  no  such  aspiration 
takes  place,  a  negative  pressure  is  still  observed,  the  pressure  in  the  left  ven- 
tricle still  sinking  as  low  as  — 24  mm.  Now,  what  the  instrument  actually 
shows  is  that  at  some  time  or  other  during  the  number  of  beats  which  took 
place  while  the  instrument  was  applied  (and  these  may  have  been  very 
few)  the  pressure  in  the  ventricle  sank  so  many  mm.  below  that  of  the 
atmosphere.  Since  the  negative  pressure  is  observed  when  the  heart  is 
beating  quite  regularly,  each  beat  being  exactly  like  the  others,  we  may 
infer  that  a  negative  pressure  occurs  at  some  period  or  other  of  each  cardiac 
cycle.  But  the  instrument  obviously  gives  us  no  information  as  to  the  exact 
phase  of  the  beat  in  which  the  negative  pressure  occurs ;  to  this  point  as  well 
as  to  the  importance  of  this  negative  pressure  we  shall  return  presently. 

§  121.  The  difficulties  due  to  the  inertia  of  the  mercury  may  be  obviated 
by  adopting  the  method  of  Chauveau  and  Marey,  which  consists  in  intro- 
ducing in  a  large  animal,  such  as  a  horse,  through  a  bloodvessel  into  a  cavity 
of  the  heart  a  tube  ending  in  an  elastic  bag  (Fig.  54,  A)  fashioned  something 
like  a  sound,  both  tube  and  bag  being  filled  with  air,  and  the  tube  being 
connected  with  a  recording  "  tambour." 

A  tube  of  appropriate  curvature,  A,  b,  Fig.  54,  is  furnished  at  its  end  with  an 
elastic  bag  or  "ampulla,"  a.  When  it  is  desired  to  explore  simultaneously  both 
auricle  and  ventricle,  the  sound  is  furnished  with  two  ampullae  with  two  small 
elastic  bags,  one  at  the  extreme  end  and  the  other  at  such  a  distance  that  when 
the  former  is  within  the  cavity  of  the  ventricle  the  latter  is  in  the  cavity  of  the 
auricle.  Such  an  instrument  is  spoken  of  as  a  "  cardiac  sound/'  Each  "  ampulla  " 
communicates  by  a  separate  air-tight  tube  with  an  air-tight  tambour  (Fig.  54,  B) 
on  which  a  lever  rests,  so  that  any  pressure  on  the  ampulla  is  communicated  to 
the  cavity  of  its  respective  tambour,  the  lever  of  which  is  raised  in  proportion. 
When  two  ampullae  are  used  the  writing  points  of  both  levers  are  brought  to  bear 
on  the  same  recording  surface  exactly  underneath  each  other.  The  tube  is  care- 
fully introduced  through  the  right  jugular  vein  into  the  right  side  of  the  heart 
until  the  lower  (ventricular)  ampulla  is  fairly  in  the  cavity  of  the  right  ventricle, 
and  consequently  the  upper  (auricular)  ampulla  in  the  cavity  of  the  right  auricle. 
Changes  of  pressure  on  either  ampulla  then  cause  movements  of  the  corresponding 
lever.  When  the  pressure,  for  instance,  on  the  ampulla  in  the  auricle  is  increased, 
the  auricular  lever  is  raised  and  describes  on  the  recording  surface  an  ascending 
curve ;  when  the  pressure  is  taken  off  the  curve  descends ;  and  so  also  with  the 
ventricle. 

The   "sound"  may  in  a  similar  manner  be  readily  introduced  through  the 

1  These  numbers  are  to  be  considered  merely  as  instances  which  have  been  observed, 
and  not  as  averages  drawn  from  a  large  number  of  cases. 


THE  HEART. 


167 


carotid  artery  into  the  left  ventricle  and  the  changes  taking  place  in  that  chamber 
also  explored. 

When  this  instrument  is  applied  to  the  right  auricle  and  ventricle  some 
such  record  is  obtained  as  that  shown  in  Fig.  55,  where  the  upper  curve  is  a 

FIG.  54. 


Marey's  Tambour,  with  Cardiac  Sound  :  A.  A  simple  cardiac  sound  such  as  may  be  used  for 
exploration  of  the  left  ventricle.  The  portion  a  of  the  ampulla  at  the  end  is  of  thin  India-rubber, 
stretched  over  an  open  framework  with  metallic  supports  above  and  below.  The  long  tube  b 
serves  to  introduce  it  into  the  cavity  which  it  is  desired  to  explore. 

B.  The  tambour.  The  metal  chamber  ra  is  covered  in  an  air-tight  manner  with  the  India- 
rubber  c,  bearing  a  thin  metal  plate  m'  to  which  is  attached  the  lever  I  moving  on  the  hinge  h. 
The  whole  tambour  can  be  placed  by  means  of  the  clamp  cl  at  any  height  on  the  upright  s'.  The 
India-rubber  tube  t  serves  to  connect  the  interior  of  the  tambour  either  with  the  cavity  of  the 
ampulla  of  A  or  with  any  other  cavity.  Supposing  that  the  tube  t  were  connected  with  b,  any 
pressure  exerted  on  a  would  cause  the  roof  of  the  tambour  to  rise  and  the  point  of  the  lever 
would  be  proportionately  raised. 

tracing  taken  from  the  right  auricle  and  the  lower  curve  from  the  right 
ventricle  of  the  horse,  both  curves  being  taken  simultaneously  on  the  same 
recording  surface. 

In  these  curves  the  rise  of  the  lever  indicates  pressure  exerted  upon  the 
corresponding  ampulla,  and  the  upper  curve  from  the  right  auricle  shows 
the  sudden  brief  pressure  (6)  exerted  by  the  sudden  and  brief  auricular 
systole.  The  lower  curve  from  the  right  ventricle  shows  that  the  pressure 
exerted  by  the  ventricular  systole  begins  almost  immediately  after  the  auricu- 
lar systole,  increases  very  rapidly  indeed,  so  that  the  lever  rises  in  almost  a 
straight  line  up  to  c',  is  continued  for  some  considerable  time,  and  then 
falls  very  rapidly  to  reach  the  base  line.  But  it  may  be  doubted  whether 
the  instrument  can  be  trusted  to  tell  much  more  than  this.  The  pressure 
recorded  by  each  lever  is  the  pressure  exerted  on  the  ampulla,  and  this 
may  continue  to  be  exerted  after  all  blood  has  been  discharged  from  the 
cavity,  the  walls  of  the  emptied  cavity  closing  round  and  pressing  on  the 
ampulla.  But,  as  we  shall  presently  see,  it  is  of  great  interest  to  determine, 
not  only  the  force  and  duration  of  the  pressure  exerted  by  the  ventricular 
systole,  but  also  whether  or  no  the  fibres  continue  contracted  and  exert- 
ing pressure  for  an  appreciable  time  after  the  blood  has  been  forced  out 
of  the  cavity.  The  figure,  moreover,  it  need  hardly  be  said,  does  not  by 


168 


THE   VASCULAR  MECHANISM. 


FIG.  55. 


d 

AU 


Simultaneous  Tracings  from  the 
Right  Auricle  and  Ventricle  of  the 
Horse.  (After  Chauveau  and  Marey.) 


itself  give  any  information  as  to  the  relative  amounts  of  pressure  ex- 
erted by  the  auricle  and  ventricle  respectively. 
In  the  curve  the  auricular  lever  rises  about 
half  as  high  as  the  ventricular  lever ;  but  we 
must  not  infer  from  this  that  the  auricular 
stroke  is  half  as  strong  as  the  ventricular 
stroke ;  the  former  is  arranged  so  as  to  move 
much  more  readily,  to  be  much  more  sensitive 
than  the  latter.  The  instrument,  it  is  true, 
may  be  experimentally  graduated,  and  may 
then  be  used  to  determine  the  actual  amount 
of  pressure  ;  but  for  this  purpose  is  not  wholly 
satisfactory.  We  may  add  that  the  irregu- 
larities seen  on  the  ventricular  curve  during 
the  ventricular  systole,  and  on  the  auricular 
curve  at  the  same  time,  have  given  rise  to 
much  debate,  and  need  not  be  discussed  here. 
On  the  whole,  the  method,  though  useful  for 
giving  a  graphic  view  of  the  series  of  events 
within  the  cardiac  cavities  during  a  cardiac  cycle,  the  short  auricular 
pressure,  the  long-continued  ventricular  pressure,  lasting  nearly  half  the 
whole  period,  and  the  subsequent  pause  when  both  parts  are  at  rest  or  in 
diastole,  cannot  with  safety  be  used  for  drawing  more  detailed  conclusions. 
Perhaps  the  least  untrustworthy  method  of  recording  the  changes  of 
endocardiac  pressure  is  that  recently  introduced  by  Roy  and  Rolleston, 
though  difficulties  present  themselves  in  the  interpretation  of  the  curves 
obtained  by  it. 

By  means  of  a  short  canula  introduced  through  a  large  vessel,  or  directly,  as 
a  trocar,  through  the  walls  of  the  ventricle  (or  auricle),  the  blood  in  the  cavity  is 
brought  to  bear  on  an  easily  moving  piston.  The  movements  of  the  piston  are 
recorded  by  a  lever,  and  the  evils  of  inertia  are  met  by  making  the  piston  and 
lever  work  against  the  torsion  of  a  steel  ribbon,  the  length  of  which,  and  conse- 
quently the  resistance  offered  by  which,  and  hence  the  excursions  of  the  piston, 
can  be  varied  at  pleasure. 

The  curves  obtained  by  this  method  vary  according  to  circumstances. 
We  may  take  as  fair  examples  two  curves  from  the  left  ventricle,  one  (Fig. 
56,  A)  of  a  rapidly  beating,  and  the  other  (Fig.  56,  H)  of  a  slowly  beating 
heart. 

§  122.  In  attempting  to  interpret  these  curves  with  the  view  of  learning  • 
the  changes  of  pressure  taking  place  in  the  heart,  it  is  desirable  to  study 
them  in  connection  with  the  tracing  of  which  we  have  already  spoken 
(Fig.  57),  taken  .by  means  of  a  light  lever  placed  on  the  exposed  ventricle, 
and  which,  as  we  have  seen,  is  a  curve  of  the  changes  taking  place  in  the 
front-to-back  diameter  of  the  ventricle ;  or  we  may  use  what  is  very  nearly 
the  same  thing,  viz.,  a  cardiographic  tracing  (Fig.  58) ;  that  is  to  say,  a 
tracing  of  the  cardiac  impulse  which  is  a  curve  of  changes  in  the  pressure 
exerted  by  the  apex  of  the  heart  on  the  chest-wall. 

Various  forms  of  cardiograph  have  been  used  to  record  the  cardiac  impulse.  In 
some  the  pressure  of  the  impulse,  as  in  the  sphygmograph,  is  transmitted  directly 
to  a  lever  which  writes  upon  a  travelling  surface.  In  others  the  impulse  is,  by 
means  of  an  ivory  button,  brought  to  bear  on  an  air-chamber,  connected  by  a 
tube  with  a  tambour,  as  in  Fig.  54  ;  the  pressure  of  the  cardiac  impulse  com- 
presses the  air  in  the  air-chamber,  and  through  this  the  air  in  the  chamber  of  the 
tambour  by  which  the  lever  is  raised.  In  such  delicate  and  complicated  move- 
ments as  those  of  the  heart,  however,  the  use  of  long  tubes  filled  with  air  is  liable 
to  introduce  various  errors. 


THE  HEART. 


169 


We  may  begin  our  study  of  these  curves  at  any  point  in  the  cycle ;  let 
it  be  the  point  b'  in  Fig.  56.    From  this  point  the  curve  rises  very  abruptly, 


FIG.  56. 


Curves  of  Endocardiac  Pressure— from  Left  Ventricle  of  Dog:  A,  a  quickly  beating,  B,  a 
more  slowly  beating,  heart.  The  letters  in  this  and  the  succeeding  Figs.  57,  58,  are  explained  in 
the  text. 

almost  in  the  vertical  line,  to  a  maximum  at  c,  and  the  same  sudden  large  rise 
to  a  maximum  occurs  in  the  front-to-back  diameter  of  the  ventricles  (Fig. 

FIG.  57. 


(See  Fig.  52.) 

57)  and  in  the  pressure  of  the  apex  against  the  chest  wall  (Fig.  58).  There 
can  be  no  doubt  that  this  corresponds  to  the  first  part  of  the  systole  of  the 
ventricles.  By  the  sudden  onset  of  the  contraction  of  the  ventricular 
fibres  pressure  is  brought  to  bear  on  the  contents  of  the  ventricle,  and 
there  being  as  yet  no  escape  for  the  blood,  by  the  increasing  contraction 
of  the  fibres  the  pressure  becomes  greater  and  greater.  At  the  point  c  a 


170  THE  VASCULAR  MECHANISM. 

change  take  place  in  all  three  curves  ;  the  rise  is  converted  into  a  fall, 
which,  however,  is  very  gradual  as  far  as  d.  In  the  case  of  the  front-to- 
back  diameter  curve  (Fig.  57)  we  may 
FIG.  58.^  ^  interpret  this  as  meaning  that  while  the 

continued  contraction  of  the  muscular 
fibres  still  maintains  that  change  in  the 
form  of  the  ventricle  by  which  the  front- 
to-back  diameter  is  increased,  that  same 
carchogram  from  Man'  '  diameter  is  somewhat  lessened  by  a  dim- 

inution or  the  volume  of  the   ventricles 

due  to  the  escape  of  blood  into  the  great  arteries ;  and  the  cardiographic 
tracing  admits  of  a  similar  interpretation — the  apex  relaxes  its  pressure  on 
the  chest-wall.  We  may  extend  the  same  interpretation  to  the  pressure 
curve  (Fig.  56).  Somewhere  about  c  the  pressure  in  the  (left)  ventricle 
has  become  higher  than  the  pressure  in  the  aorta,  and  in  consequence  blood 
escapes  from  the  former  into  the  latter.  Whether  the  exact  moment  of  the 
opening  of  the  valves  is  absolutely  identical  with  the  turn  of  the  curve 
at  c,  the  curve  beginning  to  fall  at  the  moment  when  the  area  of  high  pres- 
sure in  the  ventricle  is  made  continuous  with  the  area  of  lower  pressure 
in  the  aorta,  or  whether  it  occurs  a  little  before  c,  the  still  increasing 
contraction  of  the  ventricular  fibres  still  increasing  the  pressure  on  the 
column  of  blood  as  it  begins  to  move  from  the  cavity  of  the  ventricle  into 
the  aorta,  may  be  left  for  the  present  undecided.  The  sudden  fall  from  d 
to  a  admits  of  only  one  interpretation,  and  that  in  all  the  curves ;  this  can 
only  be  due  to  the  sudden  relaxation  of  the  muscular  fibres  of  the  ventricle, 
whereby  the  front-to-back  diameter  suddenly  diminishes,  the  apex  suddenly 
ceases  to  press  on  the  chest- wall,  and  the  pressure  which  the  ventricular 
walls  were  previously  exerting  on  the  fluid  in  the  canula  introduced 
into  its  cavity  also  suddenly  ceases.  From  b'  to  d,  then,  the  ventricular 
walls  are  still  contracting  ;  during  the  whole  of  this  time  the  real  systole  is 
being  continued,  but  gives  place  at  d  to  a  rapid  relaxation  which  ushers  in 
or  forms  the  first  part  of  the  sequent  diastole.  Some  little  time  after  the 
beginning  of  this  systole,  somewhere  about  c,  as  we  have  seen,  blood 
begins  to  escape  from  the  ventricle  into  the  aorta ;  this  escape  is  certainly 
completed  by  the  time  d  is  reached,  and  we  have  reason  to  think  that  it  is 
really  completed  some  little  time  before.  The  entrance  into  the  aorta  of 
the  column  of  blood  ejected  by  the  ventricle  distends  that  vessel,  and  the 
distention  passes  on,  as  we  have  seen,  along  the  arterial  track  as  the  pulse. 
If,  now,  we  measure  the  time  during  which  the  aorta,  even  near  the  heart, 
is  being  distended  by  the  injection  of  the  ventricular  contents,  we  find  this 
to  be  appreciably  less  than  the  time  from  c  to  d,  during  which  the  systole 
of  the  ventricle  is  still  going  on,  though  the  contents  have  already  begun 
to  escape  at  about  c.  This  means  that  the  ventricle,  though  empty,  remains 
contracted  for  some  little  time  after  its  contents  have  left  the  cavity.  It  is 
possible  that  the  point  c'  in  the  three  figures  under  discussion,  where  the 
descent  of  the  lever  changes  in  rate,  becoming  less  rapid,  corresponds  to 
the  end  of  the  outflow  from  the  ventricle ;  but  this  is  not  certain,  and, 
indeed,  the  exact  interpretation  of  this  part  of  the  curve  is  especially 
difficult. 

The  escape  from  the  ventricle  is  rapid  and  forcible ;  the  flow  ceases  sud- 
denly. Hence,  as  we  have  already  stated,  §  116,  owing  to  the  column  of 
blood  tending  to  move  on  by  virtue  of  its  inertia  after  the  propelling  force 
has  ceased  to  act,  a  negative  pressure  makes  its  appearance  behind  the 
column  of  blood  discharged  from  the  ventricle,  and  as  soon  as  the  column 
is  lodged  in  the  aorta  leads  to  a  reflux  toward  the  ventricle.  This  reflux 


THE  HEART.  171 

would  of  itself  have  the  effect  of  closing  the  valves  even  were  the  aorta 
a  rigid  tube.  But  the  aorta  is  extensible  and  elastic  and  the  effects  of  the 
movement  of  the  column  of  fluid  are  combined  with  the  effects  of  the  move- 
ment of  the  arterial  walls ;  the  elastic  action  of  the  arterial  walls,  in  a 
manner  which  we  shall  discuss  later  on  in  dealing  with  the  pulse,  also  leads  to 
a  reflux.  It  has  been  urged  that  the  reflux  due  to  the  negative  pressure  of 
the  mere  movement  of  the  column  of  blood  being  more  rapid,  occurs 
independently  of  and  earlier  than  the  reflux  due  to  the  elastic  recoil,  the 
former  closing  the  valves,  the  latter  securing  their  complete  closure.  Be 
this  so  or  no  the  valves  are  probably  closed  almost  immediately  after  the 
escape  of  the  ventricular  contents,  though  observers  are  not  agreed  upon 
this  point,  some  urging  that  the  valves  are  not  closed  until  so  late  a  period 
as  the  point  d,  just  as  relaxation  is  about  to  begin.  In  the  curves  we 
are  now  considering,  a  notch,  followed  by  a  rise,  or  at  least  a  more  or  less 
abrupt  change  in  the  course  of  the  curve  at  c',  is  sometimes  observed  in  that 
part  of  the  curve  which  intervenes  between  the  first  large  rise  and  the  final 
sudden  fall ;  and  this  secondary  rise  has  been  taken  to  indicate  the  closure 
of  the  semilunar  valves.  Sometimes  two  such  notches  and  peaks  are  seen, 
and  the  occurrence  of  the  two  has  been  attributed  to  a  want  of  synchronism 
in  the  closure  of  the  pulmonary  and  aortic  semilunar  valves,  the  latter 
closing  some  little  time  before  the  former.  But  it  is  by  no  means  clear  that 
these  notches  and  peaks  are  thus  due  to  the  closure  of  the  valves ;  they 
may  possibly  have  another  origin,  they  are  riot  always  present,  and,  indeed, 
it  does  not  seem  certain  that  the  closing  of  the  valves  should  necessarily 
make  an  impress  on  the  ventricular  curve. 

§  123.  In  the  performance  of  the  ventricle,  then  (and  what  has  been  said 
of  the  left  ventricle  applies  also  to  the  right  ventricle),  there  appear  to  be 
four  stages : 

1.  A    rapid    "getting   up"    of    pressure   within  the  ventricle,  all  the 
valves  being  as  yet  closed  ;  this  continues  until   the  pressure  within  the 
ventricle,  becoming  greater  than  that  in  the  aorta,  throws  open  the  aortic 
valves. 

2.  The  escape  of  the  contents  of  the  ventricle  into  the  aorta,  the  contrac- 
tion of  the  ventricular  walls  still  continuing. 

3.  Further  maintenance  of  the  contraction  for  some  little  time  after  the 
main  body,  at  all  events,  of  the  contents  have  passed  the  aortic  valves ;  by 
this  the  complete  emptying  of  the  ventricle  seems  assured. 

4.  Sudden  and  rapid  relaxation  of  the  ventricular  walls. 

These  four  events  together  make  up  a  large  portion,  and  in  a  quickly 
beating  heart  the  greater  portion,  of  the  whole  cardiac  cycle. 

Meanwhile,  that  is,  during  the  time  from  b'  to  «,  blood  has  been  flowing 
from  the  great  veins  into  the  auricle;  during  the  interval  from  b'  to  d  none 
of  this  can  pass  into  the  ventricle  since  this  is  still  contracted,  but  with  the 
commencement  of  relaxation  from  d  onward  there  is  no  longer  any  obstacle  ; 
on  the  contrary,  as  we  shall  see,  an  inducement  for  the  blood  to  pass  from 
the  auricle  into  the  ventricle. 

For  a  brief  time,  as  we  have  seen,  there  is  probably  an  unbroken  flow 
from  the  great  veins  (pulmonary  or  venas  cavse)  through  the  anricle  into  the 
ventricle,  leading  to  a  steady  but  slight  increase  of  the  front-to-back  diam- 
eter, to  a  slight  pressure  of  the  apex  on  the  chest-wall,  and  to  a  slight 
increase  of  intra-ventricular  pressure,  especially  shown  in  the  curve  of  the 
slowly  beating  heart  of  the  horse  (Fig.  55).  In  Fig.  57  the  sudden  rise  due 
to  the  ventricular  systole  is  preceded  by  a  rise  b  followed  by  a  fall,  forming 
thus,  as  it  were,  a  shoulder  on  the  curve.  This  has  been  interpreted  as  indi- 
cating the  sharp  transient  auricular  systole ;  the  sudden  injection  of  the 


172  THE  VASCULAR  MECHANISM. 

auricular  contents  into  the  ventricle  increases  the  front-to-back  diameter  of 
the  ventricle,  and  the  momentum  of  the  rapid  stroke  being  considerable,  the 
lever  is  in  each  case  carried  too  far  forward,  so  that  the  rise  is  followed  by  a 
fall,  producing  a  notch.  A  similar  though  somewhat  different  shoulder  is 
also  seen  in  the  cardiogram,  Fig.  58.  In  the  curve  of  ventricular  pressure 
taken  by  means  of  the  cardiac  sound  (Fig.  55)  there  is  a  similar  temporary 
increase  bf  in  the  ventricular  pressure  coincident  with  the  auricular  stroke  b, 
and  in  the  "  piston  "  pressure-curve  of  the  rapidly  beating  heart  (Fig.  56,  A) 
there  is  a  similar  shoulder  b  just  preceding  the  rise  of  the  ventricular  systole. 
The  meaning  of  the  last  curve  is,  however,  doubtful,  for  in  the  similar  curve 
of  the  more  slowly  beating  heart  (Fig.  56,  B)  it  occurs  immediately  after  the 
relaxation  of  the  ventricle,  some  time  before  the  occurrence  of  the  auricular 
systole,  and  in  many  curves  taken  by  the  same  method  is  absent  altogether. 
The  exact  meaning,  therefore,  of  the  shoulder  b  in  the  other  curves  must  be 
left  at  present  undecided. 

§  124.  We  have  still  to  consider  the  negative  pressure  shown  by  the 
minimum  manometer.  This  instrument,  as  we  have  said,  merely  shows  that 
the  pressure  in  the  ventricle  (or  auricle)  becomes  negative  at 'some  phase 
or  other  of  the  cardiac  cycle,  but  does  not  tell  us  in  which  phase  it  occurs. 

Now  there  are  two  ways  in  which  such  a  negative  pressure  might  originate. 
In  the  first  place,  as  we  have  just  seen,  a  negative  pressure  makes  its  appear- 
ance in  the  rear  of  the  column  of  blood  driven  from  the  ventricle  into  the 
aorta  with  great  suddenness  and  rapidity.  But  this  negative  pressure,  as  we 
have  also  seen,  follows  the  column  into  the  aorta  past  the  semilunar  valves, 
and  in  part,  at  all  events,  determines  the  closure  of  the  semilunar  valves. 
Hence  if  this  is  the  negative  pressure  which  the  minimum  manometer 
records,  it  ought  to  be  shown  not  only  when  the  end  of  the  tube  connected 
with  the  manometer  is  in  the  cavity  of  the  ventricle,  but  also  when  the  tube 
is  slipped  out  of  the  ventricle  just  past  the  semilunar  valves.  When  the 
tube,  however,  is  in  the  latter  situation  the  manometer  does  not  show  the 
same  marked  negative  pressure  that  it  does  when  the  tube  is  in  the  ventricle  ; 
the  negative  pressure  which  occurs  in  the  aorta  at  each  beat  is  sufficient  to 
produce  such  an  effect  on  the  minimum  manometer  as  is  produced  when  the 
instrument  is  in  the  ventricle.  Hence  we  infer  that  the  negative  pressure 
shown  by  the  minimum  manometer  is  not  produced  in  this  way.  We  may, 
moreover,  conclude  that  the  semilunar  valves  are  closed  before  this  negative 
pressure  makes  its  appearance  in  the  ventricle  ;  otherwise,  however  produced, 
it  would  be  transmitted  from  the  interior  of  the  ventricle  through  the  open 
valves  to  the  root  of  the  aorta  beyond. 

But  there  is  another  event  which  might  give  rise  to  a  negative  pressure. 
The  relaxation  of  the  ventricular  walls  is,  as  the  curves  (Figs.  56,  57,  58) 
show,  a  rapid  process,  something  quite  distinct  from  the  mere  filling  of  the 
ventricular  cavities  with  blood  from  the  auricles ;  and,  though  some  have 
objected  to  the  view,  it  may  be  urged  that  this  return  of  the  ventricle  from 
its  contracted  (and  emptied)  condition  to  its  normal  form  would  develop  a 
negative  pressure.  This  return  is  probably  simply  the  total  result  of  the 
return  of  each  fibre  or  fibre  cell  to  its  natural  condition,  though  some  have 
urged  that  the  extra  quantity  of  blood  thrown  into  the  coronary  arteries  at 
the  systole  helps  to  unfold  the  ventricles  somewhat  in  the  way  that  fluid 
driven  between  the  two  walls  of  a  double-walled  collapsed  ball  or  cup  will 
unfold  it. 

Accepting  the  return  of  the  ventricles  to  their  normal  form  as  the  cause 
of  the  negative  pressure  (and  it  may  be  remarked  that  the  return  of  the 
thick-walled  left  ventricle  naturally  exerts  a  greater  negative  pressure  than 
the  thin-walled  right  ventricle),  it  is  obvious  that  the  negative  pressure  will 


THE  HEART.  173 

assist  the  circulation  by  sucking  the  blood  which  meanwhile  has  been  accu- 
mulated in  the  auricle,  from  that  cavity  into  the  ventricle,  the  auriculo-ven- 
tricular  valves  easily  giving  way.  At  the  same  time  this  very  flow  from  the 
auricle  will  at  once  put  an  end  to  the  negative  pressure,  which  obviously  can 
be  of  brief  duration  only.  It  may  be  further  urged  in  support  of  this  view, 
that  even  when  the  thorax  is  opened,  so  that  the  respiratory  movements  can 
no  longer  act  toward  producing  a  negative  pressure  in  the  auricle  and  great 
veins,  a  minimum  manometer  placed  in  the  right  auricle  shows  frequently 
no  pressure  at  all  (that  is,  a  pressure  equal  to  that  of  the  atmosphere) 
and  sometimes  a  decidedly  negative  pressure.  Seeing  that  the  blood  under 
these  circumstances  is  being  driven  along  the  great  veins  by  a  pressure  which 
though  low  is  always  above  that  of  the  atmosphere,  we  may  conclude  that 
the  negative  pressure  produced  in  the  ventricle  is  the  cause  of  this  lowering 
of  the  pressure  in  the  auricle,  though  it  is  unable  to  make  itself  felt  along 
the  great  veins. 

§  125.  The  duration  of  the  several  phases.  We  may,  first  of  all,  distin- 
guish certain  main  phases:  (1)  The  systole  of  the  auricles.  (2)  The  systole, 
proper,  of  the  ventricles,  during  which  their  fibres  are  in  a  state  of  contrac- 
tion, lasting  to  d  in  Figs.  56,  57,  58.  (3)  The  diastole  of  the  ventricles — 
that  is  to  say,  the  time  intervening  between  their  fibres  ceasing  to  contract 
and  commencing  to  contract  again.  To  these  we  may  perhaps  add  (4)  The 
pause  or  rest  of  the  whole  heart,  comprising  the  period  from  the  end  of  the 
relaxation  of  the  ventricles  to  the  beginning  of  the  systole  of  the  auricles ; 
during  this  time  the  walls  are  undergoing  no  active  changes,  neither  con- 
tracting nor  relaxing,  their  cavities  being  simply  passively  filled  by  the 
influx  of  blood. 

The  mere  inspection  of  almost  any  series  of  cardiac  curves,  however 
taken — those,  for  instance,  which  we  have  just  discussed — will  show,  apart 
from  any  accurate  measurements,  that  the  systole  of  the  auricles  is  always 
very  brief,  that  the  systole  of  the  ventricles  is  always  very  prolonged — always 
occupying  a  considerable  portion  of  the  whole  cycle — and  that  the  diastole  of 
the  whole  heart,  reckoned  from  the  end  either  of  the  systole  or  of  the  relaxa- 
tion of  the  ventricle,  is  very  various,  being  in  quickly  beating  hearts  very 
short  and  in  slowly  beating  hearts  decidedly  longer. 

When  we  desire  to  arrive  at  more  complete  measurements,  we  are  obliged 
to  make  use  of  calculations  based  on  various  data ;  and  these  give  only 
approximate  results.  Naturally,  the  most  interest  is  attached  to  the  dura- 
tion of  events  in  the  human  heart. 

The  datum  which  perhaps  has  been  most  largely  used  is  the  interval 
between  the  beginning  of  the  first  and  the  occurrence  of  the  second  sound. 
This  may  be  determined  with  approximate  correctness,  and  is  found  to  vary 
from  0.301  to  0.327  second,  occupying  from  40  to  46  per  cent,  of  the  whole 
period,  and  being  fairly  constant  for  different  rates  of  heart-beat.  That  is 
to  say,  in  a  rapidly  beating  heart  it  is  the  pauses  which  are  shortened,  and 
not  the  duration  of  the  actual  beats. 

The  observer,  listening  to  the  sounds  of  the  heart,  makes  a  signal  at  each  event 
on  a  recording  surface,  the  difference  in  time  between  the  marks  being  measured 
by  means  of  the  vibrations  of  a  tuning-fork  recorded  on  the  same  surface.  By 
practice  it  is  found  possible  to  reduce  the  errors  of  observation  within  very  small 
limits. 

Now,  whatever  be  the  exact  causation  of  the  first  sound,  it  is  undoubtedly 
coincident  with  the  systole  of  the  ventricles,  though  possibly  the  actual  com- 
mencement of  its  becoming  audible  may  be  slightly  behind  the  actual  begin- 
ning of  the  muscular  contractions.  Similarly  the  occurrence  of  the  second 


174  THE  VASCULAR   MECHANISM. 

sound,  which,  as  we  have  seen,  is  certainly  due  to  the  closure  of  the  semi- 
lunar  valves,  has  been  taken  to  mark  the  close  of  the  ventricular  systole. 
And  on  this  supposition  the  interval  between  the  beginning  of  the  first  and 
the  occurrence  of  the  second  sound  has  been  regarded  as  indicating  approxi- 
mately the  duration  of  the  ventricular  systole — i.  e.,  the  period  during  which 
the  ventricular  fibres  are  contracting. 

The  determination  of  the  separate  duration  of  each  of  the  three  periods  of 
the  ventricular  systole — viz.,  the  getting  up  of  the  pressure,  the  discharge  of 
the  contents,  and  the  remaining  emptied  but  contracted — is  subjected  to  so 
much  uncertainty  that  it  need  not  be  insisted  on  here ;  it  may,  however,  be 
said  that,  roughly  speaking,  each  phase  occupies  probably  about  0.1  second. 
In  a  heart  beating  72  times  a  minute,  which  may  be  taken  as  the  normal 
rate,  each  entire  cardiac  cycle  would  last  about  0.8  second,  and  taking  0.3 
second  as  the  duration  of  the  ventricular  systole,  the  deduction  of  this  would 
leave  0.5  second  for  the  whole  diastole  of  the  ventricle,  including  its  relaxa- 
tion, the  latter  occupying  about  or  somewhat  less  than  0.1  second.  In  the 
latter  part  of  this  period  there  occurs  the  systole  of  the  auricles,  the  exact 
duration  of  which  it  is  difficult  to  determine,  it  being  hard  to  say  when  it 
really  begins,  but  which,  if  the  contraction  of  the  great  veins  be  included, 
may  perhaps  be  taken  as  lasting,  on  an  average,  0.1  second.  The  "  passive 
interval,"  therefore,  during  which  neither  auricle  nor  ventricle  is  undergoing 
contractions,  lasts  about  0.4  second,  and  the  absolute  pause  or  rest  during 
which  neither  auricle  nor  ventricle  is  contracting  or  relaxing,  about  0.3 
second ;  if,  however,  a  longer  period  be  allotted  to  the  ventricular  systole, 
these  periods  must  be  proportionately  shortened.  The  systole  of  the  ventricle 
follows  so  immediately  upon  that  of  the  auricles,  that  practically  no  interval 
exists  between  the  two  events. 

The  duration  of  the  several  phases  may,  for  convenience  sake,  be  arranged 
in  a  tabular  form  as  follows : 

Second.         Second. 

Systole  of  ventricle  before  the  opening  of  the  semilunar 
valves,  while  pressure  is  still  getting  up  (probably  rather 

less  than 0.1 

Escape  of  blood  into  aorta  (about) 0.1 

Continued    contraction   of    the  emptied    ventricle    (possibly 

rather  more  than) 0.1    J 

Total  systole  of  the  ventricle  (probably  rather  more  than)  0.3 

Diastole  of  both  auricle  and  ventricle,  neither  contracting, 

or  "passive  interval  "  (probably  rather  less  than) 0.4    | 

Systole  of  auricle  (about  or  less  than) 0.1    J 

Diastole  of  ventricle,  including  relaxation  and  filling,  up  to 
the  beginning  of  the  ventricular  systole  (probably  rather 
less  than) 0.5 

Total  cardiac  cycle 0.8 

Summary. 

§  126.  We  may  now  briefly  recapitulate  the  main  facts  connected  with 
the  passage  of  blood  through  the  heart.  The  right  auricle  during  its  dias- 
tole, by  the  relaxation  of  its  muscular  fibres,  and  by  the  fact  that  all  back- 
ward pressure  from  the  ventricle  is  removed  by  the  closing  of  the  tricuspid 
valves,  offers  but  little  resistance  to  the  ingress  of  blood  from  the  veins.  On 
the  other  hand,  the  blood  in  the  trunks  of  both  the  superior  and  inferior 
vena  cava  is  under  a  pressure,  which,  though  diminishing  toward  the  heart, 
remains  higher  than  the  pressure  obtaining  in  the  interior  of  the  auricle  ;  the 
blood  in  consequence  flows  into  the  empty  auricle,  its  progress  in  the  case  of 
the  superior  vena  cava  beinsj  assisted  by  gravity.  At  each  inspiration  this 
flow  (as  we  shall  see  in  speaking  of  respiration)  is  favored  by  the  diminution 


THE   HEART.  175 

of  pressure  in  the  heart  and  great  vessels  caused  by  the  respiratory  move- 
ments. Before  this  flow  has  gone  on  very  long,  the  diastole  of  the  ventricle 
begins,  its  cavity  dilates,  the  flaps  of  the  tricuspid  valve  fall  back,  and  blood 
for  some  little  time  flows  in  an  unbroken  stream  from  the  venae  cavse  into 
the  ventricle.  In  a  short  time,  however,  probably  before  very  much  blood 
has  had  time  to  enter  the  ventricle,  the  auricle  is  full ;  and  forthwith  its 
sharp  sudden  systole  takes  place.  Partly  by  reason  of  the  backward  pres- 
sure in  the  veins,  which  increases  rapidly  from  the  heart  toward  the  capil- 
laries, and  which,  at  some  distance  from  the  heart,  is  assisted  by  the  presence 
of  valves  in  the  venous  trunks,  but  still  more  from  the  fact  that  the  systole 
begins  at  the  great  veins  themselves  and  spreads  thence  over  the  auricle,  the 
force  of  the  auricular  contraction  is  spent  in  driving  the  blood,  not  back 
into  the  veins,  but  into  the  ventricle,  where  the  pressure  is  still  exceedingly 
low.  Whether  there  is  any  backward  flow  at  all  into  the  great  veins,  or 
whether  by  the  progressive  character  of  the  systole  the  flow  of  blood  con- 
tinues, so  to  speak,  to  follow  up  the  systole  without  break,  so  that  the  stream 
from  the  veins  into  the  auricle  is  really  continuous,  is  at  present  doubtful ; 
though  a  slight  positive  wave  of  pressure  synchronous  with  the  auricular 
systole,  travelling  backward  along  the  great  veins,  has  been  observed,  at  least 
in  cases  where  the  heart  is  beating  vigorously. 

The  ventricle  thus  being  filled  by  the  auricular  systole,  the  play  of  the 
tricuspid  valves  described  above  comes  into  action,  the  auricular  systole  is 
followed  by  that  of  the  ventricle,  and  the  pressure  within  the  ventricle,  cut 
off  from  the  auricle  by  the  tricuspid  valves,  is  brought  to  bear  on  the  pul- 
monary semilunar  valves  and  the  column  of  blood  on  the  other  side  of  those 
valves.  As  soon  as  by  the  rapidly  increasing  shortening  of  the  ventricular 
fibres  the  pressure  within  the  ventricle  becomes  greater  than  that  in  the  pul- 
monary artery,  the  semilunar  valves  open  and  the  still  continuing  systole 
discharges  the  contents  of  the  ventricle  into  that  vessel. 

As  the  ventricle  thus  rapidly  and  forcibly  empties  itself,  either  the  tran- 
sient negative  pressure  which  makes  its  appearance  in  the  rear  of  the  ejected 
column  of  blood,  or  the  elastic  action  of  the  aortic  walls,  leads  to  a  reflux 
of  blood  toward  the  ventricle,  the  effect  of  which,  however,  is  to  close  the 
semilunar  valves  and  thus  to  shut  off  the  blood  in  the  distended  arteries  from 
the  emptied  ventricle.  Either  immediately  at  or  more  probably  some  little 
time  after  this  closing  of  the  valves  the  ventricular  systole  ends  and  relax- 
ation begins  ;  then  once  more  the  cavity  of  the  ventricle  becomes  unfolded 
and  finally  distended  by  the  influx  of  blood,  a  negative  pressure  devel- 
oped by  the  relaxation  probably  aiding  the  flow  from  the  auricle  and 
great  veins. 

During  the  whole  of  this  time  the  left  side  has  with  still  greater  energy 
been  executing  the  same  manoeuvre.  At  the  same  time  that  the  venae  cavse 
are  filling  the  right  auricle  the  pulmonary  veins  are  filling  the  left  auricle. 
At  the  same  time  that  the  right  auricle  is  contracting  the  left  auricle  is  con- 
tracting, too.  The  systole  of  the  left  ventricle  is  synchronous  with  that  of 
the  right  ventricle,  but  executed  with  greater  force ;  and  the  flow  of  blood 
is  guided  on  the  left  side  by  the  mitral  and  aortic  valves  in  the  same  way 
that  it  is  on  the  right  by  the  tricuspid  valves  and  the  valves  of  the  pul- 
monary artery. 

The  Work  Done. 

§  127.  The  most  important  factor  of  all  in  the  determination  of  the  work 
of  the  vascular  mechanism  is  the  quantity  of  blood  ejected  from  the  ventricle 
into  the  aorta  at  each  systole.  The  general  result  of  some  of  these  calcula- 
tions gives  about  180  grammes  (6  ozs.)  as  the  quantity  of  blood  which  is 


176  THE   VASCULAR   MECHANISM. 

driven  from  each  ventricle  at  each  systole  in  a  full-grown  man  of  average 
size  and  weight,  but  this  estimate  is  probably  too  high. 

In  the  dog  the  quantity  has  been  experimentally  determined,  by  allowing  the 
heart  to  deliver  its  contents  through  one  branch  of  the  aorta,  all  others  being 
ligatured  or  blocked,  into  a  receiver,  the  contents  of  which  are  at  intervals,  lay  an 
ingenious  contrivance,  returned  to  the  right  auricle.  The  time  taken  to  fill  the 
receiver  and  the  number  of  beats  executed  during  that  time  being  noted,  the 
average  quantity  ejected  at  a  beat  is  thus  given.  It  is  found  to  vary  widely. 

Various  methods  have  been  adopted  for  calculating  the  average  amount  of 
blood  ejected  at  each  ventricular  systole.  The  simplest  method  is  to  measure  the 
capacity  of  the  recently  removed  and  as  yet  not  rigid  ventricle,  filled  with  blood 
under  a  pressure  equal  to  the  calculated  average  pressure  in  the  ventricle.  On 
the  supposition  that  the  whole  contents  of  the  ventricle  are  ejected  at  each  systole 
this  would  give  the  quantity  driven  into  the  aorta  at  each  stroke.  The  other 
methods  are  very  indirect. 

It  is  evident  that  exactly  the  same  quantity  must  issue  at  a  beat  from 
each  ventricle ;  for  if  the  right  ventricle  at  each  beat  gave  out  rather  less 
than  the  left,  after  a  certain  number  of  beats  the  whole  of  the  blood  would 
be  gathered  in  the  systemic  circulation.  Similarly,  if  the  left  ventricle  gave 
out  less  than  the  right,  all  the  blood  would  soon  be  crowded  into  the  lungs. 
The  fact  that  the  pressure  in  the  right  ventricle  is  so  much  less  than  that  in 
the  left  (probably  30  or  40  mm.  as  compared  with  200  mm.  of  mercury),  is 
due,  not  to  differences  in  the  quantity  of  blood  in  the  cavities,  but  to  the  fact 
that  the  peripheral  resistance  which  has  to  be  overcome  in  the  lungs  is  so 
much  less  than  that  in  the  rest  of  the  body. 

It  must  be  remembered  that  though  it  is  of  advantage  to  speak  of  an 
average  quantity  ejected  at  each  stroke,  it  is  more  than  probable  that  that 
quantity  may  vary  within  very  wide  limits.  Taking,  however,  180  grammes 
as  the  quantity,  in  man,  ejected  at  each  stroke  at  a  pressure  of  250  mm.1  of 
mercury,  which  is  equivalent  to  3.21  metres  of  blood,  this  means  that  the 
left  ventricle  is  capable  at  its  systole  of  lifting  180  grammes  3.21  metres 
high,  i.  e.,  it  does  578  gramme-metres  of  work  at  each  beat.  Supposing  the 
heart  to  beat  72  times  a  minute,  this  would  give  for  the  day's  work  of  the 
left  ventricle  nearly  60,000  kilogramme-metres.  Calculating  the  work  of 
the  right  ventricle  at  one-fourth  that  of  the  left,  the  work  of  the  whole  heart 
would  amount  to  75,000  kilogramme-metres,  which  is  just  about  the  amount 
of  work  done  in  the  ascent  of  Snowdon  by  a  tolerably  heavy  man. 

A  calculation  of  more  practical  value  is  the  following:  Taking  the 
quantity  of  blood  as  T^  of  the  body- weight,  the  blood  of  a  man  weighing 
75  kilos  would  be  about  5760  grammes.  If  180  grammes  left  the  ventricle 
at  each  beat,  a  quantity  equivalent  to  the  whole  blood  would  pass  through 
the  heart  in  32  beats,  i.  e.,  in  less  than  half  a  minute. 

THE  PULSE. 

§  128.  We  have  seen  that  the  arteries,  though  always  distended,  undergo 
at  each  systole  of  the  ventricle 'a  temporary  additional  distention,  so  that 
when  a  finger  is  placed  on  an  artery,  such  as  the  radial,  an  intermittent  pres- 
sure on  the  finger,  coming  and  going  with  the  beat  of  the  heart,  is  felt,  and 
when  a  light  lever  is  placed  on  the  artery,  the  lever  is  raised  at  each  beat, 
falling  between. 

This  intermittent  expansion  which  we  call  the  pulse,  corresponding  to 
the  jerking  outflow  of  blood  from  a  severed  artery,  is  present  in  the  arteries 
only,  being,  except  under  particular  circumstances,  absent  from  the  veins 
and  capillaries.  The  expansion  is  frequently  visible  to  the  eye,  and  in  some 

1 A  high  estimate  is  purposely  taken  here. 


THE  PULSE. 


177 


cases,  as  where  an  artery  has  a  bend,  may  cause  a  certain  amount  of  loco- 
motion of  the  vessel. 

The  temporary  increase  of  pressure  which  is  the  cause  of  the  temporary 
increase  of  expansion  makes  itself  felt,  as  we  have  seen,  in  the  curve  of 
arterial  pressure  taken  by  the  mercury  manometer ;  but  the  inertia  of  the 
mercury  prevents  the  special  characters  of  each  increase  becoming  visible. 
In  Fick's  spring  manometer  (Fig.  59),  in  which  the  increase  of  pressure 
unfolds  a  curved  spring  and  so  moves  a  lever,  the  inertia  is  much  less,  and 
satisfactory  tracings  may  be  taken  by  this  instrument.  Other  instruments 
have  also  been  devised  for  recording  the  special  characters  of  each  increase 


FIG.  59. 


Fick's  Spring  Manometer.  The  flattened  tube  in  the  form  of  a  hoop  is  firmly  fixed  at  one  end, 
while  the  other  free  end  is  attached  to  a  lever.  The  interior  of  the  tube,  filled  with  spirit,  is 
brought,  by  means  of  a  tube  containing  sodium  carbonate  solution,  into  connection  with  an 
artery,  in  much  the  same  way  as  in  the  case  of  the  mercury  manometer.  The  increase  of  pressure 
in  the  artery  being  transmitted  to  the  hollow  hoop,  tends  to  straighten  it,  and  correspondingly 
moves  the  attached  lever. 

of  pressure  or  of  the  expansion  of  the  artery  which  is  the  result  of  that 
increase.  The  easiest  and  most  common  method  of  registering  the  expansion 
of  an  artery  is  that  of  simply  bringing  a  light  lever  to  bear  on  the  outside 
of  the  artery. 

A  lever  specially  adapted  to  record  a  pulse  tracing  is  called  a  sphygmo- 
graph,  the  instrument  generally  comprising  a  small  travelling  recording 
surface  on  which  the  lever  writes.  There  are  many  different  forms  of sphyg- 
mograph,  but  the  general  plan  of  structure  is  the  same.  Fig.  60  represents 
in  a  diagrammatic  form  the  essential  parts  of  the  sphygmograph,  known  as 
Dudgeon's  [and  Fig.  61,  Marey's,  which  is  in  more  common  use].  The 
instrument  is  generally  applied  to  the  radial  artery  because  the  arm  affords 
a  convenient  support  to  the  fulcrum  of  the  lever,  and  because  the  position  of 
12 


178 


THE  VASCULAR  MECHANISM. 


the  artery,  near  to  the  surface  and  with  the  support  of  the  radius  below  so 
that  adequate  pressure  can  be  brought  to  bear  by  the  lever  on  the  artery,  is 


FIG. 


Diagram  of  a  Sphygmograph  ('Dudgeon's).  Certain  supporting  parts  are  omitted  so  that  the  mul- 
tiplying levers  may  be  displayed,  a  is  a  small  metal  plate  which  is  kept  pressed  on  the  artery  by  the 
spring  b.  The  vertical  movements  of  a  cause  to-and-fro  movements  of  the  lever  c  about  the  fixed 
point  d.  These  are  communicated  to  and  magnified  by  the  lever  e,  which  moves  round  the  fixed 
point/.  The  free  end  of  this  lever  carries  a  light  steel  marker  which  rests  on  a  strip  of  smoked 
paper  g.  The  paper  is  placed  beneath  two  small  wheels  and  rests  on  a  roller  which  can  be  rotated 
by  means  of  clock-work  contained  in  the  box  h.  The  paper  is  thus  caused  to  travel  at  a  uniform 
rate.  The  screw  graduated  in  ounces  (Troy)  is  brought  to  bear  on  the  spring  b  by  means  of  a  cam 
and  by  this  the  pressure  put  on  the  artery  can  be  regulated.  The  levers  magnify  the  pulse-move- 
ments fifty  times. 

favorable  for  making  observations.     It  can,  of  course,  be   applied  to  other 
arteries.     When    applied  to  the  radial  artery    some  such  tracing  as  that 


[Fio.  61. 


Marey's  Sphygmograph :  B,  B  is  where  the  sphygmograph  is  applied  to  the  arm ;  R,  spring 
which  rests  upon  radial  artery  ;  V,  screw  for  adjusting  marking  lever  L ;  H,  clock-work  ;  P,  smoked 
paper  upon  which  tracing  is  made  ;  r,  small  spring  for  causing  descent  of  lever  after  raising.] 

shown  in  Fig.  62  is  obtained.     At  each  heart  beat  the  lever  rises  rapidly  and 
then  falls  more  gradually  in  a  line  which  is  more  or  less  uneven. 


THE  PULSE. 


179 


§  129.  We  have  now  to  study  the  nature  and  characters  of  the  pulse  in 
greater  detail. 

We  may  say  at  once,  and  indeed  have  already  incidentally  seen,  that 
pulse  is  essentially  due  to  the  action  of   physical  causes ;  it  is  the 


the 


FIG.  62. 


Pulse  Tracing  from  the  Radial  Artery  of  Man  The  vertical  curved  line  L,  gives  the  tracing 
which  the  recording  lever  made  when  the  blackened  paper  was  motionless.  The  curved  inter- 
rupted lines  show  the  distance  from  one  another  in  time  of  the  chief  phases  of  the  pulse-wave, 
viz.,  x  =  commencement  and  A  end  of  expansion  of  artery;  p,  pre-dicrotic  notch,  d,  dicrotic 
notch.  C,  dicrotic  crest.  D,  Post- dicrotic  crest.  /,  the post-dicrotic  notch.  These  are  explained 
in  the  text  later  on. 

physical  result  of  the  sudden  injection  of  the  contents  of  the  ventricle  into 
the  elastic  tubes  called  arteries;  its  more  important  features  may  be  ex- 

[FiG.  63. 


Apparatus  of  Marey  for  showing  Mode  in  which  Pulse  is  Propagated  in  the  Arteries:  B  is  a 
rubber  pump,  with  valve  attachment,  to  prevent  a  regurgitant  current;  I,  V ,  I",  are  levers  resting 
on  a  gum  tube,  at  intervals  of  20  cm.  of  tubing;  C,  drum  upon  which  tracing  is  made  ;  H,  clock- 
work to  revolve  drum.] 

plained  on  physical  principles  and  may  be  illustrated   by  means  of  an  arti- 
ficial model  [Fig.  63]. 

If  two  or  more  levers  be  placed  on  the  arterial  tubes  of  an  artificial 
model,  Fig.  63,  one  near  to  the  pump,  and  the  others  equal  distances  apart, 
with  a  considerable  length  of  tubing  between  the  proximal  and  distal  levers, 
and  the  levers  be  made  to  write,  on  a  vertical  line,  on  a  recording  surface, 


180 


THE  VASCULAR  MECHANISM. 


so  that  their  curves  can  be  more  easily  compared,  the  following  facts  may  be 
observed  when  the  pump  is  set  to  work  regularly. 

At  each  stroke  of  the  pump,  each  lever  rises  until  it  reaches  a  maxi- 
mum (Fig.  64,  la,  2a,  etc.)  and  then  'falls  again,  thus  describing  a  curve. 


wfXAAAAAAAAAAA/WW 


Pulse-curve  described  by  a  series  of  Sphygmographic  Levers,  placed  at  intervals  of  20  cm. 
from  each  other  along  an  elastic  tube  into  which  fluid  is  forced  by  the  sudden  stroke  of  a  pump. 
The  pulse-wave  is  travelling  from  left  to  right,  as  indicated  by  the  arrows  over  the  primary  (a) 
and  secondary  (b,  c)  pulse-waves.  The  dotted  vertical  lines  drawn  from  the  summit  of  the  sev- 
eral primary  waves  to  the  tuning-fork  curve  below,  each  complete  vibration  of  which  occupies 
1-50  second,  allow  the  time  to  be  measured  which  is  taken  up  by  the  wave  in  passing  along  20  cm. 
of  the  tubing.  The  waves  a'  are  waves  reflected  from  the  closed  distal  end  of  the  tubing  ;  this  is 
indicated  by  the  direction  of  the  arrows.  It  will  be  observed  that  in  the  more  distant  lever  VI, 
the  reflected  wave,  having  but  a  slight  distance  to  travel,  becomes  fused  with  the  primary  wave. 
(From  Marey.) 

The  rise  is  due  to  the  expansion  of  the  part  of  the  tube  under  the  lever, 
and  the  fall  is  due  to  that  part  of  the  tube  running  after  the  expansion  to 
its  previous  calibre.  The  curve  is,  therefore,  the  curve  of  the  expansion 
(and  return)  of  the  tube  at  the  point  at  which  the  lever  rests.  We  may 
call  it  the  pulse-curve.  It  is  obvious  that  the  expansion  passes  by  the  lever 


THE  PULSE. 


181 


in  the  form  of  a  wave.  At  one  moment  the  lever  is  at  rest ;  the  tube 
beneath  it  is  simply  distended  to  the  normal  amount  indicative  of  the  mean 
pressure  which  at  the  time  obtains  in  the  arterial  tubes  of  the  model ;  at  the 
next  moment  the  pulse  expansion  reaches  the  lever,  and  the  lever  begins  to 
rise ;  it  continues  to  rise  until  the  top  of  the  wave  reaches  it,  after  which 
it  falls  again  until  finally  it  comes  to  rest,  the  wave  having  completely 
passed  by. 

It  may  perhaps  be  as  well  at  once  to  warn  the  reader  that  the  figure  which 
we  call  the  pulse-curve  is  not  a  representation  of  the  pulse-wave  itself;  it  is 
simply  a  representation  of  the  movements,  up  and  down,  of  the  piece  of  the 
wall  of  the  tubing  at  the  spot  on  which  the  lever  rests  during  the  time  that 
the  wave  is  passing  over  that  spot.  We  may  roughly  represent  the  wave 
in  the  diagram  Fig.  65  in  which  the  wave  shown  by  the  dotted  line  is 
passing  over  the  tube  (shown  in  a  condition  of  rest  by  the  thick  double 
line)  in  the  direction  from  H  to  C.  It  must,  however,  be  remembered 

FIG.  65. 


A  rough  Diagrammatic  Representation  of  a  Pulse-wave  passing  over  an  Artery. 

that  the  wave  thus  figured  is  a  much  shorter  wave  than  is  the  pulse-wave 
in  reality  (that  being,  as  we  shall  see,  about  6  metres  long),  i.  e.,  occu- 
pies a  smaller  length  of  the  arterial  system  from  the  heart  H  toward  the 
capillaries  C. 

The  curves  below,  X,  Y,  Z,  represent,  in  a  similarly  diagrammatic  fash- 
ion, the  curves  described,  during  the  passage  of  the  wave,  by  levers  placed 
on  the  points  x,  y,  z.  At  Z  the  greater  part  of  the  wave  has  already  passed 
under  the  lever,  which  during  its  passage  has  already  described  the  greater 
part  of  its  curve,  shown  by  the  thick  line,  and  has  only  now  to  describe  the 
small  part,  shown  by  the  dotted  line,  corresponding  to  the  remainder  of  the 
wave  from  Z  to  H.  At  Fthe  lever  is  at  the  summit  of  the  wave.  At  X 
the  lever  has  only  described  a  small  part  of  the  beginning  of  the  wave,  viz., 
from  Cto  x,  the  rest  of  the  curve,  as  shown  by  the  dotted  line,  having  yet 
to  be  described. 

But  to  return  to  the  consideration  of  Fig.  64. 

§  130.  The  rise  of  each  lever  is  somewhat  sudden,  but  the  fall  is  more 
gradual,  and  is  generally  marked  with  some  irregularities  which  we  shall 
study  presently.  The  rise  is  sudden  because  the  sharp  stroke  of  the  pump 
suddenly  drives  a  quantity  of  fluid  into  the  tubing  and  so  suddenly  expands 
the  tube ;  the  fall  is  more  gradual  because  the  elastic  reaction  of  the  walls 
of  the  tube,  which  brings  about  the  return  of  the  tube  to  its  former  calibre 


182 


THE  VASCULAR  MECHANISM. 


FIG. 


ABC 


Pulse-tracings  from  the  same  Ra- 
dial Artery  under  Different  Pressures 
of  the  Lever.  (The  letters  are  ex- 
plained in  a  later  part  of  the  text.) 


after  the  expanding  power  of  the  pump  has  ceased,  is  more  gradual  in  its 
action. 

These  features,  the  suddenness  of  the  rise  or  up-stroke,  and  the  more 
gradual  slope  of  the  fall  or  down-stroke,  are  seen  also  in  natural  pulse-curves 

taken  from  living  arteries  (Figs.  62,  66,  etc.). 
Indeed,  the  difference  between  the  up-stroke 
and  the  down-stroke  is  even  more  marked 
in  the  latter  than  in  the  former,  the  delivery 
of  blood  from  the  ventricle  being  more  rapid 
than  the  issue  of  water  from  a  pump  as  ordi- 
narily worked. 

It  may  here  be  noted  that  the  actual  size 
of  the  curve,  that  is,  the  amount  of  excursion 
of  the  lever,  depends  in  part  (as  does  also  to 
a  great  extent  the  form  of  the  curve)  on  the 
amount  of  pressure  exerted  by  the  lever  on 
the  tube.  If  the  lever  only  just  touches  the 
tube  in  its  expanded  state,  the  rise  will  be 
insignificant.  If,  on  the  other  hand,  the 
lever  be  pressed  down  too  firmly,  the  tube 
beneath  will  not  be  able  to  expand  as  it 
otherwise  would,  and  the  rise  of  the  lever 
will  be  proportionately  diminished.  There 
is  a  certain  pressure  which  must  be  exerted 
by  the  lever  on  the  tube,  the  exact  amount  depending  on  the  expansive 
power  of  the  tubing  and  on  the  pressure  exerted  by  the  fluid  in  the  tube, 
in  order  that  the  tracing  may  be  best  marked.  This  is  shown  in  Fig.  66r  in 
which  are  given  three  tracings  taken  from  the  same  radial  artery  with  the 
same  instrument;  in  the  lower  curve  the  pressure  of  the  lever  is  too  great, 
in  the  upper  curve  too  small,  to  bring  out  the  characters  seen  most  distinctly 
in  the  middle  curve  with  a  medium  pressure. 

§  131.  It  will  be  observed  that  in  Fig.  64,  curve  I.,  which  is  nearer  the 
pump,  rises  higher,  and  rises  more  rapidly  than  curve  II.,  which  is  further 
away  from  the  pump ;  that  is  to  say,  at  the  lever  further  away  from  the 
pump  the  expansion  is  less  and  takes  place  more  slowly  than  at  the  lever 
nearer  the  pump.  Similarly  in  curve  IV.  the  rise  is  still  less,  and  takes 
place  still  less  rapidly  than  in  II.,  and  the  same  change  is  seen  still  more 
marked  in  V.  as  compared  with  IV.  In  fact,  if  a  number  of  levers  were 
placed  at  equal  distances  along  the  arterial  tubing  of  the  model  and  the 
model  were  working  properly,  with  an  adequate  peripheral  resistance,  we 
might  trace  out  step  by  step  how  the  expansion,  as  it  travelled  along  the 
tube,  got  less  and  less  in  amount  and  at  the  same  time  became  more  gradual 
in  its  development,  the  curve  becoming  lower  and  more  flattened  out,  until 
in  the  neighborhood  of  the  artificial  capillaries  there  was  hardly  any  trace 
of  it  left.  In  other  words,  we  might 
trace  out  step  by  step  the  gradual  dis- 
appearance of  the  pulse. 

The  same  changes,  the  same  gradual 
lowering  and  flattening  of  the  curve, 
may  be  seen  in  natural  pulse-tracings, 
as  for  instance  in  Fig.  67,  which  is  a 
tracing  from  the  dorsalis  pedis  artery, 
compared  with  the  tracing  from  the  ra- 
dial artery,  Fig.  66,  taken  from  the  same  individual  with  the  same  instru- 
ment on  the  same  occasion.  This  feature  is,  of  course,  not  obvious  in  all 


FIG.  67. 


Pulse-tracing  from  Dorsalis  Pedis,  taken 
from  the  same  individual  as  Fig.  66. 


THE   PULSE.  183 

pulse-curves  taken  from  different  individuals  with  different  instruments  and 
under  varied  circumstances ;  but  if  a  series  of  curves  from  different  arteries 
were  carefully  taken  under  the  same  conditions  it  would  be  found  that  the 
aortic  tracing  is  higher  and  more  sudden  than  the  carotid  tracing,  which, 
again,  is  higher  and  more  sudden  than  the  radial  tracing — the  tibial  tracing 
being  in  turn  still  lower  and  more  flattened.  The  pulse-curve  dies  out  by 
becoming  lower  and  lower  and  more  and  more  flattened  out. 

And  a  little  consideration  will  show  us  that  this  must  be  so.  The  systole 
of  the  ventricle  drives  a  quantity  of  blood  into  the  already  full  aorta.  The 
sudden  injection  of  this  quantity  of  blood  expands  the  portion  of  the  aorta 
next  to  the  heart,  the  part  immediately  adjacent  to  the  semilunar  valves 
beginning  to  expand  first,  and  the  expansion  travelling  thence  on  to  the  end 
of  this  portion.  In  the  same  way  the  expansion  travels  on  from  this  portion 
through  all  the  succeeding  portions  of  the  arterial  system.  For  the  total 
expansion  required  to  make  room  for  the  new  quantity  of  blood  is  not  pro- 
vided by  that  portion  alone  of  the  aorta  into  which  the  blood  is  actually 
received  ;  it  is  supplied  by  the  whole  arterial  system ;  the  old  quantity  of 
blood  which  is  replaced  by  the  new  in  this  first  portion  has  to  find  room  for 
itself  in  the  rest  of  the  arterial  space.  As  the  expansion  travels  onward, 
however,  the  increase  of  pressure  which  each  portion  transmits  to  the  suc- 
ceeding portion  will  be  less  than  that  which  it  received  from  the  preceding 
portion.  For  the  whole  increase  of  pressure  due  to  the  systole  of  the  ven- 
tricle has  to  be  distributed  over  the  whole  of  the  arterial  system ;  the  general 
mean  arterial  pressure  is,  as  we  have  seen,  maintained  by  repeated  systoles, 
and  any  one  systole  has  to  make  its  contribution  to  that  mean  pressure  ;  the 
increase  of  pressure  which  starts  from  the  ventricle  must,  therefore,  leave 
behind  at  each  stage  of  its  progress  a  fraction  of  itself;  that  is  to  say,  the 
expansion  is  continually  growing  less,  as  the  pulse  travels  from  the  heart 
to  the  capillaries.  Moreover,  while  the  expansion  of  the  aorta  next  to  the 
heart  is,  so  to  speak,  the  direct  effect  of  the  systole  of  the  ventricle,  the  expan- 
sion of  the  more  distant  artery  is  the  effect  of  the  systole  transmitted  by  the 
help  of  the  elastic  reaction  of  the  arterial  tract  between  the  heart  and  the 
distant  artery  ;  and  since  this  elastic  reaction  is  slower  in  development  than 
the  actual  systole,  the  expansion  of  the  more  distant  artery  is  slower  than 
that  of  the  aorta,  the  up-stroke  of  the  pulse-curve  is  less  sudden,  and  the 
whole  pulse-curve  is  more  flattened. 

The  object  of  the  systole  is  to  supply  a  contribution  to  the  mean  pressure, 
and  the  pulse  is  an  oscillation  above  and  below  that  mean  pressure — an  oscil- 
lation which  diminishes  from  the  heart  onwards,  being  damped  by  the  elastic 
walls  of  the  arteries,  and  so,  little  by  little,  converted  into  mean  pressure 
until  in  the  capillaries  the  mean  pressure  alone  remains — the  oscillations 
having  disappeared. 

§  132.  If  in  the  model  the  points  of  the  two  levers  at  different  distances 
from  the  pump  be  placed  exactly  one  under  the  other  on  the  recording 
surface,  it  is  obvious  that,  the  levers  being  alike  except  for  their  position 
on  the  tube,  any  difference  in  time  between  the  movements  of  the  two 
levers  will  be  shown  by  an  interval  between  the  beginnings  of  the 
curves  they  describe,  the  recording  surface  being  made  to  travel  suffi- 
ciently rapidly. 

If  the  movements  of  the  two  levers  be  thus  compared,  it  will  be  seen  that 
the  far  lever  (Fig.  64,  II.)  commences  later  than  the  near  one  (Fig.  64,  I.)  ; 
the  further  apart  the  two  levers  are,  the  greater  is  the  interval  in  time 
between  their  curves.  Compare  the  series  I.  to  VI.  (Fig.  64).  This  means 
that  the  wave  of  expansion,  the  pulse-wave,  takes  some  time  to  travel  along 
the  tube.  In  the  same  way  it  would  be  found  that  the  rise  of  the  near  lever 


184  THE  VASCULAR  MECHANISM. 

began  some  fraction  of  a  second  after  the  stroke  of  the  pump,  meaning  that 
time  is  required  for  the  transmission  of  the  wave. 

The  velocity  with  which  the  pulse-wave  travels  depends  chiefly  on  the 
amount  of  rigidity  possessed  by  the  tubing.  The  more  extensible  (with  cor- 
responding elastic  reaction)  the  tube,  the  slower  is  the  wave ;  the  more  rigid 
the  tube  becomes,  the  faster  the  wave  travels ;  in  a  perfectly  rigid  tube,  what 
in  the  elastic  tube  would  be  the  pulse,  becomes  a  mere  shock  travelling 
with  very  great  rapidity.  The  width  of  the  tube  is  of  much  less  influence, 
though  according  to  some  observers  the  wave  travels  more  slowly  in  the  wider 
tubes. 

The  rate  at  which  the  normal  pulse-wave  travels  in  the  human  body 
has  been  variously  estimated  at  from  10  to  5  metres  per  second.  In  all  proba- 
bility the  lower  estimate  is  the  more  correct  one  ;  but  it  must  be  remembered 
that  the  rate  may  vary  very  considerably  under  different  conditions.  Accord- 
ing to  all  observers  the  velocity  of  the  wave  in  passing  from  the  groin  to  the 
foot  is  greater  than  in  passing  from  the  axilla  to  the  wrist  (6  metres  against 
5  metres).  This  is  probably  due  to  the  fact  that  the  femoral  artery  with  its 
branches  is  more  rigid  than  the  axillary  and  its  branches.  So  also  in  the 
arteries  of  children,  the  wave  travels  more  slowly  than  in  the  more  rigid 
arteries  of  the  adult.  The  velocity  is  also  increased  by  circumstances  which 
heighten  and  decreased  by  those  which  lessen  the  mean  arterial  pressure, 
since  with  increasing  pressure  the  arterial  walls  become  more  and  with 
diminishing  pressure  less  rigid.  Probably,  also,  the  velocity  of  the  pulse- 
wave  depends  on  conditions  of  the  arterial  walls  which  we  cannot  adequately 
describe  as  mere  differences  in  rigidity.  In  experimenting  with  artificial 
tubes  it  is  found  that  different  qualities  of  India-rubber  give  rise  to  very 
different  results. 

Care  must  be  taken  not  to  confound  the  progress  of  the  pulse-wave — 
i.  e.,  of  the  expansion  of  the  arterial  wall — with  the  actual  onward  move- 
ment of  the  blood  itself.  The  pulse-wave  travels  over  the  moving  blood 
somewhat  as  a  rapidly  moving  natural  wave  travels  along  a  sluggishly  flow- 
ing river.  Thus  while  the  velocity  of  the  pulse-wave  is  6  or  possibly  even 
10  metres  per  second,  that  of  the  current  of  the  blood  is  not  more  than 
half  a  metre  per  second  even  in  the  large  arteries,  and  is  still  less  in  the 
smaller  ones. 

§  133.  Referring  again  to  the  caution  given  above  not  to  regard  the  pulse- 
curve  as  a  picture  of  the  pulse- wave,  we  may  now  add  that  the  pulse-wave  is 
of  very  considerable  length.  If  we  know  how  long  it  takes  for  the  pulse- 
wave  to  pass  over  any  point  in  the  arteries  and  how  fast  it  is  travelling,  we 
can  easily  calculate  the  length  of  the  wave.  In  an  ordinary  pulse-curve  the 
artery,  owing  to  the  slow  return,  is.seen  not  to  regain  the  calibre  which  it  had 
before  the  expansion,  until  just  as  the  next  expansion  begins — that  is  to  say, 
the  pulse-wave  takes  the  whole  time  of  a  cardiac  cycle,  viz.,  T\ths  second  to 
pass  by  the  lever.  Taking  the  velocity  of  the  pulse- wave  as  6  metres  per 
second  the  length  of  the  wave  will  be  T8oths  of  6  metres — or  nearly  5  metres. 
And  even  if  we  took  a  smaller  estimate,  by  supposing  that  the  real  expan- 
sion and  return  of  the  artery  at  any  point  took  much  less  time,  say  T%ths 
second,  the  length  of  the  pulse-wave  would  still  be  more  than  2  metres.  But 
even  in  the  tallest  man  the  capillaries  furthest  from  the  heart,  those  in  the 
tips  of  the  toes,  are  not  2  metres  distant  from  the  heart.  In  other  words,  the 
length  of  the  pulse-wave  is  much  greater  than  the  whole  length  of  the  arte- 
rial system,  so  that  the  beginning  of  each  wave  has  become  lost  in  the  small 
arteries  and  capillaries  some  time  before  the  end  of  it  has  finally  passed  away 
from  the  beginning  of  the  aorta. 

We  must  now  return  to  the  consideration  of  certain  special  features  in 


THE  PULSE. 


185 


the  pulse,  which  from  the  indications  they  give  or  suggest  of  the  condition 
of  the  vascular  system  .are  often  of  great  interest. 

§  134.  Dicrotism.  In  nearly  all  pulse-tracings,  the  curve  of  the  expansion 
and  recoil  of  the  artery  is  broken  by  two,  three,  or  several  smaller  elevations 
and  depressions ;  secondary  waves  are  imposed  upon  the  fundamental  or 
primary  wave.  In  the  sphygmographic  tracing  from  the  carotid,  Fig.  68, 


FIG.  68. 


FIG,  69. 


Pulse-tracing  from  Carotid  Artery  of  Healthy  Man  (from  Moens) :  x,  commencement  of  expan- 
sion of  artery.  A,  summit  of  the  first  rise.  C,  dicrotic  secondary  wave.  B,  pre-dicrotic  second- 
ary wave ;  />,  notch  preceding  this.  D,  succeeding  secondary  wave.  The  curve  above  is  that  of  a 
tuning-fork  with  ten  double  vibrations  in  a  second. 

and  in  many  of  the  other  tracings  given,  these  secondary  elevations  are 
marked,  as  B,  C,  D.  When  one  such  secondary  elevation  only  is  conspicu- 
ous, so  that  the  pulse-curve  presents  two  notable 
crests  only,  the  primary  crest  and  a  secondary 
one,  the  pulse  is  said  to  be  "  dicrotic  "  ;  when  two 
secondary  crests  are  prominent,  the  pulse  is  often 
called  "tricrotic";  where  several,  "  polycrotic." 
As  a  general  rule,  the  secondary  elevations  appear 
only  on  the  descending  limb  of  the  primary  wave,  as 
in  most  of  the  curves  given,  and  the  curve  is  then 
spoken  of  as  "  katacrotic."  Sometimes,  however, 
the  first  elevation  or  crest  is  not  the  highest,  but  ap- 
pears on  the  ascending  portion  of  the  main  curve; 
such  a  curve  is  spoken  of  as  "  anacrotic,"  Fig.  69. 

Of  these  secondary  elevations  the  most  frequent,  conspicuous,  and  impor- 
tant is  the  one  which  appears  some  way  down  on  the  descending  limb,  and 
is  marked  C  on  Fig.  68  and  on  most  of  the  curves  here  given.  It  is  more  or 
less  distinctly  visible  on  all  sphygmograms,  and  may  be  seen  in  those  of  the 
aorta  as  well  as  of  other  arteries.  Sometimes  it  is  so  slight  as  to  be  hardly 
discernible ;  at  other  times  it  may  be  so  marked  as  to  give  rise  to  a  really 
double  pulse  (Fig.  70),  i.  e.,  a  pulse  which  can  be  felt  as  double  by  the 

FIG.  70. 

n 


Anacrotic  Sphygmograph- 
tracing  from  the  Ascending 
Aorta.  (Aneurism.) 


Two  Grades  of  Marked  Dicrotism  in  Radial  Pulse  of  Man.    (Typhoid  fever.) 

finger ;  hence  it  has  been  called  the  dierotic  elevation  or  the  dicrotic  wave, 
the  notch  preceding  the  elevation  being  spoken  of  as  the  "  dicrotic  notch." 
Neither  it  nor  any  other  secondary  elevations  can  be  recognized  in  the 
tracings  of  blood-pressure  taken  with  a  manometer.  This  may  be  explained, 


186  THE  VASCULAR  MECHANISM. 

as  we  have  said  (§  128)  by  the  fact  that  the  movements  of  the  mercury 
column  are  too  sluggish  to  reproduce  these  finer  variations ;  but  dicrotism 
is  also  conspicuous  by  its  absence  in  the  tracings  given  by  more  delicately 
responsive  instruments.  Moreover,  when  the  normal  pulse  is  felt  by  the 
finger,  most  persons  find  themselves  unable  to  detect  any  dicrotism.  But 
that  it  does  really  exist  in  the  normal  pulse  is  shown  by  the  fact  that  it 
appears  in  a  most  unmistakable  manner  in  the  tracing  obtained  by  allow- 
ing the  blood  to  spurt  directly  from  an  opened  small  artery,  such  as  the 
dorsalis  pedis,  upon  a  recording  surface. 

Less  constant  and  conspicuous  than  the  dicrotic  wave,  but  yet  appearing 
in  most  sphygmograms,  is  an  elevation  which  appears  higher  up  on  the 
descending  limb  of  the  main  wave  ;  it  is  marked  B  in  Fig.  68,  and  on  several 
of  the  other  curves,  and  is  frequently  called  the  pre-dicrotie  wave ;  it  may 
become  very  prominent.  Sometimes  other  secondary  waves,  often  called 
"  post-dicrotic,"  are  seen  following  the  dicrotic  wave,  as  at  D  in  Fig.  68,  and 
some  other  curves ;  but  these  are  not  often  present,  and  usually,  even  when 
present,  inconspicuous. 

When  tracings  are  taken  from  several  arteries,  or  from  the  same  artery 
under  different  conditions  of  the  body,  these  secondary  waves  are  found  to 
vary  very  considerably,  giving  rise  to  many  characteristic  forms  of  pulse- 
curve.  Were  we  able  with  certainty  to  trace  back  the  several  features  of  the 
curves  to  their  respective  causes,  an  adequate  examination  of  sphygmo- 
graphic  tracings  would  undoubtedly  disclose  much  valuable  information 
concerning  the  condition  of  the  body  presenting  them.  Unfortunately,  the 
problem  of  the  origin  of  these  secondary  waves  is  a  most  difficult  and  com- 
plex one;  so  much  so,  that  the  detailed  interpretation  of  a sphygmographic 
tracing  is  still  in  most  cases  extremely  uncertain. 

§  135.  The  chief  interest  attaches  to  the  nature  and  meaning  of  the  di- 
crotic wave.  In  general,  the  main  conditions  favoring  dicrotism  are  (1)  a 
highly  extensible  and  elastic  arterial  wall,  (2)  a  comparatively  low  mean 
pressure,  leaving  the  extensible  and  elastic  reaction  of  the  arterial  wall  free 
scope  to  act,  and  (3)  a  sufficiently  vigorous  and  sufficiently  rapid  stroke  of 
the  ventricle,  and  the  discharge  of  a  large  quantity  of  blood  into  the  aorta. 
The  development  of  the  dicrotic  wave  may  probably  be  explained  as  follows  : 

At  each  beat  the  time  during  which  the  contents  of  the  left  ventricle  are 
injected  into  the  aorta  is,  as  we  have  seen  (§  125),  very  brief.  The  expan- 
sion of  the  aorta  is  very  sudden,  and  the  cessation  of  that  expansion  is  also 
very  sudden. 

Now,  when  fluid  is  being  driven  with  even  a  steady  pressure  through  an 
elastic  tube  or  a  system  of  elastic  tubes,  levers  placed  on  the  tube  wall 
describe  curves  indicating  variations  in  the  diameter  of  the  tube,  if  the  inflow 
into  the  tube  be  suddenly  stopped,  as  by  sharply  turning  a  stop-cock  ;  and  a 
comparison  of  levers  placed  at  different  distances  from  the  stop-cock  will 
show  that  these  variations  of  diameter  travel  down  the  tube  from  the  stop- 
cock in  the  form  of  waves.  The  lever  near  the  stop-cock  will  first  of  all  fall, 
but  speedily  begin  to  rise  again,  and  this  subsequent  rise  will  be  followed  by 
another  fall,  after  which  there  may  be  one  or  more  succeeding  rises  and  falls — 
that  is,  oscillations — with  decreasing  amplitudes,  until  the  fluid  comes  to  rest. 
The  levers  further  from  the  stop-cock  will  describe  curves  similar  to  the 
above  in  form  but  of  less  amplitude,  and  it  will  be  found  that  these  occur 
somewhat  later  in  time,  the  more  so  the  further  the  lever  is  from  the  stop- 
cock. Obviously  these  waves  are  generated  at  or  near  the  stop-cock,  and 
travel  thence  along  the  tubing. 

We  may  infer  that  at  each  beat  of  the  heart  similar  waves  would  be 
generated  at  the  foot  of  the  aorta  upon  the  sudden  cessation  of  the  flow  from 


THE  PULSE.  187 

the  ventricle,  and  would  travel  thence  along  the  elastic  arteries.  The  facts 
that  each  beat  is  rapidly  succeeded  by  another,  and  that  the  flow  which  sud- 
denly ceases  is  also,  by  the  nature  of  the  ventricular  stroke,  suddenly  gen- 
erated, may  render  the  waves  more  complicated,  but  will  not  change  their 
essential  nature. 

The  exact  interpretation  of  the  generation  of  these  waves  is  perhaps  not 
without  difficulty,  but  two  factors  seem  of  especial  importance.  In  the  first 
place,  as  we  have  already  more  than  once  said,  when  a  rapid  flow  is  suddenly 
stopped  a  negative  pressure  makes  its  appearance  behind  the  column  of  fluid. 
In  a  rigid  tube  this  simply  tends  to  a  reflux  of  fluid.  In  an  elastic  tube  its 
effects  are  complicated  by  the  second  factor,  the  elastic  action  and  inertia  of 
the  walls  of  the  tube.  Upon  the  sudden  cessation  of  the  flow,  the  expansion 
of  the  tube,  or  as  we  may  at  once  say,  of  the  aorta,  ceases,  the  vessel  begins  to 
shrink,  and  the  lever  placed  on  its  walls  falls,  as  from  A  onward  in  the  pulse- 
curve.  This  shrinking  is  in  part  due  to  the  elastic  reaction  of  the  walls  of 
the  aorta,  but  is  increased  by  the  "  suction  "  action  of  the  negative  pressure 
spoken  of  above.  In  thus  shrinking,  however,  under  these  combined  causes, 
the  aorta,  through  the  inertia  of  its  walls,  overshoots  the  mark,  it  is  carried 
beyond  its  natural  calibre — i.  e.,  the  diameter  it  would  possess  if  left  to  itself 
with  the  pressure  inside  and  outside  equal ;  it  shrinks  too  much  and  conse- 
quently begins  again  to  expand.  This  secondary  expansion  (taking  for 
simplicity  sake  a  pulse-curve  in  which  the  so-called  pre-dicrotic  wave,  B,  is 
absent  or  inconspicuous)  causes  the  secondary  rise  of  the  lever  up  to  C — that 
is,  the  dicrotic  rise.  In  thus  expanding  again  the  aorta  tends  to  draw  back 
toward  the  heart  the  column  of  blood  which  by  loss  of  momentum  had  come 
to  rest,  or,  indeed,  under  the  influence  of  the  negative  pressure  spoken  of 
above,  was  already  undergoing  a  reflux.  In  this  secondary  expansion,  more- 
over, the  aorta  is  by  the  inertia  of  its  walls,  aided  by  that  of  the  blood,  again 
carried,  so  to  speak,  beyond  its  mark,  so  that  no  sooner  has  it  become  ex- 
panded and  filled  with  fluid  to  a  certain  extent  than  it  again  begins  to  shrink 
as  from  C  onward.  And  this  shrinking  may  in  a  similar  manner  to  the  first 
be  followed  by  a  further  expansion  and  shrinking,  giving  rise  to  a  post- 
dicrotic  wave,  or  it  may  be  to  post-dicrotic  waves.  And  the  successive 
changes  thus  inaugurated  at  the  root  of  the  aorta  travel  as  so  many  waves 
along  the  arterial  system,  diminishing  as  they  go.  It  will  be  observed  that 
for  the  development  of  these  waves  a  certain  quality  in  the  walls  of  the  tubing 
is  necessary.  The  tube  must  be  such  as  possesses  when  at  rest  an  open  lumen  ; 
the  walls  must  be  of  such  a  kind  that  the  tube  remains  open  when  empty — 
"  e.y  when  the  atmospheric  pressure  is  equal  inside  and  outside— so  that  when 
it  shrinks  too  much  it  expands  again  in  striving  to  retain  its  natural  calibre. 

This  we  have  seen  to  be  a  characteristic  of  the  arteries.     A  collapsible 
ube  of  thin  membrane  will  not  show  the  phenomena  ;  such  a  tube  when  the 
stop-cock  is  turned  collapses  and  empties  itself,  continuing  to  be  collapsed 
without  any  effort  to  expand  again. 

In  the  above  explanation  no  mention  has  been  made  of  the  closing  of  the 
semiluuar  valves ;  we  shall  have  to  speak  of  these  a  little  later  on  in  refer- 
•ing  to  the  pre-dicrotic  wave,  and  shall  see  that,  under  the  view  we  have 
just  given,  the  closing  of  the  semilunar  valves  is  to  be  regarded  rather  as 
the  effect  than  the  cause  of  the  dicrotic  wave.  Many  authors,  however,  give 
in  interpretation  of  the  dicrotic  wave  different  from  that  detailed  above, 
"'hus,  it  is  held  that  the  primary  shrinking  from  A  onward,  being  brought 
bear  on  the  column  of  blood  already  come  to  rest,  in  face  of  the  great 
>ressure  in  front,  drives  the  blood  back  against  the  semilunar  valves,  thus 
closing  them,  and  that  the  impact  of  the  column  of  blood  against  the  valves 
irts  a  new  wave  of  expansion,  which  reinforcing  the  natural  tendency  of 


188  THE  VASCULAR  MECHANISM. 

the  elastic  walls  to  expand  again  after  their  primary  shrinking,  produces 
tho  dicrotic  wave  C.  On  this  view,  it  is  the  blood  driven  back  from  the 
valves  which  expands  the  artery  ;  on  the  view  given  above,  it  is  the  ex- 
panding artery  which  draws  the  blood  back  toward  the  valves. 

Moreover,  quite  other  views  have  been  or  are  held  concerning  this 
dicrotic  wave.  According  to  many  authors,  it  is  what  is  called  a  "  re- 
flected "  wave.  Thus,  when  the  tube  of  the  artificial  model  bearing  two 
levers  is  blocked  just  beyond  the  far  lever,  the  primary  wave  is  seen  to  be 
accompanied  by  a  second  wave,  which  at  the  far  lever  is  seen  close  to, 
and  often  fused  into,  the  primary  wave  (Fig.  64,  VI.  af),  but  at  the  near 
lever  is  at  some  distance  from  it  (Fig.  64,  I.  a'),  being  the  further  from 
it  the  longer  the  interval  between  the  lever  and  the  block  in  the  tube.  The 
second  wave  is  evidently  the  primary  wave  reflected  at  the  block  and  travel- 
ling backward  toward  the  pump.  It  thus,  of  course,  passes  the  far  lever  be- 
fore the  near  one.  And  it  has  been  argued  that  the  dicrotic  wave  of  the 
pulse  is  really  such  a  reflected  wave,  started  either  at  the  minute  arteries 
and  capillaries,  or  at  the  points  of  bifurcation  of  the  larger  arteries,  and 
travelling  backward  to  the  aorta.  But  if  this  were  the  case,  the  distance 
between  the  primary  crest  and  the  dicrotic  crest  ought  to  be  less  in  arteries 
more  distant  from  than  in  those  nearer  to  the  heart,  just  as  in  the  artificial 
scheme  the  reflected  wave  is  fused  with  a  primary  wave  near  the  block  (Fig. 
64,  VI.  6  a.  a'),  but  becomes  more  and  more  separated  from  it  the  further 
back  toward  the  pump  we  trace  it  (Fig.  64,  I.  1  a.  a'}.  Now  this  is  not  the 
case  with  the  dicrotic  wave.  Careful  measurements  show  that  the  distance 
between  the  primary  and  dicrotic  crests  is  either  greater,  or  certainly  not 
less,  in  the  smaller  "or  more  distant  arteries  than  in  the  larger  or  nearer 
ones.  This  feature  indeed  proves  that  the  dicrotic  wave  cannot  be  due  to 
reflection  at  the  periphery,  or,  indeed,  in  any  way  a  retrograde  wave.  Be- 
sides, the  multitudinous  peripheral  division  would  render  one  large  periph- 
erally reflected  wave  impossible.  Again,  the  more  rapidly  the  primary 
wave,  is  obliterated,  or  at  least  diminished,  on  its  way  to  the  periphery,  the 
less  conspicuous  should  be  the  dicrotic  wave.  Hence  increased  extensibility 
and  increased  elastic  reaction  of  the  arterial  walls  which  tend  to  use  up 
rapidly  the  primary  wave,  should  also  lessen  the  dicrotic  wave.  But  as  a 
matter  of  fact  these  conditions,  as  we  have  said,  are  favorable  to  the  promi- 
nence of  the  dicrotic  wave. 

On  the  other  hand,  these  and  the  other  conditions-  which  favor  dicrotism 
in  the  pulse  are  exactly  those  which  would  favor  such  a  development  of 
secondary  waves  as  has  been  described  above,  and  their  absence  would  be 
unfavorable  to  the  occurrence  of  such  waves.  Thus  dicrotism  is  less  marked 
in  rigid  arteries  (such  as  those  of  old  people)  than  in  healthy  elastic  ones ; 
the  rigid  wall  neither  expands  so  readily  nor  shrinks  so  readily,  and  hence 
does  not  so  readily  give  rise  to  such  secondary  waves.  Again,  dicrotism  is 
more  marked  when  the  mean  arterial  pressure  is  low  than  when  it  is  high  ; 
indeed,  dicrotism  may  be  induced  when  absent,  or  increased  when  slightly 
marked  by  diminishing,  in  one  way  or  another,  the  mean  pressure.  Now, 
when  the  pressure  is  high,  the  arteries  are  kept  continually  much  expanded, 
and  are  therefore  the  less  capable  of  further  expansion  ;  that  is  to  say,  are, 
so  far,  more  rigid.  Hence  the  additional  expansion  due  to  the  systole  is  not 
very  great ;  there  is  a  less  tendency  for  the  arterial  walls  to  swing  backward 
and  forward,  so  to  speak,  and  hence  a  less  tendency  to  the  development  of 
secondary  waves.  When  the  mean  pressure  is  low,  the  opposite  state  of 
things  exists ;  supposing,  of  course,  that  the  ventricular  stroke  is  adequately 
vigorous  (the  low  pressure  being  due,  not  to  diminished  cardiac  force,  but  to 
diminished  peripheral  resistance),  the  relatively  empty  but  highly  distensible 


THE  PULSE.  189 

artery  is  rapidly  expanded,  and,  falling  rapidly  back,  enters  upon  a  second- 
ary (dicrotic)  expansion,  and  even  a  third. 

Moreover,  the  same  principles  may  be  applied  to  explain  why  sometimes 
dicrotisrn  will  appear  marked  in  a  particular  artery  while  it  remains  little 
marked  in  the  rest  of  the  system.  In  experimenting  with  an  artificial  tubing 
such  as  the  arterial  model,  the  physical  characters  of  which  remain  the 
same  throughout,  both  the  primary  and  the  secondary  waves  retain  the  same 
characters  as  they  travel  along  the  tubing,  save  only  that  both  gradually 
diminish  toward  the  periphery  ;  and  in  the  natural  circulation,  when  the 
vascular  conditions  are  fairly  uniform  throughout,  the  pulse-curve,  as  a  rule, 
possesses  the  same  general  characters  throughout,  save  that  it  is  gradually 
"  damped  off."  But  suppose  we  were  to  substitute  for  the  first  section  of  the 
tubing  a  piece  of  perfectly  rigid  tubing  ;  this  at  the  stroke  of  the  pump,  on 
account  of  its  being  rigid,  would  show  neither  primary  nor  secondary  ex- 
pansion, but  the  expanding  force  of  the  pump's  stroke  would  be  transmitted 
through  it  to  the  second  elastic  section,  and  here  the  primary  and  secondary 
waves  would  at  once  become  evident.  This  is  an  extreme  case,  but  the  same 
thing  would  be  seen  to  a  less  degree  in  passing  from  a  more  rigid,  that  is, 
less  extensible  and  elastic  section,  to  a  less  rigid,  more  extensible  and  elastic 
section  ;  the  primary  and  secondary  expansions,  in  spite  of  the  general  damp- 
ing effect,  would  suddenly  increase.  Similarly  in  the  living  body  a  pulse- 
curve  which,  so  long  as  it  is  travelling  along  arteries  in  which  the  mean 
pressure  is  high,  and  which  are  therefore  practically  somewhat  rigid,  is  not 
markedly  dicrotic,  may  become  very  markedly  dicrotic  when  it  comes  to  a 
particular  artery  in  which  the  mean  pressure  is  low  (and  we  shall  see  pres- 
ently that  such  a  case  may  occur),  and  the  walls  of  which  are  therefore  for 
the  time  being  relatively  more  distensible  than  the  rest. 

Lastly,  we  may  recall  the  observation  made  above  (§  130),  that  the  curve 
of  expansion  of  an  elastic  tube  is  modified  by  the  pressure  exerted  by  the 
lever  employed  to  record  it,  and  that  hence,  in  the  same  artery  and  with  the 
same  instrument,  the  size,  form,  and  even  the  special  features  of  the  curve 
vary  according  to  the  amount  of  pressure  with  which  the  lever  is  pressed 
upon  the  artery.  Accordingly  the  amount  of  dicrotism  apparent  in  a  pulse 
may  be  modified  by  the  pressure  exerted  by  the  lever.  In  Fig.  64,  for 
instance,  the  dicrotic  wave  is  more  evident  in  the  middle  than  in  the  upper 
tracing. 

§  136.  The  pre-dicrotic  wave  (marked  B  on  Fig.  68,  and  on  several 
other  of  the  pulse-curves),  which  precedes  the  dicrotic  wave  and  is  still 
more  variable  than  that  wave,  being  sometimes  slight  or  even  invisible  and 
sometimes  conspicuous,  has  given  rise  to  much  controversy.  In  the  inter- 
pretation of  the  dicrotic  wave  given  in  the  preceding  paragraph  it  was 
stated  that  the  negative  pressure  developed  on  the  cessation  of  the  flow  in  the 
rear  of  the  column  of  blood,  led  by  itself  to  a  reflux  toward  the  ventricle  ; 
and  it  has  been  suggested  that  this  reflux  meeting  and  closing  the  semi- 
lunar  valves  starts  a  small  wave  of  expansion  before  the  larger  dicrotic 
wave  has  had  time  to  develop  itself.  On  this  view  the  semilunar  valves 
would  be  actually  closed  before  the  occurrence  of  the  secondary  dicrotic 
expansion  of  the  arterial  walls,  though  the  larger,  more  powerful  reflux  of 
this  later  event  must  render  the  closure  more  complete,  and  in  doing  so 
possibly  gives  rise  to  the  second  sound.  According,  however,  to  the  second 
view  given  in  the  same  paragraph,  which  regards  the  reflux  due  to  the 
shrinking  of  the  artery  in  face  of  the  great  pressure  in  front  as  firmly 
closing  the  semilunar  valves,  and  as  thus  starting  the  secondary  dicrotic 
wave  of  expansion,  the  firm  closing  of  the  semilunar  valves  must  take  place 
before  the  beginning,  riot  during  the  development  of  the  dicrotic  wave  ;  it 


190  THE  VASCULAR  MECHANISM. 

is  still  possible,  however,  even  on  this  view,  as  on  the  other,  to  suppose  that 
an  antecedent  reflux,  due  to  the  negative  pressure  succeeding  the  cessation 
of  flow  from  the  ventricle,  closes  the  valves  and  starts  the  pre-dicrotic  wave. 
But  the  matter  is  one  not  yet  beyond  the  stage  of  controversy. 

§  137.  In  an  anacrotic  pulse  the  first  rise  is  not  the  highest,  but  a 
second  rise  (B,  Fig.  69)  which  follows  and  is  separated  from  it  by  a  notch 
is  higher  than,  or  at  least  as  high  as,  itself.  Such  an  anacrotic  wave,  though 
it  may  sometimes  be  produced  temporarily  in  healthy  persons,  is  generally 
associated  with  diseased  conditions,  usually  such  in  which  the  arteries  are 
abnormally  rigid.  In  describing  the  ventricular  systole,  we  spoke  of  the 
pressure  within  the  ventricle  as  reaching  its  maximum  just  before  the 
opening  of  the  semilunar  valves ;  and  this  is  apparently  the  normal  event ; 
but  there  are  curves  which  seem  to  show  that  after  the  first  sudden  rise  of 
pressure  which  opens  the  valves,  followed  by  a  brief  lessening  of  pressure, 
which  appears  on  the  curve  as  a  notch,  the  pressure  may  again  rise,  and 
that  to  a  point  higher  than  before.  And  a  similar  curve  is  sometimes  de- 
scribed by  the  front-to-back  diameter  of  the  ventricle.  The  systole  opens 
the  valve  as  it  were  with  a  burst ;  this  is  followed  by  a  slight  relapse, 
and  then  the  systole,  strengthening  again,  discharges  the  whole  of  the  ven- 
tricular contents  into  the  aorta  and  so  brings  about  a  tardy  maximum  ex- 
pansion. And  what  is  thus  started  in  the  aorta  travels  onward  over  the 
arterial  system.  It  is  difficult  to  see  how  these  anacrotic  events  can  be  pro- 
duced, except  by  a  certain  irregularity  in  the  ventricular  systole ;  and, 
indeed,  the  anacrotic  pulse  is  frequently  associated  with  some  disease  or 
defect  of  the  ventricle. 

§138.  Venous  pulse.  Under  certain  circumstances  the  pulse  may  be 
carried  on  from  the  arteries  through  the  capillaries  into  the  veins.  Thus, 
as  we  shall  see  later  on,  when  the  salivary  gland  is  actively  secreting,  the 
blood  may  issue  from  the  gland  through  the  veins  in  a  rapid  pulsating 
stream.  The  nervous  events  which  give  rise  to  the  secretion  of  saliva,  lead 
at  the  same  time,  by  the  agency  of  vasomotor  nerves,  of  which  we  will  pre- 
sently speak,  to  a  dilatation  of  the  small  arteries  of  the  gland.  When 
the  gland  is  at  rest  the  minute  arteries  are,  as  we  shall  see,  somewhat  con- 
stricted and  narrowed,  and  thus  contribute  largely  to  the  peripheral  resist- 
ance in  the  part ;  this  peripheral  resistance  throws  into  action  the  elastic 
properties  of  the  small  arteries  leading  to  the  gland,  and  the  remnant  of 
the  pulse  reaching  these  arteries  is,  as  we  before  explained,  finally  de- 
stroyed. When  the  minute  arteries  are  dilated,  their  widened  channels 
allow  the  blood  to  flow  more  easily  through  them  and  with  less  friction  ;  the 
peripheral  resistance  which  they  normally  offer  is  thus  lessened.  In  conse- 
quence of  this  the  elasticity  of  the  walls  of  the  small  arteries  is  brought 
into  play  to  a  less  extent  than  before,  and  these  small  arteries  cease  to  do 
their  share  in  destroying  the  pulse  which  comes  down  to  them  from  the 
larger  arteries.  As  in  the  case  of  the  artificial  model,  where  the  "  periph- 
eral "  tubing  is  kept  open,  not  enough  elasticity  is  brought  into  play  to 
convert  the  intermittent  arterial  flow  into  a  continuous  one,  and  the  pulse 
which  reaches  the  arteries  of  the  gland  passes  on  through  them  and  through 
the  capillaries,  arid  is  continued  on  into  the  veins.  A  similar  venous  pulse 
is  also  sometimes  seen  in  other  organs. 

Careful  tracings  of  the  great  veins  in  the  neighborhood  of  the  heart 
show  elevations  and  depressions,  which  appear  due  to  the  variations  of  intra- 
cardiac  (auricular)  pressure,  and  which  may,  perhaps,  be  spoken  of  as 
constituting  a  "  venous  pulse,"  though  they  have  a  quite  different  origin 
from  the  venous  pulse  just  described  in  the  salivary  gland  ;  but  at  present 
they  need  further  elucidation.  In  cases,  however',  of  insufficiency  of  the 


THE  VASCULAE   MECHANISM.  191 

tricuspid  valves,  the  systole  of  the  ventricle  makes  itself  distinctly  felt  in 
the  great  veins ;  and  a  distention  travelling  backward  from  the  heart 
becomes  very  visible  in  the  veins  of  the  neck.  This  is  sometimes  spoken  of 
as  a  venous  pulse. 

Variations  of  pressure  in  the  great  veins,  due  to  the  respiratory  move- 
ments, are  also  sometimes  spoken  of  as  a  venous  pulse ;  the  nature  of  these 
variations  will  be  explained  in  treating  of  respiration. 

THE  REGULATION  AND  ADAPTATION  OF  THE  VASCULAR  MECHANISM. 

The   Regulation  of  the  Beat  of  the  Heart. 

§  139.  So  far  the  facts  with  which  we  have  had  to  deal,  with  the  excep- 
tion of  the  heart's  beat  itself,  have  been  simply  physical  facts.  All  the 
essential  phenomena  which  we  have  studied  may  be  reproduced  on  a  dead 
model.  Such  an  unvarying  mechanical  vascular  system  would,  however, 
be  useless  to  a  living  body  whose  actions  were  at  all  complicated.  The 
prominent  feature  of  a  living  mechanism  is  the  power  of  adapting  itself  to 
changes  in  its  internal  and  external  circumstances. 

The  vascular  mechanism  in  all  animals  in  which  it  is  present  is  capable 
of  local  and  general  modifications,  adapting  it  to  local  and  general  changes 
of  circumstance.  These  modifications  fall  into  two  great  classes : 

1.  Changes  in  the  heart's  beat.     These,  being  central,  have,  of  course,  a 
general  effect;  they  influence  or  may  influence  the  whole  body. 

2.  Changes  in  the  peripheral  resistance,  due  to  variations  in  the  calibre  of 
the  minute  arteries,  brought  about  by  the  agency  of  their  contractile  mus- 
cular coats.     These  changes  may  be  either  local,  affecting  a  particular  vas- 
cular area  only,  or  general,  affecting  all  or  nearly  all  the  bloodvessels  of 
the  body. 

These  two  classes  of  events  are  chiefly  governed  by  the  nervous  system. 
It  is  by  means  of  the  nervous  system  that  the  heart's  beat  and  the  calibre 
of  the  minute  arteries  are  brought  into  relation  with  each  other,  and  with 
almost  every  part  of  the  body.  It  is  by  means  of  the  nervous  system  acting 
either  on  the  heart  or  on  the  small  arteries,  or  on  both,  that  a  change  of  cir- 
cumstances affecting  either  the  whole  or  a  part  of  the  body  is  met  by  com- 
pensating or  regulative  changes  in  the  flow  of  blood.  It  is  by  means  of  the 
nervous  system  that  an  organ  has  a  more  full  supply  of  blood  when  at  work 
than  when  at  rest,  that  the  tide  of  blood  through  the  skin  rises  and  ebbs 
with  the  rise  and  fall  of  the  temperature  of  the  air,  that  the  work  of  the 
heart  is  tempered  to  meet  the  strain  of  overfull  arteries,  and  that  the  arterial 
gates  open  and  shut  as  the  force  of  the  central  pump  waxes  and  wanes.  The 
study  of  these  changes  becomes,  therefore,  to  a  large  extent  a  study  of 
nervous  actions. 

The  circulation  may  also  be  modified  by  events  not  belonging  to  either  of 
the  above  two  classes.  Thus,  in  this  or  that  peripheral  area,  changes  in  the 
capillary  walls  and  the  walls  of  the  minute  arteries  and  veins  may  lead  to 
an  increase  of  the  tendency  of  the  blood  corpuscles  to  adhere  to  the  vascular 
walls,  and  so,  quite  apart  from  any  change  in  the  calibre  of  the  bloodvessels, 
may  lead  to  increase  of  the  peripheral  resistance.  This  is  seen  in  an  extreme 
case  in  inflammation,  but  may  possibly  intervene  to  a  less  extent  in  the 
ordinary  condition  of  the  circulation,  and  may  also  be  under  the  influence 
of  the  nervous  system.  Further,  any  decided  change  in  the  quantity  of 
blood  actually  in  circulation  must  also  influence  the  working  of  the  vascular 
mechanism.  But  both  these  changes  are  unimportant  compared  with  the 
other  two  kinds  of  changes.  Hence,  the  two  most  important  problems  for 
us  to  study  are,  1,  how  the  nervous  system  regulates  the  beat  of  the  heart, 


192  THE  VASCULAR  MECHANISM. 

and  2,  how  the  nervous  system  regulates  the  calibre  of  the  bloodvessels.    We 
will  first  consider  the  former  problem. 

The  Development  of  the  Normal  Seat. 

|  140.  The  heart  of  a  mammal,  or  of  a  warm-blooded  animal,  generally 
ceases  to  beat  within  a  few  minutes  after  being  removed  from  the  body  in 
the  ordinary  way,  the  hearts  of  newly-born  animals  continuing,  however,  to 
beat  for  a  longer  time  than  those  of  adults.  Hence,  though  by  special  pre- 
caution and  by  means  of  an  artificial  circulation  of  blood,  an  isolated 
mammalian  heart  may  be  preserved  in  a  pulsating  condition  for  a  much 
longer  time,  our  knowledge  of  the  exact  nature  and  of  the  causes  of  the 
cardiac  beat  is  as  yet  very  largely  based  on  the  study  of  the  hearts  of  cold- 
blooded animals,  which  will  continue  to  beat  for  hours,  or  under  favorable 
circumstances  even  for  days,  after  they  have  been  removed  from  the  body 
with  only  ordinary  care.  *  We  have  reason  to  think  that  the  mechanism  by 
which  the  beat  is  carried  on  varies  in  some  of  its  secondary  features  in  differ- 
ent kinds  of  animals ;  that  the  heart,  for  instance,  of  the  eel,  the  snake,  the 
tortoise,  and  the  frog,  differ  in  some  minor  details  of  behavior,  both  from 
each  other  and  from  the  bird  and  the  mammal ;  but  we  may,  at  first  at  all 
events,  take  the  heart  of  the  frog  as  illustrating  the  main  and  important 
truths  concerning  the  causes  and  mechanism  of  the  beat. 

In  studying  closely  the  phenomena  of  the  beat  of  the  heart  it  becomes  necessary 
to  obtain  a  graphic  record  of  various  movements. 

1.  In  the  frog  or  other  cold-blooded  animal,  a  light  lever  may  be  placed  directly 
on  the  ventricle  (or  on  an  auricle,  etc.),  and  changes  of  form,  due  either  to  disten- 
tion  by  the  influx  of  blood  or  to  the  systole,  will  cause  movements  of  the  lever, 
which  may  be  recorded  on  a  travelling  surface.     The  same  methods,  as  we  have 
seen,  may  be  applied  to  the  mammalian  heart. 

2.  Or,  as  in  Gaskell's  method,  the  heart  may  be  fixed  by  a  clamp  carefully  ad- 
justed around  the  auriculo- ventricular  groove,  while  the  apex  of  the  ventricle  and 
some  portion  of  one  auricle  are  attached  by  threads  to  horizontal  levers  placed 
respectively  above  and  below  the  heart.     The  auricle  and  the  ventricle  each  in  its 
systole  pulls  at  the  lever  attached  to  it,  and  the  times  and  extent  of  the  contrac- 
tions may  thus  be  recorded. 

3.  A  record  of  endocardiac  pressure  may  he  taken  in  the  frog  or  tortoise,  as  in 
the  mammal,  by  means  of  an  appropriate  manometer.     And  in  these  animals,  at 
all  events,  it  is  easy  to  keep  up  an  artificial  circulation.     A  canula  is  introduced 
into  the  sinus  venosus,  and  another  into  the  ventricle  through  the  aorta.     Serum 
or  dilute  blood  (or  any  other  fluid  which  it  may  be  desire.d  to  employ)  is  driven  by 
moderate  pressure  through  the  former ;  to  the  latter  is  attached  a  tube  connected 
by  means  of  a  side  piece  with  a  small  mercury  manometer.    So  long  as  the  exit-tube 
is  open  at  the  end  fluid  flows  freely  through  the  heart  and  apparatus.    Upon  closing 
the  exit-tube  at  its  far  end  the  force  of  the  ventricular  systole  is  brought  to  bear  on 
the  manometer,  the  index  of  which  registers  in  the  usual  way  the  movements  of  the 
mercury  column.     Newell  Martin  has  succeeded  in  applying  a  modification  of  this 
method  to  the  mammalian  heart. 

4.  The  movements  of  the  ventricle  may  be  registered  by  introducing  into  it 
through  the  auricnlo-ventricular  orifice  a  so-called  "  perfusion  "  canula.  Figs.  71 
and  72,  1.,  with  a  double  tube,  one  inside  the  other,  and  tying  the  ventricle  on  to 
the  canula  at  the  auricnlo-ventricular  groove,  or  at  any  level  below  that  which  may 
be  desired.     The  blood  or  other  fluid  is  driven  at  an  adequate  pressure  through  the 
tube  a.  enters  the  ventricle,  and  returns  by  the  tube  b.     If  b  be  connected  with  a 
manometer  as  in  method  3,  the  movements  of  the  ventricle  may  be  registered. 

5.  In  the  apparatus  of  Roy,  Fig.  72,  II.,  the  exit-tube  is  free,  but  the  ventricle 
(the  same  method  may  be  adopted  for  the  whole  heart)  is  placed  in  an  air-tight 
chamber  filled  with  oil,  or  partly  with  normal  saline  solution  and  partly  with  oil. 
By  means  of  the  tube  b  the  interior  of  the  chamber  a  is  continuous  with  that  of  a 
small  cylinder  c  in  which  a  piston  <7,  secured  by  a  thin  flexible  animal  membrane 


THE   VASCULAR  MECHANISM. 


193 


FIG.  71. 


works  up  and  down.  The  piston  again  bears  on  a  lever  e  by  means  of  which 
its  movements  may  be  registered.  When  the  ventricle  con- 
tracts, and  by  contracting  diminishes  in  volume,  there  is  a 
lessening  of  pressure  in  the  interior  of  the  chamber ;  this  is 
transmitted  to  the  cylinder,  and  the  piston  correspondingly 
rises,  carrying  with  it  the  lever.  As  the  ventricle  subsequently 
becomes  distended  the  pressure  in  the  chamber  is  increased,  and 
the  piston  and  lever  sink.  In  this  way  variations  in  the  volume 
of  the  ventricle  may  be  recorded,  without  any  great  interference 
with  the  flow  of  blood  or  fluid  through  it. 


The  heart  of  the  frog,  as  we  have  just  said,  will  con- 
tinue to  beat  for  hours  after  removal  from  the  body,  even 
after  the  cavities  have  been  cleared  of  blood,  and,  indeed, 
when  they  are  almost  empty  of  all  fluid.     The  beats  thus 
carried  out  are  in  all  important  respects  identical  with 
the  beats  executed  by  the  heart  in  its  normal  condition 
within  the  living  body.     Hence  we  may  infer  that  the    A  perfusion  Camiia. 
beat  of  the  heart  is  an  automatic  action ;  the  muscular 
contractions  which   constitute  the  beat  are  due  to  causes  which  arise  spon- 
taneously in  the  heart  itself. 

In  the  frog's  heart,  as  in  that  of  the  mammal,  §115,  there  is  a  distinct 
sequence  of  events  which  is  the  same  whether  the  heart  be  removed  from,  or 


Purely  Diagrammatic  Figures  of— I.  Perfusion  canula  tied  into  frog's  ventricle :  a,  entrance : 
b,  exit-tube  ;  A,  wall  of  ventricle  ;  B,  ligature. 

II.  Roy's  apparatus  modified  by  Gaskill ,  a,  chamber  filled  with  saline  solution  and  oil,  con- 
taining the  ventricle  A  tied  on  to  perfusion  canula  /;  6,  tube  leading  to  cylinder  c,  in  which 
moves  piston  d,  working  the  lever  e. 

be  still  in  its  normal  condition  within,  the  body.  First  comes  the  beat  of 
the  sinus  venosus,  preceded  by  a  more  or  less  peristaltic  contraction  of  the 
large  veins  leading  into  it,  next  follows  the  sharp  beat  of  the  two  auricles 
together,  then  comes  the  longer  beat  of  the  ventricle,  and  lastly,  the  cycle  is 
completed  by  the  beat  of  the  bulbus  arteriosus,  which  does  not,  like  the 
mammalian  aorta,  simply  recoil  by  elastic  reaction  after  distention  by  the 
ventricular  stroke,  but  carries  out  a  distinct  muscular  contraction  passing  in 
a  wave  from  the  ventricle  outward. 

When  the  heart  in  dying  ceases  to  beat,  the  several  movements  cease, 
as  a  rule,  in  an  order  the  inverse  of  the  above.     Omitting  the  bulbus  arte- 


194 


THE  VASCULAR  MECHANISM. 


riosus,  which  sometimes  exhibits  great  rhythmical  power,  we  may  say  that 
first  the  ventricle  fails,  then  the  auricles  "fail,  and  lastly  the  sinus  venosus 
fails. 

The  heart  after  it  has  ceased  to  beat  spontaneously  remains  for  some 
time  irritable — that  is,  capable  of  executing  a  beat,  or  a  short  series  of 
beats,  when  stimulated  either  mechanically,  as  by  touching  it  with  a  blunt 
needle  or  electrically  by  an  induction  shock  or  in  other  ways.  The  artificial 
beat  so  called  forth  may  be  in  its  main  features  identical  with  the  natural 
beat,  all  the  divisions  of  the  heart  taking  part  in  the  beat,  and  the  sequence 
of  events  being  the  same  as  in  the  natural  beat.  Thus  when  the  sinus  is 
pricked  the  beat  of  the  sinus  may  be  followed  by  a  beat  of  the  auricles  and 
of  the  ventricle ;  and  even  when  the  ventricle  is  stimulated,  the  directly 
following  beat  of  the  ventricle  may  be  succeeded  by  a  complete  beat  of  the 
whole  heart. 

Under  certain  circumstances,  however,  the  division  directly  stimulated 
is  the  only  one  to  beat;  when  the  ventricle  is  pricked  for  instance  it  alone 
beats,  or  when  the  sinus  is  pricked  it  alone  beats.  The  results  of  stimulation, 
moreover,  may  differ  according  to  the  condition  of  the  heart  and  according 
to  the  particular  spot  to  which  the  stimulus  is  applied. 

With  an  increasing  loss  of  irritability,  the  response  to  stimulation  ceases 
in  the  several  divisions  in  the  same  order  as  that  of  the  failure  of  the  natural 
beat — the  ventricle  ceases  to  respond  first,  then  the  auricles,  and  lastly  the 
sinus  venosus,  which  frequently  responds  to  stimulation  long  after  the  other 
divisions  have  ceased  to  make  any  sign. 

It  would  appear  as  if  the  sinus  venosus,  auricles,  and  ventricle  formed  a 
descending  series  in  respect  to  their  irritability  and  to  the  power  they  possess 
of  carrying  on  spontaneous  rhythmic  beats,  the  sinus  being  the  most  potent. 
This  is  also  seen  in  the  following  experiments : 

In  order  that  the  frog's  heart  may  beat  after  removal  from  the  body  with 
the  nearest  approach  in  rapidity,  regularity,  and  endurance  to  the  normal 
condition,  the  removal  must  be  carried  out  so  that  the  excised  heart  still 
retains  the  sinus  venosus  intact. 

When  the  incision  is  carried  through  the  auricles  so  as  to  leave  the  sinus 
venosus  behind  in  the  body,  the  sinus  venosus  beats  forcibly  and  regularly, 
having  suffered  hardly  any  interruption  from  the  operation.  The  excised 
heart,  however,  remains  in  the  majority  of  cases  for  some  time  motionless. 
Stimulated  by  a  prick  or  an  induction-shock,  it  will  perhaps  give  one,  two,  or 
several  beats,  and  then  come  to  rest.  In  the  majority  of  cases,  however,  the 
animal  having  previously  been  in  a  vigorous  condition,  it  will  after  a  while 
recommence  its  spontaneous  beating,  the  systole  of  the  ventricle  following 
that  of  the  auricles ;  but  the  rhythm  of  beat  will  not  be  the  same  as 
that  of  the  sinus  venosus  left  in  the  body,  but  will  be  slower,  and  the  beats 
will  not  continue  to  go  on  for  so  long  a  time  as  will  those  of  a  heart  still  re- 
taining the  sinus  venosus. 

When  the  incision  is  carried  through  the  auriculo-ventricular  groove,  so 
as  to  leave  the  auricles  and  sinus  venosus  within  the  body,  and  to  isolate  the 
ventricle  only,  the  results  are  similar  but  more  marked.  The  sinus  and 
auricles  beat  regularly  and  vigorously,  with  their  proper  sequence,  but  the 
ventricle,  after  a  few  rapid  contractions  due  to  the  incision  acting  as  a  stimu- 
lus, generally  remains  for  a  long  time  quiescent.  When  stimulated,  how- 
ever, the  ventricle  will  give  one,  two,  or  several  beats,  and  after  a  while,  in 
many  cases  at  least,  will  eventually  set  up  a  spontaneous  pulsation  with  an 
independent  rhythm  ;  and  this  may  last  for  some  considerable  time,  but  the 
beats  are  not  so  regular  and  will  not  go  on  for  so  long  a  time  as  will  those 
of  a  ventricle  to  which  the  auricles  are  still  attached. 


THE  VASCULAR  MECHANISM.  195 

If  a  transverse  incision  be  carried  through  the  ventricle  at  about  its  upper 
third,  leaving  the  base  of  the  ventricle  still  attached  to  the  auricles,  the  por- 
tion of  the  heart  left  in  the  body  will  go  on  pulsating  regularly,  with  the 
ordinary  sequence  of  sinus,  auricles,  ventricle,  but  the  isolated  lower  two- 
thirds  of  the  ventricle  will  not  beat  spontaneously  at  all,  however  long  it  be 
left.  Moreover,  in  response  to  a  single  stimulus  such  as  an  induction-shock 
or  a  gentle  prick  it  gives,  not,  as  in  the  case  of  the  entire  ventricle  when 
stimulated  at  the  base  or  of  the  ventricle  to  which  the  auricles  are  attached, 
a  series  of  beats,  but  a  single  beat. 

Lastly,  to  complete  the  story,  we  may  add  that  when  the  heart  is  bisected 
longitudinally,  each  half  continues  to  beat  spontaneously,  with  an  indepen- 
dent rhythm,  so  that  the  beats  of  the  two  halves  are  not  necessarily  syn- 
chronous, and  this  continuance  of  spontaneous  pulsations  after  longitudinal 
bisection  may  be  seen  in  the  conjoined  auricle  and  ventricle,  or  in  the  iso- 
lated auricles,  or  in  the  isolated  but  entire  ventricle.  Moreover,  the  auricles 
may  be  divided  in  many  ways  and  yet  many  of  the  segments  will  continue 
beating;  small  pieces  even  may  be  seen  under  the  microscope  pulsating, 
feebly,  it  is  true,  but  distinctly  and  rhythmically. 

In  these  experiments,  then,  the  various  parts  of  the  frog's  heart  also 
form,  as  regards  the  power  of  spontaneous  pulsation,  a  descending  series : 
sinus  venosus,  auricles,  entire  ventricle,  lower  portions  of  ventricle,  the  last 
-exhibiting  under  ordinary  circumstances  no  spontaneous  pulsations  at  all. 

§  141.  Now  we  have  seen  (§  139)  that  these  parts  form  to  a  certain 
•extent  a  similar  descending  series  as  regards  the  presence  of  ganglia ;  at 
least  so  far  that  the  ganglia  are  very  numerous  in  the  sinus  venosus,  that 
they  occur  in  the  auricles,  and  that  while  Bidder's  ganglia  are  present  at 
the  junction  of  the  ventricle  with  the  auricles,  ganglia  are  wholly  absent 
from  the  rest  of  the  ventricle.  Hence,  on  the  assumption  (which  we  have 
already,  §  96,  seen  reason  to  doubt)  that  the  nerve  cells  of  ganglia  are  sim- 
iilar  in  general  functions  to  the  nerve  cells  of  the  central  nervous  system,  the 
view  very  naturally  presents  itself  that  the  rhythmic  spontaneous  beat  of 
the  heart  of  the  frog  is  due  to  the  spontaneous  generation  in  the  ganglionic 
nerve  cells  of  rhythmic  motor  impulses,  which,  passing  down  to  the  muscular 
fibres  of  the  several  parts,  cause  rhythmic  contractions  of  these  fibres, 
the  sequence  and  coordination  of  the  beating  of  the  several  divisions  of  the 
heart  being  the  result  of  a  coordination  between  the  several  ganglia  in  re- 
gard to  the  generation  of  impulses.  Under  this  view  the  cardiac  muscular 
fibre  simply  responds  to  the  motor  impulses  reaching  it  along  its  motor  nerve 
iibre  in  the  same  way  as  the  skeletal  muscular  fibre  responds  to  the  motor 
impulses  reaching  it  along  its  motor  nerve  fibre  ;  in  both  cases  the  muscular 
iibre  is,  as  it  were,  a  passive  instrument  in  the  hands  of  the  motor  nerve,  or 
rather  of  the  nervous  centre  (ganglion  or  spinal  cord)  from  which  the 
motor  nerve  proceeds.  And  the  view,  thus  based  on  the  fact  of  the  frog's 
heart,  has  been  extended  to  the  hearts  of  (vertebrate)  animals  generally. 

There  are  reasons,  however,  which  show  that  this  view  is  not  tenable. 

For  instance,  the  lower  two-thirds,  or  lower  third,  or  even  the  mere  tip 
of  the  frog's  ventricle — that  is  to  say,  parts  which  are  admitted  not  to  con- 
tain nerve  cells — may,  by  special  means,  be  induced  to  carry  on  for  a  con- 
siderable time  a  rhythmic  beat,  which  in  its  main  features  is  identical  with 
the  spontaneous  beat  of  the  ventricle  of  the  intact  heart.  If  such  a  part 
of  the  frog's  ventricle  be  tied  on  to  the  end  of  a  perfusion  canula  (Fig.  71), 
the  portion  of  the  ventricular  cavity  belonging  to  the  part  may  be  ade- 
quately distended  and  at  the  same  time  be  "  fed  "  with  a  suitable  fluid, 
such  as  blood,  made  to  flow  through  the  canula  ;  it  will  then  be  found  that 
the  portion  of  ventricle  so  treated  will,  after  a  preliminary  period  of  qui- 


196  THE  VASCULAR  MECHANISM. 

escence,  commence  to  beat,  apparently  spontaneously,  and  will  continue  so 
beating  for  a  long  period  of  time.  It  may  be  said  that  in  this  case  the  dis- 
tention  of  the  cavity  and  the  supply  of  blood  or  other  fluid  acts  as  a  stim- 
ulus ;  but  if  so  the  stimulus  is  a  continuous  one,  or  at  least  not  a  rhythmic 
one,  and  yet  the  beat  is  most  regularly  rhythmic. 

Then,  again,  the  reluctance  of  the  ventricle  to  execute  spontaneous 
rhythmic  beats  is  to  a  certain  extent  peculiar  to  the  frog.  The  ventricle  of 
the  tortoise,  for  instance,  the  greater  part  of  the  substance  of  which  is  as 
free  from  nerve  cells  as  is  that  of  the  i'rog,  will  beat  spontaneously  when 
isolated  from  the  auricles  with  great  ease  and  for  a  long  time.  Further,  a 
mere  strip  of  this  ventricular  muscle  tissue,  if  kept  gently  extended  and 
continually  moistened  with  blood  or  other  suitable  fluid,  will  continue  to 
beat  spontaneously  with  very  great  regularity  for  hours,  or  even  days, 
especially  if  the  series  be  started  by  the  preliminary  application  of  induc- 
tion-shocks rhythmically  repeated. 

In  connection  with  this  question  we  may  call  attention  to  the  fact  that 
the  cardiac  muscular  fibre  is  not  wholly  like  the  skeletal  muscular  fibre ;  in 
many  respects  the  contraction  or  beat  of  the  former  is  in  its  very  nature 
different  from  the  contraction  of  the  latter  ;  the  former  cannot  be  considered, 
like  the  latter,  a  mere  instrument  in  the  hands  of  the  motor  nerve  fibre. 
The  features  of  the  beat  or  contraction  of  cardiac  muscle  maybe  studied  on 
the  isolated  and  quiescent  ventricle,  or  part  of  the  ventricle,  of  the  frog. 
When  such  a  ventricle  is  stimulated  by  a  single  stimulus,  such  as  a  single 
induction-shock  or  a  single  touch  with  a  blunt  needle,  a  beat  may  or  may 
not  result.  If  it  follows  it  resembles,  in  all  its  general  features  at  least, 
a  spontaneous  beat.  Between  the  application  of  the  stimulus  and  the  first 
appearance  of  any  contraction  is  a  very  long  latent  period,  varying  accord- 
ing to  circumstances,  but  in  a  vigorous  fresh  frog's  ventricle  being  about 
0.3  second.  The  beat  itself  lasts  a  variable  but  considerable  time,  rising 
slowly  to  a  maximum  and  declining  slowly  again.  Of  course,  when  the  beat 
is  recorded  by  means  of  a  light  lever  placed  on  the  ventricle,  what  the 
tracing  shows  is  really  the  increase  in  the  front-to-back  diameter  of  the 
ventricle  during  the  beat — that  is  to  say,  one  of  the  results  of  the  contrac- 
tion of  the  cardiac  fibres — and  gives,  in  an  indirect  manner  only,  the  extent 
of  the  contraction  of  the  fibres  themselves ;  and  the  same  is  the  case  with 
the  other  methods  of  recording  the  movements  of  the  whole  ventricle.  We 
may,  however,  study  in  a  more  direct  way  the  contraction  of  a  few  fibres 
by  taking  a  slip  of  the  ventricle  (and  for  this  purpose  the  tortoise  is  prefer- 
able to  the  frog)  and  suspending  it  to  a  lever  after  the  fashion  of  a  muscle- 
nerve  preparation.  We  then  get  a  curve  of  contraction,  characterized 
by  a  long  latent  period,  a  slow  long-continued  rise,  and  a  slow  long-con- 
tinued fall — a  contraction,  in  fact,  more  like  that  of  a  plain  muscular  fibre 
than  of  a  skeletal  muscular  fibre.  In  the  tortoise  the  contraction  is  partic- 
ularly long,  the  contraction  of  even  the  skeletal  muscles  being  long  in  that 
animal ;  it  is  less  long,  but  still  long,  in  the  frog ;  shorter  still,  but  yet  long 
as  compared  with  the  skeletal  muscles,  in  the  mammal. 

The  beat  of  the  ventricle,  then,  is  a  single  but  relatively  slow,  prolonged 
contraction  wave  sweeping  over  the  peculiar  cardiac  muscle-cell,  passing 
through  the  cement  substance  from  cell  to  cell  along  the  fibre,  from  fibre  to 
fibre  along  the  bundle,  and  from  bundle  to  bundle  over  the  labyrinth  of  the 
ventricular  walls. 

Like  the  case  of  the  skeletal  muscle,  this  single  contraction  is  accom- 
panied by  an  electric  change,  a  current  of  action.  The  intact  ventricle  at 
rest  is,  as  we  have  already  said  (§  66),  isoelectric,  but  each  part  just  as  it  is 
about  to  enter  into  a  state  of  contraction  becomes  negative  toward  the  rest. 


THE  VASCULAR  MECHANISM.  197 

Hence,  when  the  electrodes  of  a  galvanometer  are  placed  on  two  points, 
A,  B,  of  the  surface  of  the  ventricle,  a  diphasic  variation  of  the  galvanom- 
eter needle  is  seen  just  as  a  beat,  natural  or  excited,  is  about  to  occur.  Sup- 
posing that  the  wave  of  contraction  reaches  A  first,  this  will  become  nega- 
tive toward  the  rest  of  the  ventricle,  including  B,  but  when  the  wave  some 
time  afterward  reaches  B,  B  will  become  negative  toward  the  rest  of  the 
ventricle,  including  A.  Compare  §  67. 

The  beat  of  the  auricles,  that  of  the  sinus  venosus,  and  that  of  thebulbus 
arteriosus  are  similar  in  their  main  features  to  that  of  the  ventricle,  so  that 
the  whole  beat  may  be  considered  to  be  a  wave  of  contraction  sweeping 
through  the  heart  from  sinus  to  bulbus ;  but  the  arrangement  of  fibres  is 
such  that  this  beat  is  cut  up  into  sections  in  such  a  way  that  the  sinus,  the 
auricles,  the  ventricle,  and  thebulbus  have  each  a  beat,  so  to  speak,  to  them- 
selves. In  a  normal  state  of  things  these  several  parts  of  the  whole  beat 
follow  each  other  in  the  sequence  we  have  described,  but  under  abnormal 
conditions  the  sequence  may  be  reversed,  or  one  section  may  beat  while  the 
others  are  at  rest,  or  the  several  sections  may  beat  out  of  time  with  each 
other. 

So  far  the  description  of  the  contraction  which  is  the  foundation  of  the 
beat  differs  from  that  of  a  skeletal  muscle  in  degree  only ;  but  now  comes  an 
important  difference.  When  we  stimulate  a  skeletal  muscle  with  a  strong 
stimulus  we  get  a  large  contraction  ;  when  we  apply  a  weak  stimulus  we  get 
a  small  contraction ;  within  certain  limits  (see  §  77)  the  contraction  is  pro- 
portional to  the  stimulus.  This  is  not  the  case  with  the  quiescent  ventricle 
or  heart.  When  we  apply  a  strong  induction-shock  we  get  a  beat  of  a  cer- 
tain strength ;  if  we  now  apply  a  weak  shock,  we  get  either  no  beat  at  all  or 
quite  as  strong  a  beat  as  with  the  stronger  stimulus.  That  is  to  say,  the 
magnitude  of  the  beat  depends  on  the  condition  of  the  ventricle  (or  heart), 
and  not  on  the  magnitude  of  the  stimulus.  If  the  stimulus  can  stir  the  ven- 
tricle up  to  beat  at  all,  the  beat  is  the  best  which  the  ventricle  at  the  time 
can  accomplish  ;  the  stimulus  either  produces  its  maximum  effect  or  none  at 
all.  It  would  seem  as  if  the  stimulus  does  not  produce  a  contraction  in  the 
same  way  that  it  does  when  it  is  brought  to  bear  on  a  skeletal  muscle,  but 
rather  stirs  up  the  heart  in  such  a  way  as  to  enable  it  to  execute  a  spon- 
taneous beat,  which,  without  the  extra  stimulus,  it  could  not  bring  about. 
And  this  is  further  illustrated  by  the  fact  that  when  a  ventricle  is  beating 
rhythmically,  either  spontaneously  or  as  the  result  of  rhythmic  stimulation, 
the  kind  of  effect  produced  by  a  new  stimulus  thrown  in  will  depend  upon 
the  exact  phase  of  the  cycle  of  the  beat  at  which  it  is  thrown  in.  If  it  is 
thrown  in  just  as  a  relaxation  is  taking  place,  a  beat  follows  prematurely, 
before  the  next  beat  would  naturally  follow,  this  premature  beat  being  obvi- 
ously produced  by  the  stimulus.  But  if  it  be  thrown  in  just  as  a  contraction 
is  beginning,  no  premature  beat  follows ;  the  ventricle  does  not  seem  to  feel 
the  stimulus  at  all.  There  is  a  period  during  which  the  ventricle  is  insen- 
sible to  stimuli,  and  that  however  strong ;  this  period  is  called  the  "  refrac- 
tory "  period.  (There  is,  it  may  be  mentioned,  a  similar  refractory  period 
in  skeletal  muscle,  but  it  is  of  exceedingly  short  duration.)  From  this  it 
results  that,  when  a  succession  of  stimuli  repeated  at  a  certain  rate  are  sent 
into  the  ventricle,  the  number  of  beats  does  not  correspond  to  the  number  of 
stimuli;  some  of  the  stimuli  falling  in  refractory  periods  are  ineffective  and 
produce  no  beat.  Hence,  also,  it  is  difficult  if  not  impossible  to  produce  a 
real  tetanus  of  the  ventricle,  to  fuse  a  number  of  beats  into  one.  And  there 
are  other  facts  tending  to  show  that  the  contraction  of  a  cardiac  muscular 
fibre,  even  when  induced  by  artificial  stimulation,  is  of  a  peculiar  nature, 
and  that  the  analogy  with  the  contraction  of  a  skeletal  muscular  fibre, 


198  THE  VASCULAR  MECHANISM. 

induced  by  motor   impulses   reaching  it  along  its  nerve,  does  not  hold 
good. 

These  and  other  considerations,  taken  together  with  the  facts  already 
mentioned,  that  portions  of  cardiac  muscular  tissue  in  which  ganglionic  cells 
are  certainly  not  present,  can  in  various  animals  be  induced,  either  easily  or 
with  difficulty,  to  execute  rhythmic  beats  which  have  all  the  appearance  of 
being  spontaneous  in  nature,  lead  us  to  conclude  that  the  beat  of  the  heart 
is  not  the  result  of  rhythmic  impulses  proceeding  from  the  cells  of  the  ganglia 
to  passive  muscular  fibres,  but  is  mainly  the  result  of  changes  taking  place 
in  the  muscular  tissue  itself.  And  here  we  may  call  attention  to  the  peculiar 
histological  features  of  cardiac  muscular  tissue  ;  though  so  far  differentiated 
as  to  be  striated,  its  cellular  constitution  and  its  "  protoplasmic  "  features, 
including  the  obscurity  of  the  striation,  show  that  the  differentiation  is  in- 
complete. Now,  one  attribute  of  undifferentiated  primordial  protoplasm  is 
the  power  of  spontaneous  movement. 

§  142.  We  have,  moreover,  evidence  that  it  is  the  muscular  tissue,  and 
not  the  arrangement  of  ganglia  and  nerves,  which  is  primarily  concerned  in 
maintaining  the  remarkable  sequence  of  sinus  beat,  auricle  beat,  and  ven- 
tricle beat.  This  is  perhaps  better  seen  in  the  heart  of  the  tortoise  than  in 
that  of  the  frog. 

In  this  animal  the  nerves  passing  from  the  sinus  to  the  ventricle  may  be 
divided,  or  the  several  ganglia  may  be  respectively  removed,  and  yet  the 
normal  sequence  is  maintained.  On  the  other  hand,  we  find  that  interference 
with  the  muscular  substance  of  the  auricle,  when  carried  to  a  certain  extent, 
prevents  the  beat  of  the  auricle  passing  over  to  the  ventricle,  so  that  the 
sequence  is  broken  after  the  auricle  beat.  If,  for  instance,  the  auricle  be  cut 
through  until  only  a  narrow  bridge  of  muscle  be  left  connecting  the  part  of 
the  auricle  adjoining  the  sinus  with  the  part  adjoining  the  auriculo-ventric- 
ular  ring,  or  if  this  part  be  compressed  with  a  clamp,  a  state  of  things  may 
be  brought  about  in  which  every  second  beat  only,  or  every  third  beat  only, 
of  the  sinus  and  auricle  is  followed  by  a  beat  of  the  ventricle  ;  and  then,  if 
the  bridge  be  still  further  narrowed  or  the  clamp  screwed  tighter,  the  ven- 
tricle does  not  at  all  follow  in  its  beat  the  sequence  of  sinus  and  auricle, 
though  it  may  after  a  while  set  up  an  independent  rhythm  of  its  own. 
This  experiment  suggests,  and  other  facts  support,  the  view  that  the  normal 
sequence  is  maintained  as  follows:  The  beat  begins  in  the  sinus  (including 
the  ends  of  the  veins)  ;  the  contraction  wave,  beginning  at  the  ends  of  the 
veins,  travels  over  the  muscular  tissue  of  the  sinus,  and  reaching  the  auricle 
starts  a  contraction  in  that  segment  of  the  heart ;  similarly  the  contraction 
wave  of  the  auricular  beat  reaching  the  ventricle  starts  a  ventricular  beat, 
which  in  turn  in  like  fashion  starts  the  beat  of  the  bulbus.  And  in  hearts 
in  a  certain  condition  it  is  possible  by  stimulation  to  reverse  this  sequence, 
or  to  produce,  by  alternate  stimulation,  an  alternation  of  a  normal  and  a 
reversed  sequence  ;  thus  in  the  heart  of  the  skate,  in  a  certain  condition, 
mechanical  stimulation  of  the  bulbus  by  indicating  a  beat  of  the  bulbus 
will  start  a  sequence  of  the  bulbus.  ventricle,  auricle,  and  sinus,  and  similar 
stimulation  of  the  sinus  will  produce  a  normal  sequence  of  sinus,  auricle, 
ventricle,  and  bulbus. 

It  would,  perhaps,  be  premature  to  insist  that  the  nervous  elements  do- 
not  intervene  in  any  way  in  the  maintenance  of  this  sequence ;  but  the  evi- 
dence shows  that  they  are  not  the  main  factors,  and  we  have  at  present  no 
satisfactory  indications  of  the  way  in  which  they  do  or  may  intervene. 

Two  questions  naturally  suggest  themselves  here.  The  first  is,  Why  does 
the  cardiac  cycle  begin  with  the  sinus  beat?  We  have  previously  (§  141) 
given  the  evidence  that  the  sinus  has  a  greater  potentiality  of  beating  than 


THE  VASCULAR  MECHANISM.  199 

the  other  parts ;  in  and  by  itself  it  beats  more  readily  and  with  a  quicker 
rhythm  than  the  other  parts.  When  we  ask  the  further  question,  Why  has 
it  this  greater  potentiality  ?  the  only  answer  we  can  at  present  give  is  that 
it  is  inborn  in  the  substance  of  the  sinus.  The  problem  is  somewhat  of  the 
same  kind  as  why  the  heart  of  one  animal  beats  so  much  quicker  than  that 
of  another.  All  we  can  say  at  present  is  that  the  rate  is  the  outcome  of  the 
molecular  constitution  of  tissue,  without  being  able  to  define  that  molecular 
constitution. 

The  second  question  is,  Why  does  not  the  contraction  wave  starting  at  the 
sinus  spread  as  a  continuous  wave  over  the  whole  heart?  why  is  it  broken 
up  into  sinus  beat,  auricle  beat,  ventricle  beat  ?  We  may  here  call  to  mind 
the  fact  mentioned  in  §  140  of  the  existence,  more  or  less  marked  in  all 
hearts  and  well  seen  in  the  heart  of  the  tortoise,  of  a  muscular  ring  or  collar 
between  the  sinus  and  the  auricle,  and  of  a  similar  ring  between  the  auricle 
and  ventricle.  The  muscular  tissue  in  these  rings  seems  to  be  of  a  somewhat 
different  nature  from  the  muscular  tissue  forming  the  body  of  the  sinus,  or 
of  the  auricle,  or  of  the  ventricle.  If  we  suppose  that  this  tissue  has  a  low 
conducting  power,  it  may  offer  sufficient  resistance  to  the  progress  of  the 
contraction  to  permit  the  sinus,  for  example,  to  carry  out  or  to  be  far  on  in 
the  development  of  its  beat  before  the  auricle  begins  its  beat  (and  thus  bisect, 
so  to  speak,  the  beat  which  would  otherwise  be  common  to  the  two),  and  yet 
not  offer  so  much  resistance  as  to  prevent  the  contraction  wave  passing  ulti- 
mately on  from  the  sinus  to  the  auricle.  We  may  in  the  tortoise  by  careful 
clamping  or  section  of  the  auricle  in  its  middle,  by  which  an  obstacle  to  the 
contraction  wave  is  introduced,  bisect  the  single  auricular  beat  into  two  beats, 
one  of  the  part  between  the  sinus  and  the  obstacle  and  another  between  the 
obstacle  and  the  ventricular.  We  may  thus  consider  the  breaking  up  the 
primitive  unbroken  peristaltic  wave  of  contraction  from  sinus  to  bulbus  to  be 
due  to  the  introduction  of  tissue  of  lower  conducting  power  at  the  junctions 
of  the  several  parts. 

We  do  not  say  that  this  is  the  complete  solution  of  the  problem,  but 
it  at  least  offers  an  approximate  solution ;  and  here  as  elsewhere  we 
have  no  satisfactory  evidence  of  nervous  elements  being  main  factors  in 
the  matter. 

In  the  above  we  have  dealt  chiefly  with  the  heart  of  the  cold-blooded 
animal,  but  as  far  as  we  know  the  same  conclusions  hold  good  for  the 
mammalian  heart  also. 

The  question  now  arises,  If  the  ganglia  are  not  the  prime  cause  of  the 
heart's  rhythmic  beat,  or  of  the  maintenance  of  the  normal  sequence,  what 
purposes  do  they  serve  ?  But  before  we  even  attempt  to  answer  this 
question  we  must  deal  with  the  nervous  mechanisms  by  which  the  beat 
of  the  heart,  thus  arising  spontaneously  within  the  tissues  of  the  heart 
itself,  is  modified  and  regulated  to  meet  the  requirements  of  the  rest  of 
the  body. 

The   Government  of  the  Heart-beat  by  the  Nervous  System. 

§  143.  It  will  be  convenient  to  begin  with  the  heart  of  the  frog,  which, 
as  we  have  seen,  is  connected  with  the  central  nervous  system  through 
and  therefore  governed  by  the  two  vagi  nerves,  each  of  which  though 
apparently  a  single  nerve  contains,  as  we  shall  see,  fibres  of  different  origin 
and  nature. 

If  while  the  beats  of  the  heart  of  a  frog  are  being  carefully  registered  an 
interrupted  current  of  moderate  strength  be  sent  through  one  of  the  vagi, 
the  heart  is  seen  to  stop  beating.  It  remains  for  a  time  in  diastole,  perfectly 


200 


THE  VASCULAR   MECHANISM. 


motionless  and  flaccid  ;  all  the  muscular  fibres  of  the  several  chambers  are 
for  the  time  being  in  a  state  of  relaxation.  The  heart  has  been  inhibited 
by  the  impulses  descending  down  the  vagus  from  the  part  of  the  nerve 
stimulated. 

If  the  duration  of  the  stimulation  be  short  and  the  strength  of  the  current 
great,  the  standstill  may  continue  after  the  current  has  been  shut  off;  the 
beats  when  they  reappear  are  generally  at  first  feeble  and  infrequent,  but 
soon  reach  or  even  go  beyond  their  previous  vigor  and  frequency.  If  the 
duration  of  the  current  be  very  long,  the  heart  may  recommence  beating 
while  the  stimulation  is  still  going  on,  but  the  beats  are  feeble  and  infrequent 
though  gradually  increasing  in  strength  and  frequency.  The  effect  of  the 
stimulation  is  at  its  maximum  at  or  soon  after  the  commencement  of  the 
application  of  the  stimulus,  gradually  declining  afterward  ;  but  even  at  the 
end  of  a  very  prolonged  stimulation  the  beats  may  still  be  less  in  force  or  in 
frequency,  or  in  both,  than  they  were  before  the  nerve  was  stimulated,  and 
on  the  removal  of  the  current  may  show  signs  of  recovery  by  an  increase  in 
force  and  frequency.  The  effect  is  not  produced  instantaneously  ;  if  on  the 
curve  the  point  be  exactly  marked  when  the  current  is  thrown  in,  as  at  on, 
Fig.  73,  it  will  frequently  be  found  that  one  beat  at  least  occurs  after  the 

FIG  73. 


A/\M) 


Inhibition  of  Frog's  Heart  by  Stimulation  of  Vagus  Nerve:  on  marks  the  time  at  which  the 
interrupted  current  was  thrown  into  the  vagus,  off  when  it  was  shut  off.  The  time-marker  below 
marks  seconds.  The  beats  were  registered  by  suspending  the  ventricle  from  a  clamp  attached  to 
the  aorta  and  attaching  a  light  lever  to  the  tip  of  the  ventricle. 

current  has  passed  into  the  nerve;  the  development  of  that  beat  has  taken 
place  before  the  impulses  descending  the  vagus  have  had  time  to  affect  the 
heart. 

The  stimulus  need  not  necessarily  be  the  interrupted  current ;  mechanical, 
chemical,  or  thermal  stimulation  of  the  vagus  will  also  produce  inhibition  ; 
but  in  order  to  get  a  marked  effect  it  is  desirable  to  make  use  of  not  a  single 
nervous  impulse  but  a  series  of  nervous  impulses  ;  thus  it  is  difficult  to  obtain 
any  recognizable  result  by  employing  a  single  induction-shock  of  moderate 
intensity  only.  As  we  shall  see  later  on,  "natural"  nervous  impulses 
descending  the  vagus  from  the  central  nervous  system,  and  started  there, 
by  afferent  impulses  or  otherwise,  as  parts  of  a  reflex  act,  may  produce 
inhibition. 

The  stimulus  maybe  applied  to  any  part  of  the  course  of  the  vagus  from 
high  up  in  the  neck  right  down  to  the  sinus  ;  indeed,  very  marked  results  are 
obtained  by  applying  the  electrodes  directly  to  the  sinus,  where,  as  we  have 
seen,  the  two  nerves  plunge  into  the  substance  of  the  heart.  The  stimulus 
may  also  be  applied  to  either  vagus,  though  in  the  frog  and  some  other 
animals,  one  vagus  is  sometimes  more  powerful  than  the  other.  Thus,  it 
not  infrequently  happens  that  even  strong  stimulation  of  the  vagus  on  one 


THE  VASCULAR  MECHANISM.  201 

side  produces  no  change  of  the  rhythm,  while  even  moderate  stimulation 
of  the  nerve  on  the  other  side  of  the  neck  brings  the  heart  to  a  stand- 
still at  once. 

If  during  the  inhibition  the  ventricle  or  other  part  of  the  heart  be  stimu- 
lated directly — for  instance,  mechanically  by  the  prick  of  a  needle — a  beat 
may  follow  ;  that  is  to  say,  the  impulses  descending  the  vagus,  while  inhibit- 
in  n  the  spontaneous  beats,  have  not  wholly  abolished  the  actual  irritability 
of  the  cardiac  tissues. 

With  a  current  of  even  moderate  intensity,  such  a  current,  for  instance, 
as  would  produce  a  marked  tetanus  of  a  muscle-nerve  preparation,  the  stand- 
still is  complete,  that  is  to  say,  a  certain  number  of  beats  are  entirely 
dropped  ;  but  with  a  weak  current  the  inhibition  is  partial  only,  the  heart 
does  not  stand  absolutely  still  but  the  beats  are  slowed,  the  intervals  between 
them  being  prolonged,  or  weakened  only  without  much  slowing,  or  both 
slowed  and  weakened.  Sometimes  the  slowing  and  sometimes  the  weaken- 
ing is  the  more  conspicuous  result. 

It  sometimes  happens  that,  when  in  the  frog  the  vagus  is  stimulated  in  the 
neck,  the  effect  is  very  different  from  that  just  described  ;  for  the  beats  are 
increased  in  frequency,  though  they  may  be  at  first  diminished  in  force. 
And,  occasionally,  the  beats  are  increased  both  in  force  and  in  frequency ; 
the  result  is  augmentation,  not  inhibition.  But  this  is  due  to  the  fact  that 
in  the  frog  the  vagus  along  the  greater  part  of  its  course  is  a  mixed  nerve 
and  contains  fibres  other  than  those  of  the  vagus  proper. 

§  144.  If  we  examine  the  vagus  nerve  closely,  tracing  it  up  to  the 
brain,  we  find  that  just  as  the  nerve  has  pierced  the  cranium,  just  where  it 
passes  through  the  ganglion  ( G.  V.,  Fig.  74),  certain  fibres  pass  into  it  from 
the  sympathetic  nerve  of  the  neck,  Sy.,  of  the  further  connections  of  which 
we  shall  speak  presently. 

This  being  the  case,  we  may  expect  that  we  should  get  different  results 
according  as  we  stimulated  (1)  the  vagus  in  the  cranium  before  it  was  joined 
by  the  sympathetic,  (2)  the  sympathetic  fibres  before  they  join  the  vagus, 
and  (3)  the  vagus  trunk  containing  the  real  vagus  and  the  sympathetic  fibres 
added.  What  we  have  previously  described  «re  the  ordinary  results  of 
stimulating  the  mixed  trunk,  and  these,  as  we  have  said,  are  not  wholly 
constant,  though  usually  and  in  the  main  most  distinct  inhibitory  results 
follow. 

If  we  stimulate  the  sympathetic  in  the  neck,  as  at  Sy.,  Fig.  74,  cutting 
the  nerve  below,  so  as  to  block  all  impulses  from  passing  downward,  and  only 
allow  impulses  to  pass  up  to  the  vagus  and  thence  down  the  mixed  vagus 
trunk  to  the  heart,  we  get  very  remarkable  results.  The  beat  of  the  heart, 
instead  of  being  inhibited,  is  augmented  ;  the  beats  are  increased  either  in 
frequency  or  in  force,  or  most  generally  both  in  frequency  and  in  force. 
The  effect  is,  perhaps,  best  seen  when  the  heart  before  stimulation  is  beating 
slowly  and  feebly ;  upon  stimulation  of  the  cervical  sympathetic  the  beats 
at  once  improve  in  vigor  and  frequency  ;  indeed,  a  heart  which,  for  one 
reason  or  another,  has  almost  ceased  to  beat  may,  by  proper  stimulation  of 
the  sympathetic,  be  called  back  into  vigorous  activity. 

If,  on  the  other  hand,  we  stimulate  the  vagus  before  it  has  been  joined 
by  the  sympathetic  fibres  (and  to  insure  the  result  not  being  marred  by  any 
escape  of  the  stimulating  current  on  to  the  sympathetic  fibres  it  is  necessary 
to  stimulate  the  vagus  within  the  cranium),  we  get  pure  and  constant  inhib- 
itory results — the  beats  are  for  a  time  wholly  abolished,  or  are  slowed,  or  are 
weakened,  or  are  both  slowed  and  weakened. 

Obviously,  then,  the  heart  of  the  frog  is  supplied  through  the  vagus  by 
two  sets  of  fibres  coming  from  the  central  nervous  system,  the  one  by  the 


202 


THE  VASCULAR  MECHANISM. 


vagus  proper  and  the  other  by  the  cervical  sympathetic  nerve,  and  these  two 
sets  have  opposite  and  antagonistic  effects  upon  the  heart.  We  find  upon 
examination  that  we  can  make  the  following  statements  concerning  them  : 

The  one  set,  those  belonging  to  the  vagus  proper,  are  inhibitory ;   they 
weaken  the  systole  and  prolong  the  diastole,  the  effect  with  a  strong  stimula- 


FIG.  74. 


IX 


s.  v,  a 


in 


Diagrammatic  Representation  of  the  Course  of  Cardiac  Augmentor  Fibres  in  the  Frog:  V.r., 
roots  of  vagus  (and  ixth)  nerve.  G.  V.,  ganglion  of  same.  Cr.,  line  of  cranial  wall.  Vg..  vagus 
trunk,  ix.,  ninth,  glosso-pharyngeal  nerve.  S.  V.  C.,  superior  vena  cava.  Sy.,  sympathetic  nerve 
in  neck.  G.  c.,  junction  of  sympathetic  ganglion  with  vagus  ganglion  sending  i.  c.  intra-cranial 
fibres  passing  to  Gasserian  ganglion.  The  rest  of  the  fibres  pass  along  the  vagus  trunk.  G1, 
splanchnic  ganglion  connected  with  the  first  spinal  nerve.  (?n,  splanchnic  ganglion  of  the  sec- 
ond spinal  nerve.  An.  V.,  annulus  of  Vieussens.  A.sb.,  subclavian  artery.  G111,  splanchnic  gan- 
glion of  the  third  spinal  nerve.  ///.,  third  spinal  nerve,  r.c.,  ramus  communicans. 

The  course  of  the  augmentor  fibres  is  shown  by  the  thick  black  line.  They  may  be  traced  from 
the  spinal  cord  by  the  anterior  root  of  the  third  spinal  nerve,  through  the  ramus  communicans 
to  the  corresponding  splanchnic  ganglion  Gm  and  thence  by  the  second  ganglion  Gn  the  annulus 
of  Vieussens,  and  the  first  ganglion  G1  to  the  cervical  sympathetic  Sy.  and  so  by  the  vagus  trunk 
to  the  superior  vena  cava  5.  V.  C. 

tion  being  complete,  so  that  the  heart  is  for  a  time  brought  to  a  standstill. 
Sometimes  the  slowing,  sometimes  the  weakening,  is  the  more  prominent. 
When  the  nerve  and  the  heart  are  in  good  condition  it  needs  only  a  slight 
stimulus,  a  weak  current,  to  produce  a  marked  effect ;  and  it  may  be  men- 
tioned that  the  more  vigorous  the  heart,  the  more  rapidly  it  is  beating,  the 
easier  it  is  to  bring  about  inhibition.  Although,  as  we  have  said,  the  effect 


THE  VASCULAR  MECHANISM.  203 

is  at  its  maximum  soon  after  the  beginning  of  stimulation,  a  very  prolonged 
inhibition  may  be  produced  by  prolonged  stimulation ;  indeed  by  rhythmi- 
cal stimulation  of  the  vagus  the  heart  may  be  kept  perfectly  quiescent  for  a 
very  long  time  and  yet  beat  vigorously  upon  the  cessation  of  the  stimulus. 
In  other  words,  the  mechanism  of  inhibition — that  is,  the  fibres  of  the  vagus 
and  the  part  or  substance  of  the  heart  upon  which  these  act  to  produce  in- 
hibition, whatever  that  part  or  substance  may  be — is  not  readily  exhausted. 
Further,  the  inhibition  when  it  ceases  is,  frequently  at  all  events,  followed 
by  a  period  of  reaction,  during  which  the  heart  for  a  while  beats  more  vig- 
orously and  rapidly  than  before.  Indeed,  the  total  effect  of  stimulating  the 
vagus  fibres  is  not  to  exhaust  the  heart,  but  rather  to  strengthen  it ;  and  by 
repeated  inhibitions  carefully  administered,  a  feebly  beating  heart  may  be 
nursed  into  vigorous  activity. 

The  other  set,  those  joining  the  vagus  from  the  sympathetic,  are  "aug- 
mentor  "  or  "  accelerating  "  fibres ;  the  latter  name  is  the  more  common,  but 
the  former  is  more  accurate,  since  the  effect  of  stimulating  these  fibres  is  to 
increase  not  only  the  rapidity  but  the  force  of  the  beat ;  not  only  is  the 
diastole  shortened,  but  the  systole  is  strengthened,  sometimes  the  one  result 
and  sometimes  the  other  being  the  more  prominent.  In  contrast  with  the 
case  of  the  vagus  fibres,  a  somewhat  strong  stimulation  is  required  to  produce 
an  effect ;  the  time  required  for  the  maximum  effect  to  be  produced  is  also 
remarkably  long.  Moreover,  at  all  events,  in  the  case  of  a  heart  in  which 
the  circulation  is  not  maintained,  and  which  is  therefore  cut  off  from  its 
normal  nutritive  supply,  the  augmentor  fibres  are  far  less  easily  exhausted 
than  are  the  inhibitory  fibres.  Hence,  when  in  such  a  heart  both  sets  of 
fibres  are  stimulated  together,  as  when  the  vagus  trunk  in  the  neck  is  stimu- 
lated, the  first  effects  produced  are  those  of  inhibition  ;  but  these  on  con- 
tinued stimulation  may  become  mixed  with  those  of  augmentation,  and 
finally  the  latter  alone  remain.  Lastly,  the  contrast  is  completed  by  the 
fact  that  the  augmentation  resulting  from  the  stimulation  of  the  sympathetic 
is  followed  by  a  period  of  reaction  in  which  the  beats  are  feebler ;  in  other 
words,  augmentation  is  followed  by  exhaustion  ;  and,  indeed,  by  repeated 
stimulation  of  these  sympathetic  fibres  a  fairly  vigorous  bloodless  heart  may 
be  reduced  to  a  very  feeble  condition. 

By  watching  the  effects  of  stimulating  the  sympathetic  nerve  at  various 
points  of  its  course  we  may  trace  these  augmentor  fibres  from  their  junction 
with  the  vagus  down  the  short  sympathetic  of  the  neck  through  the  first 
splanchnic  or  sympathetic  ganglion  connected  with  the  first  spinal  nerve,  6rl 
(Fig.  74),  through  one  or  both  the  loops  of  the  annulus  of  Vieussens,  An.  V, 
through  the  second  ganglion  connected  with  the  second  spinal  nerve,  6rn, 
to  the  third  ganglion  connected  with  the  third  spinal  nerve,  6rni,  and  thence 
through  the  ramus  communicans  or  visceral  branch  of  that  ganglion,  r.  c., 
to  the  third  spinal  nerve,  ///.,  by  the  anterior  root  of  which  they  reach  the 
spinal  cord. 

§  145.  Both  sets  of  fibres  may  then  be  traced  to  the  central  nervous 
system ;  and  we  find  accordingly  that  the  heart  may  be  inhibited  or  aug- 
mented by  nervous  impulses  which  are  started  in  the  nervous  system  either 
by  afferent  impulses  as  part  of  a  reflex  act  or  otherwise,  and  which  pass  to 
the  heart  by  the  inhibitory  or  by  the  augmenting  tract. 

Thus,  if  the  medulla  oblongata,  or  a  particular  part  of  the  medulla  oblon- 
gata  which  is  specially  connected  with  the  vagus  nerve,  be  stimulated,  the 
heart  is  inhibited  ;  if,  for  instance,  a  needle  be  thrust  into  this  part,  the  heart 
stands  still.  This  region  in  question  may  be  stirred  into  action  in  a  "  reflex  " 
manner  by  afferent  impulses  reaching  it  from  various  parts  of  the  body.  Thus, 
if  the  abdomen  of  a  frog  be  laid  bare,  and  the  intestines  be  struck  sharply 


204  THE  VASCULAK  MECHANISM. 

with  the  handle  of  a  scalpel,  the  heart  will  stand  still  in  diastole  with  all 
the  phenomena  of  vagus  inhibition.  If  the  nervi  mesenteriei,  or  the  connec- 
tions of  these  nerves  with  the  spinal  cord,  be  stimulated  with  the  interrupted 
current,  cardiac  inhibition  is  similarly  produced.  If  in  these  two  experiments 
both  vagi  are  divided,  or  the  medulla  oblongata  is  destroyed,  inhibition  is 
not  produced,  however  much  either  the  intestine  or  the  mesenteric  nerves  be 
stimulated.  This  shows  that  the  phenomena  are  caused  by  impulses  ascend- 
ing along  the  mesenteric  nerves  to  the  medulla,  and  so  affecting  a  portion  of 
that  organ  as  to  give  rise  by  reflex  action  to  impulses  which  descend  the 
vagi  as  inhibitory  impulses.  The  portion  of  the  medulla  thus  mediating 
between  the  afferent  and  efferent  impulses  may  be  spoken  of  as  the  cardio- 
inhibitory  centre. 

Reflex  inhibition  through  one  vagus  may  be  brought  about  by  stimula- 
tion of  the  central  end  of  the  other.  In  general  the  alimentary  tract  seems 
in  closer  connection  with  the  cardio-inhibitory  centre  than  other  parts  of  the 
body  ;  and  if  the  peritoneal  surface  of  the  intestine  be  inflamed,  very  gentle 
stimulation  of  the  inflamed  surface  will  produce  marked  inhibition.  But 
apparently  stimuli,  if  sufficiently  powerful,  will  through  reflex  action  pro- 
duce inhibition,  whatever  be  the  part  of  the  body  to  which  they  are  applied. 
Thus,  crushing  a  frog's  foot  will  stop  the  heart,  and  adequate  stimulation  of 
most  afferent  nerves  will  produce  some  amount  of  inhibition. 

The  details  of  the  reflex  chain  and  the  portion  of  the  centre  concerned  in 
the  development  of  augmenting  impulses  have  not  been  worked  out  so  fully 
as  in  the  case  of  inhibitory  impulses,  but  there  can  be  little  doubt  that  the 
former,  like  the  latter,  are  governed  by  the  central  nervous  system. 

§  146.  So  far  we  have  been  dealing  with  the  heart  of  the  frog,  but  the 
main  facts  which  we  have  stated  regarding  inhibition  and  augmentation  of 
the  heart  beat  apply  also  to  other  vertebrate  animals,  including  mammals ; 
and,  indeed,  we  meet  similar  phenomena  in  the  hearts  of  invertebrate 
animals. 

If  in  a  mammal  the  heart  be  exposed  to  view  by  opening  the  thorax,  and 
the  vagus  nerve  be  stimulated  in  the  neck,  the  heart  may  be  seen  to  stand 
still  in  diastole,  with  all  the  parts  flaccid  and  at  rest.  If  the  current  em- 
ployed be  too  weak,  the  result  as  in  the  frog  is  not  an  actual  arrest,  but  a 
slowing  or  weakening  of  the  beats.  If  a  light  lever  be  placed  on  the  heart 
a  graphic  record  of  the  standstill  or  of  the  slowing,  of  the  complete  or  incom- 
plete inhibition,  may  be  obtained  The  result  of  stimulating  the  vagus  is 
also  well  shown  on  the  blood-pressure  curve,  the  effect  of  complete  cardiac 
inhibition  on  blood-pressure  being  most  striking.  If,  while  a  tracing  of 
arterial  pressure  is  being  taken,  the  beat  of  the  heart  be  suddenly  arrested, 
some  such  curve  as  that  represented  in  Fig.  75  will  be  obtained.  It  will  be 
observed  that  two  beats  follow  the  application  of  the  current  marked  by  the 
point  a,  which  corresponds  to  the  signal  x  on  the  line  below.  Then  for  a 
space  of  time  no  beats  at  all  are  seen,  the  next  beat  b  taking  place  almost 
immediately  after  the  shutting  off  the  current  at  y.  Immediately  after  the 
last  beat  following  a  there  is  a  sudden  fall  of  the  blood-pressure.  At  the 
pulse  due  to  the  last  systole  the  arterial  system  is  at  its  maximum  of  disten- 
tion  ;  forthwith  the  elastic  reaction  of  the  arterial  walls  propels  the  blood  for- 
ward into  the  veins,  and,  there  being  no  fresh  fluid  injected  from  the  heart, 
the  fall  of  the  mercury  is  unbroken,  being  rapid  at  first,  but  slower  afterward, 
as  the  elastic  force  of  the  arterial  walls  is  more  and  more  used  up.  With  the 
returning  beats  the  pressure  correspondingly  rises  in  successive  leaps  until 
the  normal  mean  pressure  is  regained.  The  size  of  these  returning  leaps  of 
the  mercury  may  seem  disproportionately  large,  but  it  must  be  remembered 
that  by  far  the  greater  part  of  the  force  of  the  first  few  strokes  of  the  heart 


THE   VASCULAR  MECHANISM.  205 

is  expended  in  distending  the  arterial  system,  a  small  portion  only  of  the 
blood  which  is  ejected  into  the  arteries  passing  on  into  the  veins.  As  the 
arterial  pressure  rises,  more  and  more  blood  passes  at  each  beat  through  the 
capillaries,  and  the  rise  of  the  pressure  at  each  beat  becomes  less  and  less, 
until  at  last  the  whole  contents  of  the  ventricle  pass  at  each  stroke  into  the 
veins,  and  the  mean  arterial  pressure  is  established.  To  this  it  may  be 
added  that,  as  we  have  seen,  the  force  of  the  individual  beats  may  be  some- 
what greater  after  than  before  inhibition.  Besides,  when  the  mercury 


Tracing  showing  the  Influence  of  Cardiac  Inhibition  on  Blood-pressure.  From  a  Rabbit,  x, 
the  marks  on  the  signal  line  when  the  current  is  thrown  into,  and  y,  shut  off  from  the  vagus.  The 
time-marker  below  marks  seconds,  the  heart,  as  is  frequently  the  case  in  the  rabbit,  beating  very 
rapidly. 

monometer  is  used,  the  inertia  of  the  mercury  tends  to  magnify  the  effects 
of  the  initial  beats. 

In  the  mammal  inhibition  may  be  brought  about  by  impulses  passing 
along  fibres  which,  starting  in  the  medulla  oblongata,  run  down  over  the 
vagus  nerve  and  reach  the  heart  by  the  cardiac  nerves.  It  would  appear, 
however,  that  the  inhibitory  fibres  do  not  belong  to  the  vagus  proper,  but 
leave  the  central  nervous  system  by  the  spinal  accessory  nerve.  Thus  if  the 
roots  of  the  spinal  accessory  be  divided,  those  of  the  vagus  proper  being  left 
intact,  the  spinal  accessory  fibres  in  the  vagus  trunk  degenerate,  and  when 
this  takes  place  stimulation  of  the  vagus  trunk  fails  to  produce  the  ordinary 
inhibitory  effects.  In  the  mammal,  as  in  the  frog,  inhibition  may  be  brought 
about  not  only  by  artificial  stimulation  of  the  vagus  trunk,  but  by  stimula- 
tion in  a  reflex  manner  or  otherwise  of  the  cardio-inhibitory  centre.  Thus 
the  fainting  which  often  follows  upon  a  blow  on  the  stomach  is  a  repetition 
of  the  result  just  mentioned  as  obtained  on  the  frog  by  striking  the  stomach 
or  stimulating  the  nervi  mesenterici.  So  also  the  fainting,  complete  or 
partial,  which  accompanies  severe  pain  or  mental  emotion,  is  an  illustration 
of  cardiac  inhibition  by  the  vagus.  In  fact,  cardiac  inhibition  so  far  from 
being  a  mere  laboratory  experiment  enters  repeatedly  into  the  every-day 
working  of  our  own  organism  as  well  as  that  of  other  living  beings. 

Indeed  there  is  some  reason  for  thinking  that  the  central  nervous  system 
by  means  of  the  cardiac  inhibitory  fibres  keeps  as  it  were  a  continual  rein  on 
the  heart,  for,  in  the  dog  at  least,  section  of  both  vagi  causes  a  quickening 
of  the  heart's  beat. 

In  the  dog  the  augmentor  fibres  (Fig.  76)  leave  the  spinal  cord  by  the 
anterior  roots  of  the  second  and  third  dorsal  nerves,  possibly  also  to  some 
extent  by  the  fourth  and  fifth,  pass  along  the  rami  communicantes  of  those 
nerves  to  the  ganglion  stellatum,  first  thoracic  ganglion,  or  respectively  to 
one  or  other  of  the  ganglia  forming  part  of  the  thoracic,  splanchnic,  or 
sympathetic  chain  immediately  below,  and  thence  upward  through  the 


206 


THE  VASCULAR  MECHANISM. 


O.Tr.Vg.- 


Vg _ 


annulus  of  Vieussens,  passing  along  one  or  other  or  both  loops,  to  the  inferior 
cervical  ganglion.     Their  further  course  to  the  heart  is  along  the  nerves 

springing  either  from  the  inferior  cer- 

FIG.  76.  vical  ganglion  or  from  the  loop  of 

Vieussens  directly.  Their  exact  path 
from  the  ganglia  in  fact  seems  to  vary 
in  different  individuals. 

The  path  of  the  augmentor  fibres 
has  not  been  worked  out  so  fully  in 
other  mammals  as  in  the  dog,  but  it 
\  r.Sp.Ac.'ls  most  probable  that  in  all  cases  they 
leave  the  spinal  cord  by  the  anterior 
roots  of  the  second  and  third  dorsal 
nerves  (possibly  also  by  the  fourth 
and  fifth)  and,  passing  up  the  sympa- 
thetic chain  to  the  ganglion  stellatum 
and  annulus  of  Vieussens,  proceed  to 
the  heart  by  nerves  branching  off 
from  some  part  or  other  of  the  annu- 
lus or  from  the  lower  and  middla 
cervical  ganglia. 

Diagrammatic  Representation  of  the  Cardial 
Inhibitory  and  Augmentor  Fibres  in  the  Dog. 
The  upper  portion  of  the  figure  represents  the 
inhibitory,  the  lower  the  augmentor  fibres. 
r.Vg.,  roots  of  the  vagus;  r.Sp.Ac.,  roots  of  the 
spinal  accessory ;  both  drawn  very  diagram- 
matically.  G.J.,  ganglion  jugulare ;  G.Tr.Vg., 
ganglion  trunci  vagi;  Sp.Ac.,  spinal  accessory 
trunk;  ext.Sp.Ac.,  external  spinal  accessory; 
i.Sp.Ac ,  internal  spinal  accessory ;  V(j.,  trunk 
of  vagus  nerve;  n.c.,  branches  going  to  heart; 
C.Sy.,  cervical  sympathetic;  O.C.,  lower  cervi- 
cal ganglion:  A.sb.,  subclavian  artery;  An.  V., 
annulus  of  Vieussens;  G.St.(Th.1),  ganglion 
stellatum  or  first  thoracic  ganglion;  G.Th.*, 
G.Th3,  G.Th*,  second,  third,  and  fourth  tho- 
racic ganglia ;  D.IL,  D.III.,  D.IV.,  D.V.,  second 
third,  fourth,  and  fifth  thoracic  spinal  nerves ; 
r.c.,  ramus  communicans ;  n.c.,  nerves  (car- 
diac) passing  to  heart  (superior  vena  cava)  from 
cervical  ganglion  and  from  the  annulus  of 
Vieussens. 

The  inhibitory  fibres,  shown  by  black  line, 
run  in  the  upper  (medullary  roots)  of  the  spinal 
accessory,  by  the  internal  branch  of  the  spinal 
accessory,  past  the  ganglion  trunci  vagi,  along 
the  trunk  of  the  vagus,  and  so  by  branches  to 
the  superior  vena  cava  and  the  heart. 

The  augmentor  fibres,  also  shown  by  black 
line,  pass  from  the  spinal  cord  by  the  anterior 
roots  of  the  second  and  third  thoracic  nerves 
(possibly  also  from  fourth  and  fifth  as  indicated 
by  broken  black  line),  pass  the  second  and  first 
(stellate)  thoracic  ganglia  by  the  annulus  of 
Vieussens  to  the  lower  cervical  ganglion,  from 
whence,  as  also  from  the  annulus  itself,  they 
pass  along  the  cardiac  nerves  to  the  superior 
vena  cava. 

The  effects  of  stimulating  these  augmentor  fibres  in  the  mammal  are,  in 
general,  the  same  as  those  witnessed  in  the  frog.  In  the  mammal,  as  in  the 
frog,  impulses  along  these  augmentor  fibres  may  be  originated  in  the  central 


THE  VASCULAR  MECHANISM.  207 

nervous  system,  and  that  probably  in  various  ways.  That  palpitation  of  the 
heart  which  is  so  conspicuous  an  effect  of  certain  emotions  is  probably  due 
to  the  sudden  positive  action  of  augmenting  impulses,  though  it  may  possibly 
be  due,  in  part  at  least,  to  sudden  withdrawal  of  normal,  continuous,  tonic, 
and  inhibitory  impulses. 

In  the  mammal,  then,  as  in  the  frog,  the  heart  is  governed  by  two  sets 
of  nerves,  the  one  antagonistic  to  the  other.  In  the  dog  the  roots  of  the 
spinal  accessory  nerve,  by  which  inhibitory  fibres  leave  the  central  nervous 
system,  consists  entirely  of  inedullated  fibres.  Among  these  are  fibres  of 
fine  calibre,  2/J.-&/J.  in  diameter,  which  may  be  traced  down  the  trunk  of 
the  vagus,  along  the  branches  going  to  the  heart,  right  down  to  the  heart 
itself.  There  can  be  little  doubt  that  these  medullated  fibres  of  fine  calibre 
are  the  inhibitory  fibres  of  the  vagus,  and  indeed  there  is  evidence  which 
renders  it  probable  that  the  inhibitory  fibres  of  the  heart  are  always  medul- 
lated fibres  of  fine  calibre,  which  continue  as  medullated  fibres  right  dow?n 
to  the  heart,  but  eventually  lose  their  medulla  in  the  heart  itself. 

The  anterior  roots  of  the  second  and  third  dorsal  nerves,  and  the  (white) 
rami  communicantes  belonging  to  them,  which,  as  we  have  just  seen,  con- 
tain in  the  dog  augmentor  fibres,  also  consist  exclusively  of  medullated 
fibres.  But  the  nerves  which  convey  the  augmenting  impulses  from  the 
lower  cervical  ganglion,  or  from  the  annulus  of  Vieussens  to  the  heart, 
consist  of  non-medullated  fibres.  Hence,  the  augmentor  fibres  must  have 
lost  their  medulla,  and  become  continuous  with  non-medullated  fibres  some- 
where in  their  course  along  the  sympathetic  chain.  It  is  probable  that  the 
change  occurs  in  the  ganglion  stellatum  and  lower  cervical  ganglion,  and 
it  is  further  probable  that  the  change  is  effected  by  the  medullated  fibre 
passing  into  one  of  the  ganglion  cells,  and  so  losing  its  medulla,  the  im- 
pulses which  it  conveys  passing  out  of  the  nerve  cell  by  one  or  more  of  the 
other  processes  of  the  cell  which  are  continued  on  as  non-medullated  fibres. 
Cf.  §  94. 

In  the  dog  then  these  two  sets  of  nerve  fibres,  antagonistic  to  each  other 
in  function,  differ  in  structure,  the  augmentor  fibres  early  losing  their 
medulla,  and  hence  being  over  a  large  part  of  their  course  non-medullated 
fibres,  whereas  the  inhibitory  fibres  are  medullated  fibres  which,  though  they 
may  pass  by  or  through  ganglia  (as  the  ganglion  jugulare  and  ganglion 
trunci  vagi),  do  not  lose  their  medulla  in  these  ganglia,  but  remain  as 
medullated  fibres  right  down  to  the  heart.  And  this  difference  in  structure 
appears  to  hold  good  for  all  mammals,  and  is  possibly  true  for  vertebrates 
generally. 

§  147.  The  question,  What  is  the  exact  nature  of  the  change  brought 
about  by  the  inhibitory  and  augmenting  impulses  respectively  on  their 
arrival  at  the  heart?  or,  in  other  words,  by  virtue  of  what  events  produced 
in  the  heart  itself  do  the  impulses  of  one  kind  bring  about  inhibition, 
of  the  other  kind  augmentation  ?  is  a  very  difficult  one,  which  we  can- 
not attempt  to  discuss  fully  here.  We  may,  if  we  please,  speak  of  an 
"  inhibitory  mechanism  "  placed  in  the  heart  itself,  but  we  have  no  exact 
knowledge  of  the  nature  of  such  a  mechanism.  Still  less  do  we  possess 
any  satisfactory  information  as  to  an  augmenting  mechanism.  It  has  been 
suggested  that  some  of  the  ganglia  in  the  heart  serve  as  such  an  inhibi- 
tory (or  augmenting)  mechanism  ;  but  there  is  evidence  that  the  inhibitory 
impulses  produce  their  effect  by  acting  directly  on  the  muscular  fibres,  or  at 
all  events  do  not  produce  their  effect  by  acting  exclusively  on  any  gan- 
glia. One  evidence  of  this  kind  is  supplied  by  the  action  of  the  drug 
atropine. 

If,  either  in  a  frog  Or  a  mammal,  or  other  animal,  after  the  vagus   fibres 


208  THE  VASCULAR  MECHANISM. 

have  been  proved  by  trial  to  produce  upon  stimulation  the  usual  inhibitory 
effects,  a  small  quantity  of  atropine  be  introduced  into  the  circulation  (when 
the  experiment  is  conducted  on  a  living  animal,  or  be  applied  in  a  weak 
solution  to  the  heart  itself  when  the  experiment  is  conducted,  as  in  the  case 
of  a  frog,  on  an  excised  heart,  or  after  the  circulation  has  ceased),  it  will 
after  a  short  time  be  found  not  only  that  the  stimulation,  the  application  of 
a  current,  for  instance,  which  previously  when  applied  to  the  vagus  pro- 
duced marked  inhibition,  now  produces  no  inhibition,  but  even  that  the 
strongest  stimulus,  the  strongest  current  applied  to  the  vagus  will  wholly 
fail  to  affect  the  heart,  provided  that  there  be  no  escape  of  current  on  to  the 
cardiac  tissues  themselves ;  under  the  influence  of  even  a  small  dose  of 
atropine,  the  strongest  stimulation  of  the  vagus  will  not  produce  standstill 
or  appreciable  slowing  or  weakening  of  the  beat. 

Now  it  might  be  supposed  that  the  atropine  produces  this  remarkable 
effect  by  acting  on  some  ganglionic  or  other  mechanism  intervening  between 
the  vagus  fibres  and  the  cardiac  muscular  tissue ;  but  we  have  evidence 
that  the  atropine  acts  either  on  the  muscular  tissue  itself  or  on  the  very 
endings  of  the  nerves  in  the  muscular  fibres.  We  have  said  (§  140)  that  a 
properly  prepared  strip  of  the  ventricle  of  the  tortoise  will  execute  for  a 
long  time  spontaneous  rhythmic  contractions,  it  will  go  on  "beating"  for 
a  long  time.  A  strip  of  the  auricle  will  exhibit  the  same  phenomena  even 
still  more  readily.  If  now,  while  such  a  strip  from  the  auricle  is  satisfac- 
torily beating,  a  gentle  interrupted  current  be  passed  through  it,  it  will 
stop  beating ;  the  current  inhibits  the  spontaneous  beats ;  a  very  gentle 
interrupted  current  must  be  used,  otherwise  the  effect  is  obscured  by  the 
more  direct  stimulating  action  of  the  current.  If  now  the  strip  be  gently 
bathed  with  a  weak  solution  of  atropine  no  such  inhibitory  effect  is  pro- 
duced by  the  interrupted  current ;  the  beats  go  on  regardless  of  the  action 
of  the  current.  The  interruption  of  this  experiment  is  that  in  the  first  case 
the  interrupted  current  stimulated  the  fine  termination  of  the  inhibitory 
fibres  in  the  muscular  strip,  and  that  in  the  second  case  the  atropine  pro- 
duced some  effect  either  on  these  fine  fibres,  or  on  their  connections  with  the 
muscular  substance  or  on  the  actual  muscular  substance  itself,  by  virtue  of 
which  they  ceased  to  act.  But  if  this  be  so,  if  the  same  inhibitory  effects 
are  produced  alike  by  stimulating  the  vagus  trunk  and  stimulating  the 
very  endings  of  the  nerves  in  the  muscles  of  the  heart,  if  not  the  actual 
muscular  tissue  itself,  then  there  is  no  need  to  suppose  the  existence  of  any 
special  inhibitory  mechanism  placed  between  the  fibres  in  the  vagus  branches 
and  the  cardiac  muscular  tissue. 

The  action  of  atropine  on  the  heart  is,  so  to  speak,  complemented  by  the 
action  of  muscarine,  the  active  principle  of  many  poisonous  mushrooms.  If 
a  small  quantity  of  muscarine  be  introduced  into  the  circulation,  or  applied 
directly  to  the  heart,  the  beats  become  slow  and  feeble,  and  if  the  dose 
be  adequate  the  heart  is  brought  to  a  complete  standstill.  The  effect  is 
in  some  respects  like  that  of  powerful  stimulation  of  the  vagus,  but  the 
standstill  is  much  more  complete,  the  effect  is  much  more  profound.  Now 
if,  in  a  frog,  the  heart  be  brought  to  a  standstill  by  a  dose  of  muscarine.  the 
application  of  an  adequate  quantity  of  atropine  will  bring  back  the  beats 
to  quite  their  normal  strength.  The  one  drug  is,  as  far  as  the  heart  is  con- 
cerned (and,  indeed,  in  many  other  respects),  the  antidote  of  the  other. 
And,  as  in  the  case  of  atropine,  so  in  the  case  of  muscarine,  there  is  evi- 
dence that  the  drug  acts  not  on  any  ganglionic  mechanisms,  but  on  the  car- 
diac tissue  itself. 

The  conclusion  that  inhibition  is  the  result  of  changes  in  the  cardiac  tis- 
sue itself  may  serve  to  explain  why  in  inhibition  sometimes  the  slowing, 


THE  VASCULAR  MECHANISM.  209 

sometimes  the  weakening  is  the  more  prominent.  When  the  inhibitory 
impulses,  by  reason  of  particular  fibres  being  affected  or  otherwise,  are 
brought  to  bear  chiefly  on  those  parts  of  the  heart,  such  as  the  sinus,  which 
possessing  higher  rhythmic  potentiality  (see  §  142)  determine  the  sequence 
and  set  the  rate  of  rhythm,  it  is  the  rate  which  is  most  markedly  affected. 
When,  on  the  other  hand,  the  inhibitory  impulses  fall  chiefly  on  the  parts 
possessing  lower  rhythmic  potentiality,  the  most  marked  effect  is  a  diminu- 
tion in  the  force  of  the  contractions. 

There  is  no  adequate  evidence  then  that  the  cardiac  ganglia  act  as  an 
inhibitory  mechanism  in  the  sense  that  they  produce  important  changes  in 
the  nature  of  the  impulses  reaching  them  along  vagus  inhibitory  fibres  before 
those  impulses  pass  on  to  the  muscular  tissue.  We  may  add  that  there  is 
similarly  no  adequate  evidence  that  any  of  the  ganglia  act  as  an  "  aug- 
menting "  mechanism.  We  have  previously  seen  (§§  141,  142)  reasons 
for  thinking  the  ganglia  are  not  centres  for  the  origination  or  regula- 
tion of  the  spontaneous  beats.  The  question  then  arises,  What  are  their 
functions?  To  this  question  we  cannot  at  present  give  a  wholly  satisfac- 
tory answer. 

The  inhibitory  fibres  remain,  as  we  have  seen,  medullated  fibres  until  they 
reach  the  heart,  but  it  would  appear  that  they  lose  their  medulla  somewhere 
in  the  heart  before  they  actually  reach  the  muscular  tissue,  and  it  is  probable 
that  the  loss  takes  place  in  connection  with  some  of  the  cardiac  ganglia 
much  in  the  same  way  that  the  augmenting  fibres  lose  their  medulla  in  the 
ganglia  of  the  sympathetic  chain  ;  but  we  do  not  know  what  is  the  physi- 
ological effect  or  the  purpose  of  this  loss  of  the  medulla,  and  we  cannot 
suppose  that  this  is  the  sole  or  even  chief  use  of  the  ganglia.  Coincident 
with  the  loss  of  the  medulla  an  increase  of  fibres  frequently  takes  place, 
more  than  one  non-medullated  fibre  leaving  a  nerve  cell  into  which  one 
medullated  fibre  enters ;  and  we  may  suppose  that  this  mode  of  branching 
has  purposes  not  fulfilled  by  the  mere  division  of  a  fibre.  Then  again, 
bearing  in  mind  the  nutritive  or  "trophic"  function  of  the  spinal  gan- 
glia alluded  to  in  §  96,  we  may  suppose  that  the  cardiac  ganglia  are  in 
some  way  concerned  in  the  nutrition  of  the  cardiac  nerve  fibres.  But 
our  knowledge  is  not  yet  sufficiently  ripe  to  allow  exact  statements  to 
be  made. 

Other  Influences  Regulating  or  Modifying  the  Beat  of  the  Heart. 

§  148.  Important  as  is  the  regulation  of  the  heart  by  the  nervous  system, 
it  must  be  borne  in  mind  that  other  influences  are  or  may  be  at  work.  The 
beat  of  the  heart  may,  for  instance,  be  modified  by  influences  bearing  directly 
on  the  nutrition  of  the  heart.  The  tissues  of  the  heart,  like  all  other  tissues, 
need  an  adequate  supply  of  blood  of  a  proper  quality ;  if  the  blood  vary  in 
quality  or  quantity  the  beat  of  the  heart  is  correspondingly  affected.  The 
excised  frog's  heart,  as  we  have  seen,  continues  to  beat  for  some  considerable 
time,  though  apparently  empty  of  blood.  After  a  while,  however,  the  beats 
diminish  and  disappear ;  and  their  disappearance  is  greatly  hastened  by 
washing  out  the  heart  with  a  normal  saline  solution,  which  when  allowed 
to  flow  through  the  cavities  of  the  heart  readily  permeates  the  tissues  on 
account  of  the  peculiar  construction  of  the  ventricular  walls.  If  such  a 
"  washed  out  "  quiescent  heart  be  fed  with  a  perfusion  canula,  in  the  manner 
described  (§  141),  with  diluted  blood  (of  the  rabbit,  sheep,  etc.),  it  may  be 
restored  to  functional  activity.  A  similar  but  less  complete  restoration  may 
be  witnessed  if  serum  be  used  instead  of  blood ;  and  a  heart  fed  regularly 
with  fresh  supplies  of  blood  or  even  of  serum  may  be  kept  beating  for  a 

14 


210  THE  VASCULAR  MECHANISM. 

very  great  length  of  time.  In  treating  of  the  skeletal  muscles  we  saw  that 
in  their  case  the  exhaustion  following  upon  withdrawal  of  the  blood-stream 
might  be  attributed  either  to  an  inadequate  supply  of  new  nutritive  mate- 
rial and  oxygen,  or  to  an  accumulation  in  the  muscular  substance  of  the 
products  of  muscular  metabolism,  or  to  both  causes  combined.  And  the 
same  considerations  hold  good  for  the  nervous  and  muscular  structures 
of  the  heart,  though  the  subject  has  not  yet  been  sufficiently  well  worked 
out  to  permit  any  very  definite  statements  to  be  made.  It  seems  prob- 
able, however,  that  an  important  factor  in  the  matter  is  the  accumulation 
in  the  muscular  fibres  and  in  the  surrounding  lymph  of  carbonic  acid,  and 
especially  of  the  substances  which  give  rise  to  the  acid  reaction. 

When  the  frog's  heart  is  thus  "  fed  "  with  various  substances  the  interest- 
ing fact  is  brought  to  light  that  some  substances,  such,  for  instance,  as  very 
dilute  lactic  acid,  lead  to  increased  expansion,  and  others,  such,  for  instance, 
as  very  dilute  solutions  of  sodium  hydrate,  to  diminished  expansion,  that  is 
to  continued  contraction,  of  the  quiescent  ventricle.  It  would  appear  that 
the  muscular  fibres  of  the  ventricle  over  and  above  their  rhythmic  contrac- 
tions are  capable  of  varying  in  length,  so  that  at  one  time  they  are  longer, 
and  the  ventricle  when  pressure  is  applied  to  it  internally  dilates  beyond  the 
normal,  while  at  another  time  they  are  shorter,  and  the  ventricle,  with  the 
same  internal  pressure,  is  contracted  beyond  the  normal.  Further,  in  the 
frog  at  least,  when  the  pause  between  two  beats  is  lengthened  the  relaxa- 
tion of  the  ventricle  goes  on  increasing,  so  that  apparently  the  ventricle 
when  beating  normally  is  already  somewhat  contracted  when  a  new  beat 
begins.  In  other  words,  the  ventricle  possesses  what  we  shall  speak  of  in 
reference  to  arteries  as  tonicity  or  tonic  contraction,  and  the  amount  of 
this  tonic  contraction,  and  in  consequence  the  capacity  of  the  ventricle, 
varies  according  to  circumstances.  We  have,  moreover,  evidence  that 
inhibitory  impulses  diminish  and  augmenting  impulses  increase  this  tonic 
contraction. 

When  the  frog's  ventricle  is  thus  artificially  fed  with  serum  or  even  with 
blood,  the  beats,  whether  spontaneous  or  provoked  by  stimulation,  are  apt  to 
become  intermittent  and  to  arrange  themselves  into  groups.  This  intermit- 
tence  is  possibly  due  to  the  serum  or  blood  being  unable  to  carry  on  nutrition 
in  a  completely  normal  manner,  and  to  the  consequent  production  of  abnor- 
mal chemical  substances ;  and  it  is  probable  that  cardiac  intermittences  seen 
during  life  have  often  a  similar  causation.  Various  chemical  substances  in 
the  blood,  natural  or  morbid,  may  thus  affect  the  heart's  beat  by  acting  on 
its  muscular  fibres  or  its  nervous  elements,  or  both,  and  that  probably  in 
various  ways,  modifying  in  different  directions  the  rhythm  or  the  individual 
contractions,  or  both. 

The  physical  or  mechanical  circumstances  of  the  heart  also  affect  its  beat ; 
of  these  perhaps  the  most  important  is  the  amount  of  the  distention  of  its 
cavities.  The  contractions  of  cardiac  muscle,  like  those  of  ordinary  muscle 
(see  §  79),  are  increased  up  to  a  certain  limit  by  the  resistance  which  they 
have  to  overcome ;  a  full  ventricle  will,  other  things  being  equal,  contract 
more  vigorously  than  one  less  full ;  though,  as  in  ordinary  muscle,  the  limit 
at  which  resistance  is  beneficial  maybe  passed,  and  an  overfull  ventricle  will 
fail  to  beat  at  all. 

Under  normal  conditions  the  ventricle  probably  empties  itself  completely 
at  each  systole.  Hence  an  increase  in  the  quantity  of  blood  in  the  ventricle 
would  augment  the  work  done  in  two  ways :  the  quantity  thrown  out  would 
be  greater,  and  the  increased  quantity  would  be  ejected  with  greater  force. 
Further,  since  the  distention  of  the  ventricle  is  (at  the  commencement  of 
the  systole  at  all  events)  dependent  on  the  auricular  systole,  the  work  of  the 


VASOMOTOR  ACTIONS.  211 

ventricle  (and  so  of  the  heart  as  a  whole)  is  in  a  measure  governed  by  the 
auricle. 

An  interesting  combination  of  direct  mechanical  effects  and  indirect  ner- 
vous effects  is  seen  in  the  relation  of  the  heart's  beat  to  blood-pressure.  When 
the  blood -pressure  is  high,  not  only  is  the  resistance  to  the  ventricular  systole 
increased,  but  other  things  being  equal,  more  blood  flows  (in  the  mamma- 
lian heart)  through  the  coronary  artery.  Both  these  events  would  increase 
the  activity  of  the  heart,  and  we  might  expect  that  the  increase  would  be 
manifest  in  the  rate  of  the  rhythm  as  well  as  in  the  force  of  the  individual 
beats.  As  a  matter  of  fact,  however,  we  do  not  find  this.  On  the  contrary 
the  relation  of  heart-beat  to  pressure  may  be  put  almost  in  the  form  of  a 
law,  that  the  "  rate  of  the  beat  is  in  inverse  ratio  to  the  arterial  pressure," 
a  rise  of  pressure  being  accompanied  by  a  diminution,  and  fall  of  pressure 
with  an  increase,  of  the  pulse-rate.  This,  however,  only  holds  good  if  the 
vagi  be  intact.  If  these  be  previously  divided,  then  in  whatever  way  the 
blood-pressure  be  raised — whether  by  injecting  blood  or  clamping  the  aorta 
or  increasing  the  peripheral  resistance,  through  that  action  of  the  vasomotor 
nerves  which  we  shall  have  to  describe  directly — or  in  whatever  way  it  be 
lowered,  no  such  clear  and  decided  inverse  relation  between  blood-pressure 
and  pulse-rate  is  observed.  It  is  inferred,  therefore,  that  increased  blood- 
pressure  causes  a  slowing  of  the  pulse,  when  the  vagi  are  intact,  because  the 
cardio-inhibitory  centre  in  the  medulla  is  stimulated  by  the  high  pressure, 
either  directly  by  the  pressure  obtaining  in  the  bloodvessels  of  the  medulla, 
or  in  some  indirect  manner,  and  the  heart  in  consequence  to  a  certain  extent 
inhibited. 

CHANGES  IN  THE  CALIBRE  OF  THE  MINUTE  ARTERIES.     VASOMOTOR 

ACTIONS. 

§  149.  All  arteries  contain  plain  muscular  fibres,  for  the  most  part  cir- 
cularly disposed,  and  most  abundant  in,  or  sometimes  almost  entirely  con- 
iined  to,  the  middle  coat.  Moreover,  as  the  arteries  become  smaller  the 
muscular  element,  as  a  rule,  becomes  more  and  more  prominent  as  compared 
with  the  other  elements,  until,  in  the  minute  arteries,  the  middle  coat  consists 
almost  entirely  of  a  series  of  plain  muscular  fibres  wrapped  around  the  in- 
ternal coat.  Nerve  fibres,  of  whose  nature  and  course  we  shall  presently 
speak,  are  distributed  largely  to  the  arteries  and  appear  to  end  chiefly  in 
fine  plexuses  around  the  muscular  fibre,  but  their  exact  terminations  have 
not  as  yet  been  clearly  made  out.  By  mechanical,  electrical,  or  other 
stimulation,  this  muscular  coat  may,  in  the  living  artery,  be  made  to  con- 
tract. During  this  contraction,  which  has  the  slow  character  belonging  to 
the  contractions  of  all  plain  muscles,  the  calibre  of  the  vessel  is  diminished. 
The  veins  also,  as  we  have  seen,  possess  muscular  elements,  but  these  vary 
in  amount  and  distribution  very  much  more  in  the  veins  than  in  the  arteries. 
Most  veins,  however,  are  contractile,  and  may  vary  in  calibre  according  to 
the  condition  of  their  muscular  elements.  Veins  are  also  supplied  with 
nerves.  It  will  be  of  advantage,  however,  to  consider  separately  the  little 
we  know  concerning  the  changes  in  the  veins,  and  to  confine  ourselves  at 
present  to  the  changes  in  the  arteries. 

If  the  web  of  a  frog's  foot  be  watched  under  the  microscope,  any  indi- 
vidual small  artery  will  be  found  to  vary  considerably  in  calibre  from  time 
to  time,  being  sometimes  narrowed  and  sometimes  dilated  ;  and  these  changes 
may  take  place  without  any  obvious  changes  either  in  the  heart-beat  or  in 
the  general  circulation  ;  they  are  clearly  changes  of  the  artery  itself.  Dur- 
ing the  narrowing,  which  is  obviously  due  to  a  contraction  of  the  muscular 


212  THE  VASCULAR  MECHANISM. 

coat  of  the  artery,  the  capillaries  fed  by  the  artery  and  the  veins  into  which 
these  lead  become  less  filled  with  blood  and  paler.  During  the  widening, 
which  corresponds  to  the  relaxation  of  the  muscular  coat,  the  same  parts  are 
fuller  of  blood  and  redder.  It  is  obvious  that,  the  pressure  at  the  entrance 
into  any  given  artery  remaining  the  same,  more  blood  will  enter  the  artery 
when  relaxation  takes  place,  and  consequently  the  resistance  offered  by  the 
artery  is  diminished,  and  less  when  contraction  occurs  and  the  resistance  is 
consequently  increased  ;  the  blood  flows  in  the  direction  of  least  resistance. 

The  extent  and  intensity  of  the  narrowing  or  widening,  the  constriction 
or  dilatation  which  may  thus  be  observed  in  the  frog's  web,  vary  very  largely. 
Variations  of  slight  extent,  either  more  or  less  regular  and  rhythmic  or 
irregular,  occur  even  when  the  animal  is  apparently  subjected  to  no  disturb- 
ing causes,  and  may  be  spoken  of  as  spontaneous ;  larger  changes  may  follow 
events  occurring  in  various  parts  of  the  body  ;  while  as  the  result  of  experi- 
mental interference  the  arteries  may  become  either  constricted,  in  some  cases 
almost  to  obliteration,  or  dilated  until  they  acquire  double  or  more  than 
double  their  normal  diameter.  This  constriction  or  dilation  may  be 
brought  about  not  only  by  treatment  applied  directly  to  the  web,  but  also 
by  changes  affecting  the  nerve  of  the  leg  or  other  parts  of  the  body.  Thus, 
section  of  the  sciatic  nerve  is  generally  followed  by  a  widening  which  may 
be  slight  or  which  may  be  very  marked,  and  which  is  sometimes  preceded 
by  a  passing  constriction ;  while  stimulation  of  the  peripheral  stump  of  the 
divided  nerve  by  an  interrupted  current  of  moderate  intensity  generally 
gives  rise  to  constriction,  often  so  great  as  almost  to  obliterate  some  of  the 
minute  arteries. 

Obviously,  then,  the  contractile  muscular  elements  of  the  minute  arteries 
of  the  web  of  the  frog's  foot  are  capable  by  contraction  or  relaxation  of 
causing  decrease  or  increase  of  the  calibre  of  the  arteries ;  and  this  condition 
of  constriction  or  dilation  may  be  brought  about  through  the  agency  of  the 
nerves.  Indeed,  not  only  in  the  frog,  but  also,  and  still  more  so,  in  warm- 
blooded animals,  have  we  evidence  that  in  the  case  of  nearly  all,  if  not  all, 
the  arteries  of  the  body,  the  condition  of  the  muscular  coat,  and  so  the  calibre 
of  the  artery,  is  governed  by  means  of  nerves ;  these  nerves  have  received 
the  general  name  of  vasomotor  nerves. 

§150.  If  the  ear  of  a  rabbit,  preferably  a  light-colored  one,  be  held  up 
before  the  light,  a  fairly  conspicuous  artery  will  be  seen  running  up  the 
middle  line  of  the  ear  accompanied  by  its  broader  and  more  obvious  veins. 
If  this  artery  be  carefully  watched  it  will  be  found,  in  most  instances,  to  be 
undergoing  rhythmic  changes  of  calibre,  constriction  alternating  with  dila- 
tation. At  one  moment  the  artery  appears  as  a  delicate,  hardly  visible,  pale 
streak,  the  whole  ear  being  at  the  same  time  pallid.  After  a  while  the 
artery  slowly  widens  out,  becomes  broad  and  red,  the  whole  ear  blushing, 
and  many  small  vessels  previously  invisible  coming  into  view.  Again  the 
artery  narrows  and  the  blush  fades  away ;  and  this  may  be  repeated  at 
somewhat  irregular  intervals  of  a  minute,  more  or  less.  The  extent  and 
regularity  of  the  rhythm  are  usually  markedly  increased  if  the  rabbit  be 
held  up  by  the  ears  for  a  short  time  previous  to  the  observation.  Similarly 
rhythmic  variations  in  the  calibre  of  the  arteries  have  been  observed  in 
several  places,  e.g.,  in  the  vessels  of  the  mesentery  and  elsewhere;  probably 
they  are  widely  spread. 

Sometimes  no  such  variations  are  seen  ;  the  artery  remains  constant  in  a 
condition  intermediate  between  the  more  extreme  widening  and  extreme 
narrowing  just  described.  In  fact,  we  may  speak  of  an  artery  as  being  at 
any  given  time  in  one  of  three  phases.  It  may  be  very  constricted,  in  which 
case  its  muscular  fibres  are  very  much  contracted ;  or  it  may  be  dilated,  in 


VASOMOTOR  ACTIONS.  213 

which  case  its  muscular  fibres  are  relaxed ;  or  it  may  be  moderately  con- 
stricted, the  muscular  fibres  being  contracted  to  a  certain  extent,  and 
remaining  in  such  a  condition  that  they  may,  on  the  one  hand,  pass  into 
stronger  contraction,  leading  to  marked  constriction,  or,  on  the  other  hand, 
into  distinct  relaxation,  leading  to  dilatation.  We  have  reason  to  think,  as 
we  shall  see,  that  many  arteries  of  the  body  are  kept  habitually,  or  at  least 
for  long  periods  together,  in  this  intermediate  condition,  which  is  frequently 
spoken  of  as  tonic  contraction,  or  tonus,  or  arterial  tone. 

§  151.  If,  now,  in  a  vigorous  rabbit,  in  which  the  heart  is  beating  with 
adequate  strength  and  the  whole  circulation  is  in  a  satisfactory  condition, 
the  cervical  sympathetic  nerve  be  divided  on  one  side  of  the  neck,  remark- 
able changes  may  be  observed  in  the  bloodvessels  of  the  ear  of  the  same  side. 
The  arteries  and  veins  widen,  they  together  with  the  small  veins  and  the 
capillaries  become  full  of  blood,  many  vessels  previously  invisible  come  into 
view,  the  whole  ear  blushes,  and  if  the  rhythmic  changes  described  above 
were  previously  going  on,  these  now  cease  ;  and,  in  consequence  of  the  extra 
supply  of  warm  blood,  the  whole  ear  becomes  distinctly  warmer.  Now  these 
changes  take  place,  or  may  take  place,  without  any  alteration  in  the  heart- 
beat or  in  the  general  circulation.  Obviously  the  arteries  of  the  ear  have, 
in  consequence  of  the  section  of  the  nerve,  lost  the  tonic  contraction  which 
previously  existed ;  their  muscular  coats,  previously  somewhat  contracted, 
have  become  quite  relaxed,  and  whatever  rhythmic  contractions  were  pre- 
viously going  on  have  ceased.  The  more  marked  the  previous  tonic 
contraction,  and  the  more  vigorous  the  heart-beats,  so  that  there  is  an 
adequate  supply  of  blood  to  fill  the  widened  channels,  the  more  striking 
the  results.  Sometimes,  as  when  the  heart  is  feeble,  or  the  pre-existing 
tonic  contraction  is  slight,  the  section  of  the  nerve  produces  no  very  obvious 
change. 

If,  now,  the  upper  segment  of  the  divided  cervical  sympathetic  nerve — 
that  is,  the  portion  of  the  nerve  passing  upward  to  the  head  and  ear— be 
laid  upon  the  electrodes  of  an  induction  machine  and  a  gentle  interrupted 
current  be  sent  through  the  nerve,  new  changes  take  place  in  the  blood- 
vessels of  the  ear.  A  short  time  after  the  application  of  the  current,  for  in 
this  effect  there  is  a  latent  period  of  very  appreciable  duration,  the  ear  grows 
paler  and  cooler,  many  small  vessels  previously  conspicuous  become  again 
invisible,  the  main  artery  shrinks  to  the  thinnest  thread,  and  the  main  veins 
become  correspondingly  small.  When  Jhe  current  is  shut  off  from  the  nerve 
these  effects  still  last  some  time,  but  eventually  pass  off;  the  ear  reddens, 
blushes  once  more,  and  indeed  may  become  even  redder  and  hotter,  with 
the  vessels  more  filled  with  blood  than  before.  Obviously  the  current  has 
generated  in  the  cervical  sympathetic  nerve  impulses  which,  passing  upward 
to  the  ear  and  finding  their  way  to  the  muscular  coats  of  the  arteries  of  the 
ear,  have  thrown  the  muscles  of  those  coats  into  forcible  contractions,  and 
have  thus  brought  about  a  forcible  narrowing  of  the  calibre  of  the  arteries 
— a  forcible  constriction.  Through  the  narrowed  constricted  arteries  less 
blood  finds  its  way,  and  hence  the  paleness  and  coldness  of  the  ear.  If  the 
impulses  thus  generated  be  very  strong,  the  constriction  of  the  arteries  may 
be  so  great  that  the  smallest  quantity  only  of  blood  can  make  its  way  through 
them,  and  the  ear  may  become  almost  bloodless.  If  the  impulses  be  weak, 
the  constriction  induced  may  be  slight  only;  and,  indeed,  by  careful  manipu- 
lation the  nerve  may  be  induced  to  send  up  to  the  ear  impulses  only  just 
sufficiently  strong  to  restore  the  moderate  tonic  constriction  which  existed 
before  the  nerve  was  divided. 

We  infer  from  these  experiments  that  among  the  various  nerve  fibres 
making  up  the  cervical  sympathetic,  there  are  certain  fibres  which,  passing 


214 


THE  VASCULAR  MECHANISM. 


FIG.  77. 


V.M.C.. 


Sp.C. 


upward  to  the  head,  become  connected  with  the  arteries  of  the  ear,  and  that 
these  fibres  are  of  such  a  kind  that  impulses  generated  in  them  and  passing 
upward  to  the  ear  lead  to  marked  contraction  of  the  muscular  fibres  of  the 
arteries,  and  thus  produce  constriction.  These  fibres  are  vasomotor  fibres 
for  the  bloodvessels  of  the  ear.  From  the  loss  of  tone,  so  frequently  follow- 
ing section  of  the  cervical  sympathetic,  we  may  further  infer  that,  normally 
during  life,  impulses  of  a  gentle  kind  are  continually  passing  along  these 
fibres  upward  through  the  cervical  sympathetic,  which  impulses,  reaching 
the  arteries  of  the  ear,  maintain  the  normal  tone  of  those  arteries.  But,  as 
we  said,  the  existence  of  this  tone  is  not  so  constant,  and  these  tonic  impulses 
are  not  so  conspicuous  as  the  artificial  constrictor 
impulses  generated  by  stimulation  of  the  nerve. 

§  151.  The  above  results  are  obtained  what- 
ever be  the  region  of  the  cervical  sympathetic 
which  we  divide  or  stimulate  from  the  upper  cervi- 
cal ganglion  to  the  lower.  We  may,  therefore,  de- 
scribe these  vasomotor  impulses  as  passing  upward 
from  the  lower  cervical  ganglion  along  the  cervical 
sympathetic  to  the  upper  cervical  ganglion,  from 
which  they  issue  by  branches  which  ultimately 
find  their  way  to  the  ear.  But  these  impulses  do 
not  start  from  the  lower  cervical  ganglion ;  on 
the  contrary,  by  repeating  the  experiments  of  di- 
vision and  stimulation  in  a  series  of  animals,  we 
may  trace  the  path  of  these  impulses  from  the 
lower  cervical  ganglion  (Fig.  77)  through  the 
annulus  of  Vieussens  to  the  ganglion  stellatum  or 
first  thoracic  ganglion,  and  thence  either  along  the 
ramus  communicans  (visceral  branch)  to  the  an- 
terior root  of  the  second  dorsal  nerve,  and  thus  to 
the  spinal  cord,  or  lower  down  along  the  thoracic 
sympathetic  chain,  and  thence  by  other  rami  com- 
municantes  to  some  other  of  the  upper  dorsal 
nerves,  and  thus  to  the  spinal  cord.  The  path 
taken  by  these  vasomotor  impulses  for  the  ear  is 
in  fact  very  similar  to  that  of  the  augmentor  fibres 
for  the  heart  (cf.  Fig.  76),  from  the  spinal  cord 
up  to  the  annulus  of  Vieussens  and  to  the  lower 
cervical  ganglion ;  but  there  they  part  company. 
We  can  thus  trace  these  impulses  along  the  cer- 
vical sympathetic  to  the  anterior  roots  of  certain 
dorsal  nerves,  and  through  these  to  a  particular 
part  of  the  spinal  cord,  where  we  will  for  the  pres- 
ent leave  them.  We  may  accordingly  speak  of 
vasomotor  fibres  for  the  ear  as  passing  from  the 
dorsal  spinal  cord  to  the  ear  along  the  track  just 
marked  out ;  stimulation  of  these  fibres  at  their 
origin  in  the  spinal  cord  or  at  any  part  of  their 
course  (along  the  anterior  roots  of  the  second, 
third,  or  other  upper  dorsal  nerves,  visceral 
branches  of  those  nerves,  ganglion  stellatum  or 
upper  part  of  thoracic  sympathetic  chain,  annulus 
of  Vieussens,  etc.)  leads  to  constriction  in  the 
bloodvessels  of  the  ear  of  that  side ;  and  section 
of  these  fibres  at  any  part  of  the  same  course  tends  to  abolish  any  previously 


Diagram  illustrating  the 
Paths  of  Vasoconstrictor  Fi- 
bres along  the  Cervical  Sym- 
pathetic and  (part  of)  the  Ab- 
dominal Splanchnic  :  Aur., 
artery  of  ear ;  G.  C.s.,  superior 
cervical  ganglion  ;  Abd.  Spl., 
upper  roots  of  and  part  of  ab- 
dominal splanchnic  nerve ;  V. 
M.  0.,  vasomotor  centre  in  me- 
dulla. The  other  references 
are  the  same  as  in  Fig.  76, 1 
146.  The  paths  of  the  constric- 
tor fibres  are  shown  by  the 
arrows.  The  dotted  line  in 
the  spinal  cord,  Sp.  C.,  is  to  in- 
dicate the  passage  of  constric- 
tor impulses  down  the  cord 
from  the  vasomotor  centre  in 
the  medulla. 


VASOMOTOR  ACTIONS.  215 

existing  tonic  constriction  of  the  bloodvessels  of  the  ear,  though  this  effect 
is  not  so  constant  or  striking  as  that  of  stimulation. 

§  153.  We  must  now  turn  to  another  case.  In  dealing  with  digestion  we 
shall  have  to  study  the  submaxillary  salivary  gland.  We  may  for  the 
present  simply  say  that  this  is  a  glandular  mass  well  supplied  with  blood- 
vessels, and  possessing  a  double  nervous  supply.  On  the  one  hand  it  receives 
fibres  from  the  cervical  sympathetic,  Fig.  78,  v.  sym.  (in  the  dog,  in  which 
the  effects  which  we  are  about  to  describe  are  best  seen,  the  vagus  and  cervical 

FIG.  78. 

v.sym. 


fsn.sym.f.  n.sym.sm. 


Diagrammatic  Representation  of  the  Submaxillary  Gland  of  the  Dog,  with  its  Nerves  and 
Bloodvessels.  The  dissection  has  been  on  an  animal  lying  on  its  back,  but  since  all  the  parts 
shown  in  the  figure  cannot  be  seen  from  any  one  point  of  view,  the  figure  does  not  give  the  exact 
anatomical  relations  of  the  several  structures. 

sm.  gld.  Thesnbmaxillary  gland,  into  the  duct  (sin.  d.)  of  which  a  canula  has  been  tied.  The 
sublingual  gland  and  duct  are  not  shown.  n.l.,  n.l'.  The  lingual  branch  of  the  fifth  nerve,  the 
part  n.l.  is  going  to  the  tongue,  ch.t.,  ch.t'.,  ch.t".  The  chorda  tympani.  The  part  ch.t".  is  pro- 
ceeding from  the  facial  nerve  ;  at  ch.  V.  it  becomes  conjoined  with  the  lingual  n.l'.,  and  afterward 
diverging  passes  as  ch.  t.  to  the  gland  along  the  duct;  the  continuation  of  the  nerve  in  company 
with  the  lingual  n.l.  is  not  shown,  sm.  gl.  The  submaxillary  ganglion  with  its  several  roots. 
a.  car.  The  carotid  artery,  two  small  branches  of  which,  a.  sm.  a.  and  r.  sm.  p.,  pass  to  the  anterior 
and  posterior  parts  of  the  gland,  v.  sm.  The  anterior  and  posterior  veins  from  the  gland,  falling 
into  v.j.,  the  jugular  vein.  v.  sym.  The  conjoined  vagus  and  sympathetic  trunks,  g.  cer.  s.  The 
upper  cervical  ganglion,  two  branches  of  which,  forming  a  plexus  (a./.)  over  the  facial  artery, 
are  distributed  (n.  sym.  sm.)  along  the  two  glandular  arteries  to  the  anterior  and  posterior  portions 
of  the  gland. 

The  arrows  indicate  the  direction  taken  by  the  nervous  impulses  during  reflex  stimulation  of 
the  gland.  They  ascend  to  the  brain  by  the  lingual  and  descend  by  the  chorda  tympani. 

sympathetic  are  enclosed  in  a  common  sheath  so  as  to  form  what  appears  to 
be  a  single  trunk),  which  reach  the  gland  in  company  with  the  arteries  sup- 
plying the  gland  (n.  sym.  sm.}.  On  the  other  hand,  it  receives  fibres  from  a 
small  nerve  called  the  chorda  tympani  (ch.  t.),  which,  springing  from  the 
seventh  cranial  (facial)  nerve,  crosses  the  tympanum  of  the  ear  (hence  the 
name)  and,  joining  the  lingual  branch  of  the  fifth  nerve,  runs  for  some  dis- 
tance in  company  with  that  nerve,  and  then  ends  partly  on  the  tongue, 
and  partly  in  a  small  nerve  which,  leaving  the  lingual  nerve  before  reach- 
ing the  tongue,  runs  along  the  duct  of  the  submaxillary  gland,  and  is  lost  in 
the  substance  of  the  gland  ;  a  small  branch  is  also  given  off  to  the  sub- 
lingual  gland. 


216  THE   VASCULAR  MECHANISM. 

Now  when  the  chorda  tympani  is  simply  divided  no  very  remarkable 
changes  take  place  in  the  bloodvessels  of  the  gland,  but  if  the  peripheral 
segment  of  the  divided  nerve,  that  still  in  connection  with  the  gland,  be 
stimulated,  very  marked  results  follow.  The  small  arteries  of  the  gland 
become  very  much  dilated  and  the  whole  gland  becomes  flushed.  (As  we 
shall  see  later  on,  the  gland  at  the  same  time  secretes  saliva  copiously,  but 
this  does  not  concern  us  just  now.)  Changes  in  the  calibre  of  the  blood- 
vessels are  of  course  not  so  readily  seen  in  a  compact  gland  as  in  a  thin, 
extended  ear;  but  if  a  fine  tube  be  placed  in  one  of  the  small  veins  by 
which  the  blood  returns  from  the  gland,  the  effects  on  the  bloodvessels  of 
stimulating  the  chorda  tympani  become  very  obvious.  Before  stimulation 
the  blood  trickles  out  in  a  thin  slow  stream  of  a  dark  venous  color ; 
during  stimulation  the  blood  rushes  out  in  a  rapid  full  stream,  often  with  a 
distinct  pulsation  and  frequently  of  a  color  which  is  still  scarlet  and  arte- 
rial in  spite  of  the  blood  having  traversed  the  capillaries  of  the  gland ;  the 
blood  rushes  so  rapidly  through  the  widened  bloodvessels  that  it  has  not 
time  to  undergo  completely  that  change  from  arterial  to  venous  which  nor- 
mally occurs  while  the  blood  is  traversing  the  capillaries  of  the  gland. 
This  state  of  things  may  continue  for  some  time  after  the  stimulation 
has  ceased,  but  before  long  the  flow  from  the  veins  slackens,  the  issuing 
blood  becomes  darker  and  venous,  and  eventually  the  circulation  becomes 
normal. 

Obviously  the  chorda  tympani  contains  fibres  which  we  may  speak  of  as 
"  vasomotor,"  since  stimulation  of  them  produces  a  change  in,  and  brings 
about  a  movement  in  the  bloodvessels  ;  but  the  change  produced  is  of  a 
character  the  very  opposite  to  that  produced  in  the  bloodvessels  of  the  ear 
by  stimulation  of  the  cervical  sympathetic.  There  stimulation  of  the  nerve 
caused  contraction  of  the  muscular  fibres,  constriction  of  the  small  arteries ; 
here  stimulation  of  the  nerve  causes  a  widening  of  the  arteries,  which 
widening  is  undoubtedly  due  to  relaxation  of  the  muscular  fibres.  Hence 
we  must  distinguish  between  two  kinds  of  vasomotor  fibres,  fibres  the  stim- 
ulation of  which  produces  constriction,  vaso-constrictor  fibres,  and  fibres 
the  stimulation  of  which  causes  the  arteries  to  dilate,  vaso-dilator  fibres, 
the  one  kind  being  the  antagonist  of  the  other. 

The  reader  can  hardly  fail  to  be  struck  with  the  analogy  between  these 
two  kinds  of  vasomotor  fibres  on  the  one  hand,  and  the  inhibitory  and  aug- 
mentor  fibres  of  the  heart  on  the  other  hand.  The  augmentor  cardiac  fibres 
increase  the  rhythm  and  the  force  of  the  heart-beats ;  the  vaso-constrictor 
fibres  increase  the  contractions  of  the  muscular  fibres  of  the  arteries ;  the 
one  works  upon  a  rhythmically  active  tissue,  the  other  upon  a  tissue  whose 
work  is  more  or  less  continuous,  but  the  effect  is  in  each  case  similar — an 
increase  of  the  work.  The  inhibitory  cardiac  fibres  slacken  or  stop  the 
rhythm  of  the  heart  and  diminish  the  beats ;  the  vaso-dilator  fibres  di- 
minish the  previously  existing  contraction  of  the  muscular  fibres  of  the 
arteries  so  that  these  expand  under  the  pressure  of  the  blood. 

§  154.  But  we  must  return  to  the  vasomotor  nerves.  The  cervical  sympa- 
thetic contains  vaso-constrictor  fibres  for  the  ear,  and  we  may  now  add  for 
other  regions,  also  of  the  head  and  face.  Thus  the  branches  of  the  cervical 
sympathetic,  going  to  the  submaxillary  gland  of  which  we  just  spoke  (Fig. 
78,  n.  sym.  sm.\  contain  vaso-constrictor  fibres  for  the  vessels  of  the  gland  ; 
stimulation  of  these  fibres  produces  on  the  vessels  of  the  gland  an  effect 
exactly  the  opposite  of  that  produced  by  stimulation  of  the  chorda  tympani. 
But  to  this  particular  point  we  shall  have  to  return  when  we  deal  with  the 
gland  in  connection  with  digestion.  A  more  important  fact  for  our  present 
purpose  is  that  the  cervical  sympathetic  appears  to  contain  only  vaso-con- 


VASOMOTOR  ACTIONS.  217 

stricter  fibres  ;  if  we  put  aside  as  exceptional  and  doubtful  the  result  of 
certain  observers  who  obtained  vaso-dilator  effects  in  the  mouth  and  face, 
we  may  say  that  in  no  region  to  which  the  fibres  of  the  cervical  sympathetic 
are  distributed  can  any  vaso-dilator  action  be  observed  as  the  result  of 
stimulation  of  the  nerve  at  any  part  of  its  course.  In  the  chorda  tympani, 
on  the  other  hand,  the  vasomotbr  fibres  are  exclusively  vaso-dilator  fibres, 
and  this  is  true  both  of  the  part  of  the  nerve  ending  in  the  submaxillary 
and  sublingual  glands  and  the  rest  of  the  ending  of  the  nerve  in  the  tongue. 
Stimulation  of  the  chorda  tympani  (as  far  as  the  vasomotor  functions  of 
the  nerve  are  concerned,  for  it  has,  as  we  shall  see,  other  functions)  at  any 
part  of  its  course,  from  its  leaving  the  facial  nerve  to  its  endings  in  the 
tongue  or  gland,  produces  only  vaso-dilator  effects,  never  vaso- constrictor 
effects. 

With  many  other  nerves  of  the  body  the  case  is  different.  In  the  frog, 
division  of  the  sciatic  nerve  leads  to  a  widening  of  the  arteries  of  the  web 
of  the  foot  of  the  same  side,  and  stimulation  of  the  peripheral  end  of  the 
nerve  causes  a  constriction  of  the  vessels,  which,  if  the  stimulation  be  strong, 
may  be  so  great  that  the  web  appears  for  the  time  being  to  be  devoid  of 
blood.  Also,  in  a  mammal,  division  of  the  sciatic  nerve  causes  a  similar 
widening  of  the  small  arteries  of  the  skin  of  the  leg.  Where  the  condition 
of  the  circulation  can  be  readily  examined,  as,  for  instance,  in  the  hairless 
balls  of  the  toes,  especially  when  these  are  not  pigmented,  the  vessels  are 
seen  to  be  dilated  and  injected,  and  a  thermometer  placed  between  the 
toes  shows  a  rise  of  temperature  amounting,  it  may  be,  to  several  degrees. 
If,  moreover,  the  peripheral  end  of  the  divided  nerve  be  stimulated,  the 
vessels  of  the  skin  become  constricted,  the  skin  grows  pale,  and  the  temper- 
ature of  the  foot  falls.  And  very  similar  results  are  obtained  in  the  fore- 
limb  by  division  and  subsequent  stimulation  of  the  nerves  of  the  brachial 
plexus. 

The  quantity  of  blood  present  in  the  bloodvessels  of  the  mammal,  though  it 
may  sometimes  be  observed  directly,  has  frequently  to  be  determined  indirectly. 
The  temperature  of  passive  structures  subject  to  cooling  influences,  such  as  the 
skin,  is  largely  dependent  on  the  supply  of  blood  ;  the  more  abundant  the  supply 
the  warmer  the  part.  Hence,  in  these  parts  variations  in  the  quantity  of  blood 
may  be  inferred  from  variations  of  temperature  ;  but  in  dealing  with  more  active 
structures  there  are  obviously  sources  of  error  in  the  possibility  of  the  treatment 
adopted,  such  as  the  stimulation  of  a  nerve  giving  rise  to  an  increase  of  temper- 
ature due  to  increased  metabolism,  independent  of  variations  in  blood-supply. 

The  quantity  of  blood  may  also  be  determined  by  the  pkthy smog raph.  In  this 
instrument  a  part  of  the  body,  such  as  the  arm,  is  introduced  into  a  closed  chamber 
filled  with  fluid,  ex.  gr.,  a  large  glass  tube,  the  opening  by  which  the  arm  is  intro- 
duced being  secured  with  a  stout  caoutchouc  membrane.  An  increase  or  decrease 
of  blood  sent  into  the  arm  will  lead  to  an  increase  or  decrease  of  the  volume  of  the 
arm,  and  this  will  make  itself  felt  by  an  increase  or  diminution  of  pressure  in  the 
fluid  of  the  closed  chamber,  which  may  be  registered  and  measured  in  the  usual 
way.  We  shall  have  to  speak  again  of  a  modification  of  this  instrument  when  we 
are  dealing  with  the  kidney. 

So  far  the  results  are  quite  like  those  obtained  by  division  and  stimulation 
of  the  cervical  sympathetic,  and  we  might  infer  that  the  sciatic  nerve  and 
brachial  plexus  contain  vaso-constrictor  fibres  for  the  vessels  of  the  skin  of 
the  hind-limb  and  fore-limb,  vaso-dilator  fibres  being  absent.  But  some- 
times a  different  result  is  obtained  :  on  stimulating  the  divided  sciatic  nerve 
the  vessels  of  the  foot  are  not  constricted,  but  dilated — perhaps  widely  dilated. 
And  this  vaso-dilator  action  is  almost  sure  to  be  manifested  when  the  nerve 
is  divided,  and  the  peripheral  stump  stimulated  some  days  after  division,  by 
which  time  commencing  degeneration  has  begun  to  interfere  with  the  irrita- 


218  THE  VASCULAR  MECHANISM. 

bility  of  the  nerve.  For  example,  if  the  sciatic  be  divided,  and  some  days 
afterward,  by  which  time  the  flushing  and  increased  temperature  of  the  foot 
following  upon  the  section  has  wholly  or  largely  passed  away,  the  peripheral 
stump  be  stimulated  with  an  interrupted  current,  a  renewed  flushing  and 
rise  of  temperature  is  the  result.  We  are  led  to  conclude  that  the  sciatic 
nerve  (and  the  same  holds  good  for  the  brachial  plexus)  contains  both  vaso- 
constrictor and  vaso-dilator  fibres,  and  to  interpret  the  varying  results  as  due 
to  variations  in  the  relative  irritability  of  the  two  sets  of  fibres.  The  con- 
strictor fibres  appear  to  predominate  in  these  nerves,  and  hence  constriction 
is  the  more  common  result  of  stimulation  ;  the  constrictor  fibres  also  appear 
to  be  more  readily  affected  by  a  tetanizing  current  than  the  dilator  fibres. 
When  the  nerve  after  division  commences  to  degenerate,  the  constrictor 
fibres  lose  their  irritability  earlier  than  the  dilator  fibres,  so  that  at  a  certain 
stage  a  stimulus,  such  as  the  interrupted  current,  while  it  fails  to  affect  the 
constrictor  fibres,  readily  throws  into  action  the  dilator  fibres.  The  latter, 
indeed,  in  contrast  to  ordinary  motor  nerves  (§  81),  retain  their  irritability 
after  section  of  the  nerve  for  very  many  days.  The  result  is,  perhaps,  even 
still  more  striking  if  a  mechanical  stimulus,  such  as  that  of  "  crimping  "  the 
nerve  by  repeated  snips  with  the  scissors,  be  employed.  Exposure  to  a  low 
temperature  again  seems  to  depress  the  constrictors  more  than  the  dilators  ; 
hence,  when  the  leg  is  placed  in  ice-cold  water  stimulation  of  the  sciatic,  even 
when  the  nerve  has  been  but  recently  divided,  throws  the  dilator  only  into 
action  and  produces  flushing  of  the  skin  with  blood.  Rhythmical  stimula- 
tion, moreover,  of  even  a  freshly  divided  nerve  produces  dilatation.  And 
there  are  other  facts  which  support  the  same  view  that  the  sciatic  nerve 
(and  brachial  plexus)  contains  both  vaso-constrictor  and  vaso-dilator  fibres 
which  are  differently  affected  by  different  circumstances.  We  may  point 
out  that  the  case  of  the  vagus  of  the  frog  is  a  very  analogous  one ;  in  it  are 
both  cardiac  inhibitory  (true  vagus)  and  cardiac  augmentor  (sympathetic) 
fibres,  but  the  former,  like  the  vaso-constrictor  fibres  in  the  sciatic,  are 
predominant,  and  special  means  are  required  to  show  the  presence  of  the 
latter. 

In  the  splanchnic  nerve  (abdominal  splanchnic),  which  supplies  fibres  to 
the  bloodvessels  of  so  large  a  part  of  the  abdominal  viscera,  there  is  abun- 
dant evidence  of  the  presence  of  vaso-constrictor  fibres,  but  the  presence  of 
vaso-dilator  fibres  has  not  clearly  been  shown.  Division  of  this  nerve  leads 
to  a  widening  of  the  bloodvessels  of  the  abdominal  viscera,  stimulation  of 
the  nerve  to  a  constriction  ;  and,  as  we  shall  see,  since  the  amount  of  blood- 
vessels thus  governed  by  this  nerve  is  very  large  indeed,  interference  either 
in  the  one  direction  or  the  other  with  its  vasomotor  functions  produces  very 
marked  results,  not  only  on  the  circulation  in  the  abdomen,  but  on  the  whole 
vascular  system. 

In  nerves  going  to  muscles  vaso-dilator  fibres  predominate ;  indeed,  in 
these  the  presence  of  any  vaso-constrictor  fibres  at  all  has  not  at  present  been 
satisfactorily  established.  When  a  muscle  contracts  there  is  always  an 
increased  flow  of  blood  through  the  muscle;  this  may  be  in  part  a  mere 
mechanical  result  of  the  change  of  form,  the  shortening  and  thickening  of 
the  fibres  opening  out  the  minute  bloodvessels,  but  is  not  wholly,  and  prob- 
ably not  even  largely,  thus  produced.  A  notable  feature  of  vasomotor 
fibres  is  that,  in  very  many  cases  at  all  events,  their  action  is  not  affected  by 
small  or  moderate  doses  of  urari  such  as  render  the  motor  nerves  of  striated 
muscle  powerless.  Thus,  in  a  frog  placed  under  the  influence  of  a  moderate 
amount  of  urari,  stimulation  of  a  nerve  going  to  a  muscle  will  produce 
vasomotor  effects  unaccompanied  and  unobscured  by  any  contraction  of  the 
striated  fibres.  By  placing  a  thin  muscle  of  a  frog,  such  as  the  mylo-hyoid, 


VASOMOTOR  ACTIONS.  219 

under  the  microscope,  and  watching  the  calibre  of  the  small  arteries  and  the 
circulation  of  the  blood  through  them  while  the  nerve  is  being  stimulated, 
the  widening  of  the  bloodvessels  as  the  result  of  the  stimulation  may  be  ac- 
tually observed.  This  experiment  appears  not  to  succeed  in  a  mammal ;  and 
it  has  been  suggested  that  when  a  muscle  contracts  some  of  the  chemical 
products  of  the  metabolism  of  the  muscle  may,  by  direct  action  on  the  mi- 
nute bloodvessels  apart  from  any  nervous  agency,  lead  to  a  widening  of  those 
bloodvessels  ;  this,  however,  is  doubtful.  With  regard  to  the  vaso-constrictor 
fibres,  the  only  evidence  that  they  exist  in  muscles  is  that  when  the  nerve  of 
a  muscle  is  divided  the  bloodvessels  of  the  muscle  widen,  somewhat  like 
bloodvessels  of  the  ear  after  division  of  the  cervical  sympathetic.  This  sug- 
gests the  presence  of  vaso-constrictor  fibres  carrying  the  kind  of  influence 
which  we  called  tonic,  leading  to  an  habitual  moderate  constriction ;  it  can- 
not, however,  be  regarded  by  itself  as  conclusive  evidence  ;  but  we  must  not 
discuss  the  matter  here. 

Speaking  generally,  then,  most  if  not  all  the  arteries  of  the  body  are  sup- 
plied with  vasomotor  fibres  running  in  this  or  that  nerve,  the  fibres  being 
either  vaso-constrictor  or  vaso-dilator,  and  some  nerves  containing  one  kind 
of  fibres  only,  some  both  in  varying  proportion.  Almost  every  nerve  in  the 
body,  therefore,  may  be  looked  upon  as  influencing  a  certain  set  of  blood- 
vessels, as  governing  a  vascular  area,  the  area  being  large  or  small,  and  the 
government  being  exclusively  constrictor  or  exclusively  dilator,  or  mixed. 

The  Course  of  Vaso-constrictor  and  Vaso-dilator  Fibres. 

§  155.  Both  the  vaso-constrictor  and  the  vaso-dilator  fibres  have  their 
origin  in  the  central  nervous  system,  the  spinal  cord,  or  the  brain,  but  the 
course  of  the  two  sets  appears  to  be  very  different. 

In  the  mammal,  as  far  as  we  know  at  present,  all  the  vaso-constrictor 
fibres  for  the  whole  body  take  their  origin  in  the  middle  region  of  the  spinal 
cord,  or  rather  leave  the  spinal  cord  by  the  nerves  belonging  to  this  middle 
region.  Thus  in  the  dog  the  vaso-constrictor  fibres,  not  only  for  the  trunk 
but  for  the  limbs,  head,  face,  and  tail,  leave  the  spinal  cord  by  the  anterior 
roots  of  the  spinal  nerves  reaching  from  about  the  second  dorsal  to  the  fourth 
lumbar  nerve,  both  inclusive.  Running  in  the  case  of  each  nerve  root  to 
the  mixed  nerve  trunk  they  pass  along  the  visceral  branch,  white  ramus 
communicans,  to  the  chain  of  splanchnic  ganglia  lying  in  the  thorax  and 
abdomen — the  so-called  thoracic  and  abdominal  sympathetic  chain  (Fig.  77). 
From  these  ganglia  they  reach  their  destination  in  various  ways.  Thus 
those  going  to  the  head  and  neck  pass  chiefly  through  the  second  and  third 
dorsal  and  partly  through  the  fourth,  fifth  and  first  dorsal  nerves,  thence 
upward  through  the  annulus  of  Vieussens  to  the  lower  cervical  ganglion, 
and  thence,  as  we  have  seen,  up  the  cervical  sympathetic.  Those  for  the 
abdominal  viscera  pass  off  in  a  similar  way  to  the  abdominal  splanchnic 
nerves  (Fig.  77,  abd.  spl.).  Those  destined  for  the  fore  limbs  pass  along  in  the 
fourth  to  the  ninth  dorsal  nerves,  both  inclusive,  chiefly  in  the  seventh,  and 
sometimes  a  few  in  the  tenth,  and  so  reach  the  brachial  plexus  ;  while  those 
for  the  leg  pass  through  the  eleventh  dorsal  to  the  third  lumbar,  both  inclusive, 
a  few  passing  through  the  tenth  dorsal  and  the  fourth  lumbar,  and  finally 
to  the  sciatic  plexus.  Those  for  the  tail  pass  through  in  the  first  to 
third  lumbar  inclusive.  The  constrictor  fibres  of  the  skin  of  the  trunk 
probably  reach  the  spinal  nerves  in  which  they  ultimately  run  in  a  similar 
manner.  All  the  vaso-constrictor  fibres,  whatever  their  destination,  leave 
the  spinal  cord  by  the  anterior  roots  of  spinal  nerves,  and  then,  passing 
through  the  appropriate  visceral  branches,  join  the  thoracic  or  abdominal 


220  THE  VASCULAK  MECHANISM. 

chain  of  splanchnic  ganglia.  In  these  ganglia  the  fibres  undergo  a  remark- 
able change.  Along  the  anterior  root  and  along  the  visceral  branch  they 
are  medullated  fibres,  but  long  before  they  reach  the  bloodvessels  for  which 
they  are  destined  they  become  non-medullated  fibres  ;  they  appear  to  lose 
their  medulla  in  the  system  of  splanchnic  ganglia.  We  may  add  that  in  the 
anterior  roots  and  along  the  visceral  branches,  white  rarni  communicantes, 
these  fibres  are  invariably  of  small  diameter,  not  more  than  1.8/x  to  3.6  //. 

§  156.  The  course  of  the  vaso-dilator  fibres  appears  to  be  a  somewhat 
different  one,  some  apparently  accompanying  the  vaso-constrictor  fibres,  and 
others  running  an  independent  course,  though  the  details  have  as  yet  been 
fully  worked  out  in  the  case  of  only  a  few  of  the  fibres.  It  is  chiefly  in  the 
nerves  belonging  to  the  cranial  and  sacral  regions  of  the  central  nervous 
system,  whence,  as  we  have  seen,  no  vaso-constrictor  fibres  are  known  to  issue, 
that  the  course  of  the  vaso-dilator  fibres  has  been  successfully  traced.  Thus 
the  vaso-dilator  fibres  for  the  submaxillary  gland  running  in  the  chorda 
tympani  may  be  traced,  as  we  have  seen,  back  to  the  facial  or  seventh  nerve ; 
and  the  continuation  of  the  chorda  tympani  along  the  lingual  nerve  to  the 
tongue  contains  vaso-dilator  fibres  for  that  organ ;  when  the  lingual  is  stim- 
ulated, the  bloodvessels  of  the  tongue  dilate  owing  to  the  stimulation  of  the 
conjoined  corda  tympani  fibres.  The  ramus  tympanicus  of  the  glosso-pharyn- 
geal  nerve  contains  vaso-dilator  fibres  for  the  parotid  gland,  and  it  appears 
probable  that  the  trigeminal  nerve  contains  vaso-dilator  fibres  for  the  eye 
and  nose  and  possibly  for  other  parts.  In  the  anterior  roots  of  the  sacral 
nerves  run  vaso-dilator  fibres  which  pass  into  the  so-called  nervi  erigentes,  the 
nerves  stimulation  of  which  by  leading  to  a  widening  of  the  arteries  of  the 
penis,  brings  about  the  erection  of  that  organ,  the  effect  being  assisted  by  a 
simultaneous  hindrance  to  the  venous  outflow.  Though  vaso-dilator  fibres 
are,  as  we  have  seen,  present  in  the  nerves  of  the  limbs,  and  probably  also 
in  those  of  the  trunk,  the  investigation  of  their  several  paths  is  rendered 
very  difficult  by  the  concomitant  presence  of  vaso-constrictor  fibres.  There 
are  some  reasons  for  thinking  that  the  vaso-dilator  fibres  in  these  nerves  pur- 
sue a  direct  course  from  the  spinal  cord  through  the  anterior  spinal  roots, 
and  thus  afford  a  contrast  with  the  constrictor  fibres  of  the  same  nerves, 
which,  as  we  have  seen,  take  a  roundabout  course,  passing  into  the  splanch- 
nic system  before  they  join  the  nerve  trunk.  Our  information,  however, 
is  too  imperfect  to  allow  any  very  positive  statement  to  be  made.  Accept- 
ing this  view,  however,  we  may  say  that  while  all  the  vaso-constrictor  fibres, 
as  far  as  we  know,  come  from  a  particular,  though  considerable,  part  of  the 
spinal  cord  and  pass  into  the  splanchnic  system  on  their  way  to  their  sev- 
eral destinations,  the  vaso-dilator  fibres  arise  from  all  parts  of  the  spinal 
cord  as  well  as  from  the  medulla  oblongata,  and  pursue  a  more  or  less  direct 
course  to  their  destination. 

Further,  while  the  vaso-dilator  fibres,  as  they  leave  the  central  nervous 
system,  are,  like  the  vaso-constrictor  fibres,  fine  medullated  fibres,  unlike 
the  vaso-constrictors  they  retain  their  medulla  for  the  greater  part  of  their 
course,  and  only  lose  it  near  their  termination  in  the  tissue  whose  blood- 
vessels they  supply. 

Lastly,  while  the  vaso-constrictor  fibres,  as  in  the  case  of  the  cervical 
sympathetic,  of  the  abdominal  splanchnic,  and  of  the  nerves  of  the  skin, 
and  probably  in  all  cases,  are  normally  in  a  state  of  moderate  activity  (so 
long  as  they  remain  in  connection  with  the  central  nervous  system),  the 
moderate  activity  maintaining  that  moderate  constriction  which  we  spoke 
of  above  as  "  tone,"  the  vaso-dilators  appear  to  possess  no  such  continued 
activity.  Section  of  vaso-constrictor  fibres  leads  to  loss  of  tone,  diminution 
of  constriction,  lasting,  as  we  shall  see,  for  some  considerable  time;  but 


VASOMOTOR  ACTIONS.  221 

section  of  vaso-dilators,  according,  at  all  events,  to  most  observers,  does  not 
lead  to  analogous  constriction  or  diminution  of  dilatation  ;  all  that  is  observed 
is  a  transient  increase  of  dilatation  due  probably  to  the  section  acting  as  a 
transient  stimulus  to  the  nerve  at  the  place  of  section.  But  before  we  study 
the  use  made  by  the  central  nervous  system  of  vasomotor  nerves,  it  will  be 
best  to  consider  briefly  some  features  of 

The  Effects  of  Vasomotor  Actions. 

§  157.  A  very  little  consideration  will  show  that  vasomotor  action  is  a 
most  important  factor  in  the  circulation.  In  the  first  place  the  whole  flow 
of  blood  in  the  body  is  adapted  to  and  governed  by  what  we  may  call  the 
general  tone  of  the  arteries  of  the  body  at  large.  In  a  normal  condition 
of  the  body  a  very  large  number  of  the  minute  arteries  of  the  body  are  in 
a  state  of  tonic,  i.  e.,  of  moderate,  contraction,  and  it  is  the  narrowing  due  to 
this  contraction  which  forms  a  large  item  of  that  peripheral  resistance  which 
we  have  seen  to  be  one  of  the  great  factors  of  blood-pressure.  The  normal 
general  blood-pressure,  and,  therefore,  the  normal  flow  of  blood,  is  in  fact 
dependent  on  the  "  general  tone  "  of  the  minute  arteries. 

In  the  second  place  local  vasomotor  changes  in  the  condition  of  the 
minute  arteries  changes — i.  e.,  of  any  particular  vascular  area — have  very 
decided  effects  on  the  circulation.  The  changes,  though  local  themselves, 
may  have  effects  which  are  both  local  and  general,  as  the  following  con- 
siderations will  show : 

Let  us  suppose  that  the  artery  A  is  in  a  condition  of  normal  tone,  is  mid- 
way between  extreme  constriction  and  dilatation.  The  flow  through  A  is 
determined  by  the  resistance  in  A,  and  in  the  vascular  tract  which  it  sup- 
plies in  relation  to  the  mean  arterial  pressure,  which  is  again  dependent  on 
the  way  in  which  the  heart  is  beating  and  on  the  peripheral  resistance  of 
all  the  small  arteries  and  capillaries,  A  included.  If,  while  the  heart  and 
the  rest  of  the  arteries  remain  unchanged,  A  be  constricted,  the  peripheral 
resistance  in  A  will  increase,  and  this  increase  of  resistance  will  lead  to  an 
increase  of  the  general  arterial  pressure.  Since,  as  we  have  seen  (§  108),  it 
is  arterial  pressure  which  is  the  immediate  cause  of  the  flow  from  the  arteries 
to  the  veins,  this  increase  of  arterial  pressure  will  tend  to  drive  more  blood 
from  the  arteries  into  the  veins.  The  constriction  of  A,  however,  by  in- 
creasing the  resistance,  opposes  any  increase  of  the  flow  through  A  itself,  in 
fact  will  make  the  flow  through  A  less  than  before.  The  whole  increase  of 
discharge  from  the  arterial  into  the  venous  system  will  take  place  through 
the  arteries  in  which  the  resistance  remains  unchanged,  that  is,  through 
channels  other  than  A.  Thus,  as  the  result  of  the  constriction  of  an  artery 
there  occur  (1)  diminished  flow  through  the  artery  itself,  (2)  increased  gen- 
eral arterial  pressure,  leading  to  (3)  increased  flow  through  the  other  arteries. 
If,  on  the  other  hand,  A  be  dilated,  while  the  heart  and  other  arteries  remain 
unchanged,  the  peripheral  resistance  in  A  is  diminished.  This  leads  to  a 
lowering  of  the  general  arterial  pressure,  which  in  turn  tends  to  drive  less 
blood  from  the  arteries  into  the  veins.  The  dilatation  of  A,  however,  by 
diminishing  the  resistance,  permits,  even  with  the  lowered  pressure,  more 
blood  to  pass  through  A  itself  than  before.  Hence  the  diminished  flow  tells 
all  the  more  on  the  rest  of  the  arteries  in  which  the  resistance  remains 
unchanged.  Thus,  as  the  result  of  the  dilatation  of  any  artery,  there  occur 
(1)  increased  flow  of  blood  through  the  artery  itself,  (2)  diminished  general 
pressure,  and  (3)  diminished  flow  through  the  other  arteries.  Where  the 
artery  thus  constricted  or  dilated  is  small,  the  local  effect,  the  diminution  or 
increase  of  flow  through  itself,  is  much  more  marked  than  the  general  effects, 


222  THE  VASCULAR  MECHANISM. 

the  change  in  blood-pressure  and  the  flow  through  other  arteries.  When, 
however,  the  area,  the  arteries  of  which  are  affected,  is  large,  the  general 
effects  are  very  striking.  Thus  if  while  a  tracing  of  the  blood- pressure  is 
being  taken  by  means  of  a  manometer  connected  with  the  carotid  artery, 
the  abdominanl  splanchnic  nerves  be  divided,  a  conspicuous  but  steady  fall 
of  pressure  is  observed,  very  similar  to  but  more  marked  than  that  which  is 
seen  in  Fig.  99.  The  section  of  the  abdominal  splanchnic  nerves  causes 
the  mesenteric  and  other  abdominal  arteries  to  dilate,  and  these  being  very 
numerous,  a  large  amount  of  peripheral  resistance  is  taken  away,  and  the 
blood-pressure  falls  accordingly ;  a  large  increase  of  flow  into  the  portal 
veins  takes  place,  and  the  supply  of  blood  to  the  face,  arms,  and  legs  is  pro- 
portionately diminished.  It  will  be  observed  that  the  dilatation  of  the  arteries 
is  not  instantaneous  but  somewhat  gradual,  as  shown  by  the  pressure  sinking 
not  abruptly  but  with  a  gentle  curve. 

The  general  effects  on  blood-pressure  by  vasomotor  changes  are  so 
marked  that  the  manometer  may  be  used  to  detect  vasomotor  actions. 
Thus,  if  the  stimulation  of  a  particular  nerve  or  any  other  operation  leads 
to  a  marked  rise  of  the  mean  blood-pressure,  unaccompanied  by  any 
changes  in  the  heart-beat,  we  may  infer  that  constriction  has  taken  place  in 
the  arteries  of  some  considerable  vascular  area ;  and  similarly,  if  the  effect 
be  a  fall  of  blood-pressure,  we  may  infer  that  constriction  has  given  way  to 
dilatation. 

Vasomotor  Functions  of  the  Central  Nervous  System. 

§  158.  The  central  nervous  system,  to  which  we  have  traced  the  vaso- 
motor nerves,  makes  use  of  these  nerves  to  regulate  the  flow  of  blood  through 
the  various  organs  and  parts  of  the  body  ;  by  the  local  effects  thus  produced 
it  assists  or  otherwise  influences  the  functional  activity  of  this  or  that  tissue; 
by  the  general  effects  it  secures  the  well-being  of  the  body. 

The  use  of  the  vaso-dilator  nerves,  which  is  more  simple  than  that  of  the 
vaso-constrictors  since  it  appears  not  to  be  complicated  by  the  presence  of 
habitual  tonic  influences,  is  frequently  conspicuous  as  part  of  a  reflex  act. 
Thus,  when  food  is  placed  in  the  mouth,  afferent  impulses,  generated  in  the 
nerves  of  taste,  give  rise  in  the  central  nervous  system  to  efferent  impulses, 
which  descend  the  chorda  tympani  and  other  nerves  to  the  salivary  glands, 
and,  by  dilating  the  bloodvessels,  secure  a  copious  flow  of  blood  through  the 
glands,  while,  as  we  shall  see  later  on,  they  excite  them  to  secrete.  The 
centre  of  this  reflex  action  appears  to  lie  in  the  medulla  oblongata,  and  may 
be  thrown  into  activity  not  only  by  impulses  reaching  it  along  the  specific 
nerves  of  taste,  but  also  by  impulses  passing  along  other  channels ;  thus, 
emotions  started  in  the  brain  by  the  sight  of  food  or  otherwise  may  give  rise 
to  impulses  passing  down  along  the  central  nervous  system  itself  to  the 
medulla  oblongata,  or  even  in  the  stomach  may  send  impulses  up  the  vagus 
nerve,  or  stimulation  of  one  kind  or  another  may  send  impulses  up  almost 
any  sentient  nerve,  and  these  various  impulses  reaching  the  medulla  may,  by 
reflex  action,  throw  into  activity  the  vaso-dilator  fibres  of  the  chorda  tym- 
pani and  other  analogous  nerves,  and  bring  about  a  flushing  of  the  salivary 
glands,  while  at  the  same  time  they  cause  the  glands  to  secrete. 

The  vaso-dilator  fibres  of  the  nervi  erigentes  may  be  thrown  into  activity 
in  a  similar  reflex  way,  the  centre  in  this  case  being  placed  in  the  sacral  or 
lumbar  portion  of  the  spinal  cord,  though  it  is  easily  thrown  into  activity 
by  impulses  descending  down  the  spinal  cord  from  the  brain  that  such  a 
centre  does  exist,  is  shown  by  the  fact  that  when  in  a  dog  the  spinal  cord  is 
completely  divided  in  the  dorsal  region,  erection  of  the  penis  may  readily  be 


VASOMOTOR  ACTIONS.  223 

brought  about  by  stimulation  of  the  sentient  surfaces.  And  other  instances 
might  be  quoted  in  which  vaso-dilator  fibres  appear  to  be  connected  with  a 
"  centre  "  soon  after  their  entrance  into  the  nervous  system. 

If,  as  seems  probable  (§  153),  the  bloodvessels  of  a'muscle  dilate  by  vaso- 
motor  action  whenever  the  muscle  is  thrown  into  contraction,  either  in  a 
reflex  or  voluntary  movement,  the  vaso-dilator  fibres  of  the  muscle  would 
seem  to  be  thrown  into  action  by  impulses  arising  in  the  spinal  cord  not  far 
from  the  origin  of  the  ordinary  motor  impulses  and  accompanying  those 
motor  impulses  along  the  motor  nerve. 

§  159.  The  case  of  the  vaso-constrictor  fibres  is  somewhat  more  com- 
plicated on  account  of  the  existence  of  tonic  influences  ;  since  the  same  fibres 
may,  on  the  one  hand,  by  an  increase  in  the  impulses  passing  along  them, 
be  the  means  of  constriction,  and,  on  the  other  hand,  by  the  removal  or 
diminution  of  the  tonic  influences  passing  along  them,  be  the  means  of  dila- 
tation. We  have  already  traced  all  the  vaso-constrictor  fibres  from  the 
middle  region  of  the  spinal  cord  to  the  splanchnic  system  in  the  thorax  and 
abdomen,  from  whence  they  pass  (1)  by  the  abdominal  splanchnic  and  by  the 
hypogastric  nerves  to  the  viscera  of  the  abdomen  and  pelvis  (concerning  the 
vasomotor  nerves  of  the  thoracic  viscera  we  know  at  present  very  little)  ; 
(2)  by  the  cervical  sympathetic  or  cervical  splanchnic,  as  it  might  be  called, 
to  the  skin  of  the  head  and  neck,  the  salivary  glands  and  mouth,  the  eyes 
and  other  parts,  and  probably  the  brain,  including  its  membranes ;  (3)  by 
the  brachial  and  sciatic  plexuses  to  the  skin  of  the  fore  and  hind  limbs,  and 
by  various  other  nerves  to  the  skin  of  the  trunk.  The  chief  parts  of  the 
body  supplied  by  vaso-constrictor  fibres  appear  to  be  the  skin,  with  its  ap- 
pendages, and  the  alimentary  canal,  with  its  appendages,  glandular  and 
other  ;  the  great  mass  of  skeletal  muscles  appears  to  receive  an  insignificant 
supply  of  vaso-constrictor  fibres. 

If,  now,  in  an  animal,  the  spinal  cord  be  divided  in  the  lower  dorsal 
region,  the  skin  of  the  legs  becomes  flushed,  their  temperature  frequently 
rises,  and  there  is  a  certain  amount  of  fall  in  the  general  blood-pressure  as 
measured,  for  instance,  in  the  carotid  ;  and  this  state  of  things  may  last  for 
some  considerable  time.  Obviously,  the  section  of  the  spinal  cord  has  cut 
off  the  usual  tonic  influences  descending  to  the  lower  limbs ;  in  consequence 
the  bloodvessels  have  become  dilated,  thus  causing  the  general  peripheral 
resistance  to  become  proportionately  diminished,  and  the  general  blood- 
pressure  to  fall.  The  tonic  vaso-constrictor  impulses  for  the  lower  limbs, 
therefore,  have  their  origin  in  the  central  nervous  system  higher  up  than 
the  lower  dorsal  region  of  the  spinal  cord. 

If  the  spinal  cord  be  divided  between  the  roots  of  the  fifth  and  sixth 
dorsal  nerves  (that  is  to  say,  at  the  level  where  the  path  of  the  splanchnic 
fibres  from  the  cord  seems  to  divide  (see  Fig.  77)  those  issuing  above  pass- 
ing upward  to  the  fore  limbs  and  head,  and  those  issuing  below  passing  to 
the  abdomen  and  lower  limbs),  the  cutaneous  bloodvessels  of  the  lower  limbs 
dilate,  as  in  the  former  case,  and  on  examination  it  will  be  found  that  the 
bloodvessels  of  the  abdomen  are  also  largely  dilated  ;  at  the  same  time  the 
blood-pressure  undergoes  a  very  marked  fall ;  it  may,  indeed,  be  reduced  to 
a  very  few  millimetres  of  mercury.  Obviously,  the  tonic  vaso-constrictor 
impulses  passing  to  the  abdomen  and  to  the  lower  limbs  take  origin  in  the 
central  nervous  system  higher  up  than  the  level  of  the  fifth  dorsal  nerve. 

If  the  section  of  the  spinal  cord  be  made  above  the  level  of  the  second 
dorsal  nerve,  in  addition  to  the  above-mentioned  results  the  vessels  of  the 
head  and  face  also  become  dilated  ;  but,  in  consequence  of  the  fall  of  general 
blood-pressure  just  mentioned,  these  vessels  never  become  so  full  of  blood, 
the  loss  of  tone  is  not  so  obvious  in  them  as  after  simple  division  of  the 


224  THE  VASCULAR  MECHANISM. 

cervical  sympathetic,  since  the  latter  operation  produces  little  or  no  effect  on 
the  general  blood-pressure. 

Obviously,  then,  the  tonic  vaso-constrictor  impulses,  which  passing  to  the 
skin  and  viscera  of  the  body  maintain  that  tonic  narrowing  of  so  many  small 
arteries  by  which  the  general  peripheral  resistance,  and  so  the  general  blood- 
pressure,  is  maintained,  proceed  from  some  part  of  the  central  nervous 
system  higher  up  than  the  upper  dorsal  region  of  the  spinal  cord.  And, 
since  exactly  the  same  results  follow  upon  section  of  the  spinal  cord  in  the 
cervical  region  right  up  to  the  lower  limit  of  the  medulla  oblongata,  we  infer 
that  these  tonic  impulses  proceed  from  the  medulla  oblongata. 

On  the  other  hand,  we  may  remove  the  whole  of  the  brain  right  down  to 
the  upper  parts  of  the  medulla,  and  yet  produce  no  flushing,  or  only  a  slight 
transient  flushing,  of  any  part  of  the  body,  and  no  fall  at  all,  or  only  a  slight 
transient  fall,  of  the  general  blood-pressure.  We,  therefore,  seem  justified 
in  assuming  the  existence  in  the  medulla  oblongata  of  a  nervous  centre, 
which  we  may  speak  of  as  a  vasomotor  centre,  or  the  medullary  vasomotor 
centre,  from  which  proceed  tonic  vaso-constrictor  impulses,  or  which  regu- 
lates the  emission  and  distribution  of  such  tonic  vaso-constrictor  impulses  or 
influences  over  various  parts  of  the  body. 

§  160.  The  existence  of  this  vasomotor  centre  may,  moreover,  be  shown 
in  another  way.  The  extent  or  amount  of  the  tonic  constrictor  impulses 
proceeding  from  it  may  be  increased  or  diminished,  the  activity  of  the  centre 
may  be  augmented  or  inhibited  by  impulses  reaching  it  along  various  afferent 
nerves  ;  and  provided  no  marked  changes  in  the  heart-beat  take  place  at  the 
same  time,  a  rise  or  fall  of  general  blood-pressure  may  be  taken  as  a  token 
of  an  increase  or  decrease  of  the  activity  of  the  centre. 

In  the  rabbit  there  is  found  in  the  neck,  lying  side  by  side  with  the 
cervical  sympathetic  nerve  and  running  for  some  distance  in  company  with 
it,  a  slender  nerve  which  may  be  ultimately  traced  down  to  the  heart,  and 
which  if  traced  upward  is  found  to  come  off  somewhat  high  up  from  the 
vagus,  by  two  or  more  roots,  one  of  which  is  generally  a  branch  of  the 
superior  laryngeal  nerve.  This  nerve  (the  fibres  constituting  which  are,  in 
the  dog,  bound  up  with  the  vagus,  and  do  not  form  an  independent  nerve) 
appears  to  be  exclusively  an  afferent  nerve  ;  when,  after  division  of  the  nerve 
the  peripheral  end,  the  end  still  in  connection  with  the  heart,  is  stimulated, 
no  marked  results  follow.  The  beginnings  of  the  nerve  in  the  heart  are 
therefore  quite  different  from  the  endings  of  the  inhibitory  fibres  of  the 
vagus,  or  of  the  augmentor  fibres  of  the  splanchnic  (sympathetic)  system  ; 
the  nerve  has  nothing  to  do  with  the  nervous  regulation  of  the  heart  (see 
§§  139  et  seq.*).  If,  now,  while  the  pressure  in  an  artery  such  as  the  carotid  is 
being  registered,  the  central  end  of  the  nerve — i.  e.,  the  one  connected  with 
the  brain — be  stimulated  with  the  interrupted  current,  a  gradual  but  marked 
fall  of  pressure  (Fig.  79)  in  the  carotid  is  observed,  lasting,  when  the  period 
of  stimulation  is  short,  some  time  after  the  removal  of  the  stimulus.  Since 
the  beat  of  the  heart  is  not  markedly  changed,  the  fall  of  pressure  must  be 
due  to  the  diminution  of  peripheral  resistance  occasioned  by  the  dilatation 
of  some  arteries.  And  it  is  probable  that  the  arteries  thus  dilated  are 
chiefly,  if  not  exclusively,  those  arteries  of  the  abdominal  viscera  which  are 
governed  by  the  abdominal  splanchnic  nerves ;  for  if  these  nerves  are 
divided  on  both  sides  previous  to  the  experiment,  the  fall  of  pressure  when 
the  nerve  is  stimulated  is  very  small — in  fact,  almost  insignificant.  The 
inference  we  draw  is  as  follows :  The  afferent  impulses,  passing  upward  along 
the  nerve  in  question,  have  so  affected  some  part  of  the  central  nervous 
system  that  the  influences  which,  in  a  normal  condition  of  things,  passing 
along  the  abdominal  splanchnic  nerves  keep  the  minute  arteries  of  the 


VASOMOTOK  ACTIONS.  225 

abdominal  viscera  in  a  state  of  moderate  tonic  constriction,  fail  altogether, 
and  those  arteries  in  consequence  dilate  just  as  they  do  when  the  abdominal 

FIG.  79. 


Tracing  showing  the  Effect  on  Blood-pressure  of  Stimulating  the  Central  End  of  the  Depressor 
Nerve  in  the  Rabbit.  On  the  time-marker  below  the  intervals  correspond  to  seconds.  At  x  an 
interrupted  current  was  thrown  into  the  nerve. 

splanchnic  nerves  are  divided,  the  effect  being  possibly  increased  by  the 
similar  dilatation  of  other  vascular  areas.  Since  stimulation  of  the  nerve 
of  which  we  are  speaking  always  produces  a  fall,  never  a  rise,  of  blood-pres- 
sure— the  amount  of  fall,  of  course,  being  dependent  on  circumstances,  such 
as  the  condition  of  the  nervous  system,  state  of  blood-pressure,  etc. — the 
nerve  is  known  by  the  name  of  the  depressor  nerve.  As  we  shall  point  out 
later  on,  by  means  of  this  afferent  nerve  from  the  heart  the  peripheral  resist- 
ance is,  in  the  living  body,  lowered  to  suit  the  weakened  powers  of  a  laboring 
heart. 

This  gradual  lowering  of  blood-pressure  by  diminution  of  peripheral 
resistance  affords  a  marked  contrast  to  the  sudden  lowering  of  blood-pres- 
sure by  cardiac  inhibition.  (Compare  Fig.  79  with  Fig.  75.) 

§  161.  But  the  general  blood-pressure  may  be  modified  by  afferent  im- 
pulses passing  along  other  nerves  than  the  depressor,  the  modification 
taking  on,  according  to  circumstances,  the  form  either  of  decrease  or  of 
increase. 

Thus,  if,  in  an  animal  placed  under  the  influence  of  urari  (some  anaes- 
thetic other  than  chloral,  etc.,  being  used),  the  central  stump  of  the  divided 
sciatic  nerve  be  stimulated,  an  increase  of  blood-pressure  (Fig.  80)  almost 

FIG.  80. 


Effect  on  Blood-pressure  Curve  of  Stimulating  Sciatic  Nerve  under  Urari.  (Cat.)  x  marks 
the  moment  in  which  the  current  was  thrown  into  the  nerve.  Artificial  respiration  was  car- 
ried on,  and  the  usual  respiratory  undulations  are  absent. 

exactly  the  reverse  of  the  decrease  brought  about  by  stimulating  the  de- 
pressor, is  observed.  The  curve  of  the  blood-pressure,  after  a  latent  period 
during  which  no  changes  are  visible,  rises,  steadily  without  any  corre- 
sponding change  in  the  heart's  beat,  reaches  a  maximum,  and  after  a 

15 


226  THE  VASCULAR  MECHANISM. 

while  slowly  falls  again,  the  fall  sometimes  beginning  to  appear  before  the 
stimulus  has  been  removed.  There  can  be  no  doubt  that  the  rise  of  pres- 
sure is  due  to  the  constriction  of  certain  arteries ;  the  arteries  in  question 
being  those  of  the  abdominal  splanchnic  area  certainly,  and  possibly  those 
of  other  vascular  areas  as  well.  The  effect  is  not  confined  to  the  sciatic  ; 
stimulation  of  any  nerve  containing  afferent  fibres  may  produce  the  same 
rise  of  pressure,  and  so  constant  is  the  result  that  the  experiment  has  been 
made  use  of  as  a  method  of  determining  the  existence  of  afferent  fibres  in 
any  given  nerve  and  even  the  paths  of  centripetal  impulses  through  the 
spinal  cord. 

If,  on  the  other  hand,  the  animal  be  under  the  influence  not,  of  urari,' 
but  of  a  large  dose  of  chloral,  instead  of  a  rise  of  blood-pressure,  a  fall 
quite  similar  to  that  caused  by  stimulating  the  depressor  is  observed  when 
an  afferent  nerve  is  stimulated.  The  condition  of  the  central  nervous 
system  seems  to  determine  whether  the  effect  of  afferent  impulses  on  the 
central  nervous  system  is  one  leading  to  an  augmentation  of  vaso-constrictor 
impulses  and  so  to  a  rise,  or  one  leading  to  a  diminution  of  vaso-constrictor 
impulses  and  so  to  a  fall  of  blood-pressure. 

§  162.  We  have  used  the  words  "  central  nervous  system  "  in  speaking 
of  the  above  ;  we  have  evidence,  however,  that  the  part  of  the  central  nervous 
system  acted  on  by  the  afferent  impulses  is  the  vasomotor  centre  in  the 
medulla  oblongata,  and  that  the  effects  in  the  way  of  diminution  (depressor) 
or  of  augmentation  (pressor)  are  the  results  of  afferent  impulses  inhibiting 
or  augmenting  the  tonic  activity  of  this  centre  or  of  a  part  of  this  centre 
especially  connected  with  abdominal  splanchnic  nerves.  The  whole  brain  may 
be  removed  right  down  to  the  medulla  oblongata,  and  yet  the  effects  of  stimu- 
lation in  the  direction  either  of  diminution  or  of  augmentation  may  still  be 
brought  about.  If  the  medulla  oblongata  be  removed,  these  effects  vanish 
too,  though  all  the  rest  of  the  nervous  system  be  left  intact.  Nay,  more,  by 
partially  interfering  with  the  medulla  oblongata,  we  may  partially  diminish 
these  effects  and  thus  mark  out,  so  to  speak,  the  limits  of  the  centre  in  ques- 
tion within  the  medulla  itself.  Thus,  in  an  intact  animal  under  urari,  stimu- 
lation of  the  sciatic  nerve  with  a  stimulus  of  a  certain  strength  will  produce 
a  rise  of  blood-pressure  up  to  a  certain  extent.  After  removal  of  the  whole 
brain  right  down  to  the  medulla  oblongata,  the  same  stimulation  will  produce 
the  same  rise  as  before ;  the  vasomotor  centre  has  not  been  interfered  with. 
Directly,  however,  in  proceeding  downward,  the  region  of  the  centre  in  ques- 
tion is  reached,  stimulation  of  the  sciatic  produces  less  and  less  rise,  until  at 
last  when  the  lower  limit  of  the  centre  is  arrived  at  no  effect  at  all  on  blood- 
pressure  can  be  produced  by  even  strong  stimulation  of  the  sciatic  or  other 
afferent  nerve.  In  this  way  the  lower  limit  of  the  medullary  vasomotor 
centre  has  been  determined  in  the  rabbit  at  a  horizontal  line  drawn  about 
4  or  5  mm.  above  the  point  of  the  calamus  scriptorius,  and  the  upper  limit 
at  about  4  mm.  higher  up — i.  e.,  about  1  or  2  mm.  below  the  corpora  quadri- 
gemina.  When  transverse  sections  of  the  brain  are  carried  successively 
lower  and  lower  down,  an  effect  on  blood-pressure  in  the  way  of  lowering  it 
and  also  of  diminishing  the  rise  of  blood-pressure  resulting  from  stimulation 
of  the  sciatic,  is  first  observed  when  the  upper  limit  is  reached.  On  carrying 
the  sections  still  lower,  the  effects  of  stimulating  the  sciatic  become  less  and 
less,  until  when  the  lower  limit  is  reached  no  effects  at  all  are  observed.  The 
centre  appears  to  be  bilateral,  the  halves  being  placed  not  in  the  middle  line, 
but  more  sideways  and  rather  nearer  the  anterior  than  the  posterior  surface. 
It  may  perhaps  be  more  closely  defined  as  a  small  prismatic  space  in  the 
forward  prolongation  of  the  lateral  columns  after  they  have  given  off  their 
fibres  to  the  decussating  pyramids.  This  space  is  largely  occupied  by  a  mass 


VASOMOTOR  ACTIONS.  227 

of  gray  matter,  called  by  Clarke  the  antero-lateral  nucleus,  and  containing 
large  multipolar  cells  ;  but  it  is  by  no  means  certain  that  this  group  of  nerve 
cells  really  acts  as  the  centre  in  question. 

§  163.  The  above  experiments  appear  to  afford  adequate  evidence  that,  in 
a  normal  state  of  the  body,  the  integrity  of  the  medullary  vasomotor  centre 
is  essential  to  the  production  and  distribution  of  those  continued  constrictor 
impulses  by  which  the  general  arterial  tone  of  the  body  is  maintained,  and 
that  an  increase  or  decrease  of  vaso-constrictor  action  in  particular  arteries, 
or  in  the  arteries  generally,  is  brought  about  by  means  of  the  same  medullary 
vasomotor  centre.  But  we  must  not,  therefore,  conclude  that  this  small  por- 
tion of  the  medulla  oblongata  is  the  only  part  of  the  central  nervous  system 
which  can  act  as  a  centre  for  vaso-constrictor  fibres  ;  and,  as  we  have  seen, 
there  is  no  evidence  at  present  that  the  vaso-dilator  fibres  are  connected 
with  either  this  or  any  other  one  centre.  In  the  frog  reflex  vasomotor  effects 
may  be  obtained  by  stimulating  various  afferent  nerves  after  the  whole 
medulla  has  been  removed,  and,  indeed,  even  when  only  a  comparatively 
small  portion  of  the  spinal  cord  has  been  left  intact  and  connected,  on  the 
one  hand,  with  the  afferent  nerve  which  is  being  stimulated,  and,  on  the 
other,  with  the  efferent  nerves  in  which  run  the  vasomotor  fibres  whose  action 
is  being  studied.  In  the  mammal  such  effects  do  not  so  readily  appear,  but 
may  with  care  and  under  special  conditions  be  obtained.  Thus  in  the  dog, 
when  the  spinal  cord  is  divided  in  the  dorsal  region,  the  arteries  of  the  hind 
limbs  and  hinder  part  of  the  body,  as  we  have  already  said  (§  158),  become 
dilated.  This  one  would  naturally  expect  as  the  result  of  their  severance 
from  the  medullary  vasomotor  centre.  But  if  the  animal  be  kept  in  good 
condition  for  some  time,  a  normal  or  nearly  normal  arterial  tone  is  after  a 
while  re-established  ;  and  the  tone  thus  regained  may,  by  afferent  impulses 
reaching  the  cord  below  the  section,  be  modified  in  the  direction  certainly 
of  diminution — i.  e.,  dilatation — and  possibly,  but  this  is  by  no  means  so  cer- 
tain, of  increase — i.  e.,  constriction.  Dilatation  of  various  cutaneous  vessels 
of  the  limbs  may  be  readily  produced  by  stimulation  of  the  central  stump 
of  one  or  another  nerve. 

These  remarkable  results,  which,  though  they  are  most  striking  in  connec- 
tion with  the  lower  part  of  the  spinal  cord,  hold  good  apparently  for  other 
parts  also  of  the  spinal  core,  naturally  suggest  a  doubt  whether  the  explana- 
tion just  given  above  of  the  effects  of  section  of  the  medulla  oblongata  is  a 
valid  one.  When  we  come  to  study  the  central  nervous  system,  we  shall 
again  and  again  see  that  the  immediate  effect  of  operative  interference  with 
these  delicate  structures  is  a  temporary  suspension  of  nearly  all  their  func- 
tions. This  is  often  spoken  of  as  "shock"  and  may  be  regarded  as  an 
extreme  form  of  inhibition.  An  example  of  it  occurs  in  the  above  experi- 
ment of  section  of  the  dorsal  cord.  For  some  time  after  the  operation  the 
vaso-dilator  nervi  erigentes  (which,  as  far  as  we  know,  have  no  special  connec- 
tion with  the  medullary  vasomotor  centre)  cannot  be  thrown  into  activity  as 
part  of  a  reflex  action ;  their  centre  remains  for  some  time  inactive.  After 
a  while,  however,  it  recovers,  and  erection  of  the  penis  through  the  nervi 
erigentes  may  then  still  be  brought  about  by  suitable  stimulation  of  sensory 
surfaces.  Hence  the  question  may  fairly  be  put  whether  the  effects  of  cut- 
ting and  injuring  the  structures  which  we  have  spoken  of  as  the  medullary 
vasomotor  centre,  are  not  in  reality  simply  those  of  shock,  whether  the  vas- 
cular dilatation  which  follows  upon  sections  of  the  so-called  medullary  vaso- 
motor centre,  does  not  come  about  because  section  of  or  injury  to  this^ region 
exercises  a  strong  inhibitory  influence  on  all  the  vasomotor  centres  situated 
in  the  spinal  cord  below.  Owing  to  the  special  function  of  the  medulla 
oblongata  in  carrying  on  the  all-important  work  of  respiration,  a  mammal 


228  THE  VASCULAR  MECHANISM. 

whose  medulla  has  been  divided  cannot  be  kept  alive  for  any  length  of  time. 
We  cannot,  therefore,  put  the  matter  to  the  simple  experimental  test  of  extir- 
pating the  supposed  medullary  vasomotor  centre  and  seeing  what  happens 
when  the  animal  has  completely  recovered  from  the  effects  of  the  operation ; 
we  have  to  be  guided  in  our  decision  by  more  or  less  indirect  arguments. 
And  against  the  argument  that  the  effects  are  those  of  shock,  we  may  put 
the  argument,  evidence  for  which  we  shall  meet  with  in  dealing  with  the 
central  nervous  system,  that  when  one  part  of  the  central  nervous  system  is 
removed  or  in  any  way  placed  hors  de  combat,  another  part  may  vicariously 
take  on  its  function  ;  in  the  absence  of  the  medullary  vasomotor  centre,  its 
function  may  be  performed  by  other  parts  of  the  spinal  cord  which  in  its 
presence  do  no  such  work. 

And  we  may,  in  connection  with  this,  call  attention  to  the  fact  that  the 
dilatation  or  loss  of  tone  which  follows  upon  section  of  the  cervical  sympathetic 
(and  the  same  is  true  of  the  abdominal  splanchnic)  is  not  always,  though  it 
may  be  sometimes,  permanent ;  in  a  certain  number  of  cases  "it  has  been 
found  that  after  a  while,  it  may  not  be  until  after  several  days,  the  dilatation 
disappears  and  the  arteries  regain  their  usual  calibre ;  on  the  other  hand,  in 
some  cases  no  such  return  has  been  observed  after  months  or  even  years. 
This  recovery,  when  it  occurs,  cannot  always  be  attributed  to  any  regenera- 
tion of  vasomotor  fibres  in  the  sympathetic,  for  it  is  stated  to  have  been 
observed  when  the  whole  length  of  the  nerve  including  the  superior  cervical 
ganglion  has  been  removed.  When  recovery  of  tone  has  thus  taken  place, 
dilatation  or  increased  constriction  may  be  occasioned  by  local  treatment ;  the 
ear  may  be  made  to  blush  or  pale  by  the  application  of  heat  or  cold,  by 
gentle  stroking  or  rough  handling  and  the  like  ;  but  neither  the  one  nor  the 
other  condition  can  be  brought  about  by  the  intervention  of  the  central 
nervous  system.  So  also  the  spontaneous  rhythmic  variations  in  the  calibre 
of  the  arteries  of  the  ear  of  which  we  speak,  though  they  cease  for  a  time 
after  division  of  the  cervical  sympathetic,  may  in  some  cases  eventually  reap- 
pear and  that  even  if  the  superior  cervical  ganglion  be  removed ;  in  other 
cases  they  do  not.  And  the  analogous  rhythmic  variations  of  the  veins  of 
the  bat's  wing  have  been  proved  experimentally  to  go  on  vigorously  when 
all  connection  with  the  central  nervous  system  has  been  severed ;  they  may 
continue,  in  fact,  in  isolated  pieces  of  the  wing  provided  that  the  vessels  are 
adequately  filled  and  distended  with  blood  or  fluid.  From  these  and  other 
facts,  even  after  making  allowance  for  the  negative  cases,  we  may  conclude 
that  what  we  have  spoken  of  as  the  tone  of  the  vessels  of  the  face,  though 
influenced  by  and  in  a  measure  dependent  on  the  central  nervous  system, 
is  not  simply  the  result  of  an  effort  of  that  system.  The  muscular  walls  of 
the  arteries  are  not  mere  passive  instruments  worked  by  the  central  nervous 
system  through  the  vasomotor  fibres ;  they  appear  to  have  an  intrinsic  tone 
of  their  own,  and  it  seems  natural  to  suppose  that  when  the  central  nervous 
system  causes  dilatation  or  constriction  of  the  vessels  of  the  face,  it  makes  use, 
in  so  doing,  of  this  intrinsic  local  tone.  It  has  been  supposed  that  this 
intrinsic  tone  is  dependent  on  some  local  nervous  mechanism  ;  in  the  ear  at 
least  no  such  mechanism  has  yet  been  found  ;  and,  indeed,  as  we  have  said 
above  (§  176),  no  such  peripheral  nervous  mechanism  is  really  necessary.  In 
the  case  both  of  a  vessel  governed  by  vaso-dilator  fibres  and  one  governed 
by  vaso-constrictor  fibres,  we  may  suppose  a  certain  natural  condition  of  the 
muscular  fibres  which  we  may  call  a  condition  of  equilibrium.  In  a  vessel 
governed  only  by  vaso-dilator  fibres,  if  there  be  such,  this  condition  of 
equilibrium  is  the  permanent  condition  of  the  muscular  fibre,  from  which  it 
is  disturbed  by  vaso-dilator  impulses,  but  to  which  it  speedily  returns.  In 
a  vessel  governed  by  vaso-constrictor  fibres,  and  subject  to  tone,  the  muscu- 


VASOMOTOR  ACTIONS.  229 

lar  fibre  is  habitually  kept  on  the  constrictor  side  of  this  equilibrium,  and, 
as  in  the  cases  quoted  above,  may  strive  of  itself  toward  some  amount  of 
active  constriction  even  when  separated  from  the  central  nervous  system. 

But  to  return  to  the  medullary  vasomotor  centre.  Without  attempting 
to  discuss  the  matter  fully,  we  may  say  that,  after  all  due  weight  has  been 
attached  to  the  play  of  inhibitory  impulses  or  "  shock  "  as  a  result  of  opera- 
tive interference,  there  still  remains  a  balance  of  evidence  in  favor  of  the 
view  that  the  region  of  the  medulla  of  which  we  are  speaking  does  really 
act  as  a  general  vasomotor  centre  in  the  manner  previously  explained,  and 
plays  an  important  part  in  the  vasomotor  regulation  of  the  living  body. 

It  is  not,  however,  to  be  regarded  as  the  single  vasomotor  centre,  whence 
alone  can  issue  tonic-constrictor  impulses  or  whither  afferent  impulses  from 
all  parts  of  the  body  must  always  travel  before  they  can  affect  the  vaso- 
motor impulses  passing  along  this  or  that  nerve.  We  are  rather  to  suppose 
that  the  spinal  cord  along  its  whole  length  contains,  interlaced  with  the 
reflex  and  other  mechanisms  by  which  the  skeletal  muscles  are  governed, 
vasomotor  centres  and  mechanisms  of  varied  complexity,  the  details  of 
whose  functions  and  topography  have  yet  largely  to  be  worked  out ;  and 
though,  as  we  have  seen,  the  medullary  centre  is  essentially  a  centre  of  im- 
pulses issuing  along  vaso-constrictor  fibres,  it  is  possible  that  there  are  ties 
between  it  and  vaso-dilator  fibres  also.  As  in  the  absence  of  the  sinus 
venosus,  the  auricles  and  ventricle  of  the  frog's  heart  may  still  continue  to 
beat,  so  in  the  absence  of  the  medulla  oblongata  these  spinal  vasomotor 
centres  provide  for  the  vascular  emergencies  which  arise.  As,  however,  in 
the  normal  entire  frog's  heart,  the  sinus,  so  to  speak,  gives  the  word  and 
governs  the  work  of  the  whole  organ,  so  the  medullary  vasomotor  centre 
rules  and  co-ordinates  the  lesser  centres  of  the  cord,  and  through  them  pre- 
sides over  the  chief  vascular  areas  of  the  body.  By  means  of  these  vaso- 
motor central  mechanisms,  by  means  of  the  head  centre  in  the  medulla,  and 
the  subsidiary  centres  in  the  spinal  cord,  the  delicate  machinery  of  the  circu- 
lation which  determines  the  blood-supply,  and  so  the  activity  of  each  tissue 
and  organ,  is  able  to  respond  by  narrowing  or  widening  arteries  to  the  ever- 
varying  demands  and  to  meet  by  compensating  changes  the  shocks  and 
strains  of  daily  life. 

§  164.  We  may  sum  up  the  history  of  vasomotor  actions  somewhat  as 
follows : 

All,  or  nearly  all — or,  as  far  as  we  know,  all — the  arteries  of  the  body  are 
connected  with  the  central  nervous  system  by  nerve  fibres,  called  vaso- 
motor fibres,  the  action  of  which  varies  the  amount  of  contraction  of  the 
muscular  coats  of  the  arteries,  and  so  leads  to  changes  in  calibre.  The 
action  of  these  vasomotor  fibres  is  more  manifest  and  probably  more 
important  in  the  case  of  small  and  minute  arteries  than  in  the  case  of 
larger  ones. 

These  vasomotor  fibres  are  of  two  kinds :  the  one  kind,  vaso-constrictor 
fibres,  are  of  such  a  nature  or  have  such  connections  at  their  central  origin 
or  peripheral  endings  that  stimulation  of  them  produces  narrowing,  con- 
striction of  the  arteries  ;  and  during  life  these  fibres  appear  to  be  the  means 
by  which  the  central  nervous  system  exerts  a  continued  tonic  influence  on 
the  arteries  and  maintains  an  arterial  "  tone."  The  other  kind,  the  vaso- 
dilator fibres,  are  of  such  a  kind  or  have  such  connections  that  stimulation 
of  them  produces  widening,  dilation  of  the  arteries.  There  is  no  adequate 
evidence  that  these  vaso-dilator  fibres  serve  as  channels  for  tonic  dilating 
impulses  or  influences. 

The  vaso-constrictor  fibres  leave  the  spinal  cord  by  the  anterior  roots  of 
the  nerves  coming  from  middle  regions  only  of  the  spinal  cord  (in  the  dog, 


230  THE  VASCULAR  MECHANISM. 

and  probably  in  other  mammals  from  about  the  first  dorsal  to  the  fifth 
lumbar  nerve),  pass  into  the  splanchnic  ganglia  connected  with  those  nerves 
(thoracic  and  abdominal  chain  of  sympathetic  ganglia),  where  the  fibres  lose 
their  medulla,  and  proceed  to  their  destination  as  non-medullated  fibres, 
either  still  in  so-called  sympathetic  nerves,  such  as  splanchnic,  cervical 
sympathetic,  hypogastric,  etc.,  or  along  recurrent  branches  of  the  splanchnic 
system,  to  join  the  spinal  nerves  of  the  arm,  leg,  and  trunk. 

In  the  intact  organism  the  emission  and  distribution  along  these  vaso- 
constrictor fibres  of  tonic-constrictor  impulses,  by  which  general  and  local 
arterial  tone  is  maintained  and  regulated,  is  governed  by  a  limited  portion 
of  the  medulla  oblongata  known  as  the  medullary  vasomotor  centre ;  and 
when  some  change  of  conditions  or  other  natural  stimulus  brings  about  a 
change  in  the  activity  of  the  vaso-constrictor  fibres  of  one  or  more  vascular 
areas,  or  of  all  the  arteries  supplied  with  vaso-constrictor  fibres,  this  same 
medullary  vasomotor  centre  appears  in  such  cases  to  play  the  part  of  a 
centre  of  reflex  action.  Nevertheless,  in  cases  where  the  nervous  connections 
of  this  medullary  vasomotor  centre  with  a  vascular  area  are  cut  off  by  an 
operation,  as  by  section  of  the  cord,  other  parts  of  the  spinal  cord  may  act 
as  centres  for  the  vaso-constrictor  fibres  of  the  area,  and  possibly  these 
subordinate  centres  may  be  to  a  certain  extent  in  action  in  the  intact 
organism. 

The  vaso-dilator  fibres  appear  to  take  origin  in  various  parts  of  the 
central  nervous  system  and  to  proceed  in  a  direct  course  to  their  destination 
along  the  (anterior)  roots  and  as  part  of  the  trunks  and  branches  of  various 
cerebro-spinal  nerves ;  they  do  not  lose  their  medulla  until  they  approach 
their  termination.  They  do  not  appear  to  serve  as  channels  of  tonic  dilating 
influences ;  they  are  thrown  into  action  generally  as  part  of  a  reflex  action, 
and  their  centre  in  the  reflex  act  appears  in  each  case  to  lie  in  the  central 
nervous  system  not  far  from  the  centre  of  the  ordinary  motor  fibres  which 
they  accompany. 

The  effects  of  the  activity  of  the  vaso-dilator  fibres  appear  to  be  essentially 
local  in  nature ;  when  any  set  of  them  come  into  action  the  vascular  area 
which  these  govern  is  dilated.  And  the  vascular  areas  so  governed  are 
relatively  so  small  that  changes  in  them  produce  little  or  no  effect  on  the 
vascular  system  in  general. 

The  effects  of  changes  in  the  activity  of  the  vaso-constrictor  fibres  are 
both  local  and  general,  and  may  be  also  double  in  nature.  By  an  inhibition 
of  tonic-constrictor  impulses  a  certain  amount  of  dilation  may  be  effected ; 
by  an  augmentation  of  constrictor  impulses,  constriction,  it  may  be  of  con- 
siderable extent,  may  be  brought  about.  When  the  vascular  area  so  affected 
is  small,  the  effects  are  local,  more  or  less  blood  is  distributed  through  the 
area ;  when  the  vascular  area  affected  is  large,  the  inhibition  of  constriction 
may  lead  to  a  marked  fall,  and  an  augmentation  of  constriction  to  a  marked 
rise  of  general  blood-pressure. 

§  165.  We  shall  have  occasion  later  on  again  and  again  to  point  out 
instances  of  the  effects  of  vasomotor  action,  both  local  and  general,  but  we 
may  here  quote  one  or  two  characteristic  ones.  "  Blushing  "  is  one.  Ner- 
vous impulses  started  in  some  parts  of  the  brain  by  an  emotion  produce  a 
powerful  inhibition  of  that  part  of  the  medullary  vasomotor  centre  which 
governs  the  vascular  areas  of  the  head  supplied  by  the  cervical  sympathetic, 
and  hence  has  an  effect  on  the  vasomotor  fibres  of  the  cervical  sympathetic 
almost  exactly  the  same  as  that  produced  by  section  of  the  nerve.  In  conse- 
quence the  muscular  walls  of  the  arteries  of  the  head  and  face  relax,  the 
arteries  dilate,  and  the  whole  region  becomes  suffused.  Sometimes  an  emo- 
tion gives  rise  not  to  blushing,  but  to  the  opposite  effect,  viz.,  to  pallor. 


THE  CAPILLARY  CIRCULATION.  231 

In  a  great  number  of  cases  this  has  quite  a  different  cause,  being  due  to  a 
sudden  diminution  or  even  temporary  arrest  of  the  heart's  beats ;  but  in 
some  cases  it  may  occur  without  any  change  in  the  beat  of  the  heart,  and  is 
then  due  to  a  condition  the  very  converse  of  that  of  blushing,  that  is,  to  an 
increased  arterial  constriction ;  and  this  increased  constriction,  like  the  dila- 
tation of  blushing,  is  effected  through  the  agency  of  the  central  nervous 
system  and  the  cervical  sympathetic. 

The  vascular  condition  of  the  skin  at  large  affords  another  instance. 
When  the  temperature  of  the  air  is  low  the  vessels  of  the  skin  are  con- 
stricted and  the  skin  is  pale ;  when  the  temperature  of  the  air  is  high  the 
vessels  of  the  skin  are  dilated  and  the  skin  is  red  and  flushed.  In  both 
these  cases  the  effect  is  mainly  a  reflex  one,  it  being  the  central  nervous 
system  which  brings  about  augmentation  of  constriction  in  the  one  case  and 
inhibition  in  the  other,  though  possibly  some  slight  effect  is  reproduced  by 
the  direct  action  of  the  cold  or  heat  on  the  vessels  of  the  skin  simply.  More- 
over, the  vascular  changes  in  the  skin  are  accompanied  by  corresponding 
vascular  changes  in  the  viscera  (chiefly  abdominal)  of  a  reverse  kind.  When 
the  vessels  of  the  skin  are  dilated  those  of  the  viscera  are  constricted,  and 
vice  versa,  so  that  a  considerable  portion  of  the  whole  blood  ebbs  and  flows, 
so  to  spaak,  according  to  circumstances  from  skin  to  viscera  and  from  viscera 
to  skin.  By  these  changes,  as  we  shall  see  later  on,  the  maintenance  of  the 
normal  temperature  of  the  body  is  in  large  measure  secured. 

When  food  is  placed  in  the  mouth  the  bloodvessels  of  the  salivary  glands, 
as  we  have  seen,  are  flushed  with  blood  as  an  adjuvant  to  the  secretion  of 
digestive  fluid ;  and  as  the  food  passes  along  the  alimentary  canal,  each 
section  in  turn,  with  the  glandular  appendages  belonging  to  it,  welcomes  its 
advent  by  flushing  with  blood,  the  dilatation  being  sometimes,  as  in  the  case 
of  the  salivary  gland,  the  result  of  the  activity  chiefly  of  vaso-dilator  fibres, 
but  sometimes  the  result  of  the  cessation  of  constrictor  impulses  and  some- 
times the  result  of  the  two  combined.  So  also  when  the  kidney  secretes 
urine,  its  vessels  become  dilated,  and  in  general,  wherever  functional  activity 
comes  into  play,  the  metabolism  of  tissue  which  is  the  basis  of  that  activity 
is  assisted  by  a  more  generous  flow  of  blood  through  the  tissue. 

§  166.  Vasomotor  nerves  of  the  veins.  Although  the  veins  are  provided 
with  muscular  fibres  and  are  distinctly  contractile,  and  although  rhythmic 
variations  of  calibre  due  to  contractions  may  be  seen  in  the  great  veins 
opening  into  the  heart,  in  the  veins  of  the  bat's  wing,  and  elsewhere,  and 
similar  rhythmic  variations,  also  possibly  due  to  active  rhythmic  contrac- 
tions, but  possibly  also  of  an  entirely  passive  nature,  have  been  observed  in 
the  portal  veins,  very  little  is  known  of  any  nervous  arrangements  govern- 
ing the  veins.  When  in  the  frog  the  brain  and  spinal  cord  are  destroyed, 
very  little  blood  comes  back  to  the  heart  as  compared  with  the  normal 
supply,  and  the  heart  in  consequence  appears  almost  bloodless  and  beats 
feebly.  This  has  been,  by  some,  regarded  as  more  than  can  be  accounted  for 
by  mere  loss  of  arterial  tone,  and  accordingly  interpreted  as  indicating  the 
existence  of  a  normal  tone  in  the  veins  dependent  on  the  central  nervous 
system.  When  the  latter  is  destroyed,  the  veins  become  abnormally  distended 
and  a  large  quantity  of  blood  becomes  lodged  and  hidden,  as  it  were,  in  them. 

THE  CAPILLARY  CIRCULATION. 

§  167.  We  have  already  some  time  back  (§  106)  mentioned  some  of  the 
salient  features  of  the  circulation  through  the  capillaries,  viz.,  the  difficult 
passage  of  the  corpuscles  (generally  in  single  file,  though  sometimes  in  the 
larger  channels  two  or  more  abreast)  and  plasma  through  the  narrow  chan- 


232  THE  VASCULAR  MECHANISM. 

nels,  in  a  stream  which  though  more  or  less  irregular  is  steady  and  even, 
not  broken  by  pulsations,  and  slower  than  that  in  either  the  arteries  or  the 
veins.  We  have  further  seen  that  the  capillaries  vary  very  much  in  width 
from  time  to  time ;  and  there  can  be  no  doubt  that  the  changes  in  their 
calibre  are  chiefly  of  a  passive  nature.  They  are  expanded  when  a  large 
supply  of  blood  reaches  them  through  the  supplying  arteries,  and,  by  virtue 
of  their  elasticity,  shrink  again  when  the  supply  is  lessened  or  withdrawn ; 
they  may  also  become  expanded  by  an  obstacle  to  the  venous  outflow. 

On  the  other  hand,  as  we  have  also  stated,  there  is  a  certain  amount  of 
evidence  that,  in  young  animals  at  all  events,  the  calibre  of  a  capillary 
canal  may  vary,  quite  independently  of  the  arterial  supply  or  the  venous 
outflow,  in  consequence  of  changes  in  the  form  of  the  epithelioid  cells,  allied 
to  the  changes  which  in  a  muscle-fibre  or  muscle-cell  constitute  a  contrac- 
tion ;  and  though  the  matter  requires  further  investigation,  it  is  possible  that 
these  active  changes  play  an  important  part  in  determining  the  quantity  of 
blood  passing  through  a  capillary  area ;  but  there  is  as  yet  no  satisfactory 
evidence  that  they,  like  the  corresponding  changes  in  the  arteries,  are 
governed  by  the  nervous  system. 

Over  and  above  these  changes  of  form,  the  capillaries  and  minute  vessels 
are  subject  to  changes  and  exert  influences  by  virtue  of  which  they  play  an 
important  part  in  the  work  of  the  circulation.  Their  condition  determines 
the  amount  of  resistance  offered  by  their  channels  to  the  flow  of  blood 
through  those  channels,  and  determines  the  amount  and  character  of  that 
interchange  between  the  blood  and  the  tissues  which  is  the  main  fact  of  the 
circulation. 

If  the  web  of  the  frog's  foot,  or,  better  still,  if  some  transparent  tissue  of 
a  mammal  be  watched  under  the  microscope,  it  will  be  observed  that,  while 
in  the  small  capillaries  the  corpuscles  are  pressed  through  the  channel  in 
single  file,  one  after  the  other,  each  corpuscle  as  it  passes  occupying  the 
whole  bore  of  the  capillary,  in  the  larger  capillaries  (of  the  mammal),  and 
especially  in  the  small  arteries  and  veins  which  permit  the  passage  of  more 
than  one  corpuscle  abreast,  the  red  corpuscles  run  in  the  middle  of  the 
channel,  forming  a  colored  core,  between  which  and  the  sides  of  the  vessels 
all  around  is  a  colorless  layer,  containing  no  red  corpuscles,  called  the 
"  plasmatic  layer  "  or  "  peripheral  zone."  This  division  into  a  peripheral 
zone  and  an  axial  stream  is  due  to  the  fact  that  in  any  stream  passing  through 
a  closed  channel  the  friction  is  greatest  at  the  sides,  and  diminishes  toward 
the  axis.  The  corpuscles  pass  where  the  friction  is  least,  in  the  axis.  A 
quite  similar  axial  core  is  seen  when  any  fine  particles  are  driven  with  a 
sufficient  velocity  in  a  stream  of  fluid  through  a  narrow  tube.  As  the  veloc- 
ity is  diminished  the  axial  core  becomes  less  marked  and  disappears. 

In  the  peripheral  zone,  especially  in  that  of  the  veins,  are  frequently 
seen  white  corpuscles,  sometimes  clinging  to  the  sides  of  the  vessel,  some- 
times rolling  slowly  along,  and  in  general  moving  irregularly,  stopping  for 
a  while  and  then  suddenly  moving  on.  The  greater  the  velocity  of  the  flow 
of  blood,  the  fewer  the  white  corpuscles  in  the  peripheral  zone,  and  with  a 
very  rapid  flow  they,  as  well  as  the  red  corpuscles,  may  be  all  confined  to 
the  axial  stream.  The  presence  of  the  white  corpuscles  in  the  peripheral 
zone  has  been  attributed  to  their  being  specially  lighter  than  the  red  corpus- 
cles, since  when  fine  particles  of  two  kinds,  one  lighter  than  the  other,  are 
driven  through  a  narrow  tube,  the  heavier  particles  flow  in  the  axis  and  the 
lighter  in  the  more  peripheral  portions  of  the  stream.  But,  besides  this,  the 
white  corpuscles  have  a  greater  tendency  to  adhere  to  surfaces  than  have 
the  red,  as  is  seen  by  the  manner  in  which  the  former  become  fixed  to  the 
glass  slide  and  cover-slip  when  a  drop  of  blood  is  mounted  for  microscopical 


THE  CAPILLAKY  CIRCULATION.  233 

examination.  They  probably  thus  adhere  by  virtue  of  the  amoeboid  move- 
ments of  their  protoplasm,  so  that  the  adhesion  is  to  be  considered  not  so 
much  a  mere  physical  as  a  physiological  process,  and  hence  may  be  expected 
to  vary  with  the  varying  nutritive  conditions  of  the  corpuscles  and  of  the 
bloodvessels.  Thus  while  the  appearance  of  the  white  corpuscles  in  the 
peripheral  zone  may  be  due  to  their  lightness,  their  temporary  attachment 
to  the  sides  of  the  vessels  and  characteristic  progression  is  the  result  of  their 
power  to  adhere  ;  and  as  we  shall  presently  see,  their  amoeboid  movements 
may  carry  them  on  beyond  mere  adhesion. 

§  168.  These  are  the  phenomena  of  the  normal  circulation,  and  may  be 
regarded  as  indicating  a  state  of  equilibrium  between  the  blood  on  the  one 
hand  and  the  bloodvessels  with  the  tissues  on  the  other;  but  a  different  state 
of  things  sets  in  when  that  equilibrium  is  overthrown  by  causes  leading  to 
what  is  called  inflammation  or  to  allied  conditions. 

If  an  irritant,  such  as  a  drop  of  chloroform  or  a  little  diluted  oil  of 
mustard,  be  applied  to  a  small  portion  of  a  frog's  web,  tongue,  mesen- 
tery, or  some  other  transparent  tissue,  the  following  changes  may  be  ob- 
served under  the  microscope ;  they  may  also  be  seen  in  the  mesentery  or 
other  transparent  tissue  of  a  mammal.  The  first  effect  that  is  noticed  is  a 
dilatation  of  the  arteries,  accompanied  by  a  quickening  of  the  stream.  The 
irritant,  probably  by  a  direct  action  on  the  muscular  fibres  of  the  arteries, 
has  led  to  a  relaxation  of  the  muscular  coat  and  hence  to  a  widening ;  and 
we  have  already  (§  112),  explained  how  such  a  widening  in  a  small  artery 
may  lead  to  a  temporary  thickening  of  the  stream.  In  consequence  of  the 
greater  flow  through  the 'arteries,  the  capillaries  become  filled  with  corpus- 
cles, and  many  passages,  previously  invisible  or  nearly  so  on  account  of 
their  containing  no  corpuscles,  now  come  into  view.  The  veins  at  the 
same  time  appear  enlarged  and  full.  If  the  stimulus  be  very  slight,  this 
may  all  pass  away,  the  arteries  gaining  their  normal  constriction  and  the 
capillaries  and  veins  returning  to  their  normal  condition  ;  in  other  words, 
the  effect  of  the  stimulus  in  such  a  case  is  simply  a  temporary  blush. 
Unless,  however,  the  chloroform  or  mustard  be  applied  with  especial  care 
the  effects  are  much  more  profound,  and  a  series  of  remarkable  changes 
sets  in. 

In  the  normal  circulation,  as  we  have  just  said,  white  corpuscles  may 
be  seen  in  the  peripheral,  plasmatic  zone,  but  they  are  scanty  in  number, 
and  each  one  after  staying  for  a  little  time  in  one  spot  suddenly  gets  free, 
sometimes  almost  by  a  jerk,  as  it  were,  and  then  rolls  on  for  a  greater  or 
less  distance.  In  the  area  now  under  consideration  a  large  number  of  white 
corpuscles  soon  gather  in  the  peripheral  zones,  especially  of  the  veins  and 
venous  capillaries  (that  is,  of  the  larger  capillaries  which  are  joining  to 
form  veins),  but  also,  to  a  less  extent,  of  the  arteries  ;  and  this  takes  place 
although  the  vessels  still  remain  dilated  and  the  stream  still  continues 
rapid  though  not  so  rapid  as  at  first.  Each  white  corpuscle  appears  to  ex- 
hibit a  greater  tendency  to  stick  to  the  sides  of  the  vessels,  and  though 
driven  away  from  the  arteries  by  the  stronger  arterial  stream,  becomes 
lodged,  so  to  speak,  in  the  veins.  Since  new  white  corpuscles  are  continu- 
ally being  brought  by  the  blood-stream  on  to  the  scene,  the  number  of  them 
in  the  peripheral  zones  of  the  veins  increases  more  and  more,  and  this  may 
go  on  until  the  inner  surface  of  the  veins  and  venous  capillaries  appears 
to  be  lined  with  a  layer  of  white  corpuscles.  The  small  capillaries,  too, 
contain  more  white  corpuscles  than  usual,  and  even  in  the  arteries  these 
are  abundant,  though  not  forming  the  distinct  layer  seen  in  the  veins.  The 
white  corpuscles,  however,  are  not  the  only  bodies  present  in  the  peripheral 
zone.  Though  in  the  normal  circulation  blood-platelets  (see  §  33)  cannot 


234  THE  VASCULAR  MECHANISM. 

be  seen  in  the  peripheral  zone,  and  hence  must  be  confined  (on  the  view, 
which  has  the  greater  support,  that  these  bodies  are  really  present  in  quite 
normal  blood)  to  the  axial  stream,  they  make  their  appearance  in  that 
zone  with  the  changes  which  we  are  now  describing.  Indeed,  in  many  cases 
they  are  far  more  abundant  than  the  white  corpuscles,  the  latter  appearing 
imbedded  at  intervals  in  masses  of  the  former.  Soon  after  their  appearance 
the  individual  platelets  lose  their  outline  and  run  together  into  formless 
masses. 

§  169.  This  much,  the  appearance  of  numerous  white  corpuscles  and 
platelets  in  the  peripheral  zones  may  take  place  while  the  stream,  though 
less  rapid  than  at  the  very  first,  still  remains  rapid  ;  so  rapid  at  all  events 
that,  owing  to  the  increased  width  of  the  passages,  in  spite  of  the  obstruc- 
tion offered  by  the  adherent  white  corpuscles,  the  total  quantity  of  blood 
flowing  in  a  given  time  through  the  inflamed  area  is  greater  than  normal. 
But  soon,  though  the  vessels  still  remain  dilated,  the  stream  is  observed 
most  distinctly  to  slacken  and  then  a  remarkable  phenomenon  makes  its 
appearance.  The  white  corpuscles  lying  in  contact  with  the  walls  of  the 
veins  or  of  the  capillaries  are  seen  to  thrust  processes  through  the  walls  ; 
and,  the  process  of  a  corpuscle  increasing  at  the  expense  of  the  rest  of  the 
body  of  the  corpuscle,  the  whole  corpuscle,  by  what  appears  to  be  an  ex- 
ample of  amoeboid  movement,  makes  its  way  through  the  wall  of  the  ves- 
sel into  the  lymph  space  outside ;  the  perforation  appears  to  take  place  in 
the  cement  substance  joining  the  epithelioid  plates  together.  This  is  the 
migration  of  the  white  corpuscles  to  which  we  alluded  in  §  32,  and  takes 
place  chiefly  in  the  veins  and  capillaries,  not  at  all  or  to  a  very  slight  ex- 
tent in  the  arteries.  Through  this  migration  the  lymph  spaces  around  the 
vessels  in  the  inflamed  area  become  crowded  with  white  corpuscles.  At  the 
same  time  the  lymph  in  the  same  spaces  not  only  increases  in  amount  but 
changes  somewhat  in  its  chemical  characters ;  it  becomes  more  distinctly 
and  readily  coagulable,  and  is  sometimes  spoken  of  as  "  exudation  fluid," 
or  by  the  older  writers  as  "  coagulable  lymph."  This  turgescence  of  the 
lymph  spaces,  together  with  the  dilated,  crowded  condition  of  the  blood- 
vessels, gives  rise  to  the  swelling  which  is  one  of  the  features  of  inflam- 
mation. 

If  the  inflammation  now  passes  off  the  white  corpuscles  cease  to  emi- 
grate, cease  to  stick  for  any  length  of  time  to  the  sides  of  the  vessels,  the 
stream  of  blood  through  the  vessels  quickens  again,  and  the  vessels  them- 
selves, though  they  may  remain  for  a  long  time  dilated,  eventually  regain 
their  calibre,  and  a  normal  circulation  is  re-established.  The  migrated  cor- 
puscles move  away  from  the  region,  along  the  labyrinth  of  lymph  spaces, 
and  the  surplus  lymph  also  passes  away  along  the  lymph  spaces  and  lym- 
phatic vessels. 

§  170.  The  condition  of  things,  however,  instead  of  passing  off  may  go 
on  to  a  further  stage.  More  and  more  white  corpuscles,  arrested  in  their 
passage,  crowd  the  channels  and  block  the  way,  so  that  though  the  vessels 
remain  dilated  the  stream  becomes  slower  and  slower,  until  at  last  it  stops 
altogether  and  "  stagnation  "  or  "  stasis  "  sets  in.  The  red  corpuscles  are 
driven  in,  often  in  masses,  among  the  white  corpuscles  and  platelets,  the 
distinction  between  axial  stream  and  peripheral  zone  becoming  lost;  and 
arteries,  veins,  and  capillaries,  all  distended,  sometimes  enormously  so,  are 
filled  with  a  mass  of  mingled  red  and  white  corpuscles  and  platelets. 
When  actual  stagnation  occurs  the  red  corpuscles  run  together  so  that 
their  outlines  are  no  longer  distinguishable ;  they  appear  to  become 
fused  into  a  homogeneous  red  mass.  And  it  may  now  be  observed  that, 
not  only  white  corpuscles  but  also  red  corpuscles  make  their  way  through 


THE  CAPILLAKY  CIRCULATION.  235 

the  distended  and  altered  walls  of  the  capillaries,  chiefly,  at  all  events,  at 
the  junctions  of  the  epithelioid  plates,  into  the  lymph  spaces  beyond.  This 
is  spoken  of  as  the  diapedesis  of  the  red  corpuscles. 

This  latter  "  stagnation  "  stage  of  inflammation  may  be  the  prelude  to 
further  mischief  and  indeed  to  the  death  of  the  inflamed  tissue,  but  it,  too, 
like  the  earlier  stages,  may  pass  away.  As  it  passes  away  the  outlines  of 
the  corpuscles  become  once  more  distinct,  those  on  the  venous  side  of  the 
block  gradually  drop  away  into  the  neighboring  currents — little  by  little 
the  whole  obstruction  is  removed,  and  the  current  through  the  area  is 
re-established. 

The  slowing  and  final  arrest  of  the  blood  current  described  above  is  not 
due  to  any  lessening  of  the  heart's  beat ;  the  arterial  pulsations,  or  at  least 
the  arterial  flow,  may  be  seen  to  be  continued  in  full  force  down  to  the 
affected  area,  and  there  to  cease  very  suddenly.  It  is  not  due  to  any  con- 
striction of  the  small  arteries  increasing  the  peripheral  resistance,  for  these 
continue  dilated,  sometimes  exceedingly  so.  It  must,  therefore,  be  due  to 
some  new  and  unusual  resistance  occurring  in  the  area  itself,  and  there  can 
be  no  doubt  that  this  is  to  be  found  in  an  increased  tendency  of  the 
corpuscles,  especially  of  the  white  corpuscles,  to  stick  to  the  sides  of  the 
vessels.  The  increase  of  adhesiveness  is  not  caused  by  any  change  con- 
fined to  the  corpuscles  themselves;  for  if,  after  a  temporary  delay,  one 
set  of  corpuscles  has  managed  to  pass  away  from  the  affected  area,  the 
next  set  of  corpuscles  brought  to  the  area  in  the  blood-stream  is  sub- 
jected to  the  same  delay  and  the  same  apparent  fusion.  The  cause  of 
the  increased  adhesiveness  must,  therefore,  lie  in  the  walls  of  the  blood- 
vessels or  in  the  tissue  of  which  these  form  a  part.  That  the  increased 
adhesion  is  due  to  the  vascular  walls  and  not  primarily  to  the  corpuscles 
themselves  is  further  shown  by  the  fact  that  if  in  the  frog,  an  artificial 
blood  of  normal  saline  solution  to  which  milk  has  been  added  be  substituted 
for  normal  blood,  a  stasis  may  by  irritants  be  induced  in  which  oil-globules 
play  the  part  of  corpuscles,  and  by  their  aggregation. bring  about  an  arrest 
of  the  flow. 

We  are  driven  to  conclude  that  there  exist  in  health  certain  relations 
between  the  blood,  on  the  one  hand,  and  the  walls  of  the  vessel  on  the  other, 
by  which  the  tendency  of  the  corpuscles  to  adhere  to  the  bloodvessels  is  kept 
within  certain  limits;  these  relations  consequently  determine  the  normal 
flow,  with  its  axial  stream  and  peripheral  zone,  and  the  normal  amount  of 
peripheral  resistance ;  in  inflammations  these  relations,  in  a  manner  we 
cannot  as  yet  fully  explain,  are  disturbed  so  that  the  tendency  of  the 
corpuscles  to  adhere  to  the  sides  of  the  vessel  is  largely  and  progressively 
increased.  Hence  the  tarrying  of  the  corpuscles  in  spite  of  the  widening 
of  their  path,  and  finally  their  agglomeration  and  fusion  in  the  distended 
channels. 

We  may  add  that  the  changes  occurring  in  the  vascular  walls  also  at  least 
facilitate  the  migration  of  the  corpuscles,  and  modify  the  passage  from  the 
blood  to  the  tissue  of  the  fluid  parts  of  the  blood,  the  lymph  of  inflamed 
areas  being  richer  in  proteids  than  normal  lymph. 

We  must  not,  however,  pursue  this  subject  of  inflammation  any  further. 
We  have  said  enough  to  show  that  the  peripheral  resistance  (and  consequently 
all  that  depends  on  that  peripheral  resistance)  is  not  wholly  determined  by  the 
varying  width  of  the  minute  passages,  but  is  also  dependent  on  the  vital 
condition  of  the  tissue  of  which  the  walls  of  the  passage  form  a  part.  When 
the  tissue  is  in  health,  a  certain  resistance  is  offered  to  the  passage  of  blood 
through  the  capillaries  and  other  minute  vessels,  and  the  whole  vascular 
mechanism  is  adapted  to  overcome  this  resistance  to  such  an  extent  that  a 


236  THE  VASCULAR  MECHANISM. 

normal  circulation  can  take  place.  When  the  tissue  becomes  affected,  the 
disturbance  of  the  relations  between  the  tissue  and  the  blood  may,  as  in  the 
later  stages  of  inflammation,  so  augment  the  resistance  that  the  passage  of 
the  blood  becomes  at  first  difficult  and  ultimately  impossible.  And  it  is 
quite  open  to  us  to  suppose  that  under  certain  circumstances  the  reverse  of 
the  above  may  occur  in  this  or  that  area,  conditions  in  which  the  resist- 
ance may  be  lowered  below  the  normal  and  the  circulation  in  the  area 
quickened.  Thus  the  vital  condition  of  the  tissue  becomes  a  factor  in  the 
maintenance  of  the  circulation  ;  and  it  is  possible,  though  not  yet  proved, 
that  these  vital  conditions  are  directly  under  the  dominion  of  the  nervous 
system. 

§  171.  Changes  in  the  peripheral  resistance  may  also  be  brought  about  by 
changes  in  the  character  of  the  blood,  especially  by  changes  in  the  relative 
amount  of  gases  present.  When  a  stream  of  defibrinated  blood  is  artificially 
driven  through  a  perfectly  fresh  excised  organ  such  as  the  kidney,  it  is  found 
that  the  resistance  to  the  flow  of  blood  through  the  organ,  measured  for  in- 
stance by  the  amount  of  outflow  in  relation  to  the  pressure  exerted,  varies 
considerably  owing  to  changes  taking  place  in  the  organ,  and  may  be  in- 
creased by  increasing  the  venous  character  and  diminished  by  increasing 
the  arterial  character  of  the  blood.  Remarkable  changes  in  the  resistance 
are  also  brought  about  by  the  addition  of  small  quantities  of  certain  drugs 
such  as  chloral,  atropine,  etc.,  to  the  blood. 

These  changes  have  been  attributed  to  the  altered  blood  acting  on  the 
walls  of  the  vessels,  inducing,  for  instance,  constriction  or  widening  of  the 
small  arteries,  or  it  may  be  affecting  the  capillaries,  for  it  has  been  asserted 
that  the  epithelioid  plates  of  the  capillaries  vary  in  form  according  to  the 
relative  quantities  of  carbonic  acid  and  oxygen  present  in  the  blood.  But 
this  is  not  the  whole  explanation  of  the  matter,  since  similar  variations  in 
resistance  are  met  with  when  blood  is  driven  through  fine  capillary  tubes  of 
inert  matter.  In  such  experiments  it  is  found  that  the  resistance  to  the  flow 
increases  with  a  diminution  of  the  oxygen  carried  by  the  red  corpuscles, 
and  is  modified  by  the  addition  to  the  blood  of  even  small  quantities  of 
certain  drugs. 

It  is  obvious  then  that  in  the  living  body  the  peripheral  resistance,  being 
the  outcome  of  complex  conditions,  may  be  modified  in  many  ways.  Ex- 
perience teaches  us  that,  even  in  dealing  with  non-living  inert  matter,  the 
flow  of  fluid  through  capillary  tubes  may  be  modified  on  the  one  hand  by 
changes  in  the  substance  of  which  the  tubes  are  composed,  and  on  the  other 
hand  by  changes  in  the  chemical  nature  (even  independent  of  the  specific 
gravity)  of  the  fluid  which  is  used.  In  the  living  body  both  the  fluid,  the 
blood,  and  the  walls  of  the  minute  vessels  being  both  alive,  are  incessantly 
subject  to  change  ;  the  changes  in  the  one,  moreover,  are  capable  of  reacting 
upon  and  inducing  changes  in  the  other ;  and,  lastly,  the  changes  both  of 
the  one  and  of  the  other  may  be  primarily  set  going  by  events  taking  place 
in  some  part  of  the  body  far  away  from  the  region  in  which  these  changes 
are  modifying  the  resistance  to  the  flow. 

CHANGES  IN  THE  QUANTITY  OF  BLOOD. 

§  172.  In  an  artificial  scheme  changes  in  the  total  quantity  of  fluid  in 
circulation  will  have  an  immediate  and  direct  effect  on  the  arterial  pressure, 
increase  of  the  quantity  heightening  and  decrease  diminishing  it.  This 
effect  will  be  produced  partly  by  the  pump  being  more  or  less  filled  at  each 
stroke,  and  partly  by  the  peripheral  resistance  being  increased  or  diminished 
by  the  greater  or  less  fulness  of  the  small  peripheral  channels.  The  venous 


CHANGES  IN  THE  QUANTITY  OF  BLOOD.  237 

pressure  will  under  all  circumstances  be  raised  with  the  increase  of  fluid,  but 
the  arterial  pressure  will  be  raised  in  proportion  only  so  long  as  the  elastic 
walls  of  the  arterial  tubes  are  able  to  exert  their  elasticity. 

In  the  natural  circulation  the  direct  results  of  change  of  quantity  are 
modified  by  compensatory  arrangements.  Thus  experiment  shows  that 
when  an  animal  with  normal  blood-pressure  is  bled  from  one  carotid,  the 
pressure  in  the  other  carotid  sinks  so  long  as  ihe  bleeding  is  going  on,1  and 
remains  depressed  for  a  brief  period  after  the  bleeding  has  ceased.  In  a 
short  time,  however,  it  regains  or  nearly  regains,  the  normal  height.  This 
recovery  of  blood-pressure,  after  hemorrhage,  is  witnessed  so  long  as  the 
loss  of  blood  does  not  amount  to  more  than  about  3  per  cent,  of  the  body- 
weight.  Beyond  that  a  large  and  frequently  a  sudden  dangerous  permanent 
depression  is  observed. 

The  restoration  of  the  pressure  after  the  cessation  of  the  bleeding  is  too 
rapid  to  permit  us  to  suppose  that  the  quantity  of  fluid  in  the  bloodvessels 
is  repaired  by  the  withdrawal  of  lymph  from  the  extra-vascular  elements  of 
the  tissues.  In  all  probability  the  result  is  gained  by  an  increased  action  of 
the  vasomotor  nerves,  increasing  the  peripheral  resistance,  the  vasomotor 
centres  being  thrown  into  increased  action  by  the  diminution  of  their  blood- 
supply.  When  the  loss  of  blood  has  gone  beyond  a  certain  limit,  this  vaso- 
motor action  is  insufficient  to  compensate  the  diminished  quantity  (possibly 
the  vasomotor  centres  in  part  become  exhausted),  and  a  considerable  de- 
pression takes  place ;  but  at  this  epoch  the  loss  of  blood  frequently  causes 
anaemic  convulsions. 

Similarly  when  an  additional  quantity  of  blood  is  injected  into  the  vessels, 
no  marked  increase  of  blood-pressure  is  observed  so  long  as  the  vasomotor 
centre  in  the  medulla  oblongata  is  intact.  If,  however,  the  cervical  spinal 
cord  be  divided  previous  to  the  injection,  the  pressure,  which  on  account  of 
the  removal  of  the  medullary  vasomotor  centre  is  very  low,  is  permanently 
raised  by  the  injection  of  blood.  At  each  injection  the  pressure  rises,  falls 
somewhat  afterward,  but  eventually  remains  at  a  higher  level  than  before. 
This  rise  is  stated  to  continue  until  the  amount  of  blood  in  the  vessels  above 
the  normal  quantity  reaches  from  2  to  3  per  cent,  of  the  body-weight,  be- 
yond which  point  it  is  said  no  further  rise  of  pressure  occurs. 

These  facts  seem  to  show,  in  the  first  place,  that  when  the  volume  of  the 
blood  is  increased,  compensation  is  effected  by  a  lessening  of  the  peripheral 
resistance  by  means  of  a  vaso-dilator  action  of  the  vasomotor  centres,  so 
that  the  normal  blood-pressure  remains  constant.  They  further  show  that  a 
much  greater  quantity  of  blood  can  be  lodged  in  the  bloodvessels  than  is 
normally  present  in  them.  That  the  additional  quantity  injected  does  re- 
main in  the  vessels  is  proved  by  the  absence  of  extravasations  and  of  any 
considerable  increase  of  the  extra-vascular  lymphatic  fluids.  It  has  already 
been  insisted  that,  in  health,  the  veins  and  capillaries  must  be  regarded  as 
being  far  from  filled,  for  were  they  to  receive  all  the  blood  which  they  can, 
even  at  a  low  pressure,  hold,  the  whole  quantity  of  blood  in  the  body  would 
be  lodged  in  them  alone.  In  these  cases  of  large  addition  of  blood  the 
extra  quantity  appears  to  be  lodged  in  the  small  veins  and  capillaries  (espe- 
cially of  the  internal  organs),  which  are  abnormally  distended  to  contain 
the  surplus. 

We  learn  from  these  facts  the  two  practical  lessons,  first,  that  blood- 
pressure  cannot  be  lowered  directly  by  bleeding,  unless  the  quantity  removed 
be  dangerously  large,  and  secondly,  that  there  is  no  necessary  connection 

1  Chiefly  in  consequence  of  the  free  opening  in  the  vessel  from  which  the  bleeding  is 
going  on  cutting  off  a  great  deal  of  the  peripheral  resistance  and  so  leading  to  a  general 
lowering  of  the  blood-pressure. 


238  THE  VASCULAR  MECHANISM. 

between  a  high  blood-pressure  and  fulness  of  blood  or  plethora,  since  an 
enormous  quantity  of  blood  may  be  driven  into  the  vessels  without  any 
marked  rise  of  pressure. 

A  REVIEW  OF  SOME  OF  THE  FEATURES  OF  THE  CIRCULATION. 

§  173.  The  facts  dwelt  on  in  the  foregoing  sections  have  shown  us  that 
the  factors  of  the  vascular  mechanism  may  be  regarded  as  of  two  kinds: 
one  constant  or  approximately  constant,  the  other  variable. 

The  constant  factors  are  supplied  by  the  length,  natural  bore,  and  distri- 
bution of  the  bloodvessels,  by  the  extensibility  and  elastic  reaction  of  their 
walls,  and  by  such  mechanical  contrivances  as  the  valves.  By  the  natural 
bore  of  the  various  bloodvessels  is  meant  the  diameter  which  each  would 
assume  if  the  muscular  fibres  were  wholly  at  rest,  and  the  pressure  of  fluid 
within  the  vessel  were  equal  to  the  pressure  outside.  It  is  obvious,  however, 
that  even  these  factors  are  only  approximately  constant  for  the  life  of  an 
individual.  The  length  and  distribution  of  the  vessels  change  with  the 
growth  of  the  whole  body  or  parts  of  the  body,  and  the  physical  qualities 
of  the  walls,  especially  of  the  arterial  walls,  their  extensibility,  and  elastic 
reaction  change  continually  with  the  age  of  the  individual.  As  the  body 
grows  older  the  once  supple  and  elastic  arteries  become  more  and  more  stiff 
and  rigid,  and  often  in  middle  life,  or  it  may  be  earlier,  a  lessening  of 
arterial  resilience  which  proportionately  impairs  the  value  of  the  vascular 
mechanism  as  an  agent  of  nutrition,  marks  a  step  toward  the  grave.  The 
valvular  mechanisms,  too,  also  show  signs  of  age  as  years  advance,  and 
more  or  less  marked  and  increasing  imperfections  diminish  the  usefulness  of 
the  machine. 

The  chief  variable  factors  are,  on  the  one  hand,  the  beat  of  the  heart,  and, 
on  the  other,  the  peripheral  resistance,  the  variations  in  the  latter  being 
chiefly  brought  about  by  muscular  contraction  or  relaxation  in  the  minute 
arteries,  but  also,  though  to  what  extent  has  not  yet  been  accurately  deter- 
mined, by  the  condition  of  the  walls  of  the  minute  vessels  according  to 
which  the  blood  can  pass  through  them  with  less  or  with  greater  ease,  as 
well  as  by  the  character  of  the  circulating  blood. 

§  174.  These  two  chief  variables,  the  beat  of  the  heart  and  the  width 
of  the  minute  arteries,  are  known  to  be  governed  and  regulated  by  the  cen- 
tral nervous  system,  which  adapts  each  to  the  circumstance  of  the  moment, 
and  at  the  same  time  brings  the  two  into  mutual  interdependence  ;  but  the 
central  nervous  system  is  not  the  only  means  of  government ;  there  are  other 
modes  of  regulation,  and  so  other  safeguards. 

Thus  while  undoubtedly  the  two  prominent  governors  of  the  heart  are 
the  inhibitory  fibres  of  the  vagus  and  the  augmentor  fibres  from  the  splanch- 
nic system,  the  one  slowing  the  rhythm  and  weakening  the  stroke,  the 
other  quickening  the  rhythm  and  strengthening  the  stroke,  other  causes  may 
vary  the  beat  in  the  absence  of  any  action  of  either  of  these  two  nerves. 
Mere  distention  of  the  ventricle,  by  increasing  the  tension  of  the  ventricular 
fibres,  and  so  increasing  the  force  of  the  contraction  of  each  fibre  (see  § 
148),  brings  about  a  more  forcible  beat.  As  we  shall  see  in  dealing  with 
respiration,  a  powerful  inspiration  leads  to  a  larger  flow  of  blood  into  the 
heart,  and  forthwith  the  ventricle,  out  of  its  very  fulness,  gives  stronger 
beats  for  the  time.  So  also  when  by  valvular  disease  or  otherwise  an  un- 
usual obstacle  is  presented  to  the  outflow  from  the  ventricle,  increased  vigor 
in  the  strokes  of  the  distended  organ  strives  to  compensate  the  mischief.  As, 
however,  in  the  case  of  the  skeletal  muscle,  the  tension,  if  too  great,  may  be 
injurious.  In  a  similar  manner  the  auricle,  by  a  stronger  or  a  weaker  con- 


SOME  FEATURES  OF  THE  CIRCULATION.  239 

traction,  may  distend  the  ventricle  to  a  greater  or  to  a  less  extent,  and  so 
produce  a  stronger  or  weaker  ventricular  systole. 

§  175.  Still  more  efficient,  perhaps,  as  a  direct  governor  of  the  heart's 
beat  is  the  quality  and  quantity  of  blood  passing  in  the  mammal  through 
the  coronary  arteries  and  regulating  the  nutrition  of  the  cardiac  substance. 
In  the  absence  of  all  interference  by  inhibitory  and  augmentor  fibres  the 
heart  will  continue  beating  with  a  certain  rhythm  and  force,  determined  by 
the  metabolism  going  on  certainly  in  its  muscular,  and  possibly  to  a  certain 
extent  also  in  its  nervous  elements.  We  have  seen  that  the  energy  set  free 
in  an  ordinary  skeletal  muscle,  in  response  to  a  stimulus,  may  vary  from 
nothing  to  a  maximum  according  to  the  metabolism  going  on — according  to 
the  nutritive  vigor  of  the  muscular  fibres.  The  spontaneous  rhythmic  beat 
of  the  cardiac  substance  may  be  regarded  as  the  outcome  of  a  metabolism 
more  highly  pitched,  more  elaborate,  of  a  higher  order  than  that  which 
simply  furnishes  the  ordinary  skeletal  fibre  with  mere  irritability  toward 
stimuli.  All  the  more,  therefore,  may  the  beat  be  expected  to  be  influenced 
by  any  change  in  the  metabolism  of  the  cardiac  substance,  and  so  by  any 
change  in  the  blood  which  furnishes  the  basis  for  that  metabolism.  Hence 
the  beat  of  the  heart,  quite  apart  from  extrinsic  nervous  influences,  may 
vary  largely  in  consequence  of  changes  in  its  own  metabolism,  which  in  turn 
may  result  from  alterations  in  the  blood-supply,  or  may  have  a  deeper  origin, 
and  be  due  to  the  fact  that  the  cardiac  substance,  owing  to  failure  in  its 
molecular  organization  (a  failure  which  may  be  temporary  or  permanent),  is 
unable  to  avail  itself  properly  of  the  nutritive  opportunities  afforded  by  a 
normal  quantity  of  normal  blood. 

§  176.  As  is  well  known,  the  beat  of  the  heart  may  become  temporarily 
or  permanently  irregular ;  that  many  hearts  go  on  beating  day  after  day, 
year  after  year,  without  any  such  irregularity  is  a  striking  proof  of  the  com- 
plete balance  which  usually  obtains  between  the  several  factors  of  which  we 
are  speaking.  Sometimes  such  cardiac  irregularities,  those  of  a  transient 
nature  and  brief  duration,  are  the  results  of  extrinsic  nervous  influences. 
Some  events  taking  place  in  the  stomach,  for  instance,  give  rise  to  afferent  im- 
pulses which  ascending  from  the  mucous  membrane  of  the  stomach  along  cer- 
tain afferent  fibres  of  the  vagus  to  the  medulla  oblongata,  so  augment  the 
action  of  the  cardio-inhibitory  centre  as  to  stop  the  heart  for  a  beat  or  two, 
the  stoppage  being  frequently  followed  by  a  temporary  increase  in  the  rapid- 
ity and  force  of  the  beat.  Such  a  passing  failure  of  the  heart-beat,  in  its  sud- 
den onset,  in  its  brief  duration,  and  in  the  reaction  which  follows,  very 
closely  resembles  the  temporary  inhibition  brought  about  by  artificial  stimu- 
lation of  the  vagus.  But  these  characters  are  not  essential  to  cardiac  inhibi- 
tion. For  it  must  be  remembered  that  the  central  nervous  system  possesses, 
in  the  form  of  natural  nervous  impulses  of  various  origin,  a  means  of  stimu- 
lation far  finer,  more  delicate  and  more  varied  than  anything  we  can  effect 
by  our  rough  means  of  induction  coils  and  electrodes.  Thus  in  many  cases 
of  fainting,  the  heart-beats,  instead  of  stopping  abruptly,  gradually  die  away 
or  fade  away  it  may  be  to  an  absolute  brief  arrest,  but  more  frequently 
merely  down  to  a  feebleness  which  is  insufficient  to  supply  the  brain  with  a 
quantity  of  blood  adequate  to  maintain  consciousness,  and  then  in  many 
cases,  at  all  events,  are  resumed,  or  recover  strength  gradually  and  quietly 
without  any  boisterous  reaction.  In  all  probability  all  cases  of  simple  faint- 
ing from  emotion,  pain,  digestive  troubles,  etc.,  as  distinguished  from  the 
syncope  of  actual  heart  disease,  are  instances  of  vagus  inhibition,  and  though 
we  cannot  accurately  reproduce  their  varied  phases  by  direct  stimulation  of 
the  vagus  trunk,  we  may  approach  them  more  nearly  by  producing  reflex 
inhibition,  as  by  mechanical  irritation  of  the  abdomen.  (See  §  145.) 


240  THE  VASCULAK  MECHANISM. 

Whether  definite  temporary  irregularity  is  ever  brought  about  by  means 
of  the  augmentor  fibres  we  have  at  present  no  clear  evidence  ;  but  cases  do 
occur  of  palpitation  without  previous  stoppage,  cases  in  which  a  few  hurried 
strong  beats  come  on,  pass  off,  and  are  followed  by  feebler  beats ;  and  these 
may  possibly  be  due  to  some  transient  influence  of  augmentor  fibres  thrown 
into  activity  as  part  of  a  reflex  act  or  otherwise.  And  though  we  have  no 
direct  experimental  evidence,  it  is  very  probable  that  the  acceleration  or 
augmentation  of  the  beat,  or  a  combination  of  the  two,  which  so  often  follows 
emotion,  is  carried  out  by  augmentor  fibres. 

In  all  probability,  however,  irregularity  in  the  heart-beat  is  much  more 
frequently  the  result  of  intrinsic  events,  or  the  product  of  a  disordered  nutri- 
tion of  the  cardiac  substance.  The  normal  nutrition  sets  the  pace  of  the 
normal  rhythm.  We  cannot  explain  how  this  is  affected  ;  nor  can  we  explain 
why  in  one  individual  the  normal  pace  is  set  so  low  as  50  or  even  30  beats 
a  minute,  and  in  another  as  high  as  90  a  minute  or  even  more,  while  in  most 
persons  it  is  about  70  a  minute.  The  slower  or  the  quicker  the  pace,  though 
not  normal  to  the  species,  must  be  considered  as  normal  to  the  individual, 
for  it  may  be  kept  up  through  long  years  in  an  organism  capable  of  carry- 
ing on  a  normal  man's  duties  and  work.  So  long  as  we  cannot  explain  these 
differences  we  cannot  hope  to  explain  how  it  is  that  a  disordered  nutrition 
brings  about  an  irregular  heart-beat,  either  the  more  regular  irregularity  of 
a  "  dropping  "  pulse,  that  is,  a  failure  of  sequence  rather  than  an  irregularity, 
or  a  more  actively  irregular  rhythm,  such  as  that  accompanying  a  dilated 
ventricle.  We  may,  however,  distinguish  two  kinds  of  irregularity  :  one  in 
which,  in  spite  of  all  favorable  nutritive  conditions,  the  cardiac  substance 
cannot  secure,  even  perhaps  for  a  minute,  a  steady  rhythm  ;  and  another  in 
which  the  rhythm,  though  normal  under  ordinary  circumstances,  is,  so  to 
speak,  in  a  condition  of  unstable  equilibrium,  so  that  a  very  slight  change 
in  conditions,  too  much  or  too  little  blood,  or  some  small  alteration  in  the 
composition  of  the  blood,  or  the  advent  of  some,  it  may  be  slight,  nervous 
impulse,  augmentor  or  inhibitory,  develops  a  temporary  irregularity. 

§  177.  No  one  thing,  perhaps,  concerning  the  heart  is  more  striking 
than  the  fact  that  a  heart  which  has  gone  on  beating  for  many  years,  with 
only  temporary  irregularities,  and  those  few  and  far  between,  a  heart  which 
must,  therefore,  have  executed  with  long-continued  regularity,  many  mill- 
ions of  beats,  should  suddenly,  apparently  without  warning,  after  a  brief, 
flickering  struggle,  cease  to  beat  any  more.  But  we  must  remember  that 
each  beat  is  an  effort — an  effort,  moreover,  which,  as  we  have  seen 
(§  141),  is  the  best  that  the  heart  can  make  at  the  moment ;  the  accom- 
plishment of  each  beat  is,  so  to  speak,  a  hurdle  which  has  to  be  leaped 
— one  of  the  long  series  of  hurdles  which  make  up  the  steeple-chase  of  life. 
At  any  one  leap  failure  may  occur ;  so  long  as  failure  does  not  occur,  so 
long  as  the  beat  is  made,  and  a  fair  proportion  of  the  ventricular  contents 
are  discharged  into  the  great  vessels,  the  chief  end  is  gained,  and  whether 
the  leap  is  made  clumsily  or  well  is,  relatively  considered,  of  secondary  im- 
portance. But  if  the  beat  be  not  made,  everything  almost  (provided 
that  the  miss  be  due,  not  to  vagus  inhibition,  but  to  intrinsic  events)  is 
unfavorable  for  a  succeeding  beat ;  the  mysterious  molecular  changes,  by 
which  the  actual  occurrence  of  one  beat  prepares  the  way  for  the  next,  are 
missing,  the  favorable  influences  of  the  extra  rush  of  blood  through  the 
coronary  arteries  due  to  a  preceding  beat  are  missing  also,  and  even  the  dis- 
tention  of  the  cardiac  cavities,  at  first  favorable,  speedily  passes  the  limit 
and  becomes  unfavorable.  And  these  untoward  influences  accumulate 
rapidly  as  the  first  miss  is  followed  by  a  second  and  by  a  third.  In  this 
way  a  heart,  which  has  been  brought  into  a  state  of  unstable  equilibrium 


SOME  FEATUKES  OF  THE  CIRCULATION.  241 

by  disordered  nutrition  (as,  for  instance,  by  imperfect  coronary  circulation, 
such  as  seems  to  accompany  diseases  of  the  aortic  valves  leading  to  regur- 
gitation  from  the  aorta  into  the  ventricle,  in  which  cases  sudden  death  i& 
not  uncommon),  which  is  able  just  to  accomplish  each  beat,  but  no  more, 
which  has  but  a  scanty  saving  store  of  energy,  under  some  strain  or  other 
untoward  influence,  misses  a  leap,  falls,  and  is  no  more  able  to  rise.  Doubt- 
less in  such  cases  could  adequate  artificial  aid  be  promptly  applied  in  time, 
could  the  fallen  heart  be  stirred  even  to  a  single  good  beat,  the  favorable 
reaction  of  that  beat  might  bring  a  successor,  and  so  once  more  start  the 
series  ;  but  such  a  period  of  grace,  of  potential  recovery,  is  a  brief  one. 
Even  a  coarse  skeletal  muscle,  when  cut  off  from  the  circulation,  soon  loses 
its  irritability  beyond  all  recovery,  and  the  heart  cut  off  from  its  own  influ- 
ence on  itself  runs  down  so  rapidly  that  the  period  of  possible  recovery  is 
measured  chiefly  by  seconds. 

§  178.  Turning  now  to  the  minute  arteries  and  the  peripheral  resist- 
ance which  they  regulate,  we  may  call  to  mind  the  existence  of  the  two 
kinds  of  mechanism,  the  vaso-constrictor  mechanism,  which,  owing  to  the 
maintenance  by  the  central  nervous  system  of  a  tonic  influence,  can  be 
worked  both  in  a  positive  constrictor  and  in  a  negative  dilator  direc- 
tion, and  the  vaso-dilator  mechanism,  which,  as  far  as  we  know,  exerts  its 
influence  in  one  direction  only,  viz.,  to  dilate  the  bloodvessels.  The  latter 
dilator  mechanism  seems,  as  we  have  seen,  to  be  used  in  special  instances 
only,  as  seen  in  the  cases  of  the  chorda  tympani  and  nervi  erigentes ;  the 
use  of  the  former  constrictor  mechanism  appears  to  be  more  general.  Thus 
the  relaxation  of  the  cutaneous  arteries  of  the  head  and  neck,  which  is  the 
essential  feature  in  blushing,  seems  due  to  mere  loss  of  tone,  to  the  removal 
of  constrictor  influences  previously  exerted  through  the  vaso-constrictor 
fibres  of  the  cervical  sympathetic.  Though  probably  dilator  fibres  pass 
directly  along  the  roots  of  the  cervical  and  of  certain  cranial  nerves  to  the 
nerves  of  the  head  and  neck,  we  have  no  evidence  that  these  come  into 
play  in  blushing ;  as  we  have  seen,  blushing  may  be  imitated  by  mere  sec- 
tion of  the  cervical  sympathetic.  So  also  the  "  glow  "  and  redness  of 
the  skin  of  the  whole  body — i.  e.,  dilatation  generally  of  the  cutaneous 
arteries — which  is  produced  by  external  warmth,  is  probably  another  in- 
stance of  diminished  activity  of  tonic  constrictor  influences ;  though  the 
result,  that  the  dilatation  produced  by  warming  an  animal  in  an  oven  is 
greater  than  that  produced  by  section  of  nerves,  seems  to  point  to  the 
dilator  fibres  for  the  cutaneous  vessels  which,  as  we  have  seen,  probably 
exist  in  the  sciatic  and  brachial  plexuses  and  possibly  in  all  the  spinal 
nerves,  also  taking  part  in  the  action.  A  similar  loss  of  constrictor  action 
in  the  cutaneous  vessels  may  be  the  result  of  certain  emotions,  whether  going 
so  far  as  actual  blushing  of  the  body,  or  merely  producing  a  "  glow."  The 
effect  of  cold,  on  the  other  hand,  and  of  certain  emotions,  or  of  emotions 
under  certain  conditions,  is  to  increase  the  constrictor  action  on  the  cutane- 
ous vessels,  and  the  skin  grows  pale.  It  may  be  worth  while  to  point  out 
that  in  both  the  above  cases,  while  both  the  cold  and  warmth  produce  their 
effect  chiefly  at  all  events  through  the  central  nervous  system,  and  very 
slightly,  if  at  all,  by  direct  action  on  the  skin,  their  action  on  the  central 
nervous  system  is  not  simply  a  general  augmentation  or  inhibition  of  the 
whole  vasomotor  centre.  On  the  contrary,  the  cold,  while  it  constricts  the 
cutaneous  vessels,  so  acts  on  the  vasomotor  centre  as  to  inhibit  that  portion 
of  the  vasomotor  centre  which  governs  the  abdominal  splanchnic  area ; 
while  less  blood  is  carried  to  the  colder  skin,  by  the  opening  up  of  the 
splanchnic  area  more  blood  is  turned  on  to  the  warmer  regions  of  the  body,, 
and  the  rise  of  blood-pressure  which  the  constriction  of  the  cutaneous  vessels 
16 


242  THE  VASCULAR  MECHANISM. 

tended  to  produce,  and  which  might  be  undesirable,  is  thereby  prevented. 
Conversely  when  warmth  dilates  the  cutaneous  vessels,  it  at  the  same  time 
constricts  the  abdominal  splanchnic  area,  and  prevents  an  undesirable  fall 
of  pressure. 

The  warm  and  flushed  condition  of  the  skin,  which  follows  the  drink- 
ing of  alcoholic  fluids,  is  probably  in  a  similar  manner  the  result  of  an  in- 
hibition of  that  part  of  the  vasomotor  centre  which  governs  the  cutaneous 
arteries  ;  and  it  is  probable  also  that  except  for  the  local  effect  of  the  fluid 
on  the  gastric  mucous  membrane,  whereby  some  amount  of  blushing  of  the 
gastric  bloodvessels  takes  place  as  a  reflex  act,  this  effect  on  the  vessels  of 
the  skin  is  accompanied  by  an  inverse  constrictor  action  in  the  splanch- 
nic area.  This  last  point,  however,  has  not  been  proved  experimentally 
and  may  not  occur,  since  the  influence  of  the  alcohol  is  at  the  same  time  to 
increase  the  heart's  action,  and  thus  to  obviate  the  fall  of  pressure  which 
would  certainly  occur  were  the  cutaneous  and  splanchnic  vascular  areas 
to  be  dilated  at  the  same  time.  This  effect  of  the  alcohol  on  the  heart 
may  be  a  direct  action  of  the  alcohol  on  the  cardiac  substance,  being  car- 
ried thither  by  the  blood  ;  but  the  effect,  in  being  an  augmentation  of  the 
force,  and  acceleration  of  the  pace  of  the  heart-beat  of  a  temporary  character, 
followed  by  a  reaction  in  the  direction  of  feebleness  and  slowness,  so  strik- 
ingly resemble  the  effects  of  artificially  stimulating  the  cardiac  augmentor 
fibres,  that  it  is  at  least  probable  that  the  alcohol  does  act  upon  the  cardiac 
augmentor  mechanism. 

§  179.  The  influence  on  the  body  of  exercise  illustrates  both  the  manner 
in  which  the  two  vascular  factors,  the  heart-beat  and  the  peripheral  resist- 
ance, are  modified  by  circumstances,  and  the  mutual  action  of  these  on  each 
other. 

When  the  body  passes  from  a  condition  of  comparative  rest  and  quiet  to 
one  of  exertion  and  movement,  the  metabolism  of  the  skeletal  muscles  (and 
of  the  nervous  system)  is  increased  and  more  heat  is  generated  in  them. 
We  know  for  certain  that  the  increased  metabolism  throws  into  the  blood  of 
the  veins  coming  from  the  muscles  an  increased  amount  of  carbonic  acid, 
and  it  is  probable,  but  not  so  certain,  that  it  also  loads  the  blood  with  lactic 
acid  and  other  metabolic  products ;  at  the  same  time,  there  is  an  increased 
consumption  of  oxygen  ;  the  blood  of  the  body  tends  to  become  less  arterial 
and  more  venous.  In  dealing  with  respiration,  we  shall  see  that  the  influ- 
ence thus  exerted  on  the  blood  leads  to  an  increase  in  the  respiratory  move- 
ments, and  we  shall  further  see  that  the  more  vigorous  working  of  the 
respiratory  pump,  since  it  promotes  the  flow  of  blood  to  and  through  the 
heart  and  lungs,  quickens  and  strengthens  the  heart-beats.  Possibly  this 
mere  mechanical  effect  of  the  more  vigorous  breathing  is  sufficient  by  itself 
to  account  for  the  increase  in  the  frequency  and  vigor  of  the  heart's  action, 
but  it  is  more  than  probable  that  it  is  the  changed  condition  of  the  blood, 
which,  while  it  hurries  on  the  respiratory  pump,  also  stimulates  the  vascular 
pump,  either  by  a  direct  action  on  the  cardiac  substance,  or  through  the 
medium  of  the  central  nervous  system  and  the  augmentor  fibres.  If,  as 
experiments  seem  to  show,  the  increased  vigor  of  the  respiratory  movements 
compensates,  or  even  over-compensates,  the  tendency  of  the  whole  blood  to 
become  more  venous,  so  that  during  exercise  the  blood,  which  is  distributed 
by  the  aorta,  actually  does  not  contain  more  carbonic  acid  and  less  oxygen 
than  the  rest,  but  even  the  reverse,  then  these  efforts  must  be  due  to  some  of 
the  products  of  muscular  metabolism  other  than  carbonic  acid. 

The  same  changed  condition  of  blood,  while  it  thus  excites  the  heart,  di- 
lates the  cutaneous  vessels,  as  is  clearly  shown  by  the  warm  flushed  skin,  and 
at  the  same  time  throws  into  activity  the  perspiratory  mechanism  with  which 


SOME  FEATURES  OF  THE  CIRCULATION.  243 

we  shall  hereafter  have  to  deal.  There  can  be  no  doubt,  as  we  shall  see 
later  on,  that  the  perspiration  which  accompanies  muscular  exercise  is 
brought  about  by  means  of  the  central  nervous  system,  and  we  may  almost 
with  certainty  conclude  that  the  dilatation  of  the  cutaneous  arteries  is  also 
brought  about  by  means  of  the  central  nervous  system,  and  most  probably 
by  means  of  an  inhibition  of  that  part  of  the  vasomotor  centre  which  main- 
tains under  ordinary  circumstances  a  greater  or  less  tonic  constriction  of  the 
cutaneous  arteries ;  how  far  this  may  be  assisted  by  the  special  action  of 
vaso-dilator  fibres  we  do  not  know. 

This  widening  of  the  cutaneous  arteries  diminishes  largely  the  peripheral 
resistance,  and  so  tends  to  lower  the  blood-pressure.  Moreover,  with  each 
effort  of  each  skeletal  muscle  the  minute  arteries  of  that  muscle  are  dilated, 
so  that  during  exercise,  and  especially  during  vigorous  exercise  calling  into 
action  many  skeletal  muscles,  there  must  be  in  the  body  at  large  a  very  con- 
siderable widening  of  the  minute  arteries  distributed  to  the  various  muscles, 
and  in  consequence  a  very  considerable  diminution  of  the  peripheral  resist- 
ance. These  two  diminutions  of  peripheral  resistance,  cutaneous  and  mus- 
cular, would  tend  to  lower  the  blood-pressure — a  result  which  would  be  most 
injurious,  since  the  increased  metabolism  of  the  muscles  demands  a  more 
rapid  circulation  in  order  to  get  rid  of  the  products  of  metabolism,  and  for 
a  rapid  circulation  a  high  blood-pressure  is  in  most  cases  necessary,  and  in  all 
cases  advantageous.  The  evil  is,  in  part  at  all  events,  met  by  the  increased 
force  and  frequency  of  the  heart's  beats,  for,  as  we  have  said  again  and 
again,  the  mean  blood-pressure  is  the  product  of  the  heart-beat  working 
against  the  peripheral  resistance,  and  may  remain  constant  when  one  factor 
is  increased  or  diminished,  provided  that  the  other  factor  be  proportionately 
diminished  or  increased.  It  is  possible,  then,  that  the  mere  increase  in  the 
heart's  beats  are,  during  exercise,  sufficient  to  neutralize  the  diminution  of 
peripheral  resistance,  or  even  to  raise  the  blood-pressure  above  the  normal ; 
and,  indeed,  we  find,  as  a  matter  of  fact,  that  during  exercise  there  is  such 
an  increase  of  the  mean  blood-pressure.  But  it  is  more  than  probable  that 
much  valuable  labor  of  the  heart  is  economized  by  neutralizing  the  imminent 
fall  of  blood-pressure  in  another  manner.  It  would  appear  that  while  that 
part  of  the  vasomotor  centre  which  governs  the  cutaneous  vascular  area  is 
being  inhibited,  that  part  which  governs  the  abdominal  splanchnic  area  is, 
on  the  contrary,  being  augmented.  And  in  this  way  a  double  end  is  gained. 
On  the  one  hand,  the  mean  blood-pressure  is  maintained  or  increased  in  a 
more  economical  manner  than  by  increasing  the  heart-beats,  and,  on  the 
other  hand,  the  blood  during  the'exercise  is  turned  away  from  the  digestive 
organs,  which  at  the  time  are,  or  ought  to  be,  at  rest,  and  therefore  requiring 
comparatively  little  blood.  These  organs  certainly,  at  all  events,  ought  not 
during  exercise  to  be  engaged  in  the  task  of  digesting  and  absorbing  food, 
and  the  old  saying,  "  after  dinner  sit  a  while,"  may  serve  as  an  illustration  of 
the  working  of  the  vascular  mechanism  with  which  we  are  dealing.  The 
duty  which  some  of  the  digestive  organs  have  to  carry  out  in  the  way  of 
excretion  of  metabolic  waste  products  is  during  exercise  probably  taken  on 
by  the  flushed  and  perspiring  skin.  It  is  true  that  at  the  beginning  of  a 
period  of  exercise,  before  the  skin,  so  to  speak,  has  settled  down  to  its  work, 
an  increased  flow  of  urine,  dependent  on  or  accompanied  by  an  increased  flow 
of  blood  through  the  kidney,  may  make  its  appearance  ;  but  in  this  case,  as 
we  shall  see  later  on  in  dealing  with  the  kidney,  the  flow  of  blood  through 
the  kidney  may  be  increased  in  spite  of  constriction  of  the  rest  of  the 
splanchnic  area*  and,  besides,  such  an  initial  increase  of  urine  speedily  gives 
way  to  a  decrease. 

§  180.  The  effect  of  food  on  the  vascular  mechanism  affords  a  marked 


244  THE  VASCULAR  MECHANISM. 

contrast  to  the  effect  of  bodily  labor.  The  most  marked  result  is  a  widening 
of  the  whole  abdominal  splanchnic  area,  accompanied  by  so  much  constric- 
tion of  the  cutaneous  vascular  area,  and  so  much  increase  of  the  heart's  beat, 
as  is  sufficient  to  neutralize  the  tendency  of  the  widening  of  the  splanchnic 
area  to  lower  the  mean  pressure,  or  perhaps  even  sufficient  to  raise  slightly 
the  mean  pressure. 

Any  large  widening  of  the  cutaneous  area,  especially  if  accompanied  by 
muscular  labor  and  the  incident  widening  of  the  arteries  of  the  muscles, 
would  tend  so  to  lower  the  general  blood-pressure  (unless  met  by  a  wasteful 
use  of  cardiac  energy)  as  injuriously  to  lessen  the  flow  through  the  active 
digesting  viscera.  A  moderate  constriction  of  the  cutaneous  vessels,  on  the 
other  hand,  by  throwing  more  blood  on  the  abdominal  splanchnic  area  with- 
out tasking  the  heart,  is  favorable  to  digestion,  and  is  probably  the  physio- 
logical explanation  of  the  old  saying,  "  If  you  eat  till  you're  cold,  you'll 
live  to  be  old." 

In  fact,  during  life  there  seems  to  be  a  continual  give-and-take  between 
the  bloodvessels  of  the  somatic  and  those  of  the  splanchnic  divisions  of  the 
body ;  to  fill  the  one,  the  other  is  proportionately  emptied,  and  vice  versa.  ^ 

§  181.  We  have  seen  (§  160)  that  certain  afferent  fibres  of  the  vagus 
forming  in  the  rabbit  a  separate  nerve,  the  depressor  nerve,  are  associated 
with  the  vaso-constrictor  nerves  and  the  vasomotor  centre  in  such  a  way  that 
impulses  passing  centripetally  along  them  from  the  heart  lower  the  blood- 
pressure  by  diminishing  the  peripheral  resistance,  probably  inhibiting  the 
tonic  constrictor  influences  exerted  along  the  abdominal  splanchnic  nerves, 
and  so,  as  it  were,  opening  the  splanchnic  flood-gates.  We  do  not  possess 
much  exact  information  about  the  use  of  these  afferent  depressor  fibres  in 
the  living  body,  but  probably  when  the  heart  is  laboring  against  the  blood- 
pressure  which  is  too  high  for  its  powers,  the  condition  of  the  heart  starts 
impulses  which,  passing  along  the  depressor  fibres  up  to  the  medulla 
oblongata,  temper  down,  so  to  speak,  the  blood-pressure  to  suit  the  cardiac 
strength. 

We  have,  moreover,  reason  to  think  that  not  only  does  the  heart  thus 
regulate  the  blood-pressure  by  means  of  the  depressor  fibres,  but  also  that 
the  blood-pressure,  acting,  as  it  were,  in  the  reverse  direction,  regulates  the 
heart-beat ;  a  too  high  pressure,  by  acting  directly  on  the  cardio-inhibitory 
centre  in  the  medulla  oblongata  (either  directly — that  is,  as  the  result  of  the 
vascular  condition  of  the  medulla  itself — or  indirectly,  by  impulses  reaching 
the  medulla  along  afferent  nerves  from  various  parts  of  the  body)  may 
send  inhibitory  impulses  down  the  vagus,  or  so  slacken  or  tone  down  the 
heart-beats. 

In  the  following  sections  of  this  work  we  shall  see  repeated  instances, 
similar  to  or  even  more  striking  than  the  above,  of  the  management  of  the 
vascular  mechanism  by  means  of  the  nervous  system,  and  we,  therefore,  need 
dwell  no  longer  on  the  subject. 

We  may  simply  repeat  that  at  the  centre  lies  the  cardiac  muscular  fibrer 
and  at  the  periphery  the  plain  muscular  fibre  of  the  minute  artery.  On 
these  two  elements  the  central  nervous  system,  directed  by  this  or  that 
impulse  reaching  it  along  afferent  nerve  fibres,  or  affected  directly  by  this  or 
that  influence,  is  during  life  continually  playing,  now  augmenting,  now 
inhibiting,  now  the  one,  now  the  other,  and  so,  by  help  of  the  elasticity  of 
the  arteries  and  the  mechanism  of  the  valves,  directing  the  blood-flow  ac- 
cording to  the  needs  of  the  body. 


BOOK  II. 

THE  TISSUES  OF  CHEMICAL  ACTION,    THEIR  RESPECTIVE 
MECHANISMS,    NUTRITION. 


CHAPTER    I. 

THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

§  182.  THE  food  in  passing  along  the  alimentary  canal  is  subjected  to 
the  action  of  certain  juices  supplied  by  the  secretory  activity  of  the  epithe- 
lial cells  which  line  the  canal  itself  or  which  form  part  of  its  glandular 
appendages.  These  juices,  viz.,  saliva,  gastric  juice,  bile,  pancreatic  juice, 
and  the  secretions  of  the  small  and  large  intestines,  poured  upon  and  min- 
gling with  the  food  produce  in  it  such  changes  that,  from  being  largely  insol- 
uble, it  becomes  largely  soluble,  or  otherwise  modify  it  in  such  a  way  that 
the  larger  part  of  what  is  eaten  passes  into  the  blood,  either  directly  by 
means  of  the  capillaries  of  the  alimentary  canal,  or  indirectly  by  means  of 
the  lacteal  system,  while  the  smaller  part  is  discharged  as  excrement. 

Those  parts  of  the  food  which  are  thus  digested,  absorbed,  and  made  use 
of  by  the  body,  are  spoken  of  as  food-stuffs  (they  have  also  been  called 
alimentary  principles'),  and  may  be  conveniently  divided  into  four  great 
classes : 

1.  Proteids.     We  have  previously  (§  15)  spoken  of  the  chief  characters 
of  this  class,  and  have  dealt  with  several  members  in  treating  of  blood  and 
muscle.     We  may  here  repeat  that  in  general  composition  they  contain  in 
100  parts  by  weight  "  in   round  numbers"  rather  more  than   15  parts  of 
nitrogen,  rather  more  than  50  parts  of  carbon,  about  7  parts  of  hydrogen, 
and  rather   more   than  20  parts  of  oxygen ;  though   essentially  the  nitro- 
genous bodies  of  food  and  of  the  body,  they  are  made  up  of  carbon  to  the 
extent  of  more  than  half  their  weight. 

The  nitrogenous  body  gelatin,  which  occurs  largely  in  animal  food,  and 
some  other  bodies  of  less  importance,  while  more  closely  allied  to  proteid 
bodies  than  to  any  other  class  of  organic  substances,  differ  considerably  from 
proteids  in  composition  and  especially  in  their  behavior  in  the  body  ;  they 
are  not  of  sufficient  importance  to  form  a  class  by  themselves. 

2.  Fats,  frequently  but  erroneously  called  hydrocarbons.     These  vary 
very  widely  in  chemical  composition,  ranging  from  such  a  comparatively 
simple  fat  as  butyrin  to  the  highly  complex  lecithin  (§  69)  ;  they  all  possess, 
in  view  of  the  oxidation  of  both  their  carbon  and  their  hydrogen,  a  large 
amount  of  potential  energy. 

3.  Carbohydrates,  or  sugars  and  starches.      These  possess,  weight  for 
weight,  relatively  less  potential  energy  than  do  fats ;  they  already  contain 
in  themselves  a  large  amount  of  combined  oxygen,  and  when  completely 
oxidized  give  out,  weight  for  weight,  less  heat  than  do  fats. 

245 


246  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

4.  Saline  or  mineral  bodies,  and  water.  These  salts  are  for  the  most  part 
inorganic  salts,  and  this  class  differs  from  the  three  preceding  classes  inas- 
much as  the  usefulness  of  its  members  to  the  body  lies  not  so  much  in  the 
amount  of  energy  which  may  be  given  put  by  their  oxidation  as  in  the 
various  influences  which,  by  their  presence,  they  exercise  on  the  metabolic 
events  of  the  body. 

These  several  food-stuffs  are  variously  acted  upon  in  the  several  parts  of 
the  alimentary  canal,  and  we  may  distinguish,  as  the  food  passes  along  the 
digestive  tract,  three  main  stages :  digestion  in  the  mouth  and  stomach, 
digestion  in  the  small  intestine,  and  digestion  in  the  large  intestine.  In 
many  animals  the  first  stage  is,  to  a  large  extent,  preparatory  only  to  the 
second,  which  in  all  the  animals  is  the  stage  in  which  the  food  undergoes 
the  greatest  change ;  in  the  third  stage  the  changes  begun  in  the  previous 
stages  are  completed,  and  this  stage  is  especially  characterized  by  the  absorp- 
tion of  fluid  from  the  interior  of  the  alimentary  canal. 

It  will  be  convenient  to  study  these  stages  more  or  less  apart,  though  not 
wholly  so,  and  it  will  also  be  convenient  to  consider  the  whole  subject  of 
digestion  under  the  following  heads  : 

First,  the  characters  and  properties  of  the  various  juices  and  the  changes 
which  they  bring  about  in  the  food  eaten. 

Secondly,  the  nature  of  the  processes  by  means  of  which  the  epithelium 
cells  of  the  various  glands  and  various  tracts  of  the  canal  are  able  to  manu- 
facture so  many  various  juices  out  of  the  common  source,  the  blood,  and  the 
manner  in  which  the  secretory  activity  of  the  cells  is  regulated  and  subjected 
to  the  needs  of  the  economy. 

Thirdly,  the  mechanisms,  here  as  elsewhere,  chiefly  of  a  muscular  nature, 
by  which  the  food  is  passed  along  the  canal  and  most  efficiently  brought 
into  contact  with  the  several  juices. 

Fourthly  and  lastly,  the  means  by  which  the  nutritious  digested  material 
is  separated  from  the  undigested  or  excremental  material  and  absorbed  into 
the  blood. 

THE  CHARACTERS  AND  PROPERTIES  OF  SALIVA  AND  GASTRIC  JUICE. 

Saliva. 

§  183.  Mixed  saliva,  as  it  appears  in  the  mouth,  is  a  thick,  glairy,  gen- 
erally frothy  and  turbid  fluid.  Under  the  microscope  it  is  seen  to  contain, 
beside  the  molecular  debris  of  food,  bacteria  and  other  organisms  (fre- 
quently cryptogamic  spores),  epithelium  scales,  mucous  corpuscles  and 
granules,  and  the  so-called  salivary  corpuscles.  Its  reaction  in  a  healthy 
subject  is  alkaline,  especially  when  the  secretion  is  abundant.  When  the 
saliva  is  scanty,  or  when  the  subject  suffers  from  dyspepsia,  the  reaction  of 
the  mouth  may  be  acid.  Saliva  contains  but  little  solid  matter,  on  an  aver- 
age probably  about  5  per  cent.,  the  specific  gravity  varying  from  1002  to 
1006.  Of  these  solids,  rather  less  than  half,  about  2  per  cent.,  are  salts 
(including  at  times  a  minute  quantity  of  potassium  sulphocyanate).  The 
organic  bodies  which  can  be  recognized  in  it  are  globulin  and  serum  albumin 
(see  §§  16,  17),  found  in  small  quantities  only — other  obscure  bodies  occur- 
ring in  minute  quantity,  and  mucin ;  the  latter  is  by  far  the  most  conspicu- 
ous organic  constituent,  the  glairiness  or  ropiness  of  mixed  and  other  kinds 
of  saliva  being  due  to  its  presence. 

Mucin.  If  acetic  acid  be  cautiously  added  to  mixed  saliva,  the  viscidity 
of  the  saliva  is  increased,  and  on  further  addition  of  the  acid  a  semi-opaque 
ropy  mass  separates  out,  leaving  the  rest  of  the  saliva  limpid.  This  ropy 


SALIVA  AND  GASTRIC  JUICE.  247 

mass,  which  is  mucin,  if  stirred  carefully  with  a  glass  rod,  shrinks,  becoming 
opaque,  clings  to  the  glass  rod,  and  may  be  thus  removed  from  the  fluid.  If 
the  quantity  of  mucin  be  small  and  the  saliva  be  violently  shaken  or  stirred 
while  the  acid  is  being  added,  the  mucin  is  apt  to  be  precipitated  in  flakes, 
and  may  then  be  separated  by  filtration.  It  may  be  added  that  the  precipi- 
tation of  mucin  by  acid  is  greatly  influenced  by  the  presence  of  sodium 
chloride  and  other  salts ;  thus,  after  the  addition  of  sodium  chloride,  acetic 
acid,  even  in  considerable  excess,  will  not  cause  a  precipitate  of  mucin. 

Mucin,  thus  prepared  and  purified  by  washing  with  acetic  acid,  swells 
out  in  water  without  actually  dissolving ;  it  will,  however,  dissolve  into  a 
viscid  fluid  readily  in  dilute  (0.1  per  cent.)  solutions  of  potassium  hydrate, 
more  slowly  in  solutions  of  alkaline  salts.  In  order  to  filter  a  mucin  solu- 
tion, great  dilution  with  water  is  necessary. 

Mucin  is  precipitated  by  strong  alcohol  and  by  various  metallic  salts  ;  it 
may  also  be  precipitated  by  dilute  mineral  acids,  but  the  precipitate  is  then 
soluble  in  excess  of  the  acid. 

Mucin  gives  the  three  proteid  reactions  mentioned  in  §  15,  but  it  is  a 
very  complex  body,  more  complex  even  than  proteids,  for  by  treatment  with 
dilute  mineral  acids  and  in  other  ways,  it  may  be  converted  into  some  form 
of  proteid  (acid-albumin  when  dilute  mineral  acid  is  used),  while  at  the 
same  time  there  is  formed  a  body  which  appears  to  be  carbohydrate  and 
resembles  a  sugar  in  having  the  power  of  reducing  cupric  sulphate  solutions. 
Solutions  of  mucin,  moreover,  on  mere  keeping  are  apt  to  lose  their  viscidity 
and  to  become  converted  into  a  proteid  not  unlike  the  body  peptone,  which, 
as  we  shall  see,  is  the  result  of  gastric  digestion,  and  into  a  reducing  body. 
Several  kinds  of  mucin  appear  to  exist  in  various  animal  bodies,  but  they 
seem  all  to  agree  in  the  character  that  they  can  by  appropriate  treatment 
be  split  up  into  a  proteid  of  some  kind  and  into  a  carbohydrate  or  allied 
body. 

§  184.  The  chief  purpose  served  by  the  saliva  in  digestion  is  to  moisten 
and  soften  the  food  and  to  assist  in  mastication  and  deglutition.  In  some 
animals  this  is  its  only  function.  In  other  animals  and  in  man  it  has  a 
specific  solvent  action  on  some  of  the  food-stuffs.  Such  minerals  as  are 
soluble  in  slightly  alkaline  fluids  are  dissolved  by  it.  On  fats  it  has  no  effect 
save  that  of  producing  a  very  feeble  emulsion.  On  proteids  it  has  also  no 
specific  action,  though  pieces  of  meat,  cooked  or  uncooked,  appear  greatly 
altered  after  they  have  been  masticated  for  some  time ;  the  chief  alteration, 
however,  which  thus  takes  place  is  a  change  in  the  haemoglobin  and  a 
general  softening  of  the  muscular  fibres  by  aid  of  the  alkalinity  of  the  saliva. 
Of  course,  when  particles  of  food  are  retained  for  a  long  time  in  the  mouth, 
as  in  the  interstices  or  in  cavities  of  the  teeth,  the  bacteria  or  other  organ- 
isms which  are  always  present  in  the  mouth  may  produce  much  more  pro- 
found changes,  but  these  are  not  the  legitimate  products  of  the  action  of 
saliva.  The  characteristic  property  of  saliva  is  that  of  converting  starch 
into  some  form  of  sugar. 

Action  of  saliva  on  starch.  If  to  a  quantity  of  boiled  starch,  which  is 
always  more  or  less  viscid  and  somewhat  opaque  or  turbid,  a  small  quantity 
of  saliva  be  added,  it  will  be  found  after  a  short  time  that  an  important 
change  has  taken  place,  inasmuch  as  the  mixture  has  lost  its  previous 
viscidity  and  become  thinner  and  more  transparent.  In  order  to  under- 
stand this  change  the  reader  must  bear  in  mind  the  existence  of  the  follow- 
ing bodies,  all  belonging  to  the  class  of  carbohydrates : 

1.  Starch,  which  forms  with  water  not  a  true  solution  but  a  more  or  less 
viscid  mixture,  and  gives  a  characteristic  blue  color  with  iodine.  The  for- 
mula is  C6H10O5,  or  more  correctly  (C6H10O5)n  since  the  molecule  of  starch 


248  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

is  some  multiple  (n  being  not  less  than  5)  of  the  simpler  formula.  A  kind 
of  starch  known  as  soluble  starch,  while  giving  a  blue  color  with  iodine, 
forms,  unlike  ordinary  starch,  a  clear  solution. 

2.  Dextrin*,  differing  from  starch  in  forming  a  clear  solution.     Of  these 
there  are  at  least  two  ;  one,  erythrodextrin,  often  spoken  of  simply  as  dextrin, 
giving  a  port-wine  red  color  with  iodine,  and  second,  achroodextrin,  which 
gives  no  color  at  all  with  iodine.     The  formula  for  dextrin  is  the  same  as 
that  for  starch,  but  has  a  smaller  molecule  and  might  be  represented  by 

(c6H10o5r. 

3.  Dextrose,  also  called  glucose  or  grape-sugar,  giving  no  coloration  with 
iodine,  but  characterized  by  the  power  of  reducing  cupric  and  other  metallic 
salts ;  thus,  when  dextrose  is  boiled  with  a  fluid  known  as  Fehling's  fluid, 
which  is  a  solution  of  hydrated  cupric  oxide  in  an  excess  of  caustic  alkali 
and  double  tartrate  of  sodium  and  potassium,  the  cupric  oxide  is  reduced 
and  a  red  or  yellow  deposit  of  cuprous  oxide  is  thrown  down.    This  reaction 
serves  with  others  as  a  convenient  test  for  dextrose.    Neither  starch  nor  that 
commonest  form  of  sugar  known  as  cane-sugar  gives  the  reaction  ;  whether 
the  dextrins  do  is  doubtful.    The  formula  for  dextrose  is  C6H,2O6 ;  it  is  more 
simple  than  that  of  starch  or  dextrin  and  contains  an  additional  H2O  for 
every  C6.     Unlike  starch  and  dextrin  it  can  be  obtained  in  a  crystalline 
form,  either  from  aqueous  solutions  (it  being  readily  soluble  in  water),  in 
which  case  the  crystals  contain  water  of  crystallization,  or  from  its  solutions 
in  alcohol  (in  which  it  is  sparingly  soluble),  in  which  case  the  crystals  have 
no  water  of  crystallization.     Solutions  of  dextrose  have  a  marked  dextro- 
rotatory power  with  rays  of  light. 

4.  Maltose,  very  similar  to  dextrose,  and  like  it  capable  of  reducing 
cupric  salts.     The  formula  is  somewhat  different,  being  C^H^On.     Besides 
this,  it  differs  from  dextrose  chiefly  in  its  smaller  power — i.  e.,  a  given  weight 
will  not  convert  so  much  cupric  oxide  into  cuprous  oxide  as  will  the  same 
weight  of  dextrose — and  in  having  a  stronger  rotatory  action  on  rays  of  light. 
Like  dextrose  it  can  be  crystallized,  the  crystals  from  aqueous  solutions  con- 
taining water  of  crystallization. 

Now,  when  a  quantity  of  starch  is  boiled  with  water  we  may  recognize 
in  the  viscid  imperfect  solution,  on  the  one  hand,  the  presence  of  the  starch 
by  the  blue  color  which  the  addition  of  iodine  gives  rise  to ;  and  on  the 
other  hand,  the  absence  of  sugar  (maltose,  dextrose),  by  the  fact  that  when 
boiled  with  Fehling's  fluid  no  reduction  takes  place  and  no  cuprous  oxide 
is  precipitated. 

If,  however,  the  boiled  starch  be  submitted  for  a  while  to  the  action  of 
saliva,  especially  at  a  somewhat  high  temperature  such  as  35°  or  40°  C.,  it 
is  found  that  the  subsequent  addition  of  iodine  gives  no  blue  color  at 
all  or  very  much  less  color,  showing  that  the  starch  has  disappeared  or 
diminished ;  on  the  other  hand,  the  mixture  readily  gives  a  precipitate  of 
cuprous  oxide  when  boiled  with  Fehling's  fluid,  showing  that  maltose  or 
dextrose  is  present.  That  is  to  say,  the  saliva  has  converted  the  starch  into 
maltose  or  dextrose.  The  presence  of  the  previously  absent  sugar  may  also 
be  shown  by  fermentation  arid  by  the  other  tests  for  sugar.  Moreover,  if  an 
adequately  large  quantity  of  starch  be  subjected  to  the  charge,  the  sugar 
formed  may  be  isolated,  and  its  characters  determined.  When  this  is  done 
it  is  found  that  while  some  dextrose  is  formed  the  greater  part  of  the  sugar 
which  appears  is  in  the  form  of  maltose.  As  is  well  known,  starch  may,  by 
the  action  of  dilute  acid,  be  converted  into  dextrin,  and  by  further  action 
into  sugar ;  but  the  sugar  thus  formed  is  always  wholly  dextrose,  and  not 
maltose  at  all.  The  action  of  saliva  in  this  respect  differs  from  the  action 
of  dilute  acid. 


SALIVA  AND  GASTRIC  JUICE.  249 

While  the  conversion  of  the  starch  by  the  saliva  is  going  on  the  addition 
of  iodine,  frequently  gives  rise  to  a  red  or  violet  color  instead  of  a  pure  blue, 
but  when  the  conversion  is  complete  no  coloration  at  all  is  observed.  The 
appearance  of  this  red  color  indicates  the  presence  of  dextrin  (erythrodex- 
trin)  ;  the  violet  color  is  due  to  the  red  being  mixed  with  the  blue  of  still 
unchanged  starch. 

The  appearance  of  dextrin  shows  that  the  action  of  the  saliva  on  the 
starch  is  somewhat  complex ;  and  this  is  still  further  proved  by  the  fact 
that  even  when  the  saliva  has  completed  its  work  the  whole  of  the  starch 
does  not  reappear  as  maltose  or  dextrose.  A  considerable  quantity  of  the 
other  dextrin  (achroodextrin)  always  appears  and  remains  unchanged  to  the 
end  ;  and  there  are  probably  several  other  bodies  also  formed  out  of  the 
starch,  the  relative  proportions  varying  according  to  circumstances.  The 
change,  therefore,  though  perhaps  we  may  speak  of  it  in  a  general  way  as 
one  of  hydration,  cannot  be  exhibited  under  a  simple  formula,  and  we  may 
rest  content  for  the  present  with  the  statement  that  starch  when  subjected 
to  the  action  of  saliva  is  converted  chiefly  into  the  sugar  known  as  maltose 
with  a  comparatively  small  quantity  of  dextrose  and  to  some  extent  into 
achroodextrin  (erythrodextrin  appearing  temporarily  only  in  the  pro- 
cess), other  bodies  on  which  we  need  not  dwell  being  formed  at  the 
same  time. 

Raw  unboiled  starch  undergoes  a  similar  change  but  at  a  much  slower 
rate.  This  is  due  to  the  fact  that  in  the  curiously  formed  starch  grain  the 
true  starch,  or  granulose,  is  invested  with  coats  of  cellulose.  This  latter 
material,  which  requires  previous  treatment  with  sulphuric  acid  before  it 
will  give  the  blue  reaction  on  the  addition  of  iodine,  is  apparently  not  acted 
upon  by  saliva.  Hence  the  saliva  can  only  get  at  the  grauulose  by  travers- 
ing the  coats  of  cellulose,  and  the  conversion  of  the  former  is  thereby  much 
hindered  and  delayed. 

§  185.  The  conversion  of  starch  into  sugar,  and  this  we  may  speak  of  as 
the  amylolytic  action  of  saliva,  will  go  on  at  the  ordinary  temperature  of  the 
atmosphere.  The  lower  the  temperature  the  slower  the  change,  and  at  about 
0°  C.  the  conversion  is  indefinitely  prolonged.  After  exposure  to  this  cold 
for  even  a  considerable  time  the  action  recommences  when  the  temperature 
is  again  raised.  Increase  of  temperature  up  to  about  35°-40°,  or  even  a 
little  higher,  favors  the  change,  the  greatest  activity  being  said  to  be 
manifested  at  about  40°.  Much  beyond  this  point,  however,  increase  of 
temperature  becomes  injurious,  markedly  so  at  60°  or  70°  ;  and  saliva 
which  has  been  boiled  for  a  few  minutes  not  only  has  no  action  on 
starch  while  at  that  temperature,  but  does  not  regain  its  powers  on 
cooling.  By  being  boiled,  the  amylolytic  activity  of  saliva  is  permanently 
destroyed. 

The  action  of  saliva  on  starch  is  most  rapid  when  the  reaction  of  the 
mixture  is  neutral  or  nearly  so ;  it  is  hindered  or  arrested  by  a  distinctly 
acid  reaction.  Indeed,  the  presence  of  even  a  very  small  quantity  of  free 
acid,  at  all  events  of  hydrochloric  acid,  at  the  temperature  of  the  body,  not 
only  suspends  the  action  but  speedily  leads  to  permanent  abolition  of  the 
activity  of  the  juice.  The  bearing  of  this  will  be  seen  later  on. 

The  action  of  saliva  is  hampered  by  the  presence  in  a  concentrated  state 
of  the  product  of  its  own  action — that  is,  of  sugar  If  a  small  quantity  of 
saliva  be  added  to  a  thick  mass  of  boiled  starch,  the  action  will  after  a  while 
slacken,  and  eventually  come  to  almost  a  standstill,  long  before  all  the  starch 
has  been  converted.  On  diluting  the  mixture  with  water,  the  action  will 
recommence.  If  the  products  of  action  be  removed  as  soon  as  they  are 
formed,  dialysis  for  example,  a  small  quantity  of  saliva  will,  if  sufficient  time 


250  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

be  allowed,  convert  into  sugar  a  very  large,  one  might  almost  say  an  indef- 
inite, quantity  of  starch.  Whether  the  particular  constituent  on  which  the 
activity  of  saliva  depends  is  at  all  consumed  in  its  action  has  not  at  present 
been  definitely  settled. 

On  what  constituents  do  the  amylolytic  virtues  of  saliva  depend  ? 

If  saliva,  filtered  and  thus  freed  from  much  of  its  mucin  and  from  other 
formed  constituents,  be  treated  with  ten  or  fifteen  times  its  bulk  of  alcohol, 
a  precipitate  is  formed  containing,  besides  other  substances,  all  the  proteid 
matters.  Upon  standing  under  the  alcohol  for  some  time  (several  days),  the 
proteids  thus  precipitated  become  coagulated  and  insoluble  in  water.  Hence, 
an  aqueous  extract  of  the  precipitate,  made  after  this  interval,  contains  very 
little  proteid  material ;  yet  it  is  exceedingly  active.  Moreover,  by  other 
more  elaborate  methods  there  may  be  obtained  from  saliva  solutions  which 
appear  to  be  almost  entirely  free  from  proteids  and  yet  are  intensely  amylo- 
lytic. But  even  these  probably  contain  other  bodies  beside  the  really  active 
constituent.  Whatever  the  active  substance  be  in  itself,  it  exists  in  such 
extremely  small  quantities  that  it  has  never  yet  been  satisfactorily  isolated ; 
and  indeed  the  only  clear  evidence  we  have  of  its  existence  is  the  manifesta- 
tion of  its  peculiar  powers. 

The  salient  features  of  this  body,  this  amylolytic  agent,  which  we  may  call 
ptyalin,  are  then  :  1st,  its  presence  in  minute  and  almost  inappreciable  quan- 
tity. 2d,  the  close  dependence  of  its  activity  on  temperature.  3d,  its  perma- 
nent and  total  destruction  by  a  high  temperature  and  by  various  chemical 
reagents.  4th,  the  want  of  any  clear  proof  that  it  itself  undergoes  any 
change  during  the  manifestations  of  its  power — that  is  to  say,  the  energy 
necessary  for  the  transformation  which  it  effects  does  not  come  out  of  itself; 
if  it  is  at  all  used  up  in  its  action,  the  loss  is  rather  that  of  simple  wear  and 
tear  of  a  machine  than  that  of  a  substance  expended  to  do  work.  5th,  the 
action  which  it  induces  is  probably  of  such  a  kind  (splitting  up  of  a  mole- 
cule with  assumption  of  water)  as  is  effected  by  that  particular  class  of  agents 
called  "  hydrolytic." 

These  features  mark  out  the  amylolytic  active  body  of  saliva  as  belong- 
ing to  the  class  of  ferments;1  and  we  may  henceforward  speak  of  the 
amylolytic  ferment  of  saliva.  The  fibrin-ferment  (§  20)  is  so  called 
because  its  action  in  many  ways  resembles  that  of  the  ferment  of  which 
we  are  now  speaking. 

§  186.  Mixed  saliva,  whose  properties  we  have  just  discussed,  is  the 
result  of  the  mingling  in  various  proportions  of  saliva  from  the  parotid, 
submaxillary,  and  sublingual  glands  with  the  secretion  from  the  buccal 
glands.  These  constituent  juices  have  their  own  special  characters,  and 
these  are  not  the  same  in  all  animals.  Moreover,  in  the  same  individual 
the  secretion  differs  in  composition  and  properties  according  to  circum- 
stances ;  thus,  as  we  shall  see  in  detail  hereafter,  the  saliva  from  the  sub- 
maxillary  gland  secreted  under  the  influence  of  the  chorda  tympani  nerve 
is  different  from  that  which  is  obtained  from  the  same  gland  by  stimulating 
the  sympathetic  nerve. 

1  Ferments  may,  for  the  present  at  least,  be  divided  into  two  classes,  commonly  called 
organised  and  unorganised.  Of  the  former,  yeast  may  be  taken  as  a  well-known  example. 
The  fermentative  activity  of  yeast  which  leads  to  the  conversion  of  sugar  into  alcohol, 
is  dependent  on  the  life  of  the  yeast-cell.  Unless  the  yeast-cell  be  living  and  functional, 
fermentation  does  not  take  place ;  when  the  yeast-cell  dies  fermentation  ceases  ;  and  no 
substance  obtained  from  the  fluid  parts  of  yeast,  by  precipitation  with  alcohol  or  other- 
wise, will  give  rise  to  alcoholic  fermentation.  The  salivary  ferment  belongs  to  the  latter 
class ;  it  is  a  substance,  not  a  living  organism  like  yeast.  It  may  be  added,  however, 
that  possibly  the  organized  ferment,  the  yeast  for  instance,  produces  its  effect  by  means 
of  an  ordinary  unorganized  ferment  which  it  generates,  but  which  is  immediately  made 
away  with. 


SALIVA  AND  GASTRIC  JUICE.  251 

In  man  pure  parotid  saliva  may  easily  be  obtained  by  introducing  a  fine  canula 
into  the  opening  of  the  Stenonian  duct,  and  submaxillary  saliva,  or  rather  a 
mixture  of  submaxillary  and  sublingual  saliva,  by  similar  catheterization  of  the 
Whartonian  duct.  In  animals  the  duct  may  be  dissected  out  and  a  canula 
introduced. 

Parotid  saliva  in  man  is  clear  and  limpid,  not  viscid ;  the  reaction  of  the 
first  drops  secreted  is  often  acid,  the  succeeding  portions,  at  all  events  when 
the  flow  is  at  all  copious,  are  alkaline ;  that  is  to  say,  the  natural  secretion 
is  alkaline,  but  this  may  be  obscured  by  acid  changes  taking  place  in  the 
fluid  which  has  been  retained  in  the  duct,  possibly  by  the  formation  of  an 
excess  of  carbonic  acid.  On  standing  the  clear  fluid  becomes  turbid  from  a 
precipitate  of  calcic  carbonate,  due  to  an  escape  of  carbonic  acid.  It  con- 
tains globulin  and  some  other  forms  of  albumin,  with  little  or  no  mucin. 
Potassium  sulphocyanate  may  also  sometimes  be  detected,  but  structural 
elements  are  absent. 

Submaxillary  saliva,  in  man  and  in  most  animals,  differs  from  parotid 
saliva  in  being  more  alkaline,  and  from  the  presence  of  mucin  more  viscid ; 
it  contains  salivary  corpuscles,  that  is  bodies  closely  resembling  if  not  iden- 
tical with  leucocytes,  and,  often  in  abundance,  amorphous  masses.  The  so- 
called  chorda  saliva  in  the  dog,  that  is  to  say,  saliva  obtained  by  stimulating 
the  chorda  tympani  nerve  (of  which  we  shall  presently  speak),  is  under 
ordinary  circumstances  thinner  and  less  viscid,  contains  less  mucin  and 
fewer  structural  elements  than  the  so-called  sympathetic  saliva,  which  is 
remarkable  for  its  viscidity,  its  structural  elements,  and  for  its  larger  total 
of  solids. 

Sublingual  saliva  is  more  viscid  and  contains  more  salts  (in  the  dog 
about  1  per  cent.)  than  the  submaxillary  saliva. 

The  action  of  saliva  varies  in  intensity  in  different  animals.  Thus  in 
man,  the  pig,  the  guinea-pig,  and  the  rat,  both  parotid  and  submaxillary 
and  mixed  saliva  are  amylolytic ;  the  submaxillary  saliva  being  in  most 
cases  more  active  than  the  parotid.  In  the  rabbit,  while  the  submaxillary 
saliva  has  scarcely  any  action,  that  of  the  parotid  is  energetic.  The  saliva 
of  the  cat  is  much  less  active  than  the  above ;  that  of  the  dog  is  still  less 
active,  indeed  is  almost  inert.  In  the  horse,  sheep,  and  ox,  the  amolytic 
powers  of  either  mixed  saliva  or  of  any  one  of  the  constituent  juices  are 
extremely  feeble. 

Where  the  saliva  of  any  gland  is  active,  an  aqueous  infusion  of  the  same 
gland  is  also  active.  The  importance  and  bearing  of  this  statement  will  be 
seen  later  on.  From  the  aqueous  infusion  of  the  gland,  as  from  saliva  itself, 
the  ferment  may  be  approximately  isolated.  In  some  cases  at  least  some 
ferment  may  be  extracted  from  the  gland  even  when  the  secretion  is  itself 
inactive.  In  fact,  a  ready  method  of  preparing  a  highly  amylolytic  liquid 
tolerably  free  from  proteid  and  other  impurities  is  to  mince  finely  a  gland 
known  to  have  an  active  secretion,  such,  for  instance,  as  that  of  a  rat,  to 
dehydrate  it  by  allowing  it  to  stand  under  absolute  alcohol  for  some  days, 
and  then,  having  poured  off  most  of  the  alcohol  and  removed  the  remainder 
by  evaporation  at  a  low  temperature,  to  cover  the  pieces  of  gland  with  strong 
glycerin.  Though  some  of  the  ferment  appears  to  be  destroyed  by  the 
alcohol,  a  mere  drop  of  such  a  glycerin  extract  rapidly  converts  starch 
into  sugar. 

Gastric  Juice. 

§  187.  There  is  no  difficulty  in  obtaining  what  may  be  fairly  considered 
as  a  normal  saliva ;  but  there  are  many  obstacles  in  the  way  of  determining 
the  normal  characters  of  the  secretion  of  the  stomach.  When  no  food  is 


252  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

taken  the  stomach  is  at  rest  and  no  secretion  takes  place.  When  food  is 
taken,  the  characters  of  the  gastric  juice  secreted  are  obscured  by  the  food 
with  which  it  is  mingled.  The  gastric  membrane  may,  it  is  true,  be  arti- 
ficially stimulated,  by  touch,  for  instance,  and  a  secretion  obtained.  This 
we  may  speak  of  as  gastric  juice,  but  it  may  be  doubted  whether  it  ought 
to  be  considered  as  normal  gastric  juice.  And  indeed,  as  we  shall  see,  even 
the  juice  which  is  poured  into  the  stomach  during  a  meal,  varies  in  compo- 
sition as  digestion  is  going  on.  Hence  the  characters  which  we  shall  give 
of  gastric  juice  must  be  considered  as  having  a  general  value  only. 

Gastric  juice,  obtained  in  as  normal  a  condition  as  possible  from  the 
healthy  stomach  of  a  fasting  dog  by  means  of  a  gastric  fistula,  is  a  thin, 
almost  colorless  fluid  with  a  sour  taste  and  odor. 

In  the  operation  for  gastric  fistula  an  incision  is  made  through  the  abdominal 
walls,  along  the  linea  alba,  the  stomach  is  opened,  and  the  lips  of  the  gastric 
wound  securely  sewn  to  those  of  the  incision  in  the  abdominal  walls.  TJnion 
soon  takes  place,  so  that  a  permanent  opening  from  the  exterior  into  the  inside 
of  the  stomach  is  established.  A  tube  of  proper  construction,  introduced  at  the 
time  of  the  operation,  becomes  firmly  secured  in  place  by  the  contraction  of 
healing.  Through  the  tube  the  contents  of  the  stomach  can  be  received,  and  the 
mucous  membrane  stimulated  at  pleasure. 

When  obtained  from  a  natural  fistula  in  man,  its  specific  gravity  has 
been  found  to  diflfer  little  from  that  of  water,  varying  from  1.001  to  1.010, 
and  the  amount  of  solids  present  to  be  correspondingly  small.  In  animals 
pure  gastric  juice  seems  to  be  equally  poor  in  solids,  the  higher  estimates 
which  some  observers  have  obtained  being  probably  due  to  admixture  with 
food,  etc. 

Of  the  solid  matters  present  about  half  are  inorganic  salts,  chiefly  alka- 
line (sodium)  chlorides,  with  small  quantities  of  phosphates.  The  organic 
material  consists  of  pepsin,  a  body  to  be  described  immediately,  mixed  with 
other  substances  of  undetermined  nature.  In  a  healthy  stomach  gastric  juice 
contains  a  very  small  quantity  only  of  mucin,  unless  some  submaxillary 
saliva  has  been  swallowed. 

The  reaction  is  distinctly  acid,  and  the  acidity  is  normally  due  to  free 
hydrochloric  acid.  This  is  shown  by  various  proofs,  among  which  we  may 
mention  the  conclusive  fact  that  the  amount  of  chlorine  present  in  gastric 
juice  is  more  than  would  suffice  to  form  chlorides  with  all  the  bases  present, 
and  that  the  excess,  if  regarded  as  existing  in  the  form  of  hydrochloric  acid, 
corresponds  exactly  to  the  quantity  of  free  acid  present.  Lactic  and  butyric 
and  other  acids  when  present  are  secondary  products,  arising  either  by  their 
respective  fermentations  from  articles  of  food,  or  from  the  decomposition  of 
their  alkaline  or  other  salts.  In  man  the  amount  of  free  hydrochloric  acid 
in  healthy  juice  may  be  stated  to  be  about  0.2  per  cent.,  but  in  some  animals 
it  is  probably  higher. 

§  188.  On  starch  gastric  juice  has  no  amylolytic  action ;  on  the  con- 
trary, when  saliva  is  mixed  with  gastric  juice  any  amylolytic  ferment  which 
may  be  present  in  the  former  is  at  once  prevented  from  acting  by  the  acidity 
of  the  mixture.  Moreover,  in  a  very  short  time,  especially  at  the  tempera- 
ture of  the  body,  the  amylolytic  ferment  is  destroyed  by  the  acid,  so  that 
even  on  neutralization  the  mixture  is  unable  to  convert  starch  into  sugar. 

On  dextrose  healthy  gastric  juice  has  no  effect.  And  its  power  of  con- 
verting cane-sugar  seems  to  be  less  than  that  of  hydrochloric  acid  diluted  to 
the  same  degree  of  acidity  as  itself.  In  an  unhealthy  stomach,  however, 
containing  much  mucus,  the  gastric  juice  is  very  active  in  converting  cane- 
sugar  into  dextrose.  This  power  seems  to  be  due  to  the  presence  in  the 
mucus  of  a  special  ferment,  analogous  to,  but  quite  distinct  from,  the 


SALIVA   AND  GASTRIC  JUICE.  253 

ptyalin  of  saliva.  An  excessive  quantity  of  cane-sugar  introduced  into 
the  stomach  causes  a  secretion  of  mucus,  and  hence  provides  for  its  own 
conversion. 

On  fats  gastric  juice  has  at  most  a  limited  action.  When  adipose  tissue 
is  eaten,  the  chief  change  which  takes  place  in  the  stomach  is  that  the  pro- 
teid  and  gelatiniferous  envelopes  of  the  fat-cells  are  dissolved,  and  the  fats 
set  free.  Though  there  is  experimental  evidence  that  emulsion  of  fats  to  a 
certain  extent  does  take  place  in  the  stomach,  the  great  mass  of  the  fat  of 
a  meal  is  not  so  changed. 

Such  minerals  as  are  soluble  in  free  hydrochloric  acid  are  for  the  most 
part  dissolved  ;  though  there  is  a  difference  in  this  and  in  some  other  re- 
spects between  gastric  juice  and  simple  free  hydrochloric  acid  diluted  with 
water  to  the  same  degree  of  acidity  as  the  juice,  the  presence  either  of  the 
pepsin  or  of  other  bodies  apparently  modifying  the  solvent  action  of  the 
acid. 

The  essential  property  of  gastric  juice  is  the  power  of  dissolving  proteid 
matters  and  of  converting  them  into  a  substance  called  peptone. 

Action  of  gastric  juice  on  proteids.  The  results  are  essentially  the  same, 
whether  natural  juice  obtained  by  means  of  a  fistula,  or  artificial  juice,  i.  e. 
an  acid  infusion  of  the  mucous  membrane  of  the  stomach,  be  used. 

Artificial  gastric  juice  may  be  prepared  in  any  of  the  following  ways  : 

1.  The  mucous  membrane  of  a  pig's  or  dog's  stomach  is  removed  from  the  mus- 
cular coat,  finely  minced,  rubbed  in  a  mortar  with  pounded  glass  and  extracted 
with  water.     The  aqueous  extract  filtered  and  acidulated  (it  is  in  itself  somewhat 
acid),  until  it  has  a  free  acidity  corresponding  to  0.2  per  cent,   of  hydrochloric 
acid,  contains  but  little  of  the  products  of  digestion,  such  as  peptone,  but  is  fairly 
potent. 

2.  The  mucous  membrane  similarly  prepared  and  minced  is  allowed  to  digest 
at  35°  C.  in  a  large  quantity  of  hydrochloric  acid  diluted  to  0.2  per  cent.     The 
greater  part  of  the  membrane  disappears,  shreds  only  being  left,  and  the  some- 
what opalescent  liquid  can  be  decanted  and  filtered.     The  filtrate  has  powerful 
digestive  (peptic)  properties,  but  contains  a  considerable  amount  of  the  products 
of  digestion  (peptone,  etc.)  arising  from  the  digestion  of  the  mucous  membrane 
itself.1 

3.  The  mucous  membrane,  similarly  prepared  and  minced,  is  thrown  into  a  com- 
paratively large  quantity  of  concentrated  glycerin,  and  allowed  to  stand.    The  mem- 
brane may  be  previously  dehydrated  by  being  allowed  to  stand  under  alcohol,  but 
this  is  not  necessary,  and  a  too  prolonged  action  of  the  alcohol  injures  or  even  de- 
stroys the  activity  of  the  product.     The  decanted  clear  glycerin,  in  which  a  com- 
paratively small  quantity  of  the  ordinary  proteids  of  the  mucous  membrane  are 
dissolved,  if  added  to  hydrochloric  acid  of  0.2  per  cent,  (about  1  c.c.  of  the  glycerin 
to  100  c.c.  of  the  dilute  acid  are  sufficient),  makes  an  artificial  juice  tolerably  free 
from  ordinary  proteids  and  peptone,  and  of  remarkable  potency,  the  presence  of 
the  glycerin  not  interfering  with  the  results. 

Before  proceeding  to  study  the  action  of  gastric  juice  on  proteids,  it  will 
be  useful  to  review  very  briefly  the  chief  characters  of  the  more  important 
members  of  the  group. 

The  more  important  proteids  which  we  have  thus  far  studied  are :  1. 
Fibrin,  insoluble  in  water  and  not  really  soluble  (i.  e.,  without  change)  in 
saline  solutions.  2.  Myosin,  insoluble  in  water,  but  soluble  in  saline  solu- 
tions, provided  these  are  not  too  dilute  or  too  concentrated.  3.  Globulin 
(including  paraglobulin,  fibrinogen,  etc.),  insoluble  in  water,  but  readily 
soluble  in  even  very  dilute  saline  solutions.  4.  Albumin,  serum-albumin, 
soluble  in  water  in  the  absence  of  all  salts.  5.  Acid-albumin,  into  which 
globulins  and  myosin  are  rapidly  converted  by  the  action  of  dilute  acids, 

1  These,  however,  may  be  removed  by  concentration  at  40°  C.  and  subsequent  dialysis. 


254  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

the  particular  acid-albumin  into  which  the  myosin  of  muscle  is  changed, 
being  sometimes  called  syntonin.  If  the  reagent  used  be  not  dilute  acid, 
but  dilute  alkali,  the  product  is  called  alkali-albumin.  The  two  bodies, 
acid-albumin  and  alkali-albumin,  are  very  parallel  in  their  characters,  and 
may  readily  be  converted,  the  one  into  the  other,  by  the  use  of  dilute  alkali 
or  dilute  acid  respectively.  Their  most  important  common  characters  are 
insolubility  in  water  and  in  saline  solutions  and  ready  solubility  in  dilute 
acids  and  alkalies.  6.  Coagulated  proteids.  As  we  have  seen,  when  fibrin 
suspended  in  water,  serum-albumin  in  solution,  acid-albumin  or  alkali- 
albumin  suspended  in  water,  or  paraglobulin  suspended  in  water  or  dis- 
solved in  a  dilute  saline  solution,  are  heated  to  a  temperature  which  for  the 
whole  group  may  be  put  down  at  about  75°  to  80°  C.,  each  of  them  be- 
comes coagulated,  and  after  the  change  is  insoluble  in  water,  saline  solu- 
tions, dilute  acids,  etc.,  in  fact  in  everything  but  very  strong  acids.  Myosin 
and  fibrinogen  undergo  a  similar  change  at  a  lower  temperature,  viz.,  about 
56°  C.  We  may,  for  present  purposes,  speak  of  all  these  proteids  thus 
changed  under  the  one  term  of  coagulated  proteids. 

To  the  above  list  we  may  now  add  two  other  proteids,  viz. :  7.  A  kind 
of  albumin  which  forms  the  great  bulk  of  the  proteid  matter  present  in  raw 
"  white  of  egg,"  and  which,  since  it  differs  in  minor  characters  from  the 
albumin  of  blood  and  of  the  tissues,  is  called  egg-albumin.  8.  The  peculiar 
proteid  casein,  an  important  constituent  of  milk.  This  may  perhaps  be 
regarded  as  a  naturally  occurring  alkali-albumin,  since  it  has  many  resem- 
blances to  the  artificial  alkali-albumin ;  but  for  several  reasons  it  is  desir- 
able to  consider  it  as  an  independent  body. 

Egg-albumin,  like  serum-albumin,  becomes  coagulated  at  a  temperature 
of  about  75°  to  80°  C.,  and  though  casein  as  it  naturally  exists  in  milk  is 
not  coagulated  on  boiling,  when  separated  out  in  a  special  way,  and  sus- 
pended in  water  in  which  it  is  insoluble,  it  becomes  coagulated  at  about  75° 
to  80°  C. 

It  will  be  observed  that  all  these  proteids  form,  as  regards  their  solubili- 
ties, a  descending  series  in  the  following  order :  coagulated  proteids ;  fibrin ; 
acid-albumin  with  alkali-albumin,  and  casein ;  myosin,  globulin  ;  serum- 
albumin  with  egg-albumin. 

We  must  now  return  to  the  action  of  gastric  juice. 

If  a  few  shreds  of  fibrin,  obtained  by  whipping  blood,  after  being  thor- 
oughly washed  and  boiled  and  thus  by  the  boiling  coagulated,  be  thrown 
into  a  quantity  of  gastric  juice,  and  the  mixture  be  exposed  to  a  tempera- 
ture of  from  35°  to  40°  C.,  the  fibrin  will  speedily,  in  some  cases  in  a  few 
minutes,  be  dissolved.  The  shreds  first  swell  up  and  become  transparent, 
then  gradually  dissolve,  and  finally  disappear  with  the  exception  of  some  gran- 
ular debris,  the  amount  of  which,  though  generally  small,  varies  according  to 
circumstances.  If  raw,  that  is,  unboiled,  uncoagulated  fibrin  be  employed 
the  same  changes  may  be  observed,  but  they  take  place  much  more  rapidly. 

If  small  morsels  of  coagulated  albumin,  such  as  white  of  egg,  be  treated 
in  the  same  way,  the  same  solution  is  observed.  The  pieces  become  trans- 
parent at  their  surfaces  ;  this  is  especially  seen  at  the  edges,  which  gradually 
become  rounded  down ;  and  solution  steadily  progresses  from  the  outside 
of  the  piece  inward. 

If  any  other  form  of  coagulated  albumin  (e.  g.,  precipitated  acid-  or 
alkali-albumin,  suspended  in  water  and  boiled)  be  treated  in  the  same  way, 
a  similar  solution  takes  place.  The  readiness  with  which  the  solution  is 
effected,  will  depend,  cceteris  paribus,  on  the  smallness  of  the  pieces,  or  rather 
on  the  amount  of  surface  as  compared  with  bulk,  which  is  presented  to  the 
action  of  the  juice. 


SALIVA  AND  GASTRIC  JUICE.  255 

Gastric  juice  then  readily  dissolves  coagulated  proteids  which  other- 
wise are  insoluble,  or  soluble  only,  and  that  with  difficulty,  in  very  strong 
acids. 

When  proteids  which  are  soluble  in  water,  or  in  dilute  acid,  are  treated 
with  gastric  juice,  no  visible  change  takes  place ;  but  nevertheless,  it  is  found 
on  examination  that  the  solutions  have  undergone  a  remarkable  change,  the 
nature  of  which  is  easily  seen  by  contrasting  it  with  the  change  effected  by 
dilute  acid  alone.  If  raw  white  of  egg,  largely  diluted  with  water  and 
strained,  be  treated  with  a  sufficient  quantity  of  dilute  hydrochloric  acid,  the 
opalescence  or  turbidity  which  appeared  in  the  white  of  egg  on  dilution  (and 
which  is  due  to  the  precipitation  of  various  forms  of  globulin  accompanying 
the  egg-albumin  in  the  raw  white)  disappears^  and  a  clear  mixture  results. 
If  a  portion  of  the  mixture  be  at  once  boiled,  a  large  deposit  of  coagulated 
albumin  occurs.  If,  however,  the  mixture  be  exposed  to  50°  to  55°  C. 
for  some  time,  the  amount  of  coagulation  which  is  produced  by  boiling  a 
specimen  becomes  less,  and,  finally,  boiling  produces  no  coagulation  whatever. 
By  neutralization,  however,  the  whole  of  the  albumin  (with  such  restrictions 
as  the  presence  of  certain  neutral  salts  may  cause)  may  be  obtained  in  the 
form  of  acid-albumin,  the  filtrate  after  neutralization  containing  no  proteids 
at  all  (or  a  very  small  quantity).  Thus  the  whole  of  the  albumin  present 
in  the  white  of  egg  may  be,  in  time,  converted,  by  the  simple  action  of  dilute 
hydrochloric  acid,  into  acid-albumin.  Serum-albumin  similarly  treated 
undergoes  in  course  of  time  a  similar  conversion  into  acid-albumin,  and  we 
have  already  seen  (§  59)  that  solutions  of  myosin  or  of  any  of  the  globulins 
are  with  remarkable  rapidity  converted  into  acid-albumin.  Thus  simple 
dilute  hydrochloric  acid  of  the  same  degree  of  acidity  as  gastric  juice,  merely 
converts  these  proteids  into  acid-albumin,  the  rapidity  of  the  change  differ- 
ing with  the  different  proteids,  being  in  some  cases  very  slow,  and  requiring 
a  relatively  high  temperature. 

If  the  same  white  of  egg  or  serum-albumin  be  treated  with  gastric  juice 
instead  of  simple  dilute  hydrochloric  acid,  the  events  for  some  time  seem  the 
same.  Thus  after  a  while  boiling  causes  no  coagulation,  while  neutraliza- 
tion gives  a  considerable  precipitate  of  a  proteid  body,  which  being  insoluble 
in  water  and  in  sodium  chloride  solutions  and  soluble  in  dilute  alkali  and 
acids,  at  least  closely  resembles  acid-albumin.  But  it  is  found  that  only  a 
portion  of  the  proteid  originally  present  in  the  white  of  egg  or  serum-albumin 
can  thus  be  regained  by  precipitation.  Though  the  neutralization  be  carried 
out  with  the  greatest  care  it  will  be  found,  on  filtering  off  the  neutralization 
precipitate,  that  is,  the  acid-albumin,  that  the  filtrate,  as  shown  on  employ- 
ing the  various  tests  for  proteid  (see  §  15)  or  on  adding  an  adequate  quan- 
tity of  strong  alcohol,  still  contains  a  very  considerable  quantity  of  proteid 
matter  ;  and,  on  the  whole,  the  longer  the  digestion  is  carried  on,  the  greater 
is  the  proportion  borne  by  the  proteid  remaining  in  solution  to  the  precipi- 
tate thrown  down  on  neutralization  ;  indeed,  in  some  cases  at  all  events,  all 
the  proteid  matter  originally  present  remains  in  solution,  and  there  is  no 
neutralization  precipitation  at  all,  or  at  most  a  wholly  insignificant  one. 

§  189.  The  proteid  matter,  thus  remaining  in  solution  after  neutraliza- 
tion differs  from  all  the  proteids  which  we  have  hitherto  studied,  inasmuch 
as,  though  existing  in  a  neutral  solution,  it  is  not  coagulated  by  heat,  like 
the  egg-albumin  or  serum-albumin  from  which  it  has  been  produced ;  the 
solution,  after  the  neutralization  precipitate  has  been  filtered  off,  remains 
quite  clear  when  boiled.  The  only  other  solutions  of  proteids  which  do  not 
coagulate  on  boiling  are  solutions  of  acid-  or  alkali-albumin  ;  but  these  solu- 
tions must  be  acid  or  alkali  respectively  ;  the  acid-albumin  or  alkali-albumin 
is  insoluble  in  a  neutral  solution,  and  when  simply  suspended  in  water  is 


256  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

readily  coagulated  at  a  temperature  of  75°  C.  This  new  proteid  matter  of 
•which  we  are  speaking  is  soluble  in  neutral  solutions,  indeed  in  distilled 
water,  and  can  under  no  circumstances  be  coagulated  by  heat. 

Upon  examination  we  find  that  the  new  proteid  matter  thus  left  in  solu- 
tion consists  of  at  least  two  distinct  proteid  bodies.  If  to  the  solution  ammo- 
nium sulphate  be  added,  part  of  the  proteid  matter  is  precipitated  while  part 
is  still  left  in  solution.  The  proteid  body  thus  thrown  down  is  called  albu- 
mose  (there  are  several  varieties  of  albumose,  but  these  need  not  now  detain 
us).  It  approaches  albumin  in  nature  by  reason  of  the  fact  that  it  will  not 
diffuse  through  membranes  ;  that  it  differs,  however,  widely  from  that  proteid 
is  shown  by  its  solutions  not  coagulating  on  boiling.  The  body  which  is  not 
thrown  down  by  ammonium  sulphate  is  called  peptone;  it  differs  from  albu- 
mose in  being  diffusible,  for  it  will  pass  through  membranes.  The  diffusion 
is  not  nearly  so  rapid  as  that  of  salts,  sugar,  and  other  similar  substances  ; 
indeed  solutions  of  peptones  may  be  freed  from  salts  by  dialysis.  But  it  is 
very  marked  as  compared  with  that  of  other  proteids ;  these  pass  through 
membranes  with  the  greatest  difficulty,  if  at  all.  Peptone  is  insoluble  in 
alcohol,  and  may  be  precipitated  from  its  solutions  by  the  addition  of  an 
adequate  quantity  of  this  reagent ;  but  for  this  purpose  a  very  large  excess 
of  alcohol  is  needed,  otherwise  much  of  the  peptone  remains  in  solution.  It 
may  be  kept  under  alcohol  for  a  long  time  without  undergoing  change, 
whereas  other  proteids  are  more  or  less  slowly  coagulated  by  alcohol.  A 
useful  test  for  peptone  is  furnished  by  the  fact  that  a  solution  of  peptone, 
mixed  with  a  strong  solution  of  caustic  potash,  gives  on  addition  of  a  mere 
trace  of  cupric  sulphate  in  the  cold  a,  pink  color,  whereas  other  proteids  give 
a  violet  color.  In  applying  this  test,  however,  care  must  be  taken  not  to  add 
too  much  cupric  sulphate,  since  in  that  case  a  violet  color,  deepening  on 
boiling,  that  is,  the  ordinary  proteid  reaction  (see  §  15),  is  obtained. 

There  are  reasons  for  thinking  that  there  are  several  kinds,  or  at  least 
more  than  one  kind,  of  peptone  ;  but  we  may  for  the  present  regard  the  sub- 
stance as  one.  For  a  long  time  albumose  was  confounded  with  peptone,  and 
many  of  the  commercial  forms  of  "peptone"  consist  largely  of  albumose; 
indeed,  the  two  are  closely  allied  and  have  many  reactions  in  common,  the 
most  striking  differences  being  that  peptone  is  diffusible,  while  albumose  is 
not,  or  hardly  at  all,  and  that  peptone  is  not,  like  albumose,  precipitated  by 
ammonium  sulphate.  The  amount  of  albumose  appearing  in  a  digestion 
experiment,  relative  to  the  amount  of  true  peptone,  depends  on  the  activity 
of  the  juice,  and  other  circumstances.  We  may  regard  albumose  as  a  less 
complete  product  of  digestion  than  peptone. 

The  precipitate  thrown  down  by  neutralization  after  the  action  of  gastric 
juice  on  egg-  or  serum-albumin  resembles,  in  its  general  characters,  acid- 
albumin.  Since,  however,  it  probably  is  distinguishable  from  the  body  or 
bodies  produced  by  the  action  of  simple  acid  on  muscle  or  white  of  egg,  it  is 
best  to  reserve  for  it  the  name  of parapeptone,  which  was  originally  applied  to  it. 

Thus  the  digestion  by  gastric  juice  of  solutions  of  egg-albumin  or  serum- 
albumin  results  in  the  conversion  of  all  the  proteids  present  into  peptone, 
albumose,  and  parapeptone,  of  which  the  first  may  be  considered  as  the  final 
and  chief  product,  and  the  other  two  as  intermediate  products,  occurring  in 
varying  quantity,  possibly  not  always  formed,  and  probably  of  secondary 
importance.  When  fibrin,  either  raw  or  boiled,  or  any  form  of  coagulated 
proteid  is  dissolved  and  seems  to  disappear  under  the  influence  of  gastric 
juice,  the  same  products,  peptone,  albumose,  and  parapeptone  make  their 
appearance.  The  same  bodies  result  when  myosin  or  any  of  the  globulins 
are  subjected  to  the  action  of  the  juice  ;  and  acid-albumin  or  alkali-albumin 
is  similarly  converted  into  albumose  and  peptone. 


SALIVA  AND  GASTRIC  JUICE.  257 

It  is  obvious  that  the  effect  of  the  action  of  the  gastric  juice  is  to  change 
the  less  soluble  proteid  into  a  more  soluble  form,  the  change  being  either 
completed  up  to  the  stage  of  peptone,  the  most  soluble  of  all  proteids,  or 
being  left  in  part  incomplete.  This  will  be  seen  from  the  following  tabular 
arrangement  of  proteids  according  to  their  solubilities : 

Soluble  in  distilled  water. 

Aqueous  solutions  not  coagulated  on  boiling : 

Diffusible Peptone. 

Not  diffusible Albumose. 

Aqueous  solutions  coagulated  on  boiling Albumin. 

Insoluble  in  distilled  water. 

Readily  soluble  in  dilute  saline  solutions  (NaCl  1  per  )  pi  i    r 

cent, ) f  u 

Soluble  only  in  stronger  saline  solutions  (NaCl  5  to  10  )  M 

percent.) j  My(M 

Insoluble  in  dilute  saline  solutions. 
Readily  soluble  in  dilute  acid  (HC1  0.1  percent.)  in  j  Acid-albumin. 

thecold-  -IcLt' 

Soluble  with  difficulty  in  dilute  acid,  that  is  at  high  ) 
temperature  (60°  C. )  and  after  prolonged  treatment  V  Fibrin, 
only j 

Insoluble  in  dilute  acids,  soluble  only  in  strong  acids  .      Coagulated  proteid. 

Milk  when  treated  with  gastric  juice  is  first  of  all  "  curdled."  This  is  the 
result  partially  of  the  action  of  the  free  acid,  but  chiefly  of  the  special  action 
of  a  particular  constituent  of  gastric  juice,  of  which  we  shall  speak  here- 
after. The  curd  consists  of  a  particular  proteid  matter  mixed  with  fat ;  and 
this  proteid  matter  is  subsequently  dissolved  with  the  same  appearance  of  pep- 
tone, albumose,  and  parapeptone  as  in  the  case  of  other  proteids.  In  fact,  the 
digestion  by  gastric  juice  of  all  the  varieties  of  proteids  consists  in  the  con- 
version of  the  proteid  into  peptone,  with  the  concomitant  appearance  of  a 
certain  variable  amount  of  albumose  and  parapeptone. 

§  190.  Circumstances  affecting  gastric  digestion.  The  solvent  action  of 
gastric  juice  on  proteids  is  modified  by  a  variety  of  circumstances.  The 
nature  of  the  proteid  itself  makes  a  difference,  though  this  is  determined 
probably  by  physical  rather  than  by  chemical  characters.  Hence  in  making 
a  series  of  comparative  trials  the  same  proteid  should  be  used,  and  the  form 
of  proteid  most  convenient  for  the  purpose  is  fibrin.  If  it  be  desired  simply 
to  ascertain  whether  any  given  specimen  has  any  digestive  powers  at  all,  it 
is  best  to  use  boiled  fibrin,  since  raw  fibrin  is  eventually  dissolved  by  dilute 
hydrochloric  acid  alone,  probably  on  account  of  some  pepsin  previously 
present  in  the  blood  becoming  entangled  with  the  fibrin  during  clotting. 
But  in  estimating  quantitatively  the  peptic  power  of  two  specimens  of 
gastric  juice  under  different  conditions,  raw  fibrin  prepared  by  Gru'tzner's 
method  is  most  convenient. 

Portions  of  well-washed  fibrin  are  stained  with  carmine  and  again  washed  to 
remove  the  superfluous  coloring  matter.  A  fragment  of  this  colored  fibrin  thrown 
into  an  active  juice  on  becoming  dissolved,  gives  up  its  color  to  the  fluid. 
Hence  if  the  same  stock  of  colored  fibrin  be  used  in  a  series  of  experiments,  and 
the  same  bulks  of  fibrin  and  of  fluid  be  used  in  each  case,  the  amount  of  fibrin 
dissolved  may  be  fairly  estimated  by  the  depth  of  tint  given  to  the  fluid.  Fibrin 
thus  colored  with  carmine  may  be  preserved  in  ether. 
17 


258  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

Since,  if  sufficient  time  be  allowed,  even  a  small  quantity  of  gastric  juice 
will  dissolve  at  least  a  very  large  if  not  an  indefinite  quantity  of  fibrin,  we 
are  led  to  take,  as  a  measure  of  the  activity  of  a  specimen  of  gastric  juice, 
not  the  quantity  of  fibrin  which  it  will  ultimately  dissolve,  but  the  rapidity 
with  which  it  dissolves  a  given  quantity. 

The  greater  the  surface  presented  to  the  action  of  the  juice,  the  more 
rapid  the  solution  ;  hence  minute  division  and  constant  movement  favor  diges- 
tion. And  this  is  probably,  in  part  at  least,  the  reason  why  a  fragment  of 
spongy  filamentous  fibrin  is  more  readily  dissolved  than  a  solid  clump  of 
boiled  white  of  egg  of  the  same  size.  Neutralization  of  the  juice  wholly 
arrests  digestion  ;  fibrin  may  be  submitted  for  an  almost  indefinite  time  to 
the  action  of  neutralized  gastric  juice  without  being  digested.  If  the  neu- 
tralized juice  be  properly  acidified,  it  may  again  become  active ;  when  gas- 
tric juice,  however,  has  been  made  alkaline,  and  kept  for  some  time  at  a 
temperature  of  35°  C.,  its  solvent  powers  are  not  only  suspended  but  actually 
destroyed.  Digestion  is  most  rapid  with  dilute  hydrochloric  acid  of  0.2  per 
cent,  (the  acidity  of  natural  gastric  juice).  If  the  juice  contains  much  more 
or  much  less  free  acid  than  this,  its  activity  is  distinctly  impaired.  Other 
acids,  lactic,  phosphoric,  etc.,  may  be  substituted  for  hydrochloric ;  but  they 
are  not  so  effectual,  and  the  degree  of  acidity  most  useful  varies  with  the 
different  acids.  The  presence  of  neutral  salts,  such  as  sodium  chloride,  in 
excess  is  injurious.  The  action  of  mammalian  gastric  juice  is  most  rapid  at 
35°-40°  C. ;  at  the  ordinary  temperature  it  is  much  slower,  and  at  about 
0°  C.  ceases  altogether.  The  juice  may  be  kept,  however,  at  0°  C.  for  an 
indefinite  period  without  injury  to  its  powers.  The  gastric  juice  of  cold- 
blooded vertebrates  is  relatively  more  active  at  low  temperatures  than  that 
of  warm-blooded  mammals  or  birds. 

At  temperatures  much  above  40°  or  45°  the  action  of  the  juice  is  im- 
paired. By  boiling  for  a  few  minutes  the  activity  of  the  most  powerful 
juice  is  irrevocably  destroyed.  The  presence  in  a  concentrated  form  of  the 
products  of  digestion  hinders  the  process  of  solution.  If  a  large  quantity 
of  fibrin  be  placed  in  a  small  quantity  of  juice,  digestion  is  soon  arrested; 
on  dilution  with  the  normal  hydrochloric  acid  (0.2  per  cent.),  or  if  the  mix- 
ture be  submitted  to  dialysis  to  remove  the  peptones  formed,  and  its  acidity 
be  kept  up  to  the  normal,  the  action  recommences.  By  removing  the  prod- 
ucts of  digestion  as  fast  as  they  are  formed,  and  by  keeping  the  acidity  up 
to  the  normal,  a  given  amount  of  gastric  juice  maybe  made  to  digest  a  very 
large  quantity  of  proteid  material.  Whether  the  quantity  is  really  un- 
limited is  disputed ;  but  in  any  case  the  energies  of  the  juice  are  not  rapidly 
exhausted  by  the  act  of  digestion. 

§  191.  Nature  of  the  action.  All  these  facts  go  to  show  that  the  digestive 
action  of  gastric  juice  on  proteids,  like  that  of  saliva  on  starch,  is  a  ferment- 
action  ;  in  other  words,  that  the  solvent  action  of  gastric  juice  is  essentially 
due  to  the  presence  in  it  of  a  ferment-body.  To  this  ferment-body,  which 
as  yet  has  been  only  approximately  isolated,  the  name  of  pepsin  has  been 
given.  It  is  present  not  only  in  gastric  juice,  but  also  in  the  glands  of  the 
gastric  mucous  membrane,  especially  in  certain  parts  and  under  certain 
conditions  which  we  shall  study  presently.  The  glycerin  extract  of  gastric 
mucous  membrane,  at  any  rate  of  that  which  has  been  dehydrated,  contains 
a  minimal  quantity  of  proteid  matter,  and  yet  is  intensely  peptic.  Other 
methods,  such  as  the  elaborate  one  of  Briicke,  give  us  a  material  which, 
though  containing  nitrogen,  exhibits  none  of  the  ordinary  proteid  reactions, 
and  yet  in  concert  with  normal  dilute  hydrochloric  acid  is  peptic  in  a  very 
high  degree.  We  seem,  therefore,  justified  in  asserting  that  pepsin  is  not  a 
proteid,  but  it  would  be  hazardous  to  make  any  dogmatic  statement  con- 


SALIVA  AND  GASTRIC  JUICE.  259 

cerning  a  substance  obtained  in  so  small  a  quantity  at  a  time  that  its 
exact  chemical  characters  have  not  yet  been  ascertained.  At  present 
the  manifestation  of  peptic  powers  is  our  only  safe  test  of  the  presence 
of  pepsin. 

In  one  important  respect  pepsin,  the  ferment  of  gastric  juice,  differs  from 
.ptyalin,  the  ferment  of  saliva.  Saliva  is  active  in  a  perfectly  neutral 
medium,  and  there  seems  to  be  no  special  connection  between  the  ferment 
and  any  alkali  or  acid.  In  gastric  juice,  however,  there  is  a  strong  tie 
between  the  acid  and  the  ferment,  so  strong  that  some  writers  speak  of 
pepsin  and  hydrochloric  acid  as  forming  together  a  compound,  pepto- 
hydrochloric  acid. 

In  the  absence  of  exact  knowledge  of  the  constitution  of  proteids,  we 
cannot  state  distinctly  what  is  the  precise  nature  of  the  change  into  pep- 
tone ;  the  various  proteids  differ  from  each  other  in  elementary  composition 
quite  as  widely  as  does  peptone  from  any  of  them.  Judging  from  the 
analogy  with  the  action  of  saliva  on  starch,  we  may  fairly  suppose  that 
the  process  is  at  bottom  one  of  hydration ;  and  this  view  is  further  sug- 
gested by  the  fact  that  peptone  closely  resembling,  if  not  identical  with, 
that  obtained  by  gastric  digestion,  may  be  obtained  by  the  action  of  the  strong 
acids,  by  the  prolonged  action  of  dilute  acids  especially  at  a  high  tempera- 
ture, or  simply  by  digestion  with  superheated  water  in  a  Papin's  digester, 
that  is  to  say,  by  means  of  agents  which,  in  other  cases  produce  their 
effects  by  bringing  about  hydrolytic  changes ;  beyond  this  we  cannot  at 
present  go. 

§  192.  All  proteids,  as  far  as  we  know,  are  converted  by  pepsin  into  pep- 
tone. Concerning  the  action  of  gastric  juice  on  other  nitrogenous  sub- 
stances more  or  less  allied  to  proteids,  but  not  truly  proteid  in  nature,  our 
knowledge  is  at  present  imperfect.  Mucin,  nuclein,  and  the  chemical  basis 
of  horny  tissues  are  wholly  unaffected  by  gastric  juice.  The  gelatiniferous 
tissues  are  dissolved  by  it ;  and  the  bundles  and  membranes  of  connective 
tissue  are  very  speedily  so  far  affected  by  it  that  at  a  very  early  stage  of 
digestion  the  bundles  and  elementary  fibres  of  muscles  which  are  bound 
together  by  connective  tissue  fall  asunder ;  moreover,  both  prepared  gelatin 
and  the  gelatiniferous  basis  of  connective  tissue  in  its  natural  condition, 
that  is  without  being  previously  heated  with  water,  are  by  it  changed  into 
a  substance  so  far  analogous  with  peptone  that  the  characteristic  property 
of  gelatinization  is  entirely  lost.  Chondrin  and  the  elastic  tissues  undergo 
a  similar  change. 

§  193.  Action  of  gastric  juice  on  milk.  It  has  long  been  known  that  ah 
infusion  of  calves'  stomach,  called  rennet,  has  a  remarkable  effect  in  rapidly 
curdling  milk,  and  this  property  is  made  use  of  in  the  manufacture  of 
cheese.  Gastric  juice  has  a  similar  effect:  milk  when  subjected  to  the 
action  of  gastric  juice  is  first  curdled  and  then  digested.  If  a  few  drops  of 
gastric  juice  be  added  to  a  little  milk  in  a  test-tube,  and  the  mixture  ex- 
posed to  a  temperature  of  40°  C.,the  milk  will  curdle  into  a  complete  clot  in 
a  very  short  time.  If  the  action  be  continued  the  curd  or  clot  will  be  ulti- 
mately dissolved  and  digested.  Milk  contains,  besides  a  peculiar  form,  or 
peculiar  forms  of  albumin,  fats,  milk-sugar,  and  various  salines,  the  peculiar 
proteid  casein.  In  natural  milk  casein  is  present  in  solution,  and  "  curd- 
ling" consists  essentially  in  the  soluble  casein  being  converted  (or  more 
probably  as  we  shall  see  presently,  split  up)  into  an  insoluble  modification 
of  casein,  which  as  it  is  being  precipitated  carries  down  with  it  a  great  deal 
of  the  fat  and  so  forms  the  "  curd."  Now  casein  is  readily  precipitated 
from  milk  upon  the  addition  of  a  small  quantity  of  acid,  and  it  might  be 
supposed  that  the  curdling  effect  of  gastric  juice  was  due  to  its  acid  reaction. 


260  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

But  this  is  not  the  case,  for  neutralized  gastric  juice,  or  neutral  rennet,  is 
equally  efficacious. 

The  curdling  action  of  rennet  is  closely  dependent  on  temperature,  being 
like  the  peptic  action  of  gastric  juice  favored  by  a  rise  of  temperature  up  to 
about  40°  C.  Moreover  the  curdling  action  is  destroyed  by  previous  boiling 
of  the  juice  or  rennet.  These  facts  suggest  that  a  ferment  is  at  the  bottom 
of  the  matter;  and,  indeed,  all  the  features  of  the  action  support  this  view. 
Moreover,  as  a  matter  of  fact,  a  curdling  ferment  may  be  extracted  by 
glycerin  and  by  the  other  methods  used  for  preparing  ferments.  The  fer- 
ment, however,  is  not  pepsin,  but  some  other  body ;  and  the  two  may  be 
separated  from  each  other. 

It  might  be  thought  that  the  rennet-ferment,  rennin  we  may  call  it,  acted 
by  inducing  a  fermentation  in  the  sugar  of  milk,  giving  rise  to  lactic  acid 
which  precipitated  the  casein  by  virtue  of  its  being  an  acid.  But  this  view 
is  disproved  by  the  following  facts  which  show  that  the  ferment  produces  its 
curdling  effect  by  acting  directly  on  the  natural  casein  itself.  Casein  maybe 
precipitated  unchanged,  that  is,  capable  of  redissolving  in  water  (the  presence 
of  calcic  phosphate  being  assumed)  by  saturating  milk  with  neutral  saline 
bodies  (such  as  sodium  chloride  or  magnesium  sulphate) ;  and  by  being  pre- 
cipitated and  redissolved  more  than  once  may  be  obtained  largely  free  from 
fat  and  wholly  free  from  milk-sugar.  Such  solutions  of  isolated'casein  freed 
from  milk-sugar  may  be  made  to  curdle  like  natural  milk  by  the  addition 
of  rennin,  showing  that  the  milk-sugar  has  nothing  to  do  with  the  matter. 
Moreover,  the  precipitate  thrown  down  from  milk  by  dilute  acids,  lactic  acid 
included,  is  itself  unaltered  or  very  slightly  altered  casein,  not  curd,  and 
with  care  may  be  so  prepared  as  to  be  redissolved  into  solutions  which 
curdle  with  rennin,  like  solutions  of  casein  prepared  by  means  of  neutral 
salts. 

When  isolated  casein  is  curdled  by  means  of  reunin,  two  proteids,  it  is 
stated,  make  their  appearance,  one  which  is  soluble  and  allied  to  albumin, 
and  another  which  is  insoluble  and  forms  the  curd.  Curdling,  therefore, 
according  to  this  result  appears  to  be  the  splitting  up  by  a  ferment  of  a  more 
complex  body  ;  and  it  is  interesting  to  observe,  as  perhaps  throwing  light  on 
the  somewhat  analogous  formation  of  the  fibrin,  that  this  curdling  action 
will  not  take  place  if  calcic  phosphate  be  wholly  absent  from  the  mixture. 
The  calcic  phosphate  appears  to  play  a  peculiar  part  in  determining  the 
insolubility  of  the  curd,  for  there  is  evidence  that  in  the  absence  of  calcic 
phosphate  the  ferment  has  power  to  attack  the  casein  and  split  it  up,  but 
that  both  products  remain  in  solution  ;  if  calcic  phosphate  be  present,  the 
one,  viz.,  the  curd,1  becomes  insoluble. 

Rennin  is  abundant  in  the  gastric  juice  and  in  the  gastric  mucous  mem- 
brane of  ruminants,  but  is  also  found  in  the  gastric  juice  of  other  animals, 
and  either  it,  or  what  we  shall  presently  have  occasion  to  speak  of  as  the 
antecedent  of  the  ferment  or  zymogen,  is  present  also  in  the  mucous  membrane 
of  the  stomach  of  most  animals.  A  very  similar  if  not  identical  ferment  has 
also  been  found  in  many  plants. 

THE  ACT   OF   SECRETION    OF  SALIVA  AND  GASTRIC  JUICE  AND  THE 
NERVOUS  MECHANISMS  WHICH  REGULATE  IT. 

§  194.  The  saliva  and  gastric  juice  whose  properties  we  have  studied, 
though  so  different  from  each  other,  are  both  drawn  ultimately  from  one 

1  It  might  be  useful,  in  order  to  distinguish  the  curd  from  the  natural  soluble 
casein,  to  call  the  former  tyre/in  (rvpoi,  cheese),  and  so  reserve  the  name  of  casein  for 
the  latter. 


SECRETION  OF  SALIVA  AND  GASTRIC  JUICE.  261 

common  source,  the  blood,  and  they  are  poured  into  the  alimentary  canal, 
not  in  a  continuous  flow,  but  intermittently  as  occasion  may  demand.  The 
epithelial  cells  which  supply  them  have  their  periods  of  rest  and  of  activ- 
ity, and  the  amount  and  quality  of  the  fluids  which  the  cells  secrete  are 
determined  by  the  needs  of  the  economy  as  the  food  passes  along  the  canal. 
We  have  now  to  consider  how  the  epithelial  cell  manufactures  its  special 
secretion  out  of  the  materials  supplied  to  it  by  the  blood,  and  how  the  cell 
is  called  into  activity  by  the  presence  of  food,  it  may  be  as  in  the  case  of 
saliva  at  some  distance  from  itself,  or  by  circumstances  which  do  not  bear 
directly  on  itself.  In  dealing  with  these  matters  in  connection  with  the 
digestive  juices,  we  shall  have  to  enter  at  some  length  into  the  physiology 
of  secretion  in  general. 

The  question  which  presents  itself  first  is :  By  what  mechanism  is  the 
activity  of  the  secreting  cells  brought  into  play  ? 

While  fasting,  a  small  quantity  only  of  saliva  is  poured  into  the  mouth ; 
the  buccal  cavity  is  just  moist  and  nothing  more.  When  food  is  taken,  or 
when  any  sapid  or  stimulating  substance,  or  indeed  a  body  of  any  kind,  is 
introduced  into  the  mouth,  a  flow  is  induced  which  may  be  very  copious. 
Indeed  the  quantity  secreted  in  ordinary  life  during  24  hours  has  been 
roughly  calculated  at  as  much  as  from  1  to  2  litres.  An  abundant  secretion 
in  the  absence  of  food  in  the  mouth  may  be  called  forth  by  an  emotion,  as 
when  the  mouth  waters  at  the  sight  of  food,  or  by  a  smell,  or  by  events  oc- 
curring in  the  stomach,  as  in  some  cases  of  nausea.  Evidently  in  these  iiir 
stances  some  nervous  mechanism  is  at  work.  In  studying  the  action  of  this 
nervous  mechanism,  it  will  be  of  advantage  to  confine  our  attention  at  first 
to  the  submaxillary  gland. 

§  195.  The  submaxillary  gland  is  supplied  with  two  sets  of  nerves. 
These  are  represented  in  Fig.  81,  which  is  a  very  diagrammatic  rendering 
of  the  appearances  presented  when  the  submaxillary  gland  is  prepared  for 
an  experiment  in  a  dog,  the  animal  being  placed  on  its  back  and  the 
gland  exposed  from  the  neck.  The  one  set,  and  that  the  more  important, 
belongs  to  the  chorda  tympani  nerve  (ch.  </').  This  is  a  small  nerve,  which 
branches  off  from  the  facial  or  seventh  cranial  nerve  in  the  Fallopian  canal 
before  the  nerve  issues  from  the  skull.  Whether  it  really  belongs  to  the 
facial  proper  has  been  doubted  ;  in  man  the  fibres  which  form  it  are  either 
fibres  coming  not  from  the  roots  of  the  facial  proper,  but  from  the  portio 
intermedia  Wrisbergi,  or,  according  to  some,  fibres  which,  though  joining 
the  facial  in  the  Fallopian  canal,  are  ultimately  derived  from  another 
(the  fifth)  cranial  nerve.  Leaving  the  facial  nerve,  the  chorda  tympani 
passes  through  the  tympanic  cavity  or  drum  of  the  ear  (hence  the  name) 
and  joins  or  rather  runs  in  company  (ch.  £/)  with  the  lingual  or  gustatory 
branch  of  the  fifth  nerve.  Some  of  the  fibres  run  on  with  the  lingual  right 
down  to  the  tongue  (these  are  not  shown  in  the  figure),  but  many  leave  the 
lingual  as  a  slender  nerve  (ch.  £.),  which,  reaching  Wharton's  duct  or  duct 
of  the  submaxillary  gland  (sm.  d.)  runs  along  the  duct  to  the  gland.  As 
the  nerve  courses  along  the  duct  nerve-cells  make  their  appearance  among 
the  fibres,  and  these  are  especially  abundant  just  after  the  duct  enters  the 
hilus  of  the  gland.  The  fibres  may  be  traced  into  the  gland  for  some  dis- 
tance, but,  as  we  have  said,  their  ultimate  ending  has  not  yet  been  definitely 
made  out.  Along  its  whole  course  up  to  the  gland,  the  fibres  of  the  chorda 
are  very  fine  medullated  fibres,  but  they  lose  their  medulla  in  the  gland. 

The  other  set  of  nerve-fibres  reaches  the  gland  along  the  small  arteries 
of  the  gland.  These  are  non-medullated  fibres  mixed  with  a  few  medullated 
fibres,  and  may  be  traced  back  to  the  superior  cervical  ganglion.  From 
thence  they  may  be  traced  still  further  back  down  the  cervical  sympathetic 


262 


THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 


to  the  spinal  cord,  following  apparently  the  same  tract  as  the  vaso-constrictor 
fibres,  treated  of  in  §  152. 

§  196.  If  a  tube  be  placed  in  the  duct,  it  is  seen  that  when  sapid  sub- 
stances are  placed  on  the  tongue,  or  the  tongue  is  stimulated  in  any  other 
way,  or  the  lingual  nerve  is  laid  bare  and  stimulated  with  an  interrupted 
current,  a  copious  flow  of  saliva  takes  place.  If  the  sympathetic  be  divided, 
stimulation  of  the  tongue  or  lingual  nerve  still  produces  a  flow.  But  if  the 
small  chorda  nerve  be  divided,  stimulation  of  the  tongue  or  lingual  nerve 
produces  no  flow. 

Evidently  the  flow  of  saliva  is  a  nervous  reflex  action,  the  lingual  nerve 
serving  as  the  channel  for  the  afferent  and  the  small  chorda  nerve  for  the 
efferent  impulses.  If  the  trunk  of  the  lingual  be  divided  above  the  point 
where  the  chorda  leaves  it,  as  at  n.  I.',  Fig.  81,  stimulation  of  the  (front  part 


v.sym. 


/u 


FIG.  81. 


^n.sym.f.  n.sym.sm.  vi.T^ 

-"'  g.sni.a.   / 


a.car.   a-f- 


-~.sm.gl. 


n.sym.srri. 

r.sm.pi 


\ch.t. 


Diagrammatic  Representation  of  the  Submaxillary  Gland  of  the  Dog,  with  its  Nerves  and 
Bloodvessels.  The  dissection  has  been  on  an  animal  lying  on  its  back,  but  since  all  the  parts 
shown  in  the  figure  cannot  be  seen  from  any  one  point  of  view,  the  figure  does  not  give  the  exact 
anatomical  relations  of  the  several  structures. 

sm.  gld.  The  submaxillary  gland,  into  the  duct  (sm.  d.)  of  which  a  canula  has  been  tied.  The 
sublingual  gland  and  duct  are  not  shown,  n.l.,  n.l'.  The  lingual  branch  of  the  fifth  nerve,  the 
part  n.l.  is  going  to  the  tongue.  ch.t.,  ch.t'.,  ch.t".  The  chorda  tympani.  The  part  ch.t".  is  proceeding 
from  the  facial  nerve  ;  at  ch.  t'.  it  becomes  conjoined  with  the  lingual  n.l'.,  and  afterward  diverging 
passes  as  ch.  t.  to  the  gland  along  the  duct;  the  continuation  of  the  nerve  in  company  with  the 
lingual  n.l.,  is  not  shown,  sm.  gl.  The  submaxillary  ganglion  with  its  several  roots,  a.  car.  The 
carotid  artery,  two  small  branches  of  which,  a.  sm.  a.  and  r.  sm.p.,  pass  to  the  anterior  and  poste- 
rior parts  of  the  gland,  v.  sm.  The  anterior  and  posterior  veins  from  the  gland,  falling  into  v.j., 
the  jugular  vein.  v.  sym.  The  conjoined  vagus  and  sympathetic  trunks,  g.  cer.  s.  The  upper  cer- 
vical ganglion,  two  branches  of  which,  forming  a  plexus  (a/.)  over  the  facial  artery,  are  dis- 
tributed (w.  sym.  sm.)  along  the  two  glandular  arteries  to  the  anterior  and  posterior  portions  of  the 
gland. 

The  arrows  indicate  the  direction  taken  by  the  nervous  impulses  during  reflex  stimulation  of 
the  gland.  They  ascend  to  the  brain  by  the  lingual  and  descend  by  the  chorda  tympani. 

of)  tongue  produces,  under  ordinary  circumstances,  no  flow.  This  shows 
that  the  centre  of  the  reflex  action  is  higher  up  than  the  point  of  section  ;  it 
lies  in  fact  in  the  brain. 

In  the  angle  between  the  lingual  and  the  chorda,  where  the  latter  leaves  the 
former  to  pass  to  the  gland,  lies  the  small  submaxillary  ganglion  (represented  dia- 
grammatically  in  Fig.  81,  sm.  gl).  This  consists  of  small  masses  of  nerve-cells 


SECRETION  OF  SALIVA   AND  GASTRIC  JUICE.  263 

lying  on  the  small  bundles  of  nerve-fibres  which  spread  out  like  a  fan  from  the 
lingual  and  chorda  tympani  nerves  (ch.  t.)  toward  the  ducts  of  the  subm axillary  and 
sublingual  glands.  It  has  been  much  debated  whether  this  ganglion  can  act  as  a 
centre  of  reflex  action  in  connection  with  the  submaxillary  gland,  but  no  conclusive 
evidence  that  it  does  so  act  has  as  yet  been  shown ;  it  probably  belongs  in  reality  to 
the  sublingual  gland. 

Stimulation  of  the  glosso-pharyngeal  is  even  more  effectual  than  that  of 
the  lingual.  Probably  this  indeed  is  the  chief  afferent  nerve  in  ordinary 
secretion.  Stimulation  of  the  mucous  membrane  of  the  stomach  (as  by  food 
introduced  through  a  gastric  fistula)  or  of  the  vagus  may  also  produce  a  flow 
of  saliva,  as  indeed  may  stimulation  of  the  sciatic,  and  probably  of  many 
other  afferent  nerves.  All  these  cases  are  instances  of  reflex  action,  the 
cerebro-spinal  system  acting  as  a  centre.  We  may  further  define  the  centre 
as  a  part  of  the  medulla  oblongata,  apparently  not  far  removed  from  the  vaso- 
motor  centre.  When  the  brain  is  removed  down  to  the  medulla  oblongata, 
that  organ  being  left  intact,  a  flow  of  saliva  may  still  be  obtained  by 
adequate  stimulation  of  various  afferent  nerves ;  when  the  medulla  is  de- 
stroyed no  such  action  is  possible.  And  a  flow  of  saliva  may  be  produced 
by  direct  stimulation  of  the  medulla  itself.  When  a  flow  of  saliva  is  ex- 
cited by  ideas,  or  by  emotions,  the  nervous  processes  begin  in  the  higher 
parts  of  the  brain,  aud  descend  thence  to  the  medulla  before  they  give  rise 
to  distinctly  efferent  impulses  ;  and  it  would  appear  that  these  higher  parts 
of  the  brain  are  called  into  action  when  a  flow  of  saliva  is  excited  by 
distinct  sensations  of  taste. 

Considering,  then,  the  flow  of  saliva  as  a  reflex  act,  the  centre  of  which 
lies  in  the  medulla  oblongata,  we  may  imagine  the  efferent  impulses  passing 
from  that  centre  to  the  gland  either  by  the  chorda  tympani  or  by  the  sym- 
pathetic nerve.  Although  it  would  perhaps  be  rash  to  say  that  in  this 
relation  the  sympathetic  nerve  never  acts  as  an  efferent  channel,  as  a  matter 
of  fact  we  have  no  satisfactory  experimental  evidence  that  it  does  so  ;  and 
we  may,  therefore,  state  that,  practically,  the  chorda  tympani  is  the  sole 
efferent  nerve.  Section  of  that  nerve,  either  where  the  fibres  pass  from  the 
lingual  nerve  and  the  submaxillary  ganglion  to  the  gland,  or  where  it  runs 
in  the  same  sheath  as  the  lingual,  or  in  any  part  of  its  course  from  the  main 
facial  trunk  to  the  lingual,  puts  an  end,  as  far  as  we  know,  to  the  possibility 
of  any  flow  being  excited  by  stimuli  applied  to  the  sensory  nerves,  or  to  the 
sentient  surfaces  of  the  mouth  or  of  other  parts  of  the  body. 

The  natural  reflex  act  of  secretion  may  be  inhibited,  like  the  reflex  action 
of  the  vasomotor  nerves,  at  its  centre.  Thus  when,  as  in  the  old  rice  ordeal, 
fear  parches  the  mouth,  it  is  probable  that  the  afferent  impulses  caused  by 
the  presence  of  food  in  the  mouth  cease,  through  emotional  inhibition  of 
their  reflex  centre,  to  give  rise  to  efferent  impulses. 

§  197.  In  life,  then,  the  flow  of  saliva  is  brought  about  by  the  advent  to 
the  gland  along  the  chorda  tympani  of  efferent  impulses,  started  chiefly  by 
reflex  actions/  The  inquiry  thus  narrows  itself  to  the  question:  In  what 
manner  do  these  efferent  impulses  cause  the  increase  of  flow  ? 

If  in  a  dog  a  tube  be  introduced  into  Wharton's  duct,  and  the  chorda  be 
divided,  the  flow,  if  any  be  going  on,  is  from  the  lack  of  efferent  impulses 
arrested.  On  passing  an  interrupted  current  through  the  peripheral  portion 
of  the  chorda,  a  copious  secretion  at  once  takes  place,  and  the  saliva  begins 
to  rise  rapidly  in  the  tube ;  a  very  short  time  after  the  application  of  the 
current  the  flow  reaches  a  maximum  which  is  maintained  for  some  time,  and 
then,  if  the  current  be  long  continued,  gradually  lessens.  If  the  current  be 
applied  for  a  short  time  only,  the  secretion  may  last  for  some  time  after  the 
current  has  been  shut  off.  *  The  saliva  thus  obtained  is  but  slightly  viscid, 


264  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

and  under  the  microscope  a  very  few  salivary  corpuscles,  and  occasionally 
only,  amorphous  lumps  of  peculiar  material,  probably  mucous  in  nature,  are 
seen.  If  the  gland  itself  be  watched,  while  its  activity  is  thus  roused,  it 
will  be  seen  (as  we  have  already  said,  §  153)  that  its  arteries  are  dilated  and 
its  capillaries  filled,  and  that  the  blood  flows  rapidly  through  the  veins  in  a 
full  stream  and  of  bright  arterial  hue,  frequently  with  pulsating  movements. 
If  a  vein  of  the  gland  be  opened,  this  large  increase  of  flow,  and  the  lessen- 
ing of  the  ordinary  deoxygenatiou  of  the  blood  consequent  upon  the  rapid 
stream  will  be  still  more  evident.  It  is  clear  that  excitation  of  the  chorda 
largely  dilates  the  arteries ;  the  nerve  acts  energetically  as  a  vaso-dilator 
nerve. 

Thus  stimulation  of  the  chorda  brings  about  two  events:  a  dilatation  of 
the  bloodvessels  of  the  gland,  and  a  flow  of  saliva.  This  question  at  once 
arises,  Is  the  latter  simply  the  result  of  the  former  or  is  the  flow  caused  by 
some  direct  action  on  the  secreting  cells,  apart  from  the  increased  blood- 
supply  ?  In  support  of  the  former  view  we  might  argue  that  the  activity  of 
the  epithelial  secreting  cell,  like  that  of  any  other  form  of  protoplasm,  is 
dependent  on  blood-supply.  When  the  small  arteries  of  the  gland  dilate, 
while  the  pressure  in  the  arteries  on  the  side  toward  the  heart  is  (as  we  have 
previously  seen  when  treating  generally  of  blood-pressure,  §  109)  corre- 
spondingly diminished,  the  pressure  on  the  far  side  in  the  capillaries  and 
veins  is  increased  ;  hence  the  capillaries  become  fuller,  and  more  blood  passes 
through  them  in  a  given  time.  From  this  we  might  infer  that  a  larger 
amount  of  nutritive  material  would  pass  away  from  the  capillaries  into  the 
surrounding  lymph-spaces,  and  so  into  the  epithelial  cells,  the  result  of 
which  would  naturally  be  to  quicken  the  processes  going  on  in  the  cells,  and 
to  stir  these  up  to  greater  activity.  But  even  admitting  all  this  it  does  not 
necessarily  follow  that  the  activity  thus  excited  should  take  on  the  form  of 
secretion.  It  is  quite  possible  to  conceive  that  the  increased  blood-supply 
should  lead  only  to  the  accumulation  in  the  cell  of  the  constituents  of  the 
saliva,  or  of  the  raw  materials  for  their  construction,  and  not  to  a  discharge 
of  the  secretion.  A  man  works  better  for  being  fed,  but  feeding  does  not 
make  him  work  in  the  absence  of  any  stimulus.  The  increased  blood-supply, 
therefore,  while  favorable  to  active  secretion,  need  not  necessarily  bring  it 
about.  Moreover,  the  following  facts  distinctly  show  that  it  need  not. 
When  a  canula  is  tied  into  the  duct  and  the  chorda  is  energetically  stimu- 
lated, the  pressure  acquired  by  the  saliva  accumulated  in  the  canula  and  in 
the  duct  may  exceed  for  the  time  being  the  arterial  blood-pressure,  even  that 
of  the  carotid  artery  ;  that  is  to  say,  the  pressure  of  fluid  in  the  gland  out- 
side the  bloodvessels  is  greater  than  that  of  the  blood  inside  the  bloodvessels. 
This  must,  whatever  be  the  exact  mode  of  transit  of  nutritive  material 
through  the  vascular  walls,  tend  to  check  that  transit.  Again,  if  the  head 
of  an  animal  be  rapidly  cut  off,  and  the  chorda  immediately  stimulated,  a 
flow  of  saliva  takes  place  far  too  copious  to  be  accounted  for  by  the  empty- 
ing of  the  salivary  channels  through  any  supposed  contraction  of  their  walls. 
In  this  case  secretion  is  excited  in  the  gland  though  the  blood-supply  is 
limited  to  the  small  quantity  still  remaining  in  the  bloodvessels.  Lastly,  if 
a  small  quantity  of  atropine  be  injected  into  the  veins,  stimulation  of  the 
chorda  produces  no  secretion  of  saliva  at  all,  though  the  dilatation  of  the 
bloodvessels  takes  place  as  usual ;  in  spite  of  the  greatly  increased  blood- 
supply  no  secretion  at  all  takes  place.  These  facts  prove  that  the  secretory 
activity  is  not  simply  the  result  of  vascular  changes,  but  may  be  called  forth 
independently  ;  they  further  lead  us  to  suppose  that  the  chorda  contains  two 
sets  of  fibres,  one  which  we  may  call  secretory  fibres,  acting  directly  on  the 
secreting  structures  only,  and  the  other  vaso-dilator  fibres,  acting  on  the 


SECRETION   OF  SALIVA   AND  GASTRIC  JUICE.  265 

bloodvessels  only,  and  further  that  atropine,  while  it  has  no  effect  on  the 
latter,  paralyzes*  the  former  just  as  it  paralyzes  the  inhibitory  fibres  of  the 
vagus.  Hence  when  the  chorda  is  stimulated  there  pass  down  the  nerve,  in 
addition  to  impulses  affecting  the  blood-supply,  impulses  affecting  directly 
the  protoplasm  of  the  secreting  cells,  and  calling  it  into  action,  just  as  similar 
impulses  call  into  action  the  contractility  of  the  substance  of  a  muscular 
fibre.  Indeed,  the  two  things,  secreting  activity  and  contracting  activity, 
are  very  parallel. 

Since  the  chorda  acts  thus  directly  on  the  secreting  cells,  we  should 
expect  to  find  an  anatomical  connection  between  the  cells  and  the  nerve  ;  and 
some  authors  have  maintained  that  the  nerve-fibres  may  be  traced  into  the 
cells.  But,  save  perhaps  in  the  case  of  certain  glands  of  invertebrates 
(so-called  salivary  glands  of  BlattaJ,  the  evidence,  as  we  have  said,  is  as  yet 
not  convincing. 

§  198.  When  the  cervical  sympathetic  is  stimulated,  the  vascular  effects, 
as  we  have  already  said  (§  154),  are  the  exact  contrary  of  those  seen  when 
the  chorda  is  stimulated.  The  small  arteries  are  constricted,  and  a  small 
quantity  of  dark  venous  blood  escapes  by  the  veins.  Sometimes,  indeed, 
the  flow  through  the  gland  is  almost  arrested.  The  sympathetic,  therefore, 
acts  as  a  vaso-constrictor  nerve,  and  in  this  sense  is  antagonistic  to  the 
chorda. 

As  concerns  the  flow  of  saliva  brought  about  by  stimulation  of  the  sym- 
pathetic, in  the  case  of  the  submaxillary  gland  of  the  dog  the  effects  are 
very  peculiar.  A  slight  flow  results,  and  the  saliva  so  secreted  is  remark- 
ably viscid,  of  higher  specific  gravity,  and  richer  in  corpuscles  and  in  the 
above-mentioned  amorphous  lumps  than  is  the  chorda  saliva.  This  action 
of  the  sympathetic  is  little  or  not  at  all  affected  by  atropine. 

In  the  submaxillary  gland  of  the  dog,  then,  the  contrast  between  the 
effects  of  chorda  stimulation  and  those  of  sympathetic  stimulation  are  very 
marked  ;  the  former  gives  rise  to  vascular  dilatation  with  a  copious  flow  of 
fairly  limpid  saliva  poor  in  solids,  the  latter  to  vascular  constriction  with  a 
scanty  flow  of  viscid  saliva  richer  in  solids.  And  in  other  animals  a  simi- 
lar contrast  prevails,  though  with  minor  differences.  Thus,  in  the  rabbit 
both  chorda  saliva  and  sympathetic  saliva  are  limpid  and  free  from  mucus, 
though  the  latter  contains  more  proteids ;  in  the  cat,  chorda  saliva  is  more 
viscid  than  sympathetic  saliva ;  but  in  both  these  cases,  as  in  the  dog,  stimu- 
lation of  the  chorda  causes  a  copious  flow  with  dilated  bloodvessels,  and  stimu- 
lation of  the  sympathetic  a  scanty  flow  with  vascular  constriction.  We  shall 
return  again  presently  to  these  different  actions  of  the  two  nerves;  mean- 
while we  have  seen  enough  of  the  history  of  the  submaxillary  gland  to  learn 
that  secretion  in  this  instance  is  a  reflex  action,  the  efferent  impulses  of 
which  directly  affect  the  secreting  cells,  and  that  the  vascular  phenomena 
may  assist,  but  are  not  the  direct  cause  of  the  flow. 

§  199.  We  have  dwelt  long  on  this  gland  because  it  has  been  more 
fruitfully  studied  than  any  other.  But  the  nervous  mechanisms  of  the  other 
salivary  glands  are  in  their  main  features  similar.  Thus  the  secretion  of 
the  parotid  gland,  like  that  of  the  submaxillary,  is  governed  by  two  sets  of 
fibres ;  one  of  cerebro-spinal  origin,  running  along  the  auriculo-temporal 
branch  of  the  fifth  nerve,  but  originating  possibly  in  the  glosso-pharyngeal, 
and  the  other  of  sympathetic  origin  coming  from  the  cervical  sympathetic. 
Stimulation  of  the  cerebro-spinal  fibres  produces  a  copious  flow  of  limpid 
saliva,  free  from  mucus;  stimulation  of  the  cervical  sympathetic  gives  rise 
in  the  rabbit  to  a  secretion  also  free  from  mucus,  but  rich  in  proteids  and  of 
greater  amylolytic  power  than  the  cerebro-spinal  secretion  ;  in  the  dog  little 
or  no  secretion  is  produced,  though,  as  we  shall  see  later  on,  certain  changes 


266  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

are  brought  about  in  the  gland  itself.  In  both  animals  the  cerebro-spi- 
ual  fibres  are  vaso-dilator,  and  the  sympathetic  fibres  vaso-constrictor  in 
action. 

§  200.  The  secretion  of  gastric  juice.  Though  a  certain  amount  of  gastric 
juice  may  sometimes  be  found  in  the  stomachs  of  fasting  animals,  it  may  be 
stated  generally  that  the  stomach,  like  the  salivary  glands,  remains  inactive, 
yielding  no  secretion,  so  long  as  it  is  not  stimulated  by  food  or  otherwise. 
The  advent  of  food  into  the  stomach,  however,  at  once  causes  a  copious  flow 
of  gastric  juice;  and  the  quantity  secreted  in  the  twenty-four  hours  is  pro- 
bably very  considerable,  but  we  have  no  trustworthy  data  for  calculating  the 
exact  amount.  So  also  when  the  gastric  mucous  membrane  is  stimulated 
mechanically,  as  with  a  feather,  secretion  is  excited ;  but  to  a  very  small 
amount  even  when  the  whole  interior  surface  of  the  stomach  is  thus  repeat- 
edly stimulated.  The  most  efficient  stimulus  is  the  natural  stimulus,  viz., 
food ;  though  dilute  alkalies  seem  to  have  unusually  powerful  stimulating 
effects ;  thus  the  swallowing  of  saliva  at  once  provokes  a  flow  of  gastric 
juice. 

During  fasting  the  gastric  membrane  is  of  a  pale  gray  color,  somewhat 
dry,  covered  with  a  thin  layer  of  mucus,  and  thrown  into  folds ;  during 
digestion  it  becomes  red,  flushed,  and  tumid,  the  folds  disappear,  and  minute 
drops  of  fluid  appearing  at  the  mouths  of  the  glands,  speedily  run  together 
into  small  streams.  When  the  secretion  is  very  active,  the  blood  flows  from 
the  capillaries  into  the  veins  in  a  rapid  stream  without  losing  its  bright 
arterial  hue.  The  secretion  of  gastric  juice  is,  in  fact,  accompanied  by 
vascular  dilatation  in  the  same  way  as  is  the  secretion  of  saliva. 

§  201.  Seeing  that,  unlike  the  case  of  the  salivary  secretion,  food  is 
brought  into  the  immediate  neighborhood  of  the  secreting  cells,  it  is  exceed- 
ingly probable  that  a  great  deal  of  the  secretion  is  the  result  of  the  working 
of  a  local  mechanism  ;  and  this  view  is  supported  by  the  fact  that  when  a 
mechanical  stimulus  is  applied  to  one  spot  of  the  gastric  membrane  the 
secretion  is  limited  to  the  neighborhood  of  that  spot  and  is  not  excited  in 
distant  parts.  This  local  mechanism  may  be  nervous  in  nature  or  the  effect  of 
the  stimulus  may  perhaps  be  conveyed  directly  from  cell  to  cell,  from  the 
mouth  of  the  gland  to  its  extreme  base,  without  the  intervention  of  any 
nervous  elements ;  but  the  vascular  changes  at  least  would  seem  to  imply 
the  presence  of  a  nervous  mechanism. 

The  stomach  is  supplied  with  nerve  fibres  from  the  two  vagi  nerves  and 
from  the  solar  plexus  of  the  splanchnic  system.  The  two  vagi  after  forming 
the  cesophageal  plexus  on  the  oesophagus  are  gathered  together  again  as  two 
main  trunks  which  run  along  the  oesophagus — the  left  in  the  front,  the  right 
at  the  back — to  the  stomach.  The  left  or  anterior  nerve  is  distributed  to  the 
smaller  curvature  and  the  front  surface  of  the  stomach,  forming  a  plexus  in 
which  nerve-cells  are  present ;  and  branches  pass  on  to  the  liver  and  proba- 
bly to  the  duodenum.  The  right  or  posterior  nerve  is  distributed  to  the 
hinder  surface  of  the  stomach,  but  only  to  the  extent  of  about  one-third  of 
its  fibres  ;  about  two-thirds  of  the  fibres  pass  on  to  the  solar  plexus.  The 
fibres  of  the  vagus  nerves  thus  distributed  to  the  stomach  are  for  the  most 
part  non-medullated  fibres ;  by  the  time  the  vagus  reaches  the  abdomen  it 
consists  almost  exclusively  of  non-medullated  fibres,  medu Hated  fibres  being 
very  few ;  the  large  number  of  medullated  fibres  which  the  nerve  contains 
in  the  upper  part  of  the  neck  pass  off  into  the  laryngeal,  cardiac,  and  other 
branches. 

From  the  solar  plexus  nerves,  arranged  largely  in  plexuses,  pass  in  com- 
pany with  the  divisions  of  the  coeliac  artery,  coronary  artery  of  the  stomach 
and  branches  of  the  hepatic  artery,  to  the  stomach.  Though  the  two  abdo- 


SECRETION  OF  SALIVA  AND  GASTRIC  JUICE.  267 

minal  splanchnic  nerves  which  join  the  solar  plexus  (semilunar  ganglia)  are 
chiefly  composed  of  medullated  fibres,  the  nerves  which  pass  from  the  plexus 
to  the  stomach  are  to  a  large  extent  composed  of  non-medullated  fibres. 
All  these  nerves,  both  branches  of  the  vagi  and  those  from  the  solar  plexus, 
lie  at  first  in  company  with  the  arteries  on  the  surface  of  the  stomach  be- 
neath the  peritoneum.  From  thence  they  pass  inward,  still  in  company 
with  arteries,  and  form,  on  the  one  hand,  a  plexus  containing  nerve-cells 
between  the  longitudinal  and  circular  muscular  coats  corresponding  to  what 
in  the  intestine  we  shall  have  to  speak  of  as  the  plexus  of  Auerbach,  whence 
fibres  are  distributed  to  the  two  muscular  coats ;  and,  on  the  other  hand,  a 
plexus  in  the  submucous  coat,  also  containing  nerve-cells,  corresponding  to 
what  is  known  in  the  intestine  as  Meissner's  plexus.  From  this  latter  plexus 
fibres  pass  to  the  mucous  membrane ;  some  of  these  end  in  the  mascularis 
mucosse ;  whether  any  are  connected  with  the  gastric  glands,  and  if  so  how, 
is  not  at  present  known. 

There  are  no  facts  which  afford  satisfactory  evidence  that  any  part  of  this 
arrangement  of  nerves  supplies  such  a  local  nervous  mechanism  as  was  sug- 
gested above.  The  importance,  however,  of  such  a  local  mechanism  what- 
ever its  nature,  and  the  subordinate  value  of  any  connection  between  the 
gastric  membrane  and  the  central  nervous  system,  is  further  shown  by  the 
fact  that  a  secretion  of  quite  normal  gastric  juice  will  go  on  after  both  vagi, 
or  the  nerves  from  the  solar  plexus  going  to  the  stomach  have  been  divided, 
and,  indeed,  when  all  the  nervous  connections  of  the  stomach  are  so  far  as 
possible  severed.  And  all  attempts  to  provoke  or  modify  gastric  secretion 
by  the  stimulation  of  the  nerves  going  to  the  stomach  have  hitherto  failed. 
On  the  other  hand,  in  cases  of  gastric  fistula,  where  by  complete  occlusion 
of  the  oesophagus  stimulation  by  the  descent  of  saliva  has  been  avoided,  the 
mere  sight  or  smell  of  food  has  been  seen  to  provoke  a  lively  secretion  of 
gastric  juice.  This  must  have  been  due  to  some  nervous  action  ;  and  the 
same  may  be  said  of  the  cases  where  emotions  of  grief  or  anger  suddenly 
arrest  the  secretion  going  on  or  prevent  the  secretion  which  would  otherwise 
have  taken  place  as  the  result  of  the  presence  of  food  in  the  stomach.  So 
that  much  has  yet  to  be  learned  in  this  matter. 

§  202.  The  contrast  presented  between  the  scanty  secretion  resulting  from 
mechanical  stimulation  and  the  copious  flow  which  actual  food  induces  is 
interesting  because  it  seems  to  show  that  the  secretory  activity  of  the  cells  is 
heightened  by  the  absorption  of  certain  products  derived  from  the  portions 
of  food  first  digested.  This  is  well  illustrated  by  the  following  experiment 
of  Heidenhain.  This  observer,  adopting  the  method  employed  for  the  intes- 
tine, of  which  we  shall  speak  later  on,  succeeded  in  isolating  a  portion  of  the 
fundus  from  the  rest  of  the  stomach  ;  that  is  to  say,  he  cut  out  a  portion  of 
the  fundus,  sewed  together  the  cut  edges  of  the  main  stomach,  so  as  to  form 
a  smaller  but  otherwise  complete  organ,  while  by  sutures  he  converted  the 
excised  piece  of  fundus  into  a  small  independent  stomach  opening  on  to  the 
exterior  by  a  fistulous  orifice.  When  food  was  introduced  into  the  main 
stomach  secretion  also  took  place  in  the  isolated  fundus.  This  at  first  sight 
might  seem  the  result  of  a  nervous  reflex  act ;  but  it  was  observed  that  the 
secondary  secretion  in  the  fundus  was  dependent  on  actual  digestion  taking 
place  in  the  main  stomach.  If  the  material  introduced  into  the  main  stomach 
were  indigestible  or  digested  with  difficulty,  so  that  little  or  no  products  of 
digestion  were  formed  and  absorbed  into'  the  blood,  such  ex.  gr.  as  pieces 
of  ligamentum  nuchse,  very  little  secretion  took  place  in  the  isolated  fundus. 
We  quote  this  now  as  bearing  on  the  question  of  a  possible  nervous  mechan- 
ism of  gastric  secretion,  but  we  shall  have  to  return  to  it  under  another 
aspect. 


268  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

The  Changes  in  a  Gland  constituting  the  Act  of  Secretion. 

§  203.  We  have  now  to  consider  what  are  the  changes  in  the  glandular 
cells  and  their  surroundings  which  cause  this  flow  of  fluid  possessing  specific 
characters  into  the  lumen  of  an  alveolus,  and  so  into  a  duct.  It  will  be  con- 
venient to  begin  with  the  pancreas. 

The  thin  extended  pancreas  of  a  rabbit  may,  by  means  of  special  precau- 
tions, be  spread  out  on  the  stage  of  a  microscope  and  examined  with  even 
high  powers,  while  the  animal  is  not  only  alive  but  under  such  conditions 
that  the  gland  remains  in  a  nearly  normal  state,  capable  of  secreting  vigor- 
ously. It  is  possible  under  these  circumstances  to  observe  even  minutely  the 
appearances  presented  by  the  gland  when  at  rest  and  loaded,  and  to  watch 
the  changes  which  take  place  during  secretion. 

When  the  animal  has  not  been  digesting  for  some  little  time,  and  the  gland 
is  therefore  "  loaded,"  the  outlines  of  the  individual  cells  are  very  indistinct, 
the  lumen  of  the  alveolus  is  invisible  or  very  inconspicuous,  and  each  cell  is 
crowded  with  small  refractive  spherical  granules,  forming  an  irregular  gran- 
ular mass  which  hides  the  nucleus  and  leaves  only  a  very  narrow  clear  outer 
zone  next  to  the  basement  membrane,  or  it  may  be  hardly  any  such  zone  at 
all.  (Fig.  82,  A.) 

The  blood-supply,  moreover,  is  scanty,  the  small  arteries  being  constricted 
and  the  capillaries  imperfectly  filled  with  corpuscles. 

If.  however,  the  same  pancreas  be  examined  while  it  is  in  a  state  of  activity, 
either  from  the  presence  of  food  in  the  stomach  or  from  the  injection  of  some 
stimulating  drug,  such  as  pilocarpine,  a  very  different  state  of  things  is  seen. 
The  individual  cells  (Fig.  82,  ^)  have  become  smaller  and  much  more  dis- 


FIG.  82. 


B 

A  Portion  of  the  Pancreas  of  the  Rabbit.  (Kuhne  and  Sheridan  Lea.)  A  at  rest,  B  in  a  state 
of  activity,  o  the  inner  granular  zone,  in  which  A  is  larger,  and  more  closely  studded  with  fine 
granules,  than  in  B,  in  which  the  granules  are  fewer  and  coarser,  b  the  outer  transparent  zone, 
small  in  A,  larger  in  B,  and  in  the  latter  marked  with  faint  striae,  c  the  lumen,  very  obvious  in 
B,  but  indistinct  in  A.  d  an  indentation  at  the  junction  of  two  cells,  seen  in  B,  but  not  occurring 
in  A. 

tinct  in  outline  and  the  contour  of  the  alveolus,  which  previously  was  even, 
is  now  wavy,  the  basement  membrane  being  indented  at  the  junction  of  the 
cells,  also  the  lumen  of  the  alveolus  is  now  wider  and  more  conspicuous. 
In  each  cell  the  granules  have  become  much  fewer  in  number  and,  as  it  were, 
have  retreated  to  the  inner  margin,  so  that  the  inner  granular  zone  is  much 
narrower  and  the  outer  transparent  zone  much  broader  than  before ;  the 
latter,  too,  is  frequently  marked  at  its  inner  part  by  delicate  striae  running 
into  the  inner  zone.  At  the  same  time  the  bloodvessels  are  largely  dilated 
and  the  stream  of  blood  through  the  capillaries  is  full  and  rapid. 

With  care  the  change  from  the  one  state  of  things  to  the  other  may  be 


SECRETION  OF  SALIVA   AND  GASTRIC  JUICE.  269 

watched  under  the  microscope.  The  vascular  changes  can,  of  course,  be 
easily  appreciated,  but  the  granules  may  also  be  seen  to  diminish  in  number. 
Those  at  the  inner  margin  seem  to  be  discharged  into  the  lumen,  and  those 
nearer  the  outer  margin  to  travel  inward  through  the  cell-substance  toward 
the  lumen,  the  faint  striae  spoken  of  above,  apparently,  at  all  events,  being 
the  marks  of  their  paths.  Obviously,  during  secretion,  the  granules  with 
whicli  the  cell-substance  was  "  loaded  "  are  "  discharged  "  from  the  cell  into 
the  lumen  of  the  alveolus.  What  changes  these  granules  may  undergo  dur- 
ing the  discharge  we  shall  consider  presently. 

Sections  of  the  prepared  and  hardened  pancreas  of  any  animal  tell  nearly 
the  same  tale  as  that  thus  told  by  the  living  pancreas  of  the  rabbit.  In 
sections,  for  instance,  of  the  pancreas  of  a  dog  which  has  not  been  fed,  and 
therefore  has  not  been  digesting,  for  some  hours  (twenty-four  or  thirty),  the 
cells  are  seen  to  be  crowded  with  granules  (which,  however,  are  usually 
shrunken  and  irregular  owing  to  the  influence  of  the  hardening  agent), 
leaving  a  very  narrow  outer  zone.  In  similar  sections  of  the  pancreas  of  a 
dog  which  has  been  recently  fed,  six  hours  before  for  example,  and  in  which, 
therefore,  the  gland  has  been  for  some  time  actively  secreting,  the  granules 
are  far  less  numerous,  and  the  clear  outer  zone  accordingly  much  broader 
and  more  conspicuous.  With  osmic  acid  these  granules  stain  well,  and  are 
preserved  in  their  spherical  form,  so  that  the  cell  thus  stained  maintains 
much  of  the  appearance  of  a  living  cell.  But  with  carmine,  hsematoxylin, 
etc.,  the  granules  do  not  stain  nearly  so  readily  as  does  the  cell-substance  of 
the  cells,  so  that  a  discharged  cell  stains  more  deeply  than  does  a  loaded  cell 
because  the  staining  of  the  "  protoplasmic  "  cell-substance  is  not  so  much 
obscured  by  the  unstained  granules  ;  besides  which,  however,  the  actual  cell- 
substance  stains  probably  somewhat  more  deeply  in  the  discharged  cell.  It 
may  be  added  that  in  the  discharged  cell  the  nucleus  is  conspicuous  and  well 
formed ;  in  the  loaded  cell  it  is  generally  in  prepared  sections,  more  or  less 
irregular,  possibly  because  in  these  it  is  less  dense  and  more  watery  than  in 
the  discharged  cell,  and  so  shrinks  under  the  influence  of  the  reagents 
employed. 

These  several  observations  suggest  the  conclusion  that  in  a  gland  at  rest 
the  cell  is  occupied  in  forming  by  means  of  the  metabolism  of  its  cell-sub- 
stance and  lodging  in  itself  (§  30)  certain  granules  of  peculiar  substance 
intended  to  be  a  part  and  probably  an  important  part  of  the  secretion. 
This  goes  on  until  the  cell  is  more  or  less  completely  "  loaded."  In  such  a 
cell  the  amount  of  actual  living  cell-substance  is  relatively  small,  its  place 
is  largely  occupied  by  granules,  and  in  itself  has  been  partly  consumed  in 
forming 'the  granules.  During  the  act  of  secretion  the  granules  are  dis- 
charged to  form  part  of  the  secretion,  other  matters,  including  water,  as  we 
shall  see,  making  up  the  whole  secretion ;  and  the  cell  would  be  proportion- 
ately reduced  in  size  were  it  not  that  the  act  of  the  discharge  seems  to  stimu- 
late the  cell-substance  to  a  new  activity  of  growth,  so  that  the  new  cell-sub- 
stance is  formed ;  this,  however,  is  in  turn  soon  in  part  consumed  in  order  to 
form  new  granules.  And  what  is  thus  seen  with  considerable  distinctness 
and  ease  in  the  pancreas,  is  seen  with  more  or  less  distinctness  in  other 
glands. 

§  204.  When  we  study  an  albuminous  gland,  the  parotid  gland,  for  in- 
stance, in  a  living  state,  we  find  that  the  changes  which  take  place  during 
activity  are  quite  comparable  to  those  of  the  pancreas.  During  rest  (Fig. 
83,  JL),  the  cells  are  large,  their  outlines  very  indistinct,  in  fact  almost  in- 
visible, and  the  cell-substance  is  studded  with  granules.  During  activity 
(Fig.  83,  J5)  the  cells  become  smaller,  their  outlines  more  distinct,  and  the 
granules  disappear,  especially  from  the  outer  portions  of  each  cell.  After 


270 


THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 


prolonged  activity,  as  in  Fig.  83,  C,  the  cells  are  still  smaller,  with  their 
outlines  still  more  distinct,  and  the  granules  have  disappeared  almost 
entirely,  a  few  only  being  left  at  the  extreme  inner  margin  of  each  cell, 
abutting  upon  the  conspicuous,  almost  gaping  lumen  of  the  alveolus.  And 


FIG.  83. 


Changes  in  the  Parotid  during  Secretion.  (Langley.)  The  figure,  which  is  somewhat  dia- 
grammatic, represents  the  microscopic  changes  which  may  be  observed  in  the  living  gland.  A. 
During  rest.  The  obscure  outlines  of  the  cells  are  introduced  to  show  the  relative  size  of  the 
cells;  they  could  not  be  readily  seen  in  the  specimen  itself.  B.  After  moderate  stimulation.  C. 
After  prolonged  stimulation.  The  nuclei  are  diagrammatic,  and  introduced  to  show  their  ap- 
pearance and  position. 

upon  special  examination  it  is  found  that  the  nuclei  are  large  and  round. 
In  fact,  we  might  almost  take  the  parotid,  as  thus  studied,  to  be  more  truly 
typical  of  secretory  changes  than  even  the  pancreas.  For  the  demarcation 
of  an  inner  and  outer  zone  is  not  a  necessary  feature  of  a  secreting  cell  at 
rest.  What  is  essential  is  that  the  cell-substance  manufactures  material, 
which  for  a  while,  that  is  during  rest,  is  deposited  in  the  cell,  generally  in 
the  form  of  granules  but  not  necessarily  so,  and  that  during  activity  this 
material  is  used  up,  the  disappearance  of  the  granules,  when  these  are 
visible,  being  naturally  earliest  and  most  marked  at  the  outer  portions  of 
each  cell,  and  progressing  inward  toward  the  lumen,  the  whole  cell  becoming 
smaller  and,  as  it  were,  shrunken. 

In  the  cells  of  the  parotid  gland  and  other  albuminous  cells  the  granules 
seen  in  the  living  or  fresh  cell  differ  from  the  granules  seen  in  the  pancreatic 
cell,  inasmuch  as  they  are  easily  dissolved  or  broken  up  by  the  action  of 
alcohol,  chromic  acid,  and  the  other  usual  hardening  reagents,  and  hence  in 
hardened  specimens  have  disappeared.  In  consequence,  in  sections  of  har- 
dened and  prepared  albuminous  glands  the  difference  between  resting  or 
loaded  and  active  or  discharged  cells  may  appear  not  very  conspicuous ; 
and  this  is  especially  the  case  in  the  parotid  gland  of  the  rabbit  when  the 


FIG.  84. 


After  stimulation 


Sections  of  the  Parotid  of  the  Rabbit.    (After  Heidenhain.)    A.  At  rest, 
of  the  cervical  sympathetic.    Both  sections  are  from  hardened  gland. 

activity  has  been  called  into  play  by  stimulation  of  the  auriculo-temporal 
nerve.  When,  however,  either  in  the  rabbit  or  the  dog  the  cervical  sym- 
pathetic is  stimulated,  though  the  stimulation  gives  rise  in  the  rabbit  to 


SECRETION  OF  SALIVA  AND  GASTRIC  JUICE. 


271 


FIG.  85. 


little  secretion  of  saliva  and  in  the  dog  to  none  at  all,  a  marked  effect  on 
the  gland  is  produced,  and  changes  in  the  same  direction  as  those  already 
described  may  be  observed.  During  rest  the  cells  of  the  parotid  as  seen  in 
sections  of  the  gland  hardened  in  alcohol  (Fig.  84,  A)  are  pale,  transpa- 
rent, staining  with  difficulty,  and  the  nuclei  possess  irregular  outlines  as  if 
shrunken  by  the  reagents  employed.  After  stimulation  of  the  sympathetic 
the  protoplasm  of  the  cells  becomes  turbid  (Fig.  84,  B),  and  stains  much 
more  readily,  while  the  nuclei  are  no  longer  irregular  in  outline,  but  round 
and  large,  with  conspicuous  nucleoli,  the  whole  cell  at  the  same  time,  at 
least  after  prolonged  stimulation,  becoming  distinctly  smaller. 

f205.  In  a  mucous  gland  the  changes  which  take  place  are  of  a  like 
,  though  apparently  somewhat  more  complicated,  owing  probably  to 
the  peculiar  characters  of  the  mucin  which  is  so  conspicuous  a  constituent 
of  the  secretion. 

If  a  piece  of  resting  loaded  submaxillary  gland  be  teased  out,  while 
fresh  and  warm  from  the  body,  in  normal  saline  solution,  the  cell-substance 
of  the  mucous  cells  (Fig.  85,  a)  is  seen  to  be  crowded  with  granules  or 
spherules,  which  may  fairly  be  compared  with  the  granules  of  the  pancreas, 
though  perhaps  less  dense  and  solid  than  these. 

If  a  piece  of  a  gland  which  has  been  secreting  for  some  time,  and  is 
therefore  a  discharged  gland,  be  examined  in  the  same  way  (Fig.  85,  b\ 
the  granules  are  far  less  numerous  and  largely  confined  to  the  part  of  the 
cell  nearer  the  lumen,  the  outer  part  of  the  cell  around  the  nucleus  consist- 
ing of  ordinary  "  protoplasmic  "  cell-substance.  The  distinction,  however, 
between  an  inner  "  granular  zone  "  next 
to  the  lumen  and  an  outer  "  clear  zone  " 
next  to  the  basement  membrane  is  less 
distinct  than  in  the  pancreas,  partly  be- 
cause the  granules  do  not  disappear  in  so 
regular  a  manner  as  in  the  pancreas,  and 
partly  because  the  outer  zone  of  the  mu- 
cous cell,  as  it  forms,  is  less  homogeneous 
than  that  of  the  pancreatic  cell. 

The  "  granules  "  or  "  spherules  "  of 
the  mucous  cell  are  moreover  of  a  pecu- 
liar nature.  If  the  fresh  cell,  showing 
granules  (either  many  as  in  the  case  of  a 
loaded  or  few  as  in  the  case  of  a  dis- 
charged cell),  be  irrigated  with  water  or 
with  dilute  acids  or  dilute  alkalies,  the 
granules  swell  up  (Fig.  85,  a',  bf,  into  a 
transparent  mass,  giving  the  reactions  of 
mucin,  traversed  by  a  network  of  "  pro- 
toplasmic "  cell-substance.  In  this  way  is 
produced  an  appearance  very  similar  to 
that  shown  in  sections  of  mucous  glands 
hardened  and  stained  in  the  ordinary 
way. 

In  the  loaded  mucous  cell  in  hardened 
and  stained  preparations  (Fig.  86,  a) 
there  is  seen  a  small  quantity  of  protoplasmic  cell-substance  gathered 
round  the  nucleus  at  the  outer  part  of  the  cell  next  to  the  basement  mem- 
brane ;  the  rest  of  the  cell  consists  of  a  network  of  cell-substance,  the  in- 
terstices being  filled  with  transparent  material,  which,  unlike  the  network 
itself  and  the  mass  of  cell-substance  round  the  nucleus,  does  not  stain  with 


Mucous  Cells  from  a  fresh  Subni axil- 
lary Gland  of  Dog.  (Langley.)  a  and  b 
isolated  in  2  per  cent,  salt  solution:  a, 
from  loaded  gland;  b,  from  discharged 
gland  (the  nuclei  are  usually  more  ob- 
scured by  granules  than  is  here  repre- 
sented). On  teasing  out  a  fresh  fragment 
in  2  to  5  per  cent,  salt  solution,  the  cells 
usually  become  broken  up  so  that  iso- 
lated cells  are  rarely  obtained  entire ;  iso- 
lated cells  are  common  if  the  gland  be 
left  in  the  body  for  a  day  after  death  ;  a', 
b',  treated  with  dilute  acid:  a',  from 
loaded  ;  b'.from  discharged  gland. 


272 


THE  TISSUES   AND   MECHANISMS  OF  DIGESTION. 


carmine  or  with  certain  other  dyes.    The  discharged  cell  in  similar  prepara- 
tions (Fig.  86,  6)  differs  from  the  loaded  cell  in  the  amount  of  transparent 


FIG.  86. 


Alveoli  of  Dog's  Submaxillary  Gland  Hardened  in  Alcohol  and  Stained  with  Carmine.  (Lang- 
ley.)  The  network  is  diagrammatic,  a,  from  a  loaded  gland.  6,  from  a  discharged  gland  ;  the 
chorda  tympani  having  been  stimulated  at  short  intervals  during  five  hours. 

non-staining  material  being  much  less  and  chiefly  confined  to  the  inner  part 
of  the  cell,  while  the  protoplasmic  cell-substance  around  the  now  large  and 
well-formed  nucleus  is  not  only,  both  relatively  and  absolutely,  greater  in 
amount,  but  stains  still  more  deeply  than  in  the  loaded  cell. 

It  would  appear,  therefore,  that  in  the  mucous  cell,  as  in  the  pancreatic 
cell,  the  cell-substance  forms  and  deposits  in  itself  certain  material  in  the 
form  of  granules.  During  secretion  these  granules  disappear  and  presum- 
ably form  part  of  the  secretion. 

§206.  The  "  central "  or  "chief"  cells  of  the  gastric  gland  also  ex- 
hibit similar  changes.  In  such  an  animal  as  the  newt  these  cells  may,  though 
with  difficulty,  be  examined  in  the  living  state.  They  are  then  found  to  be 
studded  with  granules  when  the  stomach  is  at  rest.  During  digestion  these 
granules  become  much  less  numerous  and  are  chiefly  gathered  near  the 
lumen,  leaving  in  each  cell  a  clear  outer  zone.  And  in  many  mammals 
the  same  abundance  of  granules  in  the  loaded  cell,  the  same  paucity  of 
granules  for  the  most  part  restricted  to  an  inner  zone  in  the  discharged  cell, 
may  be  demonstrated  by  the  use  of  osmic  acid  (Fig.  87). 

When  the  stomach  is  hardened  by  alcohol  these  changes,  like  the  similar 
changes  in  an  albuminous  cell,  are  obscured  by  the  shrinking  of  the 
"  granules,"  or  by  their  swelling  up  and  becoming  diffused  through  the  rest 
of  the  cell-substance  ;  so  that,  though  in  sections  so  prepared  very  striking 
differences  are  seen  between  loaded  and  discharged  cells,  these  are  unlike 
those  seen  in  living  glands.  In  specimens  taken  from  an  animal  which  has 
not  been  fed  for  some  time,  the  central  cells  of  the  gastric  glands  are  pale, 
finely  granular,  and  do  not  stain  readily  with  carmine  and  other  dyes. 
During  the  early  stages  of  gastric  digestion,  the  same  cells  are  found  some- 
what swollen,  but  turbid  and  more  coarsely  granular  ;  they  stain  much  more 
readily.  At  a  later  stage  they  become  smaller  and  shrunken,  but  are  even 
more  turbid  and  granular  than  before,  and  stain  still  more  deeply.  This  is 
true  not  only  of  the  central  cells  in  the  cardiac  glands,  but  also  of  the  cells 
of  which  the  pyloric  glands  are  built  up.  In  a  loaded  cell  very  little 
staining  takes  place,  because  the  amount  of  living  staining  cell-substance 
is  small  relatively  to  the  amount  of  material  with  which  it  is  loaded  and 


SECRETION  OF  SALIVA   AND  GASTRIC  JUICE. 


273 


FIG.  87. 


420 


which  does  not  stain  readily.  In  the  cell  which  after  great  activity  has 
discharged  itself,  the  cell  is  smaller,  but  what  remains  is  largely  living 
cell-substance,  some  of  it  new,  and  all  stain- 
ing readily.  It  would  appear  also,  that  dur- 
ing the  activity  of  the  cell  some  substances, 
capable  of  being  precipitated  by  alcohol,  make 
their  appearance,  and  the  presence  of  this  ma- 
terial adds  to  the  turbid  and  granular  aspect 
of  the  cell ;  possibly,  also,  this  material  con- 
tributes to  the  staining.  A  similar  material 
seems  to  make  its  appearance  in  the  cells  of 
albuminous  glands. 

In  the  ovoid  or  border  cells  no  very  charac- 
teristic changes  make  their  appearance.  During 
digestion  they  become  larger,  more  swollen,  as 
it  were,  and  in  consequence  bulge  out  the  base- 
ment membrane,  but  no  characteristic  disap- 
pearance of  granules  can  be  observed.  In  the 
living  state,  the  cell-substance  of  these  ovoid 
cells  appears  finely  granular,  but  in  hardened 
and  prepared  sections  has  a  coarsely  granular, 
"  reticulate  "  look,  which  is  perhaps  less  marked 
in  the  swollen  active  cells  than  in  the  resting 
cells. 

§  207.  All  these  various  secreting  cells, 
then — pancreatic  cell,  mucous  cell,  albuminous 
cell,  and  central  gastric  cell — exhibit  the  same 
series  of  events,  modified  to  a  certain  extent  in 
the  several  cases.  In  each  case  the  "  proto- 
plasmic" cell-substance  manufactures  and 
lodges  in  itself  material  destined  to  form  part 
of  the  juice  secreted.  In  the  fresh  cell  this 
material  may  generally  be  recognized  under 
the  microscope  by  its  optical  characters  as 
granules;  these,  however,  are  apt  to  become 
altered  by  reagents.  But  we  must  guard  our- 
selves against  the  assumption  that  the  mate- 
rial which  can  thus  be  recognized  is  the  only 
material  thus  stored  up;  we  may  in  future, 
by  chemical  or  other  means,  be  able  to  differ- 
entiate other  parts  of  the  cell-body  as  being  also  material  similarly 
stored  up. 

During  activity,  while  the  gland  is  secreting,  this  material,  either 
unchanged  or  after  undergoing  change,  is  wholly  or  partially  discharged 
from  the  cell.  The  cell,  in  consequence  of  having  thus  got  rid  of  more 
or  less  of  its  load,  consists  to  a  larger  extent  of  actual  living  cell-sub- 
stance, this  being  in  many  cases  increased  by  rapid  new-growth,  though 
the  bulk  of  the  discharged  cell  may  be  less  than  that  of  the  loaded  cell. 

This  activity  of  growth  continues  after  the  act  of  secretion,  but  the  dis- 
charged cell  soon  begins  again  the  task  of  loading  itself  with  new  secretion 
material  for  the  next  act  of  secretion. 

Thus  in  most  cases  there  is,  corresponding  to  the  intermittence  of  secre- 
tion, an  alteration  of  discharge  and  loading ;  but  it  must  be  borne  in  mind 
that  such  an  alteration  is  not  absolutely  necessary,  even  in  the  case  of  inter- 
mittent  secretion.      We    can   easily  imagine   that   the   discharge,  say  of 
18 


Gastric  Gland  of  Mammal  (Bat) 
during  Activity.  (Langley.)  c,  the 
mouth  of  the  gland  with  its  cylin- 
drical cells,  n,  the  neck,  contain- 
ing conspicuous  ovoid  cells,  with 
their  coarse  protoplasmic  net- 
work. /,  the  body  of  the  gland. 
The  granules  are  seen  in  the  cen- 
tral cells  to  be  limited  to  the  inner 
portions  of  each  cell,  the  round 
nucleus  of  which  is  conspicuous. 


274  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

"granules"  during  secretion,  should  stir  up  the  cell  to  an  increased  activ- 
ity in  forming  granules,  and  that  the  formative  activity  should  cease 
when  the  secretory  activity  ceased.  In  such  a  case  the  number  of  new 
granules  formed  might  always  be  equal  to  the  number  of  old  granules 
used  up,  and  the  active  cell,  in  spite  of  its  discharge,  would  possess  as 
many  granules — that  is  to  say,  as  large  a  load — as  the  cell  at  rest.  And 
in  the  central  gastric  cells  of  some  animals  it  would  appear  that  such  a 
continued  balancing  of  load  and  discharge  does  actually  take  place,  so 
that  no  distinction  in  granules  can  be  observed  between  resting  and 
active  cells. 

§  208.  We  spoke  just  now  of  the  material  stored  up  in  the  cell  and 
destined  to  form  part  of  the  secretion  as  undergoing  change  before  it  was 
discharged.  In  the  mucous  cell  we  have  seen  that  the  material  deposited  in 
the  living  cell  has  at  first  the  form  of  granules.  These  granules,  however, 
are  easily  converted  into  a  transparent  material  lodged  in  the  spaces  of  the 
cell-substance,  which  material,  even  if  not  exactly  identical  with,  at  least 
closely  resembles,  the  mucin  found  in  the  secretion  ;  and  apparently  in  the 
act  of  secretion  the  granules  do  undergo  some  such  change.  In  the  case 
of  some  other  glands,  moreover,  we  have  chemical  as  well  as  optical 
evidence  that  the  material  stored  up  in  the  cells  is,  in  part  at  least,  not 
the  actual  substance  appearing  in  the  secretion,  but  an  antecedent  of  that 
substance. 

An  important  constituent  of  pancreatic  juice  is,  as  we  shall  see  later  on, 
a  body  called  trypsin,  a  ferment  very  similar  to  pepsin,  acting  on  proteid 
bodies  and  converting  them  into  peptone  and  other  substances.  Though  in 
many  respects  alike,  pepsin  and  trypsin  are  quite  distinct  bodies,  and  differ 
markedly  in  this,  that  while  an  acid  medium  is  necessary  for  the  action  of 
pepsin,  an  alkaline  medium  is  necessary  for  the  action  of  trypsin  ;  and  accord- 
ingly the  pancreatic  juice  is  alkaline  in  contrast  to  the  acidity  of  gastric  juice. 
Trypsin  can,  like  pepsin  (§  191),  be  extracted  with  glycerin  from  substances 
in  which  it  occurs ;  glycerin  extracts  of  trypsin,  however,  need  for  the 
manifestation  of  their  powers  the  presence  of  a  weak  alkali,  such  as  a  1 
per  cent,  solution  of  sodium  carbonate. 

Now,  trypsin  is  present  in  abundance  in  normal  pancreatic  juice ;  but  a 
loaded  pancreas,  one  which  is  ripe  for  secretion,  and  which  if  excited  to 
secrete  would  immediately  pour  out  a  juice  rich  in  trypsin,  contains  no 
trypsin  or  a  mere  trace  of  it ;  nay,  even  a  pancreas  which  is  engaged  in  the 
act  of  secreting  contains  in  its  actual  cells  an  insignificant  quantity  only  of 
trypsin,  as  is  shown  by  the  following  experiment: 

If  the  pancreas  of  an  animal,  even  of  one  in  full  digestion,  be  treated, 
while  still  warm  from  the  body,  with  glycerin,  the  glycerin  extract,  as  judged 
of  by  its  action  on  fibrin  in  the  presence  of  sodium  carbonate,  is  inert  or  nearly 
so  as  regards  proteid  bodies.  If,  however,  the  same  pancreas  be  kept  for 
twenty-four  hours  before  being  treated  with  glycerin,  the  glycerin  extract 
readily  digests  fibrin  and  other  proteids  in  the  presence  of  an  alkali.  If  the 
pancreas,  while  still  warm,  be  rubbed  up  in  a  mortar  for  a  few  minutes  with 
dilute  acetic  acid,  and  then  treated  with  glycerin,  the  glycerin  extract  is 
strongly  proteolytic.  If  the  glycerin  extract,  obtained  without  acid  from  the 
warm  pancreas,  and  therefore  inert,  be  diluted  largely  with  water  and  kept 
at  35°  C.  for  some  time,  it  becomes  active.  If  treated  with  acidulated  instead 
of  distilled  water,  its  activity  is  much  sooner  developed.  If  the  inert  glycerin 
extract  of  warm  pancreas  be  precipitated  with  alcohol  in  excess,  the  precipi- 
tate, inert  as  a  proteolytic  ferment  when  fresh,  becomes  active  when  exposed 
for  some  time  in  an  aqueous  solution,  rapidly  so  when  treated  with  acidulated 
water.  These  facts  show  that  a  pancreas  taken  fresh  from  the  body,  even 


SECRETION  OF  SALIVA  AND  GASTRIC  JUICE.  275 

during  full  digestion,  contains  but  little  ready-made  ferment,  though  there  is 
present  in  it  a  body  which,  by  some  kind  of  decomposition,  gives  birth  to  the 
ferment.  We  may  remark  incidentally,  that  though  the  presence  of  an  alkali 
is  essential  to  the  proteolytic  action  of  the  actual  ferment,  the  formation  of 
the  ferment  out  of  its  forerunner  is  favored  by  the  presence  of  a  small  quan- 
tity of  acid  ;  the  acid  must  be  used  with  care,  since  the  trypsin,  once  formed, 
is  destroyed  by  acids.  To  this  body,  this  mother  of  the  ferment,  which  has 
not  at  present  been  satisfactorily  isolated,  but  which  appears  to  be  a  complex 
body,  splitting  up  into  the  ferment,  which,  as  we  have  seen,  is  at  all  events 
not  certainly  a  proteid  body,  and  into  an  undeniably  proteid  body,  the  name 
of  zymogen  has  been  applied.  But  it  is  better  to  reserve  the  term  zymogen 
as  a  generic  name  for  all  such  bodies  as,  not  being  themselves  actual  fer- 
ments, may  by  internal  changes  give  rise  to  ferments — for  all  "  mothers  of 
ferment,"  in  fact — and  to  give  to  the  particular  mother  of  the  pancreatic 
proteolytic  ferment  the  name  trypsinogen. 

Evidence  of  a  similar  kind  shows  that  the  gastric  glands,  both  the  car- 
diac and  the  pyloric  glands,  while  they  contain  comparatively  little  actual 
pepsin,  contain  a  considerable  quantity  of  zymogen  of  pepsin,  or  pepsinogen; 
and  there  can  be  little  doubt  but  that  this  pepsinogen  is  lodged  in  the  cen- 
tral cells  of  the  cardiac  glands  and  in  the  somewhat  similar  cells  which  line 
the  whole  of  the  pyloric  glands. 

§  209.  The  act  of  secretion  itself.  The  above  discussion  prepares  us  at 
once  for  the  statement  that  the  old  view  of  secretion,  according  to  which  the 
gland  picks  out,  separates,  secretes  (hence  the  name  secretion),  and  so  filters, 
as  it  were,  from  the  common  store  of  the  blood  the  several  constituents  of  the 
juice,  is  untenable.  According  to  that  view  the  specific  activity  of  any  one 
gland  was  confined  to  the  task  of  letting  certain  constituents  of  the  blood 
pass  from  the  capillaries  surrounding  the  alveolus  through  the  cells  to  the 
channels  of  the  ducts,  while  refusing  a  passage  to  others.  We  now  know 
that  certain  important  constituents  of  each  juice,  the  pepsin  of  gastric  juice, 
the  mucin  of  saliva  and  the  like  are  formed  in  the  cell,  and  not  obtained 
ready  made  from  the  blood.  A  minute  quantity  of  pepsin  does  exist,  it  is 
true,  in  the  blood,  but  there  are  reasons  for  thinking  that  this  has  made  its 
way  back  into  the  blood,  either  being  absorbed  from  the  interior  of  the 
stomach  or,  as  seems  more  probable,  picked  up  directly  from  the  gastric 
glands ;  and  so  with  some  of  the  other  constituents  of  other  juices.  The 
chief  or  specific  constituents  of  each  juice  are  formed  in  the  cell  itself. 

But  the  juice  secreted  by  any  gland  consists  not  only  of  the  specific  sub- 
stances such  as  mucin,  pepsin,  or  other  ferment,  or  other  bodies,  found  in  it 
alone,  but  also  of  a  large  quantity  of  water,  and  of  various  other  substances, 
chiefly  salines,  common  to  it,  to  other  juices,  and  to  the  blood.  And  the 
question  arises,  Is  the  water,  are  the  salts  and  other  common  substances,  fur- 
nished by  the  same  act  as  that  which  supplies  the  specific  constituents? 

Certain  facts  suggest  that  they  are  not.  For  instance,  as  mentioned  some 
time  ago,  in  the  submaxillary  gland  of  the  dog  stimulation  of  the  chorda 
tympani  produces  a  copious  flow  of  saliva,  which  is  usually  thin  and  limpid, 
whilst  stimulation  of  the  cervical  sympathetic  produces  a  scanty  flow  of  thick 
viscid  saliva.  That  is  to  say,  stimulation  of  the  chorda  has  a  marked  effect 
in  promoting  the  discharge  of  water,  while  stimulation  of  the  sympathetic 
has  a  marked  effect  in  promoting  the  discharge  of  mucin.  To  this  we  may 
add  the  case  of  the  parotid  of  the  dog.  In  this  gland  stimulation  of  a 
cerebro-spinal  nerve,  the  auriculo-temporal,  produces  a  copious  flow  of  limpid 
saliva,  while  stimulation  of  the  sympathetic  produces  itself  little  or  no  secre- 
tion at  all ;  but  when  the  sympathetic  and  the  cerebro-spinal  nerves  are 
stimulated  at  the  same  time,  the  saliva  which  flows  is  much  richer  in  solid 


276  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

and  especially  in  organic  matter  than  when  the  cerebro-spinal  nerve  is  stimu- 
lated alone.  And  we  have  already  seen  that  in  this  gland  the  microscopic 
changes  following  upon  sympathetic  stimulation  are  more  conspicuous  than 
those  which  follow  upon  cerebro-spinal  stimulation. 

These  and  other  facts  have  led  to  the  conception  that  the  act  of  secretion 
consists  of  two  parts,  which  in  one  case  may  coincide,  in  another  may  take 
place  apart  or  in  different  proportions.  On  the  one  hand,  there  is  the  dis- 
charge of  water  carrying  with  it  common  soluble  substances,  chiefly  salines, 
derived  from  the  blood ;  on  the  other  haud,  a  metabolic  activity  of  the  cell- 
substance  gives  rise  to  the  specific  constituents  of  the  juice.  To  put  the 
matter  broadly,  the  latter  process  produces  the  specific  constituents,  the 
former  washes  these  and  other  matters  into  the  duct.  It  has  been  further 
supposed  that  two  kinds  of  nerve  fibres  exist :  one  governing  the  former 
process  and,  in  the  case  of  the  submaxillary  gland  for  instance,  prepon- 
derating, though  not  to  the  total  exclusion  of  the  other  kind,  in  the  chorda 
tympani ;  the  other  governing  the  latter  process  and  preponderating  in  the 
branches  of  the  cervical  sympathetic.  These  have  been  called  respectively 
"  secretory  "  and  "  trophic  "  fibres  ;  but  these  terms  are  not  desirable.  It 
may  here  be  remarked  that  even  the  former  process  is  a  distinct  activity  of 
the  gland  and  not  a  mere  infiltration.  For,  as  we  have  seen  in  the  case  of 
the  salivary  glands,  when  atropine  is  given,  not  only  do  the  specific  constitu- 
ents cease  to  be  ejected  as  a  consequence  of  stimulation  of  the  chorda,  but 
the  discharge  of  water,  in  spite  of  the  bloodvessels  becoming  dilated,  is  also 
arrested  :  no  saliva  at  all  leaves  the  gland.  And  what  is  true  of  the  salivary 
glands  as  regards  the  dependence  of  the  flow  of  water  on  something  else 
besides  the  mere  pressure  of  the  blood  in  the  bloodvessels  appears  to  hold 
good  with  other  glands  also.  Indeed,  it  has  been  suggested  that  the  very 
discharge  of  water  is  due  to  an  activity  of  the  cell ;  the  hypothesis  has  been 
put  forward  that  changes  in  the  cell  give  rise  to  the  formation  in  the  cell  of 
substances  which  absorb  water  from  the  blood  or  lymph  on  the  one  side 
and  give  it  up  on  the  other  side  into  the  lumen  of  the  alveolus.  Such  an 
hypothesis  cannot  be  regarded  as  proved  ;  but  the  mere  putting  it  forward 
raises  a  doubt  as  to  the  validity  of  the  distinction  on  which  we  have  been 
dwelling;  and  other  considerations  point  in  the  same  direction.  For  in- 
stance, if  the  common  soluble  salts  present  in  a  juice,  as  distinguished  from 
the  specific  constituents,  were  merely  carried  into  the  juice  by  the  rush,  so 
to  speak,  of  water,  we  should  expect  to  find  the  percentage  of  these  salts 
either  remaining  the  same  or  perhaps  decreasing  when  the  juice  was  secreted 
more  rapidly  and  in  fuller  volume.  But  under  these  circumstances  the  per- 
centage very  frequently  increases ;  and  in  general  we  find  that  under  various 
circumstances  the  proportion  of  salts  secreted  to  the  quantity  of  water 
secreted  may  vary  considerably.  Obviously,  while  something  determines 
the  quantity  of  water  passing  into  the  alveolus,  something  else  determines 
how  much  of  common  soluble  salts  that  water  contains,  and  still  something 
else  determines  to  what  extent  that  water  is  also  laden  with  specific  constitu- 
ents and  other  organic  bodies.  The  whole  action  is  too  complicated  to  be 
described  as  consisting  merely  of  the  two  processes  mentioned  above,  but  the 
time  has  not  yet  come  for  clear  and  definite  statements.  Everything,  how- 
ever, tends  to  show  that  the  cell  is  the  prime  agent  in  the  whole  business, 
though  we  cannot  at  present  define  the  nature  of  the  several  changes  in  the 
cell,  nor  can  we  say  how  those  changes  are  exactly  related  to  each  other, 
to  changes  of  the  blood-pressure  in  the  bloodvessels,  or,  we  may  add,  to 
changes  taking  place  in  the  lymph-spaces  which  lie  between  the  blood  and 
the  cell. 

We  may  perhaps  add  that  since  in  certain  cutaneous  secreting  glands 


SECRETION  OF  SALIVA  AND  GASTRIC  JUICE.  277 

the  alveolus,  or  what  corresponds  to  the  alveolus,  is  wrapped  round  with 
plain  muscular  fibres,  the  contraction  of  which  appears  to  force  the  secretion 
outward,  the  idea  has  been  suggested  that  in  glands,  such  as  we  are  now 
considering,  the  cell-substance,  making  use  of  "  protoplasmic  "  contraction 
instead  of  actual  muscular  contraction,  may  force  part  of  the  cell  contents 
into  the  lumen  of  the  alveolus.  Such  a  mode  of  secretion  would  be  com- 
parable to  the  ejection  of  undigested  material,  or  "excretion,"  by  an  amoeba. 
But  we  have  no  satisfactory  evidence  in  favor  of  this  view. 

§  210.  Throughout  the  above  we  have  spoken  as  if  the  secretion  were 
furnished  exclusively  by  the  cells  of  the  alveoli  or  secreting  portion  of  the 
gland,  as  if  the  epithelial  cells  lining  the  ducts,  or  conducting  portion  of 
the  gland,  contributed  nothing  to  the  act.  In  the  gastric  glands  the  slender 
cells  lining  the  mouths  of  the  glands  (which  correspond  to  ducts)  and  cover- 
ing the  ridges  between,  are  raucous  cells  secreting  into  the  stomach  gener- 
ally a  small,  but  under  abnormal  conditions  a  large  amount  of  mucus, 
which  has  its  uses  but  is  not  an  essential  part  of  the  gastric  juice.  In  the 
salivary  glands  we  can  hardly  suppose  that  the  long  stretch  of  character- 
istic columnar  epithelium  which  reaches  from  the  alveoli  to  the  mouth  of 
the  long  main  duct  serves  simply  to  furnish  a  smooth  lining  to  the  conduct- 
ing passages ;  but  we  have  as  yet  no  clear  indications  of  what  the  function 
of  this  epithelium  can  be. 

§  211.  Before  we  leave  the  mechanism  of  secretion  there  are  one  or 
more  accessory  points  which  deserve  attention. 

In  treating  just  now  of  the  gastric  glands  we  spoke  as  if  pepsin  were  the 
only  important  constituent  of  gastric  juice,  whereas,  as  we  have  previously 
seen,  the  acid  is  equally  essential.  The  formation  of  the  free  acid  of  the 
gastric  juice  is  very  obscure,  and  many  ingenious  but  unsatisfactory  views 
have  been  put  forward  to  explain  it.  It  seems  natural  to  suppose  that  it 
arises  in  some  way  from  the  decomposition  of  sodium  chloride  drawn  from 
the  blood ;  and  this  is  supported  by  the  fact  that  when  the  secretion  of 
gastric  juice  is  actively  going  on,  the  amount  of  chlorides  leaving  the  blood 
by  the  kidney  is  proportionately  diminished ;  but  nothing  definite  can  at 
present  be  stated  as  to  the  mechanism  of  that  decomposition. 

In  the  frog,  while  pepsin  free  from  acid  is  secreted  by  the  glands  of  the 
lower  portion  of  the  oesophagus,  an  acid  juice  is  afforded  by  glands  in  the 
stomach  itself,  which  have  accordingly  been  called  oxyntic  (dzui>£iv,t<j  sharpen, 
acidulate)  glands ;  but  these  oxyntic  glands  appear  also  to  secrete  pepsin. 
In  the  mammal  the  isolated  pylorus  secretes  an  alkaline  juice ;  in  fact,  the 
appearance  of  an  acid  juice  is  limited  to  those  portions  of  the  stomach  in 
which  the  glands  contain  both  "chief"  or  "central"  and  "ovoid"  or  "bor- 
der" cells.  Now,  from  what  has  been  previously  said,  there  can  be  no 
doubt  that  the  chief  cells  do  secrete  pepsin.  On  the  other  hand,  there  is  no 
evidence  whatever  of  the  formation  of  pepsin  by  the  "  border  "  or  "  ovoid  " 
cells,  though  this  was  once  supposed  to  be  the  case,  and  these  cells  were 
unfortunately  formerly  called  "  peptic  "  cells.  Hence  it  has  been  inferred 
that  the  border  cells  secrete  acid  ;  but  the  argument  is  at  present  one  of  ex- 
clusion only,  there  being  no  direct  proof  that  these  cells  actually  manufacture 
the  acid. 

The  rennin  appears  to  be  formed  by  the  same  cells  which  manufacture 
the  pepsin,  that  is,  by  the  chief  cells  of  the  fundus  generally,  and  to  some 
extent  by  the  cells  of  the  pyloric  glands.  We  may  add  that  we  have  evi- 
dence of  the  existence  of  a'zymogen  of  rennin  analogous  to  the  zymogen  of 
pepsin  or  of  trypsin. 

§  212.  Seeing  the  great  solvent  power  of  both  gastric  and  pancreatic  juice 
the  question  is  naturally  suggested,  Why  does  not  the  stomach  digest  itself? 


278  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

After  death,  the  stomach  is  frequently  found  partly  digested,  viz.,  in  cases 
when  death  has  taken  place  suddenly  on  a  full  stomach.  In  an  ordinary 
death,  the  membrane  ceases  to  secrete  before  the  circulation  is  at  an  end. 
That  there  is  no  special  virtue  in  living  things  which  prevents  their  being 
digested  is  shown  by  the  fact  that  the  leg  of  a  living  frog  or  the  ear  of 
a  living  rabbit  introduced  into  the  stomach  of  a  dog,  through  a  gastric 
fistula,  is  readily  digested.  It  has  been  suggested  that  the  blood-current 
keeps  up  an  alkalinity  sufficient  to  neutralize  the  acidity  of  the  juice 
in  the  region  of  the  glands  themselves ;  but  will  not  explain  why  the 
pancreatic  juice,  which  is  active  in  an  alkaline  medium,  does  not  digest  the 
proteids  of  the  pancreas  itself,  or  why  the  digestive  cells  of  the  bloodless 
actinozoon  or  hydrozoon  do  not  digest  themselves.  We  might  add,  it  does 
not  explain  why  the  amoeba,  while  dissolving  the  protoplasm  of  the  swal- 
lowed diatom,  does  not  dissolve  its  own  protoplasm.  We  cannot  answer  this 
question  at  all  at  present,  any  more  than  the  similar  one,  why  the  delicate 
protoplasm  of  the  amoeba  resists  during  life  the  entrance  into  itself  by 
osmosis  of  more  water  than  it  requires  to  carry  on  its  work,  while  a  few 
moments  after  it  is  dead  water  enters  freely  by  osmosis,  and  the  effects  of 
that  entrance  become  abundantly  evident  by  the  formation  of  bullse  and 
the  breaking  up  of  the  protoplasm. 

THE  PROPERTIES  AND  CHARACTERS  OF  BILE,  PANCREATIC  JUICE,  AND 

Succus  ENTERICUS. 

§  213.  In  the  living  body  the  food,  subjected  to  the  action  first  of  the 
saliva  and  then  of  the  gastric  juice,  undergoes  in  the  stomach  changes  which 
we  shall  presently  consider  in  detail,  and  the  food  so  changed  is  passed  on 
into  the  small  intestine,  where  it  is  further  subjected  to  the  action  of  the  bile 
secreted  by  the  liver,  of  pancreatic  juice  secreted  by  the  pancreas,  and  pos- 
sibly to  some  extent,  though  this  is  by  no  means  certain,  of  a  juice  secreted 
by  the  intestine  itself  and  called  succus  entericus.  It  will  be  convenient  to 
study  the  minute  structure  of  the  liver  in  connection  with  other  functions 
of  the  liver  more  important,  perhaps,  than  that  of  the  secretion  of  bile, 
namely,  the  formation  of  glycogen,  and  other  metabolic  events  occurring 
in  the  hepatic  cells ;  we  have  already  studied  the  structure  of  the  pan- 
creas ;  and  the  structure  of  the  intestine  will  best  be  considered  by  itself. 
We,  therefore,  turn  at  once  to  the  properties  and  characters  of  the  above- 
named  juices. 

Bile. 

Though  bile,  after  secretion  in  the  lobules  of  the  liver,  is  passed  on 
along  the  hepatic  duct,  it  is  in  the  case  of  most  animals  not  poured  at  once 
into  the  duodenum,  but  taken  by  the  cystic  duct  to  the  reservoir  of  the  gall- 
bladder. Here  it  remains  until  such  time  as  it  is  needed,  when  a  quantity 
is  poured  along  the  common  bile  duct  into  the  intestine. 

The  quality  of  bile  varies  much,  not  only  in  different  animals,  but  in 
the  same  animal  at  different  times.  It  is,  moreover,  affected  by  the  length 
of  the  sojourn  in  the  gall-bladder;  bile  taken  direct  from  the  hepatic  duct, 
especially  when  secreted  rapidly,  contains  little  or  no  mucus ;  that  taken 
from  the  gall-bladder,  as  of  slaughtered  oxen  or  sheep,  is  loaded  with  mucus. 
The  color  of  the  bile  of  carnivorous  and  omnivorous  animals,  and  of  man, 
is  generally  a  bright  golden  red  ;  of  herbivorous  animals,  a  yellowish  green 
or  a  bright  green  or  a  dirty  green,  according  to  circumstances,  being  much 
modified  by  retention  in  the  gall-bladder.  The  reaction  is  neutral  or  alka- 
line. The  following  may  be  taken  as  the  average  composition  of  human 


BILE,   PANCREATIC  JUICE,  AND  SUCCUS  ENTERICUS.          279 

bile  taken  from  the  gall-bladder,  and  therefore  containing  much  more 
mucus  as  well  as,  relatively  to  the  solids,  more  water  than  bile  from  the 
hepatic  duct. 

In  1000  parts. 

Water 859.2 

Solids : 

Bile  Salts 91.4 

Fats,  etc 9.2 

Cholesterin 2.6 

Mucus  and  Pigment 29.8 

Inorganic  Salts 7.8 

140.8 

The  entire  absence  of  proteids  is  a  marked  feature  of  bile  :  pancreatic 
juice,  as  we  shall  see,  contains  a  considerable  quantity ;  saliva,  as  we  have 
seen,  a  small  quantity;  normal  gastric  juice  probably  still  less,  and  bile  none 
at  all.  Even  the  bile  which  has  been  retained  some  time  in  the  gall-bladder, 
though  rich  in  mucus,  contains  no  proteids. 

The  constituents  which  form,  apart  from  the  mucus,  the  great  bulk  of 
the  solids  of  bile,  and  which  deserve  chief  attention,  are  the  pigments  and 
the  bile-salts ;  of  these  we  shall  speak  immediately. 

With  regard  to  the  inorganic  salts  actually  present  as  such  sodium  salts 
are  conspicuous,  sodium  chloride,  amounting  to  0.2  or  more  per  cent., 
sodium  phosphate  to  nearly  as  much,  the  rest  being  earthy  phosphates 
and  other  matters  in  small  quantity.  The  presence  of  iron,  to  the 
extent  of  about  0.006  per  cent.,  is  interesting,  since,  as  we  shall  see, 
there  are  reasons  for  thinking  that  the  pigment  of  bile,  itself  free  from 
iron,  is  derived  from  iron-holding  haemoglobin  ;  some,  at  least,  of  the 
iron  set  free  during  the  conversion  of  haemoglobin  into  bile  pigment, 
which  probably  takes  place  in  the  liver,  finds  its  way  into  the  bile. 
Bile  also  appears  to  contain  a  small  quantity,  at  all  events  occasionally, 
of  other  metals,  such  as  manganese  and  copper ;  metals  introduced  into 
the  body  are  apt  to  be  retained  in  the  liver  and  eventually  leave  it  by 
the  bile. 

The  small  quantity  of  fat  present  consists  in  part  of  the  complex  body, 
lecithin. 

The  peculiar  body,  cholesterin,  which  though  fatty-looking  (hence  the 
name  "bile  fat")  is  really  an  alcohol  with  the  composition  G26H44O,  is  con- 
spicuous by  its  quantity  and  constancy.  It  forms  the  greater  part  of  most 
gall-stones,  though  some  are  composed  chiefly  of  pigment.  Insoluble  in 
water  and  cold  alcohol,  though  soluble  in  hot  alcohol  and  readily  soluble 
in  ether,  chloroform,  etc.,  it  is  dissolved  by  the  bile-salts  in  aqueous  solution 
and  hence  is  present  in  solution  in  bile  Its  physiological  functions  are 
obscure. 

The  ash  of  bile  consists  largely  of  soda,  derived  partly  from  the  sodium 
chloride  and  partly  from  the  bile-salts,  of  sulphates  derived  chiefly  if  not 
wholly  from  the  latter,  and  of  phosphates  partly  ready  formed  and  in  part 
derived  from  the  lecithin. 

§  214.  Pigments  of  bile.  The  natural  golden-red  color  of  normal  human 
or  carnivorous  bile  is  due  to  the  presence  of  bilirubin.  This,  which  is  also 
the  chief  pigmentary  constituent  of  gall-stones,  and  occurs  largely  in  the 
urine  of  jaundice,  may  be  obtained  in  the  form  either  of  an  orange-colored 
amorphous  powder  or  of  well-formed  rhombic  tablets  and  prisms.  Insoluble 
in  water,  and  but  little  soluble  in  ether  and  alcohol,  it  is  readily^ soluble 
in  chloroform  and  in  alkaline  fluids.  Its  composition  is  C16H18N2O3.  Treated 


280  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

with  oxidizing  agents,  such  as  nitric  acid  yellow  with  nitrous  acid,  it  displays 
a  succession  of  colors  in  the  order  of  the  spectrum.  The  yellowish  golden- 
red  becomes  green,  this  a  greenish-blue,  then  blue,  next  violet,  afterward 
a  dirty  red,  and  finally  a  pale  yellow.  This  characteristic  reaction  of  bili- 
rubin  is  the  basis  of  the  so-called  Gmelin's  test  for  bile-pigments.  Each 
of  these  stages  represents  a  distinct  pigmentary  substance.  An  alkaline 
solution  of  bilirubin,  exposed  in  a  shallow  vessel  to  the  action  of  the  air, 
turns  green,  becoming  converted  into  biliverdin  (C,6H.20N.2O5  or  C16H,8N.2O4, 
Maly),  the  green  pigment  of  herbivorous  bile.  Biliverdin  is  also  found  at 
times  in  the  urine  of  jaundice,  and  is  prpbably  the  body  which  gives  to  bile 
which  has  been  exposed  to  the  action  of  gastric  juice,  as  in  biliary  vomits, 
its  characteristic  green  hue.  It  is  the  first  stage  of  the  oxidation  of  bili- 
rubin in  Gmelin's  test.  Treated  with  oxidizing  agents  biliverdin  runs 
through  the  same  series  of  colors  as  bilirubin,  with  the  exception  of  the 
initial  golden-red. 

§  215.  The  bile-salts.  These  consist,  in  man  and  many  animals,  of  sodium 
glycocholate  and  taurocholate,  the  proportion  of  the  two  varying  in  different 
animals.  In  man  both  the  total  quantity  of  bile-salts  and  the  proportion 
of  the  one  bile-salt  to  the  other  seem  to  vary  a  good  deal,  but  the  glycocholate 
is  said  to  be  always  the  more  abundant.  In  ox-gall  sodium  glycocholate  is 
abundant  and  taurocholate  scanty.  The  bile-salts  of  the  dog,  cat,  bear,  and 
other  carnivora  consist  exclusively  of  the  latter. 

Insoluble  in  ether,  but  soluble  in  alcohol  and  in  water,  the  aqueous  so- 
lutions having  a  decided  alkaline  reaction,  both  salts  may  be  obtained  by 
crystallization  in  fine  acicular  needles.  They  are  exceedingly  deliquescent. 
The  solutions  of  both  acids  have  a  dextro-rotary  action  on  polarized  light. 

Preparation.  Bile,  mixed  with  animal  charcoal,  is  evaporated  to  dryness  and 
extracted  with  alcohol.  If  not  colorless,  the  alcoholic  filtrate  must  be  further 
decolorized  with  animal  charcoal,  and  the  alcohol  distilled  off.  The  dry  residue 
is  treated  with  absolute  alcohol,  and  to  the  alcoholic  filtrate  anhydrous  ether  is 
added  as  long'  as  any  precipitate  is  formed.  On  standing  the  cloudy  precipitate 
becomes  transformed  into  a  crystalline  mass  at  the  bottom  of  the  vessel.  If  the 
alcohol  be  not  absolute,  the  crystals  are  very  apt  to  be  changed  into  a  thick  syrupy 
fluid.  This  mass  of  crystals  has  been  often  spoken  of  as  bilin.  Both  salts  are 
thus  precipitated,  so  that  in  such  a  bile  as  that  of  the  ox  or  man  bilin  consists  both 
of  sodium  glycocholate  and  sodium  taurocholate.  The  two  may  be  separated  by 
precipitation  from  their  aqueous  solutions  with  sugar  of  lead,  which  throws  down 
the  former  much  more  readily  than  the  latter.  The  acids  may  be  separated  from 
their  respective  salts  by  dilute  sulphuric  action,  or  by  the  action  of  lead  acetate  and 
sulphydric  acid. 

On  boiling  with  dilute  acids  (sulphuric,  hydrochloric),  or  caustic  potash 
or  baryta  water,  glycocholic  acid  is  split  up  into  cholalic  (cholic)  acid  and 
glycin.  Taurocholic  acid  may  similarly  be  split  up  into  cholalic  acid  and 
taurin.  Thus : 

Glycocholic  acid.  Cholalic  acid.  Glycin. 

CMH«N06  +  H.2O  =  C.24H4005  +  CH.2.NH2(CO.OH). 

Taurocholic  acid.  Cholalic  acid.  Taurin. 

C.26H45NS07  +  H,0  -  C.24H4005  +  C,H4.NH2.S03H. 

Both  acids  contain  the  same  non-nitrogenous  acid,  cholalic  acid  ;  but  this 
acid  is  in  the  first  case  associated  or  conjugated  with  the  important  nitro- 
genous body  glycin,  or  amido-acetic  acid,  which  is  a  compound  formed  from 
ammonia  and  one  of  the  "  fatty  acid  "  series,  viz.,  acetic  ;  and  in  the  second 
case  with  taurin,  or  amido-isethionic  acid,  that  is  a  compound  into  which 
representatives  of  ammonia,  of  the  ethyl  group,  and  of  sulphuric  acid  enter. 


BILE,   PANCREATIC  JUICE,   AND  SUCCUS  ENTERICUS.          281 

The  decomposition  of  the  bile  acids  into  cholalic  acid  and  tauriu  or  glycin 
respectively  takes  place  naturally  in  the  intestine,  the  glyciii  and  taurin 
being  probably  absorbed,  so  that  from  the  two  acids,  after  they  have  served 
their  purpose  in  digestion,  the  two  ammonia  compounds  are  returned  into 
the  blood.  Each  of  the  two  acids,  or  cholalic  acid  alone,  when  treated  with 
sulphuric  acid  and  cane-sugar,  gives  a  magnificent  purple  color  (Petten- 
kofer's  test),  with  a  characteristic  spectrum.  A  similar  color  may,  however, 
often  be  produced  by  the  action  of  the  same  bodies  on  albumin,  amyl  alcohol, 
and  some  other  organic  bodies. 

§  216.  Action  of  bile  on  food.  In  some  animals  at  least  bile  contains  a 
ferment  capable  of  converting  starch  into  sugar  ;  but  its  action  in  this  respect 
is  wholly  subordinate. 

On  proteids  bile  has  no  direct  digestive  action  whatever,  but  being,  gen- 
erally at  least,  alkaline,  and  often  strongly  so,  tends  to  neutralize  the  acid 
contents  of  the  stomach  as  they  pass  into  the  duodenum,  and,  as  we  shall 
see,  so  prepares  the  way  for  the  action  of  the  pancreatic  juice.  To  peptic 
action  it  is  distinctly  antagonistic  ;  the  presence  of  a  sufficient  quantity  of 
bile  renders  gastric  juice  inert  toward  proteids.  Moreover,  when  bile,  or  a 
solution  of  bile-salts,  is  added  to  a  fluid  containing  the  products  of  gastric 
digestion,  a  precipitate  takes  place,  consisting  of  parapeptone  (when  present), 
peptone,  pepsin,  and  bile-salts.  The  precipitate  is  redissolved  in  an  excess 
of  bile  or  solution  of  bile-salts ;  but  the  pepsin,  though  redissolved,  remains 
inert  toward  proteids.  This  precipitation  actually  does  take  place  in  the 
duodenum,  and  we  shall  speak  of  it  again  later  on. 

With  regard  to  the  action  of  bile  on  fats,  the  following  statements  may 
be  made : 

Bile  has  a  slight  solvent  action  on  fats,  as  seen  in  its  use  by  painters.  It 
has  by  itself  a  slight  but  only  slight  emulsifying  power ;  a  mixture  of  oil 
and  bile  separates  after  shaking  rather  less  rapidly  than  a  mixture  of  oil  and 
water.  With  fatty  acids  bile  forms  soap.  It  is,  moreover,  a  solvent  of  solid 
soaps,  and  it  would  appear  that  the  emulsion  of  fats  is  under  certain  circum- 
stances at  all  events  facilitated  by  the  presence  of  soaps  in  solution.  Hence 
bile  is  probably  of  much  greater  use  as  an  emulsion  agent  when  mixed  with 
pancreatic  juice  than  when  acting  by  itself  alone.  To  this  point  we  shall 
return.  Lastly,  the  passage  of  fats  through  membranes  is  assisted  by  wetting 
the  membranes  with  bile  or  with  a  solution  of  bile-salts.  Oil  will  pass  to  a 
certain  extent  through  a  filter-paper  kept  wet  with  a  solution  of  bile-salts, 
whereas  it  will  not  pass  or  passes  with  extreme  difficulty  through  one  kept 
constantly  wet  with  distilled  water. 

Bile  possesses  some  antiseptic  qualities.  Out  of  the  body  its  presence 
hinders  various  putrefactive  processes;  and  when  it  is  prevented  from  flow- 
ing into  the  alimentary  canal,  the  contents  of  the  intestine  undergo  changes 
different  from  those  which  take  place  under  normal  conditions,  and  leading 
to  the  appearance  of  various  products,  especially  of  ill-smelling  gases. 

These  various  actions  of  bile  seem  to  be  dependent  on  the  bile-salts  and 
not  on  the  pigmentary  or  other  constituents. 

Pancreatic  Juice. 

§  217.  Natural  healthy  pancreatic  juice  obtained  by  means  of  a  tem- 
porary pancreatic  fistula  differs  from  the  digestive  juices  of  which  we  have 
already  spoken  in  the  comparatively  large  quantity  of  proteids  which  it  con- 
tains. Its  composition  varies  according  to  the  rate  of  secretion,  for,  with  the 
more  rapid  flow,  the  increase  of  total  solids  does  not  keep  pace  with  that  of 
the  water,  though  the  ash  remains  remarkably  constant. 


282  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

By  an  incision  through  the  linea  alba  the  pancreatic  duct  (or  ducts)  can  easily 
be  found  either  in  the  rabbit  or  in  the  dog,  and  a  canula  secured  in  it.  There  is 
no  difficulty  about  a  temporary  fistula ;  but  with  permanent  fistulas  the  secretion  is 
apt  to  become  altered  in  nature,  and  to  lose  many  of  its  characteristic  properties. 
Some,  however,  have  succeeded  in  obtaining  permanent  fistulae  without  any  impair- 
ment of  the  secretion. 

Healthy  pancreatic  juice  is  a  clear,  somewhat  viscid  fluid,  frothing  when 
shaken.  It  has  a  very  decided  alkaline  reaction,  and  contains  few  or  no 
structural  constituents. 

The  average  amount  of  solids  in  the  pancreatic  juice  (of  the  dog)  obtained 
from  a  temporary  fistula  is  about  8  to  10  per  cent. ;  but  in  even  thoroughly 
active  juice  obtained  from  a  permanent  fistula  is  not  more  than  about  2  to  5 
per  cent.,  0.8  being  inorganic  matter  ;  and  this  is  probably  the  normal 
amount.  The  important  constituents  of  quite  fresh  juice  are  albumin,  a 
peculiar  form  of  proteid  allied  to  myosin,  giving  rise  to  a  sort  of  clotting,  a 
small  amount  of  fats  and  soaps,  and  a  comparatively  large  quantity  of  sodium 
carbonate,  to  which  the  alkaline  reaction  of  the  juice  is  due,  and  which  seems 
to  be  peculiarly  associated  with  the  proteids. 

Since,  as  we  shall  presently  see,  pancreatic  juice  contains  a  ferment  acting 
energetically  on  proteid  matters  in  an  alkaline  medium,  it  rapidly  digests 
its  own  proteid  constituents,  and,  when  kept,  speedily  changes  in  character. 
The  rnyosin-like  clot  is  dissolved,  and  the  juice  soon  contains  a  peculiar 
form  of  alkali-albumin  (precipitable  by  saturation  with  magnesium  sul- 
phate), as  well  as  small  quantities  of  leucin,  tyrosin,  and  peptone,  which 
seem  to  be  the  products  of  self-digestion  and  are  entirely  absent  from  the 
perfectly  fresh  juice. 

§  218.  Action  on  food-stuffs.  On  starch  pancreatic  juice  acts  with  great 
energy,  rapidly  converting  it  into  sugar  (chiefly  maltose).  All  that  has 
been  said  in  this  respect  concerning  saliva  might  be  repeated  in  the  case  of 
pancreatic  juice,  except  that  the  activity  of  the  latter  is  far  greater  than  that 
of  the  former.  Pancreatic  juice  and  the  aqueous  infusion  of  the  gland  are 
always  capable  of  converting  starch  into  sugar,  whether  the  animal  from 
which  they  were  taken  be  starving  or  well  fed.  From  the  juice,  or,  by  the 
glycerin  method  from  the  gland  itself,  an  amylolytic  ferment  may  be 


approximately  isolated. 
On  vroteids  p 


proteids  pancreatic  juice  also  exercises  a  solvent  action,  so  far  similar 
to  that  of  gastric  juice  that  by  it  proteids  are  converted  into  peptone.  If  a 
few  shreds  of  fibrin  are  thrown  into  a  small  quantity  of  pancreatic  juice, 
they  speedily  disappear,  especially  at  a  temperature  of  35°  C.,  and  the  mix- 
ture is  found  to  contain  peptone.  The  activity  of  the  juice  in  thus  converting 
proteids  into  peptone  is  favored  by  increase  of  temperature  up  to  40°  or 
thereabouts,  and  hindered  by  low  temperatures ;  it  is  permanently  destroyed 
by  boiling.  The  digestive  powers  of  the  juice  in  fact  depend,  like  those  of 
gastric  juice,  on  the  presence  of  a  ferment  which,  as  we  have  already  said, 
may  be  isolated  much  in  the  same  way  as  pepsin  is  isolated,  and  to  which 
the  name  trypsin  has  been  given. 

The  appearance  of  fibrin  undergoing  pancreatic  digestion  is,  however, 
different  from  that  undergoing  peptic  digestion.  In  the  former  case  the 
fibrin  does  not  swell  up,  but  remains  as  opaque  as  before,  and  appears  to 
suffer  corrosion  rather  than  solution.  But  there  is  a  still  more  important 
distinction  between  pancreatic  and  peptic  digestion  of  proteids.  Peptic 
digestion  is  essentially  an  acid  digestion  ;  we  have  seen  that  the  action  only 
takes  place  in  the  presence  of  an  acid,  and  is  arrested  by  neutralization. 
Pancreatic  digestion,  on  the  other  hand,  may  be  regarded  as  an  alkaline 
digestion  ;  the  action  is  most  energetic  when  some  alkali  is  present,  and  the 


BILE,   PANCREATIC  JUICE,   AND  SUCCUS   ENTERICUS.          283 

activity  of  an  alkaline  juice  is  hindered  or  delayed  by  neutralization  and 
arrested  by  acidification,  at  least  with  mineral  acids.  The  glycerin  extract 
of  pancreas  is  under  all  circumstances  as  inert  in  the  presence  of  free  mineral 
acid  as  that  of  the  stomach  in  the  presence  of  alkalies.  If  the  digestive 
mixture  be  supplied  with  sodium  carbonate  to  the  extent  of  1  per  cent., 
digestion  proceeds  rapidly,  just  as  does  a  peptic  mixture  when  acidulated 
with  hydrocholoric  acid  to  the  extent  of  0.2  per  cent.  Sodium  carbonate  of 
1  per  cent,  seems  in  fact  to  play  in  tryptic  digestion  a  part  altogether  com- 
parable to  that  of  hydrochloric  acid  of  0.2  per  cent,  in  gastric  digestion. 
And  just  as  pepsin  is  rapidly  destroyed  by  being  heated  to  about  40°  with 
a  1  per  cent,  solution  of  sodium  carbonate,  so  trypsin  is  rapidly  destroyed 
by  being  similarly  heated  with  dilute  hydrochloric  acid  of  0.2  per  cent. 
Alkaline  bile,  which  arrests  peptic  digestion,  seems,  if  anything,  favorable 
to  tryptic  digestion. 

Corresponding  to  this  difference  in  the  helpmate  of  the  ferment,  there  is 
in  the  two  cases  a  difference  in  the  nature  of  the  products.  In  both  cases 
peptone  is  produced,  and  such  differences  as  can  be  detected  between  pan- 
creatic and  gastric  peptones  are  relatively  small ;  but  in  pancreatic  digestion 
the  by-product  is  not,  as  in  gastric  digestion,  a  kind  of  acid-albumin,  but, 
as  might  be  expected,  a  body  having  more  analogy  with  alkali-albumin. 
Moreover,  before  the  alkali-albumin  is  actually  formed,  the  fibrin  becomes 
altered  and  takes  on  characters  intermediate  between  those  of  alkali-albumin 
and  of  ordinary  albumin  ;  and  when  fresh  raw,  i.  e.,  unboiled,  fibrin  is  acted 
upon  by  pancreatic  juice,  one  or  more  globulins  appear  as  initial  products. 

Further,  there  are  evidences  that  differences  of  even  a  more  profound 
nature  than  the  above  exist  between  pancreatic  and  gastric  digestion.  One 
of  these  is  the  appearance  in  the  pancreatic  digestion  of  proteids  of  two 
remarkable  nitrogenous  crystalline  bodies,  leucin  and  tyrosin.  When  fibrin 
(or  other  proteid)  is  submitted  to  the  action  of  pancreatic  juice,  the  amount 
of  peptone  which  can  be  recovered  from  the  mixture  falls  far  short  of  the 
original  amount  of  proteids,  much  more  so  than  in  the  case  of  gastric  juice  ; 
and  the  longer  the  digestive  action,  the  greater  is  this  apparent  loss.  If  a 
pancreatic  digestive  mixture  be  freed  from  the  alkali-albumin  by  neutraliza- 
tion and  filtration,  the  filtrate  yields,  when  concentrated  by  evaporation,  a 
crop  of  crystals  of  tyrosin.  If  these  be  removed  the  peptone  may  be  pre- 
cipitated from  the  concentrated  filtrate  by  the  addition  of  a  large  excess  of 
alcohol  and  separated  by  filtration.  The  second  filtrate,  upon  being  concen- 
trated by  evaporation,  yields  abundant  crystals  of  leucin  and  traces  of 
tyrosin.  Thus,  by  the  action  of  the  pancreatic  juice,  a  considerable  amount 
of  the  proteid  which  is  being  digested  is  so  broken  up  as  to  give  rise  to 
products  which  are  no  longer  proteid  in  nature.  From  this  breaking  up  of 
the  proteid  there  arise  leucin,  tyrosin,  and  probably  several  other  bodies, 
such  as  fatty  acids  and  volatile  substances. 

As  is  well  known,  leucin  and  tyrosin  are  the  bodies  which  make  their 
appearance  when  proteids  or  gelatin  are  acted  on  by  dilute  acids,  alkalies,  or 
various  oxidizing  agents.  Leucin  is  a  body  which,  in  an  impure  state,  crys- 
tallizes in  minute  round  lumps  with  an  obscure  radiate  striation,  but  when 
pure  forms  thin  glittering  flat  crystals.  It  has  the  formula  C6H13NO2  or 
C5H10.NH2(CO.OH),  and  is  amido-caproic  acid.  Now,  caproic  acid  is  one 
of  the  "  fatty  acid  "  series,  so  that  leucin  may  be  regarded  as  a  compound  of 
ammonia  with  a  fatty  acid.  Tyrosin,  C9HnNO3,  on  the  other  hand,  belongs 
to  the  "aromatic"  series ;  it  is  a  phenyl  compound,  and  hence  allied  to  ben- 
zoic  acid  and  hippuric  acid.  So  that  in  pancreatic  digestion  the  large  com- 
plex proteid  molecule  is  split  up  into  fatty  acid  and  aromatic  molecules, 
some  other  bodies  of  less  importance  making  their  appearance  at  the  same 


284  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

time.  We  infer  that  the  proteid  molecules  are  in  some  way  built  up  out  of 
"  fatty  acid  "  and  "  aromatic  "  molecules,  together  with  other  components, 
and  we  shall  later  on  see  additional  reasons  for  this  view. 

Among  the  supplementary  products  of  pancreatic  digestion  may  be  men- 
tioned the  body  indol  (C8H7N),  to  which  apparently  the  strong  and  pecu- 
liarly fecal  odor  which  sometimes  makes  its  appearance  during  pancreatic 
digestion  is  due.  Indol,  however,  unlike  the  leucin  and  tyrosin,  is  not  a 
product  of  pure  pancreatic  digestion,  but  of  an  accompanying  decomposition 
due  to  the  action  of  organized  ferments.  A  pancreatic  digestive  mixture 
soon  becomes  swarming  with  bacteria,  in  spite  of  ordinary  precautions,  when 
natural  juice  or  an  infusion  of  the  gland  is  used.  When  isolated  ferment 
is  used  and  atmospheric  germs  are  excluded,  or  when  pancreatic  digestion 
is  carried  on  in  the  presence  of  salicylic  acid  or  thymol,  which  prevent  the 
development  of  bacteria  and  like  organisms  but  permit  the  action  of  the 
trypsin,  no  odor  is  perceived  and  no  indol  is  produced. 

After  long-continued  digestion,  especially  when  accompanied  by  putre- 
factive decomposition,  the  amount  of  proteids  which  are  carried  beyond  the 
peptone  stage  and  broken  up  may  be  very  great. 

On  the  gelatiniferous  elements  of  the  tissues  as  they  actually  exist  in  the 
tissue  previous  to  any  treatment,  pancreatic  juice  appears  to  have  no  solvent 
action.  The  fibrillse  and  bundles  of  fibrillse  of  ordinary  untouched  con- 
nective tissue  are  not  digested  by  pancreatic  juice,  which  in  this  respect 
affords  a  striking  contrast  to  gastric  juice.  But  when  they  have  been  pre- 
viously treated  with  acid  or  boiled,  so  as  to  become  converted  into  actual 
gelatin,  trypsin  is  able  to  dissolve  them,  apparently  changing  them  much  in 
the  same  way  as  does  pepsin.  Trypsin,  unlike  pepsin,  will  dissolve  mucin. 
Like  pepsin,  it  is  inert  toward  nuclein,  horny  tissues,  and  the  so-called 
amyloid  matter. 

On  fats  pancreatic  juice  has  a  twofold  action.  In  the  first  place  it  emul- 
sifies fats.  If  hog's  lard  be  gently  heated  until  it  melts  and  be  then  mixed 
with  pancreatic  juice  before  it  solidifies  on  cooling,  a  creamy  emulsion  last- 
ing for  almost  an  indefinite  time  is  formed.  So  also  when  olive  oil  is  shaken 
up  with  pancreatic  juice,  the  separation  of  the  two  fluids  takes  place  very 
slowly,  and  a  drop  of  the  mixture  under  the  microscope  shows  that  the 
division  of  the  fat  is  very  minute.  An  alkaline  aqueous  infusion  of  the 
gland  has  similar  emulsifying  powers.  In  the  second  place  pancreatic  juice 
splits  up  neutral  fats  into  their  respective  acids  and  glycerin.  Thus,  palmitin 
(or  tripalmitin)  (C15H31.CO.O)3.C3H5  is  with  the  assumption  of  3H2O  split  up 
into  three  molecules  of  palmitic  acid  3(Ci5H31.CO.OH)  and  one  of  glycerin 
(C3H5)  (OH3)  ;  and  so  with  the  other  neutral  fats.  If  perfectly  neutral  fat 
be  treated  with  pancreatic  juice,  especially  at  the  body-temperature,  the 
emulsion  which  is  formed  speedily  takes  on  an  acid  reaction,  and  by  appro- 
priate means  not  only  the  corresponding  fatty  acids,  but  glycerin  may  be 
obtained  from  the  mixture.  When  alkali  is  present,  the  fatty  acids  thus  set 
free  form  their  corresponding  soaps.  Pancreatic  juice  contains  fat  and  is 
consequently  apt  after  collection  to  have  its  alkalinity  reduced,  and  an 
aqueous  infusion  of  a  pancreatic  gland  (which  always  contains  a  consider- 
able amount  of  fat)  very  speedily  becomes  acid. 

Thus  pancreatic  juice  is  remarkable  for  the  power  it  possesses  of  acting 
on  all  the  food-stuffs,  on  starch,  fats,  and  proteids. 

The  action  on  starch,  the  action  on  proteids,  and  the  splitting  up  of 
neutral  fats  appear  to  be  due  to  the  presence  of  three  distinct  ferments,  and 
methods  have  been  suggested  for  isolating  them.  The  emulsifying  power,  on 
the  other  hand,  is  connected  with  the  general  composition  of  the  juice  (or  of 
the  aqueous  infusion  of  the  gland),  being  probably  in  large  measure  depend- 


BILE,  PANCREATIC  JUICE,   AND  SUCCUS   ENTERICUS.          285 

ent  on  the  alkali  and  the  alkali-albumin  present.  The  proteolytic  ferment, 
trypsin,  as  ordinarily  prepared  seems  to  be  proteid  in  nature  and  capable  of 
giving  rise  by  digestion  to  peptone ;  but  it  may  be  doubted,  as  in  the  case 
of  pepsin  and  other  ferments,  whether  the  pure  ferment  has  yet  been  isolated. 
There  are  no  means  of  distinguishing  the  amylolytic  ferment  of  the  pan- 
creas from  ptyalin.  The  term  pancreatin  has  been  variously  applied  to 
many  different  preparations  from  the  gland,  and  its  use  had,  perhaps,  better 
be  avoided. 

The  action  of  pancreatic  juice  or  of  the  infusion  or  extract  of  the  gland, 
on  starch,  is  seen  under  all  circumstances,  whether  the  animal  be  fasting  or 
not.  The  same  may  probably  be  said  of  the  action  on  fats.  On  proteids 
the  natural  juice,  when  secreted  in  a  normal  state,  is  always  active.  The 
glycerin  extract  or  aqueous  infusion  of  the  gland,  on  the  contrary,  as  we 
have  already  explained  (§  208),  is  active  in  proportion  as  the  trypsinogen 
has  been  converted  into  trypsin. 

SUCGUS  Entericus. 

§  219.  When  in  a  living  animal  a  portion  of  the  small  intestine  is  liga- 
tured, so  that  the  secretions  coming  down  from  above  cannot  enter  its  canal, 
while  yet  the  blood-supply  is  maintained  as  usual,  a  small  amount  of  secre- 
tion collects  in  its  interior.  This  is  spoken  of  as  the  succus  entericus,  and  is 
supposed  to  be  furnished  by  the  glands  of  Lieberkiihn,  of  which  we  shall 
presently  speak. 

Succus  entericus  may  be  obtained  by  the  following  method,  known  as  that  of 
Thiry  modified  by  Vella.  The  small  intestine  is  divided  in  two  places  at  some 
distance  (30  to  50  cm.)  part.  By  fine  sutures  the  lower  end  of  the  upper  section 
is  carefully  united  with  the  upper  end  of  the  lower  section,  thus,  as  it  were,  cut- 
ting out  a  whole  piece  of  the  small  intestine  from  the  alimentary  tract.  In  suc- 
cessful cases  union  between  the  cut  surfaces  takes  place,  and  a  shortened  but  other- 
wise satisfactory  canal  is  re-established.  Of  the  isolated  piece,  the  two  ends  are 
separately  brought  through  incisions  in  the  abdominal  wall,  and  their  mouths  care- 
fully fastened  in  such  a  manner  that  each  mouth  of  the  piece  opens  on  to  the  ex- 
terior. During  the  process  of  healing  two  fistulae  are  thus  established,  one  leading 
to  the  beginning  of  and  the  other  to  the  end  of  a  short  piece  of  intestine  quite  iso- 
lated from  the  rest  of  the  alimentary  canal ;  by  means  of  these  openings  a  small 
quantity  of  fluid  can  be  obtained. 

The  quantity  secreted  is  said  to  be  considerably  increased  by  the  administration 
of  pilocarpine. 

Succus  entericus  obtained  from  the  dog  by  the  above  method  is  a  clear 
yellowish  fluid  having  a  faintly  alkaline  reaction  and  containing  a  certain 
quantity  of  mucus.  It  is  said  to  convert  starch  into  sugar  and  proteids  into 
peptone  (the  action  being  very  similar  to  that  of  pancreatic  juice),  to  split 
up  neutral  fats,  to  emulsify  fats,  and  to  curdle  milk.  It  is  also  said  to  con- 
vert rapidly  cane-sugar  into  grape-sugar,  and  by  a  fermentative  action  to 
convert  cane-sugar  into  lactic  acid,  and  this  again  into  butyric  acid,  with 
the  evolution  of  carbonic  acid  and  free  hydrogen. 

According  to  the  above  results  succus  entericus  is  to  be  regarded  as  an 
important  secretion  acting  on  all  kinds  of  food.  But  even  at  its  best  its 
actions  are  slow  and  feeble.  Moreover,  many  observers  have  obtained  nega- 
tive results,  so  that  the  various  statements  are  conflicting.  Besides,  we  have 
no  exact  knowledge  as  to  the  amount  to  which  such  a  secretion  takes  place 
under  normal  circumstances  in  the  living  body.  We  may,  therefore,  con- 
clude that,  at  present  at  all  events,  we  have  no  satisfactory  reasons  for  sup- 
posing that  the  actual  digestion  of  food  in  the  intestine  is,  to  any  great 
extent,  aided  by  such  a  juice. 


286  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

Of  the  possible  action  of  other  secretions  of  the  alimentary  canal,  as  of 
the  csecum  and  large  intestine,  we  shall  speak  when  we  come  to  consider  the 
changes  in  the  alimentary  canal. 

§  220.  Gall-stones.  Concretions,  often  of  considerable  size,  known  as  gall- 
stones, are  not  unfrequently  formed  in  the  gall-bladder,  and  smaller  concre- 
tions are  sometimes  formed  in  the  bile  passages.  In  man  two  kinds  of 
gall-stones  are  common.  One  kind  consists  almost  entirely  of  cholesterin, 
sometimes  nearly  free  from  any  admixture  with  pigment,  sometimes  more  or 
less  discolored  with  pigment.  Gall-stones  of  this  kind  have  a  crystalline 
structure,  and  when  broken  or  cut  show  frequently  radiate  and  concentric 
markings.  The  other  kind  consists  chiefly  of  bilirubin  in  combination  with 
calcium.  Gall-stones  of  this  kind  are  dark-colored  and  amorphous.  Less 
common  than  the  above  are  small,  dark-colored  stones,  having  often  a  mul- 
berry shape,  consisting  not  of  bilirubin  itself,  but  of  one  or  other  derivative 
of  bilirubin.  Gall-stones  consisting  almost  entirely  of  inorganic  salts,  calcic 
carbonates  and  phosphates,  are  also  occasionally  met  with.  In  the  lower 
animals,  in  oxen  for  instance,  bilirubin  gall-stones  are  not  uncommon,  but 
cholesterin  gall-stones  are  rare. 

A  gall-stone  appears  always  to  contain  a  more  or  less  obvious  "nucleus," 
around  which  the  material  of  the  stone  has  been  deposited,  and  which  may 
be  regarded  as  the  origin  of  the  stone ;  the  real  cause  of  the  formation  of  the 
stone  lies,  however,  in  certain  changes  in  the  bile,  by  which  the  cholesterin, 
or  bilirubin,  or  other  constituent  ceases  to  remain  dissolved  in  the  bile.  But 
we  cannot  discuss  this  matter  here. 

THE  SECRETION  OF  PANCREATIC  JUICE  AND  OF  BILE. 

§  221.  The  secretion  of  pancreatic  juice.  Although  in  some  cases,  as 
that  of  the  parotid  of  the  sheep,  the  flow  of  saliva  is  continuous  or  nearly  so, 
in  most  animals,  as  in  man,  the  intermittence  of  the  secretion  is  very  nearly 
absolute.  While  food  is  in  the  mouth  saliva  flows  freely,  but  between  meals 
only  just  sufficient  is  secreted  to  keep  the  mouth  moist,  and  probably  the 
greater  part  of  this  is  supplied  not  by  the  larger  salivary  but  by  the  small 
buccal  glands.  The  flow  of  pancreatic  juice,  on  the  other  hand,  is  much 
more  prolonged,  being  in  the  rabbit  continuous,  and  in  the  dog  lasting  for 
twenty  hours  after  food.  But  this  contrast  between  the  secretion  of  saliva 
and  that  of  pancreatic  juice  is  natural,  since  the  stay  of  food  in  the  mouth, 
even  during  a  protracted  feast,  is  relatively  short,  whereas  the  time  during 
which  the  material  of  a  meal  is  able  in  some  way  or  other  to  affect  the  pan- 
creas is  very  prolonged. 

The  flow,  though  continuous  or  nearly  so,  is  not  uniform.  In  the  dog  the 
flow  of  pancreatic  juice  begins  immediately  after  food  has  been  taken,  and 
rises  to  a  maximum  which  may  be  reached  within  the  first  or,  as  in  the  case 
furnishing  the  diagram  given  in  Fig.  88,  the  second  hour,  but  which  more 
commonly  is  not  reached  until  the  third  or  fourth  hour.  This  rise  is  then 
followed  by  a  fall,  after  which  there  is  a  secondary  rise,  reaching  a  second 
maximum  at  a  very  variable  time,  but  generally  between  the  fifth  and 
seventh  hours.  This  second  maximum,  however,  is  never  so  high  as  the 
first. 

The  second  rise  may  be  due  to  material  absorbed  from  the  intestines 
being  carried  in  the  circulation  to  the  pancreas,  and  so  directly  exciting  the 
gland  to  activity,  much  in  the  same  way  as,  in  the  case  of  the  stomach,  the 
absorption  of  digested  material  promotes  the  flow  of  gastric  juice  (see  §  202) ; 
and  a  similar  absorption  may  contribute  to  the  first  rise  also,  but  it  is  more 
probable  that  so  marked  and  sudden  a  rise  as  this  is  carried  out  by  some 


SECRETION  OF  PANCREATIC  JUICE  AND  OF  BILE. 


287 


nervous  mechanism.  The  details  of  this  mechanism  have,  however,  not  as 
yet  been  satisfactorily  worked  out. 

The  pancreas  derives  its  nerves,  which  reach  it  along  its  bloodvessels, 
from  the  solar  plexus  of  the  splanchnic  system,  but  the  ultimate  origin  of 
the  fibres  have  not  been  traced  out ;  some  of  them,  however,  certainly  come 
through  the  plexus  from  the  right  vagus. 

Stimulation  of  the  medulla  oblongata,  or  of  the  spinal  cord,  will  call  forth 
secretion  in  a  quiescent  gland,  or  increase  a  secretion  already  going  on. 
From  this  we  may  infer  the  existence  of  a  reflex  mechanism,  though  we 
cannot  as  yet  trace  out  satisfactorily  the  exact  path  of  either  the  afferent  or 
the  efferent  impulses ;  all  we  can  say  is,  that  the  latter  do  not  reach  the 

FIG.  88. 


2j  3|  4  I  5  I  6  I  7  |8  I  9  1 10  I  111  12  1 13  I  H  1 15  |lG  i  1  I  2  I  3  I  4  I  5  I  6  I  7  I  8  I  9  llO 


Diagram  illustrating  the  Influence  of  Food  on  the  Secretion  of  Pancreatic  Juice.  (N.  0. 
Bernstein.)  The  abscissae  represent  hours  after  taking  food  ;  the  ordinates  represent  in  c.c.  the 
secretion  in  ten  minutes.  A  marked  rise  is  seen  at  B  immediately  after  food  was  taken,  with  a 
secondary  rise  between  the  fourth  and  fifth  hours  afterward.  Where  the  line  is  dotted  the  obser- 
vation was  interrupted.  On  food  being  again  given  at  C,  another  rise  is  seen,  followed  in  turn 
by  a  depression  and  a  secondary  rise  at  the  fifth  hour.  A  very  similar  curve  would  represent  the 
secretion  of  bile. 

pancreas  by  the  vagus,  since  stimulation  of  the  medulla  is  effective  after 
section  of  both  vagi. 

A  secretion  already  going  on  may  be  arrested  by  stimulation  of  the  cen- 
tral end  of  the  vagus,  and  the  stoppage  of  the  secretion,  which  has  been 
observed  as  occurring  during  and  after  vomiting,  is  probably  brought  about 
in  this  way.  This  effect,  which,  however,  is  not  confined  to  the  vagus,  stimu- 
lation of  other  afferent  nerves,  such  as  the  sciatic,  producing  the  same  effect, 
maybe  regarded  (in  the  absence  of  any  proof  that  the  result  is  due  to  reflex 
constriction  of  the  pancreatic  bloodvessels  unduly  checking  the  blood-supply) 
as  an  inhibition  of  a  reflex  mechanism  at  its  centre  in  the  medulla,  or  in 
some  other  part  of  the  central  nervous  system,  much  in  the  same  way  as 
fear  inhibits  at  the  central  nervous  system  the  secretion  of  saliva  following 
food  in  the  mouth  (§  196).  But  if  so,  then  we  must  regard  the  secretion  of 
pancreatic  juice  as  closely  resembling  that  of  saliva,  inasmuch  as  it  is  called 


THE  TISSUES  AND  MECHANISMS  OF   DIGESTION. 

forth  by  a  reflex  act.  Yet  it  is  stated  that,  unlike  the  case  of  saliva,  the 
secretion  of  pancreatic  juice  continues  after  all  the  nerves  going  to  the  gland 
have  been  divided,  an  operation  which  would  do  away  with  the  possibility 
of  reflex  action.  Such  an  experiment,  however,  cannot  be  regarded  as 
decisive,  since  it  is  almost  impossible  to  be  sure  of  dividing  all  the  nerves. 

No  evidence  has  yet  been  brought  forward  to  prove  the  existence  of  any 
double  nervous  mechanism  similar  to  that  of  chorda  fibres  and  sympathetic 
fibres  in  the  salivary  gland.  All  that  can  be  said  is  that,  when  the  gland  is 
stimulated  to  secrete,  the  bloodvessels  are  dilated  as  in  the  salivary  gland ; 
and  we  have  already  (§203)  dwelt  on  the  histological  changes  which  accom- 
pany secretion.  We  may  add  that  when  the  gland  is  stimulated  to  increased 
secretion,  the  increase  is  not  merely  an  increase  of  water;  the  discharge  of 
solids  is  increased  even  more  than  the  discharge  of  water,  so  that  the  per- 
centage of  solids  in  the  juice  increases. 

The  quantity  of  pancreatic  juice  secreted,  in  the  case  of  man,  in  twenty- 
four  hours  has  been  calculated  at  300  c.c.,  but  such  a  calculation  is  of  very 
uncertain  value. 

We  have  seen  (§  197)  that  in  the  salivary  glands  the  pressure  which  may 
be  exerted  by  the  fluid  in  the  ducts  is  very  considerable,  exceeding  it  may 
be  even  the  blood-pressure  in  the  carotid  artery.  In  this  respect  the  pan- 
creas differs  from  the  salivary  glands.  When]  in  a  rabbit,  a  can u la  con- 
nected with  a  vertical  tube  or  a  manometer  is  placed  in  the  pancreatic  duct, 
the  column  of  fluid  does  not  rise  above  a  height  corresponding  to  a  pressure 
of  about  17  mm.  of  mercury.  But  at  this  pressure  the  gland  becomes  cedem- 
atous  on  account  of  the  juice  secreted  passing  back  through  the  walls  of 
the  ducts  and  alveoli  into  the  connective  tissue ;  a  much  higher  pressure  is 
needed  to  render  a  salivary  gland  cedematous  ;  and  whether  the  IOWT  pressure 
observed  in  the  pancreas  is  due  to  the  ease  with  which  oedema  takes  place, 
or  to  the  actual  secretion  not  being  able  to  reach  a  higher  pressure,  cannot 
be  stated  with  certainty. 

§  222.  The  secretion  of  bile.  The  act  of  secretion  of  bile  by  the  liver 
must  not  be  confounded  with  the  discharge  of  bile  from  the  bile-duct  into 
the  duodenum.  When  the  acid  contents  of  the  stomach  are  poured  over 
the  orifice  of  the  biliary  duct,  a  gush  of  bile  takes  place.  Indeed,  stimu- 
lation of  this  region  of  the  duodenum  with  a  dilute  acid  at  once  calls 
forth  a  flow,  though  alkaline  fluids  so  applied  have  little  or  no  effect. 
When  no  such  acid  fluid  is  passing  into  the  duodenum  no  bile  is,  under 
normal  circumstances,  discharged  into  the  intestine.  The  discharge  is  due 
to  a  contraction  of  the  muscular  walls  of  the  gall-bladder  and  ducts,  accom- 
panied by  a  relaxation  of  the  sphincter  of  the  orifice ;  both  acts  are  prob- 
ably of  a  reflex  nature,  but  the  details  of  the  mechanism  have  not  been 
worked  out. 

The  secretion  of  bile,  "on  the  other  hand,  as  shown  by  the  result  of  bili- 
ary fistulse,  is  continuous  ;  it  appears  never  to  cease.  When  no  food  is 
taken  the  bile  passes  from  the  liver  along  the  hepatic  and  then  back  along 
the  cystic  duct  (the  flow  being  aided  probably  by  peristaltic  contractions 
of  the  muscular  fibres  of  the  duct)  to  the  gall-bladder,  where  it  is  tem- 
porarily stored  ;  hence  in  starving  animals,  when  no  discharge  is  excited  by 
food,  the  gall-bladder  becomes  greatly  distended  with  bile.  But  the  secre- 
tion, though  continuous,  is  not  uniform.  The  rate  of  secretion  varies,  and 
is  especially  influenced  by  food  ;  it  is  seen  to  rise  rapidly  after  meals, 
reaching  its  maximum,  in  dogs,  in  from  four  to  eight  hours.  There  seems 
to  be  an  immediate,  sudden  rise  when  food  is  taken,  then  a  fall,  followed  sub- 
sequently by  a  more  gradual  rise  up  to  the  maximum,  and  ending  in  a  final 
fall  to  the  lowest  point.  The  curve  of  secretion,  in  fact,  resembles  that  of 


SECRETION  OF  PANCREATIC  JUICE   AND  OF  BILE.  289 

the  secretion  of  pancreatic  juice  in  having  a  double  rise ;  and  as  in  that 
case  so  in  this,  it  is  very  probable  that  the  first  rise  is  in  part  the  result  of 
nervous  action,  and  it  is  also  possible  that  nervous  influences  intervene  in 
the  second,  more  lasting  rise  ;  but,  as  we  shall  see  presently,  even  nervous 
influences  may  affect  the  liver  in  a  very  indirect  manner,  and  our  knowl- 
edge as  to  any  direct  action  of  the  nervous  system  on  the  liver  is  at  present 
very  imperfect. 

The  liver  receives  its  chief  nervous  supply  from  the  solar  plexus,  and  to  a 
great  extent  through  that  part  of  the  solar  plexus  called  the  hepatic  plexus, 
which  embraces  the  portal  vein,  hepatic  artery,  and  bile-duct,  as  these  plunge 
into  the  liver  at  the  porta.  The  solar  plexus  is  fed  by  the  two  abdominal 
splanchnic  nerves,  major  and  minor,  by  other  smaller  nerves  from  the  lower 
parts  of  the  splanchnic  (sympathetic)  chain,  and  by  the  terminal  portion 
of  the  right  vagus  nerve.  Small  branches  from  the  left  vagus,  rami  hepatici, 
also  pass  directly  to  the  liver  from  the  termination  of  that  nerve  on  the 
stomach,  finding  their  way  also  through  the  porta.  The  fibres  thus  entering 
the  liver  from  the  several  sources  are,  for  the  most  part,  non-rnedullated 
fibres  ;  with  these,  however,  are  mixed  a  certain  number  of  medullated 
fibres. 

As  to  the  functions  of  these  nerves  in  reference  to  the  secretion  of  bile, 
we  may  say  at  once  that  no  satisfactory  or  exact  statement  can  at  present  be 
made. 

§  223.  It  must  be  remembered,  however,  that  the  liver  is  so  peculiarly 
related  to  the  other  organs  of  digestion,  and  its  vascular  arrangements  so 
special  that,  with  regard  to  it,  as  compared  with  many  other  organs,  an  in- 
trinsic nervous  mechanism  must  occupy  a  more  or  less  subordinate  position. 
The  blood-supply  of  the  pancreas  for  instance  is  dependent  chiefly  on  the 
width  for  the  time  being  of  the  pancreatic  arteries ;  it  will  be  affected,  of 
course,  by  the  general  arterial  pressure  and  by  any  circumstances  which 
affect  the  outflow  by  the  pancreatic  veins,  and  therefore  by  the  condition  of 
the  portal  venous  system  of  which  those  veins  form  a  part ;  but  in  the  main, 
the  amount  of  blood  bathing  the  alveoli  of  the  pancreas  will  depend  on 
whether  the  pancreatic  arteries  are  constricted  or  dilated.  The  quality  of 
the  blood  reaching  the  pancreas,  being  arterial  blood  drawn  direct  from  the 
arterial  foundation,  will  be  modified  only  by  such  circumstances  as  modify 
the  general  mass  of  the  blood. 

Very  different  is  the  case  of  the  liver.  The  supply  of  arterial  blood 
coming  direct  through  the  hepatic  artery  is  small  compared  with  the  mass 
pouring  through  the  vena  portse ;  it  moreover,  as  we  shall  see,  is  distributed 
in  capillaries  among  the  small  interlobular  branches  of  the  vena  portee  and 
has  become  venous,  indeed  merged  with  the  portal  blood,  before  it  reaches 
the  actual  lobules.  The  supply  of  blood  for  the  liver  is  mainly  that 
through  the  vena  porta? ;  and  this  supply  is  not,  like  an  arterial  supply,  a 
fairly  uniform  one,  modified  chiefly  by  the  vasomotor  events  of  the  organ 
itself,  but  is  dependent  on  what  happens  to  be  taking  place  in  the  alimen- 
tary canal  and  in  abdominal  organs  other  than  the  liver  itself.  When  no 
food  is  being  digested  and  the  alimentary  canal  is  at  rest,  the  vessels  of  that 
canal,  as  we  have  already  said  in  speaking  of  the  stomach,  are  like  those 
of  the  pancreas  and  salivary  glands,  in  a  state  of  tonic  constriction  ;  a 
relatively  small  quantity  of  blood  passes  through  them  ;  hence  the  flow 
through  the  vena  portee  is  relatively  inconsiderable,  and  the  pressure  in 
that  vessel  is  low.  When  digestion  is  going  on  all  the  minute  arteries  of 
the  stomach,  intestine,  spleen  and  pancreas  are  dilated,  and  general  arterial 
pressure  being  by  some  means  or  other  maintained  (see  §  180),  a  relatively 
large  quantity  of  blood  rushes  into  the  vena  portse  and  the  pressure  in  that 

19 


290  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

vessel  becomes  much  increased,  though,  of  course,  remaining  lower  than 
the  general  arterial  pressure.  Moreover,  during  digestion,  peristaltic  move- 
ments of  the  muscular  coats  of  the  alimentary  canal  are,  as  we  have  seen, 
active;  and  these  movements,  serving  as  aids  to  the  circulation  (see  §  110), 
help  to  increase  the  portal  flow.  Further,  the  spleen,  as  we  shall  see  in 
speaking  of  that  organ,  is  in  many  animals  richly  provided  with  plain  mus- 
cular fibres,  and  in  such  cases  seems,  especially  during  digestion,  to  act  as  a 
muscular  pump  driving  the  blood  onward,  with  increased  vigor, along  the 
splenic  veins  to  the  liver.  So  that  even  were  the  liver  not  connected  with 
the  central  nervous  system  by  a  single  nervous  tie,  the  tide  of  blood  through 
the  liver  would  ebb  and  flow  according  to  the  abse-nce  or  presence  of  food 
in  the  alimentary  canal. 

An  increase  of  blood-supply  does  not,  of  course,  necessarily  mean  an 
increase  of  secretory  activity.  As  we  have  seen,  §  197,  in  the  presence  of 
atropine  the  secretion  of  saliva  may  stand  still  in  spite  of  dilated  blood- 
vessels and  the  consequent  rush  of  blood  ;  but  we  may  safely  assert  that, 
other  things  being  equal,  a  fuller  blood-supply  is  favorable  to  activity. 
Apparently  a  mere  change  in  the  quantity  of  blood  bathing  an  alveolus 
will  not  start  in  the  cells  the  changes  which  constitute  the  act  of  secretion, 
any  more  than  an  increase  in  the  blood  bathing  a  muscular  fibre  will  neces- 
sarily set  going  a  contraction  ;  but  unless  there  be  some  counteracting 
influence  at  work,  a  fuller  and  richer  lymph  around  a  cell  will  naturally 
lead  to  the  cell  taking  up  more  material  from  the  lymph,  and  so  will  increase 
the  cell's  store  of  energy.  Hence,  especially  in  the  hepatic  cell,  which 
appears  to  be  always  at  work,  always  undergoing  metabolism  of  such  a 
kind  as  to  give  rise  to  bile,  we  might  fairly  expect  the  greater  flow  through 
the  portal  vein  to  quicken  the  flow  through  the  bile-duct. 

And,  as  a  matter  of  fact,  we  do  find  vaso-constrictor  action  dominant 
over  the  secretion.  In  the  various  experiments  which  have  been  made  to 
ascertain  the  action  of  the  nervous  system  on  the  secretion  of  bile,  it  has 
always  been  found  that  stimulation  of  the  medulla  oblongata,  or  of  the 
spinal  cord,  or  of  the  abdominal  splanchnic  nerves,  stops,  or  at  least 
checks  the  flow  of  bile.  Now  the  effect  of  these  stimulations  is,  as  we 
have  already  seen  more  than  once,  a  powerful  constricting  action  on  the 
abdominal  bloodvessels  ;  by  such  stimulation  the  blood-supply  of  the  liver 
is  materially  diminished,  and  in  consequence  the  secretory  activity  is  slack- 
ened or  arrested. 

But  there  is  something  besides  the  mere  quantity  of  blood  to  be  consid- 
ered in  this  relation.  The  blood  which  passes  from  the  alimentary  canal  at 
rest  is  ordinary  venous  blood,  laden  simply  with  carbonic  acid  and  the  ordi- 
nary products  of  the  metabolism  of  the  muscular  and  mucous  coats  of  the 
canal.  When  digestion  is  going  on  the  portal  blood  is  laden,  as  we  shall 
see,  with  some,  at  all  events,  of  the  products  of  digestion,  with  sugar  prob- 
ably and  with  various  proteid  bodies.  And  it  is  quite  possible  or  even  prob- 
able that  some  of  these  bodies  in  the  portal  blood  reaching  the  hepatic  cells, 
stir  them  up  to  secretory  activity  ;  indeed,  this  view  may  be  regarded  as  sup- 
ported by  the  fact  that  'proteid  food  increases  the  quantity  of  bile  secreted, 
whereas  fatty  food,  which  as  we  shall  see  passes  chiefly,  if  not  wholly,  not 
by  the  portal  vein  but  by  the  lymphatics,  and  which  is  probably  largely 
disposed  of  in  some  way  or  other  before  it  can  reach  the  liver,  has  no  such 
effect. 

Hence  we  may  infer  that  at  all  events  the  second  increase  of  the  flow 
of  bile  which  occurs  during  the  later  stages  of  digestion,  may  be  to  a 
large  extent  the  direct  effect  of  blood,  laden  with  digestive  products, 
passing  from  the  stomach  and  intestines,  especially  the  latter,  to  the  liver 


SECRETION  OF  PANCREATIC  JUICE  AND  OF  BILE.  291 

by  the  portal  vein,  quite  independent  of  any  direct  nervous  action  on 
the  liver  itself;  and,  indeed,  it  is  possible  that  the  first  rise  also  may  be 
partly  due  to  the  increased  flow  of  blood  from  the  stomach,  aided  by  the 
absorption  from  that  organ  of  a  certain  amount  of  digested  material. 
Since,  however,  there  is  no  evidence  of  any  decrease  in  blood-supply,  or 
in  the  rate  of  absorption,  corresponding  to  the  fall  between  the  two  rises, 
some  influences  other  than  those  which  we  are  discussing  must  be  at  work 
in  the  matter. 

§  224.  The  blood-supply  of  the  liver  being  thus,  quite  apart  from  any 
nervous  supply  of  its  own,  so  closely  dependent  on  what  is  going  on  in  the 
alimentary  canal,  it  will  be  convenient  to  say  a  few  words  more  concerning 
the  vasomotor  nerves  of  that  canal.  As  we  have  already  said,  in  speaking 
of  the  vascular  system,  §  155,  the  vaso-constrictor  fibres  for  the  stomach  and 
intestines,  large  and  small,  issuing  from  what  we  may  call  the  vaso-constrictor 
region  of  the  cord,  pass  for  the  most  part  through  the  two  abdominal  splanch- 
nic nerves,  major  and  minor,  a  small  number  only  passing  out  below  the 
roots  of  those  nerves.  When  these  splanchnic  nerves  are  divided  the 
vessels  of  the  canal  are  dilated,  when  they  are  centrifugally  stimulated  the 
vessels  are  constricted.  Whether  there  be  any  distinct  vaso-dilator  fibres  for 
all  or  any  part  of  the  canal,  and  if  so  what  course  they  take,  is  not  known. 
When  no  food  has  for  some  time  been  taken,  the  mucous  membrane  of  the 
stomach  as  seen  through  a  gastric  fistula  is  pale ;  the  bloodvessels  are  con- 
stricted. And,  as  far  as  we  know,  a  similar  condition  obtains  throughout 
the  small  and  large  intestines.  When  food  is  taken  the  mucous  membrane 
of  the  stomach  becomes  flushed  ;  its  vessels  become  dilated.  This  appears 
to  be  the  result  of  an  inhibition  of  the  previously  existing  tonic  constric- 
tion ;  at  least  we  have  no  evidence  supporting  any  other  explanation. 
Apparently  the  presence  of  food  in  the  stomach  starts  in  the  mucous  mem- 
brane influences  which,  ascending  to  the  central  nervous  system,  inhibit  the 
vasomotor  centre  for  the  abdominal  splanchnic  nerves  or  such  part  of  that 
centre  as  governs  the  vaso-constrictor  fibres  of  the  stomach.  By  what  path 
such  afferent  impulses  reach  the  central  nervous  system  is  not  as  yet  defi- 
nitely settled  ;  but  possibly  by  the  vagus  nerve,  if  it  be  true,  as  stated,  that 
centripetal  stimulation  of  that  nerve,  while  it  raises  the  general  blood-pres- 
sure by  increasing,  in  a  reflex  manner,  vaso-constriction  in  other  regions, 
leads  to  a  dilatation  of  the  gastric  vessels.  So  also  it  is  probable  that  as  the 
food  reaches  succeeding  sections  of  the  alimentary  canal,  those  in  turn  in 
a  similar  manner  become  flushed  with  blood.  In  the  frog  there  is  some 
evidence  that  vaso-constrictors  leaving  the  spinal  cord  by  consecutive  spinal 
nerves,  govern  the  bloodvessels  of  consecutive  sections  of  the  alimentary  canal. 

All  this  flushing  of  the  canal  with  blood  leads,  we  repeat,  to  an  increased 
flow  of  blood  at  a  higher  pressure  through  the  portal  vein.  Whether  there 
be  any  additional  mechanism  set  to  work,  such  as,  for  instance,  which  some 
observations  suggest,  a  rhythmical  peristaltic  contraction  of  the  portal 
vein,  by  which  the  blood  is  still  more  rapidly  hurried  to  the  liver,  and 
whether  the  increased  venous  supply  through  the  portal  vein  is  accompanied 
by  a  corresponding  increase  of  the  lesser  supply  of  arterial  blood  through 
the  hepatic  artery,  is  not  known.  It  may,  perhaps,  be  here  remarked  that 
there  is  no  need  for  any  increase  of  arterial  blood,  since  the  blood  from  the 
alimentary  canal,  owing  to  its  more  rapid  passage  through  the  minute  vessels, 
is  probably  like  the  corresponding  blood  in  the  veins  of  an  active  salivary 
gland  (though  probably  also  not  to  the  same  extent)  less  venous  than  usual 
during  digestion,  in  spite  of  the  extra  quantity  of  carbonic  acid  thrown  into 
it  by  the  increased  metabolism  of  the  muscular  coat  during  the  peristaltic 
movements. 


292  THE  TISSUES   AND  MECHANISMS  OF  DIGESTION. 

§  225.  It  is  interesting  to  observe  that  the  pressure  under  which  the  bile 
is  secreted  is  relatively  low,  like  that  of  the  pancreatic  juice,  not  high  like 
that  of  the  saliva  ;  it  is  much  lower  than  the  arterial  pressure  in  the  same 
animal,  whereas  in  the  case  of  saliva  (§  197)  the  pressure  is  greater  than  the 
blood-pressure  in  the  carotid  artery.  But,  in  the  case  of  bile,  since  the  blood 
which  flows  through  the  hepatic  lobules  is,  mainly,  venous  portal  blood,  we 
have  to  compare  the  pressure  of  the  secretion,  not  with  arterial  pressure  but 
with  the  venous  pressure  in  the  portal  system  ;  and  in  the  dog  it  has  been 
found  that  while  the  pressure  of  the  bile  secreted  stood  at  about  200  mm.  of 
a  solution  of  sodium  carbonate — that  is,  about  15  mm.  mercury — the  blood- 
pressure  in  a  branch  of  the  superior  mesenteric  vein  stood  only  at  about  90 
mm.  of  the  same  solution — that  is,  about  7  mm.  mercury.  Now,  the  venous 
pressure  in  the  mesenteric  veins  is  higher,  though  only  slightly  higher,  than 
that  in  the  portal  vein  into  which  these  pour  their  blood  (the  difference  of 
pressure  being  the  main  cause  why  the  blood  flows  from  the  one  into  the 
other),  and  is,  therefore,  certainly  higher  than  the  pressure  in  the  portal 
capillaries  of  the  hepatic  lobules.  So  that  what  is  true  of  the  salivary  gland 
is  also  true,  on  a  different  scale,  of  the  liver,  viz.,  that  the  pressure  exerted 
by  the  secretion  is  higher  than  the  pressure  of  the  blood  in  the  vessels  feed- 
ing the  secreting  cells. 

§  226.  If  the  pressure  in  the  bile-duct  be  artificially  increased,  as  by 
pouring  fluid  into  the  glass  tube  or  manometer  with  which  the  canula  in  the 
duct  is  connected,  a  resorption  of  the  secreted  bile  takes  place ;  and  resorp- 
tion  will  also  take  place  within  the  body,  when  the  pressure  generated  by  the 
act  of  secretion  itself  reaches  and  is  maintained  at  a  sufficiently  high  level. 
Thus,  when  in  the  living  body  the  bile-duct  is  ligatured,  or  becomes  ob- 
structed by  gall-stones  or  otherwise,  fluid  is  accumulated  on  the  near  side  of 
the  ligature  at  a  pressure  which  goes  on  increasing  until  resorption  of  the 
bile  takes  place,  bile  salts  and  biliary  pigments  are  thrown  back  upon  the 
system,  and  "jaundice"  results.  It  would  appear  that  in  these  cases  re- 
sorption takes  place  through  the  interlobular  bile-ducts  and  not  through  the 
hepatic  cells  or  other  structures  within  the  lobules.  The  high  pressure  in 
the  ducts  does  not  lead  to  a  reversal  of  the  current  in  the  hepatic  cells  (at 
most  it  slackens  or  possibly  stops  the  current),  but  the  bile  secreted  into  the 
interlobular  ducts  escapes  from  these.  It  further  appears  that  the  escape  is 
not  into  the  bloodvessels  but  into  the  lymphatics ;  the  bile  salts,  pigments, 
and  other  constituents  are  carried  into  the  thoracic  duct,  and  in  an  indirect 
manner  only  find  their  way  into  the  blood  stream. 

To  complete  the  history  of  the  secretion  of  bile  we  ought  now  to  turn  to 
the  manufacture  of  the  biliary  constituents  within  the  cells.  But  since  the 
hepatic  cells  are  also  engaged  in  labors  other  and  more  important,  perhaps, 
than  that  of  secreting  bile,  it  will  be  convenient  to  defer  what  we  have  to 
say  on  this  point  until  we  come  to  speak  of  the  formation  of  glycogen  and 
of  the  general  metabolic  events  taking  place  in  the  liver. 

THE  MUSCULAR  MECHANISMS  OF  DIGESTION. 

§  227.  From  its  entrance  into  the  mouth  until  such  remnant  of  it  as  is 
undigested  leaves  the  body,  the  food  is  continually  subjected  to  movements 
having  for  their  object  the  tritu ration  of  the  food  as  in  mastication,  or  its 
more  complete  mixture  with  the  digestive  juices,  or  its  forward  progress 
through  the  alimentary  canal.  These  various  movements  may  briefly  be 
considered  in  detail : 

§  228.  Peristaltic  movements.  Putting  aside  the  somewhat  complicated 
pharyngeal  part  of  deglutition,  and  taking  the  resophageal  movements  by 


THE  MUSCULAR  MECHANISMS  OF   DIGESTION."  293 

themselves,  we  find  that  these,  together  with  the  movements  of  the  stomach 
and  of  the  small  and  large  intestines  right  down  to  the  anus  are  more  or  less 
alike,  and  may  be  described  under  the  general  name  of  "  peristaltic"  move- 
ments. We  have  already  in  §  88  spoken  of  these,  but  it  may  be  well  to 
consider  them  briefly  again  under  a  general  aspect,  before  dwelling  on  the 
special  movements  of  the  several  parts  of  the  alimentary  canal. 

The  muscular  coat  of  the  alimentary  canal  consists,  as  we  have  seen,  of 
two  layers,  separated  more  or  less  distinctly  by  a  sheet  of  connective  tissue, 
an  outer  thinner  longitudinal  layer,  and  an  inner  thicker  circular  layer  ;  and 
a  similar  arrangement  obtains  in  nearly  all  the  muscular  hollow  tubes  of  the 
body,  except  the  arteries,  in  which  the  muscular  elements  are  present  not  so 
much  for  the  purpose  of  driving  the  blood  onward  as  for  the  sake  of  regula- 
ting the  irrigation. 

The  action  of  the  circular  coat  is  fairly  simple.  A  contraction  starting 
at  any  part,  travels  onward  in  the  same  direction,  generally  downward,  that 
is  to  say,  from  a  part  nearer  the  mouth  to  a  part  nearer  the  rectum,  for  a 
greater  or  less  distance,  the  circularly  disposed  bundles  contracting  in  se- 
quence. The  result  is  a  narrowing  or  constriction  of  the  tube  which,  travel- 
ling more  or  less  slowly  along  the  tube,  drives  the  contents  onward  ;  when  a 
butcher  empties  the  intestine  of  a  slaughtered  animal  by  squeezing  it  high 
up  with  his  hand  or  with  his  thumb  and  finger,  and  carrying  the  squeezing 
action  downward  along  the  length  of  the  intestine,  he  makes  the  passive  in- 
testine do  very  much  what  the  circular  coat  does  actively,  by  contraction, 
in  the  living  animal. 

The  action  of  the  longitudinal  coat  is  perhaps  not  so  clear  ;  but  a  con- 
traction of  the  longitudinal  coat  taking  place  in  any  segment  of  the  tube 
would  tend  to  draw  the  tube  over  the  contents  lying  immediately  above,  or 
below,  the  segment,  very  much  as  a  glove  is  drawn  over  a  finger.  And  a 
succession  of  such  contractions  travelling  along  the  tube  would  lead  to  a 
movement  of  the  contents  in  the  same  direction.  Were  the  circular  coat 
absent  a  longitudinal  coat  might  by  itself  possibly  suffice  to  propel  the  con- 
tents along  the  tube.  In  the  presence  of  the  circular  coat,  the  action  of  the 
longitudinal  coat  in  any  segment  of  the  tube,  if  taking  place  immediately 
before  the  circular  contraction,  would,  by  filling  the  segment  with  contents, 
render  the  squeezing  action  of  the  circular  coat  more  efficient ;  if  taking 
place  immediately  after  the  circular  contraction,  it  would  help  in  quicken- 
ing the  return  of  the  tube  to  its  normal  calibre,  for  the  contraction  of  the 
longitudinal  coat  tends  to  shorten  and  widen  the  segment,  and  thus  would 
prepare  it  for  new  contents.  We  can  hardly  imagine  that  the  two  coats 
would  contract  at  the  same  time,  since  they  would  tend  to  neutralize  each 
other's  action.  Indeed,  we  may  probably  go  farther  and  assume  that  in 
each  segment  of  the  canal  first  the  longitudinal  coat  contracts  while  the 
circular  coat  is  relaxed,  and  that  then  the  circular  coat  contracts  while  the 
longitudinal  relaxes.  When  we  come  to  deal  with  respiration  we  shall  meet 
with  a  similar  double  antagonistic  and  successive  action  between  inspiratory 
and  expiratory  muscles  ;  we  shall  further  see  reason  to  think  that  the  pro- 
cesses which  start  the  expiratory  act  tend  to  check  or  inhibit  the  inspiratory 
act  and  vice  versa;  and  very  possibly  a  like  see-saw  of  stimulation  and  in- 
hibition obtains  in  the  muscles  of  the  alimentary  canal. 

It  must  be  remembered  that  the  circular  coat  is  always  much  thicker 
than  the  longitudinal  coat ;  and  we  may  infer  that  while  the  chief  work  of 
driving  the  contents  onward  falls  on  the  former,  the  latter  assists  the  work, 
•either  in  the  way  which  we  have  suggested  or  in  some  other  way. 

In  the  small  intestine  the  tube  is  hung  loosely  and  much  twisted,  so  that 
many  loops  are  formed ;  the  contents,  moreover,  are  largely  fluid.  Hence, 


294  THE  TISSUES   AND   MECHANISMS  OF   DIGESTION. 

the  steady  onward  movement,  such  as  is  seen  when  more  solid  contents  pass 
along  the  straight  and  somewhat  firmly  attached  oesophagus,  is  complicated 
by  movements  due  to  a  loop  being  projected  forward  by  the  entrance  of  fluid 
from  above,  or  being  dragged  down  by  the  weight  of  its  new  contents,  or, 
on  the  other  hand,  due  to  a  loop  being  retracted  by  the  driving  onward  of 
its  contents  and  the  emptying  of  itself,  and  the  like.  In  this  way  a  peculiar 
writhing  movement  of  the  bowel  is  brought  about,  and  the  phrase  "  peristal- 
tic movements  "  is  generally  used  to  denote  this  total  effect  of  the  contrac- 
tion of  the  muscular  coats ;  it  will,  however,  be  best  to  restrict  the  meaning 
to  the  progressive  contraction  of  the  circular  coat  assisted,  in  most  cases,  by 
a  similar  progressive  contraction  of  the  longitudinal  coat. 

§  229.  Mastication.  This  in  man  consists  chiefly  of  an  up-and-down 
movement  of  the  lower  jaw,  combined  in  the  grinding  action  of  the  molar 
teeth,  with  a  certain  amount  of  lateral  fore-and-aft  movement.  The  lower 
jaw  is  raised  by  means  of  the  temporal,  rnasseter,  and  internal  pterygoid 
muscles.  The  slighter  effort  at  depression  brings  into  action  chiefly  the 
digastric  muscle,  though  the  mylo-hyoid  and  genio-hyoid  probably  share  in 
the  matter.  Contraction  of  the  external  pterygoids  pulls  forward  the  con- 
dyles  and  thrusts  the  lower  teeth  in  front  of  the  upper.  Contraction  of  the 
pterygoids  on  one  side  will  also  throw  the  teeth  on  to  the  opposite  side.  The 
lower  horizontally  placed  fibres  of  the  temporal  serve  to  retract  the  jaw. 

During  mastication  the  food  is  moved  to  and  fro  and  rolled  about  by  the 
movements  of  the  tongue.  These  are  affected  by  the  muscles  of  that  organ 
governed  by  the  hypoglossal  nerve. 

The  act  of  mastication  is  a  voluntary  one,  guided  as  are  so  many  volun- 
tary acts,  not  only  by  muscular  sense,  but  also  by  contact  sensations.  The 
motor  fibres  of  the  fifth  cranial  nerve  convey  motor  impulses  from  the  brain 
to  the  above-mentioned  muscles ;  but  paralysis  of  the  sensory  fibres  of  the 
same  nerve  renders  mastication  difficult  by  depriving  the  will  of  the  aid  of 
the  usual  sensations. 

§  230.  Deglutition.  The  food  when  sufficiently  masticated  is,  by  the 
movements  of  the  tongue,  gathered  up  into  a  bolus  on  the  middle  of  the 
upper  surface  of  that  organ.  The  front  of  the  tongue  being  raised — partly 
by  its  intrinsic  muscles  and  partly  by  the  stylo-glossus — the  bolus  is  thrust 
back  between  the  tongue  and  the  palate  through  the  anterior  pillars  of  the 
fauces  or  isthmus  faucium.  Immediately  before  it  arrives  there  the  soft 
palate  is  raised  by  the  levator  palati,  and  so  brought  to  touch  the  posterior 
wall  of  the  pharynx,  which,  by  the  contraction  of  the  upper  margin  of  the 
superior  constrictor  of  the  pharynx,  bulges  somewhat  forward.  The  elevation 
of  the  soft  palate  causes  a  distinct  rise  of  pressure  in  the  nasal  chambers? 
this  can  be  shown  by  introducing  a  water  manometer  into  one  nostril  and 
closing  the  other  just  previous  to  swallowing.  By  the  contraction  of  the 
palato-pharyngeal  muscles  which  lie  in  the  posterior  pillars  of  the  fauces  the 
curved  edges  of  those  pillars  are  made  straight,  and  thus  tend  to  meet  in 
the  middle  line,  the  small  gap  between  them  being  filled  up  by  the  uvula. 
Through  these  manoeuvres  the  entrance  into  the  posterior  nares  is  blocked, 
while  the  soft  palate  is  formed  into  a  sloping  roof,  guiding  the  bolus  down 
the  pharynx.  By  the  contraction  of  the  stylo-pharyngeus  and  palato- 
pharyngeus,  the  funnel-shaped  bag  of  the  pharynx  is  brought  up  to  meet 
the  descending  morsel,  very  much  as  a  glove  may  be  drawn  up  over  the 
finger. 

Meanwhile  in  the  larynx,  as  shown  by  the  laryngoscope,  the  arytenoid 
cartilages  and  .vocal  cords  are  approximated,  the  latter  being  also  raised  so 
that  they  come  very  near  to  the  false  vocal  cords ;  and  the  cushion  at  the 
base  of  the  epiglottis  covers  the  rima  glottidis,  while  the  epiglottis  itself  is 


THE  MUSCULAR  MECHANISMS  OF  DIGESTION.  295 

depressed  over  the  larynx.  The  thyroid  cartilage  is  now,  by  the  action  of 
the  laryngeal  muscles,  suddenly  raised  up  behind  the  hyoid  bone,  and  thus 
assists  the  epiglottis  to  cover  the  glottis.  This  movement  of  the  thyroid  can 
easily  be  felt  on  the  outside.  Thus,  both  the  entrance  into  the  posterior 
nares  and  that  into  the  larynx  being  closed,  the  impulse  given  to  the  bolus 
by  the  tongue  can  have  no  other  effect  than  to  propel  it  beneath  the  sloping 
soft  palate  over  the  incline  formed  by  the  root  of  the  tongue  and  the  epi- 
glottis. The  palato-glossi  or  constrictores  isthmi  faucium,  which  lie  in  the 
anterior  pillars  of  the  fauces,  by  contracting  close  the  door  behind  the  food 
which  has  passed  them. 

When  the  bolus  of  food  is  large  it  is  received  by  the  middle  and  lower 
constrictors  of  the  pharynx,  which,  contracting  in  sequence  from  above 
downward,  thrust  it  into  the  oesophagus,  along  which  it  is  driven  by  a  similar 
series  of  successive  contractions  which  we  shall  speak  of  immediately  as 
peristaltic  action.  This  comparatively  slow  descent  of  the  food  from  the 
pharynx  into  the  stomach  may  be  readily  seen  if  animals  with  long  necks, 
such  as  horses  and  dogs,  be  watched  while  swallowing.  When,  however,  the 
morsel  is  not  large,  or  when  the  substance  swallowed  is  liquid,  the  movement 
of  the  back  part  of  the  tongue  may  be  sufficient  not  merely  to  introduce  the 
food  into  the  grasp  of  the  constrictors  of  the  pharynx,  but  even  to  propel  it 
rapidly,  to  shoot  it,  in  fact,  along  the  lax  oesophagus  before  the  muscles  of 
that  organ  have  time  to  contract.  In  such  a  mode  of  swallowing  the  middle 
and  lower  constrictors  take  little  or  no  part  in  driving  the  food  onward, 
though  they  and  the  oesophagus  appear  to  contract  from  above  downward 
after  the  food  has  passed  by  them,  as  if  to  complete  the  act  and  to  insure 
that  nothing  has  been  left  behind.  Deglutition  in  this  fashion  still  remains 
possible  after  these  constrictors  have  become  paralyzed  by  section  of  their 
motor  nerves. 

When  a  second  act  of  deglutition  succeeds  the  first  with  sufficient  rapid- 
ity, the  nervous  changes  which  start  the  pharyngeal  movements  of  the  second 
act  appear  to  inhibit  the  cesophageal  movements  of  the  first  act ;  and  when 
swallowing  is  repeated  rapidly  several  times  in  succession,  the  oesophagus 
remains  quiet  and  lax  during  the  whole  time  until  immediately  after  the  last 
swallow,  when  a  peristaltic  movement  closes  the  series. 

When  the  stethoscope  is  applied  over  the  oesophagus,  at  different  regions, 
a  sound  is  heard  during  deglutition  ;  sometimes  two  sounds  are  heard.  The 
first  and  most  constant  is  coincident  with  the  passage  of  the  bolus,  and  is  due 
to  this  and  to  the  muscular  sound  of  the  contracting  muscles.  The  later  and 
less  constant  sound  appears  to  be  caused  by  a  quantity  of  air-bubbles  with 
which  the  bolus  was  entangled,  lodged  at  the  cardiac  end  of  the  oesophagus, 
being  forced  into  the  stomach  by  the  sequent  peristaltic  contraction  of  the 
oesophagus. 

It  will  be  seen  from  what  has  been  said  that  deglutition,  though  a  con- 
tinuous act,  may  be  regarded  as  divided  into  three  stages  :  The  first  stage 
is  the  thrusting'of  the  food  through  the  isthmus  faucium  ;  this  may  be  either 
of  long  or  short  duration.  The  second  stage  is  the  passage  through  the 
upper  part  of  the  pharynx.  Here  the  food  traverses  a  region  common  both 
to  the  food  and  to  respiration,  and  in  consequence  the  movement  is  as  rapid 
as  possible.  The  third  stage  is  the  descent  through  the  grasp  of  the  con- 
strictors. Here  the  food  has  passed  the  respiratory  orifice,  and  in  conse- 
quence its  passage  again  becomes  comparatively  slow,  except  in  case  of 
fluids  and  small  morsels,  when,  as  we  have  seen,  it  may  continue  to  be  rapid. 
The  passage  along  the  oesophagus  may,  perhaps,  be  regarded  as  constituting 
a  fourth  stage  ;  but  it  will  be  more  convenient  to  consider  the  cesophageal 
movements  bv  themselves. 


296  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

The  first  stage  in  this  complicated  process  is  undoubtedly  a  voluntary 
act.  The  raising  of  the  soft  palate  and  the  approximation^  the  poste- 
rior pillars  may  also  be  at  times  voluntary,  since  they  have  been  seen,  in  a 
case  where  the  pharynx  was  laid  bare  by  an  operation,  to  take  place  before 
the  food  had  touched  these  parts ;  but  the  movement  may  take  place 
without  any  exercise  of  the  will  and  in  the  absence  of  consciousness. 
Indeed,  the  second  stage,  taken  as  a  whole,  though  some  of  the  earlier 
component  movements  are,  as  it  were,  on  the  borderland  between  the  volun- 
tary and  involuntary  kingdoms,  must  be  regarded  as  a  reflex  act.  The 
third  and  last  stage,  whatever  be  the  exact  form  which  it  takes,  is  undoubt- 
edly reflex  ;  the  will  has  no  power  whatever  over  it,  and  can  neither  origi- 
nate, stop,  nor  modifiy  it. 

Deglutition  in  fact  as  a  whole  is  a  reflex  act ;  it  cannot  take  place  unless 
some  stimulus  be  applied  to  the  mucous  membrane  of  the  fauces.  When 
we  voluntarily  bring  about  swallowing  movements  with  the  mouth  empty, 
we  supply  the  necessary  stimulus  by  forcing  with  the  tongue  a  small 
quantity  of  saliva  into  the  fauces,  or  by  touching  the  fauces  with  the  tongue 
itself. 

In  the  reflex  act  of  deglutition,  caused  in  the  ordinary  way  by  the  food 
coming  in  contact  with  the  fauces,  the  afferent  impulses  originated  in  the 
fauces  are  carried  up  to  the  nervous  centre  by  the  glosso-pharyngeal  nerve, 
by  branches  of  the  fifth,  and  by  the  pharyngeal  branches  of  the  superior 
laryngeal  division  of  the  vagus.  The  latter  seem  of  special  importance, 
since  the  act  of  swallowing,  quite  apart  from  the  presence  of  food  in  the 
mouth,  may  be  brought  about  by  centripetal  stimulation  of  the  superior  laryn- 
geal nerve.  The  efferent  impulses  descend  the  hypoglossal  to  the  muscles 
of  the  tongue,  and  pass  down  the  glosso-pharyngeal,  the  vagus  through  the 
pharyngeal  plexus,  the  fifth,  and  the  spinal  accessory,  to  the  muscles  of  the 
fauces  and  pharynx  ;  their  exact  paths  being  as  yet  not  fully  known,  and 
probably  varying  in  different  animals.  The  laryngeal  muscles  are  governed 
by  the  laryngeal  branches  of  the  vagus. 

The  centre  of  the  reflex  act  lies  in  the  medulla  oblongata.  Deglutition 
can  be  excited,  by  tickling  the  fauces,  in  an  animal  rendered  unconscious 
by  removal  of  the  brain,  provided  the  medulla  be  left.  If  the  medulla  be 
destroyed,  deglutition  is  impossible.  The  centre  for  deglutition  lies  higher 
up  than  that  of  respiration,  so  that  in  the  diseases  or  injuries  involving 
the  upper  part  of  the  medulla  oblongata,  the  former  act  may  be  impaired 
or  rendered  impossible  while  the  latter  remains  untouched.  It  has  been 
said  to  form  part  of  the  superior  olivary  bodies,  but  this  view  is  based  on 
anatomical  grounds  only.  We  shall  have  to  deal  with  this  and  similar 
matters  in  treating  of  the  central  nervous  system.  It  is  probable  that,  as 
is  .the  case  in  so  many  other  reflex  acts,  the  whole  movement  can  be  called 
forth  by  stimuli  affecting  the  centre  directly,  and  not  acting  on  the  usual 
afferent  nerves. 

§  231.  Movements  of  the  oesophagus.  These,  as  we  have  just  said,  are 
fairly  simple.  The  circular  contraction  begun  by  the  constrictors  of  the 
pharynx  is  continued  along  the  circular  coat  of  the  oesophagus,  and  assisted 
by  an  accompanying  contraction  of  the  longitudinal  coat,  the  direction  being 
always,  save  in  the  abnormal  action  of  vomiting,  from  above  downward. 

It  will  be  remembered  that  the  muscular  bundles  of  the  oesophagus  are 
composed  of  striated  fibres  in  the  upper  part,  and  of  plain  unstriated  fibre 
cells  in  the  lower  part,  the  transition  occupying  a  different  level  in  different 
animals.  Nevertheless,  as  far  as  the  peristaltic  movement  is  concerned,  the 
two  kinds  of  fibres  behave  in  the  same  way,  except  that  the  peristaltic  wave, 
if  we  may  so  call  it,  travels  more  rapidly  in  the  striated  region. 


THE  MUSCULAR  MECHANISMS  OF  DIGESTION.  297 

These  peristaltic  movements  of  the  oesophagus  may,  like  those  of  the  in- 
testine, be  seen  after  removal  of  the  organ  from  the  body  ;  and,  indeed,  may 
continue  to  appear,  upon  stimulation,  for  an  unusual  length  of  time.  They 
may,  therefore,  be  carried  out  by  the  muscular  elements,  with  or  without  the 
help  of  the  nervous  elements  imbedded  in  them,  apart  from  any  action  of 
the  central  nervous  system.  Nevertheless,  in  the  living  body,  the  move- 
ments of  the  oesophagus  seem  to  be  in  a  special  way  dependent  on  the  central 
nervous  system ;  the  contractions  are  not  started  and  carried  out  by  the 
walls  of  the  tube  alone,  and  so  transmitted  from  section  to  section  in  the 
walls  of  the  tube  itself;  but  afferent  impulses  started  in  the  pharvnx  and 
passing  to  the  medulla  oblongata,  give  rise  to  reflex  efferent  impulses  which 
descend  along  nervous  tracts  to  successive  portions  of  the  organ.  If  the 
oesophagus  be  cut  across  some  way  down,  or  if  a  portion  of  the  middle  region 
be  excised,  stimulation  of  the  pharynx  will  produce  a  peristaltic  contrac- 
tion, which  travelling  downward  will  not  stop  at  the  cut  or  excision,  but 
will  be  continued  on  into  the  lower  disconnected  portion  by  means  of  the 
central  nervous  system.  And  it  is  stated  that  ordinary  peristaltic  contrac- 
tions of  the  lower  part  of  the  oesophagus  can  be  readily  excited  by  stimu- 
lation of  the  pharynx,  but  not  by  stimuli  applied  to  its  own  mucous  mem- 
brane. In  the  reflex  act  which  thus  brings  about  the  peristaltic  contraction 
of  the  oesophagus  the  afferent  nerves  are  those  of  the  pharynx,  viz.,  the 
superior  laryngeal  nerve  and  pharyngeal  branches  of  the  vagus,  branches 
of  the  fifth,  and  in  some  animals,  at  least,  branches  of  the  glosso-pharyngeal, 
but  chiefly  the  first ;  and  oesophageal  movements  can  easily  be  excited  by 
centripetal  stimulation  of  the  superior  laryngeal.  -The  centre  lies  in  the 
medulla  oblongata,  being  a  part  of  the  general  deglutition  centre  ;  and  effer- 
ent impulses  pass  along  fibres  of  the  vagus,  reaching  the  upper  part  of  the 
oesophagus  by  the  recurrent  laryngeal  nerves,  and  the  lower  part  through 
the  oesophageal  plexus  of  the  vagus.  Section  of  the  trunk  of  the  vagus 
renders  difficult  the  passage  of  food  along  the  oesophagus,  and  stimulation 
of  the  peripheral  stump  causes  oesophageal  contractions. 

The  force  of  this  movement  in  the  oesophagus  is  considerable ;  thus  in 
the  dog  a  ball,  pulling  by  means  of  a  pulley  against  a  weight  of  250 
grammes,  has  been  found  to  be  readily  carried  down  from  the  pharynx  to 
the  stomach. 

At  the  junction  of  the  oesophagus  with  the  stomach  the  circular  fibres 
usually  remain  in  a  more  or  less  permanent  condition  of  tonic  or  obscurely 
rhythmic  contraction,  more  particularly  when  the  stomach  is  full  of  food, 
and  thus  serve  as  a  sphincter  to  prevent  the  return  of  food  from  the  stomach 
into  the  oesophagus.  Upon  the  arrival  of  the  bolus  of  food  at  the  end  of 
the  oesophagus,  the  centre  for  this  sphincter  is  inhibited  and  the  orifice  is 
thus  opened  up.  Possibly  the  patency  of  the  orifice  is  still  further  secured 
by  a  contraction  of  the  longitudinal  muscular  fibres  which  radiate  from  the 
end  of  the  oesophagus  over  the  stomach. 

§  232.  Movements  of  the  stomach.  While  the  object  of  the  oesophageal 
movement  is  simply  to  carry  the  swallowed  bolus  with  all  due  speed  to  the 
stomach,  and  while  the  intestinal  movement  has,  in  like  manner,  simply  to 
carry  the  intestinal  contents  onward,  the  twisted  course  of  the  looped  path 
insuring  all  the  mixing  of  the  constituents  of  the  contents  that  may  be 
necessary,  the  movements  of  the  stomach  have  a  double  object :  on  the  one 
hand  to  provide  an  adequate  exposure  of  the  contents  of  the  dilated  chamber 
to  the  influence  of  the  gastric  juice,  and  on  the  other  to  propel  the  partially 
digested  food,  when  ready,  into  the  duodenum.  We  may,  accordingly,  dis- 
tinguish between  what  we  may  call  the  "  churning  "  and  the  "  propulsive  " 
movements  of  the  stomach. 


298  THE   TISSUES  AND   MECHANISMS   OF   DIGESTION. 

When  the  stomach  is  empty  all  the  muscular  fibres,  as  we  have  said, 
longitudinal,  circular,  and  oblique,  fall  into  a  condition  which  we  may  per- 
haps speak  of  as  an  obscure  tonic  contraction.  The  whole  stomach  is'small 
and  contracted,  its  cavity  is  nearly  obliterated,  and  the  mucous  membrane, 
owing  to  the  predominance  of  the  circular  coat,  is,  like  the  lining  membrane 
of  an  empty  artery,  thrown  into  longitudinal  folds.  As  more  and  more  food 
enters  the  stomach  all  the  coats  become  relaxed,  with  the  exception  of  the 
pyloric  sphincter,  which  remains  at  first  permanently  closed,  and  the  less 
marked  cardiac  sphincter,  which  merely  relaxes  from  time  to  time  at  each 
act  of  swallowing.  No  sooner,  however,  do  the  coats  thus  become  relaxed 
than  they  set  up  obscure  rhythmical  peristaltic  contractions,  giving  rise  to 
the  "churning"  movements.  These  movements  have  been  described  as  of 
such  a  kind  that  the  contents  flow  in  a  main  current  from  the  cardia  along 
the  greater  curvature  to  the  pylorus,  and  back  to  the  cardia  along  the  lesser 
curvature,  subsidiary  currents  mixing  the  peripheral  portions  of  the  con- 
tents with  the  more  central ;  it  may  be  doubted,  however,  whether  any  such 
regularity  of  flow  is  marked  or  constant,  and  it  is  not  easy  to  see  by  what 
combination  and  sequence  of  contractions  in  the  three  coats,  longitudinal, 
circular,  and  oblique,  such  a  regular  flow  can  be  produced.  But  in  any 
case,  by  such  rhythmical  contractions  the  food  and  gastric  juice  are 
rolled  about  and  mixed  together.  These  churning  movements  are  fee- 
ble at  first,  even  though  the  stomach  be  filled  and  distended  by  a  large 
meal  rapidly  eaten ;  they  become  more  and  more  pronounced  as  digestion 
proceeds. 

Before  digestion  has  proceeded  very  far  the  "  propulsive  "  movements 
begin.  These  occur  at  intervals,  and  are  repeated  at  first  slowly  but  after- 
ward more  rapidly.  Each  movement  consists  in  a  contraction  of  the  circu- 
lar muscular  fibres  more  powerful  than  any  taking  part  in  the  churning  move- 
ments, and  leading  to  a  circular  constriction  which,  beginning  apparently  at 
about  the  obscurely  defined  groove  which  marks  the  beginning  of  the  antrum 
pylori,  travels  down  toward  the  pylorus,  propelling  the  food  onward.  This 
movement  is  accompanied  or  rather  preceded  by  a  relaxation  of,  that  is  to 
say,  in  all  probability  an  inhibition  of  the  permanent  contraction  of,  the 
sphincter  pylori  itself*  in  order  that  the  gastric  contents  may  pass  into  the 
duodenum.  But  the  occurrence  of  this  relaxation  is  determined  by  the  na- 
ture of  the  gastric  contents ;  for  if  the  propulsive  movement  drives  large 
undigested  pieces  toward  the  pylorus,  the  sphincter  is  apt  to  close  again,  the 
result  of  which  is  that  the  undigested  morsels  are  carried  back  into  the  main 
body  of  the  stomach. 

The  combined  effect,  then,  of  the  churning  and  of  the  propulsive  move- 
ments is,  after  a  certain  part  of  the  meal  has  been  reduced  to  a  thick  fluid 
condition  somewhat  resembling  pea  soup  and  often  called  chyme,  to  strain 
off  this  more  fluid  part  into  the  duodenum,  and  to  submit  the  remaining  still 
solid  pieces  to  the  further  action  of  the  gastric  juice. 

As  digestion  proceeds,  more  and  more  material  leaves  the  stomach,  winch 
is  thus  gradually  emptied,  the  last  portions  which  are  carried  through  being 
those  parts  of  the  food  which  are  least  digestible,  and  any  wholly  indigesti- 
ble foreign  bodies  which  happen  to  have  been  swallowed  ;  the  latter  may 
perhaps  never  leave  the  stomach  at  all.  The  presence  of  food  leads  to  the 
development  of  the  movements  ;  but  evidently  it  is  not  the  mere  mechanical 
repletion  of  the  organ  which  is  the  cause  of  the  movements,  since  the  stomach 
is  fullest  at  the  beginning  when  the  movements  are  slight,  and  becomes 
emptier  as  they  grow  more  forcible.  The  one  thing  which  does  increase 
pari  passu  with  the  movements  is  the  acidity,  which  is  at  a  minimum  when 
the  (generally  alkaline)  food  has  been  swallowed,  and  increases  steadily  on- 


THE  MUSCULAR  MECHANISMS  OF  DIGESTION.  299 

ward.  It  has  not,  however,  been  definitely  shown  that  the  increasing  acidity 
is  the  efficient  stimulus  giving  rise  to  the  movements. 

The  movements  of  even  a  full  stomach  are  said  to  cease  during  sleep. 
The  nervous  mechanism  of  the  gastric  movements  had  better  be  considered 
in  connection  with  that  of  the  intestinal  movements. 

§  233.  Vomiting.  In  a  conscious  individual  this  act  is  preceded  by  feel- 
ings of  nausea,  during  which  a  copious  flow  of  saliva  into  the  mouth  takes 
place.  This  being  swallowed  carries  down  with  it  a  certain  quantity  of  air, 
the  presence  of  which  in  the  stomach,  by  assisting  in  the  opening  of  the  car- 
diac sphincter,  subsequently  facilitates  the  discharge  of  the  gastric  contents. 
The  nausea  is  generally  succeeded  at  first  by  ineffectual  retching  in  which 
a  deep  inspiratory  effort  is  made,  so  that  the  diaphragm  is  thrust  down  as  low 
as  possible  against  the  stomach,  the  lower  ribs  being  at  the  same  time  forcibly 
drawn  in ;  since  during  this  inspiratory  effort  the  glottis  is  kept  closed,  no 
air  can  enter  into  the  lungs ;  but  some  is  drawn  into  the  pharynx,  and  thence 
probably  descends  by  a  swallowing  action  into  the  stomach.  'When  retching 
passes  on  to  actual  vomiting  this  inspiratory  effort  is  succeeded  by  a  sudden 
violent  expiratory  contraction  of  the  abdominal  walls,  the  glottis  still  being 
closed,  so  that  the  whole  force  of  the  effort  is  spent,  as  we  shall  see  it  is 
in  defecation,  in  pressure  on  the  abdominal  contents.  The  stomach  is,  there- 
fore, forcibly  compressed  from  without.  At  the  same  time,  or  rather  imme- 
diately before  the  expiratory  effort,  by  a  contraction  of  its  longitudinal  fibres 
the  oesophagus  is  shortened  and  the  cardiac  orifice  of  the  stomach  brought 
close  under  the  diaphragm,  while  apparently  by  an  inhibition  of  the  circular 
sphincter,  aided  perhaps  by  a  contraction  of  the  fibres  which  radiate  from 
the  end  of  the  oesophagus  over  the  stomach,  the  cardiac  orifice,  which  is 
normally  closed,  is  somewhat  suddenly  dilated.  This  dilatation  opens  the  way 
for  the  con  tents' of  the  stomach,  which,  pressed  upon  by  the  contraction  of 
the  abdomen,  and  to  a  certain  but  probably  only  to  a  slight  extent  by  the 
contraction  of  the  gastric  walls,  are  driven  forcibly  up  the  oesophagus.  The 
mouth  being  widely  open,  and  the  neck  stretched  to  afford  as  straight  a 
course  as  possible,  the  vomit  is  ejected  from  the  body.  At  this  moment  there 
is  an  additional  expiratory  effort  which  serves  to  prevent  the  vomit  passing 
into  the  larynx.  In  most  cases,  too,  the  posterior  pillars  of  the  fauces  are 
approximated,  in  order  to  close  the  nasal  passage  against  the  ascending  stream. 
This,  however,  in  severe  vomiting  is  frequently  ineffectual. 

Thus  in  vomiting  there  are  two  distinct  acts:  the  dilatation  of  the  cardiac 
orifice  and  the  extrinsic  pressure  of  the  abdominal  walls  in  an  expiratory 
effort.  Without  the  former  the  latter,  even  when  distressingly  vigorous,  is 
ineffectual.  Without  the  latter,  as  in  urari  poisoning,  the  intrinsic  move- 
ments of  the  stomach  itself  are  rarely  efficient  to  do  more  than  eject  gas, 
and,  it  may  be,  a  very  small  quantity  of  food  or  fluid.  Pyrosis  or  water- 
brash  is,  however,  probably  brought  about  by  this  intrinsic  action  of  the 
stomach. 

During  vomiting  the  pylorus  is  generally  closed,  so  that  but  little  mate- 
rial escapes  into  the  duodenum.  When  the  gall-bladder  is  full,  a  copious 
flow  of  bile  into  the  duodenum  accompanies  the  act  of  vomiting.  Part  of 
this  may  find  its  way  into  the  stomach,  as  in  bilious  vomiting,  the  pylorus 
then  having  evidently  been  opened. 

The  nervous  mechanism  of  vomiting  is  complicated  and  in  many  aspects 
obscure.  The  efferent  impulses  which  cause  the  expiratory  effort  must  come 
from  the  respiratory  centre  in  the  medulla ;  with  these  we  shall  deal  in 
speaking  of  respiration.  The  dilatation  of  the  cardiac  orifice  is  caused,  in 
part  at  least,  by  impulses  descending  the  vagi,  since  when  these  are  cut  real 
vomiting  with  discharge  of  the  gastric  contents,  if  it  takes  place  at  all,  be- 


300  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

comes  difficult  through  want  of  readiness  in  the  dilatation.  Such  intrinsic 
movements  of  the  stomach  as  do  take  place  and  the  movements  of  the 
oesophagus  appear  to  be  carried  out  by  the  usual  nerves.  The  efferent  im- 
pulses which  cause  the  flow  of  saliva  in  the  introductory  nausea  also  descend 
along  the  usual  nerves  such  as  the  chorda  tympani.  These  various  impulses 
may  best  be  considered  as  starting  from  a  vomiting  centre  in  the  medulla, 
having  close  relations  with  the  respiratory  centre.  This  centre  may  be  ex- 
cited, may  be  thrown  into  action,  in  a  reflex  manner,  by  stimuli  applied  to 
peripheral  nerves,  as  when  vomiting  is  induced  by  tickling  the  fauces,  or  by 
irritation  of  the  gastric  membrane,  or  by  obstruction  of  the  intestine  due  to 
ligature,  hernia,  etc.  That  the  vomiting  in  the  last  instance  is  due  to  nervous 
action,  and  not  to  any  regurgitation  of  the  intestinal  contents,  is  shown  by 
the  fact  that  it  will  take  place  when  the  intestine  is  perfectly  empty  and 
may  be  prevented  by  section  of  the  mesenteric  nerves.  The  vomiting  at- 
tending renal  and  biliary  calculi  is  apparently  also  reflex  in  origin.  Vom- 
iting in  fact,  as  a  rule,  is  a  reflex  action,  the  afferent  impulses  passing  along 
one  or  other  nerves,  but  most  frequently  along  those  connected  with  the 
alimentary  canal,  that  is,  along  afferent  fibres  running  in  the  vagus  or  in  the 
splanchnic  nerves.  The  centre,  however,  may  be  affected  directly,  as  prob- 
ably in  the  cases  of  some  poisons,  and  in  some  instances  of  vomiting  from 
disease  of  the  medulla  oblongata,  Lastly,  it  may  be  thrown  into  action  by 
impulses  reaching  it  from  parts  of  the  brain  higher  up  than  itself,  as  in  cases 
of  vomiting  produced  by  smells,  tastes,  or  emotions,  or  by  the  recollection 
of  past  events,  and  in  some  cases  of  vomiting  due  to  cerebral  disease. 

Many  emetics,  such  as  tartar  emetic,  appear  to  act  directly  on  the  centre, 
since,  introduced  into  the  blood,  they  will  produce  vomiting  after  a  bladder 
has  been  substituted  for  the  whole  stomach.  Others  again,  such  as  mustard 
and  water,  act  in  a  reflex  manner  by  irritation  of  the  gastric  mucous  mem- 
brane. With  others,  again,  which  cause  vomiting  by  developing  a  nauseous 
taste,  the  action  involves  parts  of  the  brain  higher  than  the  centre  itself. 

§  234.  Movements  of  the  small  intestine.  These,  as  we  have  already  said, 
are  the  typical  peristaltic  movements,  simple  except  in  so  far  as  they  are 
complicated  by  the  existence  of  the  pendent  loops,  the  peculiar  oscillating 
movements  of  which  appear  to  be  produced  chiefly  by  the  longitudinal  fibres. 

The  peristaltic  movements,  as  a  rule,  take  place  from  above  downward, 
and  a  wave  beginning  at  the  pylorus  may  be  traced  a  long  way  down.  But 
contractions  may,  and  in  all  probability  occasionally  do,  begin  at  various 
points  along  the  length  of  the  intestine.  A  movement  started  by  artificial 
stimulation  some  way  down  the  intestine,  may  travel  not  only  downward  but 
also  upward  ;  it  has  been  disputed,  however,  whether  in  the  living  body  any 
natural  backward  peristaltic  movement  really  takes  place.  In  the  living 
body  the  intestines  have  periods  of  rest,  alternating  with  periods  of  activity, 
the  occurrence  of  the  periods  depending  on  various  circumstances ;  the 
intensity  of  the  movements  also  varies  very  considerably. 

§  235.  Movements  of  the  large  intestine.  They  are  fundamentally  the 
same  as  those  of  the  small  intestine,  but  distinct  in  so  far  as  the  latter  cease 
at  the  ileo-csecal  valve,  at  which  spot  the  former  normally  begin  ;  they  are 
simpler,  inasmuch  as  the  pendent  loops  are  absent,  and  not  so  vigorous,  since 
relatively  to  the  diameter  of  the  tube  the  amount  of  muscular  fibre  is  less. 
Along  the  colon  where  the  sacculi  are  well  developed  the  movement  may 
perhaps  be  described  as  almost  intermittent  from  sacculus  to  sacculus,  the 
contents  of  one  sacculus  being  driven  by  the  peristaltic  contractions  of  its 
circular  fibres  into  the  next  sacculus,  which  prepares  to  receive  them  by  a 
relaxation  of  its  circular  and  a  contraction  of  its  longitudinal  fibres. 

Since  the  lips  of  the  ileo-caecal  valve  are  placed  transversely  across  the 


THE  MUSCULAR  MECHANISMS  OF  DIGESTION.  301 

caecum,  not  only  does  distention  of  the  csecum,  by  stretching  the  valve  along 
the  line  of  the  lips,  bring  them  into  apposition,  but  the  pressure  exerted  by 
the  peristaltic  movement  has  the  same  effect.  In  this  way  any  return  of  the 
contents  from  the  large  to  the  small  intestine  is  prevented. 

Arrived  at  the  sigmoid  flexure,  the  contents,  now  more  or  less  solid  feces, 
are  supported  by  the  bladder  and  the  sacrum,  so  that  they  do  not  press  on 
the  sphincter  ani. 

§  236.  Defecation.  This  is  a  mixed  act,  being  superficially  the  result  of 
an  effort  of  the  will,  and  yet  carried  out  by  means  of  an  involuntary  mechan- 
ism. Part  of  the  voluntary  effort  consists  in  producing  a  pressure-effect,  by 
means  of  the  abdominal  muscles.  These  are  contracted  forcibly  as  in  ex- 
piration, but  the  glottis  being  closed  and  the  escape  of  air  from  the  lungs 
prevented,  the  whole  force  of  the  pressure  is  brought  to  bear  on  the  abdo- 
men itself,  and  so  drives  the  contents  of  the  descending  colon  onward  toward 
the  rectum.  The  sigmoid  flexure  is  by  its  position  sheltered  from  this  pres- 
sure ;  a  body  introduced  per  anum  into  the  empty  rectum  is  not  affected  by 
even  forcible  contractions  of  the  abdominal  walls. 

The  anus  is  guarded  by  the  sphincter  ani,  which  is  habitually  in  a  state 
of  normal  tonic  contraction,  capable  of  being  increased  or  diminished  by  a 
stimulus  applied,  either  internally  or  externally,  to  the  anus.  The  tonic 
contraction  is  in  part  at  least  due  to  the  action  of  a  nervous  centre  situated 
in  the  lumbar  spinal  cord.  If  the  nervous  connection  of  the  sphincter  with 
the  spinal  cord  be  broken,  relaxation  takes  place.  If  the  spinal  cord  be 
divided  somewhat  higher  up,  for  instance  in  the  dorsal  region,  the  sphincter, 
after  the  depressing  effect  of  the  operation,  which  may  last  several  days,  has 
passed  off,  regains  and  subsequently  maintains  its  tonicity,  showing  that  the 
centre  is  not  placed  higher  up  than  the  lumbar  region  of  the  cord.  The 
increased  or  diminished  contraction  following  on  local  stimulation  is  prob- 
ably due  to  reflex  augmentation  or  inhibition  of  the  action  of  this  centre. 
The  centre  is  also  subject  to  influences  proceeding  from  higher  regions  of  the 
cord,  and  from  the  brain.  By  the  action  of  the  will,  by  emotions,  or  by 
other  nervous  events,  the  lumbar  sphincter  centre  may  be  inhibited,  and 
thus  the  sphincter  itself  relaxed  ;  or  augmented,  and  thus  the  sphincter 
tightened.  A  second  item,  therefore,  of  the  voluntary  process  in  defecation 
is  the  inhibition  of  the  lumbar  sphincter  centre,  and  consequent  relaxation 
of  the  sphincter  muscle.  Since  the  lumbar  centre  may  remain  wholly  effi- 
cient when  separated  from  the  brain,  the  paralysis  of  the  sphincter  which 
occurs  in  certain  cerebral  diseases  is  probably  due  to  inhibition  of  this  lum- 
bar centre,  and  not  to  paralysis  of  any  cerebral  centre. 

Thus  a  voluntary  contraction  of  the  abdominal -walls,  accompanied  by  a 
relaxation  of  the  sphincter,  might  press  the  contents  of  the  descending  colon 
into  the  rectum  and  out  at  the  anus.  Since,  however,  as  we  have  seen,  the 
pressure  of  the  abdominal  walls  is  warded  off  the  sigmoid  flexure,  such  a 
mode  of  defecation  would  always  end  in  leaving  the  sigmoid  flexure  full. 
Hence  the  necessity  for  these  more  or  less  voluntary  acts  being  accompanied 
by  an  involuntary  augmentation  of  the  peristaltic  action  of  the  large  intes- 
tine, sigmoid  flexure,  and  rectum. 

In  the  movements  of  the  rectum  we  can  trace  out  more  distinctly  than  in 
other  regions  of  the  alimentary  canal  the  separate  actions  of  the  longitudinal 
and  circular  fibres.  The  former,  by  means  of  contractions  travelling  from 
above  downward,  shorten  the  rectum,  and,  since  the  anus  affords  a  more  or 
less  fixed  support,  pull  the  rectum  and  its  contents  down  ;  the  latter,  by 
means  of  contractions  travelling  from  above  downward  but  taking  place 
somewhat  later,  narrow  the  rectum  and  so  squeeze  the  contents  onward  and 
outward. 


302  THE  TISSUES  AND  MECHANISMS   OF  DIGESTION. 

Defecation  then  appears  to  take  place  in  the  following  manner :  The 
large  intestine  and  sigmoid  flexure  becoming  more  and  more  full,  stronger 
and  stronger  peristaltic  action  is  excited  in  their  walls.  By  this  means  the 
feces  are  driven  into  the  rectum,  and  so,  by  a  continuance  of  the  movements, 
increasing  in  vigor,  against  the  sphincter.  Through  a  voluntary  act,  or 
sometimes  at  least  by  a  simple  reflex  action,  the  lumbar  sphincter  centre  is 
inhibited  and  the  sphincter  relaxed.  At  the  same  time  the  contraction  of 
the  abdominal  muscles  presses  firmly  on  the  descending  colon,  and  thus, 
contractions  of  the  levator  ani  assisting,  the  contents  of  the  rectum  are 
ejected. 

It  must,  however,  be  remembered  that,  while  in  appealing  to  our  own  con- 
sciousness, the  contraction  of  the  abdominal  walls  and  the  relaxation  of  the 
sphincter  seem  purely  voluntary  efforts,  the  whole  act  of  defecation,  includ- 
ing both  of  these  seemingly  so  voluntary  components,  may  take  place  in  the 
absence  of  consciousness,  and  indeed,  in  the  case  of  the  dog  at  least,  after 
the  complete  severance  of  the  lumbar  from  the  dorsal  cord.  In  such  cases 
the  whole  act  must  be  purely  reflex,  excited  by  the  presence  of  feces  in  the 
rectum. 

§  237.  The  nervous  mechanisms  of  gastric  and  intestinal  movements.  Both 
the  stomach  and  intestines  when  removed  from  the  body  and  thus  wholly 
separated  from  the  central  nervous  system  may,  by  direct  stimulation,  be 
readily  excited  to  movements ;  and  indeed  in  the  absence  of  all  obvious 
stimuli  movements  which  seem  to  be  spontaneous  may  at  times  be  observed. 
The  movements  of  which  we  are  speaking  are  orderly  movements  of  a 
peristaltic  nature,  not  mere  local  contractions  of  a  few  bundles  of  plain 
muscular  fibres.  The  alimentary  canal,  therefore,  like  the  heart,  though  to 
a  less  degree,  possesses  within  itself  such  mechanisms  as  are  requisite  for 
carrying  out  its  own  movements ;  and,  as  in  the  case  of  the  heart,  there  is 
no  adequate  evidence  that  the  ganglia  scattered  in  its  muscular  walls — 
namely,  those  forming  the  plexus  of  Auerbach — play  any  prime  part  in 
developing  these  movements. 

On  the  other  hand,  powerful  movements  of  a  peristaltic  kind  may  be 
induced,  not  only  as  we  have  already  seen  in  the  oesophagus  but  also  in  the 
stomach,  in  the  small  intestines,  and  even  in  the  large  intestines  by  stimula- 
tion of  the  vagus  nerve. 

The  chief  and  usual  cause  of  the  movements  of  the  stomach  and  intestines 
is  the  presence  of  food  in  their  interior.  But  we  do  not  know  definitely  the 
exact  manner  in  which  the  food  produces  the  movement.  It  may  be  that 
the  food,  by  stimulating  the  mucous  membrane,  sends  up  afferent  impulses, 
and  that  these  give  rise  by  reflex  action  to  efferent  impulses  which  descend 
the  vagus  fibres  to  successive  portions  of  the  canal,  in  a  manner  similar  to 
that  already  described  in  reference  to  the  oesophagus.  If  this  be  so  the 
efferent  impulses  reach  the  stomach  and  upper  part  of  the  duodenum  by  the 
terminal  portions  of  the  two  vagi,  Fig.  89,  JR.  V.,  L.  K,  and  reach  the  intes- 
tines by  the  portion  of  the  right  or  posterior  vagus,  Fig.  89,  E'.  V'.,  which 
passes  into  the  solar  plexus  and  thence  by  the  mesenteric  nerves.  The 
afferent  impulses  from  the  stomach  travel  also  apparently  by  the  vagus  ;  the 
paths  of  those  from  the  intestines  have  not  yet  been  determined. 

But  that  such  a  reflex  action  through  vagus  fibres  is  not  the  only  means 
by  which  the  presence  of  food  brings  about  the  movements  in  question,  is 
shown  by  the  fact  that  these  continue  to  be  developed  after  section  of  both 
vagus  nerves.  Probably  the  whole  action  is  a  mixed  one  which  we  may 
picture  to  ourselves  somewhat  as  follows :  The  alimentary  canal  possesses  a 
power  of  spontaneous  movement,  feeble  it  is  true,  very  inferior  to  that  of  the 
heart  and  very  apt  to  be  latent,  but  still  existing.  The  presence  of  food  in 


THE  MUSCULAR   MECHANISMS  OF  DIGESTION. 


303 


some  way  or  other,  by  some  direct  action  quite  apart  from  the  central  nervous 
system,  is  able  to  increase  this  power  so  that,  without  any  aid  from  the  central 
nervous  system,  as  after  section  of  the  vagi,  adequate  peristaltic  move- 
ments can,  under  favorable  circumstances,  be  carried  out.  Nevertheless,  in 


R.V 


Ret. 


Diagram  to  illustrate  the  Nerves  of  the  Alimentary  Canal  in  the  Dog.1  The  figure  is,  for  the 
sake  of  simplicity,  made  as  diagrammatic  as  possible,  and  does  not  represent  the  anatomical  relations. 
Oe.  to  Ret.  The  alimentary  canal,  oesophagus,  stomach,  small  intestine,  large  intestine,  rectum. 
L.  V.  Left  vagus  nerve,  ending  on  front  of  stomach,  r.l.  Recurrent  laryngeal  nerve  supplying 
upper  part  of  oesophagus.  R.  V.  Right  vagus,  joining  left  vagus  in  oesophageal  plexus,  oe.  pi.,  sup- 
plying posterior  part  of  stomach  and  continued  as  R'.  V.  to  join  the  solar  plexus,  here  represented 
by  a  single  ganglion  and  connected  with  the  inferior  mesenteric  ganglion  (or  plexus)  m.gl.  a. 
Branches  from  the  solar  plexus  to  stomach  and  small  intestine  and  from  the  mesenteric  ganglion 
to  the  large  intestine.  SpL  maj.  Large  splanchnic  nerve  arising  from  the  thoracic  ganglia  and 
rami  communicantes,  r.c.,  belonging  to  dorsal  nerves  from  the  6th  to  the  9th  (or  10th).  Spl.min. 
Small  splanchnic  nerve  similarly  arising  from  10th  and  llth  dorsal  nerves.  These  both  join  the 
solar  plexus  and  thence  make  their  way  to  the  alimentary  canal,  c.r.  Nerves  from  the  ganglia, 
etc.,  belonging  to  llth  and  12th  dorsal  and  1st  and  2d  lumbar  nerves,  proceeding  to  the  inferior 
mesenteric  ganglia  (or  plexus),  in.  gl.,  and  thence  by  the  hypogastric  nerve,  n.  hyp.,  and  the  hypo- 
gastric  plexus,  pi.  h;/p.,  to  the  circular  muscles  of  the  rectum,  l.r.  Nerves  from  the  2d  and  3d 
sacral  nerves,  S.2,  S.3  (nervi  erigentes),  proceeding  by  the  hypogastric  plexus  to  the  longitudinal 
muscles  of  the  rectum. 

the  normal  course  of  events,  satisfactory  movements  are  still  further  secured 
by  the  reflex  action  through  vagus  fibres  just  described.  Thus,  in  the  dog, 
the  act  of  swallowing  food  or  even  the  mere  smell  of  food  has  been  observed 
to  increase  the  movements  of  a  piece  of  intestine  isolated  from  the  rest  of 

1  It  was  not  observed  until  too  late  that  in  the  diagram  of  the  nerves  of  the  alimentary 
canal  in  the  dog,  twelve  dorsal  nerves  had  been  represented.  The  figure,  as  stated,  makes 
no  pretence  to  anatomical  exactness ;  but  it  would  have  been  better  to  represent  either 
thirteen  or  fifteen  dorsal  nerves. 


304  THE  TISSUES   AND   MECHANISMS  OF   DIGESTION. 

the  alimentary  canal  but  retaining  its  connections  with  the  central  nervous 
system.  .  Under  this  view  the  peristaltic  movements  produced  by  centrifugal 
stimulation  of  the  vagus  in  the  neck  are  comparable  not  so  much  with  the 
contraction  of  a  skeletal  muscle  when  its  motor  nerve  is  stimulated  as  with 
the  beats  which  may  be  called  forth  in  an  inhibited  or  otherwise  quiescent 
heart  by  stimulation  of  the  cardiac  augmentor  fibres. 

Indeed,  we  may,  perhaps,  call  the  vagus  fibres  which  pass  to  the  stomach 
and  intestines  (and  these,  we  may  remark,  are,  like  the  cardiac  augmentor 
fibres,  non-medu Hated  fibres  along  the  greater  part  of  their  course)  aug- 
mentor fibres  rather  than  motor  fibres.  We  have  all  the  more  reason  to  do 
so  since  there  exist  companion,  but  antagonistic,  inhibitory  fibres.  If  while 
lively  peristaltic  action  is  going  on  in  the  bowels  the  splanchnic  nerves  be 
stimulated,  the  bowels  are  brought  to  rest,  often  in  a  very  abrupt  and 
marked  manner.  Inhibitory  fibres,  therefore,  run  in  the  splanchnic  nerves 
(Fig.  89,  Spl.  maj,  and  mm.),  passing  along  them  from  the  spinal  cord  to 
the  abdominal  plexuses,  and  thence  to  the  alimentary  canal. 

It  will  be  noticed  that  the  splanchnic  nerves,  while  containing  vaso-con- 
strictor,  i.  e.,  augmentor,  fibres  for  the  bloodvessels  of  the  intestines,  carry 
inhibitory  fibres  for  the  muscular  coat;  and  probably  the  vagus,  while  con- 
taining augmentor  fibres  for  the  muscular  coat,  carries  inhibitory  dilator 
fibres  for  the  bloodvessels.  It  may  further  be  remarked  that  the  vagus, 
while  supplying  augmentor  fibres  for  the  muscular  mechanisms  of  the  ali- 
mentary canal,  carries,  as  we  so  well  know,  inhibitory  fibres  for  the  cardiac 
muscular  mechanism. 

In  the  above  statement  we  have  purposely  used  the  general  term,  peri- 
staltic movement ;  but,  as  we  have  seen,  in  the  movements  of  the  alimentary 
canal,  two  sets  of  muscles  are  concerned — the  circular  and  the  longitudinal. 
Now,  in  the  rectum  we  are  able  to  recognize  that  the  two  sets  of  muscles 
have  quite  distinct  nervous  supplies.  The  longitudinal  coat  is  governed 
by  nerve-fibres  which,  in  the  dog,  leave  the  spinal  cord  in  the  anterior  roots 
of  the  second  and  third  sacral  nerves  (Fig.  89,  S.  2,  S.  3),  pass  along  the 
branches  of  those  nerves  frequently  spoken  of  as  the  nervi  erigentes,  I.  r.,  to 
the  hypogastric  plexus  (pi.  hyp.),  and  thence  to  the  rectum.  Stimulation 
of  these  nerves  causes  contractions  of  the  rectum,  which  are  confined  to 
the  longitudinal  coat  and,  as  we  have  said,  pull  the  rectum  down.  The  cir- 
cular coat  is  governed  by  fibres  which  leave  the  spinal  cord  by  the  anterior 
roots  of  the  lower  dorsal  and  first  two  lumbar  nerves,  Fig.  89  (coming  from 
the  lower  part  of  that  spinal  region  from  which,  as  we  have  seen  (§  155) 
the  vaso-constrictor  fibres  take  origin),  and,  early  losing  their  medulla,  pass 
to  the  rectum  by  the  inferior  mesenteric  ganglia,  the  hypogastric  nerves, 
and  hypogastric  plexus  (Fig.  89,  m.  gl.,  n.  hyp.,  pi.  hyp.).  Stimulation  of 
these  fibres  gives  rise  to  contractions  which  are  confined  to  the  circular  coat 
and  squeeze  out  the  contents  of  the  rectum.  A  similar  double  nervous 
supply  probably  governs  the  longitudinal  and  circular  coats  along  the 
whole  alimentary  canal ;  but  the  details  of  such  a  supply  are  at  present 
unknown. 

Our  knowledge,  moreover,  concerning  the  details  of  any  special  nervous 
mechanisms,  by  means  of  which  the  more  complicated  movements  of  the 
stomach,  including  the  closing  and  opening  of  the  sphincters,  are  carried  out, 
is  at  present  very  imperfect.  We  cannot  add  to  what  we  have  incidentally 
said  in  speaking  of  vomiting. 

The  movements  of  the  rectum,  including  the  sigmoid  flexure,  appear 
to  be  much  more  closely  dependent  on  the  central  nervous  system  than 
are  those  of  the  rest  of  the  alimentary  canal.  As  we  have  said,  the  move- 
ments of  both  large  and  small  intestine  are  rather  assisted  and  aug- 


THE  MUSCULAR  MECHANISMS  OF  DIGESTION.  305 

mented  than  primarily  culled  forth  by  impulses  descending  from  the  central 
nervous  system  along  the  vagus  fibres.  As  the  large  intestine,  however, 
passes  into  the  rectum,  government  by  the  vagus  is  replaced  by  government 
through  the  lumbar  cord  and  the  nerves  just  previously  mentioned ;  and 
this  government  appears  to  be  not  so  much  mere  augmentation  as  the  actual 
carrying  out  of  the  movements  through  reflex  action.  Hence,  this  is  the 
part  of  intestinal  movement  which  fails  in  diseases  of  the  central  nervous 
system,  the  failure  leading  to  obstinate  constipation,  if  not  to  actual  dif- 
ficulty of  defecation.  The  presence  of  feces  in  the  sigmoid  flexure  no 
longer  stirs  up  the  reflex  mechanism  for  their  discharge  ;  meanwhile  the 
more  independent  movements  of  the  higher  parts  of  the  canal  continue  to 
drive  the  contents  onward,  and  hence  the  feces  accumulate  in  the  sigmoid 
flexure  and  colon  awaiting  the  delayed  action  of  the  imperfect  reflex 
mechanism.  With  regard  to  the  exact  manner  in  which  the  presence  of 
food  acts  as  a  stimulus,  it  may  be  worth  while  to  remark  that,  although  in 
the  stomach,  as  we  have  seen,  mere  fulness  is  not  the  efficient  cause  of 
the  movements,  since  these  become  more  active  as  digestion  proceeds  and 
the  bulk  of  the  contents  diminishes,  yet  in  the  intestine  distention  of  the 
bowel  up  to  certain  limits  most  distinctly  increases  the  vigor  of  the  move- 
ments just  as  distention  of  the  cardiac  cavities  within  certain  limits  im- 
proves the  cardiac  stroke.  This  is  well  seen  in  obstruction  of  the  bowels, 
in  which  case  the  bowel  distended  above  the  obstruction  is  frequently 
thrown  into  violent  peristaltic  movements.  This  effect  is  in  part  at  least 
due  to  the  distention  extending  the  muscular  fibres,  and  so  in  a  direct 
manner  promoting  their  contraction  (see  §  77),  but  may  be  in  part  due 
to  augmentor  impulses  excited  in  a  reflex  manner.  Probably  in  an  in- 
testine isolated  from  the  central  nervous  system  food  provokes  peristaltic 
movements  much  more  by  causing  distention  and  so  stretching  the  muscular 
coats  than  by  acting  as  a  stimulus  to  the  mucous  membrane,  either  through 
chemical  action  or  in  any  similar  way. 

§  238.  Next  to  the  presence  of  food  in  the  interior  of  the  alimentary 
canal  a  deficient  oxygenation  of  the  blood  supplied  to  the  walls  of  the 
canal  or  the  sudden  cutting  off  of  the  supply  of  blood  may  be  regarded 
as  the  most  powerful  provocatives  of  peristaltic  action.  When  the  aorta  is 
clamped,  or  when  the  respiration  is  seriously  interfered  with,  peristaltic 
movements  become  very  pronounced.  Thus,  in  death  by  asphyxia  or  suf- 
focation, an  involuntary  discharge  of  feces,  which  is  in  part  at  least  the 
result  of  increased  peristaltic  action,  is  not  an  unfrequent  result;  and 
the  marked  peristaltic  movements  which  are  so  frequently  seen  in  an  animal 
when  the  abdomen  is  laid  open  immediately  after  death  appear  to  be  due  to 
the  cessation  of  the  circulation  and  the  consequent  failure  in  the  supply  of 
blood  to  the  walls  of  the  alimentary  canal,  and  not,  as  has  been  suggested, 
to  the  contact  with  air  of  the  peritoneal  surface.  Since  it  is  blood  which 
brings  oxygen  to  the  tissues,  failure  in  the  supply  of  blood  is  tantamount 
to  failure  in  the  supply  of  oxygen  ;  but  the  blood  current  brings  other 
things  besides  oxygen  and  also  takes  things  away ;  and  the  failure  of  this 
action  also,  probably,  as  well  as  failure  in  the  supply  of  oxygen,  provokes 
the  movements  in  question. 

The  movements  thus  produced  are  to  some  extent  the  result  of  the  defi- 
cient supply  of  blood  acting  directly  on  the  walls  of  the  canal,  though  in 
asphyxia  at  all  events  this  effect  may  be  increased  by  the  too  venous  blood 
stimulating  the  central  nervous  system,  and  thus  sending  augmentor  impulses 
down  the  vagus. 

With  regard  to  the  mode  of  action  of  the  drugs  which  promote  peri- 
staltic action,  it  will  be  sufficient  here  to  say  that  while  some,  such  as 

20 


306  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

nicotin,  appear  to  act  directly  on  the  walls  of  the  canal,  others,  such  as 
strychnia,  produce  their  effect  chiefly  by  acting  through  the  central  nervous 
system. 

THE  CHANGES  WHICH  THE  FOOD  UNDERGOES  IN  THE  ALIMENTARY 

CANAL. 

§  239.  Having  studied  the  properties  of  the  digestive  juices  as  exhibited 
outside  the  body,  and  the  various  mechanisms  by  means  of  which  the  food 
introduced  into  the  body  is  brought  under  the  influence  of  those  juices,  we 
have  now  to  consider  what,  as  matters  of  fact,  are  the  actual  changes  which 
the  food  does  undergo  in  passing  along  the  alimentary  canal ;  what  are  the 
steps  by  which  the  contents  of  the  canal  are  gradually  converted  into  feces. 
The  events  which  lead  to  this  conversion  are  twofold.  On  the  one  hand  the 
digestive  juices  do  bring  about,  inside  the  alimentary  canal,  changes  which 
in  the  main  are  the  same  as  those  observed  in  laboratory  experiments  out- 
side the  body  and  described  in  previous  sections,  though  the  results  are 
somewhat  modified  by  the  special  conditions  which  obtain  within  the  body. 
On  the  other  hand,  absorption,  that  is  to  say,  the  passage  from  the  interior 
of  the  canal  into  the  bloodvessels  and  lymphatics,  of  digested  material  in 
company  with  water  is  going  on  along  the  whole  length  of  the  canal,  and 
especially  in  the  small  and  large  intestines.  It  will  be  convenient  to  con- 
fine ourselves  at  present  to  the  study  of  the  first  class  of  events,  the  changes 
effected  in  the  canal,  merely  noting  the  disappearance  of  this  or  that  prod- 
uct, and  deferring  the  difficult  problem  of  how  absorption,  takes  place  to  a 
subsequent  and  separate  discussion. 

In.  the  mouth  the  presence  of  the  food,  assisted  by  the  movements  of  the 
jaw,  causes,  as  we  have  seen,  a  flow  of  saliva.  By  mastication,  and  by  the 
addition  of  mucous  saliva,  the  food  is  broken  into  small  pieces,  moistened, 
and  gathered  into  a  convenient  bolus  for  deglutition.  In  man  some  of  the 
starch  is,  even  during  the  short  stay  of  the  food  in  the  mouth,  converted 
into  sugar ;  for  if  boiled  starch  free  from  sugar  be  even  momentarily  held 
in  the  mouth  and  then  ejected  into  water  (kept  boiling  to  destroy  the 
ferment),  it  will  be  found  to  contain  a  decided  amount  of  sugar.  In  many 
animals  no  such  change  takes  place.  The  viscid  saliva  of  the  dog  serves 
almost  solely  to  assist  in  deglutition  ;  and  even  the  longer  stay  which  food 
makes  in  the  mouth  of  the  horse  is  insufficient  to  produce  any  marked  con- 
version of  the  starch  it  may  contain.  During  the  rapid  transit  through  the 
oesophagus  no  appreciable  change  takes  place. 

The  amount  of  absorption  of  digested  material,  or  even  of  simple  water, 
from  the  mouth  or  oesophagus  must  always  be  insignificant. 

TJie  Changes  in  the  Stomach. 

§  240.  The  arrival  of  the  food,  the  reaction  of  which  is  either  naturally 
alkaline,  or  is  made  alkaline,  or  at  least  is  reduced  in  acidity,  by  the  addi- 
tion of  saliva,  causes  a  flow  of  gastric  juice.  This,  already  commencing 
while  the  food  is  yet  in  the  mouth,  increases  as  the  food  accumulates  in  the 
stomach,  and  as,  by  the  churning  gastric  movements,  one  part  after  another 
of  the  food  is  brought  into  contact  with  the  mucous  membrane. 

The  characters  of  the  juice  appear  to  change  somewhat  as  the  act  of 
digestion  proceeds.  The  amount  of  pepsin  in  the  gastric  contents  increases 
for  some  time  after  food  is  taken,  and  probably  the  actual  secretion  increases 
also.  The  acidity  of  the  gastric  contents  is  at  first  very  feeble ;  indeed  in 
man,  in  some  cases  at  least,  for  some  little  time  after  the  beginning  of  a  meal 
no  free  acid  is  present,  and  during  this  period  the  conversion  of  starch  into 


THE  CHANGES  IN  THE  ALIMENTARY  CANAL.  307 

sugar  may  continue.  This  condition,  however,  is  temporary  only;  very 
soon  the  contents  become  acid,  arresting  the  action  of  and  ultimately  de- 
stroying the  amylolytic  ferment;  and,  since  the  rate  of  secretion  of 'acid 
appears  to  be  fairly  constant,  the  contents  of  the  stomach,  unless  fresh  alka- 
line food  be  taken,  become  more  acid  as  digestion  goes  on. 

The  gross  effect  of  gastric  digestion  is  to  break  up  and  partly  to  dissolve 
the  larger  lumps  of  masticated  food  into  a  thick  grayish  soup-like  liquid 
called  chyme,  with  which  are  still  mixed  in  variable  quantity  larger  and 
smaller  masses  of  less  changed  food.  This  is  the  result,  partly  of  the  solu- 
tion of  proteid  matters,  partly  of  the  solution  of  the  gelatiniferous  connec- 
ti  ve  tissue  holding  the  proteid  elements  together.  In  a  fragment  of  meat, 
for  instance,  the  muscular  fibres,  through  the  solution  of  the  connective 
tissue  binding  them  together,  fall  asunder,  the  sarcolemma  is  dissolved,  and 
the  fibres  themselves  split  up  sometimes  longitudinally  but  most  frequently 
by  transverse  cleavage  into  discs,  and  are  ultimately  more  or  less  reduced 
partly  into  a  granular  mass,  partly  to  actual  solution.  In  a  piece  of  tissue 
containing  fat,  the  connective  tissue  binding  the  fat  cells  together  and  the 
envelopes  of  the  fat  cells  are  dissolved,  so  that  the  fat,  fluid  at  the  tempera- 
ture of  the  body,  is  set  free  from  the  individual  cells  and  runs  together  into 
larger  and  smaller  masses.  In  vegetable  tissue  the  proteid  elements  are  in 
part  dissolved  and,  though  there  is  no  evidence  that  in  man  cellulose  is  dis- 
solved in  the  stomach,  the  whole  tissue  is  softened  and  to  a  certain  extent 
disintegrated.  Milk  is  curdled  and  the  curd  subsequently  more  or  less  dis- 
solved. 

The  thick  soup-like  acid  chyme  consists  accordingly  partly  of  substances 
which  have  entered  into  actual  solution,  partly  of  mere  particles  or  droplets 
of  proteid,  fatty  or  other  nature,  and  partly  of  masses  small  or  great  which 
may  be  recognized  under  the  microscope  as  more  or  less  changed  portions  of 
animal  or  vegetable  tissue.  The  amount  of  material  actually  dissolved  is  in 
most  specimens  of  chyme  exceedingly  small.  When  the  solid  parts  are 
removed  by  filtration  the  clear  filtrate  contains  beside  salts,  pepsin  and  free 
hydrochloric  acid  (the  constituents  of  the  gastric  juice),  a  small  amount  of 
sugar,  of  parapeptone  and  of  peptone.  The  sugar  is  often  absent,  the  para- 
peptone  is  not  always  present,  and  the  amount  of  peptone  (or  albumose)  is 
always  small. 

During  gastric  digestion  the  chyme  thus  formed  is  from  time  to  time 
ejected  through  the  pylorus,  accompanied  by  even  large  morsels  of  solid 
less-digested  matter.  This  may  occur  immediately  when  water  alone  is 
taken  and  within  a  few  minutes  of  food  having  been  taken  ;  but  the  larger 
escape  from  the  stomach  probably  does  not  in  man  begin  until  from  one  to 
two,  and  lasts  from  four  to  five,  hours  after  the  meal,  becoming  more  rapid 
toward  the  end,  and  such  pieces  as  are  the  least  broken  up  by  the  gastric 
juice  and  movements  being  the  last  to  leave  the  stomach. 

The  time  taken  up  in  gastric  digestion  probably  varies  in  the  same 
animal  not  only  with  different  articles  of  food,  but  also  with  varying  con- 
ditions of  the  stomach  and  of  the  body  at  large.  In  different  animals  it 
varies  very  considerably,  being  from  twelve  to  twenty-four  hours  in  the  dog 
after  a  full  meal,  while  the  stomachs  of  rabbits  are  never  empty  but  always 
remain  largely  filled  with  food,  even  during  starvation.  In  man  the  stomach 
probably  becomes  empty  between  the  usual  meals. 

The  total  amount  of  change  which  the  food  undergoes  in  the  stomach, 
that  is  the  share  taken  by  the  stomach  in  the  whole  work  of  digestion, 
seems  to  vary  largely  in  different  animals,  and  in  the  same  animal  differs 
according  to  the  nature  of  the  meal.  In  a  dog  fed  on  an  exclusively  meat 
diet,  a  very  large  part  of  the  digestion  is  said  to  be  carried  out  by  the 


308          THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

stomach,  very  little  work  apparently  being  left  for  the  intestines ;  that  is  to 
say,  the  larger  part  of  the  meal  is  reduced  in  the  stomach  to  actual  solution 
and  a  considerable  quantity  is  probably  absorbed  directly  from  the  stomach. 
In  such  cases  the  amount  of  peptone  found  in  the  stomach  during  the  diges- 
tion of  the  meal  is  found  to  be  fairly  constant,  from  which  it  may  be  in- 
ferred that  the  peptone  is  absorbed  as  soon  as  it  is  formed.  There  is  also 
evidence  that  fat  may  to  a  certain  extent  undergo  in  the  stomach  changes 
leading  to  emulsion,  similar  to  those  which,  as  we  shall  see,  are  carried  out 
in  the  small  intestine. 

But  such  cases  as  these  cannot  be  regarded  as  typical  cases  of  gastric 
digestion,  and  in  man,  at  all  events,  living  on  a  mixed  diet  the  work  of  the 
stomach  appears  to  be  to  a  large  extent  preparatory  only  to  the  subsequent 
labors  of  the  intestine.  It  is  true  that  our  information  on  this  matter  is 
imperfect,  being  chiefly  drawn  from  the  study  of  cases  of  gastric  or  duodena) 
fistula,  in  which  probably  the  order  of  things  is  not  normal,  or  being  in 
large  measure  deductions  from  experiments  on  animals,  whose  economy  in 
this  respect  must  be  largely  different  from  our  own  ;  but  we  are  probably 
safe  in  concluding  that,,  in  ourselves,  the  chief  effect  of  gastric  digestion  is 
by  means  of  the  disintegration  spoken  of  above  to  reduce  the  lumps  of  food 
to  the  more  uniform  chyme  and  so  to  facilitate  the  changes  which  take  place 
in  the  small  intestine.  During  the  disintegration  some  of  the  proteid  in  the 
meal  is  converted  into  peptone ;  and  the  peptone  so  formed  is  probably 
absorbed  at  once ;  but  much  proteid  remains  unchanged  or  at  least  is  not 
converted  into  peptone,  and  the  fats  and  starches  undergo  in  themselves 
very  little  change  indeed. 

In  the  act  of  swallowing,  no  inconsiderable  quantity  of  air  is  carried 
down  into  the  stomach,  entangled  in  the  saliva  or  in  the  food.  This  may 
be  returned  in  eructations.  When  the  gas  of  eructation  or  that  obtained 
directly  from  the  stomach  is  examined,  it  is  found  to  consist  chiefly  of 
nitrogen  and  carbonic  acid,  the  oxygen  of  the  atmospheric  air  having  been 
largely  absorbed.  In  most  cases  the  carbonic  acid  is  derived  by  simple 
diffusion  from  the  blood,  or  from  the  tissues  of  the  stomach,  which  similarly 
take  up  the  oxygen.  In  many  cases  of  flatulency,  however,  it  may  arise 
from  a  fermentative  decomposition  of  the  sugar  which  has  been  taken  as 
such  in  food  or  which  has  been  produced  from  the  starch,  the  gas  being 
either  formed  in  the  stomach  or  passing  upward  from  the  intestine  through 
the  pylorus. 

The  enormous  quantity  of  gas  which  is  discharged  through  the  mouth 
in  cases  of  hysterical  flatulency,  even  on  a  perfectly  empty  stomach, 
and  which  seems  to  consist  largely  of  carbonic  acid,  presents  difficulties 
in  the  way  of  explanation  ;  it  is  possible  that  it  may  be  simply  diffused 
from  the  blood,  but  it  is  also  possible  that  in  many  cases  it  is  derived 
from  air  which  the  patient  has  hysterically  swallowed,  the  oxygen  having 
been  removed,  in  the  stomach,  by  absorption  and  replaced  by  carbonic 
acid. 

In  the  Small  Intestine. 

§241.  The  semi-digested  acid  food,  or  chyme,  as  it  passes  over  the 
biliary  orifice,  causes,  as  we  have  seen  (§  223),  gushes  of  bile,  and  at  the 
same  time  the  pancreatic  juice  flows  into  the  intestine  freely.  These  two 
alkaline  fluids,  especially  the  more  strongly  and  constantly  alkaline  pan- 
creatic juice,  tend  to  neutralize  the  acidity  of  the  chyme,  but  the  con- 
tents of  the  duodenum  do  not  become  distinctly  alkaline  until  some  distance 
from  the  pylorus  is  reached.  The  rapidity  with  which  the  change  in  the 
reaction  is  completed  is  not  the  same  in  all  animals,  and  in  the  same  animal 


THE  CHANGES  IN   THE  ALIMENTARY   CANAL.  309 

appears  to  vary  according  to  the  nature  of  the  food  and  various  circum- 
stances. In  man,  living  on  a  mixed  diet,  the  contents  have  probably 
become  distinctly  alkaline  before  they  have  passed  far  down  the  duode- 
num. On  the  other  hand,  in  dogs  the  contents  of  the  small  intestine 
have  been  observed  to  be  acid  throughout,  and  that  not  only  when  fed 
on  starch  and  fat,  which  might,  by  an  acid  fermentation  of  which  we 
shall  presently  speak,  give  rise  to  an  acid  reaction,  but  even  when  fed  on 
meat. 

The  conversion  of  starch  into  sugar,  which,  as  we  have  seen,  is  sooner  or 
later  arrested  in  the  stomach,  is  resumed  with  great  activity  arid  indeed  com- 
pleted by  the  pancreatic  juice,  possibly  assisted  by  the  succus  entericus,  the 
presence  of  bile  being  said  to  increase  the  activity  of  the  pancreatic  amylo- 
lytic  ferment.  The  conversion  begins  as  soon  as  the  acidity  of  the  chyme 
is  sufficiently  reduced  and  continues  along  the  intestine;  portions,  however, 
of  still  undigested  starch  may  be  found  in  the  large  intestine,  and  even  at 
times  in  the  feces. 

The  pancreatic  juice,  as  we  have  seen,  emulsifies  fats,  and  also  splits  them 
into  their  respective  fatty  acids  and  glycerin.  The  fatty  acids  thus  set  free 
become  converted  by  means  of  the  alkaline  contents  of  the  intestine  into 
soaps ;  but  to  what  extent  saponification  thus  takes  place  is  not  exactly 
known.  Undoubtedly  soaps  have  to  a  small  extent  been  found  both  in 
portal  blood  and  in  the  thoracic  duct  after  a  meal  ;  but  there  is  no  proof 
that  any  large  quantity  of  fat  is  introduced  in  this  form  into  the  circula- 
tion. On  the  other  hand,  the  presence  of  neutral  fats  in  the  lacteals,  and 
to  a  slight  extent  in  portal  blood,  is  a  conspicuous  result  of  the  digestion  of 
fatty  matters ;  and  in  all  probability  saponification  in  the  intestine  is  a  sub- 
sidiary process,  the  effect  of  which  is  rather  to  facilitate  the  emulsion  of 
neutral  fats  than  to  introduce  soaps  as  such  into  the  blood,  for  the  pres- 
ence of  soluble  soaps  favors  the  emulsion  of  neutral  fats.  Hence  a  rancid 
fat,  L  e.,  a  fat  containing  a  certain  amount  of  free  fatty  acid,  forms  an  emul- 
sion with  an  alkaline  fluid  more  readily  than  does  a  quite  neutral  fat.  A 
drop  of  rancid  oil  let  fall  on  the  surface  of  an  alkaline  fluid,  such  as  a  solu- 
tion of  sodium  carbonate  of  suitable  strength,  rapidly  forms  a  broad  ring 
of  emulsion,  and  that  even  without  the  least  agitation.  As  saponification 
takes  place  at  the  junction  of  the  oil  and  alkaline  fluid,  currents  are  set  up 
by  which  globules  of  oil  are  detached  from  the  main  drop  and  driven  out 
in  a  centrifugal  direction  ;  the  intensity  of  the  currents  and  the  consequent 
amount  of  emulsion  depend  on  the  concentration  of  the  alkaline  medium 
and  on  the  solubility  of  the  soaps  which  are  formed.  Now  the  bile  and 
pancreatic  juice  supply  just  such  conditions  as  the  above  for  emulsionizing 
fats  ;  they  both  together  afford  an  alkaline  medium,  the  pancreatic  juice 
gives  rise  to  an  adequate  amount  of  free  fatty  acid,  and  the  bile  in  addition 
brings  into  solution  the  soaps  as  they  are  formed.  So  that  we  may  speak 
of  the  emulsion  of  fats  in  the  small  intestine  as  being  carried  on  by  the 
bile  and  pancreatic  juice  acting  in  conjunction  ;  and  as  a  matter  of  fact 
the  bile  and  pancreatic  juice  do  largely  emulsify  the  contents  of  the  small 
intestine,  so  that  the  grayish  turbid  chyme  is  changed  into  a  creamy-look- 
ing fluid,  which  has  been  sometimes  called  chyle.  It  is  advisable,  how- 
ever, to  reserve  this  name  for  the  contents  of  the  lacteals.  Many  of  the 
fats  present  in  food — for  instance,  butter — already  contain  some  fatty  acids 
when  eaten ;  for  these  fats  the  initial  action  of  the  pancreatic  juice  is  less 
necessary. 

This  mutual  help  of  bile  and  pancreatic  juice  in  producing  an  emulsion 
explains  to  a  certain  extent  the  controversy  which  long  existed  between  those 
who  maintained  that  the  bile  and  those  who  maintained  that  the  pancreatic 


310  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

juice  was  necessary  for  the  digestion  and  absorption  of  fatty  food.  That  the 
pancreatic  juice  does  produce  in  the  intestine  such  a  change  as  favors  the 
transference  of  neutral  fats  from  the  intestine  into  the  lacteals  is  shown  by 
the  fact  that  in  diseases  affecting  the  pancreas  much  fatty  food  frequently 
passes  through  the  intestine  undigested  and  great  wasting  ensues ;  but  it 
cannot  be  maintained  that  the  pancreatic  juice  is  the  sole  agent  in  this 
matter,  since  in  animals  in  which  the  pancreatic  ducts  have  been  success- 
fully ligatured  chyle  is  still  found  in  the  lacteals.  On  the  other  hand,  that 
the  bile  is  of  use  in  the  digestion  of  fat  is  shown  by  the  prevalence  of  fatty 
stools  in  case  of  obstruction  of  the  bile-ducts  ;  and  though  the  operation  of 
ligaturing  the  bile-ducts  and  leading  all  the  bile  externally  through  a  fistula 
of  the  gall-bladder  is  open  to  objection,  since  it  in  some  way  or  other  so 
exhausts  the  animal  as  indirectly  to  affect  digestion,  still  the  results  of  experi- 
ments in  which  the  resorption  of  fat  was  distinctly  lessened  (the  quantity  of 
fat  in  the  lacteals  falling  from  3.2  to  0.2  per  cent.)  by  the  ligature  and  fistula 
obviously  point  to  the  same  conclusion.  That  in  man  the  succus  entericus 
possesses  a  wholly  insufficient  emulsifying  power  is  shown  by  the  observa- 
tion of  a  case  in  which  the  duodenum  opened  on  the  surface  by  a  fistula 
in  such  a  way  that  the  lower  part  of  the  intestine  could  be  kept  free  from 
the  contents  of  the  upper  part  containing  the  bile  and  pancreatic  juice 
and  matters  proceeding  from  the  stomach.  Fats  introduced  into  the  lower 
part,  where  they  could  not  be  acted  upon  either  by  the  bile  or  by  the  pan- 
creatic juice,  were  but  slightly  digested.  Without  denying  the  possible 
assistance  of  the  succus  entericus,  or  even  of  gastric  juice,  we  may  con- 
clude that  the  digestion  of  fat  is  in  the  main  carried  out  by  the  conjoint 
action  of  bile  and  pancreatic  juice. 

§  242.  We  have  seen  (§  216)  that  the  addition  of  bile  to  a  digesting 
mixture  gives  rise  to  a  precipitate.  This  is  partly  a  coarse,  flocculent  pre- 
cipitate, consisting  of  parapeptone  with  some  amount  of  bile  acids,  and 
partly  of  a  finer,  more  granular  precipitate,  which  is  longer  in  falling  down 
and  consists  chiefly  of  bile  acids  with  a  variable  amount  of  peptone ;  the 
latter  is  redissolved  on  the  further  addition  of  bile,  even  though  the  reaction 
of  the  mixture  remain  acid.  In  the  upper  part  of  the  duodenum  the  inner 
surface,  if  examined  while  digestion  is  going  on,  is  found  to  be  lined  by  a 
colored  flocculent  and  granular  material,  which  is  probably  a  precipitate 
thus  formed  ;  the  purpose  of  this  precipitation  is  probably  to  delay  the 
passage  of  the  undigested  parapeptone  along  the  duodenum.  Moreover, 
apart  from  this  precipitation,  bile  arrests  the  action  of  pepsin,  even  while 
the  reaction  of  the  mixture  still  remains  acid ;  and  as  soon  as  an  alkaline 
reaction  is  established  the  pepsin  is  apparently  destroyed  by  the  trypsin,  so 
that  with  the  flow  of  bile  and  pancreatic  juice  into  the  duodenum  the  pro- 
cesses which  have  been  going  on  in  the  stomach  come  to  an  end.  In  fact,  it 
would  seem  that  the  juices  of  the  various  districts  of  the  alimentary  canal 
are  mutually  destructive  ;  thus,  while  pepsin  in  an  acid  solution  destroys  the 
active  constituents  of  saliva  and  of  pancreatic  juice  (probably  also  those  of 
the  succus  entericus),  it  is  in  its  turn  antagonized  or  destroyed  by  the  bile 
and  the  other  alkaline  juices  of  the  intestine.  Hence,  pancreatic  juice  intro- 
duced through  the  mouth  must  lose  its  powers  in  the  stomach,  and  can  only 
be  of  use  as  an  alkaline  medium  containing  certain  proteid  matters.  On 
the  other  hand,  if,  as  we  have  reason  to  believe,  the  contents  of  the  stomach 
as  they  issue  from  the  pylorus  still  contain  a  large  quantity  of  undigested 
proteids,  these  must  be  digested  by  the  pancreatic  juice  (with  or  without  the 
assistance  of  the  succus  entericus),  the  action  of  which  seems  to  be  assisted, 
or  at  least  not  hindered,  by  bile.  And  in  dogs  fed  through  a  duodenal  fis- 
tula, so  that  all  gastric  digestion  is  excluded,  proteids  are  completely  digested 


THE  CHANGES  IN  THE   ALIMENTARY   CANAL.  311 

and  give  rise  to  quite  normal  feces.  To  what  stage  the  pancreatic  digestion 
is  carried,  whether  peptone  is  practically  the  only  product,  or  whether  the 
pancreatic  juice  in  the  body,  as  out  of  the  body,  carries  on  its  work  in  the 
more  destructive  form,  whereby  the  proteid  material  subjected  to  it  is  so 
broken  down  as  to  give  rise  to  appreciable  quantities  of  leucin  and  tyrosin, 
is  at  present  not  exactly  known.  Leucin  and  tyrosin  have  been  found  in 
the  intestinal  contents,  and  may  therefore  be  formed  during  normal  diges- 
tion, but  whether  an  insignificant  quantity  or  a  considerable  quantity  of  the 
proteid  material  of  food  is  thus  hurried  into  a  crystalline  form  cannot  be 
definitely  stated.  The  extent  to  which  the  action  is  carried  is  probably  dif- 
ferent in  different  animals,  and  probably  varies  also  according  to  the  nature 
of  the  meal  and  the  condition  of  the  body.  Possibly  when  a  large  and  un- 
necessary quantity  of  proteid  material  is  taken  at  a  meal,  together  with 
other  substances,  no  inconsiderable  amount  of  the  proteids  undergo  this  pro- 
found change,  and,  as  we  shall  see,  rapidly  leave  the  body  as  urea  without 
having  been  used  by  the  tissues,  their  contribution  to  the  energy  of  the 
body  being  limited  to  the  heat  given  out  during  the  changes  by  which 
they  are  converted  into  urea.  To  this  apparently  wasteful  use  of  proteids 
we  shall  return  in  speaking  of  what  is  called  the  "  luxus  consumption" 
of  food. 

§  243.  In  dealing  with  the  action  of  pancreatic  juice  we  drew  attention 
(§  218)  to  the  difference  between  the  results  of  pure  tryptic  digestion  and 
those  obtained  when  bacteria  or  other  microorganisms  were  allowed  to  be 
present.  We  saw  that  indol,  for  example,  was  the  product  of  the  action  of 
these  organisms,  not  of  trypsin.  Now7  indol  is  formed  in  varying  quantity 
during  the  digestion  which  actually  takes  place  in  the  intestine,  some  of  it  at 
times  appearing  in  the  urine  as  indigo-yielding  substance  (indican).  More- 
over, bacteria  and  other  microorganisms  are  present  in  the  intestinal  con- 
tents. Hence,  we  must  regard  the  changes  taking  place  in  the  intestine  not 
as  the  pure  results  of  the  action  of  the  several  digestive  juices,  but  as  those 
results  modified  by  or  mixed  with  the  results  of  the  action  of  microorganisms. 
We  spoke  above  (§  216)  of  bile  as  being  antiseptic,  but  this  must  be  under- 
stood as  meaning  not  that  the  presence  of  bile  arrests  the  action  of  all  micro- 
organisms within  the  intestine,  but  that  it  modifies  their  action,  keeping  it 
within  certain  limits  and  along  certain  lines. 

Concerning  the  exact  nature  and  extent  of  the  changes  thus  due  to  micro- 
organisms, our  knowledge  is  at  present  very  imperfect.  The  proteids  and 
the  carbohydrates  seem  to  be  the  food-stuffs  on  which  these  organisms  pro- 
duce their  chief  effect.  Out  of  the  proteids  they  give  rise  not  only  to  indol, 
but  to  several  other  compounds,  among  which  may  be  mentioned  phenol 
(C6H6O),  of  which  a  small  quantity  may  be  recognized  in  the  feces,  the  rest 
being  absorbed  and  appearing  in  the  urine  in  the  form  of  certain  phenol- 
compounds,  such  as  phenyl-sulphuric  acid.  Out  of  proteids  they  may  also 
form  the  peculiar  poisonous  bodies  called  ptomaines,  which  appear  in  the 
ordinary  putrefaction  of  proteids.  But  their  most  conspicuous  effects  are 
those  on  the  carbohydrates.  As  the  food  descends  the  intestine,  the  presence 
of  lactic  acid  becomes  more  and  more  obvious  ;  indeed,  in  some  cases  the 
naturally  alkaline  reaction  of  the  intestinal  contents  may  in  the  lower  part 
of  the  intestine  be  changed  into  an  acid  one  by  the  presence  of  lactic  acid. 
Now,  lactic  acid  may  be  formed  out  of  sugar  by  means  of  a  special  organism 
inducing  what  is  spoken  of  as  the  lactic  acid  fermentation.  And  we  have 
every  reason  to  believe  that  in  even  normal  digestion  a  certain  quantity  of 
sugar,  either  eaten  as  such  or  arising  from  the  amylolytic  conversion  of 
starch,  does  not  pass  away  from  the  intestine  into  the  blood  as  sugar,  but 
undergoes  this  fermentation  into  lactic  acid.  To  what  extent  this  change 


312  THE  TISSUES  AND  MECHANISMS   OF   DIGESTION. 

takes  place  we  do  not  know ;  the  amount  probably  varies  according  to  the 
amount  of  carbohydrates  eaten,  the  condition  of  the  alimentary  canal,  and 
other  circumstances.  It  may  be  under  certain  circumstances  simply  a' part 
of  normal  digestion  ;  under  other  circumstances  it  may  be  excessive  and 
give  rise  to  troubles. 

That  fermentative  changes  may  occur  in  the  small  intestine  is  further 
indicated  by  the  facts  that  the  gas  there  present  may  contain  free  hydrogen, 
and  that  chyme,  after  removal  from  the  intestine,  continues  at  the  tempera- 
ture of  the  body  to  produce  carbonic  acid  and  hydrogen  in  equal  volumes. 
This  suggests  the  possibility  of  the  sugar  of  the  intestinal  contents  under- 
going the  butyric  acid  fermentation,  during  which,  as  is  well  known,  car- 
bonic anhydride  and  hydrogen  are  evolved.  By  this  change  the  sugar  is 
removed  from  the  carbohydrate  group  into  the  fatty  acid  group  ;  it  is  thus, 
so  to  speak,  put  on  its  way  to  become  fat.  We  shall  see  hereafter  that  sugar 
may  be  somewhere  in  the  body  converted  into  fat ;  this  conversion,  however, 
takes  place  chiefly  if  not  wholly  in  the  tissues,  and  such  change  as  may  take 
place  in  the  alimentary  canal  is  to  be  regarded  as  suggestive  rather  than  as 
important. 

The  hydrogen  thus  occurring  in  the  intestine  may  also  arise  from  the 
proteid  decompositions  spoken  of  above.  However  arising  it  may  act  as  a 
reducing  agent — reducing  sulphates,  for  instance — and  thus  giving  rise  to 
sulphides  and  to  sulphuretted  hydrogen ;  as  a  reducing  agent  it  assists  in 
the  formation  of  the  fecal  and  urinary  pigments. 

Thus,  during  the  transit  of  the  food  through  the  small  intestine,  by  the 
action  of  the  bile  and  pancreatic  juice,  and  possibly  to  some  extent  of  the 
succus  entericus,  assisted  by  various  microorganisms,"  the  proteids  are  largely 
dissolved  and  converted  into  peptone  and  other  products,  the  starch  is 
changed  into  sugar,  the  sugar  possibly  being  in  part  further  converted  into 
lactic  and  other  acids,  and  the  fats  are  largely  emulsified  and  to  some  extent 
saponified.  These  products  as  they  are  formed  pass  into  either  the  lacteals 
or  the  portal  bloodvessels,  so  that  the  contents  of  the  small  intestine,  by  the 
time  they  reach  the  ileo-csecal  valve,  are  largely  but  by  no  means  wholly 
deprived  of  their  nutritious  constituents.  So  far  as  water  is  concerned,  the 
secretion  of  water  into  the  small  intestine  maintains  such  a  relation  to  the 
absorption  from  it  that  the  intestinal  contents  at  the  end  of  the  ileum, 
though  much  changed,  are  about  as  fluid  as  in  the  duodenum. 

In  the  Large  Intestine. 

§  244.  The  contents,  whether  alkaline  or  not,  in  the  ileum  now  become 
once  more  distinctly  acid.  This,  however,  is  not  caused  by  any  acid  secre- 
tion from  the  mucous  membrane ;  the  reaction  of  the  intestinal  walls  in  the 
large  as  in  the  small  intestine  is  alkaline.  It  must,  therefore,  arise  from 
acid  fermentations  going  on  in  the  contents  themselves  ;  and  that  fermenta- 
tions do  go  on  is  shown  by  the  appearance  of  marsh  gas  as  well  as  hydrogen 
in  this  portion  of  the  alimentary  canal.  The  character  and  amount  of  fer- 
mentation probably  depend  largely  on  the  nature  of  the  food,  and  probably 
also  vary  in  different  animals. 

Of  the  particular  changes  which  take  place  in  the  large  intestine  we 
have  no  very  definite  knowledge  ;  but  it  is  exceedingly  probable  that  in  the 
voluminous  caecum  of  the  herbivora  a  large  amount  of  digestion  of  a  pecu- 
liar kind  goes  on.  We  know  that  in  herbivora  a  considerable  quantity  of 
cellulose  disappears  in  passing  through  the  alimentary  canal,  and  even 
in  man  some  is  digested.  It  seems  probable  that  this  cellulose  digestion 
takes  place  in  the  large  intestine,  and  is  the  result  of  fermentative  changes 


THE  LACTEALS   AND  THE   LYMPHATIC  SYSTEM.  313 

carried  out  by  means  of  microorganisms,  marsh  gas  being  one  of  the  prod- 
ucts formed  at  the  same  time. 

Be  this  as  it  may,  whether  digestion^  properly  so  called,  is  all  but  com- 
plete at  the  ileo-ca3cal  valve,  or  whether  important  changes  still  await 
the  chyme  in  the  large  intestine,  one  great  characteristic  of  the  work 
done  in  the  colon  is  absorption.  By  the  abstraction  of  all  the  soluble  con- 
stituents, and  especially  by  the  withdrawal  of  water,  the  liquid  chyme 
becomes  as  it  approaches  the  rectum  converted  into  the  firm  solid  feces,  and 
the  color  shifts  from  the  bright  orange,  which  the  gray  chyme  gradually 
assumes  after  admixture  with  bile,  into  a  darker  and  dirtier  brown. 

TJie  Feees. 

§  245.  These  consist  in  the  first  place  of  the  indigestible  and  undi- 
gested constituents  of  the  meal  ;  shreds  of  elastic  tissue,  hairs  and  other 
horny  elements,  much  cellulose  and  chlorophyll  from  vegetable,  and  some 
connective  tissue  from  animal  food,  fragments  of  disintegrated  muscular 
fibre,  fat-cells,  and  not  unfrequently  undigested  starch-corpuscles.  The 
amount  of  each  must  of  course  vary  very  largely  according  to  the  nature 
of  the  food,  and  the  digestive  powers,  temporary  or  permanent,  of  the  indi- 
vidual. In  the  second  place,  to  these  must  be  added  substances  not  dis- 
tinctly recognizable  as  parts  of  the  food,  but  derived  from  the  secretions  of 
the  alimentary  canal.  The  feces  contain  mucus  in  variable  amount,  some- 
times albumin,  cholesterin,  butyric  and  other  fatty  acids,  lime  and  magnesia 
soaps,  coloring  matters,  and  inorganic  salts,  especially  earthy  phosphates, 
crystals  of  ammonio-magnesia  phosphates  being  very  conspicuous.  The  re- 
action is  generally  but  not  always  acid.  They  also  contain  a  ferment  similar 
in  its  action  to  pepsin,  and  an  amylolytic  ferment  similar  to  that  of  saliva 
or  of  pancreatic  juice.  The  bile  salts  are  represented  by  a  small  quantity 
of  cholalic  acid,  or  some  product  of  that  body,  and  sometimes  a  very  small 
quantity  of  taurin.  The  glycin  and  most  or  all  of  the  taurin  have  been  ab- 
sorbed from  the  intestine,  and  the  cholalic  acid  has  been  partly  absorbed 
and  partly  decomposed.  The  fact  that  the  feces  become  "  clay-colored  " 
when  the  bile  is  cut  off  from  the  intestine  shows  that  the  bile-pigment  is  at 
least  the  mother  of  the  fecal  pigment ;  and  a  special  pigment,  which  has  been 
isolated  and  called  stercobilin,  is  said  to  be  identical  with  the  substance  called 
urobilin,  which  may  be  formed  from  bilirubin.  As  other  special  constitu- 
ents of  the  feces  may  be  mentioned  excretin,  a  somewhat  complex  nitrogen- 
ous body,  whose  exact  chemical  nature  is  at  present  uncertain,  and  sfcatol 
(C9H9N),  a  nitrogenous  body  which  like  indol  is  derived  from  the  decompo- 
sition of  proteids  by  means  of  microorganisms,  and  which  is  the  chief  cause 
of  the  fecal  odor,  since  only  a  small  quantity  of  indol  remains  in  the  feces. 
These  odoriferous  bodies  are  derived  directly  from  the  food ;  at  the  same 
time  it  is  quite  possible  that  other  specific  odoriferous  substances  may  be 
secreted  directly  from  the  intestinal  wall,  especially  from  that  of  the  large 
intestine. 

THE  LACTEALS  AND  THE  LYMPHATIC  SYSTEM. 

§  246.  We  have  seen  that  absorption  does,  or  at  least  may,  take  place 
from  the  stomach.  We  have  also  stated  that  a  large  absorption,  especially 
of  water,  occurs  along  the  whole  large  intestine.  Nevertheless,  it  is  during 
the  transit  of  food  along  the  small  intestine  that  the  largest  and  most  im- 
portant part  of  the  digested  material  passes  away  from  the  canal,  partly  into 
the  lacteals,  partly  into  the  portal  vessels.  The  portal  vessels  are  simply 
parts  of  the  general  vascular  system ;  the  lacteals,  into  which,  we  may  at 


314  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

once  say,  the  greater  part  of  the  fat  passes,  are  similarly  parts  of  the  general 
lymphatic  system,  being  in  fact  the  lymphatic  vessels  of  the  alimentary  canal, 
and  especially  of  the  small  intestine.  The  only  reason  for  the  special  name 
of  lacteals  is  that,  unlike  the  lymphatic  vessels  of  other  parts  of  the  body, 
the  lymphatics  of  the  intestine  contain  at  times  a  fluid  of  a  milky  white 
appearance.  Hence  for  the  better  understanding  of  absorption  by  the  lac- 
teals  it  will  be  desirable  to  study  at  some  length  the  whole  subject  of  the 
lymphatic  system. 

The  lymphatic  vessels  may  be  said  to  begin  in  minute  passages,  possess- 
ing special  characters,  known  as  lymph-capillaries.  Broadly  speaking,  these 
lymph-capillaries  are  found,  in  the  mammal,  in  all  parts  of  the  body  in 
which  connective  tissue  is  found ;  and  they  have  special  connections  with 
those  minute  spaces  in  connective  tissue  which  we  have  already  more  than 
once  spoken  of  as  lymph-spaces. 

These  lymph-capillaries,  which,  as  we  shall  see,  are  frequently  arranged 
in  plexuses,  are  continuous  with  other  passages  also  minute  but  of  a  differ- 
ent and  more  regular  structure,  the  lymphatic  vessels  proper,  which  are 
gathered  into  larger  and  larger  vessels,  all  running  like  the  bloodvessels  in 
a  bed  of  connective  tissue,  until  at  last  all  the  lymphatic  vessels  of  the  body 
join  either  the  great  thoracic  duct  which  opens  by  a  valvular  orifice  into 
the  venous  system  at  the  junction  of  the  left  jugular  and  subclavian  veins, 
or  the  small  right  lymphatic  trunk  which  similarly  opens  into  the  junction 
of  the  right  jugular  and  subclavian  veins.  Serous  cavities  may  be  regarded 
as  large  lymph-spaces  connected  with  lymphatic  vessels. 

THE  NATURE  AND  MOVEMENTS  OF  LYMPH  (INCLUDING  CHYLE). 

§  247.  We  are  led  to  regard  the  multitudinous  spaces,  both  small  and 
great,  of  connective  tissue  all  over  the  body,  including  among  these  the 
"  serous  cavities,"  as  forming  the  beginning  or  roots  of  the  lymphatic  sys- 
tem. Into  these  spaces  certain  parts  of  the  plasma  of  the  blood  transude 
and  so  become  lymph  (to  what  extent  the  epithelioid  lining  of  the  large 
serous  cavities  plays  in  regulating  the  transudation  of  serous  fluid,  i.  e.,  of 
lymph,  into  those  cavities  we  do  notknowj  ;  from  these  spaces  the  lymph  is 
continually  flowing  through  the  lymph-capillaries  into  the  lymphatic  vessels, 
and  so  by  the  thoracic  duct  and  right  lymphatic  trunk  back  into  the  blood 
system. 

The  amount  of  lymph  occupying  the  lymph-spaces,  lymph-capillaries, 
and  minute  lymphatic  vessels  of  any  region  varies  from  time  to  time  accord- 
ing to  circumstances.  A  hand,  for  instance,  which  has  been  kept  hanging 
down  for  some  time  becomes  swollen  and  the  skin  tense  ;  if  it  be  raised  the 
swelling  lessens  and  the  skin  becomes  loose  ;  and  a  similar  temporary  swell- 
ing of  the  skin  of  the  limbs,  and  of  the  skin  generally,  is  frequently  the 
result  of  active  exercise.  Such  a  swelling  is  partly  due  to  the  bloodvessels 
being  dilated,  or  to  the  return  flow  along  the  veins  being  retarded,  so  that 
the  blood  capillaries  become  distended  with  blood,  but  is  much  more  largely 
owing  to  the  lymph-spaces  and  lymphatic  vessels  of  the  skin  and  under- 
lying structures  being  unusually  filled  with  lymph.  On  the  other  hand 
the  skin  may  become  shrivelled  and  dry  from  a  deficiency  of  lymph  in 
the  lymph  spaces  and  vessels.  Under  even  normal  circumstances  the 
quantity  of  lymph  in  the  tissues  may  vary  considerably,  and  under  abnor- 
mal circumstances  a  very  large  amount  of  lymph  may  greatly  distend 
the  spaces  of  the  connective  tissue  of  the  skin  and  other  structures,  giving 
rise  to  oedema  or  dropsy.  Obviously  there  are  agencies  at  work  in  the 
body  by  which  the  appearance  of  lymph  in  the  spaces  or  its  removal 


THE  NATURE  AND  MOVEMENTS  OF  LYMPH.  315 

thence    along  the  lymph-channels,  or    both,   may   be    either    increased  or 
diminished. 

The   Characters  of  Lymph. 

§  248.  As  it  slowly  flows  from  its  origin  in  the  tissues  to  the  mouth  of 
the  thoracic  duct  (we  may  for  simplicity's  sake  omit  the  right  lymphatic 
trunk)  the  lymph  is  subjected  to  the  influence  of  the  lymphatic  glands,  and 
is  possibly  affected  by  the  walls  of  the  lymph-vessels.  Moreover,  the  lymph 
coming  from  one  tissue  differs  more  or  less  in  certain  characters  from  the 
lymph  arising  in  another  tissue,  just  as  the  venous  blood  of  one  organ  differs 
from  the  venous  blood  of  another  organ  ;  and  these  differences  may  be  exag- 
gerated by  the  activity  of  theone  or  other  tissue.  Of  these  differences  by 
far  the  most  striking  is  that  between  the  lymph  coming  from  the  alimentary 
canal  during  active  digestion  and  known  as  chyle,  and  the  lymph  coming 
from  other  parts  of  the  body.  When  digestion  is  not  going  on,  and  when, 
consequently,  no  considerable  absorption  of  material  from  the  alimen- 
tary canal  into  the  lacteals  is  taking  place,  the  fluid  flowing  along  the 
lacteals  is  lymph,  not  differing  from  the  lymph  of  other  regions  to  any 
marked  degree. 

The  food,  accordingly,  which  flows  along  the  thoracic  duct  in  an  animal 
which  has  not  been  fed  for  some  considerable  time  may  be  taken  as  illustrat- 
ing the  general  characters  of  lymph.  The  contents  of  the  thoracic  duct 
may  be  obtained  by  laying  bare  the  junction  of  the  subclavian  and  jugular 
(in  the  dog  the  junction  of  the  axillary  and  jugular)  veins,  and  introdu- 
cing a  canula  into  the  duct  as  it  enters  into  the  venous  system  at  that  point. 
The  operation  is  not  unattended  with  difficulties. 

Lymph,  so  obtained,  is  a  clear  transparent  or  slightly  opalescent  fluid 
which  left  to  itself  soon  clots.  The  clotting  is  not  so  pronounced  as  that  of 
blood,  but  clotting  is  caused  as  in  blood  by  the  appearance  of  fibrin.  The 
fibrin  which  is  formed,  though  scanty  (0.5  per  cent.),  is  identical  apparently 
with  that  of  blood,  and,  as  far  as  we  know,  all  that  has  been  said  previously, 
(§§  14-23),  concerning  the  nature  of  clotting  blood  applies  equally  well  to 
lymph. 

Examined  with  the  microscope  lymph  contains  a  number  of  corpuscles, 
lymph-corpuscles,  which  in  all  their  characters  as  far  as  is  at  present  known 
are  identical  with  white  blood-corpuscles ;  they  vary  in  size  from  5//  to  15/Jt, 
and  the  smaller  corpuscles  are  much  more  abundant  in  lymph  than  in 
blood.  Like  the  white  corpuscles  of  blood  they  exhibit  amoeboid  move- 
ments. Their  number  varies  in  different  animals,  and,  apparently,  in  the 
same  animal,  according  to  circumstances ;  on  the  whole  perhaps  it  may  be 
said  that  lymph-corpuscles  are  about  as  numerous  in  lymph  as  white  cor- 
puscles in  blood.  Even  when  every  care  is  taken  to  avoid  admixture  with 
blood,  lymph,  and  especially  chyle,  not  unfrequently  contains  a  certain 
number  of  red  blood-corpuscles ;  sometimes  these  are  sufficient  to  give 
the  lymph  (or  chyle)  a  reddish  tinge.  They  have  been  observed  within 
the  living  lymphatic  vessels,  even  within  small  ones,  and  have  probably 
in  some  manner  or  other  made  their  way  from  the  blood  into  the  lymph 
channels. 

§  249.  The  chemical  composition  of  lymph,  even  when  taken  in  each 
case  from  the  thoracic  duct,  varies  a  good  deal.  The  total  solids  are 
much  less  than  in  blood,  amounting  in  general  to  not  more  than  5  or  6 
per  cent.  Hence  the  venous  blood  of  a  vascular  area  contains  rather 
more  solids  than  the  arterial  blood  of  the  same  area,  since  the  blood  in 
giving  rise  to  the  lymph  during  its  passage  through  the  capillaries  from  the 
arteries  to  the  veins  has  parted  with  relatively  more  water  than  solid  matter. 


316  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

The  proteids  amount  on  the  average  to  about  3  or  4  per  cent.,  that  is  to 
say,  to  about  half  as  much  as  in  blood,  the  particular  proteids  present  being 
the  same  as  in  blood,  viz.,  albumin,  paraglobulin  and  antecedents  of  fibrin. 
In  lymph,  as  distinguished  from  chyle,  the  quantity  of  fat  is  small,  and  con- 
sists of  the  usual  neutral  fats  and  the  soaps  of  their  fatty  acids,  together 
with  lecithin  ;  cholesterin  may  also  be  present.  A  certain  amount  of  sugar 
(dextrose)  appears  to  be  always  present,  and  several  observers  have  found 
an  appreciable  quantity  of  urea.  The  ash  of  lymph  like  that  of  blood-serum 
contains  a  considerable  quantity  of  sodium  chloride,  while  phosphates  and 
potash  are  scanty ;  it  also  contains  iron,  apparently  in  too  great  a  quantity 
to  be  accounted  for  by  the  few  red  corpuscles  which  may  be  present.  From 
lymph  a  certain  amount  of  gas  can  be  extracted,  consisting  chiefly  or  almost 
exclusively  of  carbonic  acid,  with  a  small  quantity  of  nitrogen,  the  amount 
of  oxygen'present  being  exceedingly  small.  The  importance  of  this  we  shall 
see  when  we  come  to  study  respiration. 

Broadly  speaking,  we  may  say  that  all  the  substances  present  in  blood- 
plasma  are  present  also  in  lymph,  but  are  accompanied  by  a  larger  quantity 
of  water. 

§  250.  Lymph  may  also  be  obtained  from  separate  regions  of  the  body, 
as  from  the  lower  or  upper  limbs,  for  instance,  by  introducing  a  fine  canula 
into  a  lymphatic  vessel.  In  its  general  features  the  lymph  so  obtained  re- 
sembles that  taken  from  the  thoracic  duct.  Analyses  of  the  lymph  distend- 
ing the  subcutaneous  connective  tissue  in  cases  of  dropsy  show  that  this  con- 
tains much  less  solid  matter  than  normal  lymph  taken  from  the  thoracic  duct 
or  larger  lymphatic  vessels.  From  this  it  has  been  inferred  that  the  lymph 
normally  existing  in  the  lymph-spaces,  lymph-capillaries  and  minute  ves- 
sels contains  an  excess  of  water ;  and,  indeed,  it  has  been  asserted  that  the 
percentage  of  solids  increases  in  passing  from  the  smaller  to  the  larger  ves- 
sels ;  but  this  cannot  be  regarded  as  distinctly  proved.  The  number  of  cor- 
puscles, however,  as  we  have  already  said,  appears  to  be  increased  in  passing 
through  the  lymphatic  glands.  It  has  also  been  stated  that  the  lymph  in 
the  finer  lymph-vessels  clots  even  less  firmly  than  that  in  the  thoracic  duct. 
From  this  we  may  infer  that  some  of  the  leucocytes  in  the  adenoid  tissue 
of  the  follicles  of  a  lymphatic  gland  find  their  way  into  the  lymph-sinus, 
and  so  into  the  efferent  lymphatics,  and  that  some  of  the  fibrin  factors  are 
added  to  the  lymph,  or,  at  least,  that  some  changes  favorable  to  clotting  are 
brought  about. 

§  251.  The  large  serous  cavities  of  the  peritoneum,  pericardium,  etc., 
are  parts  of  the  lymphatic  system  ;  indeed,  pericardial  or  other  serous  fluid 
has  all  the  general  characters  of  lymph.  We  have  already  said  (§  20),  that 
these  fluids,  when  taken  fresh  from  the  body,  clot  (this  is,  at  least,  the  case 
in  most  animals)  ;  the  clot,  when  examined  microscopically,  is  found  to  con- 
sist of  colorless  corpuscles  like  those  of  lymph  or  of  blood  entangled  in  the 
meshes  of  fibrin.  Both  in  their  proteid  and  other  chemical  constituents  these 
serous  fluids  resemble  lymph.  Analyses  of  the  accumulations  of  fluid  occa- 
sionally occurring  in  these  cavities  show  that  they  contain  sometimes  less  and 
sometimes  more  solid  matter  than  ordinary  lymph.  The  aqueous  humor  of 
the  eye  contains  very  little  solid  matter ;  and  the  cerebro-spinal  fluid  is  so 
peculiar  that  it  had  better  be  considered  by  itself  in  connection  with  the 
nervous  system. 

§  252.  Chyle.  In  fasting  animals  the  fluid  flowing  along  the  lacteals, 
as  may  be  seen  by  inspection  of  the  mesentery,  is  clear  and  transparent ;  it 
is  lymph,  differing,  as  we  have  said,  in  no  essential  respects  from  the  lymph 
flowing  along  other  lymphatic  vessels.  Shortly  after  a  meal  containing  fat 
(and  every  meal  does  contain  some  fat),  the  lymph  becomes  white  and 


THE   NATURE   AND  MOVEMENTS  OF   LYMPH.  317 

opaque  like  milk,  the  more  so  the  richer  the  meal  is  in  fat ;  it  is  then  called 
chyle.  Owing  to  the  relatively  large  quantity  of  this  milky  fluid  which  for 
some  time  after  a  meal  continues  to  be  poured  into  the  thoracic  duct,  the 
contents  of  that  duct  also  become  milky,  and  are  also  called  chyle.  In  the 
thoracic  duct  the  chyle  of  the  lacteals  is  more  or  less  mixed  with  lymph  from 
other  lymphatic  vessels,  but  the  former  is  so  preponderating  that  the  contents 
of  the  duct  may  be  taken  as  illustrating  the  nature  of  chyle. 

Chyle  differs  from  lymph  in  one  important  respect,  and  one  only  :  where- 
as lymph  ordinarily  contains  a  small  quantity  only  of  fat,  chyle  contains  a 
very  large  amount.  The  actual  amount  of  fat  present  in  the  chyle  of  the 
thoracic  duct  varies,  as  may  be  expected,  very  considerably,  according  to  the 
nature  of  the  meal,  the  stage  of  digestion,  and  various  circumstances.  Five 
per  cent,  is  a  very  common  amount ;  in  the  dog  it  has  been  found  to  vary 
from  2  to  15  per  cent.  The  increase  in  fat  is  chiefly  if  not  exclusively  due 
to  an  increase  in  the  neutral  fats  ;  though  whether  the  small  quantity  of  soaps 
and  of  lecithin  present  is  greater  than  in  lymph  has  not  been  distinctly  as- 
certained. Cholesterin  is  probably  present  in  greater  amount  than  in  lymph, 
since  it  probably  comes  from  the  bile  poured  into  the  intestines  during  diges- 
tion ;  but  this  is  not  certain.  How  far  the  nature  of  the  fat,  that  is,  the 
proportion  of  the  various  kinds  of  fat,  of  stearin,  etc.,  varies  with  the  fats 
present  in  the  meal  has  not  been  definitely  ascertained. 

The  condition  of  the  fat  in  chyle  is  peculiar.  Some  of  it  exists,  like  the 
fat  in  milk,  in  the  form  of  fat-globules  of  various  sizes,  but  all  small.  A 
very  considerable  quantity,  however,  is  present  in  the  form  of  exceedingly 
minute  spherules  or  granules,  far  smaller  than  any  globules  to  be  seen  in 
milk  ;  these  exhibit  active  "  Brownian  movements."  The  fat  present  in  this 
form  is  spoken  of  as  the  "  molecular  basis  "  of  chyle,  and  is  very  distinctive 
of  chyle.  In  the  emulsified  contents  of  the  intestine,  often  called  chyle,  the 
fat  is  finely  divided,  and  to  a  large  extent  into  small  globules,  but  there  is 
nothing  corresponding  to  this  molecular  basis ;  the  fat  does  not  assume  this 
condition  until  it  has  passed  out  of  the  intestine  into  the  lacteals.  Lymph 
examined  with  the  microscope  shows  besides  the  white  corpuscles  only  very 
few  oil-globules  and  nothing  of  this  molecular  basis.  Just  as  in  fact  lymph 
is,  broadly  speaking,  blood  minus  its  red  corpuscles,  so  chyle  is  lymph  plus 
a  very  large  quantity  of  minutely  divided  neutral  fat. 

The  total  amount  of  lymph  or  of  chyle  which  enters  the  blood  system 
through  the  thoracic  duct,  though  it  probably  varies  considerably,  is  prob- 
ably also  always  very  large.  It  has  been  calculated  that  in  a  well-fed  animal 
a  quantity  equal  at  least  to  that  of  the  whole  blood  may  pass  through  the 
thoracic  duct  in  twenty-four  hours,  and  of  this  it  is  supposed  that  about  half 
comes  through  the  lacteals  from  the  alimentary  canal,  and  therefore  to  a 
large  extent  from  food,  and  the  remainder  from  the  body  at  large.  These 
calculations  are  based  on  uncertain  data,  and  cannot,  therefore,  be  taken  as 
of  exact  value,  but  we  may  use  them  for  the  sake  of  an  illustration.  Thus, 
in  a  man  of  average  weight,  that  is,  about  154  kilos.,  the  quantity  of  blood 
(§  38)  being  -^  of  the  body  weight  is  about  12  kilos.  The  quantity  of  lymph 
or  chyle,  therefore,  discharged  into  the  blood  in  an  hour  would  be  according 
to  this  calculation  half  a  kilo.,  or  something  less  than  half  a  litre;  and  since 
the  flow  must  vary  considerably  in  the  twenty-four  hours  would  be  sometimes 
less,  and  therefore  sometimes  even  more,  than  this. 

The  Movements  of  Lymph. 

§  253.  Making  every  allowance  for  the  uncertainty  of  the  calculations 
detailed  in  the  preceding  paragraph,  it  is  obvious  that  the  lymph  must  flow 


318  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

with  a  not  inconsiderable  rapidity  (if  we  take  about  half  the  above  estimate, 
the  rate  will  be  about  5  c.c.  per  minute)  through  the  thoracic  duct,  and 
therefore  must  also  be  continually  streaming  into  that  duct  along  the  various 
lymphatic  channels  from  the  manifold  lymph-spaces  of  the  body.  This 
onward  progress  of  the  lymph  is  determined  by  a  variety  of  circumstances. 
In  the  first  place,  the  remarkably  widespread  presence  of  valves  in  the 
lymphatic  vessels  causes  every  pressure  exerted  on  the  tissues  in  which  they 
lie  to  assist  in  the  propulsion  forward  of  the  lymph.  Hence  all  muscular 
movements  increase  the  flow.  If  a  canula  be  inserted  in  one  of  the  larger 
lymphatic  trunks  of  the  limb  of  a  dog,  the  discharge  of  lymph  from  the 
canula  will  be  more  distinctly  increased  by  movements,  even  passive  move- 
ments, of  the  limb  than  by  anything  else.  When  we  come  to  speak  of  the 
entrance  of  chyle  into  the  lacteal  radicles  of  the  villi,  we  shall  see  that,  at 
all  events  according  to  one  view,  the  muscular  fibres  of  the  villus  act  as  a 
kind  of  muscular  pump,  driving  the  chyle  past  the  valved  end  of  the  lacteal 
radicle  into  the  lymphatic  canals  below.  In  addition  to  the  presence  of  valves 
along  the  course  of  the  vessels,  the  opening  of  the  thoracic  duct  into  the  venous 
system  is  guarded  by  a  valve,  so  that  every  escape  of  lymph  or  chyle  from 
the  duct  into  the  veins  becomes  itself  a  help  to  the  flow.  In  the  second 
place,  we  have  already  seen  that  the  blood-pressure  in  the  capillaries  and 
minute  vessels  is  considerably  greater  than  that  in  the  large  veins,  such  as 
the  jugular;  in  fact,  this  difference  of  pressure  is  the  cause  of  the  flow  of 
blood  from  the  capillaries  to  the  heart.  Now  the  lymph  in  the  lymphatic 
spaces  outside  the  capillaries  and  minute  vessels  undoubtedly  stands  at  a 
lower  pressure  than  the  blood  inside  the  capillaries  ;  otherwise  the  transuda- 
tion  from  the  blood  into  the  tissues  would  be  checked;  but  the  difference  is 
probably  much  less  than  the  difference  between  the  pressure  in  the  capillaries 
and  that  in  the  large  venous  trunks.  So  that  the  lymph  in  the  lymph-spaces 
of  the  tissues  may  be  considered  as  standing  at  a  higher  pressure  than  the 
blood  in  the  venous  trunks,  for  instance,  in  the  jugular  vein.  That  is  to  say, 
the  lymphatic  vessels  as  a  whole  form  a  system  of  channels  leading  from  a 
region  of  higher  pressure,  viz.,  the  lymph-spaces  of  the  tissues,  to  a  region 
of  lower  pressure,  viz.,  the  interior  of  the  jugular  and  subclavian  veins.  This 
difference  of  pressure  will,  as  in  the  case  of  the  bloodvessels,  cause  the  lymph 
to  flow  onward  in  a  continuous  stream.  Further,  this  flow,  caused  by  the 
lowness  of  the  mean  venous  pressure  at  the  subclavian  vein,  will  be  assisted 
at  every  respiratory  movement,  since  at  every  inspiration  the  pressure  in  the 
venous  trunks  becomes,  as  we  shall  see  in  dealing  with  respiration,  negative, 
and  thus  lymph  will  be  sucked  in  from  the  thoracic  duct,  while  the  increase 
of  pressure  in  the  great  veins  during  expiration  is  warded  off  from  the  duct 
by  the  valve  at  its  opening.  In  the  third  place,  the  flow  may  be  increased 
by  rhythmical  contractions  of  the  walls  of  the  lymphatics  themselves,  which, 
as  we  have  seen,  are  remarkably  muscular;  and  the  peculiar  interlacing  of 
the  muscular  fibres  above  each  valve  suggests  that  the  walls  here  act  after 
the  fashion  of  a  tiny  heart  and  by  a  rhythmical  systole  drive  on  the  fluid, 
which  by  the  action  of  the  valve  below  collects  at  the  spot.  We  have,  how- 
ever, no  experimental  proof  of  this;  for,  though  rhythmic  variations  have 
been  observed  in  the  lacteals  of  the  mesentery,  it  is  maintained  that  these 
are  simply  passive,  i.  e.,  caused  by  the  rhythmic  peristaltic  action  of  the  in- 
testine, each  contraction  of  the  intestine  filling  the  lymph-channels  more 
fully,  and  are  not  due  to  contractions  of  the  walls  of  the  lacteal  vessels  them- 
selves. In  some  of  the  lower  animals,  for  instance  in  the  frog,  the  muscular 
walls  of  the  vessels  are  developed  at  places  into  distinctly  contractile  pro- 
pulsive organs,  spoken  of  as  lymph-hearts.  Lastly,  it  is  at  least  open  for 
us,  on  the  strength  of  the  analogy  that  osmosis  may  give  rise  to  increased 


THE  NATURE  AND   MOVEMENTS  OF  LYMPH.  319 

pressure  on  one  side  of  a  diffusion  septum,  to  suppose  that  the  very  processes 
which  give  rise  to  the  appearance  of  lymph  in  the  lymph-spaces  of  the  tis- 
sues, tend  themselves  to  promote  the  flow  of  lymph.  We  have  at  least,  un- 
der all  circumstances,  one  or  other  of  these  causes  at  work,  promoting  a 
continual  flow  from  the  lymphatic  roots  to  the  great  veins.  They  are  to- 
gether sufficient  to  drive,  in  man,  the  lymph  from  the  lower  limbs  and  trunk, 
against  the  effects  of  gravity,  into  the  veins  of  the  neck.  In  the  upper  limb 
the  influences  of  gravity,  owing  to  the  varied  movements  of  the  limb,  are 
as  often  favorable  to  as  opposed  to  the  natural  flow  of  the  lymph  ;  but,  as 
we  have  already  said,  a  long-continued  unfavorable  action  of  gravity,  espe- 
cially in  the  absence  of  the  aid  of  movements  in  the  skeletal  muscles,  as  when 
the  arm  hangs  down  motionless  for  some  time,  leads  to  accumulation  of  lymph 
at  its  origin  in  the  lymph-spaces.  The  strength  of  the  causes  combining  to 
drive  on  the  lymph  is  strikingly  shown  in  animals  when  the  thoracic  duct 
is  ligatured  ;  in  such  cases  a  very  great  distention  of  the  lymphatic  vessels 
below  the  ligature  is  observed. 

§  254.  Although  the  phenomena  of  disease  and,  perhaps,  general  con- 
siderations render  it  probable  that  the  nervous  system  governs  in  some  way 
the  stream  of  lymph,  regulating,  it  may  be,  not  only  the  flow  along  the 
definite  lymph-canals,  but  also  the  transit  of  plasma  into  the  lymph-spaces 
and  the  escape  of  lymph  thence  into  the  definite  canals,  our  knowledge  on 
these  points  is  very  imperfect.  We  have  no  proof  that  the  muscular  fibres 
in  the  walls  of  the  lymphatic  vessels  are  governed  by  nerves,  or  that  the 
lymph-spaces  are  influenced  directly  by  nervous  action  ;  and  most  of  the 
attempts  to  demonstrate  any  direct  action  of  the  nervous  system  on  the 
lymphatics  have  hitherto  failed. 

§  255.  The  passage  of  material,  namely,  of  water  containing  certain  sub- 
stances in  solution,  from  the  interior  of  the  bloodvessel,  where  they  form  part 
of  the  plasma,  into  the  lymph-capillary  where  they  are  called  lymph,  consists 
of  two  steps :  the  passage  from  the  bloodvessel  into  the  lymph-space,  and  the 
passage  from  the  lymph-space  into  the  lymph-capillary  ;  for,  as  we  have  seen, 
it  is  only  in  particular  places  that  the  lymph-capillary  immediately  surrounds 
the  bloodvessel.  Once  arrived  in  the  lymph-capillary  the  lymph  finds  an 
open  path  along  the  rest  of  the  lymphatic  system,  but  the  connection  between 
the  lymph-space  and  the  lymph-capillary  is,  as  we  have  seen,  peculiar  and  at 
least  not  a  free  and  open  one. 

The  passage  of  material  from  the  bloodvessel  into  the  lymph-space  we 
speak  of  as  transudation.  What  can  we  say  as  to  the  nature  of  this  process  ? 
There  are  two  known  physical  processes  with  which  we  may  compare  it : 
diffusion  through  a  membranous  or  other  porous  partition,  and  filtration 
through  a  similar  partition.  Diffusion,  though  influenced  by  fluid  pressure, 
is  not  the  direct  result  of  fluid  pressure,  but  may,  on  the  contrary,  be  the 
cause  of  differences  of  pressure  on  the  two  sides  of  the  partition,  and  may 
work  against  fluid  pressure.  When  a  strong  solution  and  a  weak  solution 
of  salt  are  separated  by  a  diffusion  septum,  diffusion  takes  place  whether  the 
columns  of  fluid  be  at  the  same  level  on  the  two  sides  of  the  septum  or  at 
different  levels ;  and  if  the  columns  be  at  the  same  level  to  start  with,  that 
of  the  stronger  solution  soon  comes  to  exceed  the  other  in  height,  on  account 
of  the  osmotic  flow  of  water  from  the  weaker  into  the  stronger  solution. 
Filtration,  on  the  other  hand,  is  the  direct  result  of  pressure;  without  dif- 
ference of  pressure  filtration  does  not  take  place ;  and,  the  filter  remaining 
of  the  same  nature  and  in  the  same  condition,  the  amount  of  filtrate  is 
dependent  on  the  amount  of  pressure.  May  we  speak  of  the  process  of 
transudation  as  a  simple  process  of  diffusion  or  a  simple  process  of  filtration  ? 
that  is  to  say,  can  all  the  phenomena  of  transudation  be  explained  as  simply 


320  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

the  results  of  one  or  other  of  these  physical  processes  ?  Diffusion  by  itself 
will  not  account  for  the  results ;  for  the  proteids  of  the  blood-plasma  are 
iudiffusible  or  very  nearly  so,  and  yet  the  lymph  contains  a  considerable 
quantity  of  these  proteids.  We  have  no  satisfactory  knowledge  of  the  exact 
composition  of  lymph  as  it  exists  in  the  lymph-spaces.  In  the  lymph  of  the 
larger  lymph-trunks  the  diffusible  saline  substances  are  present  in  about  the 
same  proportion,  and  the  indiffusible  proteids  to  about  or  less  than  half  as 
much  as  in  blood- serum  ;  and  we  may  perhaps  assume  that  the  lymph  in  the 
lymph-spaces  contains  relatively  less  proteids,  but  has  otherwise  the  same 
composition  as  blood-plasma.  Mere  diffusion  would  not  give  rise  to  a  fluid 
of  such  a  nature.  Can  we  speak  of  transudation,  then,  as  a  filtration  ?  The 
blood  is  undoubtedly  flowing  through  the  capillaries  and  other  small  vessels 
under  a  certain  pressure ;  we  have  seen  (§  105)  that  the  pressure  is  roughly 
speaking  about  30  mm.  Hg. ;  and  it  would  be  possible  to  select  such  a  filter 
or  porous  partition  as  would  at  about  this  pressure  permit  the  passage  of  a 
certain  quantity  of  the  inorganic  and  crystalline  constituents  of  blood-plasma 
to  pass  through  in  company  with  a  relatively  smaller  quantity  of  the  proteids 
and  a  large  quantity  of  the  water,  the  red  and  white  corpuscles  being  ex- 
cluded. Such  a  filtrate  would  be  more  or  less  of  the  nature  of  lymph  ;  and 
so  far  we  might  be  justified  in  speaking  of  the  transudation  of  lymph  as  a 
process  of  filtration.  But  the  transit  through  the  living  wall  of  the  blood- 
vessel is  affected  by  circumstances  in  a  manner  so  different  from  the  manner 
in  which  the  same  circumstances  affect  a  transit  through  an  ordinary  lifeless 
filter,  that  we  gain  but  little  and  may  be  led  into  error  by  speaking  of  the 
process  as  a  filtration.  Substances  in  solution  or  otherwise,  pass  through  a 
filter  when  the  pressure  is  sufficient  to  drive  them  through  the  passages 
furnished  by  the  interstices  existing  in  the  substance  of  the  filter.  In  the 
case  of  an  ordinary  filter  the  substance  of  the  filter  is  within  limits  perma- 
nent, and  the  passages  correspondingly  constant.  The  living  wall  of  a 
capillary,  however,  is  not  a  constant  unchanging  thing.  The  epithelioid 
plates  and  other  elements  which  constitute  it  are  alive,  and  being  alive  are 
continually  undergoing  change  and  are  especially  subject  to  change ;  more- 
over, as  we  have  seen  (§§  22,  23),  the  vascular  walls  appear  to  be  continu- 
ally acting  upon  and  being  acted  upon  by  the  blood.  Hence  a  change  in 
the  blood  tends  to  cause  changes  in  them ;  and  these  changes  may  materially 
affect  in  one  direction  or  another  their  action  as  filters.  In  an  ordinary  filter 
increase  of  pressure  necessarily  entails  increase  of  filtration;  in  a  living 
filter  it  may  or  may  not,  and  the  same  increase  of  pressure  may  according 
to  circumstances  produce  very  different  results  as  regards  the  transudation 
of  lymph. 

Thus  it  seems  reasonable  to  suppose,  as  we  have  suggested  (§  197),  that, 
other  things  being  the  same,  an  increase  of  blood-pressure  should  necessarily 
increase  the  transudation  of  lymph.  Hence  when  a  small  artery  dilates, 
since  the  pressure  in  the  still  smaller  branches  and  capillaries  of  that  artery 
is,  as  we  have  more  than  once  pointed  out,  increased,  more  lymph  appears  in 
the  lymph-spaces  ;  indeed  it  is  one  of  the  main  purposes  of  the  widening  of 
small  arteries  to  supply  the  elements  of  the  tissue  with  more  lymph,  that  is, 
with  more  food.  But  it  does  not  therefore  follow  that  under  all  circum- 
stances widening  of  the  artery  should  increase  the  passage  of  lymph  ;  some- 
thing may  occur  to  counteract  the  natural  effect  of  the  increased  pressure  in 
the  bloodvessels.  An  instance  of  this  seems  to  be  afforded  by  the  case  of 
the  submaxillary  gland,  when  the  chorda  nerve  is  stimulated  while  the  gland 
is  under  the  influence  of  atropine.  As  we  have  seen,  though  the  arteries 
dilate,  no  secretion  takes  place  ;  and  we  cannot  explain  the  absence  of  a  flow 
into  the  alveoli  by  supposing  that  the  extra  amount  of  lymph  which  would 


THE  NATURE  AND  MOVEMENTS  OF  LYMPH.  321 

in  normal  circumstances  form  part  of  the  secretion,  and  in  the  case  of  a 
fairly  copious  secretion  would  be  considerable,  now  passes  away  by  the  lym- 
phatics without  reaching  the  cells  of  the  alveoli,  for  in  such  cases  no  extra 
flow  in  the  lymphatics  leading  from  the  gland  has  been  observed,  and  there 
is  no  accumulation  of  lymph  in  the  connective  tissue  of  the  gland.  Ap- 
parently, for  some  reason  or  other,  in  spite  of  the  increased  pressure  in  the 
bloodvessels,  more  lymph  than  usual  does  not  pass  into  the  lymph-spaces. 

Then  again,  as  we  shall  presently  have  occasion  to  point  out,  an  increase 
of  pressure  in  the  bloodvessels  produced  by  obstruction  to  the  venous  out- 
flow is  much  more  efficient  in  promoting  an  increase  of  transudation^  at  all 
events  an  abnormal  increase,  than  is  an  increase  of  arterial  pressure ;  and 
the  difference  between  the  two  cases  appears  to  be  too  great  to  be  accounted 
for  on  the  ground  that  an  obstruction  to  the  venous  outflow  raises  the  pres- 
sure within  the  capillaries  and  small  vessels  more  readily  and  to  a  higher 
degree  than  does  the  widening  of  the  arteries.  Moreover,  that  obstruction 
to  venous  outflow  does  not  produce  its  effects  in  the  way  of  transudation 
simply  and  merely  by  raising  the  capillary  pressure  is  shown  by  the  fact  that 
the  same  amount  of  obstruction  may  or  may  not  give  rise  to  excessive  trans- 
udation, according  to  the  condition  of  the  blood  or  other  circumstances. 
For  instance,  though  the  obstruction  produced  by  ligaturing  a  vein  fre- 
quently causes  excessive  traiisudation,  it  does  not  always  cause  it,  and  the 
femoral  vein  of  a  dog  may  be  ligatured  without  any  excessive  transudatiou 
taking  place ;  yet  if,  after  the  ligature,  certain  changes  be  induced  in  the 
blood,  excessive  transudation  occurs  in  the  leg  the  vein  of  which  has  been 
ligatured,  but  not  elsewhere.  Pointing  toward  the  same  conclusion  is  the 
fact  that  excessive  transudation  more  readily  occurs  when  a  vein  is  plugged 
by  a  thrombus  arising  from  abnormal  conditions  of  the  vascular  system  than 
when  a  vein  is  simply  ligatured.  And  in  general  we  may  say,  and  this  is  a 
point  to  which  we  shall  return,  that  two  things  chiefly  determine  the  amount 
of  transudation :  the  pressure  of  the  blood  in  the  bloodvessels,  and  the  con- 
dition of  the  vascular  walls  in  relation  to  the  blood,  the  latter  being  at  least 
as  important  as  the  former. 

Another  aspect  of  the  matter,  moreover,  deserves  attention.  In  filtration 
the  movement  takes  place  through  the  filter  in  one  direction  only,  whereas 
in  the  living  body,  the  passage  of  material  through  the  capillary  wall  takes 
place  in  two  opposite  directions.  In  all  the  tissues,  though  more  perhaps  in 
certain  tissues  than  in  others,  the  passage  from  the  bloodvessel  into  the 
lymph-space  is  accompanied  by  a  passage  from  the  lymph-space  into  the 
blood ;  while  food  for  the  tissue  passes  in  one  direction,  waste  products  pass 
in  the  other.  In  a  secreting  gland  the  greater  part  of  the  lymph  coming 
from  the  bloodvessels,  the  water  and  other  matters,  pass  away  into  the  lumen 
of  the  alveolus  after  undergoing  changes  in  the  cell ;  but  even  in  such  a  case 
there  is  some  return  from  the  cells  into  the  bloodvessels,  carbonic  acid,  for 
instance,  if  nothing  else,  is  given  up  by  the  cells  to  the  blood ;  and  in  such 
organs  as  a  muscle  or  the  liver,  the  backward  stream  of  material  from  the 
tissue  to  the  blood  is  extensive  and  important.  Moreover,  this  backward 
stream  works  against  pressure ;  indeed,  as  may  be  seen  in  a  muscle,  it  is 
when  the  bloodvessels  are  dilated  and  the  pressure  in  the  capillaries  and 
small  vessels  highest,  as  during  and  after  the  contraction  of  the  muscle,  that 
the  passage  from  the  tissue  into  the  blood  is  most  energetic.  Many  of  the 
waste  products  of  the  tissue  are,  it  is  true,  diffusible,  and  we  might  be  tempted 
to  say  that  while  the  lymph  which  feeds  the  tissue  traverses  the  vascular 
wall  by  filtration  in  the  direction  of  pressure,  the  waste  products  return  to 
the  blood  against  pressure  by  diffusion  ;  but  such  a  view  cannot  at  present 
be  regarded  as  proved  ;  and  if  it  be  true,  as  is  maintained  by  some,  that 
21 


322  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

lymph,  including  the  proteids,  may  at  times  be  reabsorbed  from  the  tissue 
into  the  bloodvessels,  it  is  distinctly  contradicted.  Finally,  the  introduction 
of  certain  substances  (termed  lytnphagogues)  into  the  blood,  is  followed  by  an 
increase  in  the  quantity  of  lymph  without  any  related  change  in  the  blood- 
pressure.  We  shall  have  to  return  to  this  question  when  we  come  to  deal 
with  the  secretion  of  urine ;  but  meanwhile  we  may  adopt  the  conclusion, 
which  is  especially  supported  by  the  phenomena  of  disease,  that  while  dif- 
fusion and  filtration  play  their  respective  parts,  diffusible  substances  pass- 
ing in  and  out  of  the  blood  more  readily  than  indiffusible  substances  and  an 
increase  of  pressure  tending  to  promote  transudation,  the  condition  of  the 
vascular  wall  so  profoundly  influences  the  transit  of  material  as  to  render 
the  process  very  complex.  We  may  probably  regard  it  as  too  complex  to 
be  compared  even  with  filtration  through  a  filter  capable  of  widely  changing 
in  texture  from  time  to  time,  and  as  more  nearly  resembling  the  process  of 
secretion. 

Concerning  the  passage  of  the  lymph  from  the  confined  lymph-spaces 
into  the  open  gangways  of  the  lymph-capillaries  we  know  very  little.  If,  as 
some  think,  the  cavity  of  the  lymph-capillary  is  shut  off  on  all  sides  and 
completely  by  a  continuous  lining  of  sinuous  epithelioid  plates,  then  the 
passage  from  the  lymph-space  into  it  must  be  regarded  as  a  sort  of  repetition 
of  the  passage  from  the  blood-capillary  into  the  lymph-space,  as  a  second 
transudation.  But  if,  as  others  think,  and  as  on  the  whole  seems  more 
probable,  the  lymph-spaces  open  at  places  directly  into  the  lymph-capillaries, 
the  passage  is  a  simply  mechanical  affair  determined  by  the  freedom  of  these 
openings. 

In  either  case  the  flow  from  the  lymph-spaces  will  be  facilitated  by  all 
events  which  promote,  and  checked  by  those  which  hinder  the  flow  of  lymph 
along  the  lymph-capillaries  and  the  other  lymphatic  channels. 

We  may  here  remark  as  influencing  the  quantity  of  lymph  in  the  lymph- 
spaces  and  vessels  that  the  quantity  of  lymph  taken  up  from  the  lymph- 
spaces  by  the  actual  elements  of  the  tissue  may  vary  considerably.  We 
remarked  in  §  30  on  the  peculiar  relations  of  living  tissue  to  water,  and 
there  are  reasons  for  thinking  that  the  very  substance  of  a  cell  or  a  fibre 
(a  muscular  fibre,  for  instance)  may  hold  in  itself  a  larger  quantity  of  water 
at  one  time  than  at  another.  The  water  thus  taken  up  or  given  out,  and  the 
substances  which  may  be  carried  in  solution  by  that  water,  come  from  and 
go  to  the  lymph.  The  condition  of  the  tissue  determines  by  itself  the 
amount  of  lymph  in  the  lymph-spaces. 

§  256.  Under  the  influence  of  all  these  several  actions  the  lymph  in  the 
various  lymph-spaces  of  the  body  varies  in  amount  from  time  to  time,  but 
under  normal  circumstances  never  exceeds  certain  limits.  Under  patho- 
logical conditions  those  limits  may  be  exceeded,  and  the  result  is  known  as 
cedema  or  dropsy.  Similar  excessive  accumulations  of  lymph  may  occur  not 
in  the  ordinary  lymph-spaces,  but  in  those  larger  lymph-spaces,  the  serous 
cavities,  any  large  excess  of  fluid  in  the  peritoneal  cavity  being  known  as 
ascites. 

The  possible  causes  of  oedema  are,  on  the  one  hand,  an  obstruction  to  the 
flow  of  lymph  from  the  lymph-spaces,  and  on  the  other  hand  an  excessive 
transudation,  the  lymph  gathering  in  the  lymph-spaces  faster  than  it  can  be 
carried  away  by  a  normal  flow  ;  with  the  former  the  lymphatic  system  itself, 
with  the  latter  chiefly  the  vascular  system,  is  concerned.  As  a  matter  of 
fact,  however,  oedema  is  almost  always,  if  not  always,  due  to  abnormal  con- 
ditions of  the  vascular  system,  and  is  the  result  not  of  hindered  outflow,  but 
of  excessive  transudation. 

Owing  to  the  numerous  anastomoses  of  the  lymph-vessels  and  the  conse- 


THE  NATURE  AND  MOVEMENTS  OF  LYMPH.  323 

qtient  establishment  of  collateral  streams,  obstruction  in  the  lymph -passages 
themselves  rarely  if  ever  gives  rise  to  oedema  ;  and  it  may  be  here  remarked 
that  owing  to  the  same  free  collateral  communication  between  the  lymph- 
vessels  the  labyrinthine  passages  of  the  lymphatic  glands  do  not  offer  the 
serious  obstacle  to  the  onward  flow  of  the  general  lymph-stream,  as  might  at 
first  sight  be  supposed.  Nor  have  we  at  present  any  knowledge  which  would 
lead  us  to  suppose  that  any  physiological  changes  in  the  walls  of  the  lym- 
phatic vessels  or  of  the  lymph-capillaries,  or  in  the  lymph-spaces,  by  giving 
rise  in  some  way  to  obstacles  to  the  flow  of  lymph,  ever  lead  to  an  accumu- 
lation of  lymph  in  the  latter. 

One  kind  of  cedema  we  have  already  touched  upon  in  speaking  of  the 
capillary  circulation  (§  169),  viz.,  the  "inflammatory"  oedema.  In  this 
kind  of  oedema,  owing  to  changes  in  the  vascular  walls,  a  larger  amount  of 
transudation  passes  into  the  lymph-spaces,  and  that  transudation  is  richer  in 
proteid  matters,  and  contains  a  larger  amount  of  the  fibrin  factors,  or  at  all 
events,  is  much  more  distinctly  coagulable  than  ordinary  lymph,  as  well  as 
crowded  with  migrating  corpuscles.  Allied  to  this  inflammatory  cedema  is 
the  increase  of  lymph,  also  apparently  changed  somewhat  in  character, 
which  appears  as  "  effusion  "  in  the  serous  cavities  when  these  are  inflamed, 
as  iu  pleurisy  and  peritonitis. 

One  of  the  most  common  forms  of  oedema  is  an  cedema  of  primarily, 
though  not  wholly,  mechanical  origin — oedema  arising  from  obstruction  to 
the  venous  flow ;  under  these  circumstances  more  lymph  passes  into  the 
lymph-spaces  than  the  lymph-vessels  are  able  to  carry" away.  If  the  femoral 
vein  be  tied  the  leg  may  become  cedematous,  and,  as  we  have  said,  cedema  is 
a  common  result  of  the  plugging  or  obstruction  of  veins  through  disease ; 
the  cedema  which  is  so  common  an  accompaniment  of  heart  disease,  involv- 
ing obstruction  to  the  return  of  venous  blood  to  the  right  side  of  the  heart, 
and  the  ascites  which  follows  upon  hindrance  to  the  portal  flow,  are  instances 
of  cedema  of  this  kind.  We  have  already  remarked  on  the  relation  of 
transudation  to  blood-pressure,  and  in  venous  obstruction  the  rise  of  pressure 
within  the  small  bloodvessels  is  distinguished  from  that  due  to  arterial  dila- 
tation by  being  accompanied  with  a  want  of  adequate  renewal  of  the  blood  ; 
this  probably  affects  the  epithelioid  lining  of  the  bloodvessels  in  such  a  way 
as  to  increase  the  transudation.  And,  indeed,  as  is  seen  in  case  of  heart  dis- 
ease with  prolonged  or  repeated  venous  obstruction,  the  oedema,  as  time  goes 
on  and  the  tissues  become  impaired,  is  more  easily  excited  and  with  greater 
difficulty  removed,  though  the  actual  amount  of  obstruction,  the  actual 
increase  of  pressure  in  the  small  vessels,  remains  the  same,  or  at  least  is  not 
proportionately  increased. 

Still  another  kind  of  oedema  is  one  due  to  changes  taking  place  in  the 
blood,  quite  apart  from  variations  of  blood-pressurer  This  kind  of  oedema 
is  seen  in  some  diseases  of  the  kidney,  in  "  Bright's  disease,"  for  instance. 
In  such  cases  the  blood  contains  less  proteids,  and  indeed  less  solids,  is  more 
watery  and  of  lower  specific  gravity  than  is  normal.  But  the  oedema  is  not 
in  these  cases  to  be  explained  on  the  view  that  the  more  watery  blood  passes 
more  readily  through  the  capillary  walls,  for  it  may  be  shown  experimentally 
that  the  mere  thinning  of  the  blood,  as  by  the  injection  of  normal  saline 
solution  into  the  bloodvessels,  will  not  at  once  lead  to  cedema,  at  least  in  the 
limbs  and  trunk,  and  it  is  these  which  in  Bright's  disease  especially  become 
cedematous.  In  all  probability  the  oedema  of  Bright's  disease,  if  it  be  really 
due  to  the  abnormal  character  of  the  blood,  is  produced  by  the  abnormal 
blood  so  acting  on  the  bloodvessels  that  these  allow  a  transudation  greater 
than  the  normal.  Finally,  oedema  may  be  due  to  abnormal  conditions  of 
the  tissues  themselves. 


324  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

But  these  are  pathological  questions  into  which  we  must  not  enter  here. 
We  have  touched  upon  them  because  they  illustrate  the  important  processes 
taking  place  in  the  lymph-spaces,  and,  as  we  have  more  than  once  insisted, 
the  lymph  in  the  lymph-spaces  is  the  middleman  of  all  the  tissues,  and  hence 
facts  illustrating  the  laws  which  govern  the  flow  of  lymph  into  and  out  of 
the  lymph-spaces  are  of  fundamental  physiological  importance. 

§  257.  Lymph-hearts.  In  the  frog  and  other  amphibia  and  in  reptiles 
the  flow  of  lymph  into  the  venous  system  is  assisted  by  rhythmically  pul- 
sating muscular  lymph-hearts,  which  present  many  curious  analogies  with 
the  blood-heart.  The  frog  possesses  four  lymph-hearts.  Of  these,  two 
belonging  to  the  hind  limbs  are  placed  one  on  each  side  of  the  coccyx  near 
its  end,  and,  being  covered  only  by  aponeurosis  and  the  skin,  may  without 
dissection  be  seen  beating.  Two  anterior  ones  are  placed  on  the  transverse 
processes  of  the  third  vertebra,  and  are  covered  from  view  by  the  shoulder 
girdle.  Each  lymph-heart  is  a  more  or  less  oval  sac  lying  in  one  of  those 
lymph  sacs  or  cavities  lined  with  sinuous  epithelioid  plates,  which,  as  we 
have  said,  are  present  in  the  frog.  It  is  continued  at  one  end,  by  an  ori- 
fice guarded  with  valves,  into  a  small  vein  which  opens,  in  the  case  of  the 
posterior  heart,  into  a  crural  vein,  and  in  the  case  of  the  anterior  hearts  into 
a  jugular  vein.  The  wall  consists  of  muscular  fibres  arranged  in  a  plexi- 
form  manner  and  supported  by  a  considerable  amount  of  connective  tissue. 
These  fibres  are  striated  and*  branched  and  are  intermediate  in  character 
between  cardiac  and  skeletal  muscular  fibres.  Nerve-fibres  terminate  in 
these  muscular  fibres,  and  the  muscular  wall,  unlike  that  of  the  blood-heart, 
is  supplied  with  capillary  bloodvessels.  The  interior  is  lined  with  epithelioid 
plates  of  sinuous  outline,  and  this  lymphatic  lining  is  continued  along  a 
number  of  openings  or  pores,  by  which  the  cavity  of  the  heart  opens  into 
the  surrounding  lymph-space.  When  the  heart  contracts  the  contents  are 
driven  into  the  vein,  the  lymphatic  pores  being  closed  by  the  approximation 
of  the  contracting  muscular  fibres ;  when  the  heart  dilates  the  fluid  in  the 
vein  is  prevented  from  returning  by  the  valves  at  its  mouth,  while  the  lymph 
enters  readily  from  the  surrounding  space  through  the  now  open  pores.  In 
the  frog  regular  lymphatic  vessels  are  scanty  ;  hence  these  lymph-hearts 
become  of  considerable  importance  in  promoting  the  flow  of  lymph.  The 
lymph-hearts  of  reptilia  are  similar  in  structure  and  function.  In  the  frog, 
in  which  they  have  been  chiefly  studied,  the  action  of  the  lymph-hearts  is  in 
a  measure  dependent  on  the  spinal  cord.  The  posterior  lymph-hearts  belong- 
ing to  the  hind  limbs  are  connected  by  means  of  the  delicate  tenth  pair  of 
spinal  nerves  with  a  region  of  the  cord  opposite  the  sixth  or  seventh  vertebra 
in  such  a  way  that  section  of  the  nerve  or  destruction  of  the  particular  region 
of  the  cord  suspends  o**  destroys  their  activity.  The  anterior  pair  are  simi- 
larly connected  with  a  region  of  the  spinal  cord  opposite  the  third  vertebra. 
Each  pair  therefore  seems  to  have  a  "  centre  "  in  the  spinal  cord  ;  but  it  is 
probable,  though  observers  are  not  wholly  agreed,  that  the  hearts,  after 
destruction  of  their  spinal  centre,  ultimately  resume  their  rhythmic  beats, 
so  that  the  dependence  of  their  activity  on  the  spinal  centre  is  not  an  abso- 
lute one.  Like  the  heart  of  the  blood -system,  the  lymph-hearts  may  be  in- 
hibited, and  that  in  a  reflex  manner,  the  inhibition  centre  being  moreover 
in  the  medulla  oblongata.  If  a  frog  be  carefully  observed,  the  activity  of 
the  lymph-hearts  will  be  found  to  vary  largely,  and  these  variations  appear 
to  be  in  part  due  to  nervous  influences,  so  that  in  this  way  the  movement 
of  lymph,  and  hence  the  processes  of  absorption,  are  in  this  animal  directly 
dependent  on  the  nervous  system. 


ABSORPTION   FROM   THE    ALIMENTARY    CANAL.  325 

ABSORPTION  FROM  THE  ALIMENTARY  CANAL. 

§  258.  We  may  now  return  to  consider  the  absorption  of  the  products 
of  digestion,  that  is  to  say,  the  passage  of  these  bodies  from  the  interior  of 
the  alimentary  canal,  where  they  are  really  outside  the  body  proper,  into 
the  body  itself.  For  simplicity's  sake  we  may  consider  digestion  in  a  broad 
way  as  the  conversion  of  practically  non-diffusible  proteids  and  starch  into 
more  diffusible  peptone  and  highly  diffusible  sugar,  and  as  the  emulsifying, 
or  division  into  minute  particles  of  fats.  We  have  reason  to  believe  that 
some  of  the  sugar  may  be  changed  into  lactic  acid,  or  even  into  butyric  or 
other  acids,  that  some  of  the  proteids  are  carried  beyond  the  peptone  condi- 
tion into  leucin  and  other  bodies,  and  that  some  of  the  fat  may  be  saponified  ; 
and  it  may  be  that  some  of  the  proteid  material  of  the  food  passes  into  the 
body  as  albumose  or  even  as  parapeptone,  or  in  some  other  little  changed 
condition.  But  we  may  probably  with  safety,  for  present  purposes,  assume 
that  the  greater  part  of  the  proteid  is  absorbed  as  peptone,  that  carbohy- 
drates are  mainly  absorbed  as  sugar,  and  that  the  greater  part  of  the  fat 
passes  into  the  body  as  emulsified  but  otherwise  unchanged  neutral  fat ;  and 
we  may  neglect  the  other  conditions  of  digested  food  as  subsidiary,  and  as 
far  as  absorption  is  concerned,  unimportant. 

We  have  seen  that  two  paths  are  open  for  these  products  of  digestion, 
one  by  the  capillaries  of  the  portal  system,  the  other  by  the  lacteals.  It 
cannot  be  a  matter  of  indifference  which  course  is  taken/  For  if  the  prod- 
ucts pass  by  the  lacteals  they  fall  into  the  general  blood-current  after  hav- 
ing undergone  only  such  changes  as  they  may  experience  in  the  lymphatic 
system ;  while  if  they  pass  into  the  portal  vein  they  are  subjected  to  certain 
powerful  influences  of  the  liver  (which  we  shall  study  in  a  future  chapter) 
before  they  find  their  way  to  the  right  side  of  the  heart,  and  if  the  substances 
be  prevented  from  reaching  the  liver,  as  when  in  the  dog  the  portal  vein  is 
connected  with  the  inferior  vena  cava  and  they  are  thus  at  once  transmitted 
to  the  general  circulation,  serious  results  ensue.  We  may,  therefore,  con- 
sider first  which  of  the  two  paths  is,  as  a  matter  of  fact,  taken  by  the  several 
products,  and  subsequently  study  the  mechanism  of  absorption  in  the  two 
cases. 

The  Course  taken  by  the  Several  Products  of  Digestion. 

§  259.  From  what  has  already  been  said  we  have  been  led  to  regard  the 
villi  as  the  most  active  organs  of  absorption,  and  the  structure  of  a  villus 
leads  us  further  to  conclude  that  the  diffusible  peptones  and  sugar  pass, 
together  with  the  water  in  which  they  are  dissolved,  into  the  superficially 
placed  capillary  network  of  the  villus  and  so  into  the  portal  system,  while 
the  merely  emulsified  fat,  unable  to  traverse  the  wall  of  the  capillary,  passes 
on  to  the  deep-seated  lacteal  radicle,  and  so  finds  its  way  into  the  lymphatic 
system.  And  the  results  of  observation  and  experiment,  as  far  as  they  go, 
support  this  view. 

Fats.  After  a  meal  containing  fat  the  lymph  of  the  lacteals  contains  fat, 
and  is  now  called  chyle  ;  and  the  richer  the  meal  in  fat  the  more  conspicuous  is 
the  fat  in  the  lymph-vessels.  We  cannot,  however,  prove  that  all  the  fat  of  a 
meal  absorbed  from  the  alimentary  canal  is  poured  by  the  thoracic  duct  into 
the  venous  system.  If  a  meal  containing  a  known  quantity  of  fat  be  given 
to  a  dog  and  the  small  quantity  of  fat  present  in  the  feces  corresponding  to 
the  meal  be  subtracted  from  that  amount,  we  can  determine  the  amount  of 
fat  absorbed,  for  we  have  no  evidence  whatever  that  any  appreciable  amount 
of  fat  undergoes  a  destructive  decomposition  in  the  alimentary  canal.  Col- 
lecting by  means  of  a  canula  inserted  into  the  thoracic  duct  the  whole  of 


326  THE  TISSUES  AND  MECHANISMS  OF  DIGESTION. 

the  chyle  during  and  after  the  meal,  so  long  as  it  remains  milky,  showing 
that  fat  is  being  absorbed,  we  can  ascertain  the  quantity  of  absorbed  fat, 
which  would,  but  for  the  operation,  have  passed  into  the  venous  system. 
When  this  has  been  done,  a  very  remarkable  deficit,  amounting  it  may  be  to 
40  or  50  per  cent.,  has  been  observed ;  that  is  to  say,  of  every  100  parts  of 
fat  which  disappear  from  the  alimentary  canal  only  about  60  'parts  find  their 
way  through  the  thoracic  duct  into  the  venous  system. 

Are  we  then  to  conclude  that  the  missing  quantity  finds  its  way  into  the 
portal  system?  Now  the  portal  blood  does,  during  digestion,  contain  a  cer- 
tain quantity  of  fat;  indeed,  the  serum  is  said  at  times  to  appear  milky  from 
the  presence  of  fat.  But  the  whole  circulating  blood  during  the  digestion 
of  a  fatty  meal  contains,  for  a  while,  the  fat  poured  into  it  by  the  thoracic 
duct ;  and  it  has  been  ascertained  in  the  dog  that  the  blood  of  the  portal 
vein  during  digestion  contains  not  more  but  less  fat  than  the  blood  of  the 
carotid  artery,  so  that  the  fat  which  appears  in  the  portal  blood  during 
digestion  is,  for  the  most  part  at  least,  not  fat  absorbed  by  the  capillaries  of 
the  alimentary  canal,  but  fat  absorbed  by  the  lacteals.  Moreover,  when  the 
chyle  of  the  thoracic  duct  is  diverted  through  a  canula,  and  not  allowed  to 
flow  into  the  blood,  the  quantity  of  fat  in  the  portal  blood,  as  in  the  blood 
at  large,  is  very  small  indeed.  Lastly,  when  a  villus  of  an  intestine  in  full 
digestion  of  fat  is  treated  with  osmic  acid,  fat  cannot  be  recognized  by  the 
microscope  within  the  capillaries  or  other  bloodvessels,  though  it  abounds 
outside  them  in  the  substance  of  the  villus  and  in  the  lacteal  radicle. 

We  may  probably,  therefore,  infer  with  safety  that  all  or  at  least  very 
nearly  all  the  fat  absorbed  from  the  intestine  takes  the  path  of  the  lacteals. 
As  to  the  deficit  mentioned  above,  that  is  as  yet  without  explanation.  It 
may  be  that  in  some  way,  on  its  course,  in  the  lymphatic  glands,  for  instance, 
the  fat  is  taken  away  from  the  chyle,  hidden  so  to  speak  somewhere  away 
from  both  chyle  and  blood ;  but  on  this  point  we  have  no  exact  information. 

§  260.  Water  and  salts.  If,  in  an  animal,  the  rate  of  flow  of  lymph  or 
chyle  through  a  canula  placed  in  the  thoracic  duct  be  watched,  and  water 
or,  to  avoid  the  injurious  effect  of  simple  water  on  the  mucous  membrane, 
normal  saline  solution,  be  then  injected  in  not  too  great  quantity  into  the 
intestine,  no  marked  increase  in  the  flow  of  chyle  through  the  canula  is 
observed.  From  this  we  may  infer  that  the  water  of  the  intestinal  contents 
is  absorbed  not  into  the  lacteals  but  into  the  portal  system.  If,  however,  a 
very  large  quantity  of  the  normal  saline  solution  be  injected  so  as  to  distend 
the  intestine,  then  the  flow  of  chyle  is  increased  to  some  extent.  It  would 
appear,  therefore,  that  while  under  normal  conditions  the  water  passes  from 
the  intestine  mainly  into  the  portal  blood,  some  of  it  may  under  some  cir- 
cumstances pass  into  the  lacteals. 

With  regard  to  the  course  taken  by  ordinary  saline  matters  we  possess 
no  detailed  information.  When  special  salts,  such  as  potassium  iodide  and 
others,  easily  recognized  by  appropriate  tests,  are  introduced  into  the  intes- 
tine, they  may  be  speedily  detected  both  in  the  blood  and  in  the  contents  of 
the  thoracic  duct ;  but  whether,  in  such  cases,  these  salts  find  their  way  into 
the  thoracic  duct  by  the  lacteal  radicle  of  the  villi,  or  pass  into  the  lymph 
stream  at  some  later  part  of  its  course,  we  do  not  know.  Nor  can  we  with 
regard  to  such  a  salt  as  sodium  chloride,  state  absolutely  that  it  passes  mainly 
with  the  water  into  the  portal  blood,  though  we  may  fairly  suppose  this  to 
be  the  case. 

§  261.  Sugar.  Both  blood  and  chyle  contain,  normally,  a  certain  small 
amount  of  sugar ;  and  careful  inquiries  show  that  the  percentage  of  sugar 
in  chyle  and  in  general  blood  is  fairly  constant,  neither  being  to  any  marked 
extent  increased  by  even  amylaceous  meals ;  on  the  other  hand,  a  meal  con- 


ABSORPTION   FROM  THE  ALIMENTARY  CANAL.  327 

taining  sugar  or  starch  does  temporarily  increase  the  quantity  of  sugar  in 
the  portal  blood.  From  this  we  may  infer  that  such  portions  of  the  sugar 
of  the  intestinal  contents  as  are  absorbed  as  sugar  pass  exclusively  by  the 
portal  vein.  We  may,  however,  here  call  attention  to  the  difficulties  attend- 
ing an  argument  of  this  kind.  In  the  first  place  the  quantitative  determi- 
nation of  a  small  amount  of  sugar  in  so  complex  a  fluid  as  blood  is  attended 
with  great  difficulties  and  uncertainties.  In  the  second  place  a  very  large 
quantity  of  blood  is  at  any  one  moment  streaming  through  the  capillaries  of 
the  alimentary  canal ;  and  we  may  perhaps  speak  of  the  quantity  which 
passes  through  them  during  the  whole  period  of  digestion  as  being  enormous. 
Hence  though  each  100  c.c.  in  passing  through  the  capillaries  might  take  up 
a  quantity  of  sugar  so  small  as  to  fall  almost  within  the  limit  of  errors  of 
observation,  yet  the  whole  quantity  absorbed  during  the  hours  of  digestion 
might  be  considerable  ;  or  to  put  it  in  another  way,  an  error  of  observation, 
unavoidable  with  our  present  means  of  analysis,  on  a  sample  of  blood  taken 
from  the  portal  vessels  might  lead  to  a  wholly  unwarranted  conclusion  that 
sugar  was  or  was  not  being  absorbed.  Making  every  allowance,  however,  for 
these  difficulties,  the  increase  of  sugar  which  has  been  observed  in  the  portal 
blood  during  digestion  seems  too  great  to  permit  of  any  other  conclusion  than 
that  sugar  is  really  absorbed  from  the  alimentary  canal  by  the  bloodvessels. 

W7hen,  however,  a  large  quantity  of  sugar  dissolved  in  a  large  quantity  of 
water  is  present  in  the  intestine,  the  sugar  in  the  chyle  is  said  to  be  increased. 
In  such  a  case  the  excess  of  water,  as  stated  above,  passes  into  the  lacteals, 
and  in  so  doing  appears  to  carry  some  of  the  sugar  with  it. 

In  this  connection  it  should  be  remembered  that  the  sugar  resulting  from 
digestion  is  for  the  most  part  maltose,  while  that  in  the  portal  blood  is  dex- 
trose, the  former  being  changed  into  the  latter  probably  while  passing  through 
the  intestinal  epithelium. 

§  262.  Proteids.  The  difficulties  attending  the  experimental  determina- 
tion of  the  path  taken  by  proteids  are  greater  even  than  in  the  case  of  sugar, 
for  the  exact  quantitative  estimation  of  peptone  in  blood  (and  we  are  assuming 
that  proteids  are  mainly  absorbed  as  peptone)  is  a  task  of  the  greatest  diffi- 
culty— one  compared  with  which  that  of  estimating  sugar  appears  almost 
easy.  Bearing  this  in  mind  we  may  state  that  all  observers  are  agreed  that 
peptone  is  absent  from  chyle  or  at  least  that  its  presence  cannot  be  satisfac- 
torily proved.  On  the  other  hand,  while  some  observers  have  succeeded  in 
finding  peptone  in  the  portal  blood  after  food,  but  not  during  fasting,  many 
have  failed  to  demonstrate  the  presence  of  peptone  in  the  blood  either  of  the 
portal  vein  or  of  the  vessels  at  large  even  after  a  meal  containing  large 
quantities  of  proteids.  Of  course,  as  we  argued  in  speaking  of  the  absorp- 
tion of  sugar,  the  quantity  of  peptone  passing  into  the  portal  blood  at  any 
moment  might  be  small,  and  yet  a  considerable  quantity  might  so  pass  during 
the  hours  of  digestion.  We  may  suppose,  moreover,  that  that  which  does 
pass  is  immediately  converted,  possibly  by  some  ferment  action,  into  one  or 
other  of  the  natural  proteids  of  the  blood,  or  otherwise  disposed  of;  and, 
indeed,  peptone  injected  carefully  and  slowly  into  a  vein  disappears  from  the 
blood,  though  little  or  even  none  passes  out  by  the  kidney.  And  the  view 
that  peptone  is  so  changed,  possibly  in  the  very  act  of  absorption,  is  sup- 
ported not  only  by  the  statement  that  peptone  may  be  found  in  the  practi- 
cally bloodless  wall,  that  is,  mucous  membrane,  of  the  intestine  removed  from 
a  dead  animal  even  when  it  appears  to  be  absent  from  the  blood,  but  also 
and  especially  by  the  following  observation.  If  an  artificial  circulation  of 
blood  be  kept  up  in  the  mesenteric  arteries  supplying  a  loop  of  intestine  re- 
moved from  the  body,  the  loop  may  be  kept  alive  for  some  considerable  time. 
During  this  survival  a  considerable  quantity  of  peptone  placed  in  the  cavity 


328  THE  TISSUES  AND   MECHANISMS   OF   DIGESTION. 

of  the  loop  will  disappear,  i.  e.,  will  be  absorbed,  but  cannot  be  recovered 
from  the  blood  which  is  being  used  for  the  artificial  circulation,  and  which 
escapes  from  the  veins  after  traversing  the  intestinal  capillaries.  The  disap- 
pearance is  not  due  to  any  action  of  the  blood  itself,  for  peptone  introduced 
into  the  blood  before  it  is  driven  through  the  mesenteric  arteries  in  the  ex- 
periment may  be  recovered  from  the  blood  as  it  escapes  from  the  mesenteric 
veins.  It  would  seem  as  if  the  peptone  were  changed  before  it  actually  gets 
from  the  interior  of  the  intestine  into  the  interior  of  the  capillaries. 

But  the  argument  that  the  absence  of  peptone  from  the  blood  is  no  proof 
that  the  peptone  is  not  absorbed  into  the  blood  may  also  be  applied  to  the 
chyle,  and  thus  leaves  us  unable  to  draw  a  conclusion  as  to  the  path  of  the 
proteids.  The  following  indirect  proof  that  peptone  does  not  pass  into  the 
chyle  has  been  offered,  but  it  too  is  open  to  objection.  We  shall  see  here- 
after that  the  absorption  of  proteid  material  leads  to  an  increase  in  the 
elimination  of  urea  by  the  kidneys.  So  marked  is  this  increase,  that  unless 
there  be  clearly  some  other  causes  at  work  leading  to  an  increase  of  urea, 
such  as  fever  for  instance,  an  increase  of  urea  in  the  urine  following  upon 
the  administration  of  proteid  food  may  be  taken  as  a  proof  that  the  proteid 
food  has  been  digested  and  absorbed.  Now  if  in  a  dog  the  thoracic  duct  be 
successfully  ligatured  so  that  the  chyle  cannot  pass  as  usual  into  the  blood, 
and  the  dog  be  fed  on  proteid  food,  as  free  as  possible  from  fat,  so  as  not 
unnecessarily  to  load  the  obstructed  lacteals,  an  increase  in  the  urea  of  the 
urine  is  observed  as  usual.  Obviously  in  such  a  case  the  proteid  food  is 
absorbed,  and  obviously  also  does  not  pass  into  the  blood  through  the  tho- 
racic duct  (the  success  of  the  ligature  having  been  proved  by  post-mortem 
examination).  But  the  experiment,  though  as  far  as  it  goes  supporting,  does 
not  rigorously  prove,  the  view  that  the  proteids  are  absorbed  by  the  capilla- 
ries of  the  alimentary  canal ;  for  the  thoracic  duct  and  lymphatics  below 
the  ligature  were  found  largely  distended,  and  lymph  and  chyle  appear  to 
have  escaped  from  the  vessels ;  hence  it  is  possible  that  some  at  least  of  the 
proteids  were  absorbed  by  the  lacteals  of  the  intestine,  but  finding  their 
usual  path  blocked  made  their  way  into  the  blood  stream. 

We  may,  therefore,  say  that  the  results  of  experiment,  while  they  do  not 
definitely  prove,  give  some  support  to,  and  at  least  do  not  contradict,  the 
view  which  we  a  little  while  ago  put  forward  as  probable,  namely,  that  pro- 
teids, transformed  into  diffusible  peptones,  pass  into  the  bloodvessels  and 
not  into  the  lacteals. 

But,  if  this  view  be  provisionally  accepted,  it  must  be  on  the  understand- 
ing that  it  is  probable  only ;  and  it  may  be  that  proteids  do  not  take  the 
same  paths  and  are  not  absorbed  in  the  same  condition  in  all  animals.  The 
experiments  just  related  were  performed  on  dogs,  that  is  to  say,  on  carnivo- 
rous animals  whose  (natural)  food  contains  a  considerable  quantity  of  fat, 
and  whose  lacteals  might,  therefore,  be  considered  as  preoccupied  in  the 
absorption  of  fat.  The  food  of  herbivora  on  the  other  hand  contains  a 
relatively  small  amount  of  fat ;  and  if  in  these  animals  all  the  proteids  and 
carbohydrates  are  absorbed  by  the  bloodvessels,  there  is  comparatively  little 
left  for  the  lacteals  to  do.  Yet  in  these  animals  the  lacteals  and  lymphatics 
are  well  developed.  In  the  villus  of  a  herbivorous  guinea-pig  or  rabbit, 
though  the  reticular  tissue  is  very  scanty  as  compared  with  that  present  in 
the  villus  of  a  dog,  the  lacteal  chamber  is,  relatively  to  the  diameter  of  the 
villus,  not  merely  as  large  as,  but  much  larger  than,  in  the  dog.  It  is  diffi- 
cult to  suppose  that  this  wide  chamber  is  intended  solely  for  the  absorption 
of  the  relatively  small  amount  of  fat  present  in  vegetable  food.  The  ques- 
tion which  we  are  discussing  is  clearly  at  present  to  be  regarded  as  by  no 
means  settled. 


ABSORPTION  FROM  THE  ALIMENTARY  CANAL.  329 

The  Mechanism  of  Absorption. 

§  263.  The  absorption  of  fats.  We  have  now  to  consider  the  manner  in 
which  these  several  substances  pass  into  either  the  lacteal  radicle  or  the 
capillary  bloodvessels.  It  will  be  convenient  to  begin  with  the  absorption 
of  the  fats. 

We  have  seen  reason  (§  241),  to  think  that  the  fats,  remaining  chiefly  as 
neutral  fats,  are  emulsified  in  the  intestine  by  means  of  the  bile  and  pan- 
creatic juice,  the  small  quantity  of  soap  which  is  formed  probably  serving 
simply  the  purpose  of  facilitating  the  ernulsification. 

The  neutral  fats  so  emulsified  pass  in  the  first  instance  into  the  bodies  of 
the  columnar  cells  of  the  villi.  It  has,  it  is  true,  been  maintained  by  some 
that  they  pass  between  the  cells  and  not  into  them  ;  but  the  evidence  is 
distinctly  against  this  view.  Since  no  such  collections  of  fat  globules  are 
seen  in  the  cubical  cells  of  the  glands  of  Lieberkuhn  we  infer  that  these 
have  nothing  to  do  with  the  absorption  of  fat. 

How  the  fat  enters  into  the  substance  of  the  cell  we  do  not  know.  We 
may  presume  that  the  striated  border  plays  some  part,  but  what  part  we  do 
not  know.  Though,  as  we  have  seen,  the  rods  making  up  the  border  appear 
able  to  move,  to  change  their  form,  we  have  no  evidence  that  the  fat  is  in- 
troduced into  the  cells  by  means  of  any  movements  of  these  rods.  We  may 
imagine  that  the  globules  pass  into  the  cell-substance  by  help  in  some  way 
of  these  rods  through  amoeboid  movements  comparable  with  the  ingestive 
movements  of  the  body  of  an  amoeba ;  but  we  have  no  positive  evidence 
to  support  this  view.  We  said  that  bile  promotes  the  passage  of  fat 
through  membranes,  possibly  by  in  some  way  promoting  a  closer  contact 
between  the  particles  of  fat  and*  the  substance  of  the  membrane ;  but  even 
if  bile  has  this  effect  on  the  surface  of  the  cells,  its  action  in  this  respect  can 
be  subsidiary  only. 

When  fatty  acids  are  ingested  neutral  fats  appear  in  the  chyle,  indicating 
a  synthesis  of  fatty  acids  into  neutral  fats  in  the  epithelium  of  the  villi. 

Within  the  columnar  cell  the  fat  may  be  seen,  both  in  osmic  acid  prepa- 
rations and  in  fresh  living  cells,  to  be  disposed  in  globules  of  various  sizes, 
some  large  and  some  small,  each  globule  placed  in  a  space  of  the  proto- 
plasmic cell-substance.  It  does  not  follow  that  the  fat  actually  entered  the 
cell  exactly  in  the  form  of  these  globules ;  it  may  be  that  the  fat  passes  the 
striated  border  in  very  minute  spherules  which,  reaching  the  body  of  the 
cell,  run  together  into  larger  globules;  but  whether  this  is  so  or  not  we  do 
not  know. 

From  the  columnar  cell  the  fat  passes  into  the  spaces  of  the  reticular 
tissue  of  the  villus.  It  has,  it  is  true,  been  contended  that  it  passes  along 
the  substance  of  the  bars  of  the  reticulum ;  but  in  carefully  prepared  osmic 
acid  specimens  of  a  villus  in  active  digestion  of  fatty  food,  the  fat  may  be 
distinctly  recognized  as  largely  filling  up,  still  in  the  form  of  globules  of 
various  sizes,  the  spaces  in  the  meshes  of  the  reticulum  which  are  not  occu- 
pied by  the  leucocytes  or  allied  wandering  cells.  The  bases  of  the  columnar 
cells,  through  the  gaps  in  the  basement  membrane,  directly  abut  upon  the 
labyrinth  of  spaces ;  and  the  fat  once  out  of  the  base  of  the  cell  is  free  in 
the  spaces  of  the  labyrinth.  How  it  issues  from  the  cell  we  do  not  exactly 
know ;  possibly  by  a  process  analogous  to  the  excretion  of  solid  matters  by 
an  amoeba. 

From  the  labyrinth  of  spaces  of  the  reticulum  of  the  villus  the  fat  passes 
into  the  cavity  of  the  lacteal  radicle ;  and  it  is  worthy  of  note  that  in  the 
passage  it  undergoes  a  change.  In  the  interior  of  the  intestine,  in  the  sub- 
stance of  the  columnar  cell,  and  apparently  in  the  labyrinth  of  the  reticu- 


330  THE   TISSUES   AND  MECHANISMS  OF   DIGESTION. 

lum,  it  is  simply  emulsified  fat  consisting  of  globules  small  and  large;  within 
the  lacteal  radicle  it  consists  partly  of  the  same  easily  recognized  globules 
but  partly  of  the  extremely  divided  "  molecular  basis  "  (§  252)  ;  it  is  now 
no  longer  emulsified  fat  but  chyle.  How  and  by  what  means  this  extremely 
minute  division  of  the  globular  fat  into  the  "  molecular  basis"  takes  place 
we  do  not  know  ;  nor  do  we  know  the  exact  manner  in  which  the  fat  passes 
from  the  spaces  of  the  reticulum  into  the  interior  of  the  radicle. 

We  may  here,  perhaps,  remark  that  the  contents  of  the  lacteal  radicle 
consist  not  exclusively  of  fat,  but  of  fat  accompanied  by  the  proteid  and 
other  substances  which  go  to  make  up  the  chyle.  Proteid  and  other  sub- 
stances besides  fat  are  also  present  in  the  lymph  which  occupies  in  part  the 
labyrinth  of  the  body  of  the  villus,  and  are  derived,  like  the  lymph  else- 
where, from  the  blood  of  adjacent  capillaries  ;  at  least,  they  are  in  part  so 
derived,  though  it  may  be  not  wholly,  for,  as  we  have  just  seen,  the  passage 
of  proteid  material  from  the  intestine  into  the  substance  of  the  villus  past 
the  capillaries,  though  not  proved,  must  still  be  considered  as  possible. 

The  spaces  of  the  reticulum  of  the  villus  are  more  or  less  occupied  by 
wandering  cells  of  which  we  spoke  under  the  general  term  of  leucocytes. 
These  do  not  all  present  the  same  appearances  and  most  probably  are  not 
all  of  the  same  kind. 

Some  of  these  leucocytes  wander  not  only  through  the  labyrinth  of  the 
reticulum  but  pass  into  the  epithelium  between  the  cells,  and  may  project 
processes  into  or  even  make  their  way  eventually  into  the  interior  of  the 
intestine ;  or  following  the  reverse  course  may  wander  from  between  the 
epithelial  cells  into  the  body  of  the  villus ;  some  of  them,  moreover,  un- 
doubtedly contain  fat.  Hence  the  view  has  been  suggested  that  these  leuco- 
cytes are  important  agents,  indeed  the  chief  agents  in  the  absorption  of  fat. 
It  has  been  supposed  that  they,  receiving  the  globules  of  fat  into  their  cell- 
substance,  in  fact  eating  the  fat  exactly  after  the  manner  of  an  amoeba, 
either  while  projecting  between  the  columnar  cells,  in  which  case  they  carry 
their  burden  of  fat  through  the  epithelium  into  the  villus,  or  while  wander- 
ing in  the  labyrinth  of  the  villus  bear  it  away  bodily  into  the  lymphatic 
system.  But  the  number  of  leucocytes  really  containing  any  appreciable 
quantity  of  fat  is  too  small  to  account  for  the  amount  of  fat  absorbed  ; 
Nor  is  the  abundance  of  leucocytes  in  the  mucous  membrane  during  the 
period  of  digestion  a  sure  proof  that  they  are  concerned  in  absorption,  but 
rather  an  indication  only  that  active  changes  of  some  kind  are  going  on, 
since  after  the  administration  of  a  saline  such  as  magnesium  sulphate,  which 
produces  effects  the  very  reverse  of  absorption,  these  leucocytes  are  present 
in  unusual  numbers.  Moreover,  under  some  circumstances,  as  in  the  villi 
of  a  new-born  puppy  after  a  meal  of  milk,  they  are  absent  even  when 
digestion  of  fat  is  rapidly  going  on  and  the  lacteals  are  filling  with  fat.  In 
fact,  what  we  stated  above  concerning  the  presence  of  fat  in  the  bodies  of 
the  columnar  cells  shows  that  leucocytes  can  have  little  to  do  in  transferring 
fat  from  the  interior  of  the  intestine  into  the  body  of  the  villus ;  and  there 
are  no  adequate  reasons  for  attributing  to  them  any  real  share  in  the  trans- 
ference of  fat  from  the  body  of  the  villus  into  the  lacteal  chamber. 

§  264.  The  lacteal  chamber  opens  at  the  base  of  the  villus  into  the 
valved  lymphatic  vessels  lying  below,  and  in  these  the  flow  of  lymph  (chyle) 
is  being  promoted  by  the  various  causes  detailed  in  §  253.  The  pressure, 
for  instance,  exerted  by  the  peristaltic  contractions  of  the  intestine  helps  to 
empty  the  lymphatic  vessel  into  which  a  lacteal  chamber  opens  and  so  pro- 
motes the  emptying  of  the  latter.  In  addition  to  this  the  plain  muscular 
fibres  of  the  villus  supply  a  special  muscular  pump  for  the  emptying  and 
filling  of  the  lacteal  chamber.  These  fibres  and  small  bundles  of  fibres, 


ABSORPTION   FROM  THE  ALIMENTARY  CANAL.  331 

though  running  in  various  directions  and  varying  in  number  and  arrange- 
ment in  different  animals,  take  on  the  whole  a  longitudinal  direction 
parallel  to  the  long  axis  of  the  villus.  It  has  been  supposed  that  in  con- 
tracting and  shortening  the  villus  they  compress  the  lacteal  and  thus  empty 
it,  and  that  when  they  relax  and  the  villus  elongates  again,  the  emptied 
chamber  fills  once  more.  But  a  different  interpretation  of  their  action  has 
been  offered  somewhat  as  follows.  When  the  muscular  fibres  contract  they 
shorten  the  villus.  In  thus  becoming  shorter  the  body  of  the  villus  becomes 
proportionately  broader,  since  probably  no  great  change  of  bulk  in  the 
reticulum  takes  place ;  in  this  broadening  the  part  to  give  way  will  be  the 
lacteal  chamber,  which  thus  becomes  broader  and  larger.  When  the  mus- 
cular fibres  relax,  the  reticulum,  the  bars  of  which  have  been  put  on  the 
stretch  in  a  lateral  direction,  by  elastic  reaction  brings  back  the  villus  to  its 
former  length,  and  the  lacteal  chamber  elongates  and  narrows.  On  this 
view  the  muscular  contraction  expands  and  so  fills,  while  the  relaxation 
narrows  and  so  empties,  the  lacteal  chamber.  Whichever  view  we  adopt,  we 
may  at  least  conclude  that  contractions  and  relaxations  of  the  muscular 
fibres  in  some  way  or  other  alternately  fill  and  empty  the  lacteal  chamber, 
and  in  all  probability,  at  all  events  during  digestion,  rhythmical  contractions 
of  these  fibres  are  continually  going  on.  When  the  villus  is  shortened  by 
the  contraction  of  the  muscular  fibres,  the  columnar  cells  are  compressed, 
becoming  longer  and  narrower;  when  the  muscular  fibres  relax  and  the 
villus  elongates,  the  columnar  cells  return  to  their  previous  form.  The 
alternating  changes  of  form  to  which  the  columnar  cells  are  thus  subjected, 
and  the  alternating  changes  of  pressure  taking  place  in  the  reticulum,  may 
also  serve  to  promote  the  passage  of  material  through  the  one  and  through 
the  other. 

§  265.  The  absorption  of  diffusible  substances  and  of  water.  On  the  pro- 
visional assumption  which  we  have  made  that  the  proteids  are  converted 
into  peptone,  we  may  consider,  for  the  present  at  all  events,  peptone,  sugar, 
and  soluble  salts  as  together  forming  a  class  distinguished  from  fats  by  their 
being  diffusible,  some  more  so  than  others.  And  we  have  made  the  further 
provisional  assumption  that  these  pass  into  the  bloodvessels  and  not  into  the 
lacteals. 

The  network  of  capillary  bloodvessels  is  spread  immediately  beneath  the 
basement  membrane,  and  all  the  material  which  enters  the  lacteal  chamber 
has  to  run  the  gauntlet  of  the  meshes  of  this  network.  During  digestion 
the  capillaries  of  the  intestine  are  filled  and  distended,  so  that  at  a  time 
when  absorption  is  taking  place  these  meshes  between  the  capillaries  are 
unusually  narrow.  From  the  interior  of  these  capillaires,  as  elsewhere, 
transudation  is  taking  place ;  these  capillaries  supply  the  lymph  which 
helps  to  fill  up  the  labyrinth  of  the  reticulum  and  the  lacteal  chamber. 
But  to  a  much  greater  extent  than  elsewhere  (cf.  §  255)  this  current  of 
transudation  from  within  the  capillary  to  without  is  accompanied  by  a 
reverse  current  from  without  to  within.  The  diffusible  substances  in 
question  pass  from  the  intestine  through  the  layer  of  epithelial  cells, 
through  the  attenuated  reticular  lymph-space  between  the  basement  mem- 
brane and  the  capillary  wall,  and  through  the  capillary  wall  into  the  blood 
current.  Their  passage  consists  of  two  stages :  that  through  the  epithelial 
cells  from  the  intestine  to  the  lymph-space,  and  that  from  the  lymph-space 
into  the  bloodvessels.  These  two  stages  may  be  expected  to  differ,  seeing 
that  the  structures  concerned  are  different ;  but  we  may  at  first  consider 
them  as  one,  and  speak  of  the  passage  from  the  intestine  into  the  blood  as 
a  single  event. 

In  speaking  of  these  substances  as  diffusible,  we  are  using  the  term  in 


332  RESPIRATION. 

reference  to  the  well-known  passage  of  such  substances  through  thin  mem- 
branes or  porous  partitions.  When  a  strong  solution  of  sugar  or  of  common 
salt  is  separated  by  a  thin  membrane  (vegetable  parchment,  dead  urinary 
bladder,  dead  intestine,  etc.)  from  a  weak  solution  of  sugar  or  of  salt,  the 
sugar  or  salt  passes  with  a  certain  rapidity  from  the  stronger  to  the  weaker 
solution,  and  water  passes  from  the  weaker  solution  to  the  stronger ;  if,  to 
begin  with,  simple  water  be  substituted  for  the  weaker  solution,  the  effect  is 
at  first  still  more  striking.  Peptone  passes  in  the  same  manner,  but  as  we 
have  seen,  much  more  slowly.  The  process  is  spoken  of  as  a  physical  one, 
since  it  is  not  accompanied  necessarily  by  any  chemical  change  in  the  dif- 
fusing substance,  nor  is  there  any  necessary  change  in  the  membrane  or 
partition.  The  rate  at  which  a  substance  diffuses,  and 'the  total  amount  of 
diffusion  which  can  take  place,  are  determined  by  certain  qualities  of  the 
substance  (which  we  may  call  physical,  though  they  depend  on  the  chemical 
nature  of  the  substance)  in  relation  to  certain  qualities  of  the  membrane ; 
thus  two  salts  may  diffuse  through  the  same  membrane  at  different  rates, 
with  different  rates  in  the  associated  current  of  water,  the  osmotic  current 
as  it  is  called,  from  the  weaker  to  the  stronger  solution  ;  and  the  same  sub- 
stance may  pass  at  different  rates  through  different  membranes.  By  a  num- 
ber of  observations,  in  which  various  substances  in  solution  and  several 
known  membranes  or  partitions  have  been  employed,  a  certain  number  of 
"  laws  of  diffusion  "  have  been  established. 

Now  if,  by  the  statement  that  diffusible  substances  pass  by  diffusion  into 
the  blood-capillaries  of  the  intestine,  we  are  led  to  expect  that  the  passage 
takes  place  exactly  according  to  the  laws  established  by  observations  on 
ordinary  membranes,  we  should  be  led  into  error ;  for  the  disappearance  of 
these  substances  from  the  interior  of  the  intestine  does  not  take  place  accord- 
ing to  the  laws  which  regulate  their  disappearance  from  one  side  of  an 
ordinary  diffusion  septum.  This  can  be  ascertained  by  introducing  solutions 
of  the  substances,  of  various  strength,  into  a  loop  of  intestine,  isolated  in  the 
living  animal  by  the  method  described  in  §  219,  and  watching  their  disap- 
pearance by  analysis  of  the  contents  of  the  loop.  For  instance,  sodium 
sulphate  passes  through  an  ordinary  diffusion  septum  with  a  rapidity  rather 
greater  than  that  of  dextrose,  whereas  dextrose  disappears  from  the  intestine 
distinctly  more  rapidly  than  sodium  sulphate ;  peptone,  which  diffuses  very 
slowly  indeed  through  an  ordinary  diffusion  septum,  disappears  rapidly 
(though  not  so  rapidly  as  dextrose)  from  the  intestine ;  and  when  the  details 
of  the  disappearance  from  the  intestine  of  weak  solutions  of  two  salts  which 
diffuse  through  an  ordinary  membrane  at  differents  rates,  which  have,  as  it 
is  said,  different  osmotic  equivalents,  are  studied,  these  details  are  quite  dif- 
ferent from  those  of  ordinary  diffusion.  The  more  the  matter  is  studied,  the 
more  decidedly  apparent  becomes  the  difference  between  ordinary  diffusion 
and  the  absorption  of  diffusible  substances  from  the  intestine. 


CHAPTER    II. 

RESPIRATION. 

THE  STRUCTURE  OF  THE  LUNGS  AND  BRONCHIAL  PASSAGES. 

§  266.  ONE  particular  item  of  the  body's  income,  viz.,  oxygen,  is  pecu- 
liarly associated  with  one  particular  item  of  the  body's  waste,  viz.,  carbonic 
acid,  inasmuch  as  the  means  which  are  applied  for  the  introduction  of  the 


THE  MECHANICS  OF  PULMONARY   RESPIRATION.  333 

former  are  also  used  for  the  getting  rid  of  the  latter.  Both  are  gases,  and 
the  ingress  of  the  one  as  well  as  the  egress  of  the  other  is  far  more  depend- 
ent on  the  simple  physical  process  of  diffusion  than  on  any  active  vital  pro- 
cesses carried  on  by  means  of  tissues.  Oxygen  passes  from  the  air  into  the 
blood  mainly  by  diffusion,  and  mainly  by  diffusion  also  from  the  blood  into 
the  tissues ;  in  the  same  way  carbonic  acid  passes  mainly  by  diffusion  from 
the  tissues  into  the  blood  and  from  the  blood  into  the  air.  Whereas,  as  we 
have  seen  in  the  secretion  of  the  digestive  juices  the  epithelial  cell  plays  an 
all-important  part,  in  respiration  the  entrance  of  oxygen  from  the  lungs  into 
the  blood,  and  from  the  blood  into  the  tissue,  and  the  passage  of  carbonic 
acid  in  the  contrary  direction,  are  affected,  if  at  all,  in  a  wholly  subordinate 
manner,  by  the  behavior  of  the  pulmonary,  or  of  the  capillary  epithelium. 
What  we  have  to  deal  with  in  respiration,  then,  is  not  so  much  the  vital 
activities  of  any  particular  tissue,  as  the  various  mechanisms  by  which  a 
rapid  interchange  between  the  air  and  the  blood  is  effected,  the  means  by 
which  the  blood  is  enabled  to  carry  oxygen  and  carbonic  acid  to  and  from 
the  tissues,  and  the  manner  in  which  the  several  tissues  take  oxygen  from 
and  give  carbonic  acid  up  to  the  blood.  We  have  reasons  for  thinking  that 
oxygen  can  be  taken  into  the  blood,  not  only  from  the  lungs  but  also  to  a 
certain  small  extent  from  the  skin,  and,  as  we  have  seen,  from  the  alimen- 
tary canal  also  ;  and  carbonic  acid  certainly  passes  away  from  the  skin,  and 
through  the  various  secretions,  as  well  as  by  the  lungs.  Still  the  lungs  are 
so  eminently  the  channel  of  the  interchange  of  gases  between  the  body  and 
the  air,  that  in  dealing  at  present  with  respiration,  we  shall  confine  our- 
selves entirely  to  pulmonary  respiration,  leaving  the  consideration  of  the 
subsidiary  respiratory  processes  till  we  come  to  study  the  secretions  of  which 
they  respectively  form  part. 

THE  MECHANICS  OF  PULMONARY  RESPIRATION. 

§  267.  The  lungs  are  placed,  in  a  state  which  is  always  one  of  disten- 
tion,  sometimes  greater,  sometimes  less,  in  the  air-tight  thorax,  the  cavity 
of  which  they,  together  with  the  heart,  great  bloodvessels,  and  other  organs, 
completely  fill.  By  the  contraction  of  certain  muscles  the  cavity  of  the 
thorax  is  enlarged.  The  lungs  must  follow  this  enlargement  and  be  them- 
selves enlarged,  otherwise  the  pleural  cavities  would  be  enlarged  ;  but  this 
is  impossible  so  long  as  the  walls  are  intact.  The  enlargement  of  the  lung 
consists  chiefly  in  an  enlargement  or  expansion  of  the  pulmonary  alveoli, 
the  air  in  which  becomes,  by  the  expansion,  rarefied.  That  is  to  say,  the 
pressure  of  the  air  within  the  lungs  becomes  less  than  that  of  the  air  out- 
side the  body,  and  this  difference  of  pressure  causes  a  rush  of  air  through 
the  trachea  into  the  lungs  until  an  equilibrium  of  pressure  is  established 
between  the  air  inside  the  lungs  and  that  outside.  This  constitutes  inspir- 
ation. On  relaxation  of  the  respiratory  muscles  (the  muscles  whose  con- 
tractions have  brought  about  the  thoracic  expansion),  the  elasticity  of  the 
lungs  and  chest-walls,  aided,  perhaps,  to  some  extent  by  the  contraction  of 
certain  m.uscles,  causes  the  chest  to  return  to  its  original  size  ;  in  consequence 
of  this  the  pressure  within  the  lungs  becomes  greater  than  that  outside,  and 
thus  air  rushes  out  of  the  trachea  until  equilibrium  is  once  more  established. 
This  constitutes  expiration  ;  the  inspiratory  and  expiratory  act  together  form- 
ing a  respiration.  The  fresh  air  introduced  into  the  upper  part  of  the  pul- 
monary passages  by  the  inspiratory  movement  contains  more  oxygen  and  less 
carbonic  acid  than  the  old  air  previously  present  in  the  lungs.  By  diffusion 
the  new  or  tidal  air,  as  it  is  frequently  called,  gives  up  its  oxygen  to,  and 
takes  carbonic  acid  from,  the  old  or  stationary  air,  as  it  has  been  called, 


334  RESPIRATION. 

and  thus  when  it  leaves  the  chest  in  expiration  has  been  the  means  of  both 
introducing  oxygen  into  the  chest  and  of  removing  carbonic  acid  from 
it.  In  this  way,  by  the  ebb  and  flow  of  the  tidal  air,  and  by  diffusion 
between  it  and  the  stationary  air,  the  whole  air  in  the  lungs  is  being  con- 
stantly renewed  through  the  alternate  expansions  and  contractions  of  the 
chest. 

§  268.  In  ordinary  respiration  the  expansion  of  the  chest  never  reaches 
its  maximum ;  by  more  forcible  muscular  contraction,  by  what  is  called 
labored  inspiration,  an  additional  thoracic  expansion  can  be  brought  about, 
leading  to  an  inrush  of  a  certain  additional  quantity  of  air  before  equilib- 
rium is  established.  This  additional  quantity  is  often  spoken  of  as  eomple- 
mental  air.  In  the  same  way  in  ordinary  respiration  the  contraction  of  the 
chest  never  reaches  its  maximum.  By  calling  into  use  additional  muscles, 
by  a  labored  expiration  an  additional  quantity  of  air,  the  so-called  reserve 
or  supplemental  air,  may  be  driven  out.  But  even  after  the  most  forcible 
expiration,  a  considerable  quantity  of  air,  the  residual  air,  still  remains  in 
the  lungs.  The  natural  condition  of  the  lungs  in  the  chest  is,  in  fact,  one 
of  partial  distention.  The  elastic  pulmonary  tissue  is  always  to  a  certain 
extent  on  the  stretch  ;  it  is  always,  so  to  speak,  striving  to  pull  asunder 
the  pulmonary  from  the  parietal  pleura ;  but  this  it  cannot  do,  because 
the  air  can  have  no  access  to  the  pleural  cavity.  When,  however,  the 
chest  ceases  to  be  air-tight,  when  by  a  puncture  of  the  chest-wall  or  dia- 
phragm air  is  freely  introduced  into  the  pleural  chamber,  the  elasticity 
of  the  lungs  pulls  the  pulmonary  away  from  the  parietal  pleura  and  the 
lungs  collapse,  driving  out  by  the  windpipe  a  considerable  quantity  of 
the  residual  air.  Even  then,  however,  the  lungs  are  not  completely 
emptied,  some  air  still  remaining  in  them  ;  this  is  probably  air  impris- 
oned in  the  infundibula  by  collapse  of  the  bronchioles,  which,  as  we  have 
seen,  have  flaccid  and  not  rigid  walls.  If,  in  a  living  animal,  the  pres- 
sure of  the  atmosphere  continue  to  have  access  to  the  outside  of  a  lung, 
the  air  thus  imprisoned  is  gradually  absorbed  and  the  lung  becomes  solid. 
The  same  result  may  occur  from  the  pressure  of  fluid  accumulated  in  the 
pleural  cavity. 

It  need  hardly  be  added  that  when  the  pleura  is  punctured  and  air  can 
gain  free  admittance  from  the  exterior  in  the  pleural  chamber,  since  the  re- 
sistance to  the  entrance  of  the  air  into  the  pleural  chamber  is  far  less  than 
the  resistance  to  the  entrance  into  the  lungs,  the  effect  of  the  respiratory 
movements  is  simply  to  drive  air  in  and  out  of  that  chamber,  instead  of  in 
and  out  of  the  lung.  There  is,  in  consequence,  no  renewal  of  the  air  within 
the  lungs  under  those  circumstances.  If  there  be  a  sufficient  obstacle  to 
the  entrance  of  air  into  the  pleural  chamber,  such  as  a  fold  of  tissue  block- 
ing up  the  opening,  the  expansion  of  the  chest  may  still  lead  to  a  distention 
of  the  lungs,  and  in  this  way,  in  some  cases,  puncture  of  the  chest-walls  has 
not  seriously  interfered  with  respiration.  The  parietal  and  pulmonary  pleura 
are,  in  normal  circumstances,  separated  by  a  very  thin  layer  only  of  fluid, 
so  that  we  may,  perhaps,  speak  of  them  as  being  in  a  state  of  "  adhesion," 
such  as  obtains  between  two  wet  membranes  superimposed.  And  it  has  been 
suggested  that  this  adhesion,  having  to  be  overcome  before  the  two  surfaces 
can  separate,  assists  in  preventing  the  entrance  of  air  into  the  pleural  cavity 
after  puncture  of  the  thorax  ;  but  it  has  not  been  clearly  shown  that  this  is 
really  of  importance  in  the  matter. 

§  269.  Before  birth  the  lungs  contain  no  air  ;  they  are  in  the  condition 
called  ateledatic.  The  Avails  of  the  alveoli,  the  epithelial  lining  of  which 
is,  at  that  time,  well  developed,  consisting  of  distinctly  nucleated  cells  with 
granular  cell-substance,  are  in  contact,  the  cavity  of  the  alveolus  not  having 


THE  MECHANICS  OF  PULMONARY   RESPIRATION.  335 

as  yet  come  into  existence ;  the  walls  of  the  bronchioles  are  similarly  in  a 
collapsed  condition,  with  their  walls  touching  ;  the  more  rigid  bronchia,  like 
the  trachea,  possess  some  amount  of  lumen,  which,  however,  is  occupied  by 
fluid.  When  the  chest  expands  with  the  first  breath  taken,  the  pressure  of 
the  inspired  air  has  to  overcome  the  "  adhesion  "  obtaining  between  the  walls 
of  the  alveoli,  thus  in  contact  with  each  other  and  also  those  of  the  bronchi- 
oles. The  force  spent  in  thus  opening  out  and  unfolding,  so  to  speak,  the 
alveoli  and  bronchioles  is  considerable,  and  in  the  expiration  succeeding  the 
first  inspiration  most  of  the  air  thus  introduced  remains,  the  force  exerted 
by  the  chest  in  returning  to  its  previous  dimensions  after  the  breathing  in 
and  the  elastic  action  of  the  alveoli  being  insufficient  to  bring  the  walls  of 
the  alveoli  again  into  contact.  Succeeding  breaths  unfold  the  lungs  more 
and  more,  until  all  the  alveoli  and  bronchioles  are  opened  up,  and  then  the 
whole  force  of  the  expiratory  act  is  directed  to  driving  out  the  previously 
inspired  air. 

It  is  not,  however,  until  some  time  after  birth  that  the  lungs  pass  into 
that  further  distended  state  of  which  we  spoke  above.  In  a  newly-born 
animal  there  is  no  negative  pressure  obtaining  in  the  pleural  cavities;  the 
lungs,  when  at  rest,  are  not  on  the  stretch,  and  opening  the  thorax  does 
not  lead  to  collapse  of  the  lungs.  The  state  of  things  obtaining  later  on 
is  established,  not  at  once,  but  gradually,  and  is  apparently  brought  about 
by  the  thorax  growing  more  rapidly,  and  so  becoming  relatively  more 
capacious  than  the  lungs.  The  distention  of  the  lungs  in  the  adult  may 
be  familiarly  described  as  being  due  to  the  chest  being  too  large  for  the 
lungs. 

§  270.  In  man  the  pressure  exerted  by  the  elasticity  of  the  lungs  alone 
amounts  to  about  5  or  7  mm.  of  mercury.  This  is  estimated  by  tying 
a  manometer  into  the  windpipe  of  a  dead  subject  and  observing  the  rise 
of  mercury  which  takes  place  when  the  chest-walls  are  punctured.  If  we 
took  7.6  mm.  as  the  pressure  this  would  be  just  T^0  of  the  pressure  of 
the  atmosphere.  If  the  chest  be  forcibly  distended  beforehand,  a  much 
larger  rise  of  the  mercury  is  observed,  amounting,  in  the  case  of  a  dis- 
tention corresponding  to  a  very  forcible  inspiration,  to  30  mm.  In  the 
living  body  this  mechanical  elastic  force  of  the  lungs  may  be  assisted  by 
the  contraction  of  the  plain  muscular  fibres  of  the  bronchi ;  the  pres- 
sure, however,  which  can  be  exerted  by  these  probably  does  not  exceed 
1  or  2  mm. 

When  a  manometer  is  introduced  into  a  lateral  opening  of  the  windpipe 
of  an  animal,  the  mercury  will  fall,  indicating  a  negative  pressure,  as  it  is 
called,  during  inspiration,  and  rise,  indicating  a  positive  pressure,  during 
expiration,  both  fall  and  rise  being  slight  and  varying  according  to  the 
freedom  with  which  the  air  passes  in  and  out  of  the  chest.  When  a  man- 
ometer is  fitted  with  air-tight  closure  into  the  mouth,  or  better,  in  order  to 
avoid  the  suction-action  of  the  mouth,  into  one  nostril,  the  other  nostril  and 
the  mouth  being  closed,  and  efforts  of  inspiration  and  expiration  are  made, 
the  mercury  falls  or  undergoes  negative  pressure  with  inspiration,  and  rises 
or  undergoes  positive  pressure  during  expiration.  It  has  been  found  in  this 
way  that  the  negative  pressure  of  a  strong  inspiratory  effort  may  vary  from 
30  to  74  mm.,  and  the  positive  pressure  of  a  strong  expiration  from  62  to 
100  mm. 

The  total  amount  of  air  which  can  be  given  out  by  the  most  forcible 
expiration  following  upon  the  most  forcible  inspiration,  that  is,  the  sum  of 
the  complemental,  tidal,  and  reserve  airs,  has  been  called  the  "  vital  capa- 
city ;"  "  extreme  differential  capacity  "  is  a  better  phrase.  It  may  be  meas- 
ured by  a  modification  of  a  gas-meter  called  a  spirometer ;  and  though  it 


336  RESPIRATION. 

varies  largely,  the  average  may  be  put  down  at  3000-4000  c.c.  (200  to 
250  cubic  inches). 

Of  the  whole  measure  of  vital  capacity,  about  500  c.c.  (30  cubic  inches) 
may  be  put  down  as  the  average  amount  of  tidal  air,  the  remainder  being 
nearly  equally  divided  between  the  complemental  and  reserve  airs.  The 
quantity  left  in  the  lungs  after  the  deepest  expiration  amounts  to  about 
1400  to  2000  c.c. 

Since  the  respiratory  movements  are  so  easily  affected  by  various  circumstances, 
the  simple  fact  of  attention  being  directed  to  the  breathing  being  sufficient  to  cause 
modifications  both  of  the  rate  and  depth  of  the  respiration,  it  becomes  very  diffi- 
cult to  fix  the  volume  of  an  average  breath.  Thus  various  authors  have  given 
figures  varying  from  53  c.c.  to  792  c.c.  The  statement  made  above  is  the  mean 
of  observations  varying  from  177  to  699  c.c. 

§  271.  Graphic  records  of  respiratory  movements.  These  may  be  ob- 
tained in  various  ways. 

The  simplest,  readiest,  and  perhaps  the  most  generally  useful  method  is  that  of 
recording  the  movements  of  the  column  of  air.  This  may  be  effected  by  intro- 
ducing a  T-piece  into  the  trachea,  one  cross-piece  being  left  open  and  the  other 
connected  with  a  Marey's  tambour  or  with  a  receiver,  which  in  turn  is  connected 
with  a  tambour  (see  Fig.  54  and  Fig.  90).  The  movements  of  the  column  of  air 
in  the  trachea  are  transmitted  to  the  tambour,  the  consequent  expansions  and 
contractions  of  which  are  transmitted  to  the  recording  drum  by  means  of  a  lever 
resting  on  it. 

If,  a  receiver  being  used,  the  open  end  of  the  I—  be  closed,  the  animal  breathes 
into  and  out  of  the  receiver,  and  the  movements  of  the  tambour  are  greatly  in- 
creased. This  has  the  disadvantage  that  the  air  in  the  receiver  soon  becomes 
unfit  for  further  respiration.  A  similar  increase  of  the  movements  of  the  lever 
of  the  tambour  may  be  obtained  by  connecting  a  piece  of  India-rubber  tubing  to 
the  open  end  of  the  I—.  By  increasing  the  length  of  this  tube,  or  slightly  con- 
tracting it,  the  movements  of  the  lever  may  be  increased  without  very  seriously 
interfering  with  the  breathing  of  the  animal. 

In  another  method  the  movements  of  the  chest  are  recorded.  When  a  small 
animal,  such  as  a  rabbit,  is  used,  the  whole  animal  may  be  placed  in  an  air-tight 
box,  breathing  being  carried  on  by  means  of  a  tube  inserted  into  the  trachea  and 
carried  through  an  air-tight  orifice  in  the  wall  of  the  box.  By  another  orifice 
and  tube  the  air  in  the  box  is  brought  into  connection  with  a  tambour,  which 
accordingly  registers  the  changes  of  pressure  in  the  air  of  the  box  produced  by 
the  movements  of  the  chest  (and  body),  and  thus  indirectly  the  movements  of 
the  chest.  In  man  and  larger  animals  the  changes  in  the  girth  of  the  chest  may 
be  conveniently  recorded  by  means  of  Marey's  pneumograph.  This  consists  of  a 
hollow  elastic  cylinder,  or  a  cylinder  with  elastic  ends,  the  interior  of  which  is 
connected  with  a  tambour.  By  means  of  a  strap  attached  to  each  end  of  the 
cylinder  the  instrument  can  be  buckled  round  the  chest  like  a  girdle.  When  the 
chest  expands,  the  ends  of  the  cylinder  are  pulled  out,  and  the  air  within  the 
chamber  rarefied;  in  consequence  the  lever  of  the  tambour  connected  with  its 
interior  is  depressed ;  conversely,  when  the  chest  contracts,  the  lever  is  elevated. 
The  pneumatograph  of  Fick  is  somewhat  similar.  Or  changes  in  one  or  other 
diameter  of  the  chest  may  be  recorded  by  what  maybe  called  the  "calipers" 
method,  as  in  the  recording  stethometer  of  Burdon-Sanderson.  This  consists  of 
a  rectangular  framework  constructed  of  two  rigid  parallel  bars  joined  at  right 
angles  to  a  cross-piece.  The  free  ends  of  the  bars,  the  distance  between  which 
can  be  regulated  at  pleasure,  are  armed,  the  one  with  a  tambour,  the  other  simply 
with  an  ivory  button.  The  tambour  bears  on  the  metal  plate  of  its  membrane 
(m',  Fig.  54)  a  small  ivory  button  in  place  of  the  lever.  When  it  is  desired  to 
record  the  changes  occurring  in  any  diameter  of  the  chest,  e.  c/.,  an  antero-pos- 
terior  diameter  from  a  point  in  the  sternum  to  a  point  in  the  back,  the  instrument 
is  made  to  encircle  the  chest  somewhat  after  the  fashion  of  a  pair  of  calipers,  the 
ivory  button  at  one  free  end  being  placed  on  the  spine  of  a  vertebra  behind  and 
the  tambour  at  the  other  on  the  sternum  in  front  in  the  line  of  the  diameter  which 


THE  MECHANICS  OF  PULMONARY  RESPIRATION.  337 

FIG.  90. 


Apparatus  for  taking  Tracings  of  the  Movements  of  the  Column  of  Air  in  Respiration.  The 
recording  apparatus  shown  is  the  ordinary  cylinder  recording  apparatus.  The  cylinder  A,  covered 
with  smoked  paper,  is  by  means  of  the  friction-  plate  B  put  into  revolution  by  the  spring  clock- 
work in  C,  regulated  by  Foucault's  regulator  D.  By  means  of  the  screw  E,  the  cylinder  can  be 
raised  or  lowered,  and  by  means  of  the  screw  ^its  speed  may  be  increased  or  diminished. 

The  tracheotomy  tube  t  fixed  in  the  trachea  of  an  animal  is  connected  by  India-rubber  tubing 
a  with  a  glass  T-piece  inserted  into  the  large  jar  G.  From  the  other  end  of  the  T-piece  proceeds 
a  second  piece  of  tubing  b,  the  end  of  which  can  be  either  closed  or  partially  obstructed  at  pleas- 
22 


338  RESPIRATION. 

is  being  studied.  The  distance  between  the  free  ends  of  the  instrument  being 
carefully  adjusted  so  that  the  button  of  the  tambour  presses  lightly  on  the  sternum, 
any  variations  in  the  length  of  the  diameter  in  question  will,  since  the  framework 
of  the  tambour  is  immobile,  give  rise  to  variations  of  pressure  within  the  tambour. 
These  variations  of  the  "receiving"  tambour,  as  it  is  called,  are  conveyed  by  a 
flexible  tube  containing  air  to  a  second  or  ''recording''  tambour,  the  lever  of 
which  records  the  variations  on  a  travelling  surface.  For  the  purpose  of  measuring 
the  extent  of  the  movements  the  instrument  must  be  experimentally  graduated. 
Other  forms  of  calipers  may,  of  course,  be  used. 

By  still  another  method  the  variations  in  intra-thoracic  pressure,  by  means  of 
which  the  movements  of  the  chest  walls  produce  the  movement  of  air  in  the 
lungs,  may  be  recorded.  This  may  be  effected  by  introducing  carefully,  to  the 
total  exclusion  of  air,  into  a  pleural  cavity  or  into,  the  pericardia!  cavity,  a  canula 
connected  by  a  rigid  tube  with  a  manometer.  With  each  inspiration  a  negative 
pressure,  or  rather  an  increase  of  the  existing  negative  pressure,  is  produced,  the 
mercury,  or  fluid,  in  the  manometer  returning  at  each  expiration.  An  easier 
method  of  recording  this  intra-thoracic  pressure  is  to  introduce  into  the  ossophagus 
an  ehstic  sound  (similar  to  the  cardiac  sound,  Fig.  54)  connected  with  a  tambour. 
The  O3sophagus  within  the  thorax,  like  the  heart  and  great  vessels,  as  we  shall  see, 
is  affected  as  well  as  the  lungs  by  the  variations  of  intra-thoracic  pressure  brought 
about  by  the  respiratory  movements. 

In  yet  another  method  the  movements  of  the  diaphragm,  which,  as  we  shall  see, 
serve  as  the  prime  agent  in  bringing  about  the  enlargement  of  the  thoracic  cavity, 
are  recorded.  This  may  be  done  by  inserting,  through  an  incision  in  the  abdominal 
wall,  a  flat  elastic  bag  between  the  diaphragm  and  abdominal  organs.  When  in 
inspiration  the  diaphragm  descends,  it  exerts  on  the  bag  a  pressure  which,  by 
means  of  a  tube,  may  be  communicated  to  a  tambour.  Or  a  needle  may  be  thrust 
through  the  chest  wall  so  as  to  rest  upon  or  transfix  the  diaphragm,  and  the  head 
of  the  needle  outside  the  body  connected  by  a  thread  or  otherwise  with  a  lever ;  each 
upward  and  downward  movement  of  the  head  of  the  needle,  corresponding  to  the 
downward  and  upward  movements  of  the  diaphragm,  is  registered  by  the  lever. 

Various  modifications  of  these  several  methods  have  been  adopted  by  various 
observers.  They  all,  however,  leave  much  to  be  desired.  A  very  ingenious  method 
of  registering  the  contractions  of  the  diaphragm  has  recently  been  introduced.  In 
the  rabbit  two  slips  of  muscular  fibres  forming  part  of  the  diaphragm,  one  on  each 
side  of  the  ensiform  cartilage,  are  so  disposed  and  possess  such  attachments  that 
one  or  both  of  them  may  be  isolated  without  injury  to  either  nerves  or  bloodvessels, 
and  arranged  so  that  while  one  end  of  the  slip  is  securely  fixed  to  the  chest-wall  as 
a  fixed  point,  the  other  end  can  by  a  thread  be  brought  to  bear  on  a  lever.  The 
slip,  even  when  thus  arranged,  appears  to  contract  rhythmically  in  complete  unison 
with  the  contractions  of  the  whole  of  the  rest  of  the  diaphragm :  it  serves,  so  to 
speak,  as  a  sample  of  the  diaphragm ;  and  hence  its  contractions,  like  those  of  the 
whole  diaphragm,  may  be  taken  as  a  record  of  respiratory  movements.  The  record 
has  to  be  corrected  for  variations  in  the  position  of  the  fixed  point. 

§  272.  In  these  various  ways  curves  are  obtained,  which,  while  differing 
in  detail,  exhibit  the  same  general  features,  and  more  or  less  resemble  the 
curve  shown  in  Fig.  91. 

As  the  figure  shows,  inspiration  begins  somewhat  suddenly  and  advances 
rapidly,  being  followed  immediately  by  expiration,  which  is  carried  out  at 
first  rapidly,  but  afterward  more  and  more  slowly.  Such  pauses  as  are  seen 
usually  occur  between  the  end  of  expiration  and  the  beginning  of  inspira- 


uro  by  means  of  the  screw  clamp  c.  From  the  jar  proceeds  a  third  piece  of  tubing  d,  connected 
with  a  Marey's  tambour  m  (see  Fig.  54),  the  lever  of  which  I  writes  on  the  recording  surface. 
When  the  tube  b  is  open  the  animal  breathes  freely  through  this,  and  the  movements  in  the  air 
of  G  and  consequently  in  the  tambour  are  slight.  On  closing  the  clamp  c,  the  animal  breathes 
only  the  air  contained  in  the  jar,  and  the  movements  of  the  lever  of  the  tambour  become  conse- 
quently much  more  marked. 

Below  the  lever  is  seen  a  small  time-marker  n  connected  with  an  electro-magnet,  the  current 
through  which,  coming  from  a  battery  by  the  wires  x  and  y,  is  made  and  broken  by  a  clock-work 
or  metronome. 


THE  MECHANICS  OF  PULMONARY   RESPIRATION. 


339 


tion.  In  normal  breathing  hardly  any  such  pause  exists,  but  in  cases  where 
the  respiration  becomes  infrequent,  pauses  of  considerable  length  may  be 
observed.  As  we  shall  see  in  detail  hereafter,  the  several  parts  of  the  whole 
act  vary  much,  under  various  circumstances,  in  relation  to  each  other. 
Sometimes  expiration,  sometimes  inspiration  is  prolonged  ;  and  either  inspi- 

[Fio.  91. 


Tracing  of  Thoracic  Respiratory  Movements  obtained  by  means  of  Marey's  Pneumograph. 
A  whole  respiratory  phase  is  comprised  between  a  anda;  inspiration,  during  which  the  lever 
•descends,  extending  from  a  to  b,  and  expiration  from  6  to  a.  The  undulations  at  c  are  caused  by 
the  heart's  beat.] 

ration  or  expiration  may  be  slow  or  rapid  in  its  development.  At  times  the 
chest  may  remain  for  a  while  at  the  height  of  inspiration,  thus  making  a 
pause  between  inspiration  and  expiration. 

In  what  may  be  considered  as  normal  breathing  the  respiratory  act  is  re- 
peated about  seventeen  times  a  minute,  the  duration  of  the  inspiration  as 
compared  with  that  of  the  expiration  (and  such  pause  as  may  exist)  being 
about  as  ten  to  twelve ;  but  the  rate  varies  very  largely,  and  in  this,  as  in 
the  volume  of  each  breath,  it  is  very  difficult  to  fix  a  satisfactory  average, 
the  figures  given  varying  from  twenty  to  thirteen  a  minute.  It  varies 
according  to  age  arid  sex.  It  is  influenced  by  the  position  of  the  body, 
being  quicker  in  standing  than  in  lying,  and  in  lying  than  in  sitting.  Mus- 
cular exertion  and  emotional  conditions  affect  it  deeply.  In  fact,  almost 
every  event  which  occurs  in  the  body  may  influence  it.  We  shall  have  to 
consider  in  detail  hereafter  the  manner  in  which  these  influences  are  brought 
to  bear. 

When  the  ordinary  respiratory  movements  prove  insufficient  to  effect  the 
necessary  changes  in  the  blood,  their  rhythm  and  character  become  changed. 
Normal  respiration  gives  place  to  labored  respiration,  and  this  in  turn  to 
dyspnoea,  which,  unless  some  restorative  event  occurs,  terminates  in  asphyxia. 
These  abnormal  conditions  we  shall  study  more  fully  hereafter. 

The  Respiratory  Movements. 

§  273.  When  the  movements  of  the  chest  during  normal  breathing  are 
watched,  or  when  a  graphic  record  is  taken  by  one  or  other  of  the  methods 
just  described,  it  is  seen  that  during  inspiration  an  enlargement  takes  place 
in  the  antero-posterior  diameter,  the  sternum  being  thrown  forward,  and  at 
the  same  time  moving  upward.  The  lateral  width  of  the  chest  is  also  in- 
creased. The  vertical  increase  of  the  cavity  is  not  so  obvious  from  the  out- 
side, though  when  the  movements  of  the  diaphragm  are  watched  by  means 
of  an  inserted  needle  or  otherwise,  it  is  clear  that  the  upper  surface  of  that 
organ  descends  at  each  inspiration,  the  anterior  walls  of  the  abdomen  bulging 
out  at  the  same  time.  In  the  female  human  subject,  the  movement  of  the 
upper  part  of  the  chest  is  very  conspicuous,  the  breast  rising  and  falling 
with  every  respiration ;  in  the  male,  however,  the  movements  are  almost  en- 


340  RESPIRATION. 

tirely  confined  to  the  lower  part  of  the  chest.  In  labored  respiration  all 
parts  of  the  chest  are  alternately  expanded  and  contracted,  the  breast  rising 
and  falling  as  well  in  the  male  as  in  the  female.  We  have  now  to  consider 
these  several  movements  in  greater  detail,  and  to  study  the  means  by  which 
they  are  carried  out. 

§  274.  Inspiration.  There  are  two  chief  means  by  which  the  chest  is 
enlarged  in  normal  inspiration,  viz.,  the  descent  of  the  diaphragm  and  the 
elevation  of  the  ribs.  The  former  causes  that  movement  in  the  lower  part 
of  the  chest  and  abdomen  so  characteristic  of  male  breathing,  which  is  hence 
called  diaphragmatic ;  the  latter  causes  the  movement  of  the  upper  chest 
characteristic  of  female  breathing,  which  is  called  costal.  These  two  main 
factors  are  assisted  by  less  important  and  subsidiary  events. 

Even  in  the  female  human  subject,  the  share  taken  in  respiration  by  the 
diaphragm  is  an  important  one  ;  in  the  male  the  diaphragm  must  be  regarded 
as  the  chief  respiratory  agent,  and  in  some  animals  its  use,  for  this  purpose, 
is  so  prominent  that  the  movements  of  the  ribs  may  in  normal  breathing  be 
almost  neglected.  In  the  rabbit  for  instance,  in  normal  breathing,  almost 
all  the  respiratory  work  is  done  by  the  contractions  of  the  diaphragm. 

The  descent  of  the  diaphragm  is  effected  by  means  of  the  contraction  of 
its  muscular  fibres.  When  at  rest  the  diaphragm  presents  a  convex  surface 
to  the  thorax ;  when  contracted  it  becomes  much  flatter,  and  in  consequence 
the  level  of  the  chest-floor  is  lowered,  the  vertical  diameter  of  the  chest  being 
proportionately  enlarged.  In  descending,  the  diaphragm  presses  on  the 
abdominal  viscera,  and  so  causes  a  projection  of  the  flaccid  abdominal  walls. 
From  its  attachments  to  the  sternum  and  the  false  ribs,  the  diaphragm,  while 
contracting,  naturally  tends  to  pull  the  sternum  and  the  upper  false  ribs 
downward  and  inward,  and  the  lower  false  ribs  upward  and  inward,  toward 
the  lumbar  spine.  In  normal  breathing,  this  tendency  produces  little  effect, 
being  counteracted  by  the  accompanying  general  costal  elevation,  and  by 
certain  special  muscles  to  be  mentioned  presently.  In  forced  inspiration, 
however,  and  especially  where  there  is  any  obstruction  to  the  entrance  of  air 
into  the  lungs,  the  lower  ribs  may  be  so  much  drawn  in  by  the  contraction 
of  the  diaphragm  that  the  girth  of  the  trunk  at  this  point  is  obviously 
diminished. 

§  275.  The  elevation  of  the  ribs  is  a  much  more  complex  matter  than  the 
descent  of  the  diaphragm.  If  we  examine  any  one  rib,  such  as  the  fifth,  we 
find  that  while  it  moves  freely  on  its  vertebral  articulation,  it  inclines  when 
in  the  position  of  rest  in  an  oblique  direction  from  the  spine  to  the  sternum  ; 
hence  it  is  obvious  that  when  the  rib  is  raised,  its  sternal  attachment  must 
not  only  be  carried  upward  but  also  thrown  forward.  The  rib  may  in  fact 
be  regarded  as  a  radius,  moving  on  the  vertebral  articulation  as  a  centre, 
and  causing  the  sternal  attachment  to  describe  an  arc  of  a  circle  in  the 
vertical  plane  of  the  body ;  as  the  rib  is  carried  upward  from  an  oblique 
to  a  more  horizontal  position,  the  sternal  attachment  must  of  necessity  be 
carried  further  away  in  front  of  the  spine.  Since  all  the  ribs  have  a  down- 
ward slanting  direction,  they  must  all  tend,  when  raised  toward  the  hori- 
zontal position,  to  thrust  the  sternum  forward,  some  more  than  others, 
according  to  their  slope  and  length.  The  elasticity  of  the  sternum  and 
costal  cartilages,  assisted  by  the  articulation  of  the  sternum  to  the  clavicle 
above,  permits  the  front  surface  of  the  chest  to  be  thus  thrust  forward  as 
well  as  upward,  when  the  ribs  are  raised.  By  this  action,  the  antero-pos- 
terior  diameter  of  the  chest  is  enlarged. 

Since  the  ribs  form  arches  which  increase  in  their  sweep  as  one  pro- 
ceeds from  the  first  downward  as  far  at  least  as  the  seventh,  it  is  evident  that 
when  a  lower  rib  such  as  the  fifth  is  elevated  so  as  to  occupy  or  to  approach 


THE  MECHANICS  OF  PULMONARY  RESPIRATION.  341 

toward  the  position  of  the  one  above  it,  the  chest  at  that  level  will  become 
wider  from  side  to  side,  in  proportion  as  the  fifth  arch  is  wider  than  the 
fourth.  Thus  the  elevation  of  the  rib  increases  not  only  the  antero-posterior 
but  also  the  transverse  diameter  of  the  chest.  Further,  on  account  of  the 
resistance  of  the  sternum,  the  angles  between  the  ribs  and  their  cartilages 
are,  in  the  elevation  of  the  ribs,  somewhat  opened  out,  and  thus  also  the 
transverse  as  well  as  the  antero-posterior  diameter,  somewhat  increased.  In 
more  than  one  way,  then,  the  elevation  of  the  ribs  enlarges  the  dimensions 
of  the  chest. 

§  276.  The  ribs  are  raised  by  the  contraction  of  certain  muscles.  Of 
these  the  external  intercostals  are  perhaps  the  most  important.  Even  in  the 
case  where  two  ribs,  such  as  the  fifth  and  sixth,  are  isolated  from  the  rest  of 
the  thoracic  cage,  by  section  of  the  structures  occupying  the  intercostal  spaces 
above  and  below,  the  contraction  of  the  external  intercostal  muscle  of  the 
intervening  space  raises  the  two  ribs,  thus  bringing  them  toward  the  posi- 
tion in  which  the  fibres  of  the  muscle  have  the  shortest  length,  viz.,  the  hori- 
zontal one.  This  elevating  action  is,  in  the  entire  chest,  further  favored  by 
the  fact  that  the  first  rib  is  less  movable  than  the  second,  and  so  affords  a 
comparatively  fixed  base  for  the  action  of  the  muscles  between  the  two,  the 
second  in  turn  supporting  the  third,  and  so  on,  while  the  scaleni  muscles  in 
addition  serve  to  render  fixed,  or  to  raise,  the  first  two  ribs.  So  that  in 
normal  respiration,  the  act  may  probably  be  described  as  beginning  by  a 
contraction  of  the  scaleni.  The  first  two  ribs  being  thus  raised  or  at  least 
fixed,  the  contraction  of  the  series  of  external  intercostal  muscles  acts  at  a 
great  disadvantage. 

While  the  elevating,  i.  e.,  inspiratory,  action  of  the  external  intercostals 
is  admitted  by  nearly  all  authors,  the  function  of  the  internal  intercostals  has 
been  much  disputed.  Some  regard  their  action  as  wholly  inspiratory  ;  others 
maintain,  what  is  perhaps  the  more  commonly  adopted  view,  that  while  those 
parts  of  them  which  lie  between  the  sternal  cartilages  act  like  the  external 
intercostals  as  elevators,  i.  e.,  as  inspiratory  in  function,  those  parts  which 
lie  between  the  osseous  ribs  act  as  depressors,  i.  e.,  as  expiratory  in  func- 
tion. 

In  the  well-known  model  consisting  of  two  rigid  bars  representing  the 
ribs,  moving  vertically  by  means  of  their  articulations  within  an  upright 
representing  the  spine,  and  connected  at  their  free  ends  by  a  piece  repre- 
senting the  sternum,  it  is  undoubtedly  true  that  stretched  elastic  bands 
attached  to  the  bars  in  such  a  way  as  to  represent  respectively  the  external 
and  internal  intercostals,  viz.,  sloping  in  the  one  case  downward  and  forward, 
and  in  the  other  downward  and  backward,  do,  on  being  left  free  to  contract, 
in  the  former  case  elevate  and  in  the  latter  depress  the  ribs.  Such  a  model, 
however,  does  not  fairly  represent  the  natural  conditions  of  the  ribs,  which 
are  not  straight  and  rigid,  but  peculiarly  curved  and  of  varying  elasticity, 
capable  moreover  of  rotation  on  their  own  axes,  and  having  their  move- 
ments determined  by  the  characters  of  their  vertebral  articulations.  The 
mechanical  conditions  in  fact  of  these  muscles  are  so  complex,  that  a  deduc- 
tion of  their  actions  from  simple  mechanical  principles,  or  from  the  direc- 
tion of  the  fibres,  must  be  exceedingly  difficult  and  dangerous.  Actual  ex- 
periments on  the  cat  and  dog  tend  to  show  that  in  these  animals  the  contrac- 
tion of  the  internal  intercostals,  along  their  whole  length,  takes  place,  in 
point  of  time,  alternately  with  that  of  the  diaphragm,  and  thus  offer  an 
argument  in  favor  of  these  muscles  being  expiratory  in  function. 

Next  in  importance  to  the  external  intercostals  come  the  levatores 
costarum,  which,  though  small  muscles,  are  able,  from  the  nearness  of  their 
costal  insertions  to  the  fulcrum,  to  produce  considerable  movement  of  the 


342  RESPIRATION. 

sternal  ends  of  the  ribs.  The  external  intercostals  and  the  levatores  costarum 
\vith  the  scaleni  may  fairly  be  said  to  be  the  elevators  of  the  ribs,  i.  e.,  the 
chief  muscles  of  costal  inspiration  in  normal  breathing. 

It  must  be  added,  however,  that  some  observers  deny  that  either  set  of 
intercostal  muscles  take  any  important  part  in  raising  the  ribs.  They  hold 
that  the  chief  if  not  the  only  use  of  these  muscles  is  by  their  contraction  to 
render  the  intercostal  spaces  firm  and  the  whole  thoracic  cage  rigid,  so  that 
the  thorax  is  moved  as  a  whole  by  the  other  muscles  mentioned,  and  the 
intercostal  spaces  do  not  give  way  during  the  respiratory  movements. 

Additional  space  in  the  transverse  diameter  is  afforded  probably  by  the 
rotation  of  the  ribs  on  an  antero-posterior  axis ;  but  this  movement  is  quite 
subsidiary  and  unimportant.  When  the  chest  is  at  rest,  the  ribs  are  some- 
what inclined  with  their  lower  borders  directed  inward  as  well  as  downward. 
When  they  are  drawn  up  by  the  action  of  the  intercostal  muscles,  their 
lower  borders  are  everted.  Thus  their  flat  sides  are  presented  to  the  thoracic 
cavity,  which  is  thereby  slightly  increased  in  width. 

§  277.  Labored  inspiration.  When  respiration  becomes  labored,  other 
muscles  are  brought  into  play.  The  scaleni  are  strongly  contracted,  so  as 
distinctly  to  raise  or  at  least  give  a  very  fixed  support  to  the  first  and  second 
ribs.  In  the  same  way  the  serratus  posticus  superior,  which  descends  from 
the  fixed  spine  in  the  lower  cervical  and  upper  dorsal  regions  to  the  second, 
third,  fourth,  and  fifth  ribs,  by  its  contractions  raises  those  ribs.  In  labored 
breathing  a  function  of  the  lower  false  ribs,  not  very  noticeable  in  easy 
breathing,  comes  into  play.  They  are  depressed,  retracted,  and  fixed,  thereby 
giving  increased  support  to  the  diaphragm,  and  directing  the  whole  energies 
of  that  muscle  to  the  vertical  enlargement  of  the  chest.  In  this  way  the 
serratus  posticus  inferior,  which  passes  upward  from  the  lumbar  aponeurosis 
to  the  last  four  ribs,  by  depressing  and  fixing  those  ribs  becomes  an  adjuvant 
inspiratory  muscle.  The  quadratus  lumborum  and  lower  portions  of  the 
sacro-lumbalis  may  have  a  similar  function. 

All  these  muscles  may  come  into  action  even  in  breathing  which,  though 
deeper  than  usual,  can  hardly  perhaps  be  called  labored.  When,  however, 
the  need  for  greater  inspiratory  efforts  becomes  urgent,  all  the  muscles  which 
can,  from  any  fixed  point,  act  in  enlarging  the  chest,  come  into  play.  Thus 
the  arms  and  shoulder  being  fixed,  the  serratus  magnus  passing  from  the 
scapula  to  the  middle  of  the  first  eight  or  nine  ribs,  the  pectoralis  minor 
passing  from  the  coracoid  to  the  front  parts  of  the  third,  fourth,  and  fifth 
ribs,  the  pectoralis  major  passing  from  the  humerus  to  the  costal  cartilages, 
from  the  second  to  the  sixth,  and  that  portion  of  the  latissimus  dorsi  which 
passes  from  the  humerus  to  the  last  three  ribs,  all  serve  to  elevate  the  ribs 
and  thus  to  enlarge  the  chest.  The  sterno-mastoid  and  other  muscles  passing 
from  the  neck  to  the  sternum,  are  also  called  into  action.  In  fact,  every 
muscle  which  by  its  contraction  can  either  elevate  the  ribs  or  contribute  to 
the  fixed  support  of  muscles  which  do  elevate  the  ribs,  such  as  the  trapezius, 
levator  anguli  scapulse,  and  rhomboidei  by  fixing  the  scapula,  may,  in  the 
inspiratory  efforts  which  accompany  dyspnoea,  be  brought  into  play. 

§  278.  Expiration.  In  normal  easy  breathing,  expiration  is  in  the  main 
a  simple  effect  of  elastic  reaction.  By  the  inspiratory  effort  the  elastic  tissue 
of  the  lungs  is  put  on  the  stretch  ;  so  long  as  the  inspiratory  muscles  continue 
contracting,  the  tissue  remains  stretched ;  but  directly  those  muscles  relax, 
the  elasticity  of  the  lungs  comes  into  play  and  drives  out  a  portion  of  the 
air  contained  in  them.  Similarly  the  elastic  sternum  and  costal  cartilages 
are  by  the  elevation  of  the  ribs  put  on  the  stretch ;  they  are  driven  into  a 
position  which  is  unnatural  to  them.  When  the  intercostal  and  other  ele- 
vator muscles  cease  to  contract,  the  elasticity  of  the  sternum  and  costal  car- 


THE  MECHANICS  OF  PULMONARY  RESPIRATION.  343 

tilages  cause  them  to  return  to  their  previous  position,  thus  depressing  the 
ribs  and  diminishing  the  dimensions  of  the  chest.  When  the  diaphragm 
descends,  in  pushing  down  the  abdominal  viscera,  it  puts  the  abdominal  walls 
on  the  stretch  ;  and  hence,  when  at  the  end  of  inspiration  the  diaphragm 
relaxes,  the  abdominal  walls  return  to  their  place,  and  by  pressing  on  the 
abdominal  viscera,  push  the  diaphragm  up  again  into  its  position  of  rest. 
Expiration,  then,  during  easy  breathing  is,  in  the  main,  simple  elastic  reac- 
tion ;  but  there  is  probably  some,  though  possibly  in  most  cases  a  very  slight, 
expenditure  of  muscular  energy  to  bring  the  chest  more  rapidly  to  its  former 
condition.  This  is,  as  we  have  seen,  supposed  by  many  to  be  afforded  by  the 
internal  intercostals  acting  as  depressors  of  the  ribs.  If  these  do  not  act  in 
this  way,  we  may  suppose  that  the  elastic  return  of  the  abdominal  walls  is 
accompanied  and  assisted  by  a  contraction  of  the  abdominal  muscles.  The 
triangularis  sterni,  the  effect  of  whose  contraction  is  to  pull  down  the  costal 
cartilages,  may  also  be  regarded  as  an  expiratory  muscle. 

When  expiration  becomes  labored,  the  abdominal  muscles  become  im- 
portant expiratory  agents.  By  pressing  on  the  contents  of  the  abdomen, 
they  thrust  them  and  therefore  the  diaphragm  also,  up  toward  the  chest,  the 
vertical  diameter  of  which  is  thereby  lessened,  while  by  pulling  down  the 
sternum  and  the  middle  and  lower  ribs  they  lessen  also  the  activity  of  the 
chest  in  its  antero-posterior  and  transverse  diameters.  They  are,  in  fact, 
the  chief  expiratory  muscles,  though  they  are  doubtless  assisted  by  the  ser- 
ratus  posticus  inferior  and  portions  of  the  sacro-lumbalis,  since  when  the 
diaphragm  is  not  contracting,  the  depression  of  the  lower  ribs  which  the 
contraction  of  these  muscles  causes,  serves  only  to  narrow  the  chest.  As 
expiration  becomes  more  and  more  forced,  every  muscle  in  the  body  which 
can  either  by  contracting  depress  the  ribs  or  press  on  the  abdominal  viscera, 
or  afford  fixed  support  to  muscles  having  those  actions,  is  called  into  play. 

§  279.  Faded  and  laryngeal  respiration.  The  thoracic  respiratory  move- 
ments are  accompanied  by  associated  respiratory  movements  of  other  parts 
of  the  body,  more  particularly  of  the  face  and  of  the  glottis. 

In  normal  healthy  respiration  the  current  of  air  which  passes  in  and  out 
of  the  lungs,  travels,  not  through  the  mouth  but  through  the  nose,  chiefly 
through  the  lower  nasal  meatus.  The  ingoing  air,  by  exposure  to  the  vas- 
cular mucous  membrane  of  the  narrow  and  winding  nasal  passages,  is  more 
efficiently  warmed  than  it  would  be  if  it  passed  through  the  mouth  ;  and  at 
the  same  time  the  mouth  is  thereby  protected  from  the  desiccating  effect  of 
the  continual  inroad  of  comparatively  dry  air. 

During  each  inspiratory  effort  the  nostrils  are  expanded,  probably  by  the 
action  of  the  dilatores  naris,  and  thus  the  entrance  of  air  facilitated.  The 
return  to  their  previous  condition  during  expiration  is  effected  by  the  elas- 
ticity of  the  nasal  cartilages,  assisted  perhaps  by  the  compressors  naris. 
This  movement  of  the  nostrils,  perceptible  in  many  people  even  during  tran- 
quil breathing,  becomes  very  obvious  in  labored  respiration. 

When  the  mouth  is  closed,  the  soft  palate,  which  is  held  somewhat  tense, 
is  swayed  by  the  respiratory  current,  but  entirely  in  a  passive  manner,  and 
it  is  not  until  the  larynx  is  reached  by  the  ingoing  air  that  any  active  move- 
ments are  met  with.  When  the  larynx  (the  details  of  which  we  shall  have 
to  deal  with  at  a  later  part  of  this  work)  is  examined  with  the  laryngoscope, 
it  is  frequently  seen  that,  while  during  inspiration  the  glottis  is  widely  open, 
with  each  expiration  the  arytenoid  cartilages  approach  each  other  so  as  to 
narrow  the  glottis,  the  cartilages  of  Santorini  projecting  inward  at  the  same 
time.  Thus,  synchronous  with  the  respiratory  expansion  and  contraction  of 
the  chest,  and  the  respiratory  elevation  and  depression  of  the  alse  nasi,  there 
is  a  rhythmic  widening  and  narrowing  of  the  glottis.  Like  the  movements 


344  RESPIRATION. 

of  the  nostril,  this  respiratory  action  of  the  glottis  is  much  more  evident  in 
labored  than  in  tranquil  breathing.  Indeed,  in  the  latter  case  it  is  fre- 
quently absent.  The  manner  in  which  this  rhythmic  opening  and  narrow- 
ing is  effected  will  be  described  when  we  come  to  study  the  production  of 
the  voice.  Whether  there  exists  a  rhythmic  contraction  and  expansion  of 
the  trachea  and  bronchial  passages,  especially  the  smaller  and  more  exclu- 
sively muscular  ones,  effected  by  means  of  the  plain  muscular  tissue  of  those 
organs  and  synchronous  with  the  respiratory  movements  of  the  chest,  is  un- 
certain. 

CHANGES  OF  THE  AIR  IN  RESPIRATION. 

§  280.  During  its  stay  in  the  lungs,  or  rather  during  its  stay  in  the  bron- 
chial passages,  the  tidal  air  (by  means  of  diffusion  chiefly)  effects  exchanges 
with  the  stationary  air ;  in  consequence  the  expired  air  differs  from  inspired 
air  in  several  important  particulars. 

The  temperature  of  expired  air  is  variable,  but  under  ordinary  circum- 
stances is  higher  than  that  of  the  inspired  air.  At  an  average  temperature 
of  the  atmosphere,  for  instance  at  about  20°  C.,  the  temperature  of  expired 
air  is  in  the  mouth  33.9°,  in  the  nose  35.3°.  When  the  external  tempera- 
ture is  low,  that  of  the  expired  air  sinks  somewhat,  but  not  to  any  great 
extent,  thus  at  — 6.3°  C.  it  is  29.8°  C.  When  the  external  temperature  is 
high,  the  expired  air  may  become  cooler  than  the  inspired,  thus  at  41.9°  it 
has  been  found  to  be  38.1°.  The  expired  air  takes  its  temperature  from 
that  of  the  body,  that  is,  of  the  blood,  and  this  as  we  shall  see  later  on,  while 
generally  higher  may,  at  times,  be  lower  than  that  of  the  atmosphere.  The 
exact  temperature  of  the  expired  air  in  fact  depends  on  the  relative  tempera- 
tures of  the  blood  and  inspired  air,  and  on  the  depth  and  rate  of  breathing. 
The  change  in  temperature  takes  place  not  in  the  lungs  but  in  the  upper 
passages,  and  chiefly  in  the  nose  and  pharynx. 

§  281.  The  expired  air  is  loaded  with  aqueous  vapor.  The  point  of 
saturation  of  any  gas,  that  is,  the  utmost  quantity  of  water  which  any  given 
volume  of  gas  can  take  up  as  aqueous  vapor,  varies  with  its  temperature, 
being  higher  with  the  higher  temperature.  For  its  own  temperature  expired 
air  is,  according  to  most  observers,  saturated  with  aqueous  vapor.  The 
moisture,  like  the  warmth,  is  imparted  not  in  the  depths  of  the  lung  but  in 
the  upper  passages.  The  inspired  air  as  it  passes  into  the  bronchia  is  already 
saturated  with  moisture. 

§  282.  The  expired  air  contains  about  4  or  5  per  cent,  less  oxygen,  and 
about  4  per  cent,  more  carbonic  acid  than  the  inspired  air,  the  quantity  of 
nitrogen  suffering  but  little  change.  Thus 

Oxygen.  Nitrogen.          Carbonic  acid. 

Inspired  air  contains    ....   20.81  79.15  0.04 

Expired  ....   16.033  79.587  4.38 

The  quantity  of  nitrogen  in  the  expired  air  is  sometimes  found  to  be 
slightly  greater  than  as  in  the  above  table,  but  sometimes  equal  to,  and 
sometimes  less  than,  that  of  the  inspired  air. 

In  a  single  breath  the  air  is  richer  in  carbonic  acid  (and  poorer  in 
oxygen)  at  the  end  than  at  the  beginning  of  the  breath.  Hence,  the  longer 
the  breath  is  held,  the  greater  the  (artificial)  pause  between  inspiration  and 
expiration,  the  higher  the  percentage  of  carbonic  acid  in  the  expired  air. 
Thus,  by  increasing  the  interval  between  two  expirations  to  100  seconds,  the 
percentage  may  be  raised  to  7.5.  When  the  rate  of  breathing  remains  the 


CHANGES  OF  THE  AIR  IN  RESPIRATION.  345 

same,  by  increasing  the  depth  of  the  breathing  the  percentage  of  carbonic 
acid  in  each  breath  is  lowered,  but  the  total  quantity  of  carbonic  acid 
expired  in  a  given  time  is  increased.  Similarly,  when  the  depth  of  breath- 
ing remains  the  same,  by  quickening  the  rate  the  percentage  of  carbonic 
acid  in  each  breath  is  lowered,  but  the  quantity  expired  in  a  given  time  is 
increased. 

Taking,  as  we  have  done,  the  amount  of  tidal  air  passing  in  and  out  of 
the  chest  of  an  average  man  at  500  c.c.,  such  a  person  will  expire  about 
22  c.c.  of  carbonic  acid  at  each  breath  ;  this,  reckoning  the  rate  of  breath- 
ing at  17  a  minute,  would  give  over  500  litres  of  carbonic  acid  for  the  day's 
production.  Actual  determinations,  however,  give  a  rather  smaller  total 
than  this  ;  thus,  in  a  series  of  experiments  of  which  we  shall  have  to  speak 
hereafter,  the  total  daily  excretion  of  carbonic  acid  in  an  average  man  was 
found  to  be  800  grms.,  i.  e.,  rather  more  than  400  litres  (406),  containing 
218.1  grms.  carbon  and  581.9  grins,  oxygen,  the  oxygen  which  actually  dis- 
appeared from  the  inspired  air  at  the  same  time  being  about  700  grms. 
This  amount,  it  should  be  said,  represents,  owing  to  the  manner  in  which  the 
experiment  was  conducted,  the  gases  given  out  and  taken  in,  not  by  the 
lungs  only,  but  by  the  whole  body ;  but  the  amount  of  carbonic  acid  given 
out  by  other  channels  than  the  lungs  is,  as  we  shall  see,  very  slight  (10  grms. 
or  even  less),  so  that  800  grms.  may  be  taken  as  the  average  production  of 
carbonic  acid  by  an  average  man.  The  quantity,  however,  both  of  oxygen 
consumed  and  of  carbonic  acid  given  out,  is  subject  to  very  wide  variations  ; 
thus,  in  the  observations  of  which  we  are  speaking,  the  daily  quantity  of 
carbonic  acid  varied  from  686  to  1285  grms.,  and  that  of  the  oxygen  from 
594  to  1072  grms.  These  variations  and  their  causes  will  be  discussed  when 
we  come  to  deal  with  the  problems  of  nutrition. 

§  283.  When  the  total  quantity  of  tidal  air  given  out  at  any  expiration 
is  compared  with  that  taken  in  at  the  corresponding  inspiration,  it  is  found 
that,  both  being  dried  and  measured  at  the  same  temperature  and  pressure, 
the  expired  air  is  less  in  volume  than  the  inspired  air,  the  difference  amount- 
ing to  about  ^th  to  ^Oth  of  the  volume  of  the  latter.  Hence,  when  an 
animal  is  made  to  breathe  in  a  confined  space,  the  air  is  absolutely  diminished 
in  volume.  The  approximate  equivalence  in  volume  between  inspired  and 
expired  air  arises  from  the  fact  that  the  volume  of  any  given  quantity  of 
carbonic  acid  is  equal  to  the  volume  of  the  oxygen  consumed  to  produce  it ; 
the  slight  falling  short  of  the  expired  air  is  due  to  the  circumstances  that  all 
the  oxygen  inspired  does  not  reappear  in  the  carbonic  acid  expired,  some 
having  formed  within  the  body  other  combinations. 

§  284.  Besides  carbonic  acid,  expired  air  contains  various  substances 
which  may  be  spoken  of  as  impurities,  many  of  an  unknown  nature,  and 
all  in  small  amounts.  Traces  of  ammonia  have  been  detected  in  expired 
air,  even  in  that  taken  directly  from  the  trachea,  in  which  case  its  presence 
could  not  be  due  to  decomposing  food  lingering  in  the  mouth.  When 
the  expired  air  is  condensed  by  being  conveyed  into  a  cooled  receiver, 
the  aqueous  product  is  found  to  contain  organic  matter,  which  from  the 
presence  of  microorganisms  introduced  in  the  inspired  air,  is  very  apt 
rapidly  to  putrefy.  The  organic  substances  thus  shown  to  be  present  in  the 
expired  air  are  the  cause  in  part  of  the  odor  of  breath.  It  is  probable  that 
some  of  them  are  of  a  poisonous  nature,  either  poisonous  in  themselves  as 
coming  direct  from  and  produced  in  some  way  or  other  in  the  pulmonary 
apparatus,  or  poisonous  as  being  the  products  of  putrefactive  decomposition  ; 
for  various  animal  substances  and  fluids  give  rise  by  decomposition  to  dis- 
tinct poisonous  products  known  as  ptomaines,  and  it  is  possible  that  some  of 
the  constituents  of  expired  air  are  of  an  allied  nature.  In  any  case  the 


346  RESPIRATION. 

substances  present  have  a  deleterious  action,  for  an  atmosphere  containing 
simply  1  per  cent,  of  carbonic  acid  (with  a  corresponding  diminution  of 
oxygen)  has  very  little  effect  on  the  animal  economy,  whereas  an  atmos- 
phere in  which  the  carbonic  acid  has  been  raised  to  1  per  cent,  by  breath- 
ing is  highly  injurious.  In  fact,  air  rendered  so  far  impure  by  breathing 
that  the  carbonic  acid  amounts  to  0.08  per  cent,  is  distinctly  unwholesome, 
not  so  much  on  account  of  the  carbonic  acid  as  of  the  accompanying  impuri- 
ties. Since  these  impurities  are  of  unknown  nature  and  cannot  be  estimated, 
the  easily  determined  carbonic  acid  is  usually  taken  as  an  indirect  measure 
of  their  presence.  We  have  seen  that  the  average  man  loads  at  each  breath 
500  c.c.  of  air  with  carbonic  acid  to  the  extent  of  4  per  cent.  He  will 
accordingly  at  each  breath  load  2  litres  to  the  extent  of  1  per  cent. ;  and 
in  one  hour,  if  he  breathe  17  times  a  minute,  will  load  rather  more  than 
2000  litres  to  the  same  extent.  At  the  very  least,  then,  a  man  ought  to  be 
supplied  with  this  quantity  of  air  hourly,  and  if  the  air  is  to  be  kept  fairly 
wholesome,  that  is  with  the  carbonic  acid  reduced  below  0.01  per  cent.,  he 
should  have  even  more  than  ten  times  as  much. 

THE  RESPIRATORY  CHANGES  IN  THE  BLOOD. 

§  285.  While  the  air  in  passing  in  and  out  of  the  lungs  is  thus  robbed 
of  a  portion  of  its  oxygen  and  loaded  with  a  certain  quantity  of  carbonic 
acid,  the  blood  as  it  streams  along  the  pulmonary  capillaries  undergoes 
important  correlative  changes.  As  it  leaves  the  right  ventricle  it  is  venous 
blood  of  a  dark  purple  or  maroon  color ;  when  it  falls  into  the  left  auricle 
it  is  arterial  blood  of  a  bright  scarlet  hue.  In  passing  through  the  capil- 
laries of  the  body  from  the  left  to  the  right  side  of  the  heart  it  is  again 
changed  from  the  arterial  to  the  venous  condition.  We  have  to  inquire, 
What  are  the  essential  differences  between  arterial  and  venous  blood,  by 
what  means  is  the  venous  blood  changed  into  arterial  in  the  lungs,  and  the 
arterial  into  venous  in  the  rest  of  the  body,  and  what  relations  do  these 
changes  in  the  blood  bear  to  the  changes  in  the  air  which  we  have  already 
studied  ? 

The  facts  that  venous  blood  at  once  becomes  arterial  in  appearance  on 
being  exposed  to  or  shaken  up  with  air  or  oxygen,  and  that  arterial  blood 
becomes  venous  in  appearance  when  kept  for  some  little  time  in  a  closed 
vessel,  or  when  submitted  to  a  current  of  some  indifferent  gas  such  as 
nitrogen  or  hydrogen,  prepare  us  for  the  statement  that  the  fundamental 
difference  between  venous  and  arterial  blood  is  in  the  relative  proportion 
of  the  oxygen  and  carbonic  acid  gases  contained  in  each.  From  both 
a  certain  quantity  of  gas  can  be  extracted  by  means  which  do  not  other- 
wise materially  alter  the  constitution  of  the  blood  ;  and  this  gas  when 
obtained  from  arterial  blood  is  found  to  contain  more  oxygen  and  less 
carbonic  acid  than  that  obtained  from  venous  blood.  This  is  the  real 
differential  character  in  the  two  bloods;  all  other  differences  are  either, 
as  we  shall  see  to  be  the  case  with  the  color,  dependent  on  this,  or  are 
unimportant  and  fluctuating. 

If  the  quantity  of  gas  which  can  be  extracted  by  the  mercurial  air-pump 
from  100  volumes  of  blood  be  measured  at  0°  C.  and  a  pressure  of  760  mm., 
it  is  found  to  amount  in  round  numbers  to  60  volumes. 

The  vacuum  produced  by  the  ordinary  mechanical  air-pump  is  insufficient  to 
extract  all  the  gas  from  blood.  Hence  it  becomes  necessary  to  use  a  mercury  pump 
capable  of  producing  a  large  Torricellian  vacuum.  In  the  form  of  mercurial  pump 
which  bears  Ludwig's  name  (Fig.  92),  two  large  globes  of  glass,  one  fixed  and  the 
other  movable,  are  connected  by  a  flexible  tube  :  the  fixed  globe  is  made  to  com- 


THE  RESPIRATORY  CHANGES  IN  THE  BLOOD. 


347 


municate  by  means  of  air-tight  stopcocks  alternately  with  a  receiver  containing  the 
blood  and  with  a  receiver  to  collect  the  gas.  When  the  movable  globe  filled 
with  mercury  is  raised  above  the  fixed  one,  the  mercury  from  the  former  runs 

FIG.  92. 


Diagrammatic  illustration  of  Ludwig's  Mercurial  Gas-pump :  A  and  B  are  two  glass  globes  con- 
nected by  strong  India-rubber  tubes,  a  and  6,  with  two  similar  glass  globes,  ^1'and  B'.  A  is  further 
connected  by  means  of  the  stopcock  c  with  the  receiver  C  containing  the  blood  (or  other  fluid)  to 
be  analyzed,  and  B  by  means  of  the  stopcock  d  and  the  tube  e  with  the  receiver  D  for  receiving  the 
gases.  A  and  B  are  also  connected  with  each  other  by  means  of  the  stopcocks/  and  g,  the  latter 
being  so  arranged  that  B  also  communicates  with  B'  by  the  passage  g'.  A'  and  B'  being  full  of 
mercury,  and  the  cocks  k,f,  g,  and  d  being  open,  but  c  and  g'  closed,  on  raising  A'  by  means  of  the 
pulley  p  the  mercury  of  A'  fills  A,  driving  out  the  air  contained  in  it  into  B,  and  so  out  through 
e.  When  the  mercury  has  risen  above  g,f  is  closed,  and  gf  being  opened,  B'  is  in  turn  raised 
until  B  is  completely  filled  with  mercury,  all  the  air  previously  in  it  being  driven  out  through 
e.  Upon  closing  d  and  lowering  B'  the  whole  of  the  mercury  in  B  falls  in  B',  and  a  vacuum  con- 
sequently is  established  in  B.  On  closing  g',  but  opening  g,f,  and  k,  and  lowering  A',  a  vacuum 
.  is  similarly  established  in  A  and  in  the  junction  between  A  and  B.  If  the  cock  c  be  now  opened, 
the  gases  of  the  blood  in  C  escape  into  the  vacuum  of  A  and  B.  By  raising  A'  after  the  closure 
of  c  and  opening  of  d,  the  gases  so  set  free  are  driven  from  A  into  B,  and  by  the  raising  of  B' 
from  B,  through  e  into  the  receiver  D,  standing  over  mercury. 

into  and  completely  fills  the  latter,  the  air  previously  present  being  driven  out. 
After  adjusting  the  C9cks,  the  movable  globe  is  then  depressed  thirty  inches 
below  the  fixed  one,  in  which  the  consequent  fall  of  the  mercury  produces  an 
almost  complete  vacuum.  By  turning  the  proper  cock  this  vacuum  is  put  into 


348 


RESPIRATION. 


connection  with  the  receiver  containing  the  blood,  which  thereupon  becomes  pro- 
portionately exhausted.  By  again  adjusting  the  cocks  and  once  more  elevating 
the  movable  globe,  the  gas  thus  extracted  is  driven  out  of  the  fixed  globe  into 
a  receiver.  The  vacuum  is  then  once  more  established  and  the  operation  re- 
peated as  long  as  gas  continues  to  be  given  off  from  the  blood. 

A  modified  form  of  pump  working  on  the  same  principles  as  that  of  Ludwig, 
but  involving  the  use  of  only  one  globe  to  be  made  vacuous  and  one  movable 
reservoir  for  mercury,  has  been  constructed  by  Pfliiger.  It  presents  several  advan- 
tages over  the  one  just  described,  the  chief  being  that  (1)  non-defibrinated  blood 
may  be  used  for  the  extraction  of  gases,  (2)  the  vacuum  into  which  the  gases  are 
evolved  is  large,  (3)  this  vacuum  is  kept  dry  by  being  connected  laterally  with  a 
vacuous  chamber  containing  sulphuric  acid.  The  details  of  this  construction  are, 
however,  complicated,  and  the  greatest  care  is  required  in  its  use  to  avoid  break- 

FIG.  93. 


Diagram  of  Alvergniat's  Pump. 

age.  Of  later  years  a  simplified  form  of  pump  has  been  introduced  for  laboratory 
work.  It  was  first  used  by  Grehant  and  Paul  Bert,  and  is  now  frequently  called  an 
Alvergniat's  pump,  from  the  name  of  its  present  maker.  Fig.  93  gives  a  diagram- 
matic representation  of  its  construction. 

A  is  a  glass  bulb  some  five  inches  in  diameter,  blown  on  to  a  glass  tube  a  below 
and  on  to  a  vertical  tube  b  above.  The  lower  end  of  a  is  connected  by  a  thick- 
walled  India-rubber  tube  with  a  reservoir  for  mercury  J?,  which  can  be  raised 
and  lowered  by  means  of  a  string  passing  over  a  pulley  c.  The  vertical  tube  b 
is  thickened  at  one  place,  and  into  this  thickened  portion  a  three-way  tap  d  is 
ground.  The  upper  end  of  b  is  prolonged  (above  the  three-way  tap)  into  a  fine 
point.  This  point  passes  by  a  tight  joint  through  the  bottom  of  a  vessel  e,  which 
can  be  partly  filled  with  mercury,  and  over  which  a  receiver  /.  filled  with  mercury 


THE  RESPIRATORY  CHANGES  IN  THE  BLOOD.  349 

for  the  collection  of  the  gases,  can  be  inverted.  A  tube  g  fused  on  laterally  to 
one  opening  of  the  three-way  tap  d  places  the  latter  in  connection  with  a  thick- 
walled  Woulff's  bottle  G  containing  a  layer  of  strong  sulphuric  acid.  The  second 
tubulure  of  this  bottle  is  similarly  connected  by  an  elastic  tube  with  the  vessel 
I),  into  which  blood  or  other  fluid  may  be  introduced  by  means  of  the  tap  h. 
All  the  movable  joints  of  the  apparatus  are  protected  by  India-rubber  tubes 
into  which  water  can  be  poured,  and  a  metal  casing  around  the  tap  d,  which 
may  also  be  filled  with  water,  similarly  prevents  the  possibility  of  any  leakage 
here. 

The  pump  is  used  as  follows :  By  placing  the  tap  d  in  the  position  shown  in  the 
figure  and  raising  B,  the  bulb  A  may  be  filled  with  mercury  up  to  the  top,  the  con- 
tained air  being  expelled  through  the  upper  end  of  l>.  By  a  slight  turn  of  the  tap 
all  connection  between  A  and  either  the  tube  g  or  the  upper  part  of  b  may  be  cut 
off,  and  on  lowering  B  a  vacuum  is  established  in  the  bulb  A  and  part  of  the  tube 
(t.  A  may  now  be  connected  by  the  tap  d  with  the  tube  g,  and  hence  with  C  and 
Z>,  and,  h  being  closed,  a  partial  vacuum  is  established  in  fand  D.  By  means  of 
the  tap  d  the  air  in  A  may  be  cut  off  from  g,  and  on  raising  B  and  placing  the  plug 
of  d  as  shown  in  the  figure  this  air  may  be  expelled  through  the  upper  end  of  b. 
By  slightly  turning  d  and  lowering  B  a  vacuum  is  again  established  in  A.  and,  as 
before,  a  further  portion  of  air  in  C  and  D  may  be  allowed  to  pass  over  into  A  and 
the  vacuum  in  D  and  C  increased.  In  this  way  all  the  air  in  I)  can  be  extracted, 
the  final  stages  being  facilitated  by  the  admission  of  a  little  water  into  Z>,  the  last 
traces  of  air  being  driven  over  into  A  by  the  rush  of  vapor  from  the  water.  A 
known  volume  of  blood  having  been  collected  over  mercury  in  a  small  tube  is  now 
allowed  to  enter  D  through  the  tap  h  and  yields  up  its  gases  to  the  vacuum.  A 
repetition  of  the  processes  by  which  the  air  in  D  was  originally  extracted  will  now 
remove  the  gases  which  have  been  given  off  from  the  known  volume  of  blood,  the 
only  difference  being  that  now  the  tube  /  filled  with  mercury  is  inverted  in  the 
trough  e  over  the  upper  end  of  the  tube  b.  In  this  way  the  gases  originally  in  D 
are  not  allowed  to  escape  into  the  air,  as  was  the  case  when  the  apparatus  was 
being  originally  made  vacuous,  but  are  collected  in  /  for  subsequent  analysis. 
During  the  extraction  of  the  gases  from  the  blood  the  bulb  D  is  immersed  in  a 
vessel  of  warm  water,  to  facilitate  the  exit  of  the  gases  and,  by  causing  the  for- 
mation of  large  quantities  of  aqueous  vapor,  to  sweep  the  gases  rapidly  over  into 
A.  The  sulphuric  acid  chamber  C  dries  the  vacuum  before  the  admission  of  the 
blood  into  />,  and  hence  makes  it  more  perfect  and  causes  the  most  complete  and 
rapid  evolution  of  gases  from  the  blood. 

The  average  composition  of  the  gas  thus  obtained  from  each  of  the  two 
kinds  of  blood  (the  arterial  blood  being  taken  from  a  large  artery,  and  the 
venous  blood  from  the  right  side  of  the  heart)  is,  stated  in  round  numbers, 
as  follows: 

From  100  vols.  may  be  obtained — 

Of  oxygen.  Of  carbonic  acid.  Of  nitrogen. 

Of  arterial  blood,  20  vols.  40  vols.  1  to  2  vols. 

Of  venous  blood,  8  to  12  vols.  46  vols.  1  to  2  vols. 

all  measured  at  760  mm.  and  0°  C. 

That  is  to  say,  venous  blood,  as  compared  with  arterial  blood,  contains  8 
to  12  per  cent,  less  oxygen  and  6  per  cent,  more  carbonic  acid.  It  must  be 
remembered,  however,  that  while  arterial  blood  from  whatever  artery  taken 
has  always  nearly  the  same  proportion  of  gases,  or  at  all  events  the  same 
amount  of  oxygen,  the  amount  of  oxygen  in  venous  blood,  even  when  taken 
from  the  same  vein,  may  vary  a  good,  deal,  still  more  so  when  it  is  taken 
from  different  veins.  The  reason  of  this  we  shall  see  hereafter. 

It  will  be  convenient  to  consider  the  relations  of  each  of  these  gases  sep- 
arately. 


350  RESPIRATION. 

The  Relations  of  Oxygen  in  the  Blood. 

§  286.  When  a  liquid  such  as  water  is  exposed  to  an  atmosphere  contain- 
ing a  gas  such  as  oxygen,  some  of  the  oxygen  will  be  dissolved  in  the  water, 
that  is  to  say,  will  be  absorbed  from  the  atmosphere.  The  quantity  which 
is  so  absorbed  will  depend  on  the  pressure  of  the  oxygen  in  the  atmosphere 
above  ;  the  greater  the  pressure  of  the  oxygen,  the  larger  the  amount  which 
will  be  absorbed.  If  the  pressure  of  the  whole  atmosphere  remain  the  same, 
at  760  mm.  of  mercury  for  instance  (the  ordinary  atmospheric  pressure),  the 
pressure  of  the  oxygen  may  be  increased  or  diminished  by  increasing  or 
diminishing  the  proportion  of  oxygen  in  the  atmosphere.  So  that  with  an 
atmosphere  remaining  at  any  given  pressure  the  quantity  of  oxygen  absorbed 
will  depend  on  the  quantity  present  in  that  atmosphere.  If,  on  the  other 
hand,  water,  already  containing  a  good  deal  of  oxygen  dissolved  in  it,  be  ex- 
posed to  an  atmosphere  containing  little  or  no  oxygen,  the  oxygen  will  escape 
from  the  water  into  the  atmosphere.  The  oxygen,  in  fact,  which  is  dissolved  in 
the  water,  like  the  oxygen  in  the  atmosphere  above,  stands  at  a  certain 
pressure,  the  amount  of  pressure  depending  on  the  quantity  dissolved ;  and 
when  water  containing  oxygen  dissolved  in  it  is  exposed  to  any  atmosphere, 
the  result,  that  is,  whether  the  oxygen  escapes  from  the  water  into  the  atmo- 
sphere or  passes  from  the  atmosphere  into  the  water,  depends  on  whether 
the  pressure  of  the  oxygen  in  the  water  is  greater  or  less  than  the  pressure 
of  the  oxygen  in  the  atmosphere.  Hence,  when  water  is  exposed  to  oxygen, 
the  oxygen  either  escapes  or  is  absorbed  until  equilibrium  is  established  be- 
tween the  pressure  of  the  oxygen  in  the  atmosphere  above  and  the  pressure 
of  the  oxygen  in  the  water  below.  This  result  is,  as  far  as  mere  absorption 
and  escape  are  concerned,  quite  independent  of  what  other  gases  are  present 
in  the  water  or  in  the  atmosphere.  Suppose  a  half-litre  of  water  was  lying 
at  the  bottom  of  a  two-litre  flask,  and  that  the  atmosphere  in  the  flask  above 
the  water  was  one-third  oxygen ;  it  would  make  no  difference,  as  far  as  the 
absorption  of  oxygen  by  the  water  was  concerned,  whether  the  remaining 
two-thirds  of  the  atmosphere  was  carbonic  acid  or  nitrogen  or  hydrogen, 
or  whether  the  space  above  the  water  was  a  vacuum  filled  to  one-third  with 
pure  oxygen.  Hence,  it  is  said  that  the  absorption  of  any  gas  depends  on 
the  partial  pressure  of  that  gas  in  the  atmosphere  to  which  the  liquid  is  ex- 
posed. This  is  true  not  only  of  oxygen  and  water,  but  of  all  gases  and 
liquids  which  do  not  enter  into  chemical  combination  with  each  other. 
Different  liquids  will,  of  course,  absorb  different  gases  with  differing  readi- 
ness ;  but  with  the  same  gas  and  the  same  liquid,  the  amount  absorbed  will 
depend  directly  on  the  partial  pressure  of  the  gas  in  the  overlying  space. 
It  should  be  added  that  the  process  is  much  influenced  by  temperature. 
Hence,  to  state  the  matter  generally,  the  absorption  of  any  gas  by  any  liquid 
will  depend  on  the  nature  of  the  gas,  the  nature  of  the  liquid,  the  pressure 
of  the  gas,  and  the  temperature  at  which  both  stand. 

Now  it  might  be  supposed,  and  indeed  was  once  supposed,  that  the  oxygen 
in  the  blood  was  simply  dissolved  by  the  blood.  If  this  were  so,  then  the 
amount  of  oxygen  present  in  any  given  quantity  of  blood  exposed  to  any 
given  atmosphere  ought  to  rise  and  fall  steadily  and  regularly  as  the  par- 
tial pressure  of  oxygen  in  that  atmosphere  is  increased  or  diminished  ;  the 
absorption  (or  escape)  of  oxygen  ought  to  follow  what  is  known  as  the 
Henry-Dalton  law  of  pressures.  But  this  is  found  not  to  be  the  case.  If 
we  expose  blood  containing  little  or  no  oxygen  to  a  succession  of  atmospheres 
containing  increasing  quantities  of  oxygen,  we  find  that  at  first  there  is  a 
very  rapid  absorption  of  the  available  oxygen,  and  then  this  somewhat  sud- 
denly ceases  or  becomes  very  small ;  and  if,  on  the  other  hand,  we  submit 


THE  RESPIRATORY  CHANGES  IN  THE  BLOOD.  351 

arterial  blood  to  successively  diminishing  pressures,  we  find  that  for  a  long 
time  very  little  oxygen  is  given  off,  and  then  suddenly  the  escape  becomes 
very  rapid.  The  absorption  of  oxygen  by  blood  does  not  follow  the  general 
law  of  absorption  according  to  pressure.  The  phenomena,  on  the  other  hand, 
suggest  the  idea  that  the  oxygen  in  the  blood  is  in  some  particular  combi- 
nation with  a  substance  or  some  substances  present  in  the  blood,  the  combi- 
nation being  of  such  a  kind  that  it  holds  good  during  a  lowering  of  pressure 
down  to  a  certain  limit,  and  that  then  dissociation  readily  occurs  ;  we  may 
add  that  this  limit  is  very  closely  dependent  on  temperature.  It  is,  how- 
ever, not  to  be  supposed  that  as  the  pressure  is  lowered,  no  oxygen  what- 
ever is  given  off  from  the  substance  until  a  certain  point  is  reached,  and 
that  at  that  point  the  whole  store  is  in  an  instant  dissociated,  no  more  re- 
maining to  be  given  off.  The  case  is  rather  that  while  pressure  is  being 
lowered  down  to  a  certain  point,  no  appreciable  dissociation  takes  place,  and 
that  then  having  begun  it  increases  rapidly  with  each  further  lowering  of 
pressure  until  the  whole  of  the  oxygen  is  given  off.  During  the  narrow 
range,  between  the  first  beginning  to  give  off  oxygen  and  the  completion  of 
the  giving  off,  the  compound  of  the  oxygen  with  the  substance  or  substances 
may  be  spoken  of  as  partly,  that  is  more  or  less,  dissociated.  What  is  the 
substance  or  what  are  the  substances  with  which  the  oxygen  is  thus  pecu- 
liarly combined? 

If  serum  free  from  red  corpuscles  be  used  in  such  absorption  experiments, 
it  is  found  that,  as  compared  with  the  entire  blood,  very  little  oxygen  is 
absorbed,  about  as  much  as  would  be  absorbed  by  the  same  quantity  of 
water;  and  such  as  is  absorbed  does  follow  the  law  of  pressure.  In  natural 
arterial  blood  the  quantity  of  oxygen  which  can  be  obtained  from  serum  is 
exceedingly  small ;  it  does  not  amount  to  half  a  volume  in  one  hundred 
volumes  of  the  entire  blood  to  which  the  serum  belonged.  It  is  evident 
that  the  oxygen  which  is  present  in  blood  is  in  some  way  or  other  peculiarly 
connected  with  the  red  corpuscles.  Now,  the  distinguishing  feature  of  the 
red  corpuscles  is  the  presence  of  haemoglobin.  We  have  already  seen  (§  24) 
that  this  constitutes  90  per  cent,  of  the  dried  red  corpuscles.  There  can  be 
a  priori  little  doubt  that  this  must  be  the  substance  with  which  the  oxygen 
is  associated,  and  to  the  properties  of  this  body  we  must  therefore  direct  our 
attention. 

§  287.  Haemoglobin.  When  separated  from  the  other  constituents  of  the 
serum,  haemoglobin  appears  as  a  substance,  either  amorphous  or  crystalline, 
readily  soluble  in  water  (especially  in  warm  water)  and  in  serum. 

Since  haemoglobin  is  soluble  in  serum,  and  since  the  identity  of  the  crystals 
observed  occasionally  within  the  corpuscles  with  those  obtained  in  other  ways  shows 
that  the  haemoglobin  as  it  exists  in  the  corpuscle  is  the  same  thing  as  that  which  is 
artificially  prepared  from  blood,  it  is  evident  that  some  peculiar  relationship  between 
the  stroma  and  the  haemoglobin  must,  in  natural  blood,  keep  the  latter  from  being 
dissolved  by  the  serum.  Hence,  in  preparing  haemoglobin  it  is  necessary  first  of 
all  to  break  up  this  connection  and  to  set  the  haemoglobin  free  from  the  corpuscles. 
This  maybe  done  by  the  addition  of  water,  of  ether,  of  chloroform,  and  of  bile-salts, 
or  by  repeatedly  freezing  and  thawing ;  blood  so  treated  becomes  "  laky  "  (cf.  I  24). 
It  is  also  of  advantage  previously  to  remove  the  alkaline  serum  as  much  as  possible, 
so  as  to  operate  only  on  the  red  corpuscles.  The  stroma  and  haemoglobin  berns  thus 
separated,  a  solution  of  haemoglobin  is  the  result.  The  alkalinity  of  the  solution, 
when  present,  being  reduced  by  the  cautious  addition  of  dilute  acetic  acid,  and  the 
solvent  power  of  the  aqueous  medium  being  diminished  by  the  addition  of  one- 
fourth  its  bulk  of  alcohol,  the  mixture,  set  aside  in  a  temperature  of  0°  C.  in  order 
still  further  to  reduce  the  solubility  of  the  haemoglobin,  readily  crystallizes,  when  the 
blood  used  is  that  of  the  dog,  cat,  horse,  rat,  guinea-pig,  etc.  In  the  case  of  the 
dog,  indeed,  it  is  simply  sufficient  to  add  ether  carefully  to  the  blood  until  it  just 
becomes  "laky,"  and  then  to  let  it  stand  in  a  cool  place  ;  the  mixture  soon  becomes 


352 


RESPIRATION. 


[FIG.  94. 


a  mass  of  crystals.     The  crystals  may  be  separated  by  filtration,  redissolved  in  water, 
and  recrystallized. 

Hemoglobin  from  the  blood  of  the  rat,  guinea-pig,  squirrel,  hedgehog, 
horse,  cat,  dog,  goose,  and  some  other  animals,  crystallizes  readily,  the  crys- 
tals being  generally  slender,  four-sided  prisms  belonging  to  the  rhombic 

system  and  often  appearing  quite 
acicular.  [Figs.  94,  95,  96.]  The 
crystals  from  the  blood  of  the  guinea- 
pig  are  octahedral,  but  also  belong  to 
the  rhombic  system ;  those  of  the 
squirrel  are  six-sided  plates.  The 
blood  of  the  ox,  sheep,  rabbit,  pig, 
and  man  crystallizes  with  difficulty. 
Why  these  differences  exist  is  not 
known  ;  but  the  composition  and  the 
amount  of  water  of  crystallization 
vary  somewhat  in  the  crystals  ob- 
tained from  different  animals.  In 
the  dog  the  percentage  composition 
of  the  crystals  has  been  determined 
as  C.  53.85,  H.  7.32,  N.  16.17,  O. 
21.84,  S.  0.39,  Fe.  0.43,  with  3  to  4 
per  cent,  of  water  of  crystallization. 
Tetrahedral  from  Blood  of  the  Pig.]  Jt  wj}}  thug  be  geen  that  hsemoglo- 

bin  contains,  in  addition  to  the  other 

elements  usually  present  in  proteid  substances,  a  certain  amount  of  iron  ; 
that   is    to    say,   the   element    iron  is  a  distinct  part   of  the  haemoglobin 


aBaa**"* 

.1  -Afl  V*!^  _*•. .*  «» 

™  Tvv.. 


[FIG.  95. 


[Fio.  96. 


Hexagonal  Crystals  from  Blood  of  Squirrel. 
On  these  six-sided  plates,  prismatic  crystals, 
grouped  in  a  stellate  manner,  not  infrequently 
occur.  (After  Funke)]. 


Prismatic,  from  Human  Blood.] 


molecule,  a  fact  which  of  itself  renders  haemoglobin  remarkable  among  the 
chemical  substances  present  in  the  animal  body. 

§  288.  The  crystals  when  seen  in  a  sufficiently  thick  layer  under  the 
microscope  have  the  same  bright  scarlet  color  as  arterial  blood  has  to  the 
naked  eye ;  when  seen  in  a  mass  they  naturally  appear  darker.  An  aqueous 
solution  of  haemoglobin,  obtained  by  dissolving  purified  crystals  in  distilled 


THE  RESPIRATORY   CHANGES  IN  THE  BLOOD.  353 

water,  has  also  the  same  bright  arterial  color.  A  tolerably  dilute  solution 
placed  before  the  spectroscope  is  found  to  absorb  certain  rays  of  light  in  a 
peculiar  and  characteristic  manner.  A  portion  of  the  red  end  of  the  spec- 
trum is  absorbed,  as  is  also  a  much  larger  portion  of  the  blue  end  ;  but  what 
is  most  striking  is  the  presence  of  two  strongly  marked  absorption  bands, 
lying  between  the  solar  lines  D  and  E.  (See  Fig.  97.)  Of  these  the  one 
toward  the  red  side,  sometimes  spoken  of  as  the  band  (a),  is  the  thinnest,  but  the 
most  intense,  and  in  extremely  dilute  solutions  (Fig.  97, 1)  is  the  only  one  vis- 
ible ;  its  middle  lies  at  some  little  distance  to  the  blue  side  of  D.  Its  position 
may  be  more  exactly  defined  by  expressing  it  in  wave-lengths.  As  is  well 
known,  the  rays  of  light  which  make  up  the  spectrum  differ  in  the  length  of 
their  ways,  diminishing  from  the  red  end,  where  the  waves  are  longest,  to  the 
blue  end,  where  they  are  shortest.  Thus,  Frauenhofer's  line  D  corresponds  to 
rays  having  a  wave-length  of  589.4  millionths  of  a  millimetre.  Using  the  same 
unit,  the  centre  of  this  absorption  band,  a,  of  hemoglobin  corresponds  to  the 
wave-length  578 ;  as  may  be  seen  in  Fig.  97,  where,  however,  the  numbers 
of  the  divisions  of  the  scale  indicate  only  100,000ths  of  a  millimetre.  The 
other,  sometimes  called  b,  much  broader,  lies  a  little  to  the  red  side  of  E, 
its  blueward  edge,  even  in  moderately  dilute  solutions  (Fig.  97,  2),  coming 
close  up  to  that  line  ;  its  centre  corresponds  to  about  wave-length  539.  Each 
band  is  thickest  in  the  middle  and  gradually  thins  away  at  the  edges. 
These  two  absorption  bands  are  extremely  characteristic  of  a  solution  of 
haemoglobin.  Even  in  very  dilute  solutions  both  bands  are  visible  (they 
may  be  seen  in  a  thickness  of  1  cm.  in  a  solution  containing  1  grm.  of 
haemoglobin  in  10  litres  of  water),  and  that  when  scarcely  any  of  the  ex- 
treme red  end  and  very  little  of  the  blue  end  is  cut  off.  They  then  appear 
not  only  faint  but  narrow.  As  the  strength  of  the  solution  is  increased  the 
bands  broaden  and  become  more  intense  ;  at  the  same  time  both  the  red  end, 
and  still  more  the  blue  end,  of  the  whole  spectrum  are  encroached  upon  (Fig. 
97,  3).  This  may  go  on  until  the  two  absorption  bands  become  fused  together 
into  one  broad  band  (Fig.  97,  4).  The  only  rays  of  light  which  pass  through 
the  haemoglobin  solution  are  those  in  the  green  between  the  blueward  edge 
of  the  united  bands  and  the  general  absorption,  which  is  now  rapidly  ad- 
vancing from  the  blue  end,  and  those  in  the  red  between  the  united  bands 
and  the  general  absorption  at  the  red  end.  If  the  solution  be  still  further 
increased  in  strength,  the  interval  on  the  blue  side  of  the  united  bands 
becomes  absorbed  also,  so  that  the  only  rays  which  pass  through  are  the 
red  rays  lying  to  the  red  side  of  D ;  these  are  the  last  to  disappear,  and 
hence  the  natural  red  color  of  the  solution  as  seen  by  transmitted  light. 
Exactly  the  same  appearances  are  seen  when  crystals  of  haemoglobin  are 
examined  with  a  micro-spectroscope.  They  are  also  seen  when  arterial 
blood  itself  (diluted  with  saline  solutions,  so  that  the  corpuscles  remain  in 
as  natural  a  condition  as  possible)  is  examined  with  the  spectroscope,  as 
well  as  when  a  drop  of  blood,  which  from  the  necessary  exposure  to  air  is 
always  arterial,  is  examined  with  the  micro-spectroscope.  In  fact,  the  spec- 
trum of  haemoglobin  is  the  spectrum  of  normal  arterial  blood. 

§  289.  When  crystals  of  haemoglobin,  prepared  in  the  way  described 
above,  are  subjected  to  the  vacuum  of  the  mercurial  air-pump,  they  give  off 
a  certain  quantity  of  oxygen,  and  at  the  same  time  they  change  in  color. 
The  quantity  of  oxygen  given  off  is  definite,  1  grm.  of  the  crystals  giving  off 
1.59  c.c.  of  oxygen  measured  at  760  mm.  Hg.  and  0°  C.  In  other  words, 
the  crystals  of  haemoglobin,  over  and  above  the  oxygen  which  enters  inti- 
mately into  the  composition  of  the  molecule  (and  which  alone  is  given  in 
the  elementary  composition  previously  stated),  contain  another  quantity  of 
oxygen,  which  is  in  loose  combination  only,  and  which  may  be  dissociated  from 

28 


354 


RESPIRATION. 


them  by  subjecting  them  to  a  sufficiently  low  pressure.    The  change  of  color 
which  ensues  when  this  loosely  combined  oxygen  is  removed,  is  character- 


FIG.  97. 


The  Spectra  of  Oxy-htemoglobin  in  Different  Grades  of  Concentration,  of  (reduced)  Haemo- 
globin, and  of  Carbonic-oxide-haemoglobin.  (After  Preyer  and  Gamgee) :  1  to  4,  solution  of  oxy- 
haemoglobin  containing— (1)  less  than  0.01  per  cent.,  (2)  0.09  per  cent,,  (3)  0.37  per  cent.,  (4)  0.8  per 
cent.  5,  solution  of  (reduced)  haemoglobin  containing  about  0.2  per  cent.  6,  solution  of  carbonic- 
oxide-hEemoglobin.  In  each  of  the  six  cases  the  layer  brought  before  the  spectroscope  was  1  cm. 
in  thickness.  The  letters  (A,  a,  etc.)  indicate  Frauenhofer's  lines,  and  the  figures  wave-lengths 
expressed  in  100,000th s  of  a  millimetre. 


THE  RESPIRATORY  CHANGES  IN  THE  BLOOD.  355 

istic  ;  the  crystals  become  darker  and  more  of  a  purple  hue,  and  at  the  same 
time  dichroic,  so  that  while  the  thicker  parts  are  purple,  the  thin  edges 
appear  greenish. 

An  ordinary  solution  of  haemoglobin,  like  the  crystals  from  which  it  is 
formed,  contains  a  definite  quantity  of  oxygen  in  a  similarly  peculiar  loose 
combination  ;  this  oxygen  it  also  gives  up  when  subjected  in  the  air-pump  to 
sufficiently  low  pressure,  becoming  at  the  same  time  of  a  purplish  hue.  This 
loosely  combined  oxygen  may  also  be  removed  by  passing  a  stream  of 
hydrogen  or  other  indifferent  gas  through  the  solution ;  the  stream  of 
hydrogen  acts  like  an  oxygen-vacuum  to  the  haemoglobin  and  thus  disasso- 
ciation  is  effected.  Carbonic  acid  gas  is  unsuitable  for  this  purpose,  since, 
as  we  shall  see,  being  an  acid,  it  acts  in  another  way  on  the  haemoglobin. 
The  oxygen  may  also  be  removed  from  the  haemoglobin  not  only  by  physical 
but  also  by  chemical  means,  as  by  the  use  of  reducing  agents.  Thus  if  a 
few  drops  of  ammonium  sulphide  or  of  an  alkaline  solution  of  ferrous  sul- 
phate kept  from  precipitation  by  the  presence  of  tartaric  acid,  be  added  to 
a  solution  of  haemoglobin,  or  even  to  an  unpurified  solution  of  blood  cor- 
puscles such  as  is  afforded  by  the  washings  from  a  blood-clot,  the  oxygen  in 
loose  combination  with  the  haemoglobin  is  immediately  seized  upon  by  the 
reducing  agent.  This  may  be  recognized  at  once  by  the  characteristic 
change  of  color ;  from  a  bright  scarlet  the  solution  becomes  of  a  purplish- 
claret  color,  when  seen  in  any  thickness,  but  greenish  when  sufficiently 
thin  ;  the  color  of  the  reduced  solution  is  exactly  like  that  of  the  crystals 
from  which  the  loose  oxygen  has  been  removed  by  the  air-pump. 

Examined  by  the  spectroscope,  this  reduced  solution,  or  solution  of  re- 
duced hcemoglobin,  as  we  may  now  call  it,  offers  a  spectrum  (Fig.  97,  5)  very 
different  from  that  of  the  unreduced  solution.  The  two  absorption  bands 
have  disappeared,  and  in  their  place  there  is  seen  a  single,  much  broader, 
but  at  the  same  time  much  fainter  band,  whose  middle  occupies  a  position 
about  midway  between  the  two  absorption  bands  of  the  unreduced  solution, 
though  the  redward  edge  of  the  band  shades  away  rather  further  toward  the 
red  than  does  the  other  edge  toward  the  blue ;  its  centre  corresponds  to 
about  wave-length  555.  At  the  same  time  the  general  absorption  of  the 
spectrum  is  different  from  that  of  the  unreduced  solution  ;  less  of  the  blue 
end  is  absorbed.  Even  when  the  solutions  become  tolerably  concentrated, 
many  of  the  bluish-green  rays  to  the  blue  side  of  the  single  band  still  pass 
through.  Hence  the  difference  in  color  between  haemoglobin  which  retains 
the  loosely  combined  oxygen,1  and  haemoglobin  which  has  lost  its  oxygen 
and  become  reduced.  In  tolerably  concentrated  solutions,  or  tolerably  thick 
layers,  the  former  lets  through  the  red  and  orange-yellow  rays,  the  latter  the 
red  and  the  bluish-green  rays.  Accordingly,  the  one  appears  scarlet,  the 
other  purple.  In  dilute  solutions,  or  in  a  thin  layer,  the  reduced  haemo- 
globin lets  through  so  much  of  the  green  rays  that  they  preponderate  over 
the  red,  and  the  resulting  impression  is  one  of  green.  In  the  unreduced 
haemoglobin  or  oxy-haemoglobin,  the  potent  yellow  which  is  blocked  out  in 
the  reduced  haemoglobin  makes  itself  felt,  so  that  a  very  thin  layer  of  oxy- 
haemoglobin,  as  in  a  single  corpuscle  seen  under  the  microscope,  appears 
yellow  rather  than  red. 

It  must  be  remembered  that  when  we  speak  of  reduced  haemoglobin  (or 
more  briefly  haemoglobin)  with  a  purple  color  and  a  characteristic  one- 
banded  spectrum,  we  mean  haemoglobin  which  has  lost  all  its  loosely  asso- 
ciated oxygen.  If  a  quantity  of  oxy-haemoglobin  be  exposed  to  an  insuffi- 

1  For  brevity's  sake  we  may  call  the  haemoglobin  containing  oxygen  in  loose  combina- 
tion, oxy-hxmogloMn.  and  the  haemoglobin  from  which  this  loosely  combined  oxygen  has 
been  removed,  reduced  haemoglobin  or  simply  haemoglobin. 


356  EESPIRATION. 

ciently  low  pressure,  or  to  the  action  of  an  insufficient  quantity  of  the 
reducing  action,  it  gives  up  a  part  only  of  its  oxygen  ;  it  is  only  partly 
reduced.  Such  a  partly  reduced  solution  still  shows  the  two  bands  of  oxy- 
haemoglobin. 

§  290.  When  the  haemoglobin  solution  (or  crystal),  which  has  lost  its 
oxygen  by  the  action  either  of  the  air-pump  or  of  a  reducing  agent  or  by  the 
passage  of  an  indifferent  gas,  is  exposed  to  air  containing  oxygen,  an  absorp- 
tion of  oxygen  at  once  takes  place.  If  sufficient  oxygen  be  present,  the 
haemoglobin  seizes  upon  sufficient  oxygen  to  obtain  its  full  complement,  each 
gramme  taking  up  in  combination  1.59  c.c.  of  oxygen  ;  if  there  be  an  in- 
sufficient quantity  of  oxygen  the  haemoglobin  still  remains  partly  reduced ; 
or  perhaps  we  may  say  that  a  part  only  of  the  haemoglobin  gets  its  allowance 
while  the  remainder  continues  reduced.  If  the  amount  of  oxygen  be  suffi- 
cient, the  solution  (or  crystal),  as  it  takes  up  the  oxygen,  regains  its  bright 
scarlet  color  and  its  characteristic  absorption  spectrum,  the  single  band  being 
replaced  by  the  two.  Thus  if  a  solution  of  oxy-haemoglobin  in  a  test-tube, 
after  being  reduced  by  the  action  of  a  drop  or  two  of  ammonium  sulphide 
solution  and  thus  showing  the  purple  color  and  the  single  band,  be  shaken 
up  with  air,  the  bright  scarlet  color  at  once  returns,  and  when  the  fluid  is 
placed  before  the  spectroscope,  it  is  seen  that  the  single  faint  broad  band  of 
the  reduced  haemoglobin  has  wholly  disappeared,  and  that  in  its  place  are 
the  two  sharp  thinner  bands  of  the  oxy-haemoglobin.  If  left  to  stand  in  the 
test-tube  the  quantity  of  reducing  agent  still  present  is  generally  sufficient 
again  to  rob  the  haemoglobin  of  the  oxygen  thus  newly  acquired,  and  soon 
the  scarlet  hue  fades  back  again  into  the  purple,  the  two  bands  giving  place 
to  the  one.  Another  shake  and  exposure  to  air  will,  however,  again  bring 
back  the  scarlet  hue  and  the  two  bands ;  and  once  more  these  may  disappear. 
In  fact,  a  few  drops  of  the  reducing  fluid  will  allow  this  game  of  haemoglobin 
taking  oxygen  from  the  air  and  giving  it  up  to  the  reducer  to  be  played  over 
and  over  again ;  at  each  turn  of  the  game  the  color  shifts  from  scarlet  to 
purple  and  from  purple  to  scarlet,  while  the  two  bands  exchange  for  the  one 
and  the  one  for  the  two. 

§  291.  Color  of  venous  and  arterial  blood.  Evidently  we  have  in  these 
properties  of  haemoglobin  an  explanation  of  at  least  one-half  of  the  great 
respiratory  process,  and  they  teach  us  the  meaning  of  the  change  of 
color  which  takes  place  when  venous  blood  becomes  arterial  or  arterial 
venous. 

In  venous  blood,  as  it  issues  from  the  right  ventricle,  the  oxygen  present 
is  insufficient  to  satisfy  wholly  the  haemoglobin  of  the  red  corpuscles;  the 
haemoglobin  is,  to  a  large  extent,  reduced,  hence  the  purple  color  of  venous 
blood.  When  ordinary  venous  blood,  diluted  without  access  of  oxygen,  is 
brought  before  the  spectroscope,  the  two  bands  of  oxy-haemoglobin  are  seen. 
This  is  explained  by  the  fact  that  in  partly  reduced  haemoglobin,  which  we 
may  conveniently  regard  as  a  mixture  of  oxy-haemoglobin  and  (reduced) 
haemoglobin,  the  two  sharp  bands  of  the  former  are  always  much  more 
readily  seen  than  the  much  fainter  band  of  the  latter.  Now  in  ordinary 
venous  blood  there  is  always  some  loose  oxygen,  removable  by  diminished 
pressure  or  otherwise  ;  the  haemoglobin  is  only  partly  reduced,  there  is  always 
some,  indeed  a  considerable  quantity,  of  oxy-haemoglobin  as  well  as  (reduced) 
haemoglobin.  It  is  only  under  special  circumstances,  as,  for  instance,  after 
death  by  what  we  shall  presently  speak  of  as  asphyxia,  that  all  the  loose 
oxygen  of  the  blood  disappears ;  and  then  the  two  bands  of  oxy-haemoglobin 
vanish  too.  If  even  only  a  small  quantity  of  oxygen  be  present,  so  distinct 
are  the  two  bands  that  a  solution  of  completely  reduced  haemoglobin  may  be 
used  as  a  test  for  the  presence  of  oxygen  ;  if  oxygen  be  present  in  any  fluid 


RESPIRATORY   CHANGES  IN  THE  BLOOD.  357 

to  which  the  reduced  haemoglobin  is  added,  the  single  band  immediately 
gives  way  to  the  two  bands  of  oxy-haemoglobin. 

As  the  venous  blood  passes  through  the  capillaries  of  the  lungs,  this 
reduced  haemoglobin  takes  from  the  pulmonary  air  its  complement  of  oxygen, 
all  or  nearly  all  the  haemoglobin  of  the  red  corpuscles  becomes  oxy-haemo- 
globin,  and  the  purple  color  forthwith  shifts  into  scarlet.  For  careful 
observations  show  that  the  haemoglobin  of  arterial  blood  is  saturated  or 
nearly  saturated  with  oxygen  ;  it  probably  falls  short  of  complete  saturation 
by  about  1  vol.  of  oxygen  in  100  vols.  of  blood.  By  increasing  the  pressure 
of  the  oxygen,  an  additional  quantity  may  be  driven  into  the  blood,  but  this, 
after  the  haemoglobin  has  become  completely  saturated,  is  effected  by  simple 
absorption.  The  quantity  so  added  is  extremely  small  compared  with  the 
total  quantity  combined  with  the  haemoglobin. 

Passing  from  the  left  ventricle  to  the  capillaries  of  the  tissues  the  oxy- 
haemoglobin  gives  up  some  of  its  oxygen  to  the  tissues,  becoming,  in  part, 
reduced  haemoglobin,  and  the  blood  in  consequence  becomes  once  more 
venous,  with  a  purple  hue.  Thus  the  red  corpuscles  by  virtue  of  their 
haemoglobin  are  emphatically  oxygen-carriers.  Undergoing  no  intrinsic 
change  in  itself,  the  haemoglobin  combines  in  the  lungs  with  oxygen,  which 
it  carries  to  the  tissues ;  these,  more  greedy  of  oxygen  than  itself,  rob  it  of 
its  charge,  and  the  reduced  haemoglobin  hurries  back  to  the  lungs  in  the 
venous  blood  for  another  portion.  The  change  from  venous  to  arterial  blood 
is  then  in  part  (for  as  we  shall  see  there  are  other  events  as  well)  a  peculiar 
combination  of  haemoglobin  with  oxygen,  while  the  change  from  arterial  to 
venous  is,  in  part  also,  a  reduction  of  oxy-hsemoglobin  ;  and  the  difference  of 
color  between  venous  and  arterial  blood  depends  almost  entirely  on  the  fact 
that  the  reduced  haemoglobin  of  the  former  is  of  purple  color,  while  the  oxy- 
hsemoglobin  of  the  latter  is  of  a  scarlet  color. 

There  may  be  other  causes  of  the  change  of  color,  but  these  are  wholly 
subsidiary  and  unimportant.  When  a  corpuscle  swells,  its  refractive  power 
is  diminished,  and  in  consequence  the  number  of  rays  which  pass  into  and 
are  absorbed  by  it  are  increased  at  the  expense  of  those  reflected  from  its 
surface  ;  anything  therefore  which  swells  the  corpuscles,  such  as  the  addition 
of  water,  tends  to  darken  blood,  and  anything,  such  as  a  concentrated  saline 
solution,  which  causes  the  corpuscles  to  shrink,  tends  to  brighten  blood. 
Carbonic  acid  has  apparently  some  influence  in  swelling  the  corpuscles,  and 
therefore  may  aid  in  darkening  the  venous  blood. 

§  292.  We  have  spoken  of  the  combination  of  haemoglobin  with  oxygen 
as  being  a  peculiar  one.  The  peculiarity  consists  in  'the  facts  that  the  oxy- 
gen may  be  associated  and  dissociated,  without  any  general  disturbance  of 
the  molecule  of  haemoglobin,  and  that  dissociation  may  be  brought  about 
very  readily.  Haemoglobin  combines  in  a  wholly  similar  manner  with  other 
gases.  If  carbonic  oxide  (monoxide)  be  passed  through  a  solution  of 
haemoglobin,  a  change  of  color  takes  place,  a  peculiar  bluish  tinge  making 
its  appearance.  At  the  same  time  the  spectrum  is  altered ;  two  bands  are 
still  visible,  but  on  accurate  measurement  it  is  seen  that  they  are  placed  more 
toward  the  blue  end  than  are  the  otherwise  similar  bands  of  oxy-haemoglobin 
(see  Fig.  97,  6)  ;  their  centres  corresponding  respectively  to  about  wave- 
lengths 572  and  533,  while  those  of  oxy-haemoglobin,  as  we  have  seen,  cor- 
respond to  578  and  539.  When  a  known  quantity  of  carbonic  oxide  gas  is 
sent  through  a  haemoglobin  solution,  it  will  be  found  on  examination  that  a 
certain  amount  of  the  gas  has  been  retained,  an  equal  volume  of  oxygen  ap- 
pearing in  its  place  in  the  gas  which  issues  from  the  solution.  If  the  solution 
so  treated  be  crystallized,  the  crystals  will  have  the  same  characteristic  color 
and  give  the  same  absorption  spectrum  as  the  solution  ;  when  subjected  to 


358  RESPIRATION. 

the  action  of  the  mercurial  pump,  they  will  give  off  a  definite  quantity  of 
carbonic  oxide,  1  gramme  of  the  crystals  yielding  1.59  c.c.  of  the  gas.  In 
fact  haemoglobin  combines  loosely  with  carbonic  oxide  just  as  it  does  with 
oxygen ;  but  its  affinity  with  the  former  is  greater  than  with  the  latter. 
While  carbonic  oxide  readily  turns  out  oxygen,  oxygen  cannot  so  readily 
turn  out  carbonic  oxide.  Indeed,  carbonic  oxide  has  been  used  as  a  means 
of  driving  out  and  measuring  the  quantity  of  oxygen  present  in  any  given 
blood.  This  property  of  carbonic  oxide  explains  its  poisonous  nature. 
When  the  gas  is  breathed,  the  reduced  and  the  unreduced  hsemoglobin  of 
the  venous  blood  unite  with  the  carbonic  oxide,  and  hence  the  peculiar, 
bright  cherry-red  color  observable  in  the  blood  and  tissues  in  cases  of  poison- 
ing by  this  gas.  The  carbonic-oxide  hsemoglobin,  however,  is  of  no  use  in 
respiration  ;  it  is  not  an  oxygen-carrier,  nay  more,  it  will  not  readily,  though 
it  does  so  slowly  and  eventually,  give  up  its  carbonic  oxide  for  oxygen,  when 
the  poisonous  gas  ceases  to  enter  the  chest  and  is  replaced  by  pure  air.  The 
organism  is  killed  by  suffocation,  by  want  of  oxygen,  in  spite  of  the  blood 
not  assuming  any  dark  venous  color ;  to  adopt  a  phrase  which  has  been 
used,  the  corpuscles  are  paralyzed. 

Hsemoglobin  smilarly  forms  a  compound,  having  a  characteristic  spec- 
trum, with  nitric  oxide,  more  stable  even  than  that  with  carbonic  oxide. 

It  has  been  supposed  by  some  that  the  oxygen  thus  associated  with  hsemo- 
globin is  in  the  condition  known  as  ozone ;  but  the  arguments  urged  in  sup- 
port of  this  view  are  inconclusive. 


Products  of  the  Decomposition  of  Hemoglobin. 

§  293.  Although  a  crystalline  body,  hsemoglobin  diffuses  with  great  diffi- 
culty. This  arises  from  the  fact  that  it  is  in  part  a  proteid  body  ;  it  consists 
of  a  colorless  proteid,  associated  with  a  colored  substance,  which  may  be 
separated  out  from  the  hsemoglobin,  though  not  in  the  exact  condition  in 
which  it  naturally  exists  in  the  compound ;  this  substance  when  separated 
out  appears  as  a  brownish-red  body  known  as  hcematin.  All  the  iron  be- 
longing to  the  hsemoglobin  is  in  reality  attached  to  the  hsematin.  A  solution 
of  hsemoglobin,  when  heated,  coagulates,  the  exact  degree  at  which  the 
coagulation  takes  place  depending  on  the  amount  of  dilution  ;  at  the  same 
time  it  turns  brown  from  the  setting  free  of  the  hsematin.  If  a  strong  solu- 
tion of  hsemoglobin  be  treated  with  acetic  (or  other)  acid,  the  same  brown 
color,  from  the  appearance  of  hsematin,  is  observed.  The  proteid  constitu- 
ent, however,  is  not  coagulated,  but  by  the  action  of  the  acid  passes  into 
the  state  of  acid-albumin.  On  adding  ether  to  the  mixture,  and  shaking, 
the  hsematin  is  dissolved  in  the  supernatant  acid  ether,  which  it  co'lors  a 
dark  red,  and  which,  examined  with  the  spectroscope,  is  found  to  possess  a 
well-marked  spectrum,  the  spectrum  of  the  so-called  acid  hsematin  of  Stokes 
(Fig.  98,  6).  The  proteid  in  the  water  below  the  ether  appears  in  a  coagu- 
lated form  owing  to  the  action  of  the  ether.  In  a  somewhat  similar  manner 
alkalies  split  up  hsemoglobin  into  a  proteid  constituent  and  hsematin. 

The  exact  nature  of  the  proteid  constituent  of  hsemoglobin  has  not  as 
yet  been  clearly  determined.  It  was  supposed  to  be  globulin  (hence  the 
name  hsematoglobulin,  contracted  into  hsemoglobin),  but  though  belonging 
to  the  globulin  family,  has  characters  of  its  own  ;  it  is  possibly  a  mixture  of 
two  or  more  distinct  proteids.  It  has  been  provisionally  named  globin  and 
is  said  to  be  free  from  ash. 

§  294.  Hsematin  when  separated  from  its  proteid  fellow,  and  purified, 
appears  as  a  dark-brown  amorphous  powder,  or  as  a  scaly  mass  with  a 


RESPIRATORY  CHANGES  IN  THE  BLOOD. 


359 


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1,8 


II 

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It 


Il 


360  RESPIRATION. 

metallic  lustre,  having  the  probable  composition  of  C32,  H34,  N4,  Fe,  O-a.1  It 
is  fairly  soluble  in  dilute  acid  or  alkaline  solutions,  and  then  gives  charac- 
teristic spectra  (Fig.  98,  1,  2,  5). 

An  interesting  feature  in  haematin  is  that  its  alkaline  solution  is  capable 
of  being  reduced  by  reducing  agents,  the  spectrum  changing  at  the  same 
time  (Fig.  98,  3),  and  that  the  reduced  solution  will,  like  the  haemoglobin, 
take  up  oxygen  again  on  being  brought  into  contact  with  air  or  oxygen. 
This  would  seem  to  indicate  that  the  oxygen-holding  power  of  haemoglobin 
is  connected  exclusively  with  its  haematin  constituent. 

By  the  action  of  strong  sulphuric  acid  hsematin  may  be  robbed  of  all  its 
iron.  It  still  retains  the  feature  of  possessing  color,  the  solution  of  iron-free 
haematin  being  a  dark  rich  brownish-red ;  but  is  no  longer  capable  of  com- 
bining loosely  with  oxygen.  This  indicates  that  the  iron  is  in  some  way 
associated  with  the  peculiar  respiratory  functions  of  haemoglobin  ;  though 
it  is  obviously  an  error  to  suppose,  as  was  once  supposed,  that  the  change 
from  venous  to  arterial  blood  consists  essentially  in  a  change  from  a  ferrous 
to  a  ferric  salt. 

Though  not  crystallizable  itself,  hsematin  forms  with  hydrochloric  acid  a 
compound,  occurring  in  minute  rhombic  crystals,  known  as  hcemin  crystals. 

When  blood  is  left  until  it  decomposes,  the  haemoglobin  is  very  apt  to 
become  changed  into  a  peculiar  body  known  as  methcemoglobin,  in  the  spec- 
trum of  which  a  very  conspicuous  band  is  seen  in  the  red  between  C  and  D 
(see  Fig.  98,  4).  The  same  change  may  be  brought  about  by  the  action 
of  weak  acids,  such  as  carbonic  acid,  by  ozone,  and  by  other  agents  such  as 
nitrites  and  potassium  permanganate.  When  a  stream  of  carbonic  acid  is 
driven  through  blood  or  through  a  solution  of  haemoglobin  the  band  in  the 
red  characteristic  of  rnethaenioglobin  soon  makes  its  appearance.  Methae- 
nioglobin  differs  but  little,  if  at  all,  in  elementary  composition  from  haemo- 
globin ;  it  is  maintained  that  it  contains  the  same  quantity  of  oxygen  as 
oxy-haemoglobin  but  in  a  more  stable  condition,  more  intimately  associated 
with  the  molecule. 

In  conclusion,  the  condition  of  oxygen  in  the  blood  is  as  follows:  Of  the 
whole  quantity  of  oxygen  in  the  blood,  only  a  minute  fraction  is  simply 
absorbed  or  dissolved  according  to  the  law  of  pressure  (the  Henry-Dalton 
law).  The  great  mass  is  in  a  state  of  combination  with  the  haemoglobin, 
the  connection  being  of  such  a  kind  that  while  the  haemoglobin  readily 
combines  with  the  oxygen  of  the  air  to  which  it  is  exposed,  dissociation 
readily  occurs  at  low  pressures,  or  in  the  presence  of  indifferent  gases,  or  by 
the  action  of  substances  having  a  greater  affinity  for  oxygen  than  has  haemo- 
globin itself.  The  difference  between  venous  and  arterial  blood,  as  far  as 
oxygen  is  concerned,  is  that  while  in  arterial  blood  the  haemoglobin  holds 
nearly  its  full  complement  of  oxygen  and  may  be  spoken  of  as  nearly 
wholly  oxy-haemoglobin,  in  venous  blood  the  haemoglobin  is  to  a  large  but 
variable  extent  reduced ;  and  the  characteristic  colors  of  venous  and  arte- 
rial blood  are  in  the  main  due  to  the  fact  that  the  color  of  reduced  haemo- 
globin is  purple,  while  that  of  oxy-haemoglobin  is  scarlet. 

The  Relations  of  the  Carbonic  Acid  in  the  Blood. 

§  295.  The  presence  of  carbonic  acid  in  the  blood  appears  to  be  deter- 
mined by  conditions  more  complex  in  their  nature  and  at  present  not  so  well 
understood  as  those  which  determine  the  presence  of  oxygen.  The  carbonic 
acid  is  not  simply  dissolved  in  the  blood  ;  its  absorption  by  blood  does  not 

1  This  formula  is  the  old  one  of  Hoppe-Seyler. 


THE  RESPIRATORY   CHANGES  IN   THE  LUNGS.  361 

follow  the  law  of  pressures.  It  exists  in  association  with  some  substance  or 
substances  in  the  blood,  and  its  escape  from  the  blood  is  a  process  of  disso- 
ciation. We  cannot,  however,  speak  of  it  as  being  associated,  to  the  same 
extent  as  is  the  oxygen,  with  the  haemoglobin  of  the  red  corpuscles.  So 
fiir  from  the  red  corpuscles  containing  the  great  mass  of  the  carbonic  acid, 
the  quantity  of  this  gas  which  is  present  in  a  volume  of  serum  is  according 
to  some  observers  actually  greater  than  that  which  is  present  in  an  equal 
volume  of  blood,  i.  e.,  an  equal  volume  of  mixed  corpuscles  and  serum  ; 
that  is  to  say,  the  carbonic  acid  is  much  more  largely  associated  with 
the  serum  (or,  in  the  living  blood,  with  the  plasma)  than  with  the  red  cor- 
puscles. 

When  serum  is  subjected  to  the  action  of  the  mercurial  pump,  by  far  the 
greater  part  of  the  carbonic  acid  is  given  off;  but  a  small  additional  quan- 
tity (2  to  5  vols.  per  cent.)  may  be  extracted  by  the  subsequent  addition  of 
an  acid.  This  latter  portion  may  be  spoken  of  as  "  fixed  "  carbonic  acid  in 
distinction  to  the  larger  "  loose  "  portion  which  is  given  off  to  the  vacuum. 
When,  however,  the  whole  blood  is  subjected  to  the  vacuum  until  the  car- 
bonic acid  ceases  to  be  given  off,  the  subsequent  addition  of  acid  is  said  not 
to  set  free  any  further  quantity ;  so  that  when  serum  is  mixed  with  corpus- 
cles all  the  carbonic  acid  may  be  spoken  of  as  "loose"  ;  and  it  is  stated  that 
the  excess  of  carbonic  acid  in  a  quantity  of  serum  over  that  present  in  the 
same  bulk  of  entire  blood,  corresponds  to  the  fixed  portion  in  serum  which 
has  to  be  driven  off  by  an  acid.  Moreover,  even  those  who  maintain  that 
the  quantity  of  carbonic  acid  in  entire  blood  is  less  than  that  in  an  equal 
volume  of  serum,  admit  that  the  carbonic  acid  exists  in  some  way  or  other 
at  a  higher  pressure  in  and  is  more  readily  given  off'  from  entire  blood  than 
from  serum.  It  seems  probable  that  the  carbonic  acid  exists  associated  with 
some  substance  or  substances  in  the  serum,  or  rather  plasma,  but  that  the 
conditions  of  its  association  (and  those  of  its  dissociation)  are  determined  by 
the  action  of  some  substance  or  substances  present  in  the  corpuscles.  It  has 
been  suggested  that  the  association  of  the  carbonic  acid  in  the  plasma  is  with 
one  or  other  of  the  proteids  of  the  plasma ;  but  it  has  also  been  suggested 
that  the  association  is  one  with  sodium  as  sodium  bicarbonate,  and  further 
than  the  haemoglobin  of  the  corpuscles  plays  a  part  in  promoting  the  disso- 
ciation of  the  sodium  bicarbonate  or  even  the  carbonate,  and  thus  keeping 
up  the  carbonic  acid  of  the  entire  blood.  Other  observers,  however,  main- 
tain that  the  plasma  does  not  hold  this  exclusive  possession  of  the  carbonic 
acid,  but  that  a  considerable  quantity  of  this  gas  is  in  some  way  associated 
with  the  red  corpuscles.  Indeed,  further  investigations  are  necessary  before 
the  matter  can  be  said  to  have  been  placed  on  a  satisfactory  footing. 

The  Relations  of  the  Nitrogen  in  the  Blood. 

§  296.  The  small  quantity  of  this  gas  which  is  present  in  both  arterial 
and  venous  blood  seems  to  exist  in  a  state  of  simple  solution. 

THE  RESPIRATORY  CHANGES  IN  THE  LUNGS. 

The  Entrance  of  Oxygen. 

§  297.  We  have  already  seen  that  the  blood  in  passing  through  the 
lungs  takes  up  a  certain  variable  quantity  (from  8  to  12  vols.  per  cent.)  of 
oxygen.  We  have  further  seen  that  the  quantity  so  taken  up,  putting  aside 
the  insignificant -fraction  simply  absorbed,  enters  into  direct  but  loose  com- 
bination with  the  hemoglobin.  In  drawing  a  distinction  between  the  oxy- 


362  RESPIRATION. 

gen  simply  absorbed  and  that  entering  into  combination  with  the  haemo- 
globin, it  must  not  be  understood  that  the  latter  is  wholly  independent  of 
pressure.  On  the  contrary,  all  chemical  compounds  are  in  various  degrees 
subject  to  dissociation  at  certain  pressures  and  temperatures ;  and  the  existence 
of  the  somewhat  loose  compound  of  oxygen  and  haemoglobin  is  dependent  on 
the  partial  pressure  of  oxygen  in  the  atmosphere  to  which  the  haemoglobin 
is  exposed.  Not  only  will  a  solution  of  haemoglobin  or  a  quantity  of  blood 
either  absorb  oxygen,  and  thus  undergo  association  or  undergo  dissociation 
and  give  off  oxygen  according  as  the  partial  pressure  of  oxygen  in  the 
atmosphere  to  which  it  is  exposed  is  high  or  low,  but  also  the  amount  taken 
up  or  given  off  will  depend  on  the  degree  of  the  partial  pressure  ;  the  haemo- 
globin as  we  have  seen  may  be  partially  as  well  as  wholly  reduced.  The 
law,  however,  according  to  which  absorption  or  escape  thus  takes  place  is 
quite  different  from  that  observed  in  the  simple  absorption  of  oxygen  by 
liquids.  The  association  or  dissociation  is  further  especially  dependent  on 
temperature,  a  high  temperature  favoring  dissociation,  so  that  at  a  high 
temperature  less  oxygen  is  taken  up  than  would  be  taken  up  (or,  as  the 
case  may  be,  more  given  off  than  would  be  given  off)  at  a  lower  tempera- 
ture, the  partial  pressure  of  the  oxygen  in  the  atmosphere  remaining  the 
same. 

Moreover,  in  the  blood  we  have  to  deal  not  with  haemoglobin  in  simple 
solution,  in  which  the  molecules  are  dispersed  uniformly  through  the  solvent, 
but  with  the  haemoglobin  segregated  into  minute  isolated  masses,  bottled  up 
as  it  were  in  the  individual  corpuscles.  The  haemoglobin  of  each  corpuscle 
is  separated  from  its  fellows  by  a  layer,  thin  it  may  be  but  still  a  distinct 
layer,  of  colorless,  haemoglobinless  plasma.  As  the  corpuscle  makes  its  way 
through  the  narrow  capillary  paths  of  a  pulmonary  alveolus,  it  is  separated 
from  the  air  of  the  alveolus  by  a  thin  layer  of  plasma  as  well  as  by  the  film 
of  the  conjoined  capillary  and  alveolar  walls;  and  a  like  layer  of  plasma 
separates  it  from  its  fellows  as  it  journeys  in  company  with  them  through 
the  wider  passages  of  the  arteries  and  veins.  Through  this  layer  of  plasma, 
which  containing  no  haemoglobin  can  hold  oxygen  in  simple  solution  only, 
the  oxygen  has  to  pass  on  its  way  to  and  from  the  corpuscle ;  and  every  cor- 
puscle may  be  considered  as  governing,  as  far  as  oxygen  is  concerned,  a  zone 
of  plasma  immediately  surrounding  itself.  The  corpuscle  takes  its  oxygen 
directly  from  this  zone  and  gives  up  its  oxygen  directly  to  this  zone;  and 
the  pressure  at  which  at  any  moment  the  oxygen  exists  in  this  zone  will 
depend  on  the  pressure  of  oxygen  outside  the  zone,  in  the  air  of  the  pul- 
monary alveolus,  for  instance,  and  on  the  smaller  or  greater  amount  of 
oxygen  associated  with  the  haemoglobin  of  the  corpuscle. 

The  evidence,  so  far  as  it  goes,  seems  to  show  that  blood  absorbs  oxygen 
in  the  same  way  as  an  aqueous  solution  of  haemoglobin  of  the  same  concen- 
tration ;  the  zone  of  plasma  spoken  of  above  as  surrounding  each  corpuscle 
behaves  as  far  as  regards  the  passage  of  oxygen  to  and  from  the  corpuscles 
in  no  essentially  different  respect  from  the  way  the  molecules  of  water 
belonging  to  a  molecule  of  dissolved  haemoglobin  behave  in  regard  to 
the  absorption  or  the  giving  off  of  oxygen  by  an  aqueous  solution  of 
haemoglobin. 

The  film  of  the  conjoined  capillary  and  alveolar  wall  is  a  thin  membrane 
soaked  with  lymph  and  wet ;  we  cannot  speak  of  it  as  actually  secreting  a 
liquid  secretion  into  the  alveolus,  for  the  cavity  of  the  alveolus  is  filled  with 
air  which,  though  saturated  with  moisture,  is  air,  not  a  liquid;  still  enough 
passes  through  the  film  to  keep  it  continually  moist.  Through  this  film  the 
oxygen  has  to  make  its  way  in  order  to  gain  access  to  the  plasma  and  so  to 
the  corpuscle ;  it  makes  its  way  dissolved  in  the  fiuid,  that  is  the  lymph, 


THE  RESPIRATORY   CHANGES  IN  THE   LUNGS.  363 

which  keeps  the  film  moist.  This  film,  moreover,  is  composed  of  living 
matter,  and  the  considerations  which  a  little  while  back  (§  265)  we  urged 
concerning  the  diffusion  through  a  living  membrane  of  solid  substances  in 
solution,  hold  good  also  for  the  diffusion  of  gases  in  solution. 

We  have  now  to  consider  the  question,  Are  the  conditions  in  which 
haemoglobin  and  oxygen  exist  in  ordinary  venous  blood  as  it  flows  to  the 
lungs,  of  such  a  kind  that  the  venous  blood  in  passing  through  the  pul- 
monary capillaries  will  find  the  partial  pressure  of  the  oxygen  in  the  pul- 
monary alveoli  sufficient  to  bring  about  the  association  of  the  additional 
quantity  of  oxygen  whereby  the  venous  is  converted  into  arterial  blood  ? 

§  298.  In  man,  as  we  have  seen,  expired  air  contains  about  16  per  cent, 
of  oxygen.  The  air  in  the  pulmonary  alveoli  must  contain  less  than  this, 
since  the  expired  air  consists  of  a  tidal  air  mixed  by  diffusion  with  the 
stationary  air.  How  much  less  it  contains  we  do  not  exactly  know,  but 
probably  the  difference  is  not  very  great.  At  the  ordinary  atmospheric 
pressure  of  760  mm.  16  per  cent,  is  equivalent  to  a  partial  pressure  of  122 
mm.  The  question,  therefore,  stands  thus,  Will  venous  blood,  exposed  at 
the  temperature  of  the  body  to  a  partial  pressure  of  less  than  122  mm.  (less 
than  16  per  cent.)  of  oxygen  take  up  sufficient  oxygen  (from  8  to  12  vols. 
per  cent.)  to  convert  it  into  arterial  blood  ?  Numerous  experiments  have 
been  made  (chiefly  but  not  exclusively  on  the  dog)  to  determine  on  the  one 
hand  the  oxygen-pressure  of  both  arterial  and  venous  blood  (i.  e.,  the  partial 
pressure  of  oxygen  in  an  atmosphere  exposed  to  which  the  arterial  blood 
neither  gives  up  nor  takes  in  oxygen,  and  the  same  for  venous  blood),  and 
on  the  other  hand  the  behavior  at  the  temperature  of  the  body  or  at  ordi- 
nary temperatures  of  blood  or  of  solutions  of  haemoglobin  (for  the  two,  as  we 
have  just  said,  behave  in  this  respect  very  much  alike)  toward  an  atmos- 
phere in  which  the  partial  pressure  of  oxygen  is  made  to  vary.  Without 
going  into  detail,  we  may  state  that  these  experiments  show  that  the  partial 
pressure  of  oxygen  in  the  lungs  is  amply  sufficient  to  bring  about,  at  the 
temperature  of  the  body,  the  association  of  that  additional  amount  of  oxy- 
gen by  which  venous  blood  becomes  arterial.  When  a  solution  of  haemo- 
globin or  when  blood  is  successively  exposed  to  increasing  oxygen  pressures, 
as  the  partial  pressure  of  oxygen  is  gradually  increased,  the  curve  of  absorp- 
tion rises  at  first  very  rapidly  but  afterward  more  slowly ;  that  is  to  say, 
the  later  additions  of  oxygen  at  the  higher  pressures  are  proportionately 
less  than  the  earlier  ones  at  the  lower  pressures.  And  this  is  consonant  with 
what  appears  to  be  the  fact,  that  the  haemoglobin  of  arterial  blood,  though 
nearly  saturated  with  oxygen,  i.  e.,  associated  with  almost  its  full  comple- 
ment of  oxygen,  is  not  quite  saturated.  When  arterial  blood  is  thoroughly 
exposed  to  air  it  takes  up  rather  more  than  1  vol.  per  cent,  of  oxygen ;  and 
that  appears  to  represent  the  difference  between  exposing  blood  to  pure  air, 
such  as  enters  or  ought  to  enter  the  mouth  in  inspiration,  and  exposing 
blood  to  the  air  as  it  exists  in  the  pulmonary  alveoli.  The  greater  relative 
absorption  at  the  lower  pressures  has  a  beneficial  effect,  inasmuch  as  it  still 
permits  a  considerable  quantity  of  oxygen  to  be  absorbed  even  when  the 
partial  pressure  of  oxygen  in  the  air  in  the  lungs  is  largely  reduced,  as  in 
ascending  to  great  heights. 

Observations  made  both  with  dog's  blood  and  ox's  blood  seem  to  show 
that  arterial  blood  ceases  to  take  up  oxygen  and  begins  to  give  off  oxygen  ; 
in  other  words,  that  dissociation  begins  to  take  place  when  the  partial 
pressure  of  the  oxygen  in  the  atmosphere  to  which  it  is  exposed  sinks  to 
about  60  mm.  of  mercury ;  that  is  to  say,  when  the  whole  atmospheric 
pressure  is  reduced  from  760  mm.  to  about  300  mm.,  or  when  the  percentage 
of  oxygen  in  the  atmosphere  is  reduced  by  decidedly  more  than  half.  And 


364  RESPIRATION. 

this  accords  with  the  observation  that  in  man,  when  the  oxygen  of  inspired 
air  is  gradually  diminished  without  any  other  change  in  the  air,  symptoms 
of  dyspnoaa  do  not  make  their  appearance  until  the  oxygen  sinks  to  10  per 
cent,  in  the  inspired  air,  and  must  therefore  be  less  than  this  in  the  pul- 
monary alveoli.  We  may  remark  that  at  ordinary  altitudes,  even  taking 
into  account  the  diminution  the  oxygen  undergoes  before  it  reaches  the  pul- 
monary alveoli,  the  partial  pressure  of  the  oxygen  in  the  atmosphere  leaves 
a  wide  margin  of  safety.  But  at  an  altitude  of  5500  metres  (17,000  feet), 
at  which  the  pressure  of  the  whole  atmosphere  stands  at  about  the  limit 
given  above  of  300  mm.,  the  partial  pressure  of  the  oxygen  will  be  such 
that  the  venous  blood  cannot  take  up  the  quantity  of  oxygen  proper  to  con- 
vert it  into  arterial  blood,  since  at  this  limit  arterial  blood  begins  to  give  off 
oxygen.  We  may  add  that  it  is  at  this  altitude  that  breathing  becomes 
especially  difficult ;  but  to  this  we  shall  return. 

§  299.  The  statements  made  so  far  refer  to  ordinary  breathing,  but  the 
question  may  be  asked,  What  happens  when  the  renewal  of  the  air  in  the 
pulmonary  alveoli  ceases,  as  when  the  trachea  is  obstructed?  In  such  a 
case  the  oxygen  in  the  alveoli  is  found  to  diminish  rapidly,  so  that  the 
partial  pressure  of  oxygen  in  them  soon  falls  below  the  oxygen-pressure  of 
ordinary  venous  blood.  But  in  such  a  case  the  blood  is  no  longer  ordinary 
venous  blood ;  instead  of  being  moderately  it  is  largely  and  increasingly 
reduced ;  instead  of  containing  a  comparatively  small  amount,  it  contains 
a  large  and  gradually  increasing  amount  of  reduced  haemoglobin.  And  as 
the  reduction  continues  to  increase,  the  oxygen-pressure  of  the  venous  blood 
also  continues  to  decrease ;  it  thus  keeps  below  that  of  the  air  in  the  lungs. 
Hence,  apparently,  even  the  last  traces  of  oxygen  in  the  lungs  may  be  taken 
up  by  the  blood  and  carried  away  to  the  tissues.  Whether  or  not  the  pul- 
monary tissue  and  the  capillary  walls,  because  of  their  being  living  structures, 
have  any  influence  upon  the  transmission  of  oxygen  must  still  be  considered 
an  unsettled  question. 

The  Exit  of  Carbonic  Add. 

§  300.  It  seems  natural  to  suppose  that  the  carbonic  acid  would  escape 
by  diffusion  from  the  blood  of  the  alveolar  capillaries  into  the  air  of  the 
alveoli.  But  in  order  that  diffusion  should  thus  take  place,  the  carbonic 
acid  pressure  of  the  air  in  the  pulmonary  alveoli  must  always  be  less  than 
that  of  the  venous  blood  of  the  pulmonary  artery,  and  ought  not  to  exceed 
that  of  the  blood  of  the  pulmonary  vein.  There  are,  however,  many  prac- 
tical difficulties  in  the  way  of  an  exact  determination  of  the  carbonic  acid 
pressure  of  the  pulmonary  alveoli  (for,  though  it  must  be  greater  than  that 
of  the  expired  air,  it  is  difficult  to  say  how  much  greater),  and  of  the  car- 
bonic acid  pressure  of  the  blood  at  the  same  time,  so  as  to  be  in  a  position 
to  compare  the  one  with  the  other.  In  the  case  of  oxygen  there  is  always 
present  in  the  lungs  a  surplus  of  the  gas,  a  portion  only  being  absorbed  at 
each  breath ;  in  the  case  of  carbonic  acid  the  whole  quantity  comes  direct 
from  the  blood,  and  any  modifications  in  breathing  seriously  affect  the 
amount  given  out.  Thus,  when  the  breath  is  held  for  some  time  the  per- 
centage of  carbonic  acid  in  the  expired  air  reaches  7  or  8  per  cent.,  but  we 
cannot  take  this  as  a  measure  of  the  normal  percentage  of  carbonic  acid  in 
the  pulmonary  alveoli,  since  by  the  mere  holding  of  the  breath  the  carbonic 
acid  in  the  blood,  and  hence  in  the  pulmonary  alveoli,  is  increased  beyond 
the  normal. 

The  difficulties  of  the  problem  seem,  however,  to  have  been  overcome  by 
an  ingenious  experiment  in  which  there  is  introduced  into  the  bronchus  of 


THE  RESPIRATORY  CHANGES  IN   THE  TISSUES.  365 

the  lung  of  a  dog  a  catheter,  round  which  is  arranged  a  small  bag ;  by  the 
inflation  of  this  bag,  the  bronchus,  whenever  desired,  can  be  completely 
blocked  up.  Thus,  without  any  marked  disturbance  of  the  general  breath- 
ing, and  therefore  without  any  marked  change  in  the  normal  proportions  of 
the  gases  of  the  blood,  the  experimenter  is  able  to  stop  the  ingress  of  fresh  air 
into  a  limited  portion  of  the  lung.  At  the  same  time  he  is  enabled  by  means 
of  the  catheter  to  withdraw  a  sample  of  the  air  of  the  same  limited  portion 
and  by  analysis  to  determine  the  amount  of  carbonic  acid  which  it  contains, 
or  in  other  words,  the  partial  pressure  of  the  carbonic  acid.  The  blood 
passing  through  the  alveolar  capillaries  of  this  limited  portion  of  the  lung 
naturally  possesses  the  same  carbonic  acid  pressure  as  the  rest  of  the  venous 
blood  flowing  through  the  pulmonary  artery — a  pressure  which,  though 
varying  slightly  from  moment  to  moment,  will  maintain  a  normal  average. 
On  the  supposition  that  carbonic  acid  passes  simply  by  diffusion  from  the 
pulmonary  blood  into  the  air  of  the  alveoli,  because  the  carbonic  acid 
pressure  of  the  latter  is  normally  lower  than  that  of  the  former,  one  would 
expect  to  find  that  the  air  in  the  occluded  portion  of  the  lung  would  con- 
tinue to  take  up  carbonic  acid  until  an  equilibrium  was  established  between 
it  and  the  carbonic  acid  pressure  of  the  venous  blood.  Consequently,  if 
after  an  occlusion,  say  of  some  minutes  (by  which  time  the  equilibrium 
might  fairly  be  assumed  to  have  been  established),  the  carbonic  acid  pressure 
of  the  air  of  the  occluded  portion  were  determined,  it  ought  to  be  found  to 
be  equal  to,  and  not  more  than  equal  to,  the  carbonic  acid  pressure  of  the 
venous  blood  of  the  pulmonary  artery.  And  this  is  the  result  which  has 
been  arrived  at :  it  has  been  found  that  the  pressures  of  the  carbonic  acid 
of  the  occluded  air  and  of  the  venous  blood  of  the  right  side  of  the  heart 
are  just  about  equal.  Hence  the  evidence,  so  far  as  it  goes,  is  distinctly  in 
favor  of  the  view  that  the  escape  of  carbonic  acid  from  the  blood  into  the 
pulmonary  alveoli  is  simply  due  to  diffusion,  and  that  there  is  no  need  to 
seek  for  any  further  explanation. 

As  far  then  as  can  be  seen  at  present,  both  the  entrance  of  oxygen  and 
the  exit  of  carbonic  acid  by  which  venous  blood  is  converted  into  arterial 
are  the  simple  physical  results  of  the  exposure  of  the  blood  in  the  pulmon- 
ary capillary  to  the  air  of  the  pulmonary  alveoli. 

THE  RESPIRATORY  CHANGES  IN  THE  TISSUES. 

§  301.  In  passing  through  the  several  tissues  the  arterial  blood  becomes 
once  more  venous.  The  oxy-hsemoglobin  becomes  considerably  reduced,  and 
a  quantity  of  carbonic  acid  passes  from  the  tissues  into  the  blood.  The 
amount  of  change  varies  in  the  various  tissues,  and  in  the  same  tissue  may 
vary  at  different  times.  Thus,  in  a  gland  at  rest,  as  we  have  seen,  the  venous 
blood  is  dark,  showing  that  the  haemoglobin  is  to  a  large  extent  in  the  reduced 
condition  ;  when  the  gland  is  active,  the  venous  blood  in  its  color,  and  in 
the  extent  to  which  the  haemoglobin  is  in  the  condition  of  oxy-hsemoglobin, 
resembles  closely  arterial  blood.  The  blood  therefore  which  issues  from  a 
gland  at  rest  is  more  "  venous  "  than  that  from  an  active  gland ;  though 
owing  to  the  more  rapid  flow  of  blood  which,  as  we  saw  in  an  earlier  sec- 
tion, accompanies  the  activity  of  the  gland,  the  total  quantity  of  oxygen 
taken  up  from  and  of  carbonic  acid  discharged  into  the  blood  from  the 
gland  in  a  given  time  may  be  greater  than  the  latter.  The  blood,  on  the 
other  hand,  which  comes  from  an  active,  i.  e.,  a  contracting  muscle,  is,  in 
spite  of  the  more  rapid  flow,  not  only  richer  in  carbonic  acid,  but  also, 
though  not  to  a  corresponding  amount,  poorer  in  oxygen  than  the  blood 
which  flows  from  a  muscle  at  rest. 


366  RESPIRATION. 

In  all  these  cases  the  great  question  which  comes  up  for  our  consideration 
is  this :  Does  the  oxygen  pass  from  the  blood  into  the  tissues,  and  does  the 
oxidation  take  place  in  the  tissues,  giving  rise  to  carbonic  acid,  which  passes 
in  turn  away  from  the  tissues  into  the  blood  ?  or  do  certain  oxidizable  redu- 
cing substances  pass  from  the  tissues  into  the  blood,  and  there  become  oxidized 
into  carbonic  acid  and  other  products,  so  that  the  chief  oxidation  takes 
place  in  the  blood  itself? 

There  are,  it  is  true,  reducing  oxidizable  substances  in  the  blood,  but 
these  are  small  in  amount,  and  the  quantity  of  carbonic  acid  to  which  they 
give  rise  when  the  blood  containing  them  is  agitated  with  air  or  oxygen,  is 
so  small  as  scarcely  to  exceed  the  errors  of  observation. 

We  may  add  that  the  oxidative  power  which  the  blood  itself  removed 
from  the  body  is  able  to  exert  on  substances  which  are  undoubtedly  oxidized 
in  the  body  is  so  small  that  it  may  be  neglected  in  the  present  considera- 
tions. If  grape-sugar  be  added  to  blood  or  to  a  solution  of  haemoglobin,  the 
mixture  may  be  kept  for  a  long  time  at  the  temperature  of  the  body  with- 
out undergoing  oxidation.  Even  within  the  body  a  slight  excess  of  sugar 
in  the  blood  over  a  certain  percentage  wholly  escapes  oxidation,  and  is  dis- 
charged unchanged.  Many  easily  oxidized  substances,  such  as  pyrogallic 
acid,  pass  largely  through  the  blood  of  a  living  body  and  are  discharged  in 
the  urine  without  being  oxidized ;  though  perhaps  in  some  of  these  cases, 
what  appears  to  be  an  absence  of  oxidation  is  really  an  oxidation  followed 
by  a  subsequent  equivalent  reduction  taking  place  in  the  urine  or  elsewhere. 
The  organic  acids,  such  as  citric,  even  in  combination  with  alkaline  bases, 
are  only  partially  oxidized ;  when  administered  as  acids,  and  not  as  salts, 
they  are  hardly  oxidized  at  all.  It  is,  of  course,  quite  possible  that  the 
changes  which  the  blood  undergoes  when  shed  might  interfere  with  its 
oxidative  action,  and  hence  the  fact  that  shed  blood  has  little  or  no  oxidiz- 
ing power,  is  not  a  satisfactory  proof  that  the  unchanged  blood  within  the 
living  vessels  may  not  have  such  a  power.  But  did  oxidation  take  place 
largely  in  the  blood  itself,  one  would  expect  even  highly  diffusible  substances 
to  be  oxidized  in  their  transit ;  whereas  if  we  suppose  the  oxidation  to  take 
place  in  the  tissues,  it  becomes  intelligible  why  such  diffusible  substances  as 
those  which  the  tissues  in  general  refuse  to  take  up  largely  should  readily 
pass  unchanged  from  the  blood  through  the  excreting  organs. 

On  the  other  hand,  it  will  be  remembered  in  speaking  of  muscle,  we 
drew  attention  (§  61)  to  the  fact  that  a  frog's  muscle  removed  from  the  body 
(and  the  same  is  true  of  the  muscles  of  other  animals)  contains  no  free 
oxygen  whatever ;  none  can  be  obtained  from  it  by  the  mercurial  air-pump. 
Yet  such  a  muscle  will  not  only  when  at  rest  go  on  producing  and  discharging 
a  certain  quantity,  but  also  when  it  contracts  evolve  a  very  considerable 
quantity,  of  carbonic  acid.  Moreover,  this  discharge  of  carbonic  acid  will 
go  on  for  a  certain  time  in  muscles  under  circumstances  in  which  it  is  im- 
possible for  them  to  obtain  oxygen  from  without.  Oxygen,  it  is  true,  is 
necessary  for  the  life  of  the  muscle ;  when  venous  instead  of  arterial  blood  is 
sent  through  the  bloodvessels  of  a  muscle,  the  irritability  speedily  disappears, 
and  unless  fresh  oxygen  be  administered  the  muscle  soon  dies.  The  muscle 
may,  however,  during  the  interval  in  which  irritability  is  still  retained  after 
the  supply  of  oxygen  has  been  cut  off,  continue  to  contract  vigorously.  The 
supply  of  oxygen,  though  necessary  for  the  maintenance  of  irritability,  is  not 
necessary  for  the  manifestation  of  that  irritability,  is  not  necessary  for  that  ex- 
plosive decomposition  which  develops  a  contraction.  A  frog's  muscle  will 
continue  to  contract  and  to  produce  carbonic  acid  in  an  atmosphere  of 
hydrogen  or  nitrogen,  that  is,  in  the  total  absence  of  free  hydrogen,  both 
from  itself  and  from  the  medium  in  which  it  is  placed. 


THE  RESPIRATORY  CHANGES  IN  THE  TISSUES.  367 

Thus,  on  the  one  hand,  the  muscle  seems  to  have  the  property  of  taking 
up  and  fixing  in  some  way  or  other  the  oxygen  to  which  it  is  exposed,  of 
storing  it  up  in  its  own  substance  in  such  a  condition  that  it  cannot  be  re- 
moved by  simple  diminished  pressure  (so  that  the  pressure  of  oxygen  in  the 
muscular  substance  may  be  considered  as  always  nil}  and  yet  has  not  entered 
into  any  distinct  combination  which  we  can  speak  of  as  an  oxidation,  but  is 
still  available  for  such  a  purpose.  On  the  other  hand,  the  muscular  substance 
is  always  undergoing  a  decomposition  of  such  a  kind  that  carbonic  acid  is 
set  free,  sometimes,  as  when  the  muscle  is  at  rest,  in  small,  sometimes,  as 
during  a  contraction,  in  large  quantities.  The  oxygen  present  in  this  car- 
bonic acid,  as  an  oxidation  product,  comes  from  the  previously  existing  store 
of  which  we  have  just  spoken.  The  oxygen  taken  in  by  the  muscle,  what- 
ever be  its  exact  condition  immediately  upon  its  entrance  into  the  muscular 
substance,  in  the  phase  which  has  been  called  "  intra-molecular,"  sooner  or 
later  enters  into  a  combination,  or  perhaps  we  should  rather  say,  enters  into 
a  series  of  combinations.  We  have  previously  urged  (§  30)  that  all  living 
substance  may  be  regarded  as  incessantly  undergoing  changes  of  a  double 
kind,  changes  of  building  up  and  changes  of  breaking  down.  In  the  end- 
products  of  the  breaking  down,  in  the  carbonic  acid  given  out  by  muscle, 
for  instance,  we  can  recognize  an  oxidation  product ;  but  we  do  not  know 
exactly  at  what  stage  or  exactly  in  what  way  the  oxygen  is  combined  with 
the  carbon.  We  may  imagine  that  the  oxygen  as  it  comes  from  the  blood 
is  caught  up,  so  to  speak,  by  and  disappears  in  the  building-up  process,  and 
that  through  those  processes  it  is  made  part  of  complex  decomposable  sub- 
stances whose  decomposition  ultimately  gives  rise  to  the  carbonic  acid  ;  but, 
as  far  as  actual  knowledge  goes,  we  cannot  as  yet  trace  out  the  steps  taken 
by  the  oxygen  from  the  moment  it  slips  from  the  blood  into  the  muscular 
substance  to  the  moment  when  it  issues  united  with  carbon  as  carbonic  acid. 

But  if  the  oxygen-pressure  of  the  muscular  tissue  be  thus  always  nil, 
oxygen  will  be  always  passing  over  from  the  blood  corpuscles,  in  which  it  is 
at  a  comparatively  high  pressure,  through  the  plasma,  through  the  capillary 
walls,  the  lymph-spaces,  and  the  sarcolemma,  into  the  muscular  substance, 
and  as  soon  as  it  arrives  there  will  be  in  some  manner  or  other  hidden  away, 
leaving  the  oxygen-pressure  of  the  muscular  substance  once  more  nil.  Con- 
versely, the  carbonic  acid  produced  by  the  decomposition  of  the  muscular 
substance  will  tend  to  raise  the  carbonic  acid  pressure  of  the  muscle  until  it 
exceeds  that  of  the  blood  ;  whereupon  carbonic  acid  will  pass  from  the 
muscle  into  the  blood,  its  place  in  the  muscular  substance  being  supplied  by 
gas  freshly  generated.  There  will  always  in  fact  be  a  stream  of  oxygen 
from  the  blood  to  the  muscle  and  of  carbonic  acid  from  the  muscle  to  the 
blood.  The  respiration  of  the  muscle,  then,  does  not  consist  in  throwing  into 
the  blood  oxidizable  substances,  there  to  be  oxidized  into  carbonic  acid  and 
other  matters ;  but  it  does  consist  in  the  assumption  and  storing  up  of  oxy- 
gen somehow  or  other  in  its  substance,  in  the  building  up  by  help  of  that 
oxygen  of  explosive  decomposable  substances,  and  in  the  carrying  out  of 
decompositions  whereby  carbonic  acid  and  other  matters  are  discharged  first 
into  the  substance  of  the  muscle  and  subsequently  into  the  blood. 

§  302.  Our  knowledge  of  the  respiratory  changes  in  muscle  is  more  com- 
plete than  in  the  case  of  any  other  tissue  ;  but  we  have  no  reason  to  suppose 
that  the  phenomena  of  muscle  are  exceptional.  On  the  contrary,  all  the 
available  evidence  goes  to  show  that  in  all  tissues  the  oxidation  takes  place 
in  the  tissue,  and  not  in  the  adjoining  blood.  It  is  a  remarkable  fact  that 
lymph,  serous  fluids,  bile,  urine,  and  milk  contain  a  mere  trace  of  free  or 
loosely  combined  oxygen,  but  a  very  considerable  quantity  of  carbonic  acid. 
And  we  may  probably  assert  with  safety  with  regard  to  all  the  tissues,  that 


368  RESPIRATION. 

in  the  tissues  themselves,  in  the  lymph  which  bathes  their  lymph-spaces,  and 
in  the  secretions  which  some  of  them  pour  forth,  free  oxygen  is  either  wholly 
absent  or  so  scanty  that  their  oxygen-pressure  maybe  regarded  as  nil,  while 
carbonic  acid  is  so  abundant  that  the  pressure  of  carbonic  acid  in  them  may 
be  regarded  as  exceeding  that  of  venous  blood.  An  exception  seems  to  be 
presented  by  the  case  of  the  lymph  flowing  along  the  larger  lymphatic 
vessels,  for  in  this  the  amount  of  carbonic  acid,  while  usually  higher  than 
that  of  arterial  blood,  is  lower  than  that  of  the  general  venous  blood ;  but 
this  probably  is  due  to  the  fact  that  the  lymph  in  its  passage  onward  is 
largely  exposed  to  arterial  blood  in  the  connective  tissues  and  in  the  lym- 
phatic glands,  where  the  production  of  carbonic  acid  is  slight  as  compared 
to  that  going  on  in  muscles.  All  the  facts  point  to  the  conclusion  that  it  is 
the  tissues,  and  not  the  blood,  which  become  primarily  loaded  with  carbonic 
acid,  the  latter  simply  receiving  the  gas  from  the  former  by  diffusion,  except 
the  (probably)  small  quantity  which  results  from  the  metabolism  of  the 
blood  corpuscles ;  and  that  the  oxygen  which  passes  from  the  blood  into  the 
tissues  is  at  once  taken  up  and  placed  under  such  conditions  that  it  is  no 
longer  removable  by  diminished  pressure. 

We  have  seen  that  in  muscle  the  production  of  carbonic  acid  is  not 
directly  dependent  on  the  consumption  of  oxygen.  The  muscle  produces 
carbonic  acid  in  an  atmosphere  of  hydrogen.  What  is  true  of  muscle  is 
true  also  of  other  tissues  and  of  the  body  at  large.  It  was  shown  long  ago 
that  animals  might  continue  to  breathe  out  carbonic  acid  in  an  atmosphere 
of  nitrogen  or  hydrogen  ;  and  this  has  more  recently  been  illustrated  by  the 
remarkable  experiment  that  a  frog  kept  at  a  low  temperature  will  live  for 
several  hours,  and  continue  to  produce  carbonic  acid,  in  an  atmosphere  abso- 
lutely free  from  oxygen.  The  carbonic  acid  produced  during  this  period 
was  made  by  help  of  the  oxygen  inspired  in  the  hours  anterior  to  the  com- 
mencement of  the  experiment.  The  oxygen  then  absorbed  was  stowed  away 
from  the  haemoglobin  into  the  tissues,  it  was  made  use  of  to  build  up  the 
explosive  compounds,  whose  explosions  later  on  gave  rise  to  the  carbonic 
acid.  Or,  to  adopt  a  simile  which  has  been  suggested,  the  oxygen  helps  to 
wind  up  the  vital  clock  ;  but  once  wound  up  the  clock  will  go  on  for  a  period 
without  further  winding.  The  frog  will  continue  to  live,  to  move,  to  pro- 
duce carbonic  acid  for  a  while  without  any  fresh  oxygen,  as  we  know  of  old 
it  will  without  any  fresh  food ;  it  will  continue  to  do  so  till  the  explosive 
compounds  which  the  oxygen  built  up  are  exhausted  ;  it  will  go  on  until  the 
vital  clock  has  run  down. 

§  303.  To  sum  up,  then,  the  results  of  respiration  in  its  chemical  aspects. 
As  the  blood  passes  through  the  lungs,  the  low  oxygen  pressure  of  the  venous 
blood  permits  the  entrance  of  oxygen  from  the  air  of  the  pulmonary  alve- 
olus, through  the  thin  alveolar  walls,  through  the  thin  capillary  sheath, 
through  the  thin  layer  of  blood-plasma  to  the  red  corpuscle,  and  the  reduced 
haemoglobin  of  the  venous  blood  becomes  wholly,  or  all  but  wholly,  oxy- 
hsemoglobin.  Hurried  to  the  tissues,  the  oxygen,  at  comparatively  high  pres- 
sure in  the  arterial  blood,  passes  largely  into  them.  In  the  tissues  the  oxy- 
gen-pressure is  always  kept  at  an  exceedingly  low  pitch  by  the  fact  that  they, 
in  some  way  at  present  unknown  to  us,  pack  away  at  every  moment  into 
some  stable  combination  each  molecule  of  oxygen  which  they  receive  from 
the  blood.  With  its  oxy-hsemoglobin  largely  but  not  wholly  reduced,  the 
blood  passes  on  as  venous  blood.  To  what  extent  the  haemoglobin  is  reduced 
will  depend  on  the  activity  of  the  tissue  itself.  The  quantity  of  haemoglobin 
in  the  blood  is  the  measure  of  limit  of  the  oxidizing  power  of  the  body  at 
large ;  but  within  that  limit  the  amount  of  oxidation  is  determined  by  the 
tissue,  and  by  the  tissue  alone. 


THE  NERVOUS  MECHANISM  OF  RESPIRATION.  369 

We  cannot  trace  the  oxygen  through  its  sojourn  in  the  tissue.  We  only 
know  that  sooner  or  later  it  comes  back  combined  in  carbonic  acid  (and 
other  matters  not  now  under  consideration).  Owing  to  the  continual  pro- 
duction of  carbonic  acid,  the  pressure  of  that  gas  in  the  extra-vascular 
elements  of  the  tissue  is  always  higher  than  that  in  the  blood  ;  the  gas 
accordingly  passes  from  the  tissue  into  the  blood,  and  the  venous  blood 
passes  on  not  only  with  its  haemoglobin  more  or  less  reduced,  i.  e.,  with  its 
oxygen-pressure  decreased,  but  also  with  its  carbonic  acid  pressure  increased. 
Arrived  at  the  lungs,  the  blood  finds  the  pulmonary  air  at  a  lower  carbonic 
acid  pressure  than  itself.  The  gas  accordingly  streams  through  the  thin 
vascular  and  alveolar  walls  until  the  pressure  without  the  bloodvessel  is 
equal  to  the  pressure  within.  At  the  same  time  the  blood  finds  in  the  air  of 
the  pulmonary  alveoli  a  supply  of  oxygen,  more  than  adequate  to  convert, 
not  entirely  but  nearly  so,  the  reduced  haemoglobin  back  again  to  oxy-hsemo- 
globin.  Thus  the  air  of  the  pulmonary  alveoli,  having  given  up  oxygen  to 
the  blood  and  taken  up  carbonic  acid  from  the  blood,  having  in  consequence 
a  higher  carbonic  acid  pressure  and  a  lower  oxygen  pressure  than  the  tidal 
air  in  the  bronchial  passages,  mixes  rapidly  with  this  by  diffusion.  The 
mixture  is  further  assisted  by  ascending  and  descending  currents  ;  and  the 
tidal  air  issues  from  the  chest  at  the  breathing  out  poorer  in  oxygen  and 
richer  in  carbonic  acid  than  the  tidal  air  which  entered  at  the  breathing  in. 

THE  NERVOUS  MECHANISM  OF  RESPIRATION. 

§  304.  Breathing  is  an  involuntary  act.  Though  the  diaphragm  and  all 
the  other  muscles  employed  in  respiration  are  voluntary  muscles,  i.  e.,  mus- 
cles which  can  be  called  into  action  by  a  direct  effort  of  the  will,  and  though 
respiration  may  be  modified  within  very  wide  limits  by  the  will,  yet  we 
habitually  breathe  without  the  intervention  of  the  will ;  the  normal  breath- 
ing may  continue,  not  only  in  the  absence  of  consciousness,  but  even  after 
the  removal  of  all  the  parts  of  the  brain  above  the  medulla  oblongata. 

We  have  already  seen  how  complicated  is  even  a  simple  respiratory  act. 
A  very  large  number  of  muscles  are  called  into  play.  Many  of  these  are 
very  far  apart  from  each  other,  such  as  the  diaphragm  and  the  nasal  muscles ; 
yet  they  act  in  harmonious  sequence  in  point  of  time.  If  the  lower  inter- 
costal muscles  contracted  before  the  scaleni,  or  if  the  diaphragm  contracted 
alternately  with  the  other  chest-muscles,  the  satisfactory  entrance  and  exit  of 
air  would  be  impossible.  These  muscles,  moreover,  are  coordinated  also  in 
respect  of  the  amount  of  their  several  contractions ;  a  gentle  and  ordinary 
contraction  of  the  diaphragm  is  accompanied  by  gentle  and  ordinary  con- 
tractions of  the  intercostals,  and  these  are  preceded  by  gentle  and  ordinary 
contractions  of  the  scaleni.  A  forcible  contraction  of  the  scaleni,  followed 
by  simply  a  gentle  contraction  of  the  intercostals,  would  perhaps  hinder 
rather  than  assist  inspiration,  and  at  all  events  would  be  waste  of  power. 
Further,  the  whole  complex  inspiratory  effort  is  often  followed  by  a  less 
marked  but  still  complex  expiratory  action.  It  is  impossible  that  all  these 
so  carefully  coordinated  muscular  contractions  should  be  brought  about  in  any 
other  way  than  by  coordinate  nervous  impulses  descending  along  efferent 
nerves  from  a  coordinating  nervous  centre.  By  experiment  we  find  this  to 
be  the  case. 

When  in  a  rabbit  the  trunk  of  a  phrenic  nerve  is  cut,  the  diaphragm  on 
that  side  remains  motionless,  and  respiration  goes  on  without  it.  When  both 
nerves  are  cut,  the  whole  diaphragm  remains  quiescent,  though  the  costal 
respiration  becomes  excessively  labored. 

When  an  intercostal  nerve  is  cut,  no  active  respiratory  movements  are 

24 


370  RESPIRATION. 

seen  in  the  intercostal  muscles  of  the  corresponding  space,  and  when  the 
spinal  cord  is  divided  below  the  origin  of  the  seventh  cervical  spinal  nerve, 
that  is,  below  the  exits  of  the  roots  of  the  phrenic  nerves,  costal  respiration 
ceases,  though  the  diaphragm  continues  to  act,  and  that  with  increased  vigor. 
When  the  cord  is  divided  just  below  the  medulla,  all  thoracic  movements 
cease,  but  the  respiratory  actions  of  the  nostrils  and  glottis  still  continue. 
These,  however,  disappear  when  the  facial  and  recurrent  laryngeal  nerves 
are  divided.  We  have  already  stated  that  after  removal  of  the  brain  above 
the  medulla,  respiration  still  continues  very  much  as  usual,  the  modifications 
which  ensue  from  the  loss  of  the  brain  being  unessential.  Hence,  putting 
all  these  facts  together,  it  is  clear  that  the  respiratory  movements  are,  as  we 
suggested,  brought  about  by  coordinated  impulses  which,  developed  in  the 
central  nervous  system  and  starting  in  the  first  instance  in  the  medulla,  find 
their  way  along  the  several  efferent  nerves.  The  proof  is  completed  by  the 
fact  that  the  removal  of,  or  extensive  injury  to,  the  medulla  alone  is,  save  in 
exceptional  cases  which  we  will  discuss  presently,  at  once  followed  by  the 
cessation  of  all  respiratory  movements,  even  though  the  rest  of  the  nervous 
system  including  every  muscle  and  every  nerve  concerned  be  left  intact. 
Nay  more,  if  only  a  small  portion  of  the  medulla,  a  tract  whose  limits  have 
not  been  clearly  defined,  but  which  may  be  described  as  lying  below  the 
vasomotor  centre  in  the  immediate  neighborhood  of  the  nuclei  of  the  vagus 
nerves,  be  removed  or  injured,  respiration  ceases,  and  death  at  once  ensues. 
Hence  this  portion  of  the  nervous  system  was  called  by  Flourens  the  vital 
knot,  or  ganglion  of  life,  nceud  vital.  We  shall  speak  of  it  as  the  respiratory 
centre. 

§  305.  The  nature  of  this  centre  must  be  exceedingly  complex  ;  for  while 
,even  in  ordinary  respiration  it  gives  rise  to  a  whole  group  of  coordinate  ner- 
vous impulses  of  inspiration  followed  in  due  sequence  by  a  smaller  but  still 
coordinate  group  of  expiratory  impulses  of  an  antagonistic  nature,  in  labored 
respiration  fresh  and  larger  impulses  are  generated,  though  still  in  coordina- 
tion with  the  normal  ones,  the  expiratory  events  being  especially  augmented  ; 
and  in  the  cases  of  more  extreme  dyspnoea  and  asphyxia  impulses  overflow, 
so  to  speak,  from  it  in  all  directions,  though  only  gradually  losing  their 
coordination,  until  almost  every  muscle  in  the  body  is  thrown  into  contrac- 
tions. 

We  must  not,  however,  conceive  of  this  centre  as  one  of  such  a  kind  that 
the  impulses  leave  it  fully  coordinated  and  equipped  so  that  nothing  remains 
for  them  but  to  travel,  unchanged,  along  the  several  efferent  nerve-fibres  to 
their  several  muscular  destinations.  On  the  contrary,  we  have  reason  to 
think  that  the  respiratory  motor  nerves,  like  other  motor  nerves,  are  con- 
nected, just  as  they  are  about  to  issue  from  the  spinal  cord,  with  a  nervous 
machinery  in  which  nerve-cells  play  a  part — a  point  which  we  shall  consider 
more  fully  in  treating  of  the  spinal  cord  ;  we  have  reason  to  think  that  the 
respiratory  impulses  starting  from  the  respiratory  centre  pass  into  and  are 
modified  by  secondary  spinal  nervous  mechanisms  before  they  issue  along  the 
motor  nerve  roots.  Indeed,  observations  show  that  under  particular  condi- 
tions, and  especially  in  young  animals,  respiratory  movements  may  be  carried 
out  in  the  entire  absence  of  the  medulla  oblongatn.  Thus  if,  in  a  kitten  or 
puppy  or  young  rabbit,  after  division  of  the  spinal  cord  below  the  medulla 
artificial  respiration  be  kept  up,  and  then  pauses  be  made  in  the  artificial 
respiration,  during  these  pauses  not  only  may  what  appear  to  be  respiratory 
movements  be  induced  in  a  reflex  manner,  by  pinching  or  by  blowing  on  the 
skin,  but,  especially  if  the  excitability  of  the  spinal  cord  be  heightened  by 
small  doses  of  strychnine,  even  spontaneous  eflbrts  of  breathing  may  occa- 
sionally be  observed.  These  are  the  exceptional  instances  mentioned  above. 


THE  NERVOUS  MECHANISM  OF  RESPIRATION.  371 

Since  in  such  cases  the  rhythmically  repeated  movements  of  the  respiratory 
muscles  are  sometimes  accompanied  by  rhythmic  movements  of  the  fore  and 
hind  limbs  not  respiratory  in  nature,  it  may  be  doubted  whether  these 
experiments  really  prove  the  existence  of  distinct  respiratory  centres  in  the 
spinal  cord  ;  and  at  most  they  merely  show  that  the  respiratory  nervous 
mechanism  is  not  entirely  confined,  as  was  once  thought,  to  the  centre  in  the 
medulla,  but  also  embraces  other  subsidiary  mechanisms,  which  may  perhaps 
be  spoken  of  as  centres,  in  the  spinal  cord  below.  It  has,  indeed,  been  main- 
tained by  some  that  these  lower  spinal  centres  are  the  chief  centres,  and  that 
the  medullary  centre  acts  merely  in  the  way  of  regulating  these ;  but  it  is  diffi- 
cult to  reconcile  this  view  with  the  experience  that  interference  with  the 
medulla,  limited  entirely  to  the  medulla,  so  often  leads  to  the  entire  aboli- 
tion of  the  respiratory  movements.  This  matter  is  not  at  present  thoroughly 
worked  out,  but  we  shall  probably  not  greatly  err  in  regarding  the  respira- 
tory nervous  system  as  in  many  ways  analogous  to  the  vasomotor  nervous 
system,  with  its  head  centre  in  the  medulla  and  secondary  centres  elsewhere, 
and  in  continuing  to  speak  of  the  centre  in  the  medulla  as  being  "  the  respira- 
tion centre,"  while  admitting  that  it  works  through  other  nervous  machinery 
placed  lower  down  in  the  spinal  cord,  and  that  this  subordinate  machinery 
may  in  exceptional  cases  carry  out,  though  inadequately,  the  work  of  the 
chief  centre. 

§  306.  Admitting  then  the  existence  of  this  medullary  respiratory  centre 
the  question  naturally  arises,  Are  we  to  regard  its  rhythmic  action  as  due 
essentially  to  changes  taking  place  in  itself,  or  as  due  to  afferent  nervous 
impulses  or  other  stimuli  which  affect  it  in  a  rhythmic  manner  from  with- 
out? In  other  words,  Is  the  action  of  the  centre  automatic  or  purely 
reflex?  We  know  that  the  centre  may  be  influenced  by  impulses  proceed- 
ing from  without,  and  that  the  breathing  may  be  affected  by  the  action  of 
the  will,  or  by  an  emotion,  or  by  a  dash  of  cold  water  on  thp  skin,  or  in  a 
hundred  other  ways ;  but  the  fact  that  the  action  of  the  centre  may  be  thus 
modified  from  without,  is  no  proof  that  the  continuance  of  its  activity  is  de- 
pendent on  extrinsic  causes. 

In  attempting  to  decide  this  question  we  naturally  turn  to  the  pneumo- 
gastric  as  being  the  nerve  most  likely  to  serve  as  the  channel  of  afferent 
impulses  setting  in  action  the  respiratory  centre.  If  both  vagus  nerves  be 
divided,  respiration  still  continues,  though  in  a  modified  form.  This  proves 
distinctly  that  afferent  impulses  ascending  those  nerves  are  not  the  efficient 
cause  of  the  respiratory  movements.  We  have  seen  that  when  the  spinal 
cord  is  divided  below  the  medulla,  the  facial  and  laryngeal  movements  still 
continue.  This  proves  that  the  respiratory  centre  is  still  in  action,  though 
its  activity  is  unable  to  manifest  itself  in  any  thoracic  movement.  But 
when  the  cord  is  thus  divided,  the  respiratory  centre  is  cut  off'  from  all  sen- 
sory impulses,  save  those  which  may  pass  into  it  from  the  cranial  nerves  of 
sensory  function  ;  and  that  these  sensory  cranial  nerves  are  not  specially 
concerned  in  developing  the  activity  of  the  respiratory  centre  is  shown  by 
the  fact  that  the  division  of  these  cranial  nerves  by  themselves,  when  the 
medulla  and  spinal  cord  are  left  intact,  does  not  do  away  with  the  continu- 
ance of  respiration.  One  cranial  nerve,  as  we  shall  see,  is  especially  con- 
cerned in  respiration,  viz.,  the  vagus  nerve ;  but  if,  after  removal  of  the 
brain  above  the  medulla,  both  vagus  nerves  be  divided,  respiration  still 
goes  on  ;  indeed,  the  respiratory  impulses  proceeding  from  the  centre  are, 
though  in  a  peculiar  way,  exaggerated.  Hence,  though  we  cannot  put  the 
matter  to  an  experimental  test  by  dividing  every  sensory  nerve  in  the  body, 
while  leaving  the  motor  nerves  of  respiration  intact,  such  an  operation  being 
practically  impossible,  we  may  infer  that  the  respiratory  impulses  proceed- 


372 


RESPIRATION. 


ing  from  the  respiratory  centre  are  not  simply  afferent  impulses  reaching  the 
centre  along  afferent  nerves  and  transformed  by  reflex  action  in  that  centre. 
They  evidently  start  de  novo  from  the  centre  itself,  however  much  their 
characters  may  be  affected  by  afferent  impulses,  reaching  that  centre  at 
the  time  of  their  being  generated.  The  action  of  the  centre  is  automatic, 
not  simply  reflex. 

§  307.  We  find,  on  inquiry,  that  the  activity  of  the  centre  is  profoundly 
influenced  by  two  classes  of  events.  These,  as  we  might  expect,  are,  on  the 
one  hand,  events  producing  changes  in  the  quality  of  the  blood  distributed 
to  the  medulla  by  the  left  ventricle,  especially  as  regards  its  gases,  that  is  to 
say,  events  modifying  the  interchange  taking  place  in  the  lungs  ;  and,  on  the 
other  hand,  nervous  impulses  started  in  various  ways  and  reaching  the  centre 
along  various  nerves  or  nervous  tracts.  It  will  be  convenient  to  consider  the 
latter  first. 

Afferent  nervous  impulses  may  affect  the  centre  in  many  various  ways. 
The  whole  act  of  breathing  or  of  taking  a  breath  is  a  double  act,  consist- 
ing of  an  inspiration  and  an  expiration,  and  nervous  impulses  may  especially 
affect  the  one  or  the  other.  One  mode  of  breathing  may  differ  from  another 
in  the  depth  of  the  individual  breath,  in  the  volume  of  air  taken  in  and  given 
out ;  and  nervous  impulses  may  increase  or  may  diminish  the  depth  of  a 
breath,  the  volume  of  air  respired.  One  mode  of  breathing  again  differs 
from  another  in  the  rapidity  with  which  one  breath  succeeds  another,  that 
is,  in  the  rate  of  rhythm  ;  and  nervous  impulses  may  slow  or  may  quicken 
the  rate  of  rhythm.  Then,  again,  combination  of  affects  so  numerous  and 
varied  as  almost  to  baffle  description,  may  result  from  the  influence  of  vari- 
ous nervous  impulses.  Emotions  may  affect  a  single  breath  or  a  long 
series  of  breaths,  may  quicken  the  rhythm  while  making  each  breath 

FIG.  99. 


\ 

A 

/\ 

A 

A 

A, 

h 

n 

t 

/v 

A. 

/s 

. 

\ 

V, 

i 

I 

I 

\ 

i 

\ 

b 

r 

Effect  on  Respiration  of  Section  of  One  Vagus.  The  vagus  was  divided  at  the  point  marked 
x.  The  curve  was  obtained  by  means  of  a  tambour  connected  with  a  receiver  into  which  the 
animal  (rabbit)  breathed,  as  shown  in  Fig.  90,  the  lever  falling  in  inspiration  as  air  is  sucked  out 
of  the  tambour  and  rising  in  expiration  as  the  air  returns.  Inspiration  begins  at  a  and  ends  at 
b.  Expiration  begins  at  b  and  ends  at  c.  The  lever  gradually  falls  between  c  and  a,  owing  to  the 
escape  of  air  from  the  apparatus. 

more  shallow,  or  may,  at  the  same  time,  make  each  breath  deeper,  or  may 
slow  the  rhythm  in  either  the  one  or  the  other  manner,  and  may  bear  chiefly 
on  inspiration  or  on  expiration.  Moreover,  there  is  not  an  afferent  nerve 
in  the  body  which  by  means  of  afferent  impulses  passing  along  it  may 
not  be  the  instrument  of  influencing  the  respiratory  centre.  Of  all  the 
automatic  centres  in  the  body  the  respiratory  centre  is  the  one  whose  in- 
dependence is  most  obscured  by  the  repeated  effects  of  afferent  nervous 
impulses. 


THE  NERVOUS  MECHANISM  OF  RESPIRATION. 


373 


Certain  afferent  nerves,  however,  appear  to  be  more  closely  connected 
with  it  than  others ;  and  of  these  the  most  conspicuous  and  important  are 
the  two  vagus  nerves,  which  we  have  already  mentioned  in  this  connection. 
Their  importance  is  well  illustrated  by  the  following  experiments:  If  one 
vagus  be  divided  in  an  ordinary  way,  without  any  special  precautions,  the 
respiration  is  either  not  materially  changed,  or  if  affected  becomes  slower 
(Fig.  99).  If  both  be  divided  (Fig.  100)  it  becomes  very  slow,  the  pauses 
between  expiration  and  inspiration  being  markedly  prolonged.  The  cha- 
racter of  the  respiratory  movement,  too,  is  markedly  changed  ;  each  respira- 
tion is  fuller  and  deeper,  so  much  so,  indeed,  that,  according  to  some  observ- 
ers, what  is  lost  in  rate  is  gained  in  extent,  the  amount  of  carbonic  acid  pro- 


FIG.  100. 


f\ 


Effect  on  Respiration  of  Section  of  Both  Vagus  Nerves.  The  curve  was  obtained  in  the  same 
way  as  Fig.  99.    The  second  vagus  nerve  was  divided  at  x. 

duced  and  oxygen  consumed  in  a  given  period  remaining  after  division  of 
the  nerves  about  the  same  as  when  these  were  intact ;  but  it  is  undesirable 
to  insist  too  much  on  the  exactness  of  this  compensation. 

FIG.  101. 


Quickening  of  Respiration  by  Gentle  Stimulation  of  the  Central  End  of  the  Vagus  Trunk.  The 
curve  was  obtained  in  the  same  way  as  Figs.  99, 100.  Stimulation  of  the  vagus  began  at  x  and 
ended  at  y. 

When  after  division  of  both  vagus  nerves  in  the  neck,  the  medulla  being 
intact,  the  central  stump,  that  connected  with  the  central  nervous  system,  of 
one  of  them  is  stimulated  with  a  gentle  interrupted  current,  the  effects  are 
not  always  the  same ;  one  of  two  results  may  follow,  and  that  whichever  of 
the  two  nerves  be  used.  In  a  certain  number  of  cases,  and  these  may,  per- 
haps, be  regarded  as  the  more  typical  ones,  the  respiration,  which  from  the 


374 


RESPIRATION. 


division  of  the  nerves  had  become  slow,  is  quickened  again  (Fig.  101),  and 
with  care,  by  a  proper  application  of  the  stimulus,  the  normal  respiratory 
rhythm  may  for  a  time  be  restored.  Upon  the  cessation  of  the  stimulus  the 
slower  rhythm  returns.  If  the  current  be  increased  in  strength,  the  rhythm 
may  in  some  cases  be  so  accelerated  that  inspiration  begins  before  the  expira- 
tion of  the  preceding  breath  is  completed  (Fig.  102),  and  this  may  go  on 

FIG.  102. 


Stimulation  of  Vagus  Leading  to  Inspiratory  Increase  :  This  curve,  unlike  the  preceding,  was 
obtained  by  inserting  a  needle  through  the  body  wall,  so  as  to  rest  on  the  diaphragm,  and  attach- 
ing a  lever  to  the  needle.  (See  §271.)  The  lever  rises  with  each  contraction  of  the  diaphragm, 
so  that  inspiration  begins  at  a  and  ends  at  b,  expiration  begins  at  6  and  ends  at  c,  the  interval  be- 
tween c  and  a  corresponding  to  the  pause.  Stimulation  of  the  vagus  begins  at  x.  It  will  be  seen 
that  upon  stimulation  the  inspiratory  rises  of  the  lever  begin  long  before  the  preceding  expira- 
tions are  complete. 

until  at  last  the  diaphragm  is  brought  into  a  condition  of  prolonged  tetanus, 
and  a  standstill  of  respiration  in  an  extreme  inspiratory  phase  is  the  result. 
On  the  other  hand,  in  a  certain  number  of  cases  the  result  is  of  an  opposite 
character.  Even  though  the  respiration  be  already  slowed  by  division  of 
the  nerves,  stimulation  produces  a  still  further  slowing,  the  pauses  between 
each  expiration  and  the  succeeding  inspiration  are  prolonged  (c/.  Fig.  103), 
and  in  a  certain  number  of  cases  actual  standstill  is  brought  about,  but  a 
standstill  of  a  kind  the  opposite  of  the  one  just  described,  since  the  dia- 
phragm which  in  that  case  was  in  prolonged  tetanus  is  in  this  case  completely 
relaxed  and  remains  for  some  time  in  the  condition  in  which  it  is  at  the  close 
of  an  ordinary  breath.  In  a  certain  number  of  cases,  and  these  are  not 
uncommon,  the  result  is  intermediate  between  the  two  above  extremes  ;  the 
diaphragm  stands  still  in  a  prolonged  contraction  in  a  position  which  is  inter- 
mediate between  the  height  of  inspiration  and  expiration. 

These  results  suggest  the  conclusion  that  the  vagus  nerve  (we  are  dealing 
now  with  the  main  trunk  of  the  nerve)  contains  afferent  fibres  of  two  kinds 
connected  with  the  respiratory  centre ;  one  kind  augmenting  the  action  of 
the  centre  somewhat  in  the  same  way  as  the  augmentor  cardiac  fibres  aug- 
ment the  beat  of  the  heart,  and  the  other  kind  having  an  inhibitory  effect. 
Apparently  sometimes  the  one  and  sometimes  the  other  kind  is,  according  to 
circumstances,  most  provoked  by  the  stimulation,  much  in  the  same  way 
as  stimulation  of  the  vagus  in  the  frog,  which,  as  we  have  seen,  §  144,  is 
the  channel  for  both  inhibitory  and  augmentor  cardiac  impulses,  produces 
sometimes  inhibition,  sometimes  augmentation  of  the  heart-beat.  To  affect 
the  heart  of  course  the  stimulation  of  the  vagus  must  be  centrifugal, 
directed  toward  the  periphery,  whereas  to  affect  the  respiration  it  must  be 
centripetal,  applied  to  the  part  of  the  nerve  connected  with  the  brain ;  and 


THE  NERVOUS  MECHANISM  OF  RESPIRATION.  375 

while  the  usual  effect  on  the  heart  of  ordinary  stimulation  of  the  vagus  is 
inhibition,  augmentation  only  occurring  in  special  cases,  the  most  common 
effect  on  respiration  is  augmentation,  though  inhibition  is  not  unfrequently 
seen.  When  the  experiment  is  conducted  on  an  animal  under  the  full 
influence  of  chloral,  stimulation  of  the  vagus  generally  produces  inhibition 
of  respiration,  probably  because  the  chloral  renders  the  respiratory  centre 
more  susceptible  to  inhibitory  influences. 

§  308.  We  said  just  now  "  the  action  of  the  centre  "  ;  but  the  respiratory 
centre  is  a  double  one  ;  it  gives  rise  to  inspiratory  and  to  expiratory  afferent 
impulses,  and  these  are  antagonistic  the  one  to  the  other.  If  inspiratory 
and  expiratory  impulses  issued  from  the  centre  at  the  same  time  and  in 
equal  potency,  there  could  be  no  breathing  at  all,  they  would  neutralize 
each  other's  effects;  and,  indeed,  any  amount  of  inspiratory  impulse  is 
antagonistic  to  a  simultaneous  expiratory  impulse,  and  vice  versa.  Hence, 
for  the  adequate  services  of  the  respiratory  centre  we  might  expect  to  find 
that  each  kind  of  afferent  impulse  ascending  the  vagus  affected  the  centre 
in  a  double  and  opposite  way,  inhibiting  expiration  while  augmenting  inspira- 
tion, or  inhibiting  inspiration  while  augmenting  expiration.  If  we  allow 
ourselves  to  speak  of  the  whole  respiratory  centre  as  consisting  of  two  parts, 
one  the  inspiratory  part,  or  inspiratory  centre  concerned  in  the  issue  of 
inspiratory  impulses,  and  the  other  the  expiratory  part,  or  expiratory  centre 
concerned  in  the  issue  of  expiratory  impulses,  we  may  suppose  that  these 
centres  are  so  related  to  each  other  that  afferent  impulses,  reaching  the 
medulla,  which  augment  or  inhibit  the  one,  necessarily  inhibit  or  augment 
the  other.  We  need  perhaps  hardly  add  that  of  these  two  centres  we  should 
expect  to  find  the  inspiratory  centre  the  dominant  and  the  most  responsive 
one;  in  normal  breathing  it  comes  almost  alone  into  obvious  use,  since,  as 
we  have  seen,  the  expiratory  muscles  have  then  a  very  slight  task  only,  the 
chest  being  emptied  chiefly  by  elastic  reaction  ;  and,  speaking  generally, 
breathing  in  is  the  first  consideration — we  breathe  out  mostly  because  we 
have  already  breathed  in. 

There  are  many  facts  which  support  this  view  of  the  double  antagonistic 
action  of  afferent  respiratory  impulses.  If  the  central  end  of  the  superior 
laryngeal  branch  of  the  vagus  be  stimulated  the  effects  are  much  more  con- 
stant than  those  of  stimulating  the  main  vagus  trunk.  Whether  the  main 
trunk  of  the  nerve  be  previously  severed  or  not,  the  result  of  centripetal 
stimulation  of  the  superior  laryngeal  branch  is  always  in  the  direction  of  a 
slowing  of  the  respiration  (Fig.  103)  ;  and  this  may  by  proper  stimulation 
be  carried  so  far  that  a  complete  standstill  of  respiration  in  the  phase  of 
rest  is  brought  about.  While  the  main  trunk  of  the  vagus  contains  fibres 
of  two  kinds,  both  augmentory  and  inhibitory  of  inspiration,  the  superior 
laryngeal  branch  appears  to  contain  one  kind  only,  those  which  inhibit 
inspiration.  If  now  while  this  experiment  is  being  conducted  on  a  rabbit 
the  abdomen  be  watched,  it  will  be  seen  that  the  inhibition  of  inspiration  is 
accompanied  by  a  contraction  of  the  abdominal  muscles,  that  is  by  an  effort 
at  expiration  ;  the  stimulation  of  the  nerve  while  inhibiting  respiration  pro- 
vokes, to  a  certain  extent,  expiration. 

§  309.  That  the  trunk  of  the  vagus  is  the  channel  of  these  two  kinds 
of  impulses,  of  a  mutually  antagonistic  character,  is  further  shown  by  apply- 
ing what  may  be  considered  as  natural  stimuli  to  the  endings  of  the  nerve 
in  the  lungs ;  and  the  results  so  obtained  have  an  especial  value  since  the 
artificial  stimulation  of  a  nerve-fibre,  at  a  part  of  its  course  by  means  of  an 
electric  current  is  at  best  a  rough  process,  by  which  we  cannot  hope  to  do 
more  than  approximate  to  the  results  actually  taking  place  in  the  living 
body  when  the  nerve  is  stimulated  at  its  endings  by  natural  stimuli ;  and 


376 


RESPIRATION. 


the  approximation  is  perhaps  less  in  the  case  of  the  exquisitely  sensitive 
respiratory  centre  than  in  many  other  cases. 


FIG.  103. 


VJ 


\J 


Slowing  of  Respiration  by  Stimulation  of  Superior  Laryngeal  Nerve.  This  curve  was  obtained 
in  the  same  way  as  Figs.  99, 100, 101,  and  the  letters  have  the  same  meaning  as  in  those  figures. 
Stimulation  begins  at  x  and  ends  at  y. 

If  in  an  animal  in  which  a  careful  graphic  record  of  the  respiratory 
movements  is  being  taken,  the  trachea  be  suddenly  closed  at  the  summit  of 

FIG.  104. 


B 

Effects  of  Distention  and  Collapse  of  Lung.  (Head.)  Both  curves  are  described  by  a  lever 
attached,  as  stated  in  §  271,  to  a  slip  of  the  diaphragm  of  a  rabbit.  A  contraction  of  the  diaphragm 
(inspiration)  raises  the  lever;  during  relaxation  of  the  diaphragm  the  lever  fails.  In  A,  the 
trachea  is  closed  at  x,  the  height  of  inspiration  :  a  pause  follows  during  which  the  lever  gradually 
sinks  until  an  inspiration  (a  very  powerful  one)  sets  in.  In  B,  the  trachea  is  closed  at  the  end  of 
expiration,  x;  there  follow  powerful  inspirations. 

an  inspiration,  the  result  is  a  pause  before  the  succeeding  inspiration  follows, 
that  is  to  say,  a  partial  or  temporary  inhibition  of  inspiration  ;  and  if  during 


THE  NERVOUS  MECHANISM  OF  RESPIRATION.  377 

such  an  experiment  on  a  rabbit  a  curve  be  taken  by  means  of  the  isolated 
slip  of  the  diaphragm,  §  271,  it  will  be  seen  (Fig.  104,  A)  that  the  slip  elon- 
gates somewhat ;  that  is  to  say,  previously  in  a  state  of  slight  tonic  contrac- 
tion, it  changes  in  the  direction  of  expiration.  If,  on  the  other  hand,  the 
trachea  be  suddenly  closed  at  the  end  of  an  expiration  (Fig.  104,  B\  when 
the  lungs  have  returned  to  their  emptied  condition,  the  result  is  an  increase 
of  the  sequent  inspirations,  that  is  to  say,  an  augmentation  of  inspiratory 
impulses.  If  the  chest  or  if  the  lung  only  be  gently  inflated,  a  temporary 
cessation  of  all  inspiration  may  be  produced,  accompanied  sometimes  by  an 
attempt  at  expiration.  If,  on  the  other  hand,  air  be  sucked  out  of  the 

FIG.  105. 


Effects  of  Repeated  Inflations.  Positive  Ventilation.  (Head.)  The  lower  curve  is  described, 
as  in  Fig.  104,  by  a  lever  attached  to  a  slip  of  the  diaphragm.  The  upper  curve  shows  the  infla- 
tions from  x  to  y,  which  were  made  without  any  attempt  to  draw  the  air  out  at  each  inflation  ; 
each  rise  on  this  curve  denotes  an  inflation.  It  will  be  observed  that  as  the  inflations  are  con- 
tinued the  respiratory  movements  of  the  diaphragm  are  gradually  "  knocked  down." 

chest,  or  if  one  lung  be  made  to  collapse  by  puncture  of  one  pleural  cham- 
ber, a  prolonged  inspiration  is  the  frequent  result,  the  diaphragm  being 
thrown  into  a  prolonged  inspiratory  tetanus.  If  the  lungs  are  repeatedly 
inflated,  without  any  means  being  taken  to  draw  out  the  air  after  each  infla- 
tion (Fig.  105),  a  procedure  which  we  may  speak  of  as  positive  ventilation, 
the  result  is  that  the  inspiratory  efforts  are  diminished,  and  if  the  ventila- 
tion is  continued  may  cease  altogether.  If,  on  the  other  hand,  air  is  repeat- 
edly sucked  out  of  the  lungs  without  any  corresponding  inflations  (negative 
ventilation),  the  inspiratory  efforts  are  increased  (Fig.  106),  and  the  increase 
may  be  such  as  to  bring  the  diaphragm  to  a  state  of  tetanus.  And  in 
general,  though  several  complications  occur  which  we  cannot  discuss  here, 
the  results  of  inflation  of  the  lungs  on  the  one  hand  and  of  suction  or  col- 
lapse of  the  lungs  on  the  other  hand,  show  that  the  mere  inflation  or  per- 
haps rather  the  mere  distention  of  the  lung  tends  to  inhibit  inspiratory  and 
usher  in  expiratory  impulses,  while  collapse  of  the  lung  tends  to  inhibit 
expiratory  and  to  develop  inspiratory  impulses,  the  effect  on  the  inspiratory 
impulses,  as  might  be  expected  from  the  dominance  of  the  inspiratory  por- 
tion of  the  centre,  being  more  marked  than  the  effect  on  the  expiratory 
impulses.  That  the  instrument  by  which  these  effects  are  produced  is  the 
vagus  nerve  is  shown  by  the  fact  that  they  are  no  longer  distinctly  recog- 
nizable when  both  vagus  nerves  are  divided.  And  that  the  results  are  due 
to  the  mere  mechanical  expansion  and  collapse  of  the  lung  in  insufflation 
and  collapse,  and  not  to  any  chemical  influences  exerted  by  the  larger 
amount  or  smaller  amount  of  air  present  in  the  lung  in  the  two  cases 
increasing  or  diminishing  the  absorption  of  oxygen  and  escape  of  carbonic 
acid,  is  shown  by  the  fact  that  the  results  remain  in  their  main  features 
the  same  when  some  indifferent  gas,  such  as  hydrogen,  is  used  for  inflation 
instead  of  air  or  oxygen.  We  infer  therefore  that  the  expansion  of  the 
pulmonary  alveoli  in* some  way  or  other  so  stimulates  the  endings  in  the 


378 


RESPIRATION. 


lung  of  the  pulmonary  branches  of  the  vagus,  that  impulses  are  generated 
which  ascending  the  vagus  trunk  inhibit  the  inspiratory  processes  in  the 
respiratory  centre  ;  and  that  conversely  collapse  of  the  lung  similarly  gen- 
erates impulses  which  are  augmentative  of  inspiratory  impulses.  And, 
assuming  on  the  strength  of  analogy  the  existence  in  the  vagus  of  two  sets 
of  fibres  we  may  say  that  expansion  stimulates  the  endings  of  the  fibres 
which  inhibit  inspiration  and  concurrently  tend  to  augment  expiration, 
while  collapse  stimulates  the  fibres  which  inhibit  expiration  and  augment 
inspiration.  The  respiratory  pump  may  thus  be  looked  upon  as  a  self-regu- 


Effects  of  Repeated  Suctions  of  the  Lungs.  Negative  Ventilation.  (Head.)  The  curve  corre- 
sponds exactly  to  Fig.  105,  except  that  the  lungs  are  subjected  to  repeated  suctions  without 
corresponding  inflations.  The  result  is  that  the  inspirations  are  repeated  in  such  a  way  as  to 
lead  almost  to  an  inspiratory  tetanus  of  the  diaphragm. 

lating  mechanism ;  the  expansion  of  the  lungs  which  is  the  result  of  the 
efferent  inspiratory  impulses  tends  to  check  the  issue  of  these  impulses  and 
to  inaugurate  the  sequent  expiration ;  and  the  return  of  the  lungs  in  expi- 
ration tends  to  set  going  the  succeeding  inspiration. 

The  regulative  influence  exerted  by  impulses  normally  ascending  the 
vagus  nerves  is  further  shown  by  the  following  striking  experiment :  As  we 
have  already  seen,  the  brain  above  the  medulla  may  be  removed  without 
any  extraordinary  change  in  the  respiration  taking  place.  We  have  also 
seen  that  when  both  vagus  nerves  are  divided  the  respiration  is  slower  and 
deeper,  but  is  otherwise  regular.  If,  however,  after  the  removal  of  the  brain 
above  the  medulla  both  vagus  nerves  are  divided,  if  the  respiratory  centre 
be  cut  off  at  one  and  the  same  time  from  impulses  passing  down  from  the 
higher  parts  of  the  brain  and  from  impulses  ascending  the  vagus  nerves, 
the  result  is  that  the  respirations  take  on  the  form  of  a  series  of  long  con- 
tinued inspiratory  spasms.  It  would  seem  as  if  there  were  a  tendency  in  the 
respiratory  centre  to  go  off  into  tetanic  inspiratory  explosions,  that  this  ten- 
dency is  held  in  check  by  impulses  from  the  brain  when  the  vagus  nerves 
are  divided,  and  by  impulses  along  the  vagus  nerves  when  the  brain  is  re- 
moved, but  meets  with  no  adequate  checks  when  impulses  from  both  sources 
are  cut  off  at  the  same  time. 

§  310.  Hypotheses  have  been  put  forward  to  explain  the  changes  in  the 
respiratory  centre  which  lead  to  the  rhythmic  discharge  of  inspiratory  and 
expiratory  impulses  and  the  further  changes  which  result  from  the  advent 
of  augmenting  and  inhibitory  impulses;  but  these  as  yet  remain  mere  hy- 
potheses, and  it  would  not  be  profitable  to  discuss  them  here.  We  may  add 


THE  NERVOUS  MECHANISM  OF  RESPIRATION.  379 

that,  though  the  analogy  of  the  cardiac  nervous  mechanism,  in  which  we  can 
anatomically  distinguish  between  augmentor  and  inhibitory  fibres,  justifies 
us  in  speaking  of  augmentor  and  inhibitory  and  respiratory  fibres  as  exist- 
ing in  the  vagus  nerve,  we  are  not  as  yet  able  to  distinguish  them  by  ana- 
tomical methods.  We  may  further  add  that,  so  exquisitely  sensitive  is  the 
respiratory  centre  to  these  afferent  impulses  stimuli  too  slight  to  produce 
any  appreciable  effect  when  applied  to  afferent  nerves  connected  with 
an  ordinary  centre,  such  as  a  spinal  reflex  centre,  may  produce  marked 
effects  on  the  respiratory  centre.  For  instance,  the  feeble  electric  current 
which  is  developed  when  the  cut  end  of  a  divided  vagus  is  replaced  in  the 
wound,  the  circuit  between  the  cut  end  and  the  longitudinal  surface  of  the 
nerve  being  closed  through  the  blood  or  lymph  of  the  wound,  is  often  suffi- 
cient to  develop  inhibitory  impulses.  Again,  when  the  connection  of  the 
respiratory  centre  with  the  lungs  through  the  vagus  nerves  is  abolished,  not 
by  section  of  the  nerves,  but  by  freezing  both  nerves  at  some  part  of  the 
course  of  each  nerve  (an  operation  which,  while  completely  blocking  the 
passage  of  impulses  along  the  nerve-fibres,  does  not  itself  act  as  a  stimulus), 
the  effect  on  the  respiratory  movements  is  much  more  in  the  direction  of  in- 
creasing and  prolonging  the  inspiratory  act  than  that  of  slowing  the  rhythm. 
Hence  it  would  appear  that  what -we  have  previously  described  as  the  result 
of  dividing  both  vagus  nerves  is  partly  due  to  the  blocking  of  natural  im- 
pulses and  partly  to  the  section  of  the  nerves,  and  possibly  to  electric  cur- 
rents developed  as  suggested  above,  acting  as  stimuli  and  thus  giving  rise  to 
artificial  impulses. 

§  311.  The  double  or  alternate  respiratory  action  of  the  vagus  nerves,  on 
which  we  have  dwelt  above,  may  be  taken  as,  in  a  general  way,  illustrative 
of  the  manner  in  which  other  afferent  nerves  and  various  parts  of  the  cere- 
brum are  enabled  to  influence  respiration.  As  we  have  already  said,  and, 
indeed,  know  from  daily  experience,  of  all  the  apsychical  nervous  centres 
the  respiratory  centre  is  the  one  which  is  most  frequently  and  most  deeply 
affected  by  nervous  impulses  from  various  quarters.  Besides  the  changes 
brought  about  by  the  will  (and  when  we  breathe  voluntarily  we  probably 
make  use,  to  some  extent,  of  the  normal  nervous  machinery  of  respiration, 
working  through  this,  rather  than  sending  independent  volitional  impulses 
direct  to  the  diaphragm  and  other  respiratory  muscles),  we  find  that 
emotions  and  painful  sensations  alter  profoundly  the  character  of  the 
respiratory  movements.  And  though  these  effects  may  be  partly  indirect 
(the  emotion  modifying  the  heart-beat  or  the  tonus  arteries,  and  so  in- 
fluencing the  flow  of  blood  through  the  respiratory  centre),  they  are 
chiefly  due  to  the  direct  action  of  nervous  impulses  reaching  that  centre 
from  higher  parts  of  the  brain.  So,  also,  impulses  from  almost  every  sen- 
tient surface  or  passing  along  almost  every  sensory  nerve  may  modify  res- 
piration in  one  direction  or  another.  The  influence  in  this  way  of  stimuli 
applied  to  the  skin  is  well  known  to  all  ;  but,  perhaps,  next  to  the  vagus 
the  nerve  most  closely  connected  with  the  respiratory  centre  is  the  fifth 
nerve,  branches  of  which  guard  the  nasal  respiratory  channels ;  the 
slightest  stimulation  of  the  nostrils  at  once  affects  the  breathing  and  most 
frequently  arrests  it.  The  effects  of  stimuli  of  various  strengths  brought 
to  bear  on  various  nerves  are  very  varied.  Sometimes  the  result  is  an 
increase  of  inspiration,  and  that  either  by  a  quickening  of  the  rhythm 
or  by  an  increase  of  the  individual  breaths  or  by  a  combination  of  the 
two.  Sometimes  the  result  is  inhibition  of  inspiration,  accompanied  or 
not  by  an  increase  of  expiration,  and  sometimes,  as  when  the  stimulation 
causes  a  cough,  the  expiratory  results  may  be  out  of  all  proportion  to  the 
modifications  of  inspiration.  While  in  the  case  of  some  nerves,  for  instance, 


380  RESPIRATION. 

as  we  have  seen,  the  superior  laryngeal,  and,  it  is  said,  the  splanchnic  nerves, 
the  effects  are  exclusively,  or,  at  least,  chiefly  inhibitory  of  inspiration  and 
augmentative  of  expiration,  that  is,  expiratory,  and  in  others,  perhaps,  chiefly 
augmentative  of  inspiration,  or  inspiratory,  in  the  case  of  most  nerves  the 
effect  may  be,  according  to  circumstances,  either  in  the  one  direction  or  the 
other.  Perhaps,  as  a  rule,  weak  stimuli  tend  to  augment,  and  strong  to  in- 
hibit inspiration  ;  but  the  effects  of  artificially  stimulating  sensory  nerves 
are  complicated  and  often  confused,  because  powerful  afferent  impulses  by 
giving  rise  to  pain  may,  through  impulses  generated  by  the  pain  itself  and 
descending  to  the  medulla  from  the  brain,  act  in  an  indirect  as  well  as  in  a 
direct  manner ;  and  the  prominence  of  the  indirect  painful  impulses  will,  in 
any  experiment,  depend  on  the  anaesthetic  used.  We  may  say,  however,  that  in 
all  cases  the  effect  is  very  largely  determined  by  the  condition  at  the  time 
being  of  the  respiratory  centre  itself;  and  that,  in  turn,  is  determined  not 
only  by  things  which  affect  its  nutrition,  such  as  the  character  of  the  blood 
circulating  in  it,  but  also  by  the  nature  and  amount  of  the  other  afferent 
impulses  which  are  playing  upon  it  at  the  same  time.  Thus,  as  we  shall  pres- 
ently see,  the  effect  of  a  stimulus  applied  to  the  vagus,  when  the  respiratory 
centre  is  inadequately  supplied  with  arterial  blood,  is  not  the  same  when  the 
centre  has  its  normal  supply  of  normal  blood.  So  also  a  stimulus  which, 
applied  to  the  vagus  or  to  another  nerve  in  an  intact  animal,  simply 
quickens  inspiration,  applied  in  an  animal  whose  cerebral  hemispheres 
have  been  removed  will  call  forth  a  prolonged  tetanic  inspiratory  gasp. 
The  respiratory  centre  responds,  in  fact,  in  the  most  intricate  and  varied 
manner  to  nervous  impulses  proceeding  from  all  parts  of  the  body,  and 
thus  delicately  adjusts  the  working  of  the  respiratory  pump  to  the  needs 
of  the  economy. 

§  312.  The  complicated  nature  of  the  respiratory  centre  is  further  shown 
by  the  fact  that  it  appears  to  consist  of  two  lateral  halves  which  normally 
work  in  unison  and  yet  may  be  made  to  work  independently.  If  the  me- 
dulla oblongata  be  carefully  divided  in  the  middle  line  respiration  may  con- 
tinue to  go  on  in  quite  a  normal  fashion.  If,  however,  one  vagus  be  then 
divided,  the  respiratory  movements,  both  costal  and  diaphragmatic,  on  the 
side  of  the  body  on  which  division  of  the  vagus  has  taken  place,  become 
slower  than  those  on  the  other  side,  so  that  the  two  sides  are  no  longer 
synchronous  ;  and  a  stimulus  confined  to  one  vagus  affects  the  respiratory 
movements  of  that  side  of  the  body  only.  So,  also,  a  section  of  a  lateral 
half  of  the  cord  below  the  medulla  stops  the  respiratory  movements  on  that 
side  alone. 

§  313.  Besides  these  nervous  influences,  however,  there  is  another  cir- 
cumstance which,  perhaps,  above  all  others  affects  the  respiratory  centre,  and 
that  is  the  condition  of  the  blood  in  respect  to  its  respiratory  changes  ;  the 
more  venous  (less  arterial)  the  blood,  the  greater  is  the  activity  of  the  respira- 
tory centre.  When  by  reason  either  of  any  hindrance  to  the  entrance  of  air 
into  the  chest  or  other  interference  with  the  due  interchange  between  the 
blood  and  the  pulmonary  air  or  of  a  greater  respiratory  activity  of  the  tissues, 
as  during  muscular  exertion,  the  blood  becomes  less  arterial,  more  venous, 
i.  e.,  with  a  smaller  charge  of  oxygen  and  more  heavily  laden  with  carbonic 
acid,  the  respiration  from  being  normal  becomes  labored.  We  may  speak  of 
normal  breathing  as  eupncea,  and  say  that  this,  when  the  blood  is  insufficiently 
arterialized,  passes  into  dyspnoea,  an  intermediate  stage  in  which  the  respira- 
tory movements  are  simply  exaggerated  being  known  as  hyperpncea.  The 
modifications  of  breathing  thus  caused  by  deficient  arterialization  of  blood 
are  especially  characterized  by  an  increase  in  the  total  energy  of  the  respira- 
tory impulses  generated,  and  in  this  respect  differ  from  the  modifications 


THE  NERVOUS  MECHANISM  OF  RESPIRATION.  381 

resulting  from  interference  with  the  nervous  arrangements  such  as  those  fol- 
lowing upon  section  of  the  vagus  nerves,  in  which  case,  as  we  have  seen,  the 
rhythm  is  much  more  profoundly  affected  than  the  amount.  In  dyspnoea 
the  breathing  is  frequently  quicker  as  \vell  as  deeper,  there  is  an  increase  in 
the  sum  of  efferent  respiratory  impulses,  and  the  expiratory  impulses,  which 
in  normal  respiration  are  very  slight,  acquire  a  pronounced  importance.  As 
the  blood  becomes  in  cases  of  obstruction  less  and  less  arterial,  more  and 
more  venous,  the  discharge  from  the  respiratory  centre  becomes  more  and 
more  vehement,  and  instead  of  confining  itself  to  the  usual  tracts  and  pass- 
ing down  to  the  ordinary  respiratory  muscles,  overflows  into  other  tracts  and 
puts  into  action  other  muscles,  until  there  is,  perhaps,  hardly  a  muscle  in  the 
body  which  is  not  made  to  feel  its  effects.  The  muscles  which  are  thus  more 
and  more  thrown  into  action  are  especially  those  tending  to  carry  out  or  to 
assist  expiration  ;  and  at  last,  if  no  relief  is  afforded,  the  violent  but  still 
definite  respiratory  movements  give  way  to  general  convulsions  of  the  whole 
body,  which,  however,  have  to  a  certain  extent  an  expiratory  character. 
With  the  onset  of  these  convulsions  dyspnoea  is  said  to  have  passed  into 
asphyxia.  By  the  violence  of  these  convulsions  the  whole  nervous  system 
becomes  exhausted,  the  convulsions  cease,  and  death  is  ushered  in  through  a 
few  infrequent  and  long-drawn  breaths;  but  to  this  matter  we  shall  return. 
The  effect  of  venous  blood,  then,  is  to  augment  all  those  natural  explosive 
decompositions  of  the  substance  of  the  central  nervous  system  which  give 
rise  to  respiratory  impulses  ;  it  increases  their  amount  and  also  quickens  their 
rhythm.  The  latter  change,  however,  is  much  less  marked  than  the  former, 
the  respiration  being  much  more  deepened  than  hurried,  and  the  several 
respiratory  acts  are  never  so  much  hastened  as  to  catch  each  other  up,  and 
so  to  produce  an  inspiratory  tetanus  like  that  resulting  from  stimulation  of 
the  vagus.  On  the  contrary,  especially  as  exhaustion  begins  to  set  in,  the 
rhythm  becomes  slower  than  proportionate  to  the  weakening  of  the  individual 
movements. 

§  314.  The  question  naturally  arises,  Does  this  condition  of  the  blood 
affect  the  substance  of  the  central  nervous  system,  that  is  to  say,  the  respira- 
tory centre  in  the  medulla  (and  the  subsidiary  spinal  nervous  mechanisms) 
directly,  or  does  it  produce  its  effect  by  stimulating  the  peripheral  ends  of 
afferent  nerves  in  various  parts  of  the  body,  and,  by  the  generation  there  of 
afferent  impulses,  indirectly  modify  the  action  of  the  central  nervous  system? 
Without  denying  the  possibility  that  the  latter  mode  of  action  may  help  in 
the  matter,  as  regards  not  only  the  vagus,  but  all  afferent  nerves,  the  follow- 
ing facts  seem  to  show  that  the  main  effect  is  produced  by  the  direct  action 
of  the  blood  on  the  central  nervous  system,  and,  indeed,  on  the  medullary 
respiratory  centre  itself.  If  the  spinal  cord  be  divided  below  the  medulla 
oblongata,  and  both  vagi  be  cut,  want  of  proper  aeration  of  the  blood  still 
produces  an  increased  activity  of  the  respiratory  centre,  as  shown  by  the 
increased  vigor  of  the  facial  respiratory  movements.  If  the  supply  of  blood 
be  cut  off  from  the  medulla  by  ligature  of  the  carotid  and  intervertebral 
arteries  dyspnoea  is  produced,  though  the  operation  produces  at  first  no 
change  in  the  blood  generally,  but  simply  affects  the  respiratory  condition 
of  the  medulla  itself  by  cutting  off  its  blood-supply,  the  immediate  result 
of  which  is  an  accumulation  of  carbonic  acid  and  a  paucity  of  available 
oxygen  in  the  nervous  substance  of  that  region.  If  the  blood  in  the  carotid 
artery  in  an  animal  be  warmed  above  the  normal,  a  dyspnoea  is  produced 
which,  though  apparently  not  quite  identical  with  the  dyspnoea  caused  by 
imperfect  arterialization  of  the  blood,  shows  that  the  too  high  temperature 
of  the  blood  directly  affects  the  activity  of  the  respiratory  centre.  We  may 
conclude,  therefore,  that  the  condition  of  the  blood  affects  respiration  by 


382  BESPIKATION. 

acting  directly  on  the  respiratory  centre.  Moreover,  it  is  the  medullary 
centre  which,  at  all  events  in  adult  animals,  is  affected  by  the  too  venous 
blood,  since  after  the  division  of  the  spinal  cord  below  the  medulla,  dyspnoeic 
thoracic  respiratory  movements  and  convulsions  do  not  follow  upon  exclusion 
of  air.  They  are,  however,  stated  to  occur  in  new-born  animals,  indicating 
that  the  subsidiary  mechanisms  in  the  upper  spinal  cord,  of  which  we  spoke 
in  §  306,  may  be  also  affected  by  the  too  venous  blood  ;  but  the  doubts  which 
we  previously  urged  hold  good  in  these  cases  also. 

While  the  respiratory  centre  is  thus  being  affected  by  the  too  venous 
blood,  it  is,  until  exhaustion  begins  to  set  in,  more  irritable,  more  easily  and 
largely  affected  by  afferent  impulses  than  in  its  normal  condition.  During 
dyspnoea  a  stimulus  which  applied  to  the  vagus  or  to  some  other  sensory  nerve 
under  normal  conditions  would  produce  little  or  no  effect,  may  start  very 
powerful  respiratory  movements. 

§  315.  Deficient  aeration  produces  two  effects  in  blood  :  it  diminishes  the 
oxygen  and  increases  the  carbonic  acid.  Do  both  of  these  changes  affect 
the  respiratory  centre,  or  only  one,  and  if  so,  which  ?  When  an  animal  is 
made  to  breathe  an  atmosphere  containing  nitrogen  only,  the  exit  of  car- 
bonic acid  by  diffusion  is  not  affected,  and  the  blood,  as  is  proved  by  actual 
analysis,  contains  no  excess  of  carbonic  acid.  Yet  all  the  phenomena  of 
dyspncea  are  present,  and  if  the  experiment  be  continued,  convulsions  ensue 
and  the  animal  dies  in  asphyxia.  In  this  case  the  result  can  only  be 
attributed  to  the  deficiency  of  oxygen.  On  the  other  hand,  if  an  animal  be 
made  to  breathe  an  atmosphere  rich  in  carbonic  acid,  but  at  the  same  time 
containing  abundance  of  oxygen,  though  the  breathing  becomes  markedly 
deeper  and  also  somewhat  more  frequent,  there  is  no  culmination  in  a  con- 
vulsive asphyxia,  even  when  the  quantity  of  carbonic  acid  in  the  blood,  as 
shown  by  direct  analysis,  is  very  largely  increased.  On  the  contrary,  the 
increase  in  the  respiratory  movements  may  after  a  while  pass  off,  the  animal 
becoming  unconscious,  and  appearing  to  be  suffering  rather  from  a  narcotic 
poison  than  from  simple  dyspnoea  ;  the  excess  of  carbonic  acid  in  the  blood 
appears  to  affect  other  parts  of  the  central  nervous  system,  and  especially 
portions  of  the  brain,  more  profoundly  than  it  does  the  respiratory  centre. 
It  has  been  maintained  by  some  that  while  a  deficiency  of  oxygen  promotes 
inspiratory  movements,  an  excess  of  carbonic  acid  stimulates  the  expiratory 
movements,  the  nervous  mechanisms  being  so  arranged  that  a  lack  of  oxygen 
leads  to  an  effort  to  get  more  of  it,  and  a  too  great  load  of  carbonic  acid  to 
an  effort  to  get  rid  of  it ;  but  the  facts  are  opposed  to  the  existence  of  any 
such  teleological  adaptation.  It  is  obvious,  however,  that  a  lack  of  oxygen 
and  an  excess  of  carbonic  acid  affect  the  respiratory  centre  in  very  different 
ways,  and  that  in  ordinary  cases  of  interference  with  the  interchange  in  the 
lungs,  as  in  deficient  aeration,  it  is  the  lack  of  oxygen  which  plays  the 
principal  part  in  developing  the  abnormal  respiratory  movements.  We 
may  infer  that  it  too  is  chiefly  concerned  in  regulating  the  more  normal 
respiration,  but  cannot  as  yet  say  what  is  the  exact  share  so  be  attributed  to 
the  carbonic  acid. 

We  may  here  point  out  that  it  is  not  to  be  supposed  that  each  breath  is 
determined  by  the  condition  of  the  blood  flowing  through  the  capillaries  of 
the  medulla  at  the  moment  preceding  that  breath  ;  it  is  not  to  be  imagined 
that  each  breath  is  the  result  of  the  lack  of  oxygen  felt  immediately  before. 
On  the  contrary,  as  we  have  previously  urged,  the  respiratory  centre,  like 
the  cardiac  substance,  is  an  automatic  centre;  the  respiratory  impulses  issue 
from  it  in  rhythmic  series  as  a  result  of  the  molecular  changes  of  the 
metabolism  going  on  in  its  substance  ;  and  whatever  affects  that  rhythm, 
whether  few  or  many  beats  be  influenced,  produces  its  result  by  modifying 


THE  NERVOUS  MECHANISM    OF  RESPIRATION.  383 

that  metabolism.  A  lack  of  oxygen  in  the  blood,  or  a  nervous  impulse 
along  an  afferent  fibre,  both  affect  the  centre  by  modifying  its  metabolism ; 
but  each  probably  affects  it  in  a  different  way.  It  is  beyond  our  present 
knowledge  to  explain  how  either  the  one  or  the  other  acts.  We  may  imagine 
that  a  lack  of  oxygen  on  the  other  hand  has  a  more  profound  effect  in  modi- 
fying the  whole  complex  series  of  metabolic  changes,  the  whole  chain  of 
building  up  and  breaking  down  processes,  thus  in  some  way  or  other  ren- 
dering the  whole  edifice,  so  to  speak,  more  unstable ;  and  that  an  afferent 
augmenting  impulse  (and  possibly  an  excess  of  carbonic  acid)  acts  rather 
after  the  fashion  of  what  we  are  accustomed  to  call  a  stimulus,  and  fires  off 
a  larger  amount  of  the  already  stored  up  explosive  compounds.  And  we 
may  further  imagine  that  the  special  feature  of  the  substance  of  the  respira- 
tory centre  is  that  the  metabolism  is  so  arranged  as  to  be  thus,  unlike  that  of 
other  living  substances,  rendered  unstable  and  more  explosive,  not  simply 
diminished  or  deadened  by  a  lack  of  oxygen.  But  these  as  yet  are  matters 
of  speculation. 

We  may,  perhaps,  add  that,  under  various  nutritive  conditions,  the  sensi- 
tiveness of  the  metabolism  of  the  respiratory  centre  to  lack  of  oxygen  may 
vary  widely.  Thus,  while  undoubtedly  under  the  normal  nutritive  condi- 
tions afforded  by  the  ordinary  supply  of  normal  blood  to  the  medulla,  lack 
of  oxygen  in  that  blood  at  once  provokes  increased  respiratory  movements, 
it  need  not  do  so  under  other  nutritive  conditions  of  the  medulla.  By  trans- 
fusion a  large  proportion  of  the  haemoglobin-holding  blood  may  in  an  animal 
be  gradually  replaced  by  hsemoglobinless  normal  saline  solution.  In  such  a 
case  the  amount  of  oxygen  brought  to  the  medulla  by  the  diluted  blood 
must  be  greatly  diminished,  and  yet  if  the  change  be  made  sufficiently 
slowly,  no  conspicuous  dyspnoea  is  produced  ;  under  the  new  strange  nutri- 
trive  conditions  of  the  diluted  blood  the  medulla  is  not  affected  in  the  same 
way  as  before  by  lack  of  oxygen. 

§  316.  There  are  reasons  for  thinking  that  conditions  of  the  blood,  other 
than  variations  in  the  amount  of  oxygen  and  carbonic  acid,  may  also  mate- 
rially affect  the  working  of  the  respiratory  centre.  It  is  a  matter  of  common 
experience  that  muscular  exertion,  especially  if  at  all  excessive,  increases 
the  respiratory  movements ;  violent  exercise  soon  puts  a  man  "  out  of 
breath."  This  increased  activity  of  the  respiratory  centre  is  in  large  meas- 
ure at  all  events  caused  by  the  character  of  the  blood  which  during  and  for 
some  little  time  after  the  movements  is  carried  to  the  medulla,  and  not  by 
any  nervous  impulses  sent  up  to  the  medulla  from  the  contracting  muscles. 
This  is  shown  by  the  fact  that  if  in  an  animal  the  spinal  cord  be  divided  in 
the  dorsal  or  lumbar  region  and  the  hind  limbs  be  powerfully  tetanized,  the 
respiratory  movements  are  increased  ;  the  animal  pants  as  it  would  do  if  it 
had  been  running.  In  such  a  case  the  only  connection  between  the  hind 
limbs  and  the  respiratory  centre  is  through  the  blood ;  it  must  be  some 
change  in  the  blood  caused  by  the  muscular  contractions  which  affects  the 
medulla  when  the  blood  passes  from  the  hind  limbs  to  be  distributed  by  the 
heart  to  the  medulla.  Now  when  a  muscle  contracts,  its  consumption  of 
oxygen  and  production  of  carbonic  acid,  especially  the  latter  (§  60),  are  in- 
creased ;  the  blood  leaving  the  muscle  is  more  venous  than  usual.  Hence, 
when  many  muscles  are  contracting  powerfully,  the  blood  carried  to  the 
right  side  of  the  heart  is  more  venous  than  usual ;  and  we  might  expect 
that  it  is  this  unusually  venous  blood  failing  to  be  adequately  arterialized  in 
the  lungs,  and  hence  reaching  the  medulla  from  the  left  side  of  the  heart  in 
a  more  venous,  less  completely  arterialized,  condition  than  usual,  which  stirs 
up  the  respiratory  centre  to  increased  activity. 

On  examination,  however,  it  is  found  that  the  blood  leaving  the  left  side 


^84  RESPIRATION. 

of  the  heart  in  such  cases  is  not  less  arterialized,  but,  if  anything,  more 
arterialized  than  usual.  The  increased  respiratory  movements  induced  by 
the  changed  blood  soon  prove  sufficient,  or  even  more  than  sufficient,  to  give 
the  blood  the  extra  quantity  of  oxygen  and  to  remove  the  extra  quantity  of 
carbonic  acid.  Obviously  the  blood  corning  from  the  tetanized  muscles 
affects  the  respiratory  centre  by  virtue  of  some  quality  which,  unlike  that 
due  to  the  deficiency  of  oxygen  or  excess  of  carbonic  acid,  is  not  imme- 
diately affected  by  the  passage  through  the  lungs.  Whether  the  quality  in 
question  be  dependent  on  an  excess  of  sarcolactic  acid,  or  on  some  other 
product  or  products  of  muscular  metabolism,  we  do  not  as  yet  know.  But 
the  fact  that  substances  in  the  blood  may  so  affect  the  respiratory  centre  is 
interesting,  since  it  shows  by  how  many  safeguards  the  working  of  the  re- 
spiratory centre  is  carefully  adapted  to  the  needs  of  the  economy. 

Thus  a  change  in  the  circumstances  surrounding  an  animal  body,  or  a 
change  in  the  body  itself,  may,  in  one  or  more  of  several  ways — by  acting  as 
a  stimulus  to  some  afferent  nerves  and  so  sending  up  afferent  nervous  im- 
pulses to  the  respiratory  centre,  or  by  interfering  with  the  interchange  of 
gases  in  the  lungs,  or  by  otherwise  altering  the  proportion  of  the  gases  pres- 
ent in  the  blood  reaching  the  respiratory  centre,  or  by  generating  or  increas- 
ing in  that  blood  some  substance  or  substances  tending  to  affect  the  nutrition 
of  the  respiratory  centre — affect  the  working  of  the  all-important  breathing 
mechanism.  And  the  affection  so  wrought  has  generally  an  adaptative  cha- 
racter; it  generally  tends  to  protect  the  organism  against  the  evil  effects  of 
the  change. 

§  317.  Apncea,  When  we  attempt  to  hold  our  breath,  we  find  that  we 
can  do  this  for  a  limited  time  only  ;  sooner  or  later  a  breath  must  come  ;  but, 
as  is  well  known,  the  time  during  which  we  can  remain  without  breathing 
may  on  occasion  be  much  prolonged,  if  we  first  of  all  take  a  series  of  deep 
breaths.  It  is  probable,  though  perhaps  not  distinctly  proved,  that  when 
we  breathe  voluntarily,  or  when  by  an  act  of  the  will  we  hold  the  respiratory 
apparatus  in  any  one  respiratory  phase,  the  nervous  impulses,  generated  by 
the  will,  do  not  pass  down  by  a  direct  and  independent  course  to  the  respira- 
tory muscles,  but  that  the  will  makes  use  or  modifies  the  activity  of  the 
medullary  and  spinal  nervous  respiratory  mechanisms.  The  breath  sooner 
or  later  inevitably  follows  because  at  last  the  natural  impulses  proceeding 
from  the  respiratory  centre  become  too  imperious  to  be  any  longer  held  in 
check  by  the  impulses  of  volition  passing  down  to  the  centre  from  the  brain. 
The  fact  that  a  series  of  deep  breaths,  a  thorough  ventilation  of  the  lungs, 
postpones  the  victory  of  the  unconscious  centre,  shows  that  such  a  ventila- 
tion in  some  way  delays  the  development  of  the  natural  respiratory  impulses. 
A  similar  but  still  more  marked  delay  may  often  be  seen  in  an  animal 
under  artificial  respiration.  If  in  a  rabbit  artificial  respiration  is  carried  on 
very  vigorously  for  a  while,  and  then  suddenly  stopped,  the  animal  does  not 
immediately  begin  to  breathe.  For  a  variable  period  no  respiratory  move- 
ments at  all  take  place,  and  breathing  when  it  does  begin  occurs  gently  and 
normally,  only  passing  into  dyspnoea  if  the  animal  is  unable  to  breathe  of 
itself;  a'nd  even  then  the  transition  is  quite  gradual.  Evidently  during  this 
period  the  respiratory  centre  is  in  a  state  of  complete  rest,  no  explosions  are 
taking  place,  no  respiratory  impulses  are  being  generated,  and  the  quiet 
transition  from  this  condition  to  that  of  normal  respiration  shows  that  the 
subsequent  generation  of  impulses  is  attended  by  no  great  disturbance.  Not 
only  is  the  centre  at  rest,  but  it  is  less  irritable  than  the  normal ;  impulses 
along  t\ie  vagus  or  other  nerves  which  otherwise  would  produce  respiratory 
explosions  are  now  ineffectual.  This  state  of  things  is  known  as  that  of 
apncea,  the  converse  of  dyspnoea ;  and  the  longer  pause  in  breathing  men- 


THE  NERVOUS  MECHANISM  OF  RESPIRATION.  385 

tioned  above  as  possible  after  unusual  ventilation  of  the  lungs  may  be  re- 
garded as  a  brief  apnoea. 

Now  it  seemed  natural  to  suppose  that  such  a  state  of  rest  of  the  respira-- 
tory  centre  was  brought  about  by  the  more  than  necessarily  ample  supply  of 
oxygen  afforded  by  the  previous  increased  inspiratory  movements ;  and, 
indeed,  it  was  maintained  that  apnoea  was  the  result  of  too  great,  just  as 
dyspnoea  is  the  result  of  too  little,  arterialization  of  the  blood  reaching  the 
respiratory  centre.  It  was  argued  that  owing  to  the  increased  vigor  of  the 
artificial  respiratory  movements  the  haemoglobin  of  the  arterial  blood,  which 
in  normal  breathing  is  not  quite  saturated  with  oxygen,  became  almost  com- 
pletely so,  and  that  at  the  same  time  the  quantity  of  oxygen  simply  dissolved 
in  the  blood  became  largely  increased  and  its  tension  largely  augmented^ 
But  there  are  reasons  which  render  such  a  view  untenable.  In  the  first 
place  there  is  no  direct  and  satisfactory  proof  that  in  apnoea  the  arterial 
blood  is  overloaded  with  oxygen  as  supposed  ;  indeed,  during  the  course  of 
apnoea  before  it  has  come  to  an  end  the  blood  becomes  distinctly  less  arterial, 
more  venous  than  usual.  In  the  second  place  apnoea  if  not  entirely  impossi- 
ble, is  much  more  difficult  to  bring  about  when  both  vagus  nerves  are  divided, 
and  if  it  does  occur  after  section  of  the  vagus  nerves  has  not  the  same  cha- 
racters as  ordinary  apncea.  Now,  when  artificial  respiration  is  being  carried 
on,  section  of  the  vagus  nerves  can  have  no  effect  on  the  quantity  of  oxygen 
taken  up  by  the  blood  in  the  lungs.  But  the  vagus  nerves  are  the  channel 
of  impulses  affecting  the  respiratory  centre,  and  this  relation  of  the  apnoea 
to  the  vagus  nerves  suggests  another  and  different  interpretation  of  apnoea. 
As  we  have  seen,  expansion  of  the  lung  by  acting  in  some  way  or  other  on 
the  pulmonary  terminations  of  the  vagus  nerve  sends  up  along  that  nerve 
impulses  which  inhibit  inspiration.  And  it  is  argued  that  repeated  forcible 
inflations  of  the  lungs  produce  apnoea  by  generating  potent  inhibitory  im- 
pulses, which  by  a  kind  of  summation  of  their  effects  in  the  medulla  stop  for 
a  while  the  generation  of  respiratory  impulses  in  the  respiratory  centre. 
This  conclusion,  moreover,  is  strongly  supported  by  the  fact  that  an  apnoea 
may  be  produced,  so  long  as  the  vagus  nerves  are  intact,  by  forcible  artificial 
respiration  with  hydrogen  instead  of  atmospheric  air  ;  in  other  words,  the 
inhibitory  impulses  generated  in  the  vagus  nerves  by  the  inflation  are  suffi- 
cient wholly  to  neutralize  the  development  of  respiratory  impulses  which 
the  deficient  arterialization  of  the  blood  would  otherwise  have  produced. 
The  exact  nature  and  development  of  such  a  summation  of  inhibitory  im- 
pulses, especially  in  the  presence  of  correlative  augmentative  impulses  called 
forth  by  the  corresponding  successive  collapses  of  the  lungs,  is  too  complex  a 
matter  to  be  dwelt  on  here.  Moreover,  an  apnoea  may  be  produced,  though, 
as  we  have  said,  with  difficulty  after  section  of  both  vagus  nerves ;  but  in 
this  case  air  and  not  hydrogen  must  be  used  for  inflation,  the  use  of  the 
latter,  in  contrast  to  the  result  when  the  nerves  are  intact,  leading  to  dysp- 
noea. The  subject  cannot  as  yet  be  considered  as  fully  cleared  up.  That 
apnoea  as  ordinarily  produced  is  in  some  way  the  result  of  inhibitory  im- 
pulses generated  by  the  inflations  can,  however,  hardly  be  doubted. 

§318.  Secondary  respiratory  rhythm — Cheyne-Stokes  respiration.  A  re- 
markable abnormal  rhythm  of  respiration,  first  observed  by  Cheyne  but 
afterward  more  fully  studied  by  Stokes,  and  hence  called  by  their  combined 
names,  occurs  in  certain  pathological  cases.  The  respiratory  movements 
gradually  decrease  both  in  extent  and  rapidity  until  they  cease  altogether, 
and  a  condition  of  apnoea,  lasting  it  may  be  for  several  seconds,  ensues. 
This  is  followed  by  a  feeble  respiration,  succeeded  in  turn  by  a  somewhat 
stronger  one,  and  thus  the  respiration  returns  gradually  to  the  normal,  or 
may  even  rise  to  hyperpncea  or  slight  dyspnoea,  after  which  it  again  declines 

25 


386  RESPIRATION. 

in  a  similar  manner.  A  secondary  rhythm  of  respiration  is  thus  developed, 
periods  of  normal  or  slight  dyspnoeic  respiration  alternating  by  gradual 
transitions  with  periods  of  apnoea.  The  cause  of  the  phenomena  is  not  thor- 
oughly understood.  Whether  the  waning  and  waxing  of  the  respiratory 
movements  be  due  to  corresponding  rhythmic  changes  in  the  nutrition  of  the 
respiratory  centre  itself,  or  to  a  rhythmic  increase  and  decrease  of  inhibitory 
impulses  playing  upon  that  centre  from  other  parts  of  the  body,  for  instance 
from  higher  regions  of  the  brain,  has  not  yet  been  settled.  It  frequently  appears 
in  connection  with  a  fatty  condition  of  the  heart,  but  has  been  met  with  in 
various  maladies.  Closely  similar  phenomena  have  been  observed  during 
sleep,  under  perfectly  normal  conditions ;  and  this  fact  is  rather  in  favor  of 
the  latter  of  the  two  explanations  just  given.  The  phenomena  present  a 
striking  analogy  with  the  "  groups  "  of  heart-beats  so  frequently  seen  in  the 
frog's  ventricle  placed  under  abnormal  circumstances. 

THE  EFFECTS  OF  CHANGES  IN  THE  COMPOSITION  AND  PRESSURE 
OF  THE  AIR  BREATHED. 

§  319.  The  preceding  sections  have  shown  us  that  the  respiratory 
mechanism  is  arranged  to  work  satisfactorily  when  the  lungs  are  adequately 
supplied  with  air  of  the  ordinary  composition  of,  and  at  the  ordinary  pres- 
sure of,  the  atmosphere.  We  have  further  seen  that  the  mechanism  can 
adapt  itself  within  certain  limits  to  changes  in  the  composition  and  pressure 
of  the  air  supplied.  We  may  now  consider  briefly  what  takes  place  when 
those  limits  are  overstepped.  The  most  striking  effects  are  seen  when,  on 
account  of  occlusion  of  the  trachea,  or  by  breathing  in  a  confined  space,  or 
for  other  reasons,  a  due  supply  of  air  not  being  obtained,  normal  respira- 
tion gives  place,  through  an  intermediate  phase  of  dyspnoea,  to  the  condition 
known  as  asphyxia ;  this,  unless  remedial  measures  be  taken,  rapidly  proves 
fatal. 

Asphyxia.  As  soon  as  the  blood  becomes  less  arterial,  more  venous  than 
normal,  the  respiratory  movements  become  deeper  and  at  the  same  time  more 
frequent ;  both  the  inspiratory  and  expiratory  phases  are  exaggerated,  the 
supplementary  muscles  spoken  of  (§  276)  are  brought  into  play,  and  the  rate 
of  the  rhythm  is  hurried.  These  effects,  as  we  have  seen,  are  chiefly  to  be 
ascribed  to  the  deficiency  of  oxygen  in  the  blood. 

As  the  blood  continues  to  become  more  and  more  venous  the  respiratory 
movements  continue  to  increase  both  in  force  and  frequency,  a  larger  num- 
ber of  muscles  being  called  into  action  and  that  to  an  increasing  extent. 
Very  soon,  however,  it  may  be  observed  that  the  expiratory  movements  are 
becoming  more  marked  than  the  inspiratory.  Every  muscle  which  can  in 
any  way  assist  in  expiration  is  in  turn  brought  into  play ;  and  at  last  almost 
all  the  muscles  of  the  body  are  involved  in  the  struggle.  The  orderly  ex- 
piratory movements  culminate  in  expiratory  convulsions,  the  order  and 
sequence  of  which  are  obscured  by  their  violence  and  extent.  That  these 
convulsions,  through  which  dyspnoea  merges  into  asphyxia,  are  due  to  a 
stimulation  (by  the  venous  blood)  of  the  medulla  oblongata,  is  proved  by 
the  fact  that  they  fail  to  make  their  appearance  when  the  spinal  cord  has 
been  previously  divided  below  the  medulla,  though  they  still  occur  after 
those  portions  of  the  brain  which  lie  above  the  medulla  have  been  removed. 
It  is  usual  to  speak  of  a  "convulsive  centre"  in  the  medulla,  the  stimula- 
tion of  which  gives  rise  to  these  convulsions ;  but  if  we  accept  the  existence 
.of  such  a  centre  we  must  at  the  same  time  admit  that  it  is  connected  by  the 
.closest  ties  with  the  normal  expiratory  division  of  the  respiratory  centre, 
gince  every  intervening  step  may  be  observed  between  a  simple  slight  ex- 


EFFECTS  OF  CHANGES  IN  COMPOSITION  OF  AIR  387 

piratory  movement  of  normal  respiration  and  the  most  violent  convulsion  of 
asphyxia.  An  additional  proof  that  these  convulsions  are  carried  out  by 
the  agency  of  the  medulla  is  afforded  by  the  fact  that  convulsions  of  a 
wholly  similar  character  are  witnessed  when  the  supply  of  blood  to  the 
medulla  is  suddenly  cut  off  by  ligaturing  the  bloodvessels  of  the  head.  In 
this  case  the  nervous  centres,  being  no  longer  furnished  with  fresh  blood, 
become  rapidly  asphyxiated  through  lack  of  oxygen,  and  expiratory  con- 
vulsions quite  similar  to  those  of  ordinary  asphyxia,  and  preceded  like  them 
by  a  passing  phase  of  dyspnoea,  make  their  appearance.  Similar  "  anaemic  " 
convulsions  are  seen  after  a  sudden  and  large  loss  of  blood  from  the  body  at 
large,  the  medulla  being  similarly  stimulated  by  the  lack  of  arterial  blood- 
In  ordinary  fainting,  which  is  loss  of  consciousness  due  to  an  insufficient 
supply  of  blood  to  the  brain,  the  diminution  of  blood-supply  is  not  great 
enough  to  produce  these  convulsions. 

Such  violent  efforts  speedily  exhaust  the  nervous  system  ;  and  the  convul- 
sions after  being  maintained  for  a  brief  period  suddenly  cease  and  are  fol- 
lowed by  a  period  of  calm.  The  calm  is  one  of  exhaustion  ;  the  pupils, 
dilated  to  the  utmost,  are  unaffected  by  light ;  touching  the  cornea  calls 
forth  no  movement  of  the  eyelids,  and,  indeed,  no  reflex  actions  can  any- 
where be  produced  by  the  stimulation  of  sentient  surfaces.  All  expiratory 
active  movements  have  ceased  ;  the  muscles  of  the  body  are  flaccid  and 
quiet ;  and  though  from  time  to  time  the  respiratory  centre  gathers  sufficient 
energy  to  develop  respiratory  movements,  these  resemble  those  of  quiet 
normal  breathing,  in  being,  as  far  as  muscular  actions  are  concerned,  almost 
entirely  inspiratory  They  occur  at  long  intervals,  like  those  after  section 
of  the  vagi ;  and  like  them  are  deep  and  slow.  The  exhausted  respiratory 
centre  takes  some  time  to  develop  an  inspiratory  explosion  ;  but  the  impulse 
when  it  is  generated  is  proportionately  strong.  It  seems  as  if  the  resistance 
which  had  in  each  case  to  be  overcome  was  considerable,  and  the  effort  in 
consequence,  when  successful,  productive  of  a  large  effect. 

Very  soon  these  inspiratory  efforts  become  less  frequent ;  their  rhythm 
becomes  irregular ;  long  pauses,  each  one  of  which  seems  a  final  one,  are 
succeeded  by  several  somewhat  rapidly  repeated  inspirations.  The  pauses 
become  longer,  and  the  inspiratory  movements  shallower.  Each  inspiration 
is  accompanied  by  the  contraction  of  accessory  muscles,  especially  of  the 
face,  so  that  each  breath  becomes  more  and  more  a  prolonged  gasp.  The 
inspiratory  gasps  spread  into  a  convulsive  stretching  of  the  whole  body  ;  and 
with  extended  limbs,  and  a  straightened  trunk,  with  the  head  thrown  back, 
the  mouth  widely  open,  the  face  drawn,  and  the  nostrils  dilated,  the  last 
breath  is  taken  in. 

Thus  we  are  able  to  distinguish  three  stages  in  the  phenomena  which 
result  from  a  continued  deficiency  of  air:  1.  A  stage  of  dyspnoea,  charac- 
terized by  an  increase  of  the  respiratory  movements  both  of  inspiration  and 
expiration.  2.  A  convulsive  stage,  characterized  by  the  dominance  of  the 
expiratory  efforts,  and  culminating  in  general  convulsions.  3.  A  stage  of 
exhaustion,  in  which  lingering  and  long-drawn  inspirations  gradually  die 
out.  When  brought  about  by  sudden  occlusion  of  the  trachea  these  events 
run  through  their  course  in  about  four  or  five  minutes  in  the  dog  and  in 
about  three  or  four  minutes  in  the  rabbit.  The  first  stage  passes  gradually 
into  the  second,  convulsions  appearing  at  the  end  of  the  first  minute.  The 
transition  from  the  second  stage  to  the  third  is  somewhat  abrupt,  the  con- 
vulsions suddenly  ceasing  early  in  the  second  minute.  The  remaining  time 
is  occupied  in  the  third  stage. 

The  duration  of  asphyxia  varies  not  only  in  different  animals,  but  in  the 
same  animal  under  different  circumstances.  Newly  born  and  young  animals 


388  RESPIRATION. 

need  much  longer  immersion  in  water  before  death  by  asphyxia  occurs  than 
do  adults.  Thus  while  in  a  full-grown  dog  recovery  from  drowning  is 
unusual  after  one  and  a  half  minutes,  a  newborn  puppy  has  been  known  to 
bear  an  immersion  of  as  much  as  fifty  minutes.  The  cause  of  the  difference 
lies  in  the  fact  that  in  the  quite  young  or  rather  just-born  animal  the  respi- 
ratory changes  of  the  tissues  are  much  less  active.  These  consume  less 
oxygen,  and  the  general  store  of  oxygen  in  the  blood  has  a  less  rapid  de- 
mand made  upon  it.  The  respiratory  activity  of  the  tissues  may  also  be 
lessened  by  a  deficiency  in  the  circulation  ;  hence  bodies  in  a  state  of  syn- 
cope at  the  time  when  the  deprivation  of  oxygen  begins  can  endure  the  loss 
for  a  much  longer  period  than  can  bodies  in  which  the  circulation  is  in  full 
swing.  There  being  the  same  store  of  oxygen  in  the  blood  in  each  case,  the 
quicker  circulation  must  of  necessity  bring  about  the  speedier  exhaustion  of 
the  store.  So  also  anaesthetics  may  diminish  the  effects  and  delay  the  final 
results :  large  doses  of  anaesthetics  may  prevent  the  exaggerated  and  con- 
vulsive movements.  In  many  cases  of  drowning,  death  is  hastened  by  the 
entrance  of  water  into  the  lungs. 

By  training,  the  respiratory  centre  may  be  accustomed  to  bear  a  scanty 
supply  of  oxygen  for  a  much  longer  time  than  usual  before  dyspnoea  sets  in, 
as  is  seen  in  the  case  of  divers. 

The  phenomena  of  slow  asphyxia,  where  the  supply  of  air  is  gradually 
diminished,  are  fundamentally  the  same  as  those  resulting  from  a  sudden 
and  total  deprivation.  The  same  stages  are  seen,  but  their  development  takes 
place  more  slowly. 

§  320.  Deficiency  of  air  results  not  only  in  a  diminution  of  the  oxygen 
but  also  in  an  increase  of  the  carbonic  acid  of  the  blood.  We  have  seen, 
however  (§  315),  that  the  phenomena  of  asphyxia  are  in  the  main  due  to  the 
former,  and  that  the  accumulation  of  carbonic  acid  in  the  blood  has  subsid- 
iary effects  only. 

If  the  percentage  of  oxygen  in  the  inspired  air  be  increased  instead  of 
diminished,  the  total  pressure  of  the  atmosphere  remaining  the  same,  the 
partial  pressure  of  the  oxygen  alone  being  changed,  no  marked  results 
follow.  We  have  already  seen  (§  297)  that  the  percentage  of  oxygen  in  the 
ordinary  atmosphere  leaves  a  wide  margin  of  safety,  and  that  (§  317)  the 
phenomena  of  apncea  are  in  the  main  at  least  to  be  explained  as  the  result 
not  of  an  increase  in  the  oxygen  of  the  blood  but  of  nervous  impulses  ascend- 
ing the  vagus  nerves.  We  have  no  satisfactory  evidence  that,  provided  the 
respiratory  mechanism  is  in  good  working  order,  an  increase  of  oxygen  in 
the  inspired  air  even  to  a  whole  atmosphere  seriously  modifies  the  respiratory 
act ;  and  it  may  be  doubted  whether  any  effect  is  produced  even  when  the 
mechanism  is  impaired. 

§  321.  The  composition  of  the  atmosphere,  the  pressure  remaining  the 
same,  may  be  modified  by  the  introduction  of  foreign  gases.  To  some  of 
these  the  respiratory  mechanism  is  indifferent ;  for  instance,  hydrogen  may 
be  substituted  for  nitrogen  without  any  change  in  the  respiration,  provided, 
of  course,  that  the  oxygen  is  not  diminished.  Other  gases  may  produce 
poisonous  effects,  either  by  interfering  with  some  of  the  respiratory  processes 
or  in  other  ways.  Thus  carbon  monoxide,  by  combining  with  the  haemo- 
globin of  the  red  corpuscles,  and  so  preventing  the  corpuscles  from  acting 
as  oxygen-carriers,  produces  asphyxia  through  deficiency  of  oxygen.  Sul- 
phuretted hydrogen  interferes  with  the  oxygenation  of  the  blood  by  acting 
as  a  reducing  agent.  Some  gases  while  allowing  the  ordinary  respiratory 
changes  of  the  blood  to  go  on  as  usual  produce  toxic  effects  by  acting  on  one 
or  other  of  the  tissues.  Thus,  as  we  have  seen,  an  excess  of  carbonic  acid 
in  the  blood  seems  to  have  a  special  effect  on  the  central  nervous  system  and 


EFFECTS  OF  CHANGES  IN  ATMOSPHERIC  PRESSURE.        389 

so  acts  as  a  narcotic  poison.  The  peculiar  effects  of  nitrous  oxide  (laughing 
gas)  are  similarly  due  to  the  direct  action  of  the  gas  in  the  blood  on  the 
central  nervous  system.  Some  gases  are  irrespirable  and  may  interfere  with 
respiration,  even  causing  suffocation,  on  account  of  their  causing  spasm  of 
the  glottis,  and  this  is  said  to  be,  to  a  certain  extent,  the  case  with  an  atmos- 
phere which  is  wholly  or  largely  composed  of  carbonic  acid. 

§  322.  The  effects  of  changes  in  atmospheric  pressure.  Diminution  of 
pressure.  The  partial  pressure  of  the  oxygen  in  the  inspired  air  may  be 
changed,  not  only  by  altering  the  composition  of  the  air  entering  at  the 
ordinary  atmospheric  pressure,  but  also  by  altering  the  total  pressure  of  the 
atmosphere  without  changing  its  composition.  The  results  of  the  latter  are, 
however,  complicated ;  we  have  then  to  deal  not  merely  with  the  effects  on 
the  interchange  of  gases  in  the  lungs  but  with  the  effects  on  the  whole 
organism.  All  the  complicated  machinery  of  the  body  is  adapted  and  ar- 
ranged to  work  under  what  we  may  call  ordinary  atmospheric  pressure,  that 
is  to  say,  within  the  limits  of  760  mm.  mercury  at  the  sea  level  and  about 
500  mm.,  corresponding  to  an  altitude  of  6000  feet,  this  being  the  range  of 
ordinary  human  dwellings.  Any  great  increase  or  decrease  of  pressure  be- 
yond these  limits  will  affect  not  only  the  exit  of  carbonic  acid  from  and  the 
entrance  of  oxygen  into  the  blood,  but,  in  varying  degree,  all  the  physical 
and  chemical  processes  of  the  body.  A  gross  instance  of  this  is  seen  when 
an  animal  is  suddenly  subjected  to  a  great  diminution  of  pressure,  as  when 
it  is  placed  in  the  receiver  of  an  air-pump  and  the  receiver  rapidly  ex- 
hausted. The  animal  is  soon  thrown  into  fatal  convulsions,  which  are  in 
part,  but  only  in  part,  due  to  the  liberation  of  gas  from  the  blood  within 
the  bloodvessels  ;  the  gas  so  set  free  mechanically  interferes  with  the  circula- 
tion, as  by  obstructing  the  play  of  the  cardiac  valves,  or  by  plugging  the 
smaller  bloodvessels,  and  thus  helps  to  bring  the  machine  to  a  standstill. 
The  free  gas  found  in  the  vessels  upon  examination  after  death  is  said  to  be 
composed  chiefly  of  nitrogen,  the  carbonic  acid  and  the  oxygen,  which  pro- 
bably were  also  set  free,  having  been  reabsorbed  before  the  examination  was 
made. 

But,  quite  apart  from  gross  effects  of  this  kind,  it  is  very  obvious  that  the 
organism  must  in  many  ways  suffer  from  a  diminution  of  pressure.  The 
complex  and  delicately  balanced  vascular  system  is  constructed  to  work  at 
the  ordinary  atmospheric  pressure.  The  force  of  the  heart-beat  and  the 
tonic  contraction  of  the  small  arteries  are,  so  to  speak,  pitched  to  meet  the 
influence  exerted  on  the  outside  of  the  bloodvessels  by  the  ordinary  pressure 
of  the  atmosphere;  and  any  great  diminution  of  that  pressure  must  produce 
a  greater  or  less  disarrangement  of  the  vascular  mechanism  until  it  is  coun- 
terbalanced by  some  compensating  changes.  And  a  little  reflection  will 
supply  many  other  instances. 

We  have  already  called  attention  (§  297)  to  the  fact  that,  the  total  pres- 
sure of  the  atmosphere  remaining  the  same,  the  partial  pressure  of  the  oxygen 
in  the  inspired  air  may  be  reduced  as  low  as  about  76  mm.  (10  per  cent.) 
without  seriously  modifying  the  respiration.  In  order  to  attain  this  dimi- 
nution of  the  partial  pressure  of  the  oxygen  without  changing  the  composi- 
tion of  the  atmosphere,  the  total  pressure  of  the  atmosphere  must  be  reduced 
to  the  limit  of  300  mm.,  corresponding  to  an  altitude  of  17,000  feet.  Now 
it  is  a  matter  of  common  experience  that  in  ascending  a  mountain  "distress" 
is  felt  long  before  such  an  altitude  is  reached.  The  distress  felt  on  such 
occasions  is  probably  due  not  so  much,  if  indeed  at  all  directly,  to  the 
diminution  of  oxygen  as  to  a  general  disarrangement  of  the  organism  and 
perhaps  more  particularly  of  the  vascular  system.  The  nose-bleeding  which 
is  so  frequent  an  occurrence  under  the  circumstances  shows  that  the  minute 


390  RESPIRATION. 

bloodvessels  more  directly  exposed  to  the  diminution  of  pressure  are  pro- 
foundly affected  by  it ;  and  what  is  true  of  them  is,  probably,  in  various 
ways  and  to  different  degrees,  true  of  the  whole  vascular  system.  The 
breathlessness  which  is  so  marked  a  feature  on  these  occasions  seems  due 
not  so  much  to  the  fact  that  the  blood  which  reaches  the  respiratory  nervous 
centres  is  deficient  in  oxygen,  as  to  the  fact  that  the  troubled  vascular  sys- 
tem fails  to  deliver  to  those  centres  their  blood  in  an  adequate  fashion. 

It  is  a  feature  of  the  vascular  system,  and  indeed  of  the  other  mechan- 
isms of  the  body  in  which  nervous  factors  intervene,  that  they  possess  the 
power  of  adapting  themselves  to  changed  conditions ;  and  as  is  well  known, 
the  human  organism  somewhat  rapidly  becomes  accustomed  to  these  mode- 
rate altitudes.  Practice  and  custom  have  far  less  effect,  though  they  have 
some,  on  the  more  fundamental  processes  depending  on  the  actual  supply  of 
oxygen  ;  and  it  is  at  the  extreme  altitudes,  where  in  addition  to  the  other 
troubles  a  deficiency  of  oxygen  definitely  makes  itself  felt,  that  the  body 
seems  to  fail  in  adapting  itself  to  the  new  circumstances. 

The  addition  of  these  troubles  not  directly  respiratory  in  nature,  when 
the  supply  of  oxygen  is  diminished  by  a  diminution  of  the  total  pressure, 
perhaps  explains  why  though  an  adequate  lowering  of  pressure  will  produce 
asphyxia,  that  asphyxia  is  somewhat  different  from  the  ordinary  asphyxia 
due  to  deprivation  of  air  or  oxygen.  Convulsions  which  are  essential  to 
ordinary  asphyxia  are  at  times  wholly  absent ;  the  nervous  system  under 
the  peculiar  conditions  does  not  respond  to  the  stimulus  of  the  lack  of 
oxygen ;  and  other  nervous  symptoms,  such  as  a  rapid  onset  of  feebleness 
amounting  almost  to  paralysis,  are  apt  to  make  their  appearance. 

§  323.  The  effects  of  increase  of  atmospheric  pressure.  These  are  in 
many  ways  remarkable.  Up  to  a  pressure  of  several  atmospheres  of  air,  the 
only  symptoms  which  present  themselves  are  those  somewhat  resembling 
narcotic  poisoning.  The  animal  becomes  sleepy  and  stupid,  the  result  pro- 
bably not  so  much  of  respiratory  changes,  as  of  the  effects  of  the  increased 
pressure  on  the  whole  organism  to  which  we  have  just  alluded.  At  a  pres- 
sure, however,  of  15  atmospheres  of  air,  or  what  amounts  to  the  same  thing, 
of  3  atmospheres  of  oxygen,  and  upward,  a  very  remarkable  phenomenon 
presents  itself.  The  animals  die  of  asphyxia  and  convulsions,  exactly  in  the 
same  way  as  when  oxygen  is  deficient.  Corresponding  with  this  it  is  found 
that  the  production  of  carbonic  acid  is  diminished.  That  is  to  say,  when 
the  pressure  of  the  oxygen  is  increased  beyond  a  certain  limit,  the  oxida- 
tions of  the  body  are  diminished,  and  with  a  still  further  increase  of  oxygen 
are  arrested  altogether.  The  oxidation  of  phosphorus  is  perhaps  analo- 
gous ;  at  a  high  pressure  of  oxygen  phosphorus  will  not  burn.  Not  only 
animals,  but  plants,  bacteria,  and  organized  ferments,  are  similarly  killed  by 
too  great  a  pressure  of  oxygen. 

THE  RELATIONS  OF  THE  RESPIRATORY  SYSTEM  TO  THE  VASCULAR  AND 

OTHER  SYSTEMS. 

§  324.  Many  events  in  the  body  show  the  influence  which  the  respiratory 
movements  exert  on  the  circulation.  When  the  brain  of  a  living  mammal 
is  exposed  by  the  removal  of  the  skull,  a  rhythmic  rise  and  fall  of  the  cere- 
bral mass,  a  pulsation  of  the  brain,  quite  distinct  from  the  movements  caused 
by  the  pulse  in  the  arteries  of  the  brain,  is  observed  ;  and  upon  examination 
it  will  be  found  that  these  movements  are  synchronous  with  the  respiratory 
movements,  the  brain  rising  up  during  expiration  and  sinking  during  inspira- 
tion. They  disappear  when  the  arteries  going  to  the  brain  are  ligatured,  or 
when  the  venous  sinuses  of  the  dura  mater  are  laid  open  so  as  to  admit  of  a 


RESPIRATORY   UNDULATIONS. 


391 


free  escape  of  the  venous  blood.  They  evidently  arise  from  the  expiratory 
movements  in  some  way  hindering  and  the  inspiratory  movements  assisting 
the  return  of  blood  from  the  brain.  We  have  already  (§  105)  stated  that 
during  inspiration  the  pressure  of  blood  in  the  great  veins  may  become 
negative,  i.  e.,  may  sink  below  the  pressure  of  the  atmosphere  ;  and  a  punc- 
ture of  one  of  these  veins  may  cause  death  by  air  being  actually  drawn  into 
the  vein  and  thus  into  the  heart  during  an  inspiratory  movement.  When 
the  veins  of  an  animal  are  laid  bare  in  the  neck  and  watched,  the  so-called 
pulsus  venosus  may  be  observed  in  them,  that  is,  they  swell  up  during  expira- 
tion and  diminish  again  during  inspiration.  And  indeed  a  little  considera- 
tion will  show  that  the  expansion  and  contraction  of  the  chest  must  have  a 
decided  effect  on  the  flow  of  blood  through  the  thoracic  portion  of,  and  thus 
indirectly  on  that  through  the  whole  of,  the  vascular  system. 

This  is  well  illustrated  by  the  effects  of  respiration  on  arterial  blood- 
pressure.  We  have  seen,  while  treating  of  the  circulation,  that  the  arterial 
blood-pressure  curves  are  marked  by  undulations,  which,  since  their  rhythm 
is  synchronous  with  that  of  the  respiratory  movements,  are  evidently  in 
some  way  connected  with  respiration.  Similar  undulations  may  be  observed 
in  the  pulse-tracings  taken  from  man. 


FIG.  107. 


Comparison  of  Blood-pressure  Curve  with  Curve  of  Intra-thoracic  Pressure.  (Dog.)  a  is  the 
blood-pressure  curve  taken  by  means  of  a  mercury  manometer  ;  it  shows  the  respiratory  undula- 
tion, the  slower  beats  on  the  descent  being  very  marked.  6  is  the  curve  of  intra-thoracic  pressure 
obtained  by  connecting  one  limb  of  a  manometer  with  the  pleural  cavity.  Inspiration  begins  at 
i,  expiration  at  e.  With  the  beginning  of  inspiration  (i)  the  expansion  of  the  chest  causes  a 
marked  fall  of  the  mercury  in  the  intra-thoracic  manometer;  but  the  effect  soon  diminishes,  since 
the  lessening  of  intra-thoracic  pressure  does  not  bear  on  the  manometer  alone,  but  on  the  lungs  also ; 
and  as  the  lungs  expand  more  and  more  the  fall  in  the  mercury  becomes  less  and  less  until  toward 
the  end  of  inspiration  the  curve  becomes  very  nearly  a  straight  line.  Conversely,  the  return  of 
the  chest  at  the  beginning  of  expiration  •€]  produces  at  first  a  marked  rise  of  the  mercury  in  the 
manometer  ;  but  this  soon  ceases  as  the  air  leaves  the  chest  and  the  lungs  shrink,  whereupon  the 
mercury  falls  slowly. 

When  these  undulations  of  the  blood-pressure  curve  are  compared  care- 
fully with  the  respiratory  movements  or  with  the  variations  of  intra-thoracic 
pressure,  what  is  most  commonly  observed  is  that  while  the  blood -pressure, 
on  the  whole,  rises  during  inspiration  and  falls  during  expiration,  neither  the 
rise  nor  the  fall  is  exactly  synchronous  with  either  inspiration  or  expiration. 
Fig.  107  shows  two  tracings  from  a  dog  taken  at  the  same  time,  one,  a,  being 
the  ordinary  blood-pressure  curve  from  the  carotid,  and  the  other,  b,  repre- 
senting the  condition  of  the  intra-thoracic  pressure  as  obtained  by  carefully 
bringing  a  manometer  into  connection  with  the  pleural  cavity.  On  com- 
paring the  two  curves  it  is  evident  that  neither  the  rise  nor  the  fall  of  arterial 
pressure  coincides  exactly  either  with  inspiration  or  with  expiration.  At 


392  RESPIRATION. 

the  beginning  of  inspiration  (i)  the  arterial  pressure  is  seen  to  be  falling;  it 
soon,  however,  begins  to  rise,  but  does  not  reach  the  maximum  until  some 
time  after  expiration  (e)  has  begun ;  the  fall  continues  during  the  remainder 
of  expiration,  and  passes  on  into  the  succeeding  inspiration.  This  suggests 
the  idea  that,  while  inspiration  tends  to  increase  and  expiration  to  diminish 
the  blood-pressure,  there  are  causes  at  work  which  in  each  case  delay  the 
effect. 

Extended  observations,  however,  show  that  such  a  relation  as  that  shown 
in  the  figure,  though  frequent,  is  not  constant.  In  fact,  the  effects  of  the 
respiratory  movements  on  blood-pressure  are  found  to  vary  very  widely 
according  as  the  respiration  is  quick  or  slow,  easy  and  shallow,  or  labored 
and  deep,  and  especially  as  the  air  enters  into  the  chest  readily  or  with  diffi- 
culty. Moreover,  respiratory  undulations  of  blood-pressure  are  seen  not 
only  with  natural  but  also  with  artificial  respiration  ;  in  the  latter  the 
mechanical  conditions  are  to  a  large  extent  the  reverse  of  those  of  the  former, 
and  might  fairly  be  expected  to  affect  the  circulation  in  a  different  way. 
The  causation  of  these  respiratory  undulations  is,  in  fact,  complex.  The 
respiratory  act  affects  the  vascular  system  in  several  different  ways,  and  the 
general  effect  varies  according  as  one  or  other  influence  is  predominant. 
These  several  actions  are  sufficiently  interesting  and  important  to  deserve 
discussion. 

§  325.  The  heart  and  great  bloodvessels  are,  like  the  lungs,  placed  in  the 
air-tight  thoracic  cavity,  and  are  subject  like  the  lungs  to  the  pumping  action 
of  the  respiratory  movements.  Were  there  no  lungs  present  in  the  chest, 
the  whole  force  of  the  expansion  of  the  thorax  in  inspiration  would  be 
directed  to  drawing  blood  from  the  extra-thoracic  vessels  toward  the  heart, 
and  conversely  in  expiration  the  effect  of  the  return  of  the  thorax  to  its 
previous  dimensions  would  be  to  drive  the  blood  thus  drawn  in  back  again 
from  the  heart  toward  the  extra-thoracic  vessels.  And,  even  in  the  presence 
of  the  lungs,  some  of  this  effect  is  still  felt.  The  main  purpose  and  the  main 
result  of  the  expansion  of  the  chest  in  inspiration  is,  of  course,  to  draw  air 
into  the  lungs ;  by  that  expansion  the  air  in  the  pulmonary  alveoli  is  rare- 
fied and  brought  to  a  lower  pressure  than  that  of  the  atmosphere  outside  the 
chest ;  and  the  difference  of  pressure  thus  set  up  leads  to  an  inrush  of 
inspired  air  until  an  equilibrium  of  pressure  is  established  between  the  air 
in  the  lungs  and  that  outside  the  chest.  Before,  however,  the  inspired  air 
can  fill  a  pulmonary  alveolus  the  elastic  walls  of  the  alveolus  have  to  be  dis- 
tended, and  that  distention  is  effected  by  means  of  the  pressure  which  causes 
the  inspired  air  to  enter.  Part  of  the  atmospheric  pressure,  in  fact,  which 
causes  the  entrance  of  the  air  into  the  lung  is  spent  in  overcoming  the  elas- 
ticity of  the  pulmonary  passages  and  cells.  So  that  while  by  the  inrush  of 
inspired  air  the  difference  of  pressure  between  the  air  inside  the  pulmonary 
alveoli  and  that  outside  the  chest,  brought  about  by  the  thoracic  expansion, 
is  completely  neutralized,  the  difference  between  the  pressure  to  which  the 
parts  lying  within  the  thorax,  but  outside  the  lungs,  are  exposed  and  that 
outside  the  chest  is  not  so  completely  neutralized.  The  pressure  on  these 
parts  always  falls  short  of  the  pressure  of  the  atmosphere  by  the  amount  of 
pressure  necessary  to  counterbalance  the  elasticity  of  the  pulmonary  passages 
and  alveoli.  Consequently,  any  structure  lying  within  the  thorax,  but  out- 
side the  lungs,  is  never,  even  at  the  conclusion  of  an  inspiration,  when  the 
lungs  are  filled  with  air,  subject  to  a  pressure  as  great  as  that  of  the  atmos- 
phere. And,  since  the  fraction  of  the  atmospheric  pressure  which  is  thus 
spent  in  distending  the  lungs  increases  as  the  lungs  become  more  and  more 
stretched,  it  follows  that  the  fuller  the  inspiration  the  greater  is  the  differ- 
ence between  the  pressure  on  structures  within  the  thorax,  but  outside  the 


RESPIRATORY   UNDULATIONS.  393 

lungs,  and  the  ordinary  pressure  of  the  atmosphere.  Now,  we  have  seen 
that  the  pressure  necessary  to  counterbalance  the  elasticity  of  the  lungs, 
when  they  are  completely  at  rest  (in  the  pause  between  expiration  and  in- 
spiration), is  in  man  about  5  to  7  mm.  of  mercury,  and  that  when  the  lungs 
are  fully  distended,  as  at  the  end  of  a  forcible  inspiration,  the  pressure  rises 
to  as  much  as  30  mm.  of  mercury.  Hence,  at  the  height  of  a  forcible  in- 
spiration the  pressure  exerted  on  the  heart  and  great  vessels  within  the 
thorax  is  30  mm.  less  than  the  ordinary  atmospheric  pressure  of  760  mm., 
and  even  when  the  chest  is  completely  at  rest,  at  the  end  of  an  inspiration, 
the  pressure  on  the  heart  and  great  vessels  is  slightly  (by  about  5  mm.  of 
mercury)  below  that  of  the  atmosphere.  We  may  add  that  any  obstacle  to 
the  free  ingress  of  the  inspired  air,  any  difficulty  in  the  full  expansion  of 
the  pulmonary  alveoli,  of  course  increases  the  negative  pressure  to  which 
the  thoracic  structures  outside  the  lungs  are  subjected  by  the  expansion  of 
the  chest.  Hence,  when  the  trachea  is  closed  a  very  large  part  of  the  thoracic 
expansion  is  directed  to  increasing  the  negative  pressure  around  the  heart 
and  great  bloodvessels. 

During  an  inspiration,  then,  the  pressure  around  the  heart  and  great 
bloodvessels  becomes  considerably  less  than  that  of  the  atmosphere  on  the 
vessels  outside  the  thorax.  During  expiration  this  pressure  returns  toward 
that  of  the  atmosphere,  but  in  ordinary  breathing  never  quite  reaches  it. 
It  is  only  in  forcible  expiration  that  the  pressure  on  the  thoracic  vascular 
organs  reaches  or  exceeds  that  of  the  atmosphere.  But- if  during  inspira- 
tion the  pressure  bearing  on  the  right  auricle  and  the  venae  cavse  become 
less  than  the  pressure  which  is  bearing  on  the  jugular,  subclavian,  and  other 
veins  outside  the  thorax,  this  must  result  in  an  increased  flow  from  the  lat- 
ter into  the  former.  Hence,  during  each  inspiration  a  larger  quantity  of 
blood  enters  the  right  side  of  the  heart.  This  probably  leads  to  a  stronger 
stroke  of  the  heart,  and  at  all  events  causes  a  larger  quantity  to  be  ejected 
by  the  right  ventricle ;  this  causes  a  larger  quantity  to  escape  from  the  left 
ventricle,  and  thus  more  blood  is  thrown  into  the  aorta,  and  the  arterial 
pressure  proportionately  increased.  During  expiration  the  converse  takes 
place.  The  pressure  on  the  intra-thoracic  bloodvessels  returns  to  the 
normal,  the  flow  of  blood  from  the  veins  outside  the  thorax  into  the  venae 
cava3  and  right  auricle  is  no  longer  assisted,  and  in  consequence  less  blood 
passes  through  the  heart  into  the  aorta,  and  arterial  pressure  falls  again. 
During  forced  expiration  the  intra-thoracic  pressure  may  be  so  great  as  to 
afford  a  distinct  obstacle  to  the  flow  from  the  veins  into  the  heart. 

The  effect  of  the  respiratory  movements  on  the  arteries  is  naturally  differ- 
ent from  that  on  the  veins.  During  inspiration  the  diminution  of  pressure 
in  the  thorax  around  the  aortic  arch  tends  to  expand  the  aortic  arch  and 
thus  to  check  the  onward  flow  of  blood  and  to  diminish  the  pressure  of  blood 
within  the  aorta.  During  expiration  the  increase  of  pressure  outside  the 
aortic  arch  of  course  tends  to  increase  also  the  blood-pressure  within  the 
aorta,  acting  in  fact  just  in  the  same  way  as  if  the  coats  of  the  aorta  them- 
selves contracted.  Thus,  as  far  as  arterial  blood-pressure  is  concerned,  the 
effects  of  the  respiratory  movements  on  the  great  veins  and  great  arteries 
respectively  are  antagonistic  to  each  other ;  the  effect  on  the  veins  being  to 
increase  arterial  pressure  during  inspiration  and  to  diminish  it  during  expira- 
tion, while  the  effect  on  the  arteries  is  to  diminish  arterial  pressure  during 
inspiration  and  to  increase  it  during  expiration.  But  we  should  naturally 
expect  the  effect  on  the  thin-walled  veins  to  be  greater  than  that  on  the 
stout,  thick-walled  arteries,  so  much  so  that  the  direct  effect  on  the  arteries 
may  be  neglected.  That  is  to  say,  we  should  expect  the  blood-pressure  to 
rise  during  inspiration  and  to  fall'during  expiration.  This,  as  we  have  seen, 


394  RESPIRATION. 

is  frequently  the  case,  and,  indeed,  when  the  breathing  is  deep  and  labored, 
and  especially  during  violent  and  sudden  respiratory  movements,  the  influ- 
ence in  this  direction  on  the  blood-pressure  curve  of  the  pumping  action  of 
the  chest  is  unmistakable. 

In  attempting,  however,  to  estimate  the  effect  of  the  respiratory  move- 
ments on  blood-pressure  we  must  bear  in  mind  what  is  taking  place  in  the 
abdomen.  In  inspiration  the  descent  of  the  diaphragm  compresses  the 
abdominal  viscera,  and  so,  while  at  the  very  first  it  drives  -a  quantity  of 
blood  onward  along  the  inferior  vena  cava,  subsequently  hinders  the  upward 
flow  from  the  abdomen  and  lower  limbs ;  at  the  same  time  by  compressing 
the  abdominal  aorta,  it  tends  to  raise  the  pressure  in  the  thoracic  aorta  and 
its  branches,  while  lowering  that  of  the  abdominal  aorta  and  its  branches. 
The  effect  of  easy  expiration  would  be  the  converse  of  this ;  but  in  forced 
expiration  the  pressure  of  the  contracting  abdominal  muscles  would,  as  in 
inspiration,  first  tend  to  drive  the  blood  onward  along  the  vena  cava,  but  sub- 
sequently to  hinder  the  flow  both  along  the  vena  cava  and  the  aorta.  The 
effect  of  the  abdominal  movements  therefore  is  mixed  and  variable,  and  their 
influence  on  the  blood -pressure  in  the  femoral  artery  must  be  different  from 
that  on  the  radial  artery  or  other  branch  of  the  thoracic  aorta.  It  is  dif- 
ficult to  predict  what  in  all  cases  the  effect  would  be ;  and  the  matter  can- 
not be  settled  by  eliminating  the  movements  of  the  diaphragm  through  sec- 
tion of  the  phrenic  nerves,  since  in  such  a  case  the  whole  working  of  the 
respiratory  pump  is  materially  affected. 

§  326.  In  addition  to  the  influence  thus  exerted  by  the  thoracic  move- 
ments on  the  great  veins  leading  to  and  the  great  arteries  leading  from  the 
heart,  we  have  to  consider  the  behavior  of  the  pulmonary  vessels  themselves 
under  the  varying  thoracic  pressure.  These,  like  the  vense  cava3  and  aorta, 
tend  to  expand  under  the  influence  of  the  inspiratory  expansion  of  the  chest, 
and  thus  to  become  fuller  of  blood,  very  much  as  they  would  if  the  whole 
lung  were  placed  under  a  large  cupping-glass.  The  first  effect  of  this  in- 
creased filling  of  the  pulmonary  vessels  would  be  to  retain  for  a  while  a 
certain  quantity  of  blood  in  the  lungs  and  thus  to  lessen  the  amount  falling 
into  the  left  auricle.  But  this  would  be  temporary  only,  and  the  widening 
of  the  pulmonary  vessels  would  speedily  produce  an  exactly  contrary  effect, 
namely,  an  increased  flow  through  the  lungs  due  to  the  diminished  resistance 
offered  by  the  widened  passages.  Conversely,  the  first  effect  of  expiration 
would  be  an  increased  flow  into  the  left  auricle  due  to  the  additional  quan- 
tity of  blood  driven  onward  by  the  partial  collapse  of  the  pulmonary  ves- 
sels, followed  by  a  more  significant  diminished  flow  caused  by  the  greater 
resistance  now  offered  by  the  narrower  vascular  channels.  Thus  the  effect 
of  inspiration  in  this  way  would  be  first  to  diminish  the  flow  into  the  left 
auricle  and  so  into  the  left  ventricle,  but  afterward,  for  the  rest  of  the 
inspiration  until  the  beginning  of  expiration,  to  increase  the  flow  into  the 
ventricle ;  while  conversely  the  effect  of  expiration  would  be  first,  for  a 
brief  period,  to  increase,  and  afterward,  during  the  rest  of  the  movement, 
to  diminish  the  flow  of  blood  into  the  left  ventricle.  Further,  while  this 
may  be  considered  as  the  effect  on  the  pulmonary  vessels,  large  and  small 
taken  altogether,  the  influence  both  of  the  thoracic  negative  pressure  during 
inspiration,  and  the  return  in  a  positive  direction  during  expiration,  will  bear 
more  on  the  thin-walled  pulmonary  veins  than  on  the  stouter  pulmonary 
artery ;  that  is  to  say,  as  inspiration  becomes  established,  there  will  be  a 
diminution  of  pressure'in  the  pulmonary  veins  greater  than  that  in  the  pul- 
monary artery,  and  this  will  be  an  additional  influence  favoring  the  flow  into 
the  left  ventricle ;  during  expiration  a  similar  difference  of  effect  will  be 
felt  in  the  contrary  direction.  During  the  increase  of  flow  into  the  ventricle 


RESPIRATOKY  UNDULATIONS.  395 

the  quantity  of  blood  ejected  at  each  stroke  will  increase,  and  each  stroke 
will  (§  148)  be  increased  in  vigor,  in  consequence  of  which  the  arterial 
pressure  will  rise.  Conversely,  during  the  decrease  of  flow  into  the  ventricle 
the  arterial  pressure  will  fall.  Hence  the  general  effect  of  the  movements 
of  the  chest  on  the  pulmonary  vessels  will  be  during  the  beginning  of  inspira- 
tion to  continue  the  lowering  of  arterial  pressure  which  was  taking  place 
during  expiration,  but  subsequently  to  raise  the  arterial  pressure;  and  con- 
versely, at  the  beginning  of  expiration  to  continue  the  rise  of  arterial  pres- 
sure which  was  taking  place  during  inspiration,  but  subsequently  to  lower 
the  arterial  pressure.  In  ordinary  breathing,  as  we  have  seen,  what  may  be 
considered  as  the  normal  relations  of  blood-pressure  to  the  respiratory  move- 
ments are  precisely  of  this  kind. 

§  327.  Effects  of  the  respiratory  movements,  however,  are  seen  not  only 
in  natural  but  also  in  artificial  respiration.  When,  for  instance,  in  an  animal 
under  urari,  artificial  is  substituted  for  natural  respiration,  undulations  of 
the  blood-pressure  curve,  synchronous  with  the  respiratory  movements,  are 
still  observed  (Fig.  108),  though  generally  less  in  extent  than  those  seen 
under  natural  conditions. 

Now  in  artificial  respiration,  the  mechanical  conditions  under  which  the 
thoracic  viscera  are  placed  as  regards  pressure,  are  the  exact  opposite  of 
those  existing  during  natural  respiration,  for  when  air  is  blown  into  the 
trachea  to  distend  the  lungs,  the  pressure  within  the  chest  is  increased 
instead  of  diminished.  Under  these  circumstances,  applying  the  consid- 
erations laid  down  in  the  preceding  paragraph  with  regard  to  natural 
respiration,  we  should  expect  to  find  that  while  the  first  effect  of  an  arti- 
ficial inspiration  would  be  to  drive  an  additional  quantity  of  blood  out 
of  the  lungs  into  the  left  ventricle,  and  thus  to  raise  arterial  pressure, 
this  would  be  in  turn  followed  by  a  fall  of  arterial  pressure  due  to  the 
increased  resistance  offered  both  to  the  passage  of  blood  through  the 
lungs  and  to  the  entrance  of  blood  through  the  vense  cavse  into  the 
right  auricle.  Conversely,  the  effect  of  the  succeeding  expiration  would 
be  an  initial  continuance  of  the  fall  of  arterial  pressure  succeeded  by  a 
rise.  In  other  words,  we  should  expect  to  find  in  artificial  respiration 
effects  exactly  the  reverse  of  those  which  we  find  in  normal  respiration ; 
and,  indeed,  in  many  curves  of  blood-pressure  taken  during  artificial 
respiration  this  is  the  case. 

Both  in  natural  and  in  artificial  respiration,  however,  the  features  of  the 
blood-pressure  curve  vary  according  as  the  breathing  is  hurried  or  slow, 
shallow  or  deep,  and  according  to  the  facility  with  which  air  enters  the 
chest,  so  much  so  that  at  times  the  blood-pressure  curves  of  natural  and 
artificial  respiration  may  closely  resemble  each  other.  And  a  little  con- 
sideration would  lead  us  to  expect  this. 

We  have  seen  that  the  rise  in  arterial  pressure  which  marks  the  respir- 
atory undulation  is,  in  the  main,  due  to  a  temporary  greater  amount  of 
blood  thrown  into  the  aorta  by  the  left  ventricle,  and  that,  correspondingly, 
the  fall  of  pressure  completing  the  undulation  is  in  the  main  due  to  a  tem- 
porary lessening  of  the  amount  so  thrown.  Though  the  causes  discussed  in 
§  325  undoubtedly  make  themselves  prominent  in  labored  and  violent  re- 
spiratory movements,  we  may  conclude  that  in  ordinary  respiration,  both 
natural  and  artificial,  the  main  events  producing  the  respiratory  undula- 
tions are  those  discussed  in  §  326.  We  may  restate  the  conclusions  of 
that  discussion  by  saying  that  the  respiratory  movements  affect  the  amount 
of  flow  of  blood  into  the  left  ventricle,  and  so  the  discharge  of  blood  from 
the  left  ventricle  into  the  aorta,  in  two  main  ways.  In  the  first  place, 
through  the  widening  or  narrowing  of  the  pulmonary  vessels  they  alter 


396  RESPIRATION. 

the  capacity  of  the  vessels  to  hold  blood  for  the  time  being.  In  the 
second  place,  in  consequence  of  the  difference  of  resistance,  occasioned  by 
the  widening  or  narrowing,  they  alter  the  rate  of  flow  through  the  pul- 
monary vessels.  The  first  factor  is  a  brief  and  passing  one ;  the  extra 
room  due  to  widening  is  soon  filled  up,  the  narrowed  vessels  soon  dis- 
charge the  quantity  which  they  can  no  longer  hold.  But  the  second 
factor  is  a  more  lasting  one ;  so  long  as  in  the  respiratory  movement  the 
vessels  remain  widened  or  narrowed  so  long  is  the  rate  of  flow  increased  or 
diminished.  These  two  factors  produce  opposite  effects,  and  hence  the  total 
result  of  any  particular  kind  of  respiration  will  depend  on  their  relative 
prominence.  With  quickly  repeated  respiratory  movements  the  first  factor 
comes  to  the  front ;  when  the  respiratory  movements  are  more  slowly  re- 
peated and  more  slowly  carried  out  the  second  factor  is  the  more  potent. 
Hence  it  comes  about  that  in  quickly  repeated  artificial  respiration  where 
the  first  factor  is  predominant,  and  the  prominent  effect  of  each  inflation 
is  to  diminish  the  capacity  of,  and  so  to  empty  the  pulmonary  vessels  and 
to  increase  the  flow  into  the  ventricle  whereby  the  pressure  rises  in  infla- 
tion, that  is  in  inspiration,  the  blood-pressure  curve  stimulates  that  of  a 
slowly  repeated  natural  respiration,  where  the  pressure  also  rises  in  in- 
spiration, but  where,  the  second  factor  being  predominant,  the  rise  of 
pressure  brought*  about  by  each  inspiration  is  due  mainly  to  the  more 
rapid  flow  through  the  widened  pulmonary  vessels.  And  other  illustra- 
tions of  a  like  kind  could  be  given. 

§  328.  Besides  the  mechanical  effects  of  the  respiratory  movements  the 
vascular  system  is  influenced  by  respiration  through  the  changes  in  the  gases 
of  the  blood. 

Changes  in  the  blood  may  affect,  on  the  one  hand,  the  vasomotor  system 
and,  on  the  other  hand,  the  heart.  They  may  further  affect  the  heart  either 
directly  by  acting  on  the  cardiac  tissues,  or  indirectly  by  means  of  the  inhib- 
itory and  augmentor  cardiac  nerves.  They  may  also,  probably,  affect  the 
peripheral  vessels,  not  only  through  vasomotor  nerves,  but  by  acting 
directly  on  the  walls  of  the  smaller  vessels.  We  have  indications  of  an 
action  of  respiration  on  the  cardio-inhibitory  system,  even  in  normal  quiet 
respiration.  One  striking  feature  of  the  respiratory  undulation  in  the  blood- 
pressure  curve  of  the  dog1  is  the  fact  that  the  pulse-rate  is  quickened  during 
the  rise  of  the  undulation  and  becomes  slower  during  the  fall.  (See  Fig.  107.) 
A  similar  influence  may  be  seen  in  pulse-tracings  taken  from  man.  The 
quickening  of  the  beat  might  be  considered  as  itself  partly  accounting  for 
the  rise  of  pressure,  or,  on  the  other  hand,  it  might  be  urged  that  the  in- 
creased flow  of  blood  which  causes  the  rise  of  pressure,  at  the  same  time 
leads  to  the  quickening  of  the  beat,  were  it  not  for  one  fact,  viz.,  that  the 
difference  is  at  once  done  away  with,  without  any  other  essential  change  in 
the  undulations,  by  section  of  both  vagus  nerves.  Evidently  the  slower  pulse 
during  the  fall  is  caused  by  a  coincident  stimulation  of  the  cardio-inhibitory 
centre  in  the  medulla  oblongata,  the  quicker  pulse  during  the  rise  being  due 
to  the  fact  that,  during  that  interval,  the  centre  is  comparatively  at  rest. 
We  have  here  indications  that,  while  the  respiratory  centre  in  the  medulla  . 
oblongata  is  at  work,  sending  out  rhythmic  impulses  of  inspiration  and  ex- 
piration, the  neighboring  cardio-inhibitory  centre  is,  as  it  were,  by  sympathy, 
thrown  into  an  activity  of  such  a  kind  that  its  influence  over  the  heart  waxes 
with  each  expiration  and  wanes  with  each  inspiration.  We  cannot  as  yet 
explain  exactly  the  manner  in  which  the  activity  of  the  one  centre  influ- 
ences that  of  the  other ;  it  may  be  that  during  the  expiratory  phase  the 

1  In  the  rabbit,  the  respiratory  undulations,  though  well  marked,  present  a  very  small 
difference  of  pulse-rate  in  the  rise  and  fall. 


RESPIRATORY  UNDULATIONS.  397 

blood  reaching  the  medulla  is  not  quite  so  well  arterialized,  especially  as  far 
as  the  escape  of  carbonic  acid  is  concerned,  as  during  the  inspiratory  phase, 
and  that  the  cardio-inhibitory  centre  is  sufficiently  sensitive  to  appreciate  the 
slight  difference;  but  of  this  we  cannot  be  sure. 

§  329.  When  through  interference  with  the  pulmonary  interchange  the 
blood  sent  out  from  the  left  ventricle  becomes  and  continues  to  be  less 
arterialized  than  usual,  the  effects  on  both  the  heart  and  the  vasomotor 
system  become  conspicuous.  The  rhythm  of  the  heart-beats  is  most  distinctly 
slowed.  This,  under  ordinary  circumstances,  when  the  vagus  nerves  are 
intact,  is  probably  in  part  the  result  of  vagus  inhibition,  the  venous  blood, 
as  suggested  above,  stimulating  the  cardio-inhibitory  centre  in  the  medulla. 
But  the  slowing  is  not  wholly  caused  in  this  way,  for  it  is  still  conspicuous  in 
an  animal  placed  under  urari  and  with  both  vagus  nerves  divided.  Compare 
curves  3  and  4  with  1  and  2  in  Fig.  108.  How  this  slowing  is  brought  about 
is  not  very  clear.  When  venous  blood  is  sent  through  an  excised  heart,  the 
beat  is,  it  is  true,  slowed,  but  it  is  also  and  still  more  conspicuously  weak- 
ened. Now  when  the  blood  becomes  too  venous,  as  is  shown  in  Fig.  108, 
even  after  the  action  of  the  vagus  nerves  has  been  eliminated  by  section  and 
also  by  urari,  the  slowing  is  out  of  proportion  to  the  weakening,  since,  as  we 
shall  presently  see,  the  blood-pressure  rises ;  and  though  that  rise  is  chiefly 
due  to  vasomotor  constriction,  still  it  could  not  take  place  if  the  cardiac  stroke 
were  very  notably  weakened.  It  may  be  that  the  venous  blood  stimulates 
the  cardiac  augmentor  mechanism  in  such  a  way  as  to  bring  about  an  aug- 
mentation of  the  cardiac  stroke  rather  than  a  quickening  of  the  rhythm  ; 
but  this  has  not  been  definitely  proved.  In  any  case  a  slow  beat,  with  such 
a  maintenance  of  the  strength  of  the  cardiac  strokes  as  permits  the  contin- 
uance for  some  considerable  time  of  a  high  blood-pressure,  is  met  when  the 
arterialization  of  the  blood  is  interfered  with.  Sooner  or  later,  however,  the 
deficiency  of  oxygen  in  the  blood  diminishes  the  store  of  explosive  compounds 
in  the  cardiac  muscular  substance,  the  beats  lessen  in  force,  often  showing  a 
temporary  increase  in  frequency,  arid  soon  become  irregular. 

§  330.  The  effects  of  deficient  arterialization  on  the  vasomotor  system 
are  well  shown  when  in  an  animal  placed  under  a  moderate  dose  of  urari  so 
as  to  eliminate  the  complications  due  to  contractions  of  the  skeletal  muscles, 
with  both  vagi  divided  so  as  to  insure  the  elimination  of  inhibitory  impulses 
from  the  medulla,  artificial  respiration  is  suspended.  Soon  after  the  respira- 
tion is  stopped,  a  very  large  but  steady  rise  of  pressure  is  observed.  (See  Fig. 
108.)  The  rise  so  witnessed  is  very  similar  to  that  brought  about  by  power- 
fully stimulating  a  number  of  vaso-constrictor  nerves ;  and  there  can  be  no 
doubt  that  it  is  due  to  the  venous  blood  stimulating  the  vasomotor  centre  in 
the  medulla,  and  thus  causing  constriction  of  the  small  arteries  of  the  body, 
especially  those  of  the  splanchnic  area,  since,  as  we  shall  see,  in  speaking  of 
the  skin,  a  too  venous  blood  leads  to  a  widening  of  the  cutaneous  arteries. 
We  say  "stimulating  the  medullary  vasomotor  centre"  because,  though  we 
must  admit  that,  since  a  rise  of  pressure  follows  upon  dyspncea  when  the 
spinal  cord  has  been  previously  divided  below  the  medulla,  the  venous  blood 
may  stimulate  other  vasomotor  centres  in  the  spinal  cord  and  possibly  even 
act  directly  on  local  peripheral  mechanisms,  yet  the  fact  that  the  rise  of 
pressure  is  much  less  under  these  circumstances  shows  that  the  medullary 
centre  plays  the  chief  part.  As  we  have  just  said,  the  effect  of  this  vaso- 
constriction  in  raising  the  pressure,  if  not  assisted  by  an  increase,  at  all 
events  is  not  neutralized  by  an  adequate  decrease  of  the  cardiac  stroke. 
Upon  the  cessation  of  the  artificial  respiration,  the  respiratory  undulations  of 
course  cease  also,  so  that  the  blood-pressure  curve  rises  at  first  steadily  in 
almost  a  straight  line  broken  only  by  the  heart-beats ;  yet  after  a  while  new 


398 


RESPIRATION. 


undulations,  the  so-called  Traube  or  Traube-Hering  curves,  make  their  ap- 
pearance (Fig.  108,  2,  3),  very  similar  to  the  previous  ones,  except  that  their 
curves  are  larger  and  of  a  more  sweeping  character.  These  new  undula- 
tions, since  they  appear  in  the  absence  of  all  thoracic  or  pulmonary  move- 
ments, passive  or  active,  and  are  witnessed  even  when  both  vagi  are  cut, 
must  be  of  vasomotor  origin  ;  the  rhythmic  rise  must  be  due  to  a  rhyth- 
mic constriction  of  the  small  arteries,  and  this  probably  is  caused  by  a 
rhythmic  discharge  from  vasomotor  centres,  and  especially  from  the  medul- 
lary vasomotor  centre.  The  undulations  are  maintained  as  long  as  the  blood- 
pressure  continues  to  rise.  With  the  increasing  venosity  of  the  blood, 
however,  both  the  vasomotor  centres  and  the  heart  become  enfeebled  ;  the 
undulations  disappear,  and  the  blood  -pressure  rapidly  sinks. 


FIG.  108. 


.iiAAAIift 

\  i\/\l\i  U  II  !il« 


i  A   A  H 

Ah 


/  n 

/1I"1V1/ 


Blood-pressure  Curves  during  a  Suspension  of  Breathing.  (Traube-Hering  Curves.)  The 
curves,  1 ,  2,  3,  4,  5,  are  portions  selected  from  one  long  continuous  tracing  forming  the  record  of 
a  prolonged  observation,  so  that  the  several  curves  represent  successive  stages  of  the  same  exper- 
iment. Each  curve  is  placed  in  its  proper  position  relative  to  the  base  line,  which,  to  save  space, 
is  omitted  ;  and  it  is  obvious  that,  starting  from  the  stage  represented  by  1,  the  blood-pressure  rises 
in  stages  2,  3,  and  4,  but  falls  again  in  stage  5.  Curve  1  is  taken  from  a  period  when  artificial  res- 
piration was  being  kept  up,  and  the  undulations  visible  are  those  the  nature  of  which  have  been 
discussed ;  the  vagus  nerves  having  been  cut  the  pulsations  on  the  ascent  and  descent  of  the 
undulations  do  not  differ.  When  the  artificial  respiration  was  suspended  these  undulations  dis- 
appeared, and  the  blood-pressure  rose  steadily,  while  the  heart-beats  became  slower.  Soon,  as 
shown  in  curve  2,  new  undulations  appeared.  A  little  later  the  blood-pressure  was  still  rising, 
the  heart-beats  still  slower,  but  the  undulations  still  more  obvious  (curve  3).  Still  later  (curve  4) 
the  pressure  was  still  higher,  but  the  heart-beats  were  quicker  and  the  undulations  flatter.  The 
pressure  then  began  to  fall  rapidly  (curve  5),  and  continued  to  fall  until  some  time  later  artificial 
respiration  was  resumed. 

We  may  here  incidentally  remark  that  the  occurrence  of  long,  slow  un- 
dulations is  not  dependent  on  the  cessation  of  the  respiratory  movements, 
and  on  an  abnormally  venous  condition  of  the  blood.  They  are  sometimes 
(Fig.  109)  seen  in  animals  whose  breathing  is  fairly  normal.  We  need 
not  discuss  them  any  further  now,  and  have  introduced  them  chiefly  to 
illustrate  the  fact  that  the  vasomotor  nervous  system  is  apt  to  fall  into  a 
condition  of  rhythmic  activity.  It  has  been  suggested  that  the  normal 


RESPIRATORY  UNDULATIONS. 


399 


respiratory  undulations  may  be  due  to  a  rhythmic  rise  and  fall  of  the 
activity  of  the  vasomotor  centre,  synchronous,  like  that  of  the  cardio- 
inhibitory  centre,  with  the  respiratory  movements.  There  can,  however,  be 
no  doubt  that  the  respiratory  variations  in  blood-pressure  are  due  to  the 
mechanical  conditions  discussed  above,  and  that  vasomotor  influences  inter- 
vene but  little  if  at  all. 

FIG.  109. 


Blood-pressure  Curve  of  a  Rabbit,  Recorded  on  a  Slowly  Moving  Surface,  to  show  Traube- 
Hering  Curves.  (The  curve  was  described  not  by  means  of  a  mercury  manometer,  but  by  an  in- 
strument similar  to  but  not  identical  with  Fick's  spring  kymograph.)  In  each  heart-beat  the 
upward  and  downward  strokes  are  very  close  together,  but  may  be  easily  distinguished  by  the 
help  of  a  lens.  The  undulations  of  the  next  order  are  those  of  respiration.  The  wider  sweeps 
are  the  Traube-Hering  curves,  of  which  two  complete  curves  and  portions  of  two  others  are 
shown.  Each  Traube-Hering  curve  comprises  about  nine  respiratory  curves,  and  each  respiratory . 
curve  about  the  same  number  of  heart-beats. 

§  331.  The  further  general  effects,  similar  to  the  above,  on  the  vascular 
system  of  deficient  arterialization  of  the  blood  may  be  studied  by  taking  a 
blood-pressure  tracing  from  the  carotid  or  other  artery  of  an  animal  while 
the  interference  with  respiration  is  pushed  on  to  a  fatal  asphyxia.  During 
the  first  and  second  stages  of  the  asphyxia  the  blood-pressure  rises  rapidly, 
attaining  a  height  far  above  the  normal.  During  the  third  stage  it  falls 
even  more  rapidly,  repassing  the  normal  and  becoming  nil  as  death  ensues. 
If  the  animal,  no  urari  having  been  given,  is  breathing  of  itself,  and  if,  as 
usually  is  the  case,  the  asphyxia  is  brought  about  by  occlusion  of  the  trachea, 
so  that  the  mechanical  effects  of  the  respiratory  movements  are  exaggerated 
by  the  air  being  unable  to  enter  the  chest,  the  respiratory  undulations  of 
the  pressure-curve  due  to  the  mechanical  causes  discussed  above  are,  espe- 
cially during  the  first  stage,  extensive,  abrupt,  and  irregular,  the  inspiratory 
movements  being  accompanied  by  a  conspicuous  fall  of  pressure.  When 
the  animal  has  been  previously  placed  under  urari,  so  that  the  respiratory 
impulses  cannot  manifest  themselves  by  any  muscular  movements,  the  rise 
of  the  pressure-curve,  as  we  have  already  said,  is  at  first  steady  and  un- 
broken, but  after  a  variable  period  Traube's  curves  make  their  appearance. 
As  during  the  third  stage  the  pressure  sinks,  these  undulations  pass  away. 

The  heart-beats  are  at  first  somewhat  quickened,  but  speedily  become 
slow,  at  the  same  time,  as  we  have  seen,  not  notably  losing  force,  so  that  the 
pulse-curves  on  the  tracing  are  exceedingly  bold  and  striking.  But  the 
boldness  of  the  curve  of  the  mercury  manometer  is,  it  must  be  remembered, 
partly  the  mere  result  of  the  slowness  of  the  rhythm  ;  the  mercury  has  time 
to  fall  largely  between  each  two  beats.  (Fig.  108,  3  and  4.)  Even  while 
the  blood-pressure  is  sinking,  and  when  the  cardiac  stroke  is  now  certainly 
lessening  in  vigor,  the  slowness  of  the  cardiac  rhythm  is  still  sufficient  to 
maintain  somewhat  these  characters  of  the  curve.  The  strokes  at  last,  how- 


400  RESPIRATION. 

ever,  rapidly  fail  in  strength  arid  become  irregular,  though  the  heart  con- 
tinues to  beat  for  some  seconds  after  the  respiratory  movements  have  ceased. 

If  the  chest  of  an  animal  be  opened  under  artificial  respiration,  and 
asphyxia  brought  on  by  cessation  of  the  respiration,  it  will  be  seen  that  the 
heart  during  the  second  and  third  stages  becomes  completely  gorged  with 
venous  blood,  all  the  cavities  as  well  as  the  large  veins  being  distended  to 
the  utmost.  If  the  heart  be  watched  to  the  close  of  the  events,  it  will  be 
seen  that  the  feebler  strokes  which  come  on  toward  the  end  of  the  third  stage 
are  quite  unable  to  empty  its  cavities ;  and  when  the  last  beat  has  passed 
away  its  parts  are  still  choked  with  blood.  The  veins  spurt  out  when 
pricked ;  and  it  may  frequently  be  observed  that  the  beats  recommence 
when  the  over-distention  of  the  heart's  cavities  is  relieved  by  puncture  of 
the  great  vessels.  When  rigor  mortis  sets  in  after  death  by  asphyxia,  the 
left  side  of  the  heart  is  more  or  less  emptied  of  its  contents ;  but  not  so  the 
right  side.  Hence,  in  an  ordinary  post-mortem  examination  in  cases  of 
death  by  asphyxia,  while  the  left  side  is  found  comparatively  empty,  the 
right  appears  gorged. 

The  various  phenomena  of  asphyxia  are  probably  brought  about  in  the 
following  way : 

The  increasingly  venous  character  of  the  blood  augments  the  action  of 
the  vasomotor  centres,  both  the  medullary  centre  and  the  subsidiary  centres 
in  the  spinal  cord,  and  thus  leads  to  a  constriction  of  the  small  arteries, 
especially  of  the  splanchnic  area.  This  is  the  chief  cause  of  the  markedly 
increased  blood-pressure;  though  the  venous  blood  may  possibly  also  act 
directly  on  peripheral  vasomotor  mechanisms,  or  what  is  more  likely,  may 
increase  the  peripheral  resistance  in  the  capillaries  themselves,  since  there 
are  reasons  for  thinking  (§  171)  that  venous  blood  rich  in  carbonic  acid 
meets  with  more  friction,  and  passes  less  easily  through  the  capillaries  than 
does  blood  less  venous  in  character. 

This  increased  peripheral  resistance  and  the  high  blood-pressure  to  which 
it  gives  rise,  while  tending  to  increase  the  distention  of  the  left  ventricle  and 
so  indirectly  helping  to  augment  the  force  of  the  heart's  beat,  soon  becomes 
a  direct  obstacle  to  the  heart  emptying  itself  of  its  contents.  On  the  other 
hand,  the  labored  respiratory  movements  favor  the  flow  of  venous  blood 
toward  the  heart,  which  in  consequence  becomes  more  and  more  full.  This 
repletion  is,  moreover,  assisted  by  the  marked  infrequency  of  the  beats  which 
is  soon  developed.  This  in  turn  depends  in  part  on  the  cardio-inhibitory 
centre  in  the  medulla  being  stimulated  by  the  venous  blood,  but,  as  we 
have  previously  seen,  cannot  be  wholly  accounted  for  in  this  way.  The 
increased  resistance  in  front,  the  augmented  supply  from  behind,  and  the 
long  pauses  between  the  strokes,  all  concur  in  distending  the  heart  more 
and  more. 

When  the  large  veins  have  become  full  of  blood,  the  inspiratory  move- 
ments can  no  longer  have  their  usual  effect  in  facilitating  the  venous  flow 
into  the  right  auricle.  The  chief  effect  of  the  chest  movement,  as  far  as  the 
circulation  is  concerned,  is  to  widen  and  so  to  increase  the  capacity  of  the 
pulmonary  vessels,  and  at  the  same  time  to  diminish  the  pressure  around  the 
large  arteries ;  hence  the  marked  sinking  of  the  blood-pressure  during  each 
inspiratory  movement. 

The  distention  of  the  cardiac  cavities,  at  first  favorable  to  the  heart-beat, 
as  it  increases  becomes  injurious;  and  the  cardiac  tissues  after  a  while  be- 
come enfeebled  by  the  action  of  the  venous  blood,  so  that  the  strokes  of  the 
heart  become  weaker  and  irregular. 

On  account  of  this  increasing  feebleness  of  the  heart's  beat,  accompanied 
by  more  or  less  irregularity,  the  blood-pressure,  in  spite  of  the  continued 


EESPIRATORY  UNDULATIONS.  401 

arterial  constriction,  begins  to  fall,  since  less  and  less  blood  is  pumped  into 
the  arterial  system  ;  the  boldness  of  the  pulse-curves  at  this  stage  is  chiefly 
due  to  the  infrequency  of  the  strokes.  As  the  quantity  which  passes  from 
the  heart  into  the  arteries  becomes  less  second  by  second,  the  pressure  gets 
lower  and  lower,  the  descent  being  assisted  by  the  exhaustion  of  the  vaso- 
motor  centre,  until  almost  before  the  last  beats  it  has  sunk  to  zero.  Thus  at 
the  close  of  asphyxia,  while  the  heart  and  venous  system  are  distended  with 
blood,  the  arterial  system  is  less  than  normally  full. 

§  332.  While  changes  occurring  primarily  in  the  respiratory  system  thus 
affect  the  vascular  system,  conversely  changes  occurring  primarily  in  the 
vascular  system  affect  the  respiratory  system.  Two  kinds  of  change  in  the 
vascular  system  bearing  on  two  parts  of  the  respiratory  system  deserve 
special  attention. 

In  the  first  place  the  respiratory  mechanism  may  be  affected  by  changes 
in  the  blood-supply  to  the  respiratory  centre  in  the  medulla.  We  have 
already  seen  (§  314)  that  the  sudden  cutting  off  of  the  supply  of  blood  to 
the  medulla  gives  rise  to  dyspnoeic  respiratory  movements  and  may  lead  to 
expiratory  convulsions.  That  is  an  extreme  case ;  but,  short  of  that,  the 
activity  of  the  respiratory  centre,  the  extent  and  character  of  the  respiratory 
explosions  which  take  place  in  it,  may  be  varied  according  as  the  constricted 
or  dilated  condition  of  the  small  arteries  branching  off  from  the  basilar 
artery  or  of  the  basilar  artery  itself  allows  a  scanty  or  a  full  flow  of  blood 
through  the  medulla.  And  it  is  possible  that  some  forms  of  dyspnoea  may 
be  brought  about  in  this  way. 

Much  more  common  and  important,  however,  is  the  second  kind  of 
change,  that  affecting  the  circulation  through  the  lungs.  In  the  normal 
organism  an  adequate  supply  of  arterial  blood  to  the  tissues  is  secured  by  an 
adequate  renewal  of  the  air  in  the  pulmonary  alveoli,  and  an  adequately 
rapid  flow  of  blood  through  the  pulmonary  capillaries.  When,  as  by  ob- 
struction in  the  pulmonary  arteries,  or  by  failure  of  the  cardiac  valves,  or, 
and  perhaps  especially,  by  an  insufficient  cardiac  stroke,  the  stream  of  blood 
from  the  lungs  into  the  left  ventricle  is  lessened  either  in  amount  or  in 
rapidity,  less  oxygen  is  carried  to  the  tissues,  including  the  nervous  tissue  of 
the  medulla,  and  dyspnoea  or  "  want  of  breath  "  follows.  When  the  circu- 
lation through  the  lungs  is  in  full  healthy  swing,  the  haemoglobin  of  the  red 
corpuscles  is  as  we  have  seen  saturated  or  nearly  saturated  with  oxygen.  If 
owing  to  a  slower  stream  the  red  corpuscles  tarry  longer  in  their  passage 
along  the  walls  of  the  pulmonary  alveoli  they  cannot  thereby  take  up  a 
compensating  addition  of  oxygen,  indeed,  it  is  doubtful  if  they  can  take  up 
any  additional  oxygen  at  all.  The  blood  falling  under  these  circumstances 
into  the  left  ventricle  and  sent  thence  over  the  body  is  not  more  arterial 
than  usual ;  at  the  same  time  the  amount  of  blood  sent  out  at  each  heart- 
stroke  is  less,  often  much  less,  than  the  normal ;  and  the  medulla  as  well  as 
other  tissues  suffer  in  consequence  from  a  deficiency  of  oxygen.  The  de- 
ficient supply  to  the  medulla  manifests  itself  in  dyspnoeic  or  at  least  in 
labored  breathing,  which  sometimes,  through  the  mechanical  influences 
discussed  above,  has  the  happy  result  of  improving  the  pulmonary  circula- 
tion and  so  produces  compensating  effects.  When  the  pulmonary  artery  is 
suddenly  plugged  with  a  clot  the  primary  and  urgent  symptom  is  "  want 
of  breath,"  though  air  enters  freely  into  the  chest ;  and  "  cardiac  dyspnoea  " 
is  a  common  symptom  of  cardiac  disease. 

§  333.  Other  systems  of  the  body  are  also  related  to  the  respiratory  sys- 
tem, though  by  ties  less  striking  than  those  which  bind  to  it  the  vascular 
system.  We  have  seen  that  deficient  arterialization  of  the  blood  stirs  up  the 
muscles  of  the  alimentary  canal  to  increased  activity,  and  we  shall  presently 

26 


402  RESPIRATION. 

see  that  the  same  condition  has  a  notable  effect  in  promoting  the  perspira- 
tion; it  probably  has  a  similar  influence  over  other  secretions.  On  the 
other  hand,  as  we  have  seen  (§  316),  there  are  reasons  for  thinking  that  the 
activity  of  the  respiratory  centre  and  so  the  energy  of  the  whole  respiratory 
act  is  influenced  by  chemical  changes,  other  than  the  decrease  of  oxygen  and 
increase  of  carbonic  acid,  brought  about  in  the  blood  by  the  activity  of  the 
skeletal  muscles. 

The  closeness  and  the  intricacy  of  the  ties  which  thus  connect  the  respira- 
tory system  with  almost  all  parts  of  the  body  may  be  illustrated  by  consider- 
ing the  effects  of  muscular  work  on  the  body,  and  the  conditions  which, 
apart  from  the  capacity  of  the  muscles  themselves  and  of  the  motor  nervous 
apparatus  which  puts  them  to  work,  determine  the  power  of  the  body  to  do 
work.  During  work,  especially  arduous  work,  the  muscular  contractions  rob 
the  blood  of  much  oxygen  and  load  it  with  much  carbonic  acid.  This 
change  in  the  blood  would  itself  increase  the  activity  of  the  respiratory  centre 
and  the  energy  of  the  respiratory  movements,  and  might  be  sufficient  to 
secure  such  an  increase  of  these  movements  that  the  deficiency  of  oxygen  and 
increase  of  carbonic  acid  should  never  overstep  certain  limits.  But,  as  we 
have  said,  apparently  other  products  of  muscular  metabolism  act  so  potently 
in  stimulating  the  respiratory  centre,  that  the  respiratory  movements  are 
more  than  sufficient  to  compensate  the  changes  in  the  gases  of  the  blood. 
The  efficacy  of  the  augmented  respiratory  movements  is  much  increased  by 
a  concomitant  increase  in  cardiac  activity  and  a  swifter  or  fuller  stream  of 
blood  through  the  lungs;  indeed,  unless  backed  up  by  the  cardiac  increase, 
the  mere  increase  of  the  pulmonary  ventilation  might  prove  inadequate. 

Hence  the  capacity  for  arduous  muscular  labor  is  determined  not  by  the 
respiratory  mechanism  alone,  nor  by  the  vascular  system  alone,  but  by 
both,  and  especially  by  both  working  together  in  harmony  and  concert. 
The  increased  ventilation  would  be  idle  unless  it  were  accompanied  by  a 
quicker  circulation,  and  the  quicker  circulation  would  similarly  be  of  com- 
paratively little  use  unless  accompanied  by  increased  ventilation.  To  a 
bystander  the  working  of  the  respiratory  pump  is  much  more  obvious  than 
that  of  the  vascular  system,  and  indeed  the  subject  himself  is  much  more 
directly  conscious  of  changes  in  the  former  than  of  changes  in  the  latter. 
Hence  when  the  organism  ceases  to  be  able  to  meet  the  demands  which  the 
labor  is  making  upon  it,  the  subject  is  said  to  be  "out  of  breath,"  though  in 
a  large  number  of  cases  the  failure  lies  much  more  at  the  door  of  the  vas- 
cular than  of  the  respiratory  system.  And,  as  a  rule,  it  may  perhaps  be 
said  that  when  two  men  differ  in  their  capacity  for  strenuous  work,  such  as 
running  a  race,  the  difference,  though  it  is  often  familiarly  spoken  of  as  one 
of  "  wind  "  or  power  of  breathing,  is  in  reality  not  a  difference  in  ventilat- 
ing capacity  but  a  difference  in  the  power  of  the  heart  to  keep  up  to  and 
work  in  harmony  with  the  increased  respiratory  movements. 

Thus  there  are  two  main  factors  in  respiration,  the  respiratory  mechanism 
proper,  and  the  circulation,  the  one  bringing  the  air  to  the  blood  and  the 
other  the  blood  to  the  air.  We  may  remind  the  reader  that  there  is  also  a 
third  factor,  and  that  one  of  great  moment,  the  amount  of  haemoglobin,  that 
is,  the  number  of  red  corpuscles,  in  the  blood.  The  amount  of  oxygen  taken 
up  from  the  lungs  depends  not  only  on  the  strokes  of  the  respiratory  and  the 
vascular  pumps,  but  also  on  the  richness  of  the  blood  in  red  corpuscles.  A 
body  which  from  loss  of  blood  or  from  disease  is  ansemic  is  thrown  out  of 
breath  by  very  slight  exertion,  not  so  much  because  the  respiratory  or  the 
vascular  pump  is  weak,  but  because,  through  lack  of  oxygen-carriers,  with 
their  best  efforts  the  combined  pumps  can  only  deliver  to  the  tissues, 
including  the  medulla,  an  inadequate  supply  of  oxygen.  And  fat  persons, 


MODIFIED  RESPIRATORY  MOVEMENTS.  403 

whose  store  of  haemoglobin  in  proportion  to  their  body  weight  is  always 
below  par,  are  proverbially  "  scant  of  breath." 

MODIFIED  KESPIRATORY  MOVEMENTS. 

§  334.  The  respiratory  mechanism  with  its  adjuncts,  in  addition  to  its 
respiratory  function,  becomes  of  service,  especially  in  the  case  of  man,  as  a 
means  of  expressing  emotions.  The  respiratory  column  of  air,  moreover,  in 
its  exit  from  the  chest,  is  frequently  made  use  of  in  a  mechanical  way  to 
expel  bodies  from  the  upper  air-passages.  Hence  arise  a  number  of  pecu- 
liarly modified  and  more  or  less  complicated  respiratory  movements,  sighing, 
coughing,  laughter,  etc.,  adapted  to  secure  special  ends  which  are  not  dis- 
tinctly respiratory.  They  are  all  essentially  reflex  in  character,  the  stimulus 
determining  each  movement,  sometimes  affecting  a  peripheral  afferent  nerve 
as  in  the  case  of  coughing,  sometimes  working  through  the  higher  parts  of 
the  brain  as  in  laughter  arid  crying,  sometimes  possibly  as  in  yawning  and 
sighing,  acting  on  the  respiratory  centre  itself.  Like  the  simple  respiratory 
act,  they  may  with  more  or  less  success  be  carried  out  by  a  direct  effort  of 
the  will. 

Sighing  is  a  deep  and  long-drawn  inspiration,  chiefly  through  the  nose, 
followed  by  a  somewhat  shorter,  but  correspondingly  large  expiration. 

Yawning  is  similarly  a  deep  inspiration,  deeper  and  longer  continued 
than  a  sigh,  drawn  through  the  widely  open  mouth,  and  accompanied  by  a 
peculiar  depression  of  the  lower  jaw  and  frequently  by  an  elevation  of  the 
shoulders. 

Hiccough  consists  in  a  sudden  inspiratory  contraction  of  the  diaphragm, 
in  the  course  of  which  the  glottis  suddenly  closes,  so  that  the  further  entrance 
of  air  into  the  chest  is  prevented,  while  the  impulse  of  the  column  of  air 
just  entering,  as  it  strikes  upon  the  closed  glottis,  gives  rise  to  a  well-known 
accompanying  sound.  The  afferent  impulses  of  the  reflex  act  are  conveyed 
by  the  gastric  branches  of  the  vagus.  The  closure  of  the  glottis  is  carried 
out  by  means  of  the  inferior  laryngeal  nerve.  See  Voice. 

In  sobbing  a  series  of  similar  convulsive  inspirations  follow  each  other 
slowly,  the  glottis  being  closed  earlier  than  in  the  case  of  hiccough,  so  that 
little  or  no  air  enters  into  the  chest. 

Coughing  consists  in  the  first  place  of  a  deep  and  long-drawn  inspiration 
by  which  the  lungs  are  well  filled  with  air.  This  is  followed  by  a  complete 
closure  of  the  glottis,  and  then  comes  a  sudden  and  forcible  expiration,  in 
the  midst  of  which  the  glottis  suddenly  opens,  and  thus  a  blast  of  air  is 
driven  through  the  upper  respiratory  passages.  The  afferent  impulses  of 
this  reflex  act  are,  in  most  cases,  as  when  a  foreign  body  is  lodged  in  the 
larynx  or  by  the  side  of  the  epiglottis,  conveyed  by  the  superior  laryngeal 
nerve  ;  but  the  movement  may  arise  from  stimuli  applied  to  other  afferent 
branches  of  the  vagus,  such  as  those  supplying  the  bronchial  passages  and 
stomach  and  the  auricular  branch  distributed  to  the  meatus  extefnus.  Stimu- 
lation of  other  nerves  also,  such  as  those  of  the  skin  by  a  draught  of  cold 
air,  may  develop  a  cough.  ' 

In  sneezing  the  general  movement  is  essentially  the  same,  except  that  the 
opening  from  the  pharynx  into  the  mouth  is  closed  by  the  contraction  of  the 
anterior  pillars  of  the  fauces  and  the  descent  of  the  soft  palate,  so  that  the 
force  of  the  blast  is  driven  entirely  through  the  nose.  The  afferent  impulses 
here  usually  come  from  the  nasal  branches  of  the  fifth.  When  sneezing, 
however,  is  produced  by  a  bright  light,  the  optic  nerve  would  seem  to  be  the 
afferent  nerve. 

Laughing  consists  essentially  in  an  inspiration  succeeded,  not  by  one,  but 


404  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

by  a  whole  series,  often  long  continued,  of  short  spasmodic  expirations,  the 
glottis  being  freely  open  during  the  whole  time,  and  the  vocal  cords  being 
thrown  into  characteristic  vibrations. 

In  crying,  the  respiratory  movements  are  modified  in  the  same  way  as  in 
laughing ;  the  rhythm  and  the  accompanying  facial  expressions  are,  how- 
ever, different,  though  laughing  and  crying  frequently  become  indistin- 
guishable. 


CHAPTER    III. 

THE  ELIMINATION   OF  WASTE  PRODUCTS. 

§  335.  We  have  traced  the  food  from  the  alimentary  canal  into  the  blood, 
and,  did  the  state  of  our  knowledge  permit,  the  natural  course  of  our  study 
would  be  to  trace  the  food  from  the  blood  into  the  tissues,  and  then  to  follow 
the  products  of  the  activity  of  the  tissues  back  into  the  blood  and  so  out 
of  the  body.  This,  however,  we  cannot  as  yet  satisfactorily  do ;  and  it  will 
be  more  convenient  to  study  first  the  final  products  of  the  metabolism  of  the 
body,  and  the  manner  in  which  they  are  eliminated,  and  afterward  to  return 
to  the  discussion  of  the  intervening  steps. 

Our  food  consists  of  certain  food-stuffs,  viz.,  proteids,  fats,  and  carbo- 
hydrates, of  various  salts,  and  of  water.  In  their  passage  through  the  blood 
and  tissues  of  the  body,  the  proteids,  fats,  and  carbohydrates  are  converted 
into  urea  (or  some  closely  allied  body),  carbonic  acid,  and  water,  the  nitrogen 
of  the  urea  being  furnished  by  the  proteids  alone.  Many  of  the  proteids 
contain  sulphur,  and  also  have  phosphorus  attached  to  them  in  some  com- 
bination or  other,  and  some  of  the  fats  taken  as  food  contain  phosphorus ; 
these  elements  ultimately  undergo  oxidation  into  phosphates  and  sulphates, 
and  leave  the  body  in  that  form  in  company  with  the  other  salts. 

Broadly  speaking,  then,  the  waste  products  of  the  animal  economy  are 
urea,  carbonic  acid,  salts,  and  water.  These  leave  the  body  by  one  or  other 
of  three  main  channels,  the  lungs,  the  skin,  and  the  kidney.  Some  part,  it 
is  true,  leaves  the  body  by  the  bowels,  for,  as  we  have  seen,  the  feces  contain, 
besides  undigested  portions  of  food,  substances  which  have  been  secreted  into 
the  bowel,  and  are,  therefore,  waste  products ;  but  the  amount  of  these  is  so 
small  that  they  may  be  neglected. 

The  lungs  serve  as  the  channel  for  the  discharge  of  the  greater  part  of 
the  carbonic  acid,  and  a  considerable  quantity  of  water ;  this  discharge  we 
have  just  studied.  Through  the  skin  there  leave  the  body  a  comparatively 
small  quantity  of  salts,  a  little  carbonic  acid,  and  a  variable  but  on  the  whole 
large  quantity  of  water. 

The  kidneys  discharge  all  or  nearly  all  the  urea  and  allied  bodies,  the 
greater  portion  of  the  salts,  and  a  large  amount  of  water,  with  an  insignifi- 
cant quantity  of  carbonic  acid.  They  are  especially  important,  since  by 
them  practically  all  the  nitrogenous  waste  leaves  the  body ;  and  to  them  we 
will  turn  first. 


§336. 

ourselves 


THE  COMPOSITION  AND  CHARACTERS  OF  URINE. 

.  These  are  so  fully  dwelt  upon  in  special  works  that  we  may  confine 
here  to  salient  points.    The  healthy  urine  of  man  is  a  clear  yellowish 


THE  COMPOSITION  AND  CHARACTERS  OF  URINE.  405 

slightly  fluorescent  fluid  of  a  peculiar  odor,  saline  taste,  and  acid  reaction, 
having  a  mean  specific  gravity  of  1020,  and  generally  holding  in  suspension 
a  little  mucus.  The  mucus,  when  present,  comes  from  the  urinary  passages, 
as  do  also  the  occasional  epithelial  cells.  All  the  rest  of  the  urine  may  be 
considered  as  the  secretion  of  the  kidney. 

The  urine,  as  we  have  said,  is  the  chief  channel  by  which  solid  matters 
leave  the  body,  a  small  quantity  only  passing  by  the  skin  and  practically 
none  by  the  lungs.  Hence,  neglecting  for  the  present  the  skin,  we  may  say 
that  all  the  substances  taken  into  the  body  sooner  or  later  leave  the  body  by 
the  urine,  save  the  few  substances  which  may  be  retained  permanently  within 
the  body  and  the  substances  which  make  up  the  body  at  the  moment  of  its 
death.  We  accordingly  find  that  the  urine  contains  a  large  number  of  sub- 
stances, the  exact  amount  of  each  substance  present  in  a  given  quantity  of 
urine  varying,  in  the  case  of  every  substance  somewhat,  and  in  the  cases  of 
many  substances  very  largely,  from  time  to  time.  The  composition  of  urine 
is  not  only  complex  but  extremely  variable. 

Moreover,  a  little  consideration  will  show  that  the  several  substances 
present  in  urine  must  have  very  different  histories.  Some  of  the  con- 
stituents of  urine  appear  in  it  in  the  exact  form  in  which  they  were  intro- 
duced into  the  mouth  ;  they  have  been  simply  absorbed  from  the  alimentary 
canal  into  the  blood  and  excreted  by  the  kidney  without  undergoing  change  ; 
they  are  derived  directly  and  without  change  from  the  food. 

Others  again  are  the  products  of  changes  which  the  food  has  undergone  in 
the  body ;  and  these  changes  may  be  slight  or  may  be  extensive,  and  may 
take  place  on  the  one  hand  in  the  alimentary  canal,  or  during  a  brief  transit 
of  the  substance  in  the  blood-stream,  or  even  in  the  urine  itself,  may,  so  to 
speak,  be  superficial ;  or  on  the  other  hand  may  take  place  in  the  very  depths 
of  the  tissues  and  be  closely  associated  with  the  very  life  of  the  tissues.  We 
shall,  however,  have  to  return  to  these  matters  later  on,  and  may  here  briefly 
consider  what  substances  are,  normally  and  abnormally,  present  in  urine,  and 
the  chief  features  of  the  fluid  itself. 

§  337.  Besides  water,  the  constituents  of  urine  are : 

Nitrogenous  crystalline  bodies.  Neglecting  the  small  proportion  of  these 
bodies  which,  especially  in  the  case  of  flesh-eaters,  are  introduced  into  the 
economy  with  the  food,  as  kreatin  and  the  like,  and  so  pass  into  the  urine 
with  no  or  with  comparatively  little  change,  we  may  on  the  whole  regard  the 
substances  of  this  class  as  the  products  of  the  changes  which  the  proteid 
matters  (and  allied  substances  such  as  gelatin  and  the  like)  present  in  food 
have  undergone  either  while  the  food  was  simply  food,  still  in  the  alimentary 
canal,  for  instance,  or  after  the  food  had  been  built  up  into  the  tissues  of  the 
body. 

Of  these  by  far  the  most  important,  in  the  urine  of  man  and  mammalia, 
is  the  body  urea  (N2H4CO).  It  is  the  chief  form  in  which,  in  these  animals, 
nitrogen  leaves  the  body.  We  shall  have  to  discuss  the  relations  and  forma- 
tion of  urea  later  on,  but  meanwhile  we  will  simply  state  that  it  has  remark- 
able double  connections  with  two  great  groups.  On  the  one  hand,  it  is 
related  to  the  ammonia  group,  and  by  hydration  is  readily  converted  into 
ammonium  carbonate  (N2H4CO+2H2O  =  (NH4\CO3).  On  the  other  hand, 
it  is  related  to  the  great  cyanogen  group,  ammonium  cyanate  and  urea  being 
isomeric,  and  the  former  by  simple  heating  being  converted  into  the  latter 
(NH4.CNO  =  N2H4CO).  ' 

Though  a  base,  forming  salts  with  acids,  such  as  nitrates,  oxalates,  etc., 
urea  occurs  in  urine  in  a  free  and  independent  condition. 

Closely  allied  to  urea,  occurring  apparently  as  a  by-product  of  the 
same  line  of  metabolism,  is  uric  acid  (C6HJf4Os)  which  is  found  always  in 


406  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

the  urine  of  man,  occurring  in  small  but  variable  quantity.  In  the  urine  of 
some  animals,  such  as  birds  and  reptiles,  it  occurs  in  abundance,  and  indeed 
in  these  replaces  urea  as  the  chief  nitrogenous  excretion.  Uric  acid  is  a 
more  complex  body  than  urea,  one  molecule  of  uric  acid  splitting  up,  under 
the  influence  of  certain  reagents,  into  two  molecules  of  urea  and  a  com- 
pound of  oxalic  acid.  Its  decomposition  products,  however,  under  different 
reagents  are  very  numerous  and  complex,  though  urea  occurs  among  them 
frequently  and  characteristically.  Uric  acid  may  be  synthetically  produced 
out  of  urea  and  glycin  (glycocol). 

It  is  a  weak  dibasic  acid,  and  occurs  in  normal  human  urine,  not  as  a 
free  acid  but  as  an  acid  salt,  being  combined  with  potassium  and  sodium, 
and  to  a  less  extent  with  calcium  and  ammonium.  In  quite  normal  urine 
these  salts  are  soluble  in  the  urine,  even  after  the  fluid  has  cooled  down  to 
the  ordinary  temperature  of  the  air ;  but  not  infrequently  the  urates,  soluble 
in  the  urine  at  the  temperature  at  which  it  leaves  the  body,  are  precipitated 
when  the  fluid  cools,  forming  the  well-known  "deposit  of  urates."  On 
further  standing  the  salts  are  apt  to  be  decomposed  and  thus  to  give  rise  to 
crystals  of  uric  acid. 

Besides  urea  and  uric  acid  the  urine  contains  small  but  variable  quanti- 
ties of  more  or  less  nearly  allied  bodies,  such  as  kreatinin,  xanthin,  hypo- 
xanthin,  and  guanin.  Concerning  these  we  will  at  present  only  say  that 
kreatinin  is  a  hydrated  form  of  the  body  kreatin  which  we  spoke  of  (§  62 ) 
as  a  constituent  of  muscles.  Kreatin  by  hydration  is  readily  converted  into 
kreatinin,  and  kreatinin  by  dehydration  into  kreatin  ;  kreatin  introduced 
into  the  alimentary  canal  or  into  the  blood  appears  in  the  urine  as  kreat- 
inin ;  and  in  flesh-eaters  some  at  least  of  the  kreatinin  of  the  urine  is  derived 
directly  from  the  kreatin  present  in  the  meat  eaten  as  food;  but  we  shall 
discuss  the  subject  of  kreatin  later  on. 

Besides  the  above,  such  bodies  as  leucin,  taurin,  cystin,  allantoin,  and 
ammonium  oxalurate  are  occasionally  found  in  urine,  but  cannot  be  regarded 
as  constituents  of  normal  urine. 

In  the  urine  of  man  hippuric  acid  appears  to  be  always  present  in  small 
quantities,  and  in  the  urine  of  herbivora  occurs  in  large  quantities.  In  these 
latter  it  is  derived  more  or  less  directly,  by  changes  of  which  we  shall  have 
to  speak  in  a  succeeding  chapter,  from  constituents  of  the  food-containing 
bodies  belonging  to  the  aromatic  group  (benzoic  acid  series)  ;  but  the  small 
quantity  present  in  man  and  other  carnivora  appears  to  come  from  the 
metabolism  of  proteid  matter  which,  as  we  have  already  seen,  contains  an 
aromatic  constituent.  Another  member  of  the  aromatic  group,  ty rosin,  is 
occasionally  present  in  urine  ;  and  as  more  regular  constituents  of  normal 
urine  may  be  mentioned  certain  phenol  compounds,  such  as  phenylsulphuric 
acid,  the  phenol  constituents  of  which  are  derived  from  the  action  of  micro- 
organisms in  the  alimentary  canal  (see  §  243)  ;  these  substances,  though  they 
no  longer  contain  nitrogen,  take  origin  from  bodies  of  the  aromatic  series. 
Similar  changes  are  also  the  source  of  indigo  compounds  (indican)  in  the 
urine,  derived  from  indol  (see  §  219). 

§  338.  Inorganic  salts.  These  for  the  most  part  exist  in  urine  in  natural 
solution,  the  composition  of  the  ash  almost  exactly  corresponding  with  the 
results  of  the  direct  analysis  of  the  fluid;  in  this  respect  urine  contrasts 
forcibly  with  blood,  the  ash  of  which  is  largely  composed  of  inorganic  sub- 
stances, which  previous  to  the  incineration  existed  in  peculiar  combination 
with  proteid  and  other  complex  bodies.  In  the  ash  of  urine  there  is  rather 
more  sulphur  than  corresponds  to  the  sulphuric  acid  directly  determined ; 
this  indicates  the  existence  in  urine  of  some  sulphur-holding  complex  body. 
And  there  are  traces  of  iron,  pointing  to  some  similar  iron-holding  sub- 


THE  COMPOSITION   AND  CHARACTERS  OF  URINE.  407 

stance.  But  otherwise,  all  the  substances  found  in  the  ash  exist  as  salts  in 
the  natural  fluid. 

The  chief  bases  are  sodium,  potassium,  calcium  and  magnesium  in  the 
form  of  chlorides,  phosphates  and  sulphates.  The  exact  way  in  which  the 
several  bases  and  acids  are  combined  is  to  some  extent  a  matter  of  uncer- 
tainty ;  but  sodium  chloride  is  certainly  present  and  in  considerable  quan- 
tity ;  it  is  the  most  abundant  and  important  inorganic  constituent.  A  large 
portion  of  the  phosphoric  acid  seems  to  exist  as  acid  sodium  phosphate,  the 
rest  as  soluble  calcium  and  magnesium  phosphates.  The  remaining  chief 
salts,  occurring,  however,  in  smaller  quantity,  are  potassium  and  sodium 
sulphate  and  calcium  chloride. 

Ammonia  occurs  in  small  quantity,  alkaline  carbonates  are  frequently 
found,  traces  of  nitrates  are  at  all  events  occasionally  present,  as  also  indica- 
tions of  salicylates  and  of  sulpho-cyanates. 

The  phosphates  are  derived  partly  from  the  phosphates  taken  as  such  in 
food,  partly  from  the  phosphorus  or  phosphates  peculiarly  associated  with 
the  proteids,  and  partly  from  the  phosphorus  of  certain  complex  fats  such 
as  lecithin.  When  urine  becomes  alkaline  (and,  as  we  shall  presently  see, 
it  may  do  so  by  changes  taking  place  in  itself)  the  calcic  and  magnesic 
phosphates  are  converted  into  basic  salts  which,  being  insoluble,  are  pre- 
cipitated, the  sodium  phosphate  remaining  in  solution.  When  the  alka- 
linity, as  is  frequently  the  case,  is  due  to  ammonia,  ammonio-magnesium 
phosphate  is  formed  and  is  apt  to  appear  in  crystals.  The  sulphates  are 
derived  partly  from  the  sulphates  taken  as  such  in  food  and  partly  from  the 
sulphur  of  the  proteids.  The  carbonates,  when  occurring  in  large  quantity, 
generally  have  their  origin  in  the  oxidation  of  such  salts  as  citrates,  tartrates, 
etc.  The  bases  present  depend  largely  on  the  nature  of  the  food  taken. 
Thus  with  a  vegetable  diet,  the  excess  of  the  alkalies  in  the  food  reappears 
in  the  urine ;  with  an  animal  diet,  the  earthy  bases  in  a  similar  way  come 
to  the  front. 

§  339.  Non-nitrogenous  bodies.  These  exist  in  very  small  quantities, 
and  many  of  them  are  probably  of  uncertain  occurrence.  Some  of  these 
are  organic  acids,  the  most  constant  perhaps  being  oxalic  acid  ;  to  this  may 
be  added  glycerin-phosphoric,  lactic,  formic,  acetic,  butyric  and  possibly 
succinic  acids.  Inosit  has  also  been  said  to  occur  normally.  It  has  been 
maintained  that  minute  quantities  of  sugar  (dextrose)  are  invariably  pres- 
ent in  even  healthy  urine ;  this,  however,  has  not  as  yet  been  placed  beyond 
all  doubt.  The  nature  of  the  substances  which  give  to  urine  its  character- 
istic odor  has  not  been  made  out ;  probably  there  are  more  such  bodies  than 
one. 

§  340.  Pigments.  Urine  is  always  colored,  the  tint  varying  from  a  light 
to  a  dark  yellow  with  an  admixture  of  brown.  In  the  course  of  twenty- 
four  hours,  a  not  inconsiderable  quantity  of  pigment  must  leave  the  body 
by  the  urine ;  but  the  nature  of  the  normal  pigment  or  pigments  of  urine 
is  at  present  obscure  and  the  subject  of  much  controversy.  The  matter 
is  apparently  further  complicated  by  the  presence  in  urine  of  what  have 
been  called  "  chromogens,"  that  is  to  say,  bodies  which  are  not  colored 
themselves  but  which  readily  give  rise  to  pigments  upon  oxidation  ;  and  it 
is  probable  that  some  of  these  "  chromogens  "  of  the  urine  are  reduction 
products  of  the  respective  pigments,  the  reduction  taking  place  in  the 
urine  after  secretion,  or  during  or  even  before  secretion.  There  is  frequently 
present  in  urine,  especially  in  cases  of  fever,  a  pigment  which  has  been 
isolated  and  determined,  which  has  a  characteristic  spectrum,  and  which, 
being  maintained  by  some  to  be  a  derivative  of  bilirubin,  has  been  called 
urobilin.  It  is  not  this  urobilin,  however,  which  gives  to  urine  its  ordinary 


408  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

color.  Some  observers,  on  the  other  hand,  maintain  that  normal  urine  does 
contain  and,  in  part  at  least,  owes  its  normal  color  to  a  somewhat  similar 
but  different  body,  which  in  consequence  they  have  called  "  normal "  uro- 
bilin.  It  is  in  fact  not  possible,  at  the  present  moment,  to  make  definite  and 
satisfactory  statements  as  to  whether  urine  contains  one  or  more  than  one 
normal  pigment,  as  to  its  or  their  nature,  as  to  whether  they  are  derived 
from  bile  pigment  or  directly  from  the  hsernatin  or  haemoglobin  or  in  other 
ways,  or  as  to  the  several  steps  by  which  they  are  produced.  There  are  also 
abnormal  coloring  matters  present  on  occasion,  such  for  instance  as  the  pecu- 
liar red  coloring  matter  occurring  sometimes  in  the  urine  of  acute  rheuma- 
tism, which  has  been  called  uroerythrin ;  but  our  knowledge  concerning 
these  is  very  imperfect. 

§  341.  Ferments  and  other  bodies.  Even  normal  urine  has  frequently 
been  found  to  contain  a  small  quantity,  hardly  amounting  to  more  than  a 
trace,  of  proteid  material,  apparently  an  albumin  ;  but  the  normal  presence 
of  even  this  small  quantity  has  been  disputed.  Urine,  however,  certainly 
contains  ferment  bodies. 

When  urine  is  treated  with  many  times  its  volume  of  alcohol,  a  granular 
or  flocculent  precipitate  is  thrown  down,  consisting  chiefly  of  phosphates, 
together  with  some  other  substances  or  probably  several  other  substances,  in 
very  small  quantities.  An  aqueous  solution  of  the  precipitate,  which  may 
be  freed  from  the  phosphates,  is  both  amylolytic  and  proteolytic.  Ferments 
may  also  and  more  readily  be  extracted  from  urine  by  allowing  shreds  of 
fibrin  to  soak  in  the  urine  for  a  few  hours,  and  then  removing  and  washing 
them.  The  ferments  become  entangled  in  the  fibrin  in  such  a  way  as  not 
to  be  easily  removed  by  washing.  The  washed  shreds  will  convert  starch 
into  sugar;  and  when  treated  with  dilute  hydrochloric  acid  digest  them- 
selves, showing  the  presence  of  pepsin.  By  this  method  it  has  been  ascer- 
tained that  an  amylolytic  ferment  and  pepsin  are  present  in  quantities  which' 
vary  in  the  twenty-four  hours  according  to  the  meals.  Rennin  has  also  been 
found,  and,  at  times  at  least,  trypsin.  From  this  it  appears  that  some  of  the 
ferments  of  the  alimentary  canal  escape  from  the  body  by  the  urine,  being 
probably  reabsorbed  directly  from  the  respective  gland  ;  the  quantity  which 
thus  escapes  is  insignificant. 

A  small  quantity  of  gas,  about  15  vols.  per  cent.,  can  be  extracted  by  the 
mercurial  pump  from  urine  received  direct  from  the  body  without  exposure 
to  air.  The  gas  so  obtained  consists  chiefly  of  carbonic  acid,  nitrogen  being 
very  scanty,  and  oxygen  occurring  in  very  small  quantities  or  being  wholly 
absent.  The  meaning  of  this  we  have  already  touched  upon  in  speaking  of 
respiration  (see  §  302). 

§  342.  The  quantities  in  which  these  multifarious  bodies,  all  of  which, 
as  we  have  seen,  we  may  perhaps  regard  as  constituents  of  normal  urine,  are 
present  in  different  specimens  of  urine,  vary  within  very  wide  limits,  being 
dependent  on  the  nature  of  the  food  taken  and  on  the  conditions  of  the  body. 
The  amount  not  of  water  only,  but  of  many  of  the  other  several  constituents, 
varies  widely  and  indeed  rapidly,  so  that  the  percentage  composition  of  urine 
will  vary  from  hour  to  hour  if  not  from  minute  to  minute.  The  causes  which 
determine  these  variations  in  the  nature  and  amount  of  urine  we  shall  study 
later  on.  Meanwhile  what  may  be  called  the  average  composition  of  human 
urine  is  shown  in  the  following  table  in  which  the  acids  and  bases  are  put  down 
separately. 


URINAKY   CONSTITUENTS.  409 

AMOUNTS  OF  THE  SEVERAL  URINARY  CONSTITUENTS  PASSED  IN 
TWENTY-FOUR  HOURS.     (After  PARKES.) 

By  an  average  Per  1  kilo 

man  of  66  kilos.  of  body-weight. 

Water 1500.000  grms.     23.0000  grms. 

Total  solids 1.1000 

Urea 33.180  0.5000 

Uric  acid 0.555  0.0084 

Hippuric  acid 0.400  0.0060 

Kreatinin 0.910  0.0140 

Pigment,  and  other  substances    .    .    .10.000  0.1510 

Sulphuric  acid 2.012  0.0305 

Phosphoric  acid 3.164  0.0480 

Chlorine 7.000  0.1260 

(8.21) 

Ammonia 0.770 

Potassium 2.500 

Sodium 11.090 

Calcium 0.260 

Magnesium 0.207 

72.000 

§  343.  The  acidity  of  urine.  The  healthy  urine  of  man  is  acid,  owing 
to  the  presence  of  acid  sodium  phosphate,  the  absence  of  free  acid  being 
shown  by  the  fact  that  sodium  hyposulphite  gives  no  precipitate.  The 
amount  of  acidity  is  about  equivalent  to  2  grammes  of  oxalic  acid  in 
twenty-four  hours,  but  the  degree  of  acidity  at  any  one  time  varies  much 
during  the  day,  being  in  an  inverse  ratio  to  the  amount  of  acid  secreted  by 
the  stomach  ;  thus  it  decreases  after  food  is  taken,  and  increases  again  as 
gastric  digestion  comes  to  an  end.  It  varies  with  the  nature  of  the  food  ; 
with  a  vegetable  diet  the  excess  of  alkalies  in  the  food,  being  secreted  by 
the  urine,  leads  to  alkalinity,  or  at  least  to  diminished  acidity,  whereas  this 
effect  is  wanting  with  an  animal  diet,  in  which  the  alkalies  are  less  abundant, 
earthy  bases  preponderating.  Hence  the  urine  of  carnivora  is  generally  very 
acid,  while  that  of  herbivora  is  alkaline.  The  latter,  when  fasting,  are  for 
the  time  being  carnivorous,  living  entirely  on  their  own  bodies,  and  hence 
their  urine  becomes  under  these  circumstances  acid. 

The  natural  acidity  increases  for  some  time  after  the  urine  has  been  dis- 
charged, owing  to  the  formation  of  fresh  acid,  apparently  by  some  kind  of 
fermentation.  This  increase  of  acid  frequently  causes  a  precipitation  of 
u rates,  which  the  previous  acidity,  even  after  the  cooling  of  the  urine,  had 
been  insufficient  to  throw  down.  After  a  while,  however,  the  acid  reaction 
gives  way  to  alkalinity.  This  is  caused  by  a  conversion  of  the  urea  into 
ammonium  carbonate  through  the  agency  of  a  specific  "  organized  "  ferment. 
This  ferment  as  a  general  rule  does  not  make  its  appearance  except  in  urine 
exposed  to  the  air ;  it  is  only  in  unhealthy  conditions  that  the  fermentation 
takes  place  within  the  bladder,  and  in  such  cases  is  due  either  to  micro- 
organisms introduced  into  the  bladder  from  without,  during  the  use  of  in- 
struments for  instance,  or  to  the  action  of  an  unorganized  ferment,  secreted, 
apparently  by  the  walls  of  the  bladder. 

§  344.  Abnormal  constituents  of  urine.  The  structural  elements  found  in 
the  urine  under  various  circumstances  are  blood,  pus  and  mucous  corpuscles, 
epithelium  from  the  bladder  and  kidney,  and  spermatozoa.  To  these  may 
be  added  the  so-called  "  casts  "  which  are  either  "  epithelial  casts,"  that  is  to 
say,  cylinders  of  more  or  less  altered  epithelial  cells  shed  from  the  tubules,  or 
structureless,  "fibrinous"  casts,  which  are  cylinders  of  peculiar  material 
moulded  in  the  lumina  of  the  tubules ;  the  exact  nature  of  this  material  is 


410  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

at  present  a  matter  of  doubt ;  it  is  not  always  the  same,  but  appears  not  to 
be  fibrin. 

The  most  common  and  important  abnormal  constituents  of  urine  are 
albumin,  giving  rise  to  albuminuria,  and  sugar,  giving  rise  to  glycosuria  or 
diabetes.  The  soluble  proteids,  generally  spoken  of  as  "  albumin,"  in  the 
urine  differ  in  different  cases.  The  exact  determination  of  their  nature  is  a 
matter  of  some  difficulty,  since,  as  we  have  seen,  we  have  in  differentiating 
the  various  proteids  to  trust  largely  to  their  behavior  as  regards  precipita- 
tion upon  the  addition  of  certain  saline  bodies ;  and  the  presence  of  saline 
bodies  in  the  natural  urine  introduces  complications.  It  would  appear, 
however,  that  the  proteids  usually  present  are  serum-albumin  and  globulin  ; 
these  are  not,  however,  as  a  rule,  if  ever,  present  in  the  same  relative  pro- 
portions as  in  blood-plasma ;  and  either  the  one  or  the  other  may  be 
present  by  itself.  A  form  of  albumose  (§  189),  called  hemi-albumose,  is 
sometimes  found,  and,  indeed,  probably  very  many  distinct  kinds  of  pro- 
teids are  from  time  to  time  present.  If  egg-albumin  be  injected  into  the 
blood  it  appears  in  the  urine  as  egg-albumin,  and  peptone  similarly  injected 
appears  as  peptone. 

The  sugar  which  is  found  in  the  urine  of  diabetes  is  (indistinguish- 
able from  ordinary  dextrose ;  but  whether  it  is  absolutely  identical  with 
that  body,  or  whether  the  sugar  in  all  cases  of  diabetic  urine  is  ex- 
actly the  same  in  character,  cannot,  perhaps,  as  yet,  be  regarded  as  defi- 
nitely settled. 

When  blood  is  mingled  with  urine  in  the  kidney  and  in  the  urinary 
passages  the  constituents  of  the  former  are,  of  course,  added  to  those  of 
the  latter ;  and  when,  as  sometimes  happens,  chyle  from  the  lacteals  makes 
its  way  into  the  kidneys,  the  urine  contains  the  fats  and  other  constituents 
of  chyle.  Fats,  however,  may  be  present  without  the  urine  being  distinctly 
"  chylous." 

Cholesterin,  bile-acids,  bile-pigments,  and  one  or  other  of  a  large 
number  of  bodies  arising  from  a  disordered  metabolism  of  the  body,  such 
as  leucin,  tyrosin,  acetone  (in  cases  of  diabetes),  oxalic  acid,  taurin,  cystin, 
and  many  others  are  also  found  more  or  less  frequently ;  some  of  these, 
indeed,  have  been  regarded  as  normal  constituents.  Besides  these  the  urine 
serves  as  the  chief  channel  of  elimination  for  various  bodies,  not  proper 
constituents  of  food,  which  may  happen  to  have  been  taken  into  the  sys- 
tem. Thus  various  minerals,  alkaloids,  salts,  pigmentary  and  odoriferous 
matters,  may  be  passed  unchanged.  Many  substances  thus  occasionally 
taken  undergo,  however,  changes  in  passing  through  the  body :  the  most 
important  of  these,  since  the  changes  which  they  undergo  throw  light 
on  the  metabolic  processes  of  the  body,  will  be  considered  in  a  succeeding 
chapter. 

THE  SECRETION  OF  URINE. 

§  345.  The  kidney  consists  of  two  parts,  so  distinct  in  structure  that 
it  seems  impossible  to  resist  the  conclusion  that  their  functions  are  differ- 
ent, and  that  the  mechanism  by  which  the  urine  secreted  is  of  a  double 
kind.  On  the  one  hand,  the  tubuli  uriniferi,  with  their  characteristic  epi- 
thelium, seem  obviously  to  be  actively  secreting  structures  comparable  to 
the  secreting  alveoli  of  the  salivary  and  other  glands.  On  the  other  hand, 
the  Malpighian  capsules,  with  their  glomeruli,  are  organs  of  a  peculiar 
nature,  with  an  almost  insignificant  epithelium,  and  their  structure  irresist- 
ibly suggests  that  they  act  rather  as  what  may  be  called,  in  a  general  way, 
a  filtering  than  as  a  truly  secreting  mechanism.  Hence  has  arisen  the  view 


THE  SECRETION  OF  URINE.  411 

which  frequently  bears  the  name  of  Bowman,  since  he  was  the  first  to  put  it 
forward,  that  certain  constituents  only  of  the  urine  are  secreted  after  the  fash- 
ion of  other  secreting  glands  by  the  tubuli  uriniferi,  and  that  the  rest  of  the 
constituents,  including  a  great  deal  of  the  water,  with  such  highly  soluble  and 
diffusible  salts  as  pre-exist  in  adequate  quantity  in  the  blood,  are,  as  it  were, 
filtered  off  by  the  glomeruli  of  the  Malpighian  capsules.  We  shall  see, 
later  on,  reason  to  doubt  whether  we  are  justified  in  applying  the  term 
"  filtration,"  which  has  a  definite  physical  meaning,  to  the  process  by  which 
water  and  other  substances  pass  from  the  bloodvessels  of  the  glomerulus  into 
the  lumen  of  the  tubule ;  for  that  process  is,  as  we  shall  find,  peculiar  and 
complex.  But  such  a  doubt  need  not  prevent  us  from  recognizing  that  the 
whole  act  of  secretion  of  urine  consists  of  two  parts,  one  of  which  is  much 
more  closely  dependent  on  the  flow  of  blood  through  the  kidney  than  is  the 
ordinary  process  of  secretion,  such  as  has  hitherto  come  before  us,  and  another 
part  which  seems  to  bear  the  same  relation  to  the  flow  of  blood  as  does  ordi- 
nary secretion. 

That  the  work  of  the  kidney  is,  to  an  unusual  degree,  dependent  on  the 
flow  of  blood  through  it,  seems  suggested  by  the  vascular  arrangements ;  for 
these  are  extremely  favorable  to  a  full  and  rapid  stream  of  blood  through 
the  organ.  The  short  and  relatively  broad  renal  artery  comes  off  direct 
from  the  abdominal  aorta,  where  the  blood-pressure  is  extremely  high  ;  the 
renal  vein  opens  directly  into  the  vena  cava,  where  the  blood-pressure  is  ex- 
tremely low.  Between  the  mouth  of  the  renal  artery  and  the  mouth  of  the 
renal  vein  the  difference  of  pressure  is  very  great,  indeed  ;  and,  as  we  have 
seen  in  treating  of  the  vascular  system,  it  is  the  difference  of  pressure  be- 
tween two  points  of  the  vascular  tract  which  is  the  actual  cause  of  the  flow 
of  blood  from  the  one  point  to  the  other.  The  difference  of  pressure,  in- 
deed, which  drives  the  blood  through  the  limited  area  of  the  kidney  is  the 
same  difference  of  pressure  which  drives  the  blood  along  the  abdominal 
aorta  down  both  legs  back  again  to  the  vena  cava. 

This  free  and  abundant  supply  of  blood  is  regulated,  is  either  increased 
or  diminished,  according  to  the  needs  of  the  moment,  by  the  vasomotor  sys- 
tem ;  this  is  shown  by  experimental  and  other  results,  which  it  will  be  profit- 
able to  study  in  some  detail.  Before  entering  into  these  details,  however,  it 
will  be  well  to  call  attention  to  the  fact  that  when  vasomotor  events  modify 
the  flow  of  blood  through  an  organ,  they  produce  their  effects  in  one  direc- 
tion or  another  by  working  on  arterial  blood-pressure.  Thus,  as  we  shall  see, 
when  stimulation  or  section  of  a  nerve  increases  the  flow  of  blood  through  the 
kidney,  it  does  so  by  increasing  the  pressure  in  the  small  vessels  of  the  kid- 
ney, including  the  capillary  loops  of  the  glomeruli.  In  such  a  case  the  walls 
of  the  glomerular  loops,  through  which  the  passage  of  materials  to  form 
(part  of)  the  urine  takes  place,  are  subjected  to  two  influences — on  the  one 
hand,  to  a  fuller,  more  rapid  flow  of  blood  past  them,  and,  on  the  other,  to 
an  increase  of  the  pressure  which  that  blood,  as  it  passes  along,  exerts  on 
them.  We  shall  have,  subsequently,  to  discuss  the  share  taken  by  these  two 
influences  in  determining  and  modifying  the  passage  of  material  through 
the  walls  of  the  glomerular  loops ;  and  this  will  bear  on  the  question  of 
filtration,  to  which  we  have  above  alluded ;  but,  for  the  present,  it  will  be 
convenient  to  deal  with  the  effects  of  variation  in  blood-pressure  apart  from 
this  secondary  question. 

§  346.  The  vasomotor  mechanisms  of  the  kidney.  It  may  be  shown  ex- 
perimentally that  the  kidney  is  supplied  with  a  vasomotor  mechanism  as 
well  developed,  perhaps,  as  that  of  any  part  of  the  body.  By  means  of  a 
modification  of  the  plethysmograph  (Figs.  110,  111),  we  can  readily  observe 
the  variations  which  take  place  in  the  volume  of  the  kidney. 


412 


THE  ELIMINATION  OF  WASTE  PRODUCTS. 


The  instrument  consists  of  two  parts,  one  of  which  (Fig.  110),  called  the  oncom- 
eter,1  is  applied  to  the  organ  about  to  be  studied,  while  the  other  (Fig.  Ill), 
called  the  oncograph,  is  the  recording  part  of  the  apparatus.  Any  diminution  in 
the  volume  of  the  organ  (Fig.  110,  it),  kidney,  spleen,  etc.,  as  the  case  may  be, 
diminishes  the  pressure  on  the  fluid  in  the  chamber  a ;  some  of  the  fluid  in  the 
chamber  M  (Fig.  Ill)  accordingly  passes  through  the  tube  K  (Fig.  Ill)  and  the 
tube  T  (Fig.  110)  to  the  chamber  a  ;  the  piston  D  accordingly  falls  and  with  it  the 
lever  H.  Similarly  an  increase  in  the  volume  of  the  organ  causes  the  lever  to  rise. 

FIG.  110. 


Renal  Oncometer.  (Seen  in  section,  semi-diagrammatic.)  K,  kidney ;  V,  vessels  and  nerves 
imbedded  in  fat,  etc.,  entering  hilus  of  organ ;  O,  C,  and  I,  C,  outer  and  inner  metal  capsules 
screwed  together  by  the  screw  S,  and  holding  between  them  the  edge  of  the  membrane  M,  which 
applies  itself  to  the  surface  of  the  kidney  and  forms  with  the  metal  capsule  two  chambers,  a  and 
B,  one  of  which  (B)  is  closed  by  a  plug  filling  the  opening  B,  while  the  other  (a)  communicates 
by  a  tube  T  with  the  recording  instrument.  The  other  opening,  C  (which  is  closed  by  a  small 
tap),  is  for  the  purpose  of  filling  the  chamber  a  with  warm  oil  after  the  kidney  has  been  placed 
in  the  box,  the  other  chamber  B  having  been  previously  partly  filled,  the  quantity  introduced 
into  it  depending  upon  the  size  of  the  kidney. 

The  volume  of  the  kidney  may  be  increased  by  a  swelling  of  its  con- 
stituent cells  and  other  structural  elements,  by  an  accumulation  of  lymph  in 
its  lymph-spaces,  and  by  a  distention  of  its  bloodvessels.  Compared  with 
the  third,  the  two  former  causes  are  in  health  so  insignificant  and  prob- 
lematical that  they  may  be  disregarded.  Further,  the  distention  of  the 
bloodvessels  will  in  general  depend  on  the  constriction  or  dilatation  of  the 
renal  arteries  and  their  ramifications,  for  distention  due  to  venous  obstruction 
will  only  occur  in  special  cases.  Hence  variations  in  the  volume  of  the  kid- 
ney may  be  taken  as  a  measure  of  variations  in  its  vascular  supply — increase 
of  volume  indicating  dilated  renal  vessels,  and  decrease  of  volume  indicating 
constriction  of  the  renal  vessels. 

1  From  oncos.  bulk. 


THE  SECRETION  OF   URINE. 


413 


When  by  means  of  the  instrument  just  described  a  tracing  is  taken  of 
the  volume  of  a  kidney  in  what  maybe  considered  a  normal  condition,  some 
such  result  as  that  shown  in  Fig.  112  is  obtained. 


FIG.  ill. 


Semi-diagrammatic  Sectional  View  of  Oncograph.  (Half  natural  size.)  K,  tube  connecting 
instrument  with  oncometer;  D,  piston  floating  on  oil  contained  in  the  cavity  M;  the  oil  is  pre- 
vented from  escaping  by  the  side  of  the  piston  by  the  delicate  flexible  membrane  E,  which  does 
not  interfere  with  the  movements  of  the  piston  ;  H,  recording  lever  connected  with  the  piston  by 
a  needle  G  passing  through  the  guides  F,  F'.  The  screw  C  is  for  the  purpose  of  clamping  the 
edge  of  the  membrane  between  the  two  ring-shaped  surfaces  at  Awhile  the  side  tube  L  is  for  the 
purpose  of  filling  the  instrument. 

The  volume  of  the  kidney  is  seen  to  be  so  delicately  responsive  to 
changes  in  the  mean  arterial  pressure  that  the  curve  reproduces  almost 
exactly  a  blood-pressure  curve,  showing  not  only  the  respiratory  undulations 


FIG.  112. 


BLOOD  PRESSURE 


KIDNEY     CURVE 


Blood -pressure  Tracing,  and  Curve  from  Renal  Oncometer.  (Natural  size.)  The  blood-pres- 
sure abscissa  line  has  been  raised  2.75  cm.  (the  actual  medium  blood-pressure  having  been  115 
mm.  Hg.).  The  time-curve  gives  interruptions  recurring  every  three  seconds. 

but  even  the  rise  and  fall  due  to  the  individual  heart-beats.  With  each  rise 
of  mean  arterial  pressure  more  blood  is  driven  into  the  renal  vessels  and  the 
kidney  swells ;  with  each  fall  of  pressure  less  blood  enters  and  the  kidney 
shrinks.  On  other  tracings  taken  in  the  same  way  may  often  be  seen  (not 


414  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

shown  in  Fig.  112)  the  wider  variations  corresponding  to  the  Traube-Hering 
curves ;  but  it  will  be  observed  that  in  these  the  kidney  shrinks  with  the 
rise  of  pressure  and  swells  with  the  fall.  For  as  we  have  seen  (§  331)  the 
rise  in  the  Traube-Hering  undulation  is  due  to  an  augmentation  of  peripheral 
resistance  caused  by  the  constriction  of  minute  arteries ;  and  this  constriction 
occurs  in  the  kidney  as  elsewhere ;  the  renal  arterioles  take  their  share  in 
producing  the  result,  arid  in  consequence  of  their  constriction  the  kidney 
shrinks.  Similarly  the  relaxation  of  the  renal  vessels  contributes  to  bring 
about  the  sequent  fall. 

§  347.  In  the  course  of  a  discussion  in  an  earlier  part  of  this  work 
(§  157)  on  the  local  and  general  effects  of  arterial  constriction  and  dilata- 
tion, we  saw  that  the  local  blood-pressure  in  and  flow  of  blood  through  the 
capillaries  and  other  minute  vessels  of  this  or  that  vascular  area  may  be 
increased — 

1.  By  an  increase  of  the  general  blood-pressure  brought  about — (a)  by  an 
increased  force,  frequency,  etc.,  of  the  heart's  beat,  (6)  by  the  constriction  of 
the  small  arteries  supplying  areas  other  than  the  area  in  question. 

2.  By  a  relaxation  of  the  artery  (or  arteries)  supplying  the  area  itself, 
which,  while  diminishing  the  pressure  in  the  artery  itself,  increases  the 
pressure  in  the  capillaries  and  small  veins  which  the  artery  supplies.     It 
need  hardly  be  added  that  this  local  relaxation  must  not  be  accompanied 
by  a  too  great  dilatation  elsewhere. 

**The   same   local    blood-pressure   and   flow  of  blood   may  similarly  be 
diminished — 

1.  By  a  constriction  of  the  artery  of  the  area  itself  (and  its  branches), 
which,  while  increasing  the  pressure  on  the  cardiac  side  of  the  artery,  dimin- 
ishes the  pressure  in  the  capillaries  and  veins  which  are  supplied  by  the 
artery.     This  again  must  not  be  accompanied  by  a  too  great  constriction 
elsewhere. 

2.  By  a  lowering  of  the  general  blood-pressure,  brought  about — (a)  by 
diminished  force,  etc.,  of  the  heart's  beat,  (0)  by  a  general  dilatation  of  the 
small  arteries  of  the  body  at  large,  or  by  a  dilatation  of  vascular  areas  other 
than  the  area  in  question. 

Applying  these  considerations  to  the  bloodvessels  of  the  kidney,  we 
should  expect  to  find  the  following: 

A  rise  in  general  blood-pressure,  and  that  means  a  rise  of  pressure  in  the 
abdominal  aorta  at  the  mouth  of  the  renal  artery,  will  cause  a  greater  flow 
of  blood  through,  and  so  an  expansion  of  the  kidney,  provided  that  the  renal 
arteries  themselves  are  not  unduly  constricted  at  the  same  time.  This  is 
well  shown,  as  we  have  seen,  in  the  curve  given  above,  where  the  increase 
of  pressure  due  to  each  heart-beat,  as  well  as  that  due  to  each  respiratory 
movement,  being  of  central  origin  and  not  due  to  arterial  constriction  and 
being  unaccompanied  by  any  compensating  constriction  of  the  renal  artery, 
leads  to  expansion  of  the  kidney,  that  is,  to  a  greater  flow  of  blood  through 
the  kidney. 

If,  however,  the  rise  of  general  blood-pressure  be  due  to  events  which  at 
the  same  time  cause  a  constriction  of  the  renal  arteries,  the  flow  through  the 
kidney  may  not  only  not  be  increased,  but  even- be  diminished  ;  the  kidney 
may  shrink  instead  of  expanding.  Thus  if  dyspnoea  be  brought  about,  as 
by  stopping  artificial  respiration  during  an  experiment,  the  kidney  at  once 
shrinks  ;  the  too  venous  blood  stimulates  the  vasomotor  centre,  and  probably 
also  by  direct  action  on  the  bloodvessels  leads  to  a  general  arterial  constric- 
tion and  so  to  a  rise  of  blood-pressure ;  but  the  renal  vessels  are  involved  in 
this  constriction,  so  much  so  that  their  constricted  condition  more  than  coun- 
terbalances the  general  rise  of  blood-pressure,  and  less  blood  flows  through 


THE  SECRETION  OF  URINE.  415 

the  renal  vessels.  So  also  when  the  medulla  or  spinal  cord  is  directly  stimu- 
lated by  induction  shocks  (the  animal  being  under  urari  so  as  to  eliminate 
the  complications  due  to  contractions  of  the  skeletal  muscles)  the  renal  ves- 
sels share  so  fully  in  the  arterial  constriction  which  results  that,  in  spite  of 
the  great  rise  of  mean  pressure  which  is  induced,  less  blood  than  normal 
passes  through  the  renal  vessels,  and  the  kidney  shrinks.  Or  if  the  abdom- 
inal splanchnic  nerves  be  stimulated,  since,  as  we  shall  see,  these  carry  vaso- 
constrictor fibres  for  the  kidney,  in  spite  of  the  rise  of  blood-pressure  which 
follows,  the  kidney  shrinks  on  account  of  the  great  constriction  of  the  renal 
vessels. 

On  the  other  hand,  if  a  rise  of  blood-pressure  be  for  any  reason  not  ac- 
companied by  a  compensating  constriction  of  the  renal  arteries,  that  rise, 
whether  it  be  brought  about  by  general  constriction  of  arteries  other  than 
the  renal  or  by  an  increase  of  the  cardiac  delivery,  causes  the  kidney  to 
swell,  showing  a  greater  flow  of  blood.  Such  a  condition  of  things  may  be 
induced  by  section  of  the  nerves  of  the  renal  plexus,  whereby  the  paths  of 
all  vaso-constrictor  impulses  to  the  kidney  are  blocked.  After  this  has  been 
done,  a  rise  of  general  pressure,  whether  by  dyspnoea,  or  by  direct  stimulation 
of  the  spinal  cord,  or  by  stimulation  of  the  abdominal  splanchnic  nerves, 
leads  to  a  greater  flow  through  the  renal  vessels  and  an  increased  expansion 
of  the  kidney. 

A  rise  of  general  blood-pressure,  then,  may  be  accompanied  by  either  a 
shrinking  or  a  swelling  of  the  kidney,  by  either  a  greater  or  lesser  flow  of 
blood  through  the  kidney,  according  to  the  concomitant  condition  of  the 
renal  vessels ;  or,  indeed,  may  under  certain  circumstances  be  accompanied 
by  no  change  at  all  in  the  renal  circulation,  the  local  effects  exactly  counter- 
balancing the  general  ones. 

Conversely,  in  a  similar  way,  a  fall  of  blood- pressure  leads  to  a  lesser 
flow  through  the  renal  vessels  and  a  shrinkage  of  the  kidney  unless  it  be 
accompanied  by  a  dilatation  of  the  renal  vessels  out  of  proportion  to  the 
general  fall.  Thus  when  the  spinal  cord  is  divided  below  the  medulla  the 
fall  of  general  blood-pressure  is,  as  we  have  seen  (§  159),  very  marked, 
being  due  to  an  abolition  for  the  time  being  of  wonted  constrictor  impulses. 
The  pressure  in  the  aorta  falls  rapidly,  and  at  the  same  time,  owing  to  the 
more  open  pathway  through  the  region  of  peripheral  resistance  in  the  body 
generally,  the  pressure  in  the  vena  cava  is  increased  ;  the  difference  of  pres- 
sure between  the  mouth  of  the  renal  artery  in  the  aorta  and  the  mouth  of 
the  renal  vein  in  the  vena  cava  is  so  largely  reduced  that  in  spite  of  the 
concomitant  relaxed  condition  of  the  renal  vessels  themselves  the  flow  of 
blood  through  the  kidney  is  largely  diminished. 

It  will  of  course  be  understood  that,  the  general  blood-pressure  remain- 
ing the  same,  the  flow  through  the  kidney  will  at  once  be,  on  the  one  hand, 
increased  by  dilatation  and,  on  the  other,  decreased  by  constriction  of  the 
renal  vessels  themselves.  The  constricted  or  dilated  condition  of  the  renal 
vessels  can  by  themselves  produce  but  little  effect  on  the  pressure  either  in 
the  aorta  or  in  the  vena  cava ;  and  the  difference  between  the  pressure  at 
the  mouth  of  the  renal  artery  and  that  at  the  mouth  of  the  renal  vein  re- 
maining the  same,  the  more  open  passages  of  the  dilated  renal  vessels  must 
lead  to  a  fuller,  and  the  narrower  passages  of  the  constricted  renal  vessels  to 
a  scantier,  flow  through  the  kidney. 

§  348.  By  means  of  the  oncometer,  watching  the  shrinking  and  swelling 
of  the  kidney  and  thus  judging  of  the  flow  of  blood  through  it,  the  results 
being  always  interpreted  with  reference  to  the  general  blood-pressure  on  the 
lines  of  the  above  discussion,  the  paths  of  vasomotor  impulses  to  the  kidney 
have  been  approximately  made  out.  Vaso-coustrictor  fibres  for  the  kidney 


416  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

are  supplied  from  what  we  have  previously  (§  155  and  elsewhere)  spoken  of 
as  the  vaso-constrictor  region  of  the  spinal  cord.  They  issue  from  the  spinal 
cord  by  the  anterior  roots  of  a  large  number  of  the  spinal  nerves  taking 
origin  from  this  region,  and  may  be  traced  (in  the  dog)  as  high  up  as  the 
sixth  dorsal,  a  few  perhaps  even  to  the  fourth  dorsal,  and  as  low  down  as  the 
second  lumbar  (fourth  lumbar  if  only  thirteen  nerves  be  counted  as  dorsal)  ; 
but  most  seem  to  pass  by  the  eleventh,  twelfth,  and  thirteenth  dorsal  nerves. 
Passing  through  the  corresponding  ganglia  of  the  splanchnic  (sympathetic) 
chain,  these  fibres  reach  the  solar  plexus  and  thus  the  renal  plexus  by  the 
abdominal  splanchnic  nerve  ;  those,  however,  coming  from  some  of  the  lower 
nerves  apparently  do  not  contribute  to  the  splanchnic  nerve,  but  take  a  sepa- 
rate course.  Centrifugal  stimulation  of  these  anterior  roots  produces  shrink- 
ing of  the  kidney,  all  the  more  marked  and  distinct  in  the  case  of  the 
eleventh,  twelfth,  and  thirteenth  dorsal  roots  because  the  effect  on  the  kidney 
is  then  not  so  much  masked  by  vasomotor  effects  on  other  organs.  Stimu- 
lation of  the  higher  roots  also  produces  shrinking  of  the  kidney,  but  less 
marked,  since  in  these  cases  the  stimulation  bears  at  the  same  time  largely 
on  vaso-constrictor  fibres  for  other  abdominal  organs,  and  so  by  raising  the 
general  blood-pressure  tends  to  neutralize  the  local  effect  on  the  kidney. 
And  even  the  very  decided  shrinking  of  the  kidney  which  results  from  the 
stimulation  of  the  splanchnic  trunk  itself  is  less  than  would  take  place  if 
the  stimulation  affected  the  vessels  of  the  kidney  only. 

§  349.  We  stated  in  §  154  that  by  the  method  of  slowly  repeated  rhyth- 
mical stimulation  the  presence  of  vaso-dilator  fibres  in  the  sciatic  nerve 
might  be  detected,  though  these  are  largely  mixed  with  vaso-constrictor 
fibres ;  and  slow  rhythmical  stimulation  of  the  anterior  roots  of  the  above- 
mentioned  lower  dorsal  nerves  leads,  not,  as  does  ordinary  rapidly  inter- 
rupted stimulation,  to  shrinking,  but  to  swelling  of  the  kidney,  showing  that 
these  roots  contain  vaso-dilator  fibres  as  well  as  vaso-constrictor  fibres.  The 
higher  (anterior)  roots  also  appear  to  contain  some  renal  vaso-dilator  fibres  ; 
but  the  effect  of  stimulating  them  by  the  slow  rhythmic  method  is  more 
masked  by  a  concomitant  dilatation  of  the  vessels  of  the  other  abdominal 
organs,  the  roots  in  question  containing  vaso-dilator  as  well  as  vaso-con- 
strictor fibrjes  for  those  organs ;  this  leads  to  a  fall  of  general  blood-pressure 
whereby  the  tendency  of  the  kidney  to  swell  is  counteracted. 

§  350.  It  is  obvious  then  that  by  means  of  this  vasomotor  mechanism 
the  ffow  of  blood  through  the  kidney  is  governed  by  the  central  nervous 
system  in  such  a  way  that  afferent  impulses,  started  in  this  or  that  region  or 
surface,  and  passing  up  to  the  central  nervous  system,  may  lead  either  to 
constriction  or  to  dilatation  of  the  renal  vessels ;  and  to  such  actions  of  this 
kind  we  shall  presently  return.  Meanwhile  we  wish  to  call  attention  to  the 
fact  that  the  volume  of  the  kidney  is  remarkably  sensitive  to  chemical 
changes  taking  place  in  the  blood.  The  injection  into  the  blood  of  even  a 
small  quantity  of  water  causes  a  transient  shrinking  of  the  kidney  followed 
by  a  more  lasting  expansion.  The  injection  of  urea  and  some  other  diuretics 
produces  the  same  effect  to  a  more  marked  degree,  leading  especially  to  a 
swelling  which  lasts  for  some  considerable  time,  while  the  injection  of  normal 
saline  solution,  and  especially  of  such  diuretics  as  sodium  acetate,  causes  an 
expansion  from  the  very  first,  the  primary  shrinking  being  absent.  It  is, 
moreover,  worthy  of  note  that  these  effects  of  diuretics  and  of  chemical 
changes  in  the  blood  are  observed  even  after  all  the  renal  nerves  have 
apparently  been  completely  severed.  Hence  the  changes  in  volume  caused 
by  the  presence  of  these  substances  in  the  blood  must  be  due  to  the  sub- 
stances acting  either  upon  some  peripheral  vasomotor  mechanism,  or,  even 
more  directly,  on  the  bloodvessels  themselves.  It  may  be  added  that  they 


THE  SECRETION  OF  URINE.  417 

will  produce  considerable  effects  in  the  kidney  itself  without  appreciably 
modifying  the  general  blood-pressure. 

£  351.  If,  while  the  kidney  is  in  the  oncometer,  and  the  various  experi- 
ments on  section  and  stimulation  of  nerves  and  the  like  are  being  carried 
on,  a  canula  be  tied  in  the  ureter,  the  secretion  of  urine  may  be  watched  at 
the  same  time.  It  will  then  be  seen  that  the  flow  of  urine  through  the  end 
of  the  canuhi  is  not  equable,  and  does  not  either  increase  or  decrease  in  an 
even  manner.  On  the  contrary,  it  will  frequently  be  found  that  a  sort  of 
gush  of  urine  takes  place,  several  drops  following  each  other  in  rapid  suc- 
cession, followed  by  a  cessation  of  flow ;  and  if  the  ureter  be  watched  it 
will  be  seen  that  the  gushes  of  urine  are  synchronous  with  waves  of 
peristaltic  contraction  sweeping  down  the  ureter.  Obviously  the  urine 
collects,  to  a  certain  extent,  in  the  pelvis  of  the  kidney,  and  is  driven 
thence  by  muscular  action  from  time  to  time ;  to  this  point  we  shall  return 
later  on. 

Making  every  allowance,  however,  for  these  irregularities  of  flow,  we  may 
take  the  rate  of  flow  from  the  end  of  the  canula  as  a  measure  of  the  rate 
of  secretion  ;  and  it  is  found  that  as  a  general  rule  increased  flow  of  urine 
.is  coincident  with  swelling  of  the  kidney,  that  is  with  a  greater  flow  of 
blood  through  it,  and  diminished  or  arrested  flow  of  urine  is  coincident 
with  shrinking  of  the  kidney,  that  is,  with  a  diminished  flow  of  blood 
through  it. 

A  striking  instance  of  this  is  afforded  by  the  expriment  of  dividing  in 
the  dog  the  spinal  cord  below  the  medulla.  The  blood-pressure  then,  as  we 
know,  falls  rapidly,  owing  to  the  removal  of  constrictor  impulses  from  the 
small  arteries  and  the  general  diminution  of  peripheral  resistance  which 
follows  upon  so  many  small  arteries  becoming  dilated  ;  and  though  the  renal 
arteries  probably  share  in  the  general  relaxation,  yet,  owing  to  the  fall  of 
pressure  in  the  aorta  conjoined  as  this  is  by  a  corresponding  rise  of  pressure 
in  the  vena  cava,  the  flow  of  blood  through  the  kidney  is  largely  dimin- 
ished. We  find  that  after  the  operation  the  secretion  of  urine  is  greatly 
diminished ;  indeed,  in  most  cases  the  flow  from  the  end  of  a  canula  is 
almost  arrested.  In  fact,  we  may  almost  make  the  general  assertion  that, 
when  in  the  dog  the  blood-pressure  falls  to  about  30  mm.  Hg.  or  less,  the 
secretion  of  urine  is  for  the  time  stopped.  These  and  other  results  support 
the  view  stated  above  that  the  secretion  of  urine  is  in  quite  a  special  way 
dependent  on  the  flow  of  blood  through  the  kidney ;  and  we  may  further 
conclude  that  the  secretion  which  is  so  particularly  influenced  by  the  flow 
of  blood  is  that  special  kind  of  secretion,  allied  to  filtration,  which  takes 
place  through  the  glomeruli,  and  not  the  more  ordinary  kind  of  secretion 
by  means  of  the  epithelium  of  the  tubuli  uriniferi.  But  before  we  proceed 
to  discuss  how  the  increased  flow  of  blood  increases  the  glomerular  flow  of 
urine,  we  must  turn  to  consider  the  functions  of  the  epithelium  of  the 
tubuli. 

Secretion  by  the  Renal  Epithelium. 

§  352.  The  glomerular  mechanism  is,  after  all,  a  small  portion  only  of 
the  whole  kidney,  and  the  epithelium  over  a  large  part  of  the  course  of  the 
tubuli  uriniferi  bears  most  distinctly  the  characters  of  an  active  secreting 
epithelium.  These  facts  would  lead  us  d  priori  to  suppose  that  the  flow  of 
urine  is  in  part  the  result  of  an  active  secretion  comparable  to  that  of  the 
salivary  or  other  glands  which  we  have  already  studied.  And  we  have  ex- 
perimental and  other  evidence  that  such  is  the  case. 

In  the  first  place,  a  flow  of  urine  may  be  artificially  excited  even  when 
the  natural  flow  has  been  arrested  by  diminution  of  blood-pressure.  Thus 

27 


418"  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

if,  when  the  urine  has  ceased  to  flow  in  consequence  of  a  section  of  the 
medulla  oblongata,  certain  substances,  such  as  urea,  urates,  sodium  acetate, 
and  the  like,  be  injected  into  the  blood,  a  more  or  less  copious  secretion  is  at 
once  set  up.  This  secretion  is,  or  at  least  may  be,  unaccompanied  by  any 
rise  of  general  blood-pressure  sufficient  to  account  for  the  increased  secretion 
as  the  mere  result  of  an  increased  flow  of  blood.  It  is  true  (as  we  have 
seen,  §  350)  that  the  injection  of  these  substances  leads  to  an  expansion  of 
the  kidney,  an  expansion  which  is  probably  due  to  a  local  dilatation  of  the 
small  renal  arteries  ;  but  the  flow  of  urine  which  is  observed  in  these  cases 
is  too  great  to  be  accounted  for  by  any  increase  of  flow  of  blood  which  the 
local  dilatation  may  bring  about ;  and  hence  we  conclude  that  the  increase 
of  secretion  is  of  a  different  kind  from  that  which  follows  upon  mere  increase 
of  blood-flow.  It  seems  much  more  reasonable  to  suppose  that  the  presence 
of  the  above  substances  in  the  blood  excites  the  renal  epithelial  cells  to  an 
unwonted  activity,  causing  them  to  pour  into  the  interior  of  the  tubules  a 
copious  secretion,  just  as  the  presence  of  pilocarpine  in  the  blood  will  cause 
the  salivary  cells  to  pour  forth  their  secretion  into  the  lumen  of  their  ducts ; 
and  that  this  activity  of  the  epithelial  cells  is  accompanied,  also  as  in  the 
case  of  the  submaxillary  and  other  glands,  by  a  vascular  dilatation,  which, 
though  adjuvant  and  beneficial,  is  not  the  distinct  cause  of  the  activity. 
This  view  is  further  supported  by  the  following  remarkable  experiment, 
which  goes  far  to  show  that  of  the  various  substances  which,  having  found 
their  way  into  the  blood,  are  thrown  out  by  the  kidney,  some  pass  into  the 
urine  through  the  glomeruli,  while  others  are  distinctly  secreted  by  the 
tubuli  uriniferi,  the  discharge  of  the  latter  being  accompanied  by  a  general 
activity  of  the  secreting  cells,  as  shown  by  the  flow  of  water  taking  place  at 
the  same  time. 

In  the  amphibia  the  kidney  has  a  double  vascular  supply ;  it  receives 
arterial  blood  from  the  renal  artery,  but  there  is  also  poured  into  it  venous 
blood  from  another  source.  The  femoral  vein  divides  at  the  top  of  the  thigh 
into  two  branches,  one  of  which  runs  along  the  front  of  the  abdomen  to 
meet  its  fellow  in  the  middle  line  and  form  the  anterior  abdominal  vein, 
while  the  other  passes  to  the  outer  border  of  the  kidney  and  branches  in  the 
substance  of  that  organ,  forming  the  so-called  renal  portal  system.  Now 
the  glomeruli,  in  some  species  at  least  of  these  animals,  are  supplied  exclu- 
sively by  the  branches  of  the  renal  artery,  the  renal  vena  porta3  only  serv- 
ing to  form  the  capillary  plexus  around  the  tubuli  uriniferi,  which  is  also 
supplied  by  the  efferent  vessels  of  the  glomeruli.  From  this  it  is  obvious 
that  if  the  renal  artery  be  tied,  the  blood  is  shut  off  entirely  from  the 
glomeruli ;  and  actual  observation  of  the  kidney  has,  in  the  animals  in  ques- 
tion, shown  that  under  these  circumstance  there  is  no  reflux  from  the  capil- 
lary network  surrounding  the  tubules  back  to  the  glomeruli ;  thus  the  kidney 
by  this  simple  operation  is  transformed  into  an  ordinary  secreting  gland 
devoid  of  any  special  filtering  mechanism.  Such  a  kidney  may  be  used  to 
ascertain  what  substances  are  excreted  by  the  glomeruli,  and  what  by  the 
tubules  in  some  other  part  of  their  course.  It  is  found  that  urea  injected 
into  the  blood  gives  rise  to  a  secretion  of  urine  when  the  renal  arteries 
are  tied;  this  substance,  therefore,  is  secreted  by  the  epithelium  of  the  tu- 
bules, and  in  being  so  secreted  gives  rise  at  the  same  time  to  a  flow  of  water 
through  the  cells  into  the  interior  of  the  tubules.  Sugar  and  peptones,  on 
the  other  hand,  which  injected  into  the  blood  readily  pass  through  the  un- 
touched kidney  and  appear  in  the  urine,  do  not  pass  through  a  kidney  the 
renal  arteries  of  which  have  been  tied,  even  when  a  diuretic  such  as  urea 
is  given  at  the  same  time  in  order  to  secure  a  flow  of  urine.  These  sub- 
stances, therefore,  are  excreted  by  the  glomeruli. 


THE  SECRETION  OF  URINE.  419 

The  validity  of  this  experiment,  which  may  be  accepted  as  indicating  a 
marked  difference  between  glornerular  secretion  on  the  one  hand  and  epithe- 
lial or  tubular  secretion  on  the  other,  depends  on  the  absence  of  any  collateral 
circulation  whereby  the  glomeruli  may  be  supplied  with  blood  after  ligature 
of  the  renal  artery.  In  these  animals  anastomoses  occur  between  the  renal 
arteries  and  the  arteries  of  the  generative  organs ;  and  unless  the  renal 
artery  be  so  tied  as  to  avoid  these  collateral  communications  the  results 
of  the  experiment  are  different. 

Additional  evidence  in  favor  of  the  secretory  activity  of  the  epithelial 
cells  is  afforded  by  the  following  observation.  Into  the  veins  of  animals  in 
which  the  urinary  flow  had  been  arrested  by  section  of  the  spinal  cord  below 
the  medulla  a  quantity  of  the  blue  coloring  material  known  as  sodium 
sulphindigotate1  is  injected.  This  substance  is  rapidly  excreted  on  the  one 
hand  by  the  liver  in  the  bile,  and  on  the  other  hand  by  the  kidney.  By 
varying  the  quantity  injected,  killing  the  animals  at  appropriate  times  after 
the  injection  of  the  material,  and  examining  the  kidneys  microscopically 
and  otherwise,  it  may  be  ascertained  that  the  pigment  so  injected  passes  from 
the  blood  into  the  renal  epithelium,  and  from  thence  into  the  channels  of 
the  tubules.  There  being  no  stream  of  fluid  through  the  tubules,  owing  to 
the  arrest  of  urinary  flow  by  means  of  the  preliminary  operation,  the  pig- 
ment travels  very  little  way  down  the  interior  of  tubules,  and  remains  very 
much  where  it  was  cast  out  by  the  epithelial  cells.  There  are  no  traces 
whatever  of  the  pigment  having  passed  by  the  glomeruli ;  and  the  cells 
which  appear  most  distinctly  to  take  up  and  eject  it  are  those  lining  such 
portions  of  the  tubules  (viz.,  the  first  and  second  convoluted  tubules,  zigzag 
tubules,  and  ascending  limbs  of  the  loops  of  Henle)  as  from  their  micro- 
scopic features  have  been  supposed  to  be  the  actively  secreting  portions  of 
the  entire  tubules.  The  following  observation  which  has  been  made  is  of  a 
peculiarly  interesting  character.  After  injecting  a  certain  quantity  of  pig- 
ment, and  allowing  such  a  time  to  elapse  as  might  be  judged  from  previous 
experiments  would  suffice  for  the  passage  of  the  material  through  the  epithe- 
lium to  be  pretty  well  completed,  a  second  quantity  was  injected.  It  was 
found  that  the  excretion  of  this  second  quantity  was  most  incomplete  and 
imperfect.  It  seems  as  if  the  cells  were  exhausted  by  their  previous  efforts, 
just  as  a  muscle  which  has  been  severely  tetanized  will  not  respond  to  a 
renewed  stimulation. 

The  above  observation  may  be  objected  to  on  the  ground  that  this  color- 
ing matter  does  not  occur  as  a  constituent  of  the  blood  either  in  health  or 
disease,  and  especially  that  the  absence  of  any  concomitant  discharge  of 
fluid  from  the  cells  excites  suspicion  that  the  process  observed  was  not  really 
one  of  secretion ;  for  the  injection  of  such  substances  as  urea  or  urates  into 
the  blood  does  cause  a  copious  flow  of  fluid,  and  indeed  thus  prevents  the 
microscopic  tracking  out  of  their  passage,  which  in  the  case  of  urates  might 
otherwise  be  done  much  in  the  same  way  as  with  the  sodium  sulphindigotate. 
Moreover,  other  observers  have  maintained  that  the  sodium  sulphindigotate 
does  like  ordinary  carmine  pass  through  the  glomeruli.  But  their  results 
may  probably  be  explained  by  the  glomeruli  having  been  damaged  by  a  too 
rapid  or  too  abundant  injection  ;  and  in  the  case  of  the  amphibian  kidney, 
when  sodium  sulphindigotate  is  injected  after  ligature  of  renal  arteries,  no 
urine  is  found  in  the  bladder,  but  the  pigment  can  be  traced  through  the 
epithelium  of  the  secreting  portions  of  the  tubules.  Without  insisting  too 
much  on  the  value  of  the  sodium  sulphindigotate  experiments,  they  may  be 
taken  as  fairly  supporting  the  view  which  we  are  considering.  We  may  add 

1  Sometimes  called  indigo-carmine,  though  this  name  is  more  properly  applied  to  a 
crude,  impure  preparation  of  potassium  sulphiudigotate. 


420  THE   ELIMINATION   OF  WASTE  PRODUCTS. 

that  in  birds,  the  urine  of  which  contains  little  water,  urates  maybe  detected 
in  the  epithelium  of  the  tubules,  but  not  in  the  capsules. 

Though  much  remains  to  be  cleared  up,  we  may,  for  the  present,  con- 
clude that  the  secretion  of  urine  does  consist  of  two  separate  and  distinct 
acts;  secretion  by  the  glomeruli,  which  we  may  for  brevity's  sake  speak  of 
as  glomerular  secretion,  and  secretion  by  the  epithelium  of  the  tubuli,  which 
we  may  speak  of  similarly  as  tubular  secretion.  But  these  forms  of  secre- 
tion, especially  the  former,  but  to  a  certain  extent  the  latter  also,  differ  from 
the  secretion  of  such  a  gland  as  the  salivary,  and  both  deserve  some  special 
consideration. 

§  353.  T/ie  nature  of  glomerular  secretion.  We  have  seen  that  the  ex- 
pansion of  the  kidney  which  has  for  its  accompaniment  an  increased  flow 
of  urine  is  one  brought  about  by  the  renal  artery  and  its  various  branches 
becoming  dilated,  under  such  circumstances  that  the  difference  between  the 
blood-pressure  in  the  aorta  at  the  mouth  of  the  renal  artery  and  the  blood- 
pressure  at  the  vena  cava  at  the  mouth  of  the  renal  vein  is  at  the  same  time 
increased,  or  at  all  events  is  not  diminished. 

In  dealing  with  the  vascular  system  we  saw  that  relaxation  of  a  small 
artery,  taking  place  without  any  marked  change  in  the  general  blood-pres- 
sure and  in  neighboring  arteries,  leads  to  a  fuller  and  more  rapid  stream  of 
blood  through  the  capillaries  supplied  by  the  artery,  and  that  at  the  same 
time  the  pressure  in  the  capillaries  themselves  is  increased ;  owing  to  the 
decrease  of  peripheral  resistance  through  the  widening  of  the  artery,  the 
great  fall  of  pressure  (see  §  105)  so  characteristic  of  the  peripheral  region 
is  shifted  from  the  arterial  side  of  the  capillaries  toward  the  venous  side  and 
to  the  capillaries  themselves. 

Hence,  as  we  have  already  said,  when  the  renal  artery  dilates  two  things 
happen  in  the  loops  of  the  glomeruli :  a  fuller,  more  rapid  stream  of  blood 
passes  through  them,  and  that  blood  as  it  flows  through  them  is  exerting  a 
greater  pressure  than  before  on  their  walls.  How  does  each  of  the  events 
stand  toward  the  secretion  of  urine? 

We  have  not  at  present  the  means  of  inducing  a  fuller  and  more  rapid 
flow  without  increasing  the  pressure ;  but  we  may  easily  obtain  increase  of 
pressure  without  the  fuller  and  more  rapid  flow.  If  we  hinder  or  obstruct 
the  outflow  through  the  renal  vein  we  at  once  increase  the  pressure  in  the 
glomerular  loops  as  in  the  other  capillaries  of  the  kidney.  Now,  when  the 
blood-pressure  in  the  glomeruli  is  thus  raised  by  partial  obstruction  to  the 
venous  outflow,  the  flow  of  urine  so  far  from  being  increased  is  diminished. 
Obviously,  then,  the  passage  of  water  and  material  through  the  walls  of  the 
glomerular  loops,  to  go  to  form  the  urine,  is  not  the  result  of  mere  pressure, 
and  cannot,  therefore,  be  spoken  of  properly  as  a  process  of  filtration. 
(Cf.  §  255.)  And  we  may  here  draw  a  comparison  between  the  passage  of 
water  and  material  through  the  wall  of  a  capillary  in  an  ordinary  situation 
to  form  lymph  and  the  passage  through  the  wall  of  the  glomerular  loop  to 
form  urine  or  part  of  urine.  The  former,  as  we  have  seen  (§  255),  appears 
to  be  directly  dependent  on  pressure,  though  influenced  as  we  have  also 
seen  in  a  very  material  way  by  the  condition  of  the  vascular  wall ;  and 
hindrance  to  venous  outflow,  so  inefficient  in  promoting  a  flow  of  urine,  is 
as  we  have  seen  especially  favorable  to  the  transudation  of  lymph.  In  the 
former  case  the  substances  which  pass  through  the  capillary  wall  may  be 
described  as  the  constituents  of  the  blood  generally,  proteids  as  well  as  salts 
and  other  soluble  and  diffusible  matters.  Through  the  wall  of  the  glomer- 
ular loop  there  pass,  so  long  as  that  wall  is  sound  and  intact,  neither 
albumin  nor  globulin  nor  fibrin  factor,  but  only  water  accompanied  by 
some,  and  apparently  a  selection  of  some,  of  the  soluble  diffusible  constitu- 


THE  SECRETION  OF   URINE.  421 

ents  of  the  blood ;  for,  as  we  have  said,  the  presence  of  proteids  in  normal 
urine  is  contested,  and,  at  most,  there  is  present  a  very  small  quantity  only 
(which,  moreover,  may  come  from  the  tubular  epithelium).  This  difference 
in  the  material  which  passes  through  may  be  referred  to  the  differences  in 
the  nature  of  the  partition.  The  transudation  of  lymph  takes  place  through 
the  capillary  wall ;  between  the  blood  on  one  side  and  the  lymph  in  the 
lymph-space  on  the  other  is  only  the  thin  film  of  conjoined  epithelioid 
plates.  But  the  corresponding  wall  of  the  glornerular  loop  is  covered  over 
and  wrapped  around,  so  to  speak,  by  an  adherent  layer  of  cells,  which 
though  reduced  and  thin  are  still  epithelial  cells ;  the  materials  which  go 
to  form  urine  have  to  pass  through  these  cells  as  well  as  through  the  film 
of  epithelioid  plates.  It  seems  to  be  this  layer  of  cells  which  determines 
what  shall  pass  and  what  shall  not. 

Obviously  the  passage  through  this  epithelium  is  of  a  peculiar  nature. 
The  necessary  condition  for  the  due  accomplishment  of  the  passage  is  as  we 
have  seen  a  full  and  rapid  stream  of  (arterial)  blood ;  the  high  pressure 
which  accompanies  that  full  and  rapid  stream,  though  probably  under 
normal  circumstances  an  adjuvant,  is  by  itself  helpless.  Thus  when  the 
pressure  is  raised  by  venous  obstruction,  in  which  case  the  high  pressure  is 
accompanied  by  a  slow  stream  or  by  actual  arrest  of  the  flow,  even  the 
passage  of  mere  water  is  retarded.  Seeing  that  many  of  the  constituents 
of  urine  are  diffusible  substances  certainly  preexisting  in  the  blood,  inor- 
ganic salines  for  instance,  and  seeing  that  diffusible  abnormal  constituents 
of  blood,  such  as  peptone  and  sugar,  pass  into  the  urine  not  by  the  tubular 
epithelium  but  by  the  glomeruli,  we  might  expect  that  diffusion,  in  contrast 
to  filtration  (see  §  265),  played  an  important  part  in  the  passage ;  and  a 
full  rapid  stream  would  undoubtedly  favor  diffusion.  But  diffusion  by 
itself  will  not  explain  matters.  Egg-albumin  differs  very  slightly  as  regards 
diffusibility  from,  serum-albumin,  and  yet  while  at  the  most  a  minute  quan- 
tity only  of  the  latter  passes  into  the  urine  in  normal  circumstances,  the 
former  when  injected  into  the  blood  at  once  makes  its  way  into  the  urine, 
presumably  by  the  glomeruli.  On  the  other  hand  urea  is  an  eminently  diffus- 
ible body,  and  yet  if  we  can  trust  the  experiments  on  the  amphibian  kidney, 
the  main  mass  at  all  events  of  the  urea  of  the  urine  passes  by  the  epithelium 
of  the  tubules. 

The  important  part  played  by  the  epithelium  is  shown  when  the  epithe- 
lium is  deranged.  If  the  renal  artery  be  temporarily  ligatured  or  other- 
wise obstructed,  so  that  the  glomeruli  are  shut  off  from  the  blood-supply  for 
some  little  time,  the  secretion  of  urine  is  stopped  ;  on  reestablishment  of  the 
circulation  the  secretion  of  urine  slowly  returns,  and  the  urine  is  then  found 
to  be  albuminous,  remaining  so  for  some  little  time.  The  serum-albumin 
and  globulin  which  could  not  pass  through  the  intact  epithelium,  can  pass 
through  when  the  epithelium  is  damaged  by  interference  with  its  nutrition. 
The  appearance  of  albumin  in  the  urine  (albuminuria)  is  a  not  infrequent 
symptom  of  kidney  disease,  and  its  presence  in  other  than  minute  quantities 
indicates  imperfections  in  the  glomerular  epithelium.  But  even  under  un- 
healthy conditions  that  epithelium  still  governs  to  a  certain  extent  the  pas- 
sage of  material ;  for  the  proteids  of  the  blood-plasma  do  not  pass  through 
bodily  or  in  a  proportion  which  corresponds  either  to  the  relative  proportion 
in  which  they  exist  in  the  plasma  or  to  the  relative  ease  (or  difficulty)  with 
which  they  pass  through  membranes.  Though  the  "  albumin"  of  albumin- 
ous urine  frequently  consists  of  both  serum-albumin  and  globulin,  these  do 
not  necessarily  occur  in  the  same  proportion  as  in  blood  ;  they  vary  in  urine 
much  more  than  they  do  in  blood ;  and  indeed  the  one  or  the  other  may  be 
absent ;  moreover,  fibrin  factors  are  very  rarely  found. 


422  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

Hsernoglobinuria,  or  the  presence  of  haemoglobin  in  urine,  may  be  brought 
about  by  injecting  into  the  bloodvessels  laky  blood,  or  some  substance  such 
as  pyrogallic  acid,  which  will  "  break  up"  the  corpuscles  of  the  blood.  Now, 
in  such  cases  there  is  evidence  that  the  haemoglobin  passes  through  the 
glomeruli ;  minute  disc-like  masses  of  haemoglobin,  the  so-called  "  menisci," 
are,  by  appropriate  methods  of  preparation,  found  in  situ  in  the  capsules. 
Such  a  passage  is  very  far  removed  from  being  a  process  of  diffusion. 

We  may  conclude,  then,  that  the  passage  of  material  through  the  glom- 
eruli, like  the  transudation  of  lymph,  and  even  to  a  more  marked  extent,  is 
a  complex  affair  in  which  the  ordinary  physical  processes  of  diffusion  and 
filtration  may  play  their  part,  but  are  not  masters  of  the  situation. 

§  354.  The  work  of  the  epithelium  of  the  tubules.  As  we  have  said,  the 
structural  features  of  the  epithelial  cells  of  the  tubules  seem  to  justify  the 
conclusion  that  they  exercise  a  secretory,activity  comparable  with  that  of  a 
salivary  or  a  gastric  gland.  But  their  work  is  in  many  ways  peculiar.  In 
the  case  of  the  salivary,  gastric,  and  pancreatic  glands  there  can  be  no  doubt 
that  the  specific  constituents  of  the  several  secretions,  mucin,  pepsin,  trypsin, 
and  the  like,  are  manufactured  in  the  alveolar  cells  out  of  antecedents  of 
some  nature  or  other.  The  evidence,  as  we  have  seen,  is  all  against  the 
view  that  these  glands  merely  withdraw,  secrete  in  the  old  sense  of  the  word, 
from  the  blood  these  substances  preexisting  in  the  blood.  When  the  salivary 
glands  are  extirpated,  or  the  pancreas  or  the  stomach  removed,  there  is  no 
accumulation  in  the  blood  of  the  specific  constituents  of  the  corresponding 
secretions.  So  also  when  the  liver  is  extirpated  there  is  no  accumulation  in 
the  blood  of  either  bile  acids  or  bile  pigment.  With  regard  to  the  kidney 
and  the  most  important  constituent  of  urine,  namely,  urea,  the  case  is  differ- 
ent. If  the  kidneys  in  a  mammal  be  extirpated,  or  if  the  kidneys  by  disease 
or  by  ligature  of  the  ureters  be  so  damaged  as  to  be  unable  to  carry  on  their 
work,  an  accumulation  takes  place  in  blood,  not  as  was  once  thought,  of  some 
antecedent  of  urea,  such  as  kreatin,  but  of  urea  itself.  In  the  case  of  birds 
and  reptiles  which  excrete  not  urea,  but  chiefly  uric  acid,  the  accumulation 
is  one  of  uric  acid.  Obviously  in  secreting  urea  the  work  of  the  epithelium 
of  the  tubules  is  largely,  if  not  exclusively,  confined  to  simply  picking  the 
urea  out  of  the  blood  and  pushing  it,  so  to  speak,  into  the  lumina  of  the 
tubules.  We  might,  perhaps,  say  exclusively,  for  there  is  no  evidence  that 
any  urea  at  all  is  actually  manufactured  in  the  kidney. 

But  even  this  mere  picking  up  the  urea  is  after  all  not  a  simple  process ; 
the  epithelial  cell  of  the  tubule  is  not  a  mere  passive  sieve  of  peculiar  struc- 
ture, especially  adapted  to  strain  off  the  urea  from  the  blood.  As  we  have 
already  seen,  when  urea  or  uric  acid  is  injected  into  the  blood  the  result  is 
not  a  mere  increase  in  the  proportions  of  urea  (or  uric  acid)  present  in  the 
urine  which  is  being  secreted.  The  injection  leads  to  an  increased  flow  of 
urine,  the  whole  activity  of  the  cell  is  stirred  up,  and  other  constituents,  not 
at  the  moment  like  the'urea  existing  in  excess  in  the  blood,  are  discharged 
into  the  lumina  of  the  tubules  together  with  the  urea, 

How  the  urea,  which  is  in  this  peculiar  manner  taken  out  of  the  blood, 
comes  to  make  its  appearance  in  the  blood  is  a  problem  in  which  the  kidney 
is  not  concerned,  and  with  which  we  shall  deal  in  treating  of  the  metabolic 
events  of  the  body  generally. 

§  355.  In  the  case  of  some  other  constituents  of  the  urine  we  have  evi- 
dence that  the  cells  do  something  more  than  simply  pick  the  constituent  out 
of  the  blood.  Hippuric  acid,  as  we  have  seen,  occurs  in  small  quantity  in 
the  urine  of  man,  and  in  larger  amount*  in  the  urine  of  herbivora.  Now, 
hippuric  acid  may  be  formed  by  the  combination,  with  dehydration,  of  ben- 
zoic  acid  and  glycin  (C7H6O2+C2H5NO2~-H,O^C9H9NO3)  ;  and  benzoic 


THE  SECRETION  OF  URINE.  423 

acid  introduced  into  the  alimentary  canal  or  injected  into  the  blood  reap- 
pears in  large  measure  in  the  urine  as  hippuric  acid.  Somewhere  in  the 
body  the  benzoic  acid  meets  with  and  combines  with  glycin.  And  we  have 
experimental  proof  that  the  combination  may  and  probably  does  take  place 
in  the  kidney. 

If  a  circulation  of  blood  be  kept  up  through  the  bloodvessels  of  the  kid- 
ney freshly  removed  from  a  living  animal,  and  benzoic  acid  and  glycin  be 
added  to  the  blood  as  it  is  about  to  enter  into  the  kidney,  hippuric  acid  will 
be  found  in  the  blood  issuing  from  the  kidney,  especially  if  the  same  blood 
be  passed  through  the  kidney  several  times  ;  the  blood  used  must  be  blood 
containing  oxy-hsemoglobin,  carbonic-oxide  haemoglobin  not  producing  the 
effect.  The  mere  mixing  with  the  blood  itself  is  insufficient ;  arid  if  the 
blood  be  sent  not  through  a  kidney  just  removed  from  the  living  body,  but 
through  one  taken  from  a  dead  body  or  one  which  has  been  left  to  itself  for 
some  time  after  removal  from  a  living  body,  the  synthesis  will  not  be  effected. 
To  carry  out  the  combination  by  means  of  the  kidney  which  has  been  re- 
moved from  the  body,  the  kidney  must  retain  for  a  while  its  own  life,  it  must 
be  a  "  surviving"  kidney.  Nor  is  it  absolutely  necessary  to  bring  the  ben- 
zoic acid  and  glycin  to  the  kidney  by  means  of  a  blood-stream.  If  a  "  sur- 
viving "  kidney  be  divided  rapidly  into  small  pieces  and  the  benzoic  acid 
rapidly  mixed  with  the  pieces,  hippuric  acid  is  formed.  Nor  is  it  necessary 
to  furnish  the  glycin.  If  benzoic  acid  alone  be  used,  hippuric  acid  is  formed 
all  the  same.  Glycin,  as  we  have  previously  said,  cannot  be  recognized  as 
a  normal  constituent  of  any  of  the  tissues  ;  nevertheless,  as  we  have  seen  in 
speaking  of  glycocholic  acid  in  the  bile,  and  as  we  shall  see  later  on,  glycin 
must  make  a  momentary  appearance  in  various  metabolic  processes  of  the 
body,  being  immediately  on  its  appearance  converted  into  something  else,  so 
that  it  never  remains  as  glycin.  It  apparently  is  formed  in  the  kidney,  and 
is  thus  momentarily  available  for  the  conversion  of  benzoic  into  hippuric 
acid. 

It  seems  probable,  therefore,  that,  with  regard  to  this  particular  con- 
stituent of  urine,  hippuric  acid,  the  cells  of  the  tubules  have  the  power  of 
effecting  a  combination  between  the  benzoic  acid  brought  to  them  by  the 
blood  and  the  glycin  which  they  furnish  by  means  of  their  own  metabolism, 
and  in  this  way  produce  hippuric  acid. 

Not  only  benzoic  acid,  but  many  other  bodies  taken  into  the  system, 
reappear  in  the  urine  combined  with  glycin,  and  in  their  cases  also  the 
combination  probably  takes  place  through  the  activity  of  the  cells  of  the 
tubules  of  the  kidney.  Moreover,  other  changes  than  the  assumption  of 
glycin,  the  various  changes  which  many  chemical  substances  taken  into  the 
system  undergo  before  reappearing  in  the  urine,  probably  also  take  place  to 
a  large  extent  in  the  kidney,  and  are  also  carried  out  by  means  of  the 
epithelium  of  the  tubules. 

What  other  constituents  of  normal  urine  are  produced  in  this  or  a  similar 
manner  we  do  not  as  yet  definitely  know.  The  pigment  urobilin,  which,  as 
we  have  seen,  is  supposed  to  be  a  derivative  from  bilirubin,  may  be  brought 
ready-formed  from  the  liver  or  may  have  the  finishing  touches  given  to  it  in 
the  kidney  itself;  and  the  other  normal  or  abnormal  urinary  pigments  pos- 
sibly arise  either  directly  from  haemoglobin  or  indirectly  from  that  body 
through  the  biliary  pigment  by  a  transformation  taking  place  in  the  cells  of 
the  tubules.  There  is  also  evidence  in  frogs  that  acid  sodium  phosphate  is 
furnished  by  the  cells  of  the  tubules. 

In  conclusion,  then,  we  may  say  that  the  activity  of  the  epithelium  of  the 
kidney  appears  especially  modified,  as  compared  with  other  secreting  glands, 
to  meet  the  special  object  which  the  kidney  has  to  secure.  The  purpose  of 


424  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

the  kidney  is  not  to  provide  a  fluid,  urine,  which  can  be  made  use  of  for  the 
needs  of  the  body,  but  to  cast  out  waste  matters  from  the  body.  Hence  its 
secretory  activity  is  limited  largely  to  the  mere  discharge  of  matters  which 
reach  it  preexistent  in  the  blood,  though  in  several  cases  it  gives  the  final 
shape  to  the  excreted  substance  before  it  passes  into  the  ureter. 

§  356.  We  may  illustrate  the  preceding  discussions  by  briefly  passing  in 
review  some  of  the  more  usual  ways  in  which  the  secretion  of  urine  is  in 
ordinary  life  modified. 

In  the  preceding  section  the  composition  of  urine  was  illustrated  by  the 
daily  output  of  the  several  constituents,  rather  than  by  a  percentage  account 
of  any  specimen  of  urine,  for  the  reason  that  the  composition  of  urine  varies 
within  extremely  wide  limits.  This  is  especially  the  case  as  regards  the  pro- 
portion of  water  to  solids.  One  urine  may  be  of  high  specific  gravity  with 
a  small  amount  of  water  relatively  to  the  solids,  while  another  may  have  so 
little  color  and  such  a  low  specific  gravity  as  to  appear  hardly  more  than 
water.  The  reason  of  these  extreme  differences  lies  in  the  fact  that  the 
kidney  is  not  only  the  channel  by  which  waste  solids  leave  the  body,  but 
also  an  important  outlet  for  the  discharge  of  the  stream  of  water  which,  in 
order  that  the  various  processes  of  the  body  may  be  duly  carried  on,  is  con- 
tinually passing  through  the  system.  It  is  frequently  of  advantage  to  the 
body  to  discharge  through  the  kidney  a  large  amount  of  water,  more  or  less 
irrespective  of  the  solid  matters  which  are,  so  to  speak,  washed  away  with 
it ;  and  hence  the  advantage  of  the  glomerular  mechanism  so  specially 
adapted  for  the  discharge  of  water. 

As  we  shall  see  presently,  to  the  skin  also  falls  the  duty  of  discharging 
large  quantities  of  water.  The  respiratory  organs  also,  as  we  have  seen, 
serve  for  the  discharge  of  water ;  but  the  amount  which  the  latter  put  out 
can  only  be  varied  by  the  inconvenient  method  of  increasing  or  diminishing 
the  whole  act  of  breathing.  Hence  we  find  special  relations  between  the 
skin  and  the  kidneys,  correlating  the  work  of  the  one  to  that  of  the  other  as 
regards  this  particular  work  of  the  discharge  of  water. 

When  the  body  is  exposed  to  cold  the  discharge  of  water  from  the  skin  in 
the  form  of  sweat  is  checked,  and  the  cutaneous  vessels  are  constricted.  At 
the  same  time  the  bloodvessels  of  the  abdominal  viscera,  including  the  kid- 
neys are  dilated,  but  not  out  of  proportion  to  the  constriction  of  the  cuta- 
neous vessels,  for  the  general  blood-pressure  does  not  fall,  but  if  anything 
rises  somewhat.  Thus  there  is  established  just  the  state  of  things  which  is 
favorable  to  a  full  and  rapid  stream  of  blood  through  the  renal  glomeruli, 
and  an  increased  flow  of  urine  results.  It  is  possible,  we  may,  perhaps,  say 
probable,  that  the  nervous  system  affords  a  special  tie  between  the  skin  and 
the  kidney,  so  that  under  the  circumstances  in  question  the  renal  arteries  are 
dilated  even  more  than  those  of  the  other  abdominal  viscera ;  but  this  has 
not  been  proved  experimentally.  It  is  also  possible  that  by  another  reflex 
mechanism  of  the  central  nervous  system  the  skin  may  work  upon  the  kid- 
ney, not  by  the  vasornotor  nerves  alone,  but  also  by  nerves  governing  the 
secretory  activity  of  the  tubules  ;  but  we  have  no  satisfactory  indications  of 
any  such  mechanisms,  and  it  seems  more  probable  that  the  connections 
should  be  with  the  glomerular  mechanism,  since  the  chief  object  at  all  events 
is  to  get  rid  of  water. 

Conversely,  when  the  body  is  exposed  to  warmth  the  skin  perspires  freely 
and  the  cutaneous  vessels  are  widely  dilated  ;  and  conversely  also  the  renal 
and  other  abdominal  vessels  are  constricted,  so  that  a  slow  and  small  stream 
of  blood  trickles  through  the  glomeruli,  and  the  urine  which  is  secreted  is 
scanty. 

§  357.  Even  more  important  than  its  relations  to  the  skin  are  the  rela- 


THE  SECRETION   OF  URINE.  425 

tions  of  the  kidney  to  the  water  absorbed  by  the  alimentary  canal ;  this  is 
especially  seen  when  large  quantities  of  fluid  are  drunk.  The  whole  of  the 
water  thus  introduced  into  the  alimentary  canal  passes  into  the  blood,  for  in 
a  healthy  organism  no  amount  of  fluid  drunk,  unless  it  throws  the  economy 
out  of  order,  can  affect  the  amount  of  water  present  in  the  feces.  But  the 
addition  to  the  blood  of  even  a  very  large  quantity  of  fluid  does  not,  as  we 
have  seen,  by  its  mere  quantity  (§  172),  increase  the  general  blood-pressure, 
and  therefore  cannot  in  this  way  produce  what  it  undoubtedly  does  produce, 
an  increased  flow  of  urine. 

The  fluid  so  absorbed  may  act  on  the  kidney  in  two  ways.  On  the  one 
hand,  as  we  have  seen  (§  350),  the  injection  of  water  into  the  blood  produces 
a  local  dilatation  of  the  renal  vessels,  as  indicated  by  the  swelling  of  the 
kidney.  Thus  the  absorption  of  mere  water  from  the  alimentary  canal  may 
stir  up  to  greater  activity  the  glomerular  mechanism,  and  in  so  doing  may 
be  assisted  by  the  presence  of  various  substances  absorbed  from  the  alimen- 
tary canal  with  the  water,  for  some  of  these  also  may  similarly  lead  to 
dilatation  of  the  renal  vessels. 

On  the  other  hand,  some  or  other  of  the  chemical  bodies  thus  passing  into 
the  blood  with  the  water  drunk  may  excite  the  secretory  activity  of  the 
tubules,  and  that  either  by  acting  directly  on  the  epithelium  as  they  are  car- 
ried through  the  kidney  in  the  blood  of  the  renal  arteries,  or  indirectly 
through  some  intervention  of  the  central  nervous  system. 

Our  knowledge  is  at  present  too  scanty  to  enable  us  to  decide  which  of 
these  two  methods  is  the  one  usually  employed  by  the  organism  ;  but  the 
inordinate  flow  of  urine,  so  poor  in  solids  as  to  be  little  more  than  water, 
which  may  be  directed  through  the  kidney  by  means  of  an  adequate  "  drink- 
ing bout,"  would  lead  us  to  conclude  that  in  such  cases  the  organism,  striv- 
ing, though  too  often  in  vain,  to  free  itself  from  the  evils  to  which  it  is  being 
subjected,  has  recourse  rather  to  the  simpler  glomerular  mechanism  than  to 
the  more  expensive  tissue-wasting  activity  of  the  tubules ;  and  the  urine  in 
such  cases  is  probably  discharged  chiefly  by  the  method  of  dilating  the  renal 
vessels  and  thus  throwing  the  poisoned  blood  into  the  glomeruli. 

When,  however,  fluid  is  taken  simply  as  a  proper  accompaniment  of  solid 
food,  the  increase  of  urine  which  results  has  probably  another  origin.  As 
we  have  already  said,  and  as  we  shall  point  out  more  fully  later  on,  the 
absorption  of  proteid  material,  which  is  a  constituent  and  generally  a  con- 
spicuous constituent  of  every  meal,  leads  to  a  formation  of  urea ;  and  urea, 
as  we  have  seen  reason  to  believe,  directly  stimulates  the  epithelium  of  the 
tubules  to  secretory  activity.  And  what  seems  prominently  true  of  urea  is 
probably  true  of  many  other  products  of  digestion  ;  so  that  the  increased 
flow  of  urine  which  follows  an  ordinary  meal  accompanied  by  not  more 
than  the  ordinary  amount  of  fluid,  is  the  result  of  the  labors  of  the  epithe- 
lium of  the  tubules  as  well  as  of  the  fuller  stream  of  blood  through  the 
glomeruli. 

£  358.  What  has  just  been  said  concerning  the  influence  on  the  kidney 
of  food  and  water  may  be  applied  also  to  the  action  of  substances  which 
being  especially  efficacious  in  promoting  a  flow  of  urine  when  taken  into  the 
body  are  called  "diuretics."  The  several  actions  of  various  diuretics  are 
very  varied,  and  it  would  be  out  of  place  to  discuss  them  fully.  We  may, 
however,  say  that  while  the  action  of  some  appears  simple  that  of  others  is 
complex. 

Such  agents  as  sodium  acetate  and  potassium  nitrate  probably  produce 
their  effect  chiefly  by  acting  directly  on  the  kidney,  including,  as  we  have 
seen  (§  350),  local  vascular  dilatation  and  so  working  on  the  glomeruli,  but 
probably  at  the  same  time  also  stirring  up,  after  the  fashion  of  urea,  the 


426  THE   ELIMINATION   OF  WASTE   PRODUCTS. 

epithelium  of  the  tubules  to  secretory  activity,  the  accompanying  fuller 
stream  of  blood  through  the  whole  kidney  being,  as  in  the  case  of  the 
salivary  and  other  glands,  a  useful  adjuvant. 

The  diuretic  effect  of  such  an  agent  as  digitalis  is  probably  more  com- 
plex. By  increasing  the  cardiac  stroke,  and  at  the  same  time  constricting 
many  small  vessels,  digitalis  raises  the  general  blood-pressure ;  but  the  tend- 
ency of  the  increased  blood-pressure  to  increase  the  flow  of  urine  may  be 
counterbalanced  by  the  -constriction  of  the  renal  vessels  themselves.  And 
while  it  is  a  matter  of  common  experience  that  digitalis  is  very  effective  as 
a  diuretic  in  cardiac  disease,  there  is  great  doubt  whether  it  really  acts  as  a 
diuretic  in  health ;  in  cardiac  disease  it  probably  raises  the  blood-pressure 
by  improving  the  cardiac  stroke  and  not  by  constriction  of  the  bloodvessels. 
But  even  in  the  absence  of  cardiac  disease,  digitalis  has  been  found  in 
certain  cases  to  act  as  a  powerful  diuretic,  and  in  these  cases  either  it  must 
act  directly  on  the  tubular  epithelium  or  its  effects  in  constricting  the  renal 
arteries  must  be  less  than  its  effects  on  other  small  arteries  or  must  pass  off 
before  the  influence  of  the  heightened  blood-pressure  has  disappeared. 

§  359.  Quite  removed  from  the  intervention  of  chemical  substances  in 
the  blood  and  yet  most  striking  is  the  influence  on  the  kidney  of  the  central 
nervous  system.  The  potent  influence  of  emotions  in  promoting  the  secre- 
tion of  urine  is  proverbial,  and  the  general  features  of  "  nervous  "  urine,  the 
water  increased  out  of  proportion  to  the  solid  constituents,  especially  seen 
in  the  "  urina  hysterica,"  which  is  hardly  more  than  simple  water,  often  dis- 
charged in  enormous  quantity,  at  once  suggests  the  view  that  impulses  origi- 
nating in  the  brain  and  passing  down  to  the  kidney  along  the  vaso-dilator 
fibres,  of  whose  existence  evidence  was  given  in  §  349,  lead  to  dilated  blood- 
vessels and  great  play  of  glomerular  activity,  without  perhaps  producing 
any  other  direct  effect  on  the  economy  ;  though  possibly  the  same  emotions 
by  constricting  the  cutaneous  and,  it  may  be,  other  vessels  may  raise  the 
general  blood-pressure  and  so  help  the  dilated  renal  vessels. 


THE  DISCHARGE  OF  URINE. 

§  360.  The  urine,  like  the  bile,  is  secreted  continuously ;  the  flow  may 
rise  and  fall,  but  in  health  never  absolutely  ceases  for  any  length  of  time. 
The  cessation  of  renal  activity,  the  so-called  suppression  of  urine,  entails 
speedy  death.  The  minute  streams  passing  continuously — now  more  rapidly, 
now  more  slowly — along  the  collecting  and  discharging  tubules,  are  gathered 
into  the  renal  pelvis,  whence  the  fluid  is  carried  along  the  ureters  into  the 
bladder  partly  by  pressure  and  gravity,  and  from  time  to  time  partly,  as  we 
have  already  said  (§  351),  by  the  peristaltic  contractions  of  the  muscular 
walls  of  the  ureter. 

If  in  a  living  animal  a  ureter  be  laid  bare  and  stimulated,  mechanically 
or  otherwise,  at  a  part  of  its  course,  waves  of  peristaltic  contraction  may  be 
seen  to  pass  in  both  directions  from  the  spot  stimulated — up  toward  the 
kidney  and  down  toward  the  bladder.  In  the  absence  of  artificial  stimula- 
tion spontaneous  waves  of  contraction  make  their  appearance,  sometimes 
repeated  with  tolerable  regularity  (about  every  twenty  seconds  in  the  rabbit), 
sometimes  occurring  in  groups  with  longer  pauses  between.  These  sponta- 
neous contractions  invariably  pass  in  one  direction,  from  the  kidney  to  the 
bladder,  and  their  frequency  and  vigor  seem  to  be  determined  by  the  activity 
of  the  secretion  of  urine.  But  they  are  not  directly  called  forth  by  the 
urine,  either  mechanically  distending  the  tube  or  chemically  stimulating  the 
inner  surface,  for  regularly  recurring  contractions  may  be  observed  in  a 


THE  DISCHARGE  OF  URINE.  427 

kidney  and  ureter  removed  from  the  body,  or  even  in  an  isolated  excised 
piece  of  the  ureter. 

The  rhythmically  repeated  contractions  arise  spontaneously  in  the  mus- 
cular coat  of  the  ureter  much  in  the  same  way  as  the  similar  cardiac  con- 
tractions arise  in  the  muscular  substance  of  the  heart ;  and  it  may  here  be 
mentioned  in  support  of  what  was  urged  in  §  141  with  regard  to  the  heart- 
beats not  being  started  by  nerve-cells,  that  rhythmically  repeated  spontane- 
ous peristaltic  contractions  have  been  observed  in  isolated  pieces  of  ureter 
taken  from  the  middle  of  its  course,  in  which  no  nerve-cells  and  indeed  no 
distinct  nerve-fibres  could  be  observed. 

In  the  living  body  these  spontaneous  movements,  beats  they  might  be 
called,  are  subordinated  to  the  flow  of  urine  into  the  pelvis ;  the  more  active 
the  secretion  of  urine,  the  more  frequent  and  vigorous  are  the  beats  of  the 
pelvis  and  ureter  ;  but  the  exact  mechanism  by  which  the  secretion  and  the 
movements  are  maintained  in  harmony  has  not  yet  been  cleared  up. 

Micturition. 

%  361.  In  the  urinary  bladder  the  urine  is  collected,  its  return  into  the 
ureters  being  prevented  by  the  oblique  entrance  into  the  bladder  and  valvu- 
lar nature  of  the  orifices  of  those  tubes,  and  its  discharge  from  thence  in 
considerable  quantity  is  effected  from  time  to  time  by  a  somewhat  complex 
muscular  mechanism,  of  the  nature  and  working  of  which  the  following  is 
a  brief  account : 

The  involuntary  muscular  fibres  forming  the  greater  part  of  the  vesical 
walls  are  arranged,  partly  in  a  more  or  less  longitudinal  direction  and 
partly  in  a  circular  manner.  After  it  has  been  emptied  the  bladder  is  con- 
tracted and  thrown  into  folds;  as  the  urine  gradually  collects,  the  bladder 
becomes  mere  and  more  distended.  The  escape  of  the  fluid  is  in  part  pre- 
vented by  the  resistance  offered  by  the  elastic  fibres  in  the  walls  of  the  ure- 
thra, which  help  to  keep  the  urethral  channel  closed.  But  this  is  not  all, 
for  observation  shows  that  fluid  is  retained  within  the  bladder  up  to  a  pres- 
sure of  twenty  inches  of  water,  so  long  as  the  bladder  is  governed  by  an 
intact  spinal  cord,  but  gives  way  to  a  pressure  of  six  inches  only  when  the 
lumbar  spinal  cord  is  destroyed  or  the  vesical  nerves  are  severed.  This 
affords  very  strong  evidence  that  the  obstruction  at  the  neck  of  the  bladder 
to  the  exit  of  urine  depends  on  some  tonic  muscular  contraction  maintained 
by  a  reflex  or  automatic  action  of  the  lumbar  spinal  cord.  And  it  has  been 
maintained  that  it  is  the  circularly  disposed  fibres  specially  developed 
around  the  neck  of  the  bladder  which  are  the  subjects  of  this  tonic  contrac- 
tion and  thus  the  chief  cause  of  retention  ;  hence  the  name  sphincter  vesicse. 
The  continuity  of  these  fibres,  however,  with  the  rest  of  the  circular  fibres 
of  the  bladder  suggests  that  they  probably  do  not  act  as  a  sphincter,  but 
that  their  use  lies  in  their  contracting  after  the  rest  of  the  vesical  fibres,  and 
thus  finishing  the  evacuation  of  the  bladder.  The  resistance  in  question  is 
supplied  by  a  tonic  contraction  not  of  the  circular  fibres  of  the  bladder 
itself,  but  of  the  muscular  fibres — partly  plain,  partly  striated — surround- 
ing the  prostatic  portion  of  the  urethra,  and  constituting  the  sphincter  vesicce 
externm  or  prostaticus,  or  sphincter  of  Henle.  It  is  stated  that  artificially 
excited  contractions  of  these  fibres  will  resist  a  pressure  of  fluid  in  the 
bladder. 

When  the  bladder  has  become  full,  we  feel  the  need  of  making  water,  the 
sensation  being  heightened,  if  not  caused,  by  the  trickling  of  a  few  drops  of 
urine  from  the  full  bladder  into  the  urethra.  We  are  then  conscious  of  an 
effort;  during  this  effort  the  bladder  is  thrown  into  a  long-continued  con- 


428  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

traction  of  an  obscurely  peristaltic  nature,  the  force  of  which  is  more  than, 
sufficient  to  overcome  the  resistance  offered  by  the  urethra,  and  the  urine 
issues  in  a  stream,  the  sphincter  vesicse  externus  being  at  the  same  time  either 
relaxed  after  the  fashion  of  the  sphincter  ani,  or  at  least  overcome.  In  its 
passage  along  the  urethra,  the  exit  of  the  urine,  at  all  events  of  the  last  por- 
tions, is  forwarded  by  irregularly  rhythmic  contractions  of  the  bulbo-caver- 
nosus  or  ejaculator  urinse  muscle,  the  contractions  of  which  compress  the 
urethra ;  and  the  whole  act  is  further  assisted  by  pressure  on  the  bladder 
exerted  by  means  of  the  abdominal  muscles,  very  much  the  same  as  in 
defecation. 

In  the  case  of  the  rectum  we  were  able  (§  237)  to  distinguish  between  the 
actions  of  the  longitudinal  and  of  the  circular  coats,  and  we  said  that  the 
two  coats  had  distinct  nervous  supplies  (Fig.  89).  The  bladder  has,  as  we 
have  said,  a  similar  nerve  supply,  and  it  is  very  probable,  but  not  yet  dis- 
tinctly proven,  that  this  like  double  supply  has  a  like  double  action.  Stimu- 
lation of  the  branches  coming  from  the  sacral  nerves,  at  the  same  time  that 
it  throws  the  longitudinal  coat  of  the  rectum  into  contractions,  brings  about 
in  the  dog,  in  which  the  longitudinal  fibres  of  the  bladder  are  much  more 
pronounced  than  the  circular,  powerful  vesical  contractions.  Moreover, 
stimulation  of  the  sacral  nerves  on  one  side  produces  unilateral  contraction 
of  the  bladder.  From  this  we  may  infer  that  the  sacral  nerves  govern  the 
longitudinal  coat.  Stimulation  of  the  hypogastric  nerves  carrying  fibres 
from  the  dorsal  and  upper  lumbar  cord  (see  Fig.  109),  while  throwing  the 
circular  coat  of  the  rectum  into  strong  contractions,  gives  rise  to  vesical  con- 
tractions, but  these  are  by  no  means  so  marked  as  those  which  appear  when 
the  sacral  nerves  are  stimulated.  We  may  probably  conclude  that  the  more 
important  fibres  in  the  fundus  of  the  bladder,  which  are  for  the  most  part 
longitudinal,  are  to  be  regarded  as  governed  chiefly  by  the  sacral  nerve- 
fibres,  while  the  circular  muscular  fibres  around  the  neck  of  the  bladder, 
whose  contraction  completes,  as  it  were,  the  emptying  of  the  bladder,  are 
those  on  which  the  hypogastric  nerve-fibres  have  the  chief  influence. 

§  362.  We  said  just  now  "  when  the  bladder  has  become  full,"  but  this 
must  not  be  understood  to  mean  "  when  the  bladder  has  received  a  certain 
quantity  of  fluid."  On  the  contrary,  it  is  a  matter  of  common  experience 
that  we  feel  the  desire  to  make  water  sometimes  when  a  large  quantity  and 
sometimes  when  a  small  quantity  of  urine  has  accumulated  in  the  bladder. 
We  have  evidence  that  the  bladder  possesses  to  a  very  high  degree  that  ob- 
scure continuous  contraction  which  we  speak  of  as  "  tone  ;"  and,  further,  that 
the  amount  of  its  tone  is  exceedingly  variable,  the  organ,  quite  independ- 
ently of  distinct  efforts  at  micturition,  being  at  one  time  contracted  and  at 
another  flaccid  and  distended.  When  it  is  in  a  contracted  state,  a  small 
quantity  of  fluid  may  exert  the  same  effect  on  the  vesical  walls  as  a  larger 
quantity  when  the  bladder  is  flaccid.  Hence,  while  the  determining  cause 
of  the  desire  to  make  water  is  the  pressure  of  the  urine  upon  the  vesical 
walls,  the  quantity  needed  to  produce  the  necessary  fulness  is  dependent  on 
the  amount  of  tonic  contraction  of  the  muscular  fibres  existing  at  the  time. 
And  we  have  evidence  that  this  tone  is  regulated  by  the  nervous  system. 

§  363.  Micturition  as  sketched  above  seems  at  first  sight,  and  especially 
when  we  appeal  to  our  own  consciousness,  a  purely  voluntary  act.  A  volun- 
tary effort  throws  the  muscular  fibres  of  the  bladder  into  contractions,  an 
accompanying  voluntary  effort  lessens  the  tone  of  the  sphincter  externus, 
probably  by  inhibiting  its  centre  in  the  spinal  cord,  while  other  voluntary 
efforts  throw  the  ejaculator  and  abdominal  muscles  into  contractions,  and 
the  resistance  of  the  urethra  being  thereby  overcome,  the  exit  of  the  urine 
naturally  follows. 


THE  DISCHARGE  OF  URINE.  429 

There  are  facts,  however,  which  prevent  the  acceptance  of  so  simple  a 
view.  In  the  first  place,  in  cases  of  urethral  obstruction,  where  the  bladder 
cannot  be  emptied  when  it  reaches  its  accustomed  fulness,  the  increasing 
distention  sets  up  fruitless  but  powerful  contractions  of  the  vesical  walls, 
contractions  which  are  clearly  involuntary  in  nature,  which  wane  or  disap- 
pear, and  return  again  and  again  in  a  rhythmic  manner,  and  which  may  be 
so  strong  and  powerful  as  to  cause  great  suffering.  It  seems  that  the  fibres 
of  the  bladder,  like  all  other  muscular  fibres,  have  their  contractions  aug- 
mented in  proportion  as  they  are  subjected  to  tension.  Just  as  a  previously 
quiescent  ventricle  of  a  frog's  heart  may  be  excited  to  a  rhythmic  beat  by 
distending  its  cavity  with  blood,  so  the  quiescent  bladder  may,  quite  inde- 
pendently of  the  will,  be  excited,  by  the  distention  of  its  cavity,  to  a  peristaltic 
action  which  in  normal  cases  is  never  carried  beyond  a  first  effort,  since  with 
that  the  bladder  is  emptied  and  the  stimulus  is  removed,  but  which  in  cases 
of  obstruction  is  enabled  clearly  to  manifest  its  rhythmic  nature. 

In  the  second  place  it  has  been  shown  that  quite  normal  micturition  may 
take  place  in  a  dog  in  which  the  lumbar  region  of  the  spinal  cord  has  been 
completely  and  permanently  separated  by  section  from  the  upper  dorsal 
region.  In  such  a  case  there  can  be  no  exercise  of  volition,  and  the  whole 
process  appears  as  a,  reflex  action.  When  under  these  circumstances  the 
bladder  becomes  full  (and  otherwise  apparently  the  act  fails)  any  slight 
stimulus,  such  as  sponging  the  anus  or  slight  pressure  on  the  abdominal 
walls,  causes  a  complete  act  of  micturition  ;  the  bladder  is  entirely  emptied, 
and  the  stream  of  urine  toward  the  end  of  the  act  undergoes  rhythmical 
augmentations  due  to  contractions  of  the  ejaculator  urinse.  These  facts  can 
only  be  interpreted  on  the  view  that  there  exists  in  the  lower  spinal  cord  (of 
the  dog)  what  we  may  speak  of  as  a  micturition  centre  capable  of  being 
thrown  into  action  by  appropriate  afferent  impulses,  the  action  of  the  centre 
being  such  as  to  cause  a  contraction  of  the  walls  of  the  bladder  and  of  the 
ejaculator  urinse,  and  at  the  same  time  to  suspend  the  tone  of  the  sphincter 
vesicle  externus.  Clinical  experience  also  goes  to  show  the  existence  of  a 
similar  micturition  centre  in  man,  placed  higher  up  in  the  cord  than  the 
corresponding  "  genital  "  centre  governing  the  genital  organs. 

Moreover,  we  have,  in  the  case  both  of  man  and  of  other  animals,  experi- 
mental and  other  evidence  that  contraction  of  the  bladder  is  frequently 
brought  about  by  reflex  action.  Thus  the  pressure  within  the  bladder  when 
observed  for  any  length  of  time  is  found  to  be  subject  to  considerable  and 
manifold  variations.  Over  and  above  passive  changes  in  pressure  due  to  the 
respiratory  movements,  through  which  the  bladder  is  pressed  upon  at  each 
descent  of  the  diaphragm,  active  contractions,  of  a  strength  inadequate  to 
bring  about  micturition,  are  from  time  to  time  observed.  These  in  some 
instances  appear  to  be  spontaneous,  or  to  be  the  result  of  emotions,  but  they 
may  be  readily  induced  in  a  reflex  manner  by  stimulating  various  sentient 
surfaces  or  sensory  nerves.  And  common  experience  affords  many  instances 
where  vesical  contractions  thus  brought  about  in  a  reflex  manner  acquire 
strength  adequate  to  empty  the  bladder. 

Observation  of  vesical  pressure  may  be  most  conveniently  carried  out  by  intro- 
ducing into  the  bladder  a  catheter  connected  with  a  water  manometer  and  a  regis- 
tering apparatus,  and  so  arranged  as  to  allow  fluid  to  be  driven  into  or  received 
from  the  bladder  at  pleasure. 

§  364.  Involuntary  micturition  obviously  of  reflex  nature  has  fre- 
quently been  observed  in  cases  of  paralysis  from  disease  of  or  injury  to  the 
spinal  cord  ;  and  the  involuntary  micturition  which  is  common  in  children, 
as  the  result  of  irritation  of  the  penis  and  genital  organs,  and  which  some- 


430  THE   ELIMINATION  OF  WASTE  PRODUCTS. 

times  occurs  in  the  adult  as  the  result  of  emotions,  or  at  least  sensory  im- 
pressions, appears  to  be  the  result  of  reflex  action.  In  these  several  cases 
we  may  fairly  suppose  that  the  centre  in  the  spinal  cord  is  affected  by  affer- 
ent impulses  reaching  it  along  various  sensory  nerves  or  descending  from 
the  brain.  Hence  we  are  led  to  the  conception  that  when  we  make  water 
by  a  conscious  effort  of  the  will,  what  occurs  is  not  a  direct  action  of  the  will 
on  the  muscular  walls  of  the  bladder,  but  that  impulses  started  by  the  will 
descend  from  the  brain  after  the  fashion  of  afferent  impulses  and  thus  in  a 
reflex  manner  throw  into  action  the  micturition  centre  in  the  spinal  cord. 
We  may  draw  an  analogy  between  the  micturition  apparatus  and  the  re- 
spiratory mechanism.  We  saw  reasons  in  the  latter  case  to  think  that  when 
the  will  interfered  with  the  respiratory  movements,  it  did  so  by  acting  upon 
the  nervous  mechanism  in  the  central  nervous  system  and  not  by  acting  di- 
rectly on  the  muscular  fibres  of  the  diaphragm  and  other  respiratory  muscles. 
And  the  case  of  the  plain  muscular  fibres  of  the  bladder  seems  even  stronger 
than  that  of  respiratory  muscles  so  largely  skeletal  in  nature.  We  might 
also  draw  an  analogy  with  the  heart.  We  are  not  able  to  throw  into  action, 
by  any  direct  effort  of  the  will,  the  cardiac  augmentor  mechanism.  Were 
we  able  to  do  so  powerfully  and  suddenly,  we  might  throw  into  violent  ac- 
tion a  weakly  beating  heart  much  in  the  same  way  that  we  empty  an  ob- 
scurely contracting  bladder.  Nor  is  this  view  negatived  by  the  fact  that 
paralysis  of  the  bladder,  or  rather  inability  to  make  water  either  voluntarily 
or  in  a  reflex  manner,  is  a  common  symptom  of  cerebral  or  spinal  disease 
or  injury.  Putting  aside  the  cases  in  which  the  reflex  act  is  not  called  forth 
because  the  appropriate  stimulus  has  not  been  applied,  the  failure  in  mic- 
turition under  these  circumstances  may  be  explained  by  supposing  that  the 
shock  of  the  spinal  injury  or  some  extension  of  the  disease  has  rendered  the 
spinal  centre  unable  to  act. 

The  so-called  incontinence  of  urine  in  children  is  simply  an  easily  ex- 
cited and  frequently  repeated  reflex  micturition.  In  cases  of  cerebral  or 
spinal  disease  a  form  of  incontinence  is  frequently  met  with  which  seems  to 
be  of  a  different  nature.  The  bladder  becoming  full,  but,  owing  to  a  failure 
in  the  mechanism  of  voluntary  or  reflex  micturition,  being  unable  to  empty 
itself  by  a  complete  contraction,  a  continued  dribbling  of  urine  takes  place 
through  the  urethra,  the  fulness  of  the  bladder  being  sufficient  to  overcome 
the  resistance  at  the  neck  of  the  urethra.  It  is  probable,  however,  that 
even  in  these  cases  the  flow  is  partly  caused  by  obscure,  unfelt,  intrinsic 
contractions  of  the  bladder. 

§  365.  Whether,  under  normal  conditions,  the  urine  undergoes  any 
notable  change  during  its  stay  in  the  bladder  has  been  much  debated. 
Experiments  show  thai  poisonous  substances  injected  into  the  bladder  with 
all  due  care  to  avoid  any  abrasion  of  the  epithelium  are  absorbed  and  pro- 
duce their  usual  effects.  It  has  also  been  stated  that  if  a  solution  of  urea 
be  injected  into  the  bladder  after  ligature  of  both  ureters,  and  allowed  to 
stay  for  some  hours,  part  of  the  urea  disappears.  But  at  present  there  is 
no  very  decided  proof  that  under  ordinary  conditions  either  the  water  or 
other  constituents  of  urine  are  to  any  appreciable  extent  absorbed  by  the 
bladder. 

Under  abnormal  conditions,  as  in  inflammation  or  irritation  of  the  blad- 
der, the  urine  may  have  undergone  marked  changes  during  its  stay  in  the 
bladder,  one  of  the  most  common  being  a  change  of  some  of  the  urea  into 
ammonium  carbonate,  by  which  the  urine  also  becomes  alkaline.  Under 
abnormal  conditions  also,  the  mucus  of  the  urine,  which  in  a  healthy  man 
is  insignificant,  though  in  some  animals,  for  instance  the  horse,  it  occurs  in 
considerable  quantity,  is  largely  increased  during  the  stay  in  the  bladder. 


THE  NATURE  AND  AMOUNT  OF  PERSPIRATION.  431 

Since  there  are  in  man  no  goblet  cells  in  the  vesical  epithelium  (in  the  frog 
they  are  present)  or  mucous  glands  in  the  walls  of  the  bladder,  this  mucus 
must  be  supplied  by  an  abnormal  metabolism  of  the  ordinary  epithelial 
cells. 

THE  NATURE  AND  AMOUNT  OF  PERSPIRATION. 

§  366.  The  quantity  of  matter  which  leaves  the  human  body  by  way 
of  the  skin  is  very  considerable.  Thus,  it  has  been  estimated  that  while 
0.5  gramme  passes  away  through  the  lungs  per  minute,  as  much  as  0.8 
gramme  passes  through  the  skin.  The  amount,  however,  varies  extremely ; 
it  has  been  calculated,  from  data  gained  by  enclosing  the  arm  in  a  caout- 
chouc bag,  that  the  total  amount  of  perspiration  from  the  whole  body  in 
twenty-four  hours  might  range  from  2  to  20  kilos ;  but  such  a  mode  of 
calculation  is  obviously  open  to  many  sources  of  error. 

Of  the  whole  amount  thus  discharged,  part  passes  away  at  once  as 
watery  vapor  mixed  with  volatile  matters,  while  part  may  remain  for  a 
time  as  a  fluid  on  the  skin  ;  the  former  is  frequently  spoken  of  as  insensible, 
the  latter  as  sensible,  perspiration  or  sweat.  The  proportion  of  the  insen- 
sible to  the  sensible  perspiration  will  depend  on  the  rapidity  of  the  secre- 
tion in  reference  to  the  dryness,  temperature,  and  amount  of  movement  of 
the  surrounding  atmosphere.  Thus,  supposing  the  rate  of  secretion  to 
remain  constant,  the  drier  and  hotter  the  air,  and  more  rapidly  the  strata 
of  air  in  contact  with  the  body  are  renewed,  the  greater  is  the  amount  of 
sensible  perspiration,  which  is,  by  evaporation,  converted  into  the  insensible 
condition  ;  and,  conversely,  when  the  air  is  cool,  moist,  and  stagnant,  a  large 
amount  of  the  total  perspiration  may  remain  on  the  skin  as  sensible  sweat. 
Since,  as  the  name  implies,  we  are  ourselves  aware  of  the  sensible  perspira- 
tion only,  it  may,  and  frequently  does,  happen  that  we  seem  to  ourselves  to 
be  perspiring  freely,  when,  in  reality,  it  is  not  so  much  the  total  perspira- 
tion which  is  being  increased  as  the  relative  proportion  of  the  sensible  per- 
spiration. The  rate  of  secretion  may,  however,  be  so  much  increased  that 
no  amount  of  dryness  or  heat,  or  movement  of  the  atmosphere,  is  sufficient 
to  carry  out  the  necessary  evaporation,  and  thus  the  sensible  perspiration 
may  become  abundant  in  a  hot,  dry  air.  And,  practically,  this  is  the  usual 
occurrence,  since,  certainly,  a  high  temperature  conduces,  as  we  shall  point 
out  presently,  to  an  increase  of  the  secretion,  and  it  is  possible  that  mere 
dryness  of  the  air  has  a  similar  effect. 

The  amount  of  perspiration  given  off  is  affected  not  only  by  the  condition 
of  the  atmosphere,  but  also  by  the  circumstances  of  the  body.  Thus  it  is 
influenced  by  the  nature  and  quantity  of  food  eaten,  by  the  amount  of  fluid 
drunk,  by  the  character  of  exercise  taken,  by  the  relative  activity  of  the 
other  excreting  organs,  more  particularly  of  the  kidney,  by  mental  condi- 
tions, and  the  like.  Variations  may  also  be  induced  by  drugs  and  by  dis- 
eased conditions.  How  these  various  influences  produce  their  effects  we  shall 
study  immediately. 

The  fluid  perspiration,  or  sweat,  when  collected,  is  found  to  be  a  clear, 
colorless  fluid  of  a  distinctly  salt  taste,  with  a  strong  and  distinctive  odor, 
varying  according  to  the  part  of  the  body  from  which  it  is  taken.  Besides 
accidental  epidermic  scales,  it  contains  no  structural  elements. 

Sweat,  as  a  whole,  is  furnished  partly  by  the  sweat-glands  and  partly  by 
the  sebaceous  glands,  for,  as  we  shall  see,  the  small  amount  which  simply 
transudes  through  the  epidermis,  apart  from  the  glands,  may  be  neglected. 
Now,  the  secretions  from  these  two  kinds  of  glands  differ  widely  in  nature, 
and  the  characters  of  the  sweat,  as  a  whole,  will  vary  according  to  the  rela- 


432  THE   ELIMINATION  OF  WASTE  PRODUCTS. 

tive  proportion  of  the  two  kinds  of  secretion.  The  secretion  of  the  sebaceous 
glands  appears  to  be  fairly  constant,  the  larger  variations  of  the  total  sweat 
depending  chiefly  on  the  varying  activity  of  the  sweat-glands.  Hence,  when 
sweat  is  scanty,  the  constituents  of  the  sebum  influence  largely  the  characters 
of  the  sweat;  when,  on  the  contrary,  the  sweat  is  very  abundant,  these  may 
be  disregarded,  and  the  sweat  may  be  considered  as  the  product  of  the  sweat- 
glands. 

We  are  not  able  at  present  to  make  a  complete  statement  as  to  what 
bodies  occur  exclusively  in  the  sebum  and  what  in  the  secretion  of  the 
sweat-glands.  The  former  consists,  very  largely,  of  fats  and  fatty  acids, 
and  appears  to  contain  some  form  or  forms  of  proteids ;  but  we  have 
reason  to  think  that  the  sweat-glands  secrete,  in  some  quantity,  some 
forms  of  fat,  and  especially  volatile  fatty  acids. 

When  sweat  is  scanty,  the  reaction  is  generally  acid,  but  when  abund- 
ant, is  alkaline  ;  and  when  a  portion  of  the  skin  is  well  washed  the 
sweat  which  is  collected  immediately  afterward  is  usually  alkaline.  From 
this  we  may  infer  that  the  secretion  of  the  sweat-glands  is  naturally 
alkaline,  but  that  when  mixed,  sweat  is  acid  ;  the  acidity  being  due  to  fatty 
(or  other)  acids  of  the  sebum.  In  the  horse,  which  is  singular  in  hair- 
covered  animals  for  its  frequent  profuse  sweating,  the  sweat  is  said  to  be 
always  acid  and  to  contain  a  considerable  quantity  of  some  form  of 
proteid.  These  features  are  probaby  due  to  the  large  admixture  of 
sebum  from  the  numerous  sebaceous  glands  connected  with  the  hairs. 

Taking  ordinary  sweat,  such  as  may  be  obtained  by  enclosing  the  arm 
in  a  bag,  we  may  say  that  in  man  the  average  amount  of  solids  is  from  1 
to  2  per  cent.,  of  which  about  two-thirds  consist  of  organic  substances. 
The  chief  normal  constituents  are:  (1)  Sodium  chloride,  with  small 
quantities  of  other  inorganic  salts.  (2)  Various  acids  of  the  fatty  series, 
such  as  formic,  acetic,  butyric,  with  probably  propionic,  caproic,  and 
caprylic.  •  The  presence  of  these  latter  is  inferred  from  the  odor ;  it  is 
probable  that  many  various  volatile  acids  are  present  in  small  quanti- 
ties. Lactic  acid,  which  has  been  reckoned  as  a  normal  constituent,  is 
stated  not  to  be  present  in  health.  (3)  Neutral  fats  and  cholesterin  ; 
these  have  been  detected  even  in  places,  such  as  the  palm  of  the  hand, 
where  sebaceous  glands  are  not  present.  (4)  The  evidence  goes  to  show 
that  neither  urea  nor  any  ammonia  compound  exists  in  the  normal  secre- 
tion to  any  extent,  though  some  observers  have  found  a  considerable  quan- 
tity of  urea  (calculated  at  10  grms.  in  the  twenty-four  hours  for  the 
whole  body).  Apparently  a  small  amount  of  nitrogen  leaves  the  body 
through  the  skin,  but  this  is  probably  supplied  by  the  sebum  or  by  the 
epidermis. 

In  various  forms  of  disease  the  sweat  has  been  found  to  contain,  some- 
times in  considerable  quantities,  blood,  albumin,  urea  (particularly  in 
cholera),  uric  acid,  calcium  oxalate,  sugar  (in  diabetic  patients),  lactic 
acid,  indigo  (or  indigo-yielding  bodies,  giving  rise  to  "  blue  "  sweat),  bile, 
and  other  pigments.  Iodine  and  potassium  iodide,  succinic,  tartaric,  and 
benzoic  (partly  as  hippuric)  acids  have  been  found  in  sweat  when  taken 
internally  as  medicines. 

Cutaneous  Respiration. 

§  367.  A  frog  whose  lungs  have  been  removed  will  continue  to  live  for 
some  time ;  and  during  that  period  will  continue,  not  only  to  produce  car- 
bonic acid,  but  also  to  consume  oxygen.  In  other  words,  the  frog  is  able  to 
breathe  without  lungs,  respiration  being  carried  on  efficiently  by  means  of 


THE  NATURE  AND  AMOUNT  OF  PERSPIRATION.  433 

the  skin.  In  mammals  and  in  man  this  cutaneous  respiration  is,  by  reason 
of  the  thickness  of  the  epidermis,  restricted  within  very  narrow  limits  ;  and, 
indeed,  it  has  been  questioned  whether  it  can  be  spoken  of  at  all  as  a  true 
respiration.  When  the  body  remains  for  some  time  in  a  closed  chamber  to 
which  the  air  passing  in  and  out  of  the  lungs  has  no  access  (as  when  the 
body  is  enclosed  in  a  large  air-tight  bag  fitting  tightly  round  the  neck,  or 
where  a  tube  in  the  trachea  carries  air  to  and  from  the  lungs  of  an  animal 
placed  in  an  air-tight  box),  it  is  found  that  the  air  in  the  chamber  loses 
oxygen  and  gains  carbonic  acid.  The  amount  of  carbonic  acid  which  is  thus 
thrown  off  by  the  skin  of  an  average  man  in  twenty-four  hours  amounts  to 
about  10  grms.,  or  according  to  some  observers  to  (no  more  than)  about  4 
grms.,  increasing  with  a  rise  of  temperature  and  being  very  markedly  aug- 
mented by  bodily  exercise.  It  is  stated  that  the  amount  of  oxygen  con- 
sumed is  about  equal  in  volume  to  that  of  the  carbonic  acid  given  off,  but 
some  observers  make  it  rather  less.  It  may  be  doubted,  however,  whether 
the  carbonic  acid  comes  direct  from  the  blood ;  it  may  come  from  decom- 
position taking  place  in  the  sweat — of  carbonates,  for  instance.  Similarly 
the  oxygen  which  disappears  may  be  simply  used  in  oxidizing  some  of  the 
constituents  of  the  sweat.  It  is  evident  that  the  loss  which  the  body  suffers 
through  the  skin  consists,  besides  a  small  quantity  of  sodium  chloride,  chiefly 
of  water. 

When  an  animal,  a  rabbit  for  instance,  is  covered  over  with  an  imper- 
meable varnish,  such  as  gelatin,  so  that  all  exit  or  entrance  of  gases  or 
liquids  by  the  skin  is  prevented,  death  shortly  ensues.  This  result  cannot 
be  due,  as  once  thought,  to  arrest  of  cutaneous  respiration,  seeing  how  insig- 
nificant and  doubtful  is  the  gaseous  interchange  by  the  skin  as  compared 
with  that  by  the  lungs.  Nor  are  the  symptoms  at  all  those  of  asphyxia,  but 
rather  of  some  kind  of  poisoning,  marked  by  a  very  great  fall  of  tempera- 
ture, which,  however,  seems  to  be  the  result  not  of  diminished  production  of 
heat,  but  of  an  increase  of  the  discharge  of  heat  from  the  surface.  The  ani- 
mal may  be  restored,  or  at  all  events  its  life  may  be  prolonged  with  the 
abatement  of  the  symptoms,  if  the  great  loss  of  heat  which  is  evidently 
taking  place  be  prevented  by  covering  the  body  thickly  with  cotton-wool  or 
keeping  it  in  a  warm  atmosphere.  The  symptoms  have  not  as  yet  been 
clearly  analyzed,  but  they  seem  to  be  due  in  part  to  a  pyrexia  or  fever  pos- 
sibly caused  by  the  retention  within  or  reabsorption  into  the  blood  of  some 
of  the  constituents  of  the  sweat,  or  by  the  products  of  some  abnormal  meta- 
bolism, and  in  part  to  a  dilatation  of  the  cutaneous  vessels  caused  by  the 
application  of  varnish  ;  owing  to  the  dilated  condition  of  the  cutaneous  ves- 
sels the  loss  of  heat  through  the  skin  is  abnormally  large,  even  though  the 
varnish  may  not  be  a  good  conductor. 

§  368.  Absorption  by  the  skin.  Although  under  normal  circumstances 
the  skin  serves  only  as  a  channel  of  loss  to  the  body,  it  has  been  maintained 
that  it  may,  under  particular  circumstances,  be  a  means  of  gain,  and  the 
little  which  we  have  to  say  on  this  matter  may  perhaps  be  said  here.  Cases 
are  on  record  where  bodies  are  said  to  have  gained  in  weight  by  immersion 
in  a  bath,  or  by  exposure  to  a  moist  atmosphere  during  a  given  period,  in 
which  no  food  or  drink  was  taken,  or  to  have  gained  more  than  the  weight 
of  the  food  or  drink  taken  ;  the  gain  in  such  cases  must  have  been  due  to 
the  absorption  of  water  by  the  skin.  Direct  experiments,  however,  throw 
doubt  on  these  statements,  for  they  show  that  under  ordinary  circumstances 
such  a  gain  by  the  skin  is  slight,  being  apparently  due  to  mere  imbibition 
of  water  by  the  upper  layers  of  the  epidermis. 

Absorption  of  various  substances  takes  place  very  readily  by  abraded 
surfaces  where  the  dermis  is  laid  bare  or  covered  only  by  the  lowest  layers 

28 


434  THE  ELIMINATION  OF   WASTE  PRODUCTS. 

of  epidermis,  but  it  has  been  debated  whether  substances  in  aqueous  solution 
can  be  absorbed  by  the  skin  when  the  epidermis  is  intact,  the  evidence  on 
this  point  being  contradictory.  In  the  case  of  the  skin  of  the  frog  an  ab- 
sorption of  water  and  of  various  soluble  substances  certainly  takes  place. 
In  the  case  of  the  sound  human  skin  there  are  no  a  priori  reasons  why  water 
carrying  substances  dissolved  in  it  should  not  pass  inward  through  the 
corneous  as  well  as  the  other  layers  of  the  epidermis,  the  amount  so  passing 
depending,  among  other  things,  upon  the  condition  of  the  skin ;  and  com- 
mon experience  seems  to  show  that  it  does.  Nevertheless,  the  results  of 
actual  experiment  are  conflicting.  Some  observers  maintain  that  soluble 
non-volatile  substances  are  not  absorbed,  and  that  volatile  substances,  such 
as  iodine,  which  may  be  detected  in  the  system  after  a  bath  containing  them, 
are  absorbed  not  by  the  skin,  but  by  the  mucous  membrane  of  the  respira- 
tory organs,  the  substance  making  its  way  to  the  latter  by  volatilization  from 
the  surface  of  the  bath.  Others,  again,  have  found  evidence  of  absorption, 
especially  with  volatile  substances,  even  when  care  has  been  taken  to  avoid 
all  errors  ;  and  the  greater  weight  may  perhaps  be  given  to  these,  since  they 
accord  with  common  experience.  The  conflict  of  experimental  results,  how- 
ever, at  least  shows  that  we  do  not  fully  understand  the  conditions  under 
which  such  absorption  takes  place. 

There  is,  moreover,  evidence  that  even  solid  particles  can  pass  through 
an  intact  skin.  The  lymphatics  in  the  skin  of  a  newborn  infant  have  been 
found  crowded  with  the  particles  of  the  peculiar  fatty  secretion  which  covers 
the  skin  at  birth  ;  and  solid  particles  rubbed  into  even  the  sound  skin  may, 
especially  when  applied  in  a  fatty  vehicle,  as*  e.  g.,  in  the  well-known  mer- 
cury ointment,  find  their  way  into  the  underlying  lymphatics.  The  wander- 
ing leucocytes  which  are  at  times  found  among  the  epidermic  cells  may 
perhaps  take  part  in  this  transport. 

THE  MECHANISM  OF  THE  SECRETION  OF  SWEAT. 

§  369.  In  dealing  with  the  manner  in  which  various  circumstances 
affect  the  amount  of  sweat  secreted  we  may,  as  we  have  already  said,  con- 
sider the  sweat  as  a  whole  to  be  supplied  by  the  sweat-glands  alone.  For 
though  it  seems  evident  that  some  amount  of  fluid  must  pass  by  simple 
transudation  through  the  ordinary  epidermis  of  the  portions  of  skin  inter- 
vening between  the  mouths  of  the  glands,  yet  on  the  whole  it  is  probable 
that  the  portion  which  so  passes  is  a  small  fraction  only  of  the  total  quantity 
secreted  by  the  skin ;  and  direct  experiment  shows  that  even  the  simple 
evaporation  of  water  is  much  greater  from  those  parts  of  the  skin  in  which 
the  glands  are  abundant,  than  from  those  in  which  they  are  scanty.  We 
have  as  yet  no  evidence  that  the  sebaceous  glands  vary  in  activity ;  their 
very  peculiar  form  of  secretion,  if  we  may  speak  of  it  as  a  secretion,  is  not 
adapted  to  sudden  changes,  and  at  all  events  we  have  as  yet  no  evidence 
that  circumstances  rapidly  and  largely  modify  the  amount  of  sebum  dis- 
charged by  healthy  sebaceous  glands. 

The  secreting  activity  of  the  skin,  like  that  of  the  other  glands,  is  usually 
accompanied  and  aided  by  vascular  dilatation.  In  one  of  the  early  experi- 
ments on  division  of  the  cervical  sympathetic  it  was  observed  that,  in  the 
case  of  the  horse,  the  vascular  dilatation  of  the  face  on  the  side  operated  on 
was  accompanied  by  increased  perspiration.  Indeed,  the  connection  between 
the  state  of  the  cutaneous  bloodvessels  and  the  amount  of  perspiration  is  a 
matter  of  daily  observation.  When  the  vessels  of  the  skin  are  constricted, 
the  secretion  of  the  skin  is  diminished  ;  when  they  are  dilated,  it  becomes 
abundant.  In  this  way,  as  we  shall  later  on  point  out,  the  temperature  of 


THE  MECHANISM  OF  THE  SECRETION  OF  SWEAT.  435 

the  body  is  largely  regulated.  When  the  surrounding  atmosphere  is  warm, 
the  cutaneous  vessels  are  dilated,  the  amount  of  sweat  secreted  is  increased, 
and  the  consequently  augmented  evaporation  tends  to  cool  down  the  body. 
On  the  other  hand,  when  the  atmosphere  is  cold,  the  cutaneous  vessels  are  con- 
stricted, perspiration  is  scanty, and  less  heat  is  lost  to  the  body  by  evaporation. 

The  analogy  with  the  other  secreting  organs  which  we  have  already 
studied  leads  us,  however,  to  infer  that  there  are  special  nerves  directly 
governing  the  activity  of  the  sudoriparous  glands,  independent  of  varia- 
tions in  the  vascular  supply.  And  not  only  is  this  view  suggested  by  many 
facts,  such  as  the  profuse  perspiration  of  the  death-agony,  of  various  crises 
of  disease,  and  of  certain  mental  emotions,  and  the  cold  sweats  occurring  in 
phthisis  and  other  maladies,  in  all  of  which  the  skin  is  anaemic  rather  than 
hyperseniic,  but  we  have  direct  experimental  evidence  of  a  nervous  mechan- 
ism of  perspiration  as  complete  as  the  vasomotor  mechanism. 

If  in  the  cat1  the  peripheral  stump  of  the  divided  sciatic  nerve  be  stimu- 
lated with  the  interrupted  current,  drops  of  sweat  may  readily  be  observed 
to  gather  on  the  hairless  sole  of  the  foot  of  that  side.  The  sweating  is  not 
due  to  any  increase  of  blood-supply,  for  it  may  be  observed  when  the  cuta- 
neous vessels  are  thrown  into  a  state  of  constriction  by  the  stimulus,  or  even 
when  the  aorta  or  crural  artery  is  clamped  previous  to  the  stimulation,  and 
indeed  may  be  obtained  by  stimulating  the  sciatic  nerve  of  a  recently  ampu- 
tated leg.  Moreover,  when  atropine  has  been  injected,  the  stimulation  pro- 
duces no  sweat,  though  vasomotor  effects  follow  as  usual.  The  analogy 
between  the  sweat-glands  of  the  foot  and  such  a  gland  as  the  submaxillary 
is  in  fact  very  close,  and  we  are  justified  in  speaking  of  the  sciatic  nerve  as 
containing  secretory  fibres  distributed  to  the  sudoriparous  glands  of  the  foot. 
Similar  results  may  be  obtained  with  the  nerves  of  the  fore  limb.  And  in 
ourselves  a  copious  secretion  of  sweat  may  be  induced  by  tetanizing  through 
the  skin  the  nerves  of  the  limbs  or  the  face. 

If  a  cat  in  which  the  sciatic  nerve  has  been  divided  on  one  side  be  exposed 
to  a  high  temperature  in  a  heated  chamber,  the  limb  the  nerve  of  which  has 
been  divided  remains  dry,  while  the  feet  or  the  other  limbs  sweat  freely. 
This  result  shows  that  the  sweating  which  is  caused  by  exposure  of  the  body 
to  high  temperatures  is  brought  about  by  the  agency  of  the  central  nervous 
system,  and  not  by  a  local  action  on  the  sweat-glands  ;  for  the  foot  of  the 
limb  whose  nerve  has  been  divided  is  equally  exposed  to  the  high  tempera- 
ture. A  high  temperature  it  is  true  up  to  a  certain  limit  increases  the  irrita- 
bility of  the  epithelium  of  the  sweat-glands  and  predisposes  it  to  secrete,  just 
as  it  promotes  action  in  the  case  of  a  muscle  or  nerve  or  other  forms  of  living 
substance.  Thus  stimulation  of  the  sciatic  in  the  cat  produces  a  much  more 
abundant  secretion  in  a  limb  exposed  to  a  temperature  of  35°  or  somewhat 
above,  than  in  one  which  has  been  exposed  to  a  distinctly  lower  temperature, 
and  in  a  limb  which  has  been  placed  in  ice-cold  water  hardly  any  secretion 
at  all  can  be  gained  ;  but  apparently  mere  rise  of  temperature  without  nerve- 
stimulation  will  not  give  rise  to  a  secretory  activity  of  the  glands.  The 
sweating  caused  by  a  dyspnoBic  condition  of  blood,  and  such  appears  to  be 
the  sweat  of  the  death-agony,  is  similarly  brought  about  by  the  agency  of 
the  central  nervous  system.  When  an  animal  with  the  sciatic  nerve  divided 
on  one  side  is  made  dyspnoeic,  no  sweat  appears  in  the  hind  limb  of  that  side 
though  abundance  is  seen  in  the  other  feet. 

1  The  cat  sweats  freely  in  the  hairless  soles  of  the  feet  but  not  on  any  part  of  the  hody 
covered  with  hairs.  The  dog  also  sweats  in  the  same  regions  but  not  so  freely  as  the  cat  ? 
indeed,  sweating  is  often  absent,  the  ducts  being  stopped  by  growth  of  the  corneous  epi- 
dermis. Rabbits  and  other  rodents  appear  not  to  sweat  at  all.  The  snout  of  the  pig 
sweats  freely ;  and  the  often  profuse  sweating  of  the  horse,  a  singular  event  among  hair- 
covered  animals,  is  known  to  all. 


436  THE  ELIMINATION  OF  WASTE  PRODUCTS. 

Sweating  may  be  brought  about  as  a  reflex  act.  Thus  when  the  central 
stump  of  the  divided  sciatic  is  stimulated  sweating  is  induced  in  the  other 
limbs,  and  in  ourselves  the  introduction  of  pungent  substances  into  the  mouth 
will  frequently  give  rise  to  a  copious  perspiration  over  the  side  of  the  face. 
We  are  thus  lead  to  speak  of  sweat  centres,  analogous  to  the  vasomotor  cen- 
tres, as'  existing  in  the  central  nervous  system  ;  and  as  in  the  case  of  vaso- 
motor centres,  a  dispute  has  arisen  as  to  whether  there  is  a  dominant  sweat 
in  the  medulla  oblongata  or  whether  such  centres  are  more  generally  dis- 
tributed over  the  whole  of  the  spinal  cord. 

It  does  not  at  present  appear  certain  whether  the  sweating  caused  by  heat 
is  carried  out  by  direct  action  of  the  heated  blood  on  the  sweat  centres,  or 
by  the  higher  temperature  stimulating  the  skin  and  so  sending  up  afferent 
impulses  which  produce  the  effect  in  a  reflex  manher ;  but  in  the  case  of 
dyspnoea  at  least  we  may  fairly  suppose  that  the  action  of  the  venous  blood 
is  chiefly  if  not  exclusively  on  the  nerve  centres.  Some  drugs,  such  as  pilo- 
carpine,  which  cause  sweating,  appear  to  produce  their  effect  chiefly  by  a 
local  action  on  the  glands,  since  the  action  continues  after  the  division  of 
the  nerves  (though  pilocarpine  apparently  has  as  well  some  slight  action  on 
the  nerve  centres)  and  the  antagonistic  action  of  atropine  is  similarly  local. 
Picrotoxine  and  strychnine  appear  to  produce  their  sweating  action  chiefly 
if  not  exclusively  by  acting  on  the  central  nervous  system,  while  nicotine 
seems  to  act  both  centrally  and  peripherally. 

§  370.  The  sweat-fibres  for  the  hind  foot  (in  the  cat)  appear  to  leave  the 
spinal  cord  by  the  roots  of  the  twelfth  thoracic  to  the  third  lumbar  nerve 
inclusive,  pass  along  the  rami  communieantes  to  the  abdominal  sympathetic, 
and  thus  reach  the  sciatic  nerve.  They  thus  follow  very  much  the  course 
of  the  vaso-constrictor  fibres  of  the  lower  limb.  The  sweat-nerves  for  the 
fore-foot  leave  the  spinal  cord  by  the  roots  of  the  fourth  to  the  ninth  or 
tenth,  chiefly  in  the  sixth,  seventh,  and  eighth  dorsal  nerve,  inclusive,  pass 
into  the  thoracic  sympathetic,  thence  into  the  ganglion  stellatum,  and  so 
join  the  brachial  plexus  by  the  fine  branches  passing  from  the  ganglion  to 
the  spinal  nerves.  The  course  to  the  fore-foot  is  finally  along  the  median 
and  ulnar  nerves  respectively.  In  the  horse  the  sweat-fibres  for  the  side  of 
the  face  and  in  the  pig  those  for  the  snout  appear  to  run  in  branches  of  the 
fifth  nerve  and  not  in  the  facial ;  in  the  latter  animal  at  least  some  of  these 
fibres  reach  the  fifth  nerve  from  the  cervical  sympathetic,  but  apparently 
not  all. 

§  371.  The  fact  mentioned  above  that  in  the  horse,  after  section  of  the 
cervical  sympathetic  nerve  on  one  side  of  the  neck,  profuse  sweating  is  apt 
to  break  out  on  that  side  of  the  face,  has  suggested  the  idea  that  this  nerve 
conveys  inhibitory  impulses  to  the  sweat-glands  of  the  head  and  face,  and 
that  when  it  is  divided  the  sweat  fibres  running  in  the  fifth  nerve,  having 
nothing  to  counteract  them,  set  up  sweating.  But  it  is  probably  sufficient 
in  this  case  to  suppose  that  the  glands  predisposed  to  activity  by  the  higher 
temperature  brought  about  by  the  section  of  the  sympathetic  dilating  the 
bloodvessels,  are  more  easily  excited  by  any  stimulus  working  upon  them 
through  the  fifth  nerve.  And  though  the  idea  of  a  double  nervous  mechan- 
ism, augmenting  and  inhibitory,  governing  the  activity  of  the  sweat-glands, 
is  a  tempting  one,  there  are  at  present  no  satisfactory  reasons  for  adopting  it. 


THE  HISTORY  OF  GLYCOGEN.  437 

CHAPTER    IV. 
THE  METABOLIC  PEOCESSES  OF  THE  BODY. 

§  372.  WE  have  followed  the  food  through  its  changes  in  the  alimen- 
tary canal,  and  have  seen  it  enter  into  the  blood,  either  directly  or  by  the 
intermediate  channel  of  the  lacteals,  in  the  form  of  peptone  (or  otherwise 
modified  albumin),  sugar,  lactic  acid,  and  fats,  accompanied  by  various 
salts  and  water.  We  have  further  seen  that  the  waste  products  which 
leave  the  body  are  urea,  carbonic  acid,  salts  and  water.  We  have  now  to 
attempt  to  connect  together  the  food  and  the  waste  products ;  to  trace  out 
as  far  as  we  are  able  the  various  steps  by  which  the  one  is  transformed  into 
the  other.  There  remains  the  further  task  to  inquire  into  the  manner  in 
which  the  energy  set  free  in  this  transformation  is  distributed  and  made 
use  of. 

The  master  tissues  of  the  body  are  the  muscular  and  nervous  tissues ; 
all  the  other  tissues  may  be  regarded  as  the  servants  of  these.  And  we 
may  fairly  presume  that,  besides  the  digestive  and  excretory  tissues  which 
we  have  already  studied,  many  parts  of  the  body  are  engaged  either  in 
further  elaborating  the  comparatively  raw  food  which  enters  the  blood,  in 
order  that  it  may  be  assimilated  with  the  least  possible  labor  by  the  master 
tissues,  or  in  so  modifying  the  waste  products  which  arise  from  the  activity 
of  the  master  tissues  that  they  may  be  removed  from  the  body  as  speedily 
as  possible.  There  can  be  no  doubt  that  manifold  intermediate  changes 
of  this  kind  do  take  place  in  the  body ;  but  our  knowledge  of  the  matter 
is  at  present  very  imperfect.  In  a  few  instances  only  can  we  localize  these 
metabolic  actions  and  speak  of  distinct  metabolic  tissues.  In  the  majority 
of  cases  we  can  only  trace  out  or  infer  chemical  changes,  without  being 
able  to  say  more  than  that  they  do  take  place  somewhere ;  and  in  conse- 
quence, perhaps  somewhat  loosely,  speak  of  them  as  taking  place  in  the 
blood. 

How  little  we  know  concerning  the  metabolism  of  the  master  tissues 
themselves  was  shown  when  we  were  dealing  with  these  tissues  in  an  earlier 
part  of  this  work ;  but  success  in  the  study  of  these  can  hardly  be  ex- 
pected until  our  knowledge  is  increased  as  regards  the  changes  which  the 
blood  undergoes  before  it  reaches  and  after  it  leaves  the  muscle  or  the 
nerve.  The  fact  that  a  large  part  of  the  absorbed  food  is  carried  through 
the  liver  before  it  is  thrown  on  the  general  circulation  leads  us  to  suppose 
that  in  this  large  organ  important  metabolic  processes  are  carried  on ;  and 
observation  with  experiment  confirms  this  view.  Important  as  the  secre- 
tion of  bile  may  be,  the  other  metabolic  functions  of  the  liver  are  of  still 
greater  importance. 

THE  HISTORY  OF  GLYCOGEN. 

§  373.  If  the  liver  of  a  well-fed  animal  be  removed  immediately  after 
death,  rapidly  divided  into  small  pieces,  thrown  into  boiling  water,  rubbed 
up  and  boiled,  a  decoction  may  be  obtained  which  after  careful  neutraliza- 
tion and  filtration  will  be  tolerably  free  from  proteid  matter.  Such  a 
decoction  is  remarkably  opalescent,  milky  in  fact  in  appearance,  much  more 
so  than  a  similar  decoction  from  muscle  or  other  tissue,  and  remains  opal- 
escent even  after  repeated  filtration.  Treated  with  iodine,  the  solution 
turns  a  brownish-red,  port-wine  red  color,  not  unlike  that  given  by  dextrin 


438  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

when  iodine  is  added  ;  the  color  disappears  on  warming,  but  reappears  on 
cooling  provided  that  not  too  much  proteid  matter  has  been  left  in  the 
solution.  Treated  with  Fehling's  fluid  or  other  tests  for  sugar,  the  solution 
is  found  to  contain  a  small  and  variable,  but  only  a  small,  quantity  of  sugar. 

If  the  solution  be  exposed,  preferably  in  a  warm  room,  to  the  action  of 
saliva  or  of  some  other  amylolytic  ferment,  or  be  boiled  with  dilute  acid, 
the  opalescence  disappears ;  and  the  now  clear  transparent  solution  gives 
no  longer  the  port-wine  reaction  with  iodine.  Tested,  moreover,  with  Fehl- 
ing's fluid  or  by  other  means  it  is  now  found  to  contain  a  considerable 
quantity  of  sugar. 

If  alcohol  be  added  to  the  opalescent  solution  until  the  mixture  con- 
tains 60  per  cent,  of  the  alcohol  (previous  concentration  by  evaporation 
being  desirable)  a  white  amorphous  precipitate  is  thrown  down.  This  pre- 
cipitate, removed  by  filtration,  boiled  with  an  alcoholic  solution  of  potash 
in  which  it  is  insoluble,  but  which  dissolves  and  destroys  any  proteids 
which  may  be  present,  treated  with  ether  to  remove  fatty  impurities,  and 
washed  with  alcohol,  may  be  obtained  in  a  pure  condition.  It  then  appears 
as  a  white  amorphous  powder,  fairly  soluble  in  water,  but  always  giving 
rise  to  a  milky  opalescent  solution  unless  an  excess  of  alkali  be  present,  in 
which  case  the  opalescence  may  be  slight  or  absent. 

The  opalescent  solution  of  this  purified  material  gives  a  port-wine  reac- 
tion with  iodine,  but  no  reaction  whatever  with  Fehling's  fluid  or  the  other 
sugar  tests.  Treated  with  an  amylolytic  ferment  or  boiled  with  dilute  acid, 
the  solution,  like  the  raw  decoction  of  liver,  loses  its  opalescence  and  its 
port-wine  reaction  with  iodine,  but  now  gives  an  abundant  evidence  of  the 
presence  of  sugar,  dextrose  if  boiling  with  acid  has  been  employed,  malt- 
ose chiefly  if  an  amylolytic  ferment  has  been  used.  If  quantitative  deter- 
mination be  employed  it  will  be  found  that  the  amount  of  sugar  obtained  is 
proportionate  to  the  amount  of  the  white  powder  acted  upon ;  in  other 
words  the  substance  forming  an  opalescent  solution  is  converted  into  sugar, 
the  solution  of  which  is  clear.  Obviously  the  substance  is  a  body  allied  to 
the  starch ;  and  this  is  confirmed  by  the  elementary  composition,  which  is 
found  to  be  C6H,0O5  or  some  multiple  of  this. 

Hence  this  body  is  called  glycogen.  And  it  is  obvious  from  what  has 
been  stated  above  that  the  liver  of  a  well-fed  animal  at  the  moment  of 
death  contains  a  considerable  quantity  of  glycogen  either  in  a  free  state  or 
in  such  a  condition  that  it  is  set  free  by  subjecting  the  liver  to  the  action 
of  boiling  water.  We  may  add  that  it  occurs  in  the  liver  in  the  hepatic 
cells,  for  the  reaction  of  a  port-wine  color  given  under  certain  conditions 
by  the  hepatic  cells  is  due  to  the  presence  of  glycogen  in  them. 

§  374.  If  the  liver,  instead  of  being  treated  immediately  upon  the 
death  of  the  animal,  is  allowed  to  remain  in  the  body  of  the  dead  animal 
for  several  hours,  especially  in  a  warm  place,  before  a  decoction  is  made 
of  it,  the  decoction  will  be  found  to  have  little  or  no  opalescence,  to  be 
quite  clear,  to  give  little  or  no  port-wine  reaction  with  iodine,  but  to 
contain  a  very  considerable  quantity  of  sugar.  As  we  have  said  above, 
the  decoction  even  of  a  liver  taken  immediately  after  death  generally  con- 
tains some  little  sugar,  and  the  quantity  of  sugar  in  the  liver  appears,  as  a 
rule,  to  increase  steadily  after  death,  the  amount  of  glycogen  diminishing 
at  the  same  time.  The  glycogen  then  present  in  the  liver  at  the  moment 
of  death  is  gradually  after  death  by  some  action  or  other  converted  into 
sugar. 

The  action  is  that  of  some  agency  whose  activity  is  destroyed  by  the  tem- 
perature of  boiling  water;  hence  the  directions  repeatedly  given  above  to 
throw  the  liver  into  boiling  water.  This  naturally  suggests  the  presence  in 


THE  HISTORY   OF  GLYCOGEN.  439 

the  liver  of  an  amylolytic  ferment.  But  not  only  have  attempts  to  isolate 
from  the  liver  an  amylolytic  ferment  failed,  in  the  hands  of  most  observers 
at  least,  but  the  exact  nature  of  the  sugar  which  appears  shows  that  the 
change  is  not  effected  by  an  ordinary  amylolytic  ferment.  In  the  case  of 
the  amylolytic  ferment  of  saliva,  pancreatic  juice,  intestinal  juice,  and  indeed 
of  all  other  amylolytic  animal  fluids,  the  sugar  into  which  starch  or  glyco- 
geu  is  converted  is  maltose.  Now,  the  sugar  which  appears  in  the  liver  after 
death  is  dextrose,  identical,  as  far  at  least  as  can  at  present  be  made  out, 
with  ordinary  dextrose.  We  are  led,  therefore,  to  infer  that  the  change  of 
glycogen  into  sugar  which  appears  to  go  on  after  death  is  carried  out  by 
some  action  of  the  liver,  probably  of  the  hepatic  cell  itself,  which  is  done 
away  with  by  a  temperature  of  iOO°  C.,  but  which  is  not  the  action  of  a 
ferment  capable  of  being  isolated. 

§  375.  We  have  used  above  the  phrase  "  well-fed  "  animal  because  the 
amount  of  glycogen  present  in  the  liver  of  an  animal  at  any  one  time  is 
very  variable,  and  especially  dependent  on  the  amount  and  nature  of  the 


food  previously  taken.  When  all  food  is  withheld  from  an  animal  the  glyco- 
gen in  the  liver  diminishes,  rapidly  at  first,  but  more  slowly  afterward.  Even 
after  some  days'  starvation  a  small  quantity  is  frequently  still  found  ;  but  in 


rabbits,  at  all  events,  the  whole  may  eventually  disappear. 

If  an  animal,  after  having  been  starved  until  its  liver  may  be  assumed  to 
be  free,  or  almost  free,  from  glycogen,  be  fed  on  a  diet  rich  in  carbohydrates 
or  on  one  consisting  exclusively  of  carbohydrates,  the  liver  will  in  a  short 
time  be  found  to  contain  a  very  large  quantity  of  glycogen.  Obviously  the 
presence  of  carbohydrates  in  food  leads  to  an  accumulation  of  glycogen  in 
the  liver  ;  and  this  is  true  both  of  starch  and  of  dextrin  and  of  the  various 
forms  of  sugar  —  cane,  grape,  and  milk  sugar.  The  effect  may  be  quite  a 
rapid  one,  for  glycogen  has  been  found  in  the  liver  in  considerable  quantity 
within  a  few  hours  after  the  introduction  of  sugar  into  the  alimentary  canal 
of  a  starving  animal. 

If  an  animal  similarly  starved  be  fed  on  an  exclusively  meat  diet  a  cer- 
tain amount  of  glycogen  is  found  in  the  liver.  This  appears  to  be  especially 
the  case  with  dogs  (probably  with  other  carnivorous  animals  also),  and  in 
earlier  works  on  the  subject  the  constant  presence  of  glycogen  in  the  livers 
of  dogs  fed  on  meat  was  regarded  as  an  important  indication  of  the  forma- 
tion within  the  body  of  non-nitrogenous  from  nitrogenous  material.  But  in 
the  first  place,  the  quantity  of  glycogen  thus  stored  up  in  the  liver  as  the 
result  of  a  meat  diet  is  much  less  than  that  which  follows  upon  a  carbohy- 
drate diet  ;  and  in  the  second  place,  ordinary  meat,  especially  horseflesh  on 
which  dogs  in  such  experiments  are  usually  fed,  contains  in  itself  (§  62)  a 
certain  amount  either  of  glycogen  or  some  form  of  sugar.  Moreover,  when 
animals  are  fed  not  on  meat,  but  on  purified  proteid,  such  as  fibrin,  casein, 
or  albumin,  the  quantity  of  glycogen  in  the  liver  becomes  still  smaller, 
though,  according  to  most  observers,  remaining  greater  than  during  starva- 
tion. We  may  infer,  therefore,  that  part  of  the  glycogen  which  appears  in 
the  liver  after  a  meat  diet  is  really  due  to  carbohydrate  materials  present  in 
the  meat.  Part,  however,  would  appear  to  be  the  result  of  the  actual  pro- 
teid food  ;  and  we  have  similar  evidence  that  gelatin  taken  as  food  leads  to 
the  formation  of  some  glycogen  in  the  liver.  But  in  this  respect  these  nitro- 
genous substances  fall  far  short  of  carbohydrate  material. 

With  regard  to  fats,  all  observers  are  agreed  that  these  lead  to  no  accu- 
mulation of  glycogen  in  the  liver  ;  an  animal  fed  on  an  exclusively  fatty 
diet  has  no  more  glycogen  in  its  liver  than  a  starving  animal. 

Hence  of  the  three  great  classes  of  food-stuffs  the  carbohydrates  stand 
out  prominently  as  the  substances  which  taken  as  food  lead  to  an  accumu- 


440 


THE  METABOLIC  PROCESSES  OF  THE  BODY. 


lation  of  glycogen  in  the  liver.  We  may  remark  that  the  greatest  accumu- 
lation of  glycogen  is  effected  not  by  a  pure  carbohydrate  diet,  but  by  a 
mixed  diet  rich  in  carbohydrates.  A  quantity  of  carbohydrate  mixed  with 
a  certain  proportion  of  proteid  gives  rise  to  a  larger  amount  of  glycogen  in 
the  liver  than  the  same  quantity  of  carbohydrate  given  by  itself;  and  it  is 
possible  that  the  presence  of  an  appropriate  quantity  of  fat  still  further 
assists  the  accumulation.  But  this  result  probably  depends,  in  part  at 
least,  on  the  fact  that,  though  differences  may  be  met  with  in  different  ani- 
mals, a  mixture  of  the  several  classes  of  food-stuffs  is  more  readily  digested, 
resulting  in  more  nutritive  material  being  thrown  upon  the  blood,  than  is  a 
meal  consisting  exclusively  of  one  kind  of  food-stuff  alone. 

As  far  as  we  know  at  present  the  glycogen  which  thus  appears  in  the 
liver  as  the  result  of  feeding  either  with  any  of  the  various  forms  of  car- 
bohydrates or  with  proteids,  or  with  other  substances,  is  of  the  same  kind 
and  presents  the  same  characters ;  at  least  we  have  no  evidence  to  the  con- 
trary. 

The  storing-up  of  glycogen  in  the  liver  is  also  influenced  by  other  circum- 
stances than  the  taking  of  food.  For  instance,  in  the  frog  an  increase  of 
glycogen  takes  place  during  the  winter  months.  In  the  summer  months  the 
liver  of  a  frog  will  be  found  to  contain  very  little  glycogen  (Fig.  114,  C), 

FIG.  113. 


Section  of  Liver  of  Frog.  (Langley.)  The  figure  shows  the  tubular  structure  of  the  liver.  At 
a,  a  tubule  is  seen  in  transverse,  at  b  in  longitudinal  section  ;  I,  lumen  of  tubule.  The  liver  was 
that  of  a  winter  frog,  and  the  cells  show  an  inner  zone  of  proteid  granules  ;  the  outer  zone  was 
chiefly  occupied  by  glycogen. 

unless  the  animal  has  been  unusually  well  fed  ;  whereas  a  liver  examined  in 
midwinter  (Figs.  113,  114,  A)  will  be  found  to  contain  a  considerable  quan- 
tity, even  though  no  food  has  been  taken  for  months.  In  such  a  case  the 
material  for  the  formation  of  the  glycogen  in  the  liver  must  have  been  fur- 
nished by  some  part  of  the  body  of  the  frog,  and  could  not,  as  may  be  the  case 
when  a  *meal  leads  immediately  to  an  increase  of  glycogen,  be  supplied 


THE  HISTORY   OF  GLYCOGEN. 


441 


directly  from  the  food.  It  seems  as  if  in  the  summer  the  frog  lives  up  to  its 
capital  of  hepatic  glycogen,  spending  it  as  fast  almost  as  it  is  made,  but  that 
during  the  winter  a  quantity  is  funded  to  provide  for  the  demands  of  the 
late  winter  and  early  spring. 

This  winter  storage  of  hepatic  glycogen  in  the  frog  seems  closely  depen- 
dent on  temperature.  If  a  winter  frog,  whose  liver  is  presumably  more  or 
less  loaded  with  glycogen,  be  exposed  for  some  time  to  a  temperature  of  20° 
or  a  little  higher,  the  liver  will  afterward  be  found  to  contain  little  or  no 
glycogen  (Fig.  114,  B) ;  and  conversely,  if  a  summer  frog  be  exposed  to 
untimely  cold,  glycogen,  though  not  in  any  great  quantity,  begins  to  be 
stored  up  in  the  liver. 

§  376.  Before  we  attempt  to  discuss  further  how  food  and  other  circum- 

FIG.  114. 


Three  Phases  of  the  Hepatic  Cells  of  the  Frog.  (Langley.)  -4,  eel  Is  rich  in  glycogen.  Takenfroma 
frog  during  winter.  The  cells  are  large  and  proteid  granules  are  massed  around  the  lumen,  the  ho- 
mogeneous outer  zones  of  the  cells  are  largely  composed  of  glycogen,  which  is  present  in  con- 
siderable abundance.  The  outer  zones  contain  numerous  fat  globules,  shown  as  dark  spots ; 
but,  as  stated  in  the  text,  these  fat  globules  vary  much.  S,  cells  poor  in  glycogen.  Taken  from  a 
winter  frog  which  had  been  kept  at  22°  C.  for  ten  days.  The  cells  contain  very  little  glycogen,  and 
the  proteid  granules  are  dispersed  throughout  the  cell.  In  a  summer  frog  well  fed  on  proteids  the 
cells  would  present  a  very  similar  appearance.  C,  starved  cells.  Taken  from  a  summer  frog  after 
a  long  fast.  The  cells  are  small  and  almost  free  from  glycogen.  The  proteid  granules  are  dis- 
persed throughout  the  cell.  All  the  specimens  were  hardened  in  1  per  cent,  osmic  acid,  and  are 
drawn  to  the  same,  or  nearly  to  the  same  scale. 

stances  thus  affect  the  glycogen  in  the  liver,  it  will  be  desirable  to  take  up 
the  matter  which  we  left  on  one  side,  viz.,  the  consideration  of  the  histo- 
logical  changes  occurring  in  the  hepatic  cells  under  various  conditions.  It 
will  be  convenient  to  begin  with  the  cells  of  the  more  distinctly  tubular 
gland  of  the  frog. 

In  a  frog  which  has  not  been  subjected  to  any  special  treatment  the  cell 
substance  of  the  hepatic  cell  (cf.  Fig.  114,  A)  will  generally  be  found  to 
contain  lodged  in  itself  three  kinds  of  material,  the  presence  of  which,  if 
not  directly  recognizable  in  the  fresh  cell,  may  be  demonstrated  by  the 
use  of  various  reagents.  In  the  first  place,  oil  globules  of  variable  size 


442  THE   METABOLIC  PROCESSES   OF  THE   BODY. 

and  in  variable  amount  are  scattered  throughout  the  cell ;  sometimes,  as 
we  have  already  said,  these  are  extremely  abundant ;  but  there  is  otherwise 
nothing  very  special  about  these  fat  globules  in  the  hepatic  cell  to  demand 
any  discussion  concerning  them  apart  from  the  general  discussion  on  the 
formation  of  fat,  into  which  we  shall  enter  later  on. 

In  the  second  place,  a  number  of  small  discrete  granules  may  be  seen 
lodged  in  the  cell  substance.  These  appear  to  be  of  a  proteid  nature  and 
are  generally  most  abundant  on  the  inner  side  of  the  cell  near  the  lumen  of 
the  bile  passage.  The  presence  of  these  granules  is  closely  dependent  on  the 
activity  of  the  digestive  processes.  They  diminish  when  digestion  is  going 
on  and  accumulate  again  afterward.  Putting  aside  certain  details,  we  may 
say  that  these  granules  behave  very  much  like  the  granules  in  an  albumin- 
ous salivary  cell,  a  pancreatic  cell,  or  a  chief  gastric  cell ;  and  we  may  prob- 
ably safely  conclude  that  they,  like  the  granules  in  these  cells,  are  in  some 
way  concerned  in  the  formation  of  the  secretion  ;  that  is,  in  their  case,  bile. 

In  the  third  place,  the  cell  contains,  more  especially  in  its  outer  parts 
nearer  the  bloodvessel,  away  from  the  lumen  of  the  bile  passage,  a  variable 
quantity  of  material  which  differs  from  the  ordinary  cell  substance  in  being 
hyaline  and  refractive,  and  hence  glassy-looking,  and  in  staining  port-wine 
red  with  iodine,  instead  of  brownish-yellow,  as  does  ordinary  cell  substance. 
This  material  is,  though  with  some  little  difficulty,  soluble  in  water,  and  by 
this  means  may  be  dissolved  out  from  the  cell.  When  this  is  done  the  places 
which  it  occupied  appear  as  vacuoles  or  gaps  of  various  sizes  limited  by  bars 
of  the  cell  substance,  which  thus  take  on  the  form  of  a  network,  the  meshes 
of  which  are  wider  arid  more  conspicuous  in  the  outer  part  of  the  cell,  in 
which  the  hyaline  material  was  previously  most  abundant.  In  the  inner 
part  of  the  cell  where  the  hyaline  material  was  scanty  the  cell  substance  is 
more  dense,  and  even  in  the  outer  part  a  shell  of  more  dense,  less  reticulate 
cell  substance  affords  a  definite  outline  to  the  cell.  There  can  be  no  doubt 
that  this  hyaline  material  is  either  actual  glycogeu,  such  as  may  be  ex- 
tracted from  the  liver,  or,  as  seems  more  probable  from  its  definite  solu- 
bility, glycogen  in  some  more  or  less  loose  combination  with  some  other 
body,  a  combination,  however,  of  such  a  kind  that  the  iodine  reaction  makes 
itself  felt. 

§  377.  The  above  may  be  taken  as  a  general  description  of  a  cell  in  an 
ordinary  condition.  The  question  now  comes  before  us,  What  changes  are 
brought  about  by  various  foods  or  by  the  absence  of  food  ? 

If  a  frog  be  largely  fed  on  a  diet  containing  large  quantities  of  carbo- 
hydrates, the  liver  will  be  found  rich  in  glycogen,  and  the  cells  will  present 
the  following  characters:  The  cell  is  relatively  large  (cf.  Fig.  114,  A), and, 
as  it  were,  swollen  ;  the  cell  substance  is  largely  occupied  by  the  hyaline 
material  just  spoken  of,  especially  in  its  outer  parts,  so  that  in  sections  pre- 
pared and  mounted  in  the  ordinary  way  in  which  the  glycogen  has  been 
dissolved  out,  the  greater  part  of  the  cell  'consists  of  a  loose  open  network  of 
bars  of  stained  cell  substance  with  wide  meshes ;  a  certain  quantity  of  more 
solid,  generally  granular-looking  cell  substance  occupies  the  part  of  the  cell 
nearest  the  lumen,  and  a  thin  shell  of  cell  substance  forms  an  envelope  for 
the  rest  of  the  cell.  The  nucleus  is  large  and  distinct,  but  though  changes  in 
the  nucleus  accompanying  changes  in  the  cell  substance  have  been  described, 
they  are  not  sufficiently  important  to  detain  us  now.  When  such  a  cell  is  seen 
in  a  perfectly  fresh  state,  the  hyaline  refractive  material  (which,  we  need 
hardly  say,  gives  a  marked  reaction  with  iodine)  often  hides  the  nucleus  and 
the  greater  part  of  the  cell  substance  proper. 

If,  on  the  other  hand,  the  frog  be  fed  on  a  proteid  diet  free  from  carbo- 
hydrates— for  instance,  on  fibrin — the  liver  contains  little  or  no  glycogen, 


THE  HISTORY  OF  GLYCOGEN. 


443 


and  the  hepatic  cells  are  not  only  much  smaller,  but  present  an  appearance 
very  different  from  the  above  (cf.  Fig.  114,  B).  Little  or  no  hyaline  mate- 
rial is  visible,  the  cells  give  little  or  no  port-wine  reaction  with  iodine,  but 
only  the  usual  brown-yellow  proteid  reaction,  and  in  specimens  prepared  and 
mounted  in  the  ordinary  way  the  cell  substance  appears  densely  granular 
throughout. 

Lastly,  if  the  frog  be  starved,  and  if  to  the  effects  of  starvation  there  be 
added  those  of  exposure  to  a  high  temperature  (25°  C.),  by  which,  as  we  have 
seen,  the  hepatic  cells  are  markedly  affected,  the  liver  is  found  to  be  free 
from  glycogen  and  the  hepatic  cells  to  be  extremely  small  (cf.  Fig.  114,  C), 
only  half  the  size  or  even  less  of  those  of  the  well-fed  frog,  but  otherwise 
much  like  the  cells  in  a  frog  fed  on  proteid  material. 

§  378.  In  the  mammal  changes  in  the  hepatic  cells  similar  to  those  just 
described  as  occurring  in  the  frog  have  also  been  observed.  When  the 
animal  is  fed  on  a  diet  rich  in  carbohydrates,  and  when,  therefore,  as  we 
have  seen,  the  liver  abounds  in  glycogen,  the  hepatic  cells  (Fig.  115)  are 
larger  (so  large  that  they  have  by  some  authors  been  described  as  com- 
pressing the  lobular  capillaries)  and  loaded  with  the  same  refractive  hya- 
line material  staining  port-wine  red  with  iodine.  When  this  material  is 
dissolved  out  a  coarse  open  network  of  cell  substance  is  displayed.  The 


FIG.  115. 


FIG.  116. 


Section  of  Mammalian  Liver  rich  in  Gly-  Section  of  Mammalian  Liver  containing 

cogen.    (Langley.)    Osmic  acid  specimen,  gly-  little  or  no  Glycogen.    (Langley.)    Osmic 

cogen  not  dissolved  out.  acid  specimen.    The  granules  are  not  well 

preserved  in  some  of  the  cells. 

most  marked  point  of  difference  between  the  mammalian  and  frog's  hepatic 
cell  under  these  conditions  is  that  in  the  former,  the  hyaline  glycogenic  sub- 
stance is  gathered  at  first  centrally  around  the  nucleus  (not  more  on  the  outer 
side,  as  is  the  case  in  the  frog)  and  spreads  from  the  centre  toward  the  pe- 
riphery, always  leaving  on  the  extreme  outside  a  somewhat  thick  shell  of 
cell  substance,  which  in  hardened  and  prepared  specimens  may  strikingly 
simulate  a  thickened  cell- wall.  We  may  add  that  in  an  animal  thus  fed  the 
whole  liver  is  very  large,  and,  as  it  were,  swollen ;  it  is  also  soft  and  tears  easily. 

In  an  animal  fed  on  proteids  alone,  for  instance  on  fibrin,  the  liver  fre- 
quently contains  some  glycogen  and  the  hepatic  cells  contain  a  small  quantity 
of  hyaline  glycogenic  material.  As  in  the  corresponding  case  in  the  frog, 
the  cells  are  comparatively  small,  and  the  cell  substance  appears  finely  and 
uniformly  granular. 

In  a  starved  mammal  the  liver  is  small,  dense  to  the  touch,  and  tough ; 
it  contains  a  trace  only  of  glycogen  or  none  at  all ;  the  cells  (Fig.  116)  are 
small,  as  it  were  shrunken,  and  the  cell  substance,  which  gives  no  port-wine 
reaction,  or  a  mere  trace  only,  with  iodine,  is  still  more  finely  granular. 

§  379.  The  microscopic  appearances  just  described  show,  and  indeed 


444  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

general  considerations  lead  us  to  the  same  conclusion,  that  the  processes 
taking  place  in  an  hepatic  cell  are  very  complex.  In  the  first  place,  the  con- 
stituents of  bile  are  being  formed  and  discharged  into  the  bile  passages  after 
the  fashion  of  ordinary  secreting  glands.  In  the  second  place,  a  formation 
of  glycogen  is  also  taking  place,  and  we  shall  have  presently  to  consider 
briefly  the  relations  of  the  one  process  to  the  other.  In  the  third  place,  as 
is  especially  indicated  by  the  somewhat  peculiar  effects  on  the  hepatic  cell  of 
food  exclusively  proteid  in  nature,  other  processes,  similar  perhaps  to  the 
formation  of  glycogen,  but  not  resulting  in  the  storage  of  any  carbohydrate 
material,  and  dealing  possibly  with  proteid  substances,  also  take  place. 
Hence  the  exact  interpretation  of  all  the  changes  which  may  be  observed 
becomes  exceedingly  difficult. 

Leaving  the  processes  of  the  first  and  third  kind  wholly  on  one  side  for 
the  present,  and  confining  our  attention  entirely  to  the  glycogen,  it  is  obvi- 
ous that  the  hepatic  cell  manufactures  the  glycogen  in  some  way  or  other 
and  lodges  it  in  its  own  substance  for  the  time  very  much  in  the  same  way 
that  a  secreting  cell  manufactures  and  lodges  in  itself  for  a  time  material 
for  the  secretion  which  it  is  about  to  pour  forth.  There  is  this  difference, 
that  in  the  one  case  the  material  of  the  secretion,  after  undergoing,  as  we 
have  seen,  more  or  less  change,  is  cast  out  into  the  lumen  of  the  alveolus, 
whereas  in  the  other  case  the  glycogen,  which  must  undergo  change,  since 
it  may  be  made  to  disappear  rapidly  from  the  hepatic  cell,  is  not  when 
changed  cast  out  into  the  bile  passages ;  it  must  therefore  be  sent  back  again 
to  the  blood. 

§  380.  We  say  "  manufacture  the  glycogen  in  some  way  or  other,"  and 
we  have  now  to  inquire  what  we  know  concerning  the  nature  and  the  several 
steps  of  this  manufacture. 

We  have  already  seen  that  the  presence  of  glycogen  in  the  liver  is 
especially  favored  by  a  carbohydrate  diet ;  and  in  our  studies  on  digestion 
we  have  seen  reason  to  think  that  a  very  large  part  at  all  events  of  the 
carbohydrate  material  of  a  meal  is  absorbed  as  sugar  by  the  capillaries  of 
the  intestine  and  carried  as  sugar  to  the  liver  in  the  portal  blood.  Hence, 
it  seems  only  reasonable  to  conclude  that  the  glycogen  which  makes  its 
appearance  in  the  liver  after  an  amylaceous  meal  arises  from  a  direct  con- 
version of  the  sugar  carried  to  the  liver  by  the  portal  vein,  the  sugar 
becoming  through  some  action  of  the  hepatic  cell  substance  dehydrated  into 
glycogen,  or  animal  starch,  as  it  has  been  called,  the  process  being  a  reverse 
of  that  by  which  in  the  alimentary  canal  starch  is  hydrated  into  sugar 
through  the  action  of  the  salivary  and  pancreatic  ferments.  Vegetable  cells 
can  undoubtedly  convert  both  starch  into  sugar  and  sugar  into  starch ;  and 
there  are  no  a  priori  arguments  or  positive  facts  which  would  lead  us  to 
suppose  that  the  activity  of  animal  living  substance  cannot  accomplish  the 
latter  as  well  as  the  former  of  these  changes.  We  are  quite  ignorant,  it  is 
true,  of  the  exact  way  in  which  either  the  hydration  or  the  dehydration  is 
effected  by  living  substances;  but  we  are  equally  ignorant  of  the  exact  way 
in  which  an  amylolytic  ferment  effects  the  hydration  of  starch  into  sugar, 
which  it  carries  out  with  so  much  apparent  ease.  It  is  not  a  great  assump- 
tion to  suppose  that  the  continually  changing  living  substance,  which  in  its 
changes  is  continually  giving  out  energy,  has  the  power  of  acting  on  mole- 
cules of  starch  or  of  sugar  in  contact  with  or  even  only  near  to  itself,  and 
so  of  hydrating  starch  into  sugar  or  of  dehydrating  sugar  into  starch. 
The  latter  process  may  be  a  more  difficult  one  than  the  former,  but  not  one 
beyond  the  power  of  the  living  substance.  We  may  fairly  suppose  that  a 
quantity  of  sugar  in  solution  present  in  a  vacuole,  for  instance,  of  the  hepatic 
cell  substance  can  be,  bv  some  action  of  the  cell  substance,  converted  into 


THE  HISTORY  OF  GLYCOGEN.  445 

glycogen  in  a  solid  form,  filling  up  the  vacuole.  Again,  as  we  have  inci- 
dentally mentioned,  sugar  injected  into  the  jugular  vein  readily  gives  rise  to 
sugar  in  the  urine ;  but  a  very  considerable  quantity  can  be  slowly  injected 
into  the  portal  vein  without  any  appearing  in  the  urine.  This  suggests  the 
idea  that  the  liver,  so  to  speak,  catches  the  sugar  as  it  is  passing  through 
the  hepatic  capillaries  and  at  once  dehydrates  it  into  glycogen. 

Similar  consideration  may  also  be  applied  to  the  case  mentioned  above  of 
the  appearance  of  glycogen  in  the  hepatic  cells  of  winter  (fasting)  frogs. 
We  have  reason  to  think  that  sugar  makes  its  appearance  as  a  product  of 
the  metabolism  of  various  tissues.  The  sugar  thus  arising  finding  its  way 
into  blood  may  be  made  use  of  at  once  elsewhere,  converted  speedily,  for 
instance,  into  carbonic  acid  and  so  got  rid  of.  But  we  can  readily  imagine 
that  uuder  certain  circumstances,  as  for  instance  when  the  activities  of  the 
animal  were  lessened  by  a  low  temperature,  it  was  not  so  made  use  of  and 
remained  in  the  blood.  If  so,  it  would  in  the  course  of  the  circulation  be 
carried  to  the  liver,  and  might  be  at  once  taken  up  by  the  hepatic  cells  and 
converted  into  glycogen  ; '  and  these  might  be  so  active  that  the  blood  was 
never  at  any  time  allowed  to  remain  loaded  with  sugar  to  such  an  extent  as 
to  permit  a  loss  through  the  urine. 

§  381.  Upon  such  a  view,  the  carbohydrate  taken  as  food  would  be 
converted  into  glycogen  by  the  agency  of  the  hepatic  cell,  without  at  any 
time  becoming  an  integral  part  of  the  living  substance  of  the  cell.  Such  a 
view  may  be  the  true  one ;  but  it  is  open  for  us  to  look  at  the  matter  in 
another  light.  We  may  push  still  further  the  analogy  between  the  glycogen 
of  the  hepatic  cell  and  the  material  with  which  a  secreting  cell  is  loaded.  In 
dealing  with  secretions  we  saw  reasons  for  regarding  such  a  body  as  mucin 
to  be  a  product  of  the  metabolism  of  the  cell  substance  of  the  mucous  cell ; 
and  we  may  similarly  regard  glycogen,  or  sugar  readily  convertible  into 
glycogen,  or  at  least  some  or  other  carbohydrate  material,  as  a  normal  pro- 
duct of  the  metabolism  of  the  hepatic  cell.  We  may  thus  conceive  of  the 
hepatic  cells  as  being  continually  engaged  in  giving  rise  to  carbohydrate 
material  in  the  form  either  of  sugar  or  of  some  other  body ;  and  we  may 
suppose  that  under  certain  circumstances,  as  in  the  absence  of  adequate  food, 
the  carbohydrate  material  thus  formed  is  at  once  discharged  into  the  blood 
of  the  hepatic  vein  for  the  general  use  of  the  body,  but  that  under  other 
circumstances,  as  when  an  amylaceous  meal  has  been  taken,  the  immediate 
wants  of  the  economy  being  covered  by  the  carbohydrates  of  the  meal,  the 
carbohydrate  products  of  the  hepatic  metabolism  are  stored  up  as  glycogen. 
Under  such  a  view  the  sugar  of  the  meal  is  used  up  somewhere  in  the  body, 
and  the  glycogen  to  the  storage  of  which  in  the  liver  it  gives  rise  comes 
direct  from  the  hepatic  substance.  And  a  similar  explanation  may  be  given 
of  the  storing-up  of  the  glycogen  in  the  liver  under  such  circumstances  as 
those  of  the  winter  frog  previously  mentioned. 

We  do  not  possess  at  present  experimental  or  other  evidence  of  so  clear 
a  kind  as  to  enable  us  to  decide  dogmatically  between  these  two  views.  We 
have  seen  that  proteid  food,  though  in  this  respect  falling  far  below  carbo- 
hydrated  food,  does  or  may  give  rise  to  a  certain  amount  of  glycogen  in  the 
liver ;  and  gelatin  seems  to  have  the  same  effect.  Further,  in  certain  cases 
of  the  disease  diabetes,  of  which  we  shall  have  to  speak  presently,  and  which 
is  characterized  by  the  presence  of  a  large  amount  of  sugar  in  the  blood, 
sugar  continues  to  be  formed  in  large  quantity,  even  when  the  diet  is  re- 
stricted to  proteid  and  fatty  matters,  all  carbohydrates  being  excluded. 
Now  in  diabetes  we  have  reason  to  believe  that  the  large  quantity  of  sugar 
in  the  blood  is  accompanied  by  a  large  deposition  of  glycogen  in  the  liver, 
and  indeed  in  other  tissues ;  for  in  the  few  cases  which  have  been  examined 


446  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

sufficiently  soon  after  death,  and  in  which  owing  to  the  suddenness  of  death 
there  was-  no  opportunity  for  stored-up  glycogen  to  disappear,  a  very  large 
quantity  of  glycogen  has  been  found  in  the  liver  or  some  other  organs. 
Hence  the  phenomena  of  diabetes  may  be  taken  as  showing,  in  a  much  more 
striking  manner  than  do  any  experiments,  that  proteid  material  taken  as 
food  may  give  rise  to  hepatic  glycogen. 

§  382.  We  may  now  turn  to  another  question,  the  answer  of  which  is  in 
a  measure  dependent  on  the  one  which  we  have  just  discussed.  What  is  the 
use  and  purpose  of  this  hepatic  glycogen  ?  What  ultimately  becomes  of  the 
glycogen  thus  for  a  while  stored  up  in  the  liver? 

One  view  which  has  been  put  forward  is  as  follows :  We  have  evidence, 
as  we  shall  presently  learn,  that  a  great  deal  of  the  fat  of  the  body  is  not 
taken  as  such  in  the  food,  but  is  constructed  anew  in  the  body  out  of  other 
substances.  Both  carbohydrates  and  proteids,  taken  in  excess  or  under 
certain  circumstances,  lead  to  an  accumulation  of  fat;  and  we  have  reason 
to  believe  that  carbohydrates,  on  the  one  hand,  and  the  carbon-holding  por- 
tions of  various  proteids,  on  the  other,  may  by  some  process  or  other  be  con- 
verted into  fat.  And  it  has  been  suggested  that  the  glycogen  in  the  liver  is 
a  phase  of  a  constructive  fatty  metabolism,  that  it  is  material  on  its  way  to 
become  fat. 

Another  view,  one  which  has  already  been  suggested  while  we  were  dealing 
with  the  manner  of  formation  of  glycogen,  makes  use  of  the  formation  of  fat 
for  the  purposes  of  analogy  only.  Seeing  that  adipose  tissue  serves  as  a 
storehouse  of  fat  which  is  not  wanted  by  the  body  at  the  moment,  but  may 
be  wanted  presently,  the  question  readily  presents  itself,  May  not  the  hepatic 
glycogen  have  an  analogous  function  ?  May  we  not  regard  the  presence  of 
glycogen  in  the  liver  as  in  large  measure  due  to  the  fact  that  it  is  deposited 
there  simply  as  a  store  of  carbohydrate  material,  being  accumulated  when- 
ever amylaceous  material  is  abundant  in  the  alimentary  canal,  and  being 
converted  into  sugar  and  so  drawn  upon  by  the  body  at  large  to  meet  the 
general  demands  for  carbohydrate  material  during  the  intervals  when  food  is 
not  being  taken  ?  And  we  can  accept  this  view  without  being  able  to  say 
definitely  what  becomes  of  the  sugar  thus  thrown  into  the  hepatic  blood. 
It  was  formally  believed  that  this  sugar  underwent  an  immediate  and  direct 
oxidation  as  it  was  circulating  in  the  blood,  but  we  have  already  dwelt 
(§  360)  on  the  objections  to  such  a  view.  It  is  sufficient  for  us  at  the  present 
to  admit  that  the  sugar  is  made  use  of  in  some  way  or  other. 

Now,  many  considerations  lead  us  to  believe  that  a  certain  average  com- 
position is  necessary  for  that  great  internal  medium  the  blood,  in  order  that 
the  several  tissues  may  thrive  upon  it  to  the  best  advantage,  one  element  of 
that  composition  being  a  certain  percentage  of  sugar.  It  would  appear  that 
some  at  least  if  not  all  of  the  tissues  are  continually  drawing  upon  the  blood 
for  sugar,  and  that  hence  a  certain  supply  must  be  kept  up  to  meet  this 
demand.  On  the  other  hand  an  excess  of  sugar  in  the  blood  itself  would 
be  injurious  to  the  tissues.  Arid  as  a  matter  of  fact  we  find  that  the  quan- 
tity of  sugar  in  the  blood  is  small  but  constant ;  it  remains  about  the  same 
when  food  is  being  taken  as  in  the  intervals  between  meals.  If  sugar  be 
injected  into  the  jugular  vein  in  too  large  quantities  or  to  rapidly,  a  certain 
quantity  appears  in  the  urine,  indicating  an  effort  of  the  system  to  throw  off 
the  excess  and  so  bring  back  the  blood  to  its  average  condition.  The  main- 
tenance of  such  a  constant  percentage  of  sugar  would  obviously  be  pro- 
vided for,  or  at  least  largely  assisted  by  the  liver  acting  as  a  structure  where 
the  sugar  might  at  once  and  without  much  labor  be  packed  away  in  the 
form  of  the  less  soluble  glycogen,  at  those  times  when,  as  during  an  amyla- 
ceous meal,  sugar  is  rapidly  passing  into  the  blood,  and  there  is  danger  of 


THE  HISTORY  OF  GLYCOGEN.  447 

the  blood  becoming  loaded  with  far  more  sugar  than  is  needed  for  the  time 
being ;  and  it  may  be  incideutly  noted  that  a  larger  quantity  of  sugar  may 
be  injected  into  the  portal  than  into  the  jugular  vein  without  any  reappear- 
ing in  the  urine,  apparently  because  a  large  portion  of  it  is  in  such  a  case 
retained  in  the  liver  as  glycogen.  At  those  times,  on  the  other  hand,  when 
we  may  suppose  that  sugar  ceases  to  pass  into  the  blood  from  the  alimentary 
canal,  the  average  percentage  in  the  blood  is  maintained  by  the  glycogen 
previously  stored  up  becoming  reconverted  into  sugar,  and  being  slowly  dis- 
charged into  the  hepatic  blood. 

Moreover,  this  view,  that  the  glycogen  of  the  liver  is  a  reserve  fund  of 
carbohydrate  material,  is  strongly  supported  by  the  analogy  of  the  migra- 
tion of  starch  in  the  vegetable  kingdom.  We  know  that  the  starch  of  the 
leaves  of  a  plant,  whether  itself  having  previously  passed  through  a  glucose 
stage  or  not,  is  normally  converted  into  sugar,  and  carried  down  to  the  roots 
or  other  parts,  where  it  frequently  becomes  once  more  changed  back  again 
into  starch. 

§  383.  Glycogen  is  found  in  other  parts  of  the  body  than  the  liver,  and  a 
study  of  the  facts  relating  to  the  presence  of  glycogen  in  other  tissues  will 
help  us  to  a  true  conception  of  the  purposes  of  the  hepatic  glycogen.  Next 
to  the  liver,  the  skeletal  muscles  are  perhaps  the  most  conspicuous  glycogen- 
holders.  So  frequently  is  glycogen  found  in  muscle  that  it  may  be  regarded 
as  an  ordinary  though  not  an  invariable  constituent  of  that  tissue ;  indeed 
it  may  almost  be  considered  as  a  constituent  of  all  contractile  tissues.  The 
quantity  varies  very  largely  both  in  the  different  muscles  of  the  same  animal 
and  corresponding  muscles  of  different  animals.  It  disappears,  according 
to  some  observers,  readily  upon  starvation,  even  before  the  hepatic  glycogen 
is  exhausted ;  but  all  observers  are  not  agreed  on  this  point,  and  in  some 
muscles,  at  least,  it  appears  to  be  retained  for  a  very  long  time.  It  is  said 
to  be  increased  in  quantity  when  the  nerve  of  the  muscle  is  divided,  and  the 
muscle  thus  brought  into  a  state  of  quiescence.  On  the  other  hand  it 
diminishes  or  even  disappears,  being  apparently  converted  into  dextrose, 
when  the  muscle  enters  into  rigor  mortis.  Some  observers  have  found  that 
it  diminishes  during  tetanus,  and  maintain  that  it,  after  conversion  into  dex- 
trose, is  used  up  in  the  act  of  contraction,  forming  through  its  oxidation  the 
immediate  supply  of  the  energy  set  free  in  the  contraction.  But  even  grant- 
ing that  the  glycogen  in  a  muscle  may  be  diminished  during  prolonged 
labor,  it  cannot  be  admitted  that  the  oxidation  or  other  chemical  change  of 
glycogen  is  a  necessary  part  of  the  ordinary  metabolism  of  a  muscular  con- 
traction, since  many  muscles  wholly  free  from  glycogen  are  perfectly  well 
able  to  carry  on  long-continued  contractions. 

Another  view  of  the  use  of  glycogen  in  muscle  is  suggested  by  the  fact 
that  undeveloped  embryonic  muscles  are  peculiarly  rich  in  glycogen.  In  a 
young  embryo,  at  the  time  when  the  muscular  substance,  though  undergoing 
striation,  is  still  largely  "  protoplasmic"  in  nature,  the  quantity  of  glycogen 
present  is  enormous ;  it  frequently  amounts  to  40  per  cent,  of  the  dry  mate- 
rial. At  this  period  the  hepatic  cells  are  immature  and  very  little  glycogen 
is  present  in  them.  Later  on,  as  the  muscles  become  more  wholly  striated, 
the  glycogen  largely  disappears  from  the  muscle,  and  very  soon  afterward 
begins  to  be  stored  up  in  the  liver. 

The  meaning  of  this  can  hardly  be  mistaken.  The  glycogen  in  the  imma- 
ture muscle  is  a  store  of  carbohydrate  material,  laid  down  on  the  spot,  and 
ready  at  once  to  be  used  in  what  we  may  probably  call  the  fierce  metabolic 
struggle  by  which  the  simple  protoplasmic  cell  substance  of  the  rudiment 
of  the  muscular  fibre  is  transformed  into  the  highly  differentiated  striated 
contractile  substance.  And  we  shall  probably  not  err  in  considering  the 


448  THE   METABOLIC   PROCESSES  OF  THE   BODY. 

glycogen  of  the  mature  muscle  to  hold  a  similar  position  ;  it  is  carbohydrate 
material  stored  up  on  the  spot,  a  local  branch,  so  to  speak,  of  the  great  carbo- 
hydrate bank.  It  is  destined  to  become  part  of  the  contractile  substance, 
and  as  such  will  contribute  to  the  energy  set  free  in  a  muscular  contrac- 
tion ;  but  its  energy  is  only  available  in  this  way  after  it  has  undergone  the 
necessary  metabolism  and  become  part  of  muscular  substance ;  it  cannot  be 
fired  off  in  a  contraction  while  it  lies  as  raw  glycogen,  or  even  as  dextrose, 
in  the  interstices  of  the  muscular  fibre. 

§  384.  Glycogen  may  also  be  found  in  considerable  quantity  in  the  pla- 
centa. Here,  as  we  shall  see  in  the  latter  part  of  this  work,  it  is  laid  down 
in  epithelial  cells  which  lie  on  the  boundary  between  the  maternal  and  the 
foetal  tissues.  And  here,  too,  there  can  be  little  doubt  that  it  is  a  store  of 
carbohydrate  material  for  the  nourishment  of  the  foetus. 

It  has  also  been  found  in  leucocytes,  in  cartilage  corpuscles,  especially  in 
those  large  rapidly  growing  and  rapidly  multiplying  cartilage  corpuscles 
which  lie  in  the  outer  zone  of  endochondral  ossification,  and  in  other  situa- 
tions. In  cases  of  diabetes,  where  the  body  is  overloaded  with  carbohy- 
drate material,  it  has  been  found  in  considerable  quantity  in  the  testis,  in 
the  brain,  and  elsewhere.  Its  occurrence  in  these  situations,  and  under  these 
circumstances,  may  be  regarded  as  additional  evidence  of  the  truth  of  the 
view  which  we  have  expounded  above,  that  the  main  purpose  of  the  deposi- 
tion of  glycogen  is  to  afford  a  store,  either  general  or  local,  of  carbohydrate 
material,  which  can  be  packed  away  without  much  trouble  so  long  as  it  re- 
mains glycogen,  but  which  can  be  drawn  upon  as  a  source  of  soluble  circu- 
lating sugar  whenever  the  needs  of  this  or  that  tissue  demand  it.  It  thus 
forms  a  very  complete  analogue  to  the  vegetable  starch,  and  fitly  earns  the 
name  of  animal  starch. 

We  have  some  reasons  for  thinking  that  there  are  several  varieties  of 
glycogen,  and  that  the  glycogen  which  exists  in  muscle  is  not  quite  identi- 
cal with  that  which  occurs  in  the  liver.  Indeed,  there  seem  to  be  interme- 
diate stages  between  glycogen  and  starch,  or  dextrin.  The  physiological 
value  of  these  differences  has  not  yet,  however,  been  clearly  determined, 
and,  with  this  caution,  we  may  continue  to  speak  of  glycogen  as  a  single 
substance. 

Diabetes. 

§  385.  Natural  diabetes  is  a  disease  characterized  by  the  appearance 
of  a  large  quantity  of  sugar  in  the  urine,  due,  as  we  have  already  said, 
to  the  presence  of  an  abnormal  quantity  of  sugar  in  the  blood.  Into 
the  pathology  of  the  various  forms  of  this  disease  it  is  impossible  to  enter 
here ;  but  a  temporary  diabetes,  the  appearance  for  a  while  of  a  large 
quantity  of  sugar  in  the  urine,  may  be  artificially  produced  in  animals  in 
several  ways. 

If  the  medulla  oblongata  of  a  well-fed  rabbit  be  punctured  in  the  region 
which  we  have  previously  described  (§  162)  as  that  of  the  vasomotor  centre 
(the  area  marked  out  as  the  "  diabetic  area,"  agreeing  very  closely  with  that 
defined  as  the  vasomotor  area),  though  the  animal  need  not  necessarily  be 
in  any  other  way  obviously  affected  by  the  operation,  its  urine  will  be  found, 
in  an  hour  or  two,  or  even  less,  to  be  increased  in  amount  and  to  contain  a 
considerable  quantity  of  sugar.  A  little  later  the  quantity  of  sugar  will 
have  reached  a  maximum,  after  which  it  declines,  and  in  a  day  or  two,  or 
even  less,  the  urine  will  again  be  perfectly  normal.  The  better  fed  the 
animal,  or,  more  exactly,  the  richer  in  glycogen  the  liver,  at  the  time  of 
the  operation,  the  greater  the  amount  of  sugar.  If  the  animal  be  pre- 


THE  HISTORY  OF  GLYCOGEN.  449 

viously  starved  so  that  the  liver  contains  little  or  no  glycogen,  the  urine 
will,  after  the  operation,  contain  little  or  no  sugar.  It  is  clear  thut  the 
urinary  sugar  of  this  form  of  artificial  diabetes  comes  from  the  glyeogen 
of  the  liver.  The  puncture  of  the  medulla  causes  such  a  change  in  the 
liver  that  the  previously  stored-up  glycogen  disappears,  and  the  blood 
becomes  loaded  with  sugar,  much,  if  not  all,  of  which  passes  away  by 
the  urine.  In  the  absence  of  any  proof  to  the  contrary,  we  may  assume 
that  in  this  form  of  artificial  diabetes,  the  glycogen  previously  present 
in  the  liver  becomes  converted  into  sugar,  just  as  we  know  that  it  does 
become  so  converted  by  post-mortem  changes.  The  glycogenic  function 
of  the  liver  is,  therefore,  subject  to  the  influences  of  the  nervous  system,  and, 
in  particular,  to  the  influence  of  a  region  of  the  cerebro-spinal  centre,  which 
we  already  know  as  the  vasomotor  centre,  or  at  least  of  a  part  of  that  region. 

Before  we  attempt  to  discuss  this  nervous  influence  we  must  say  a  few 
words  on  the  nerves  of  the  liver. 

§  386.  The  liver  is  supplied  with  nerves  from  the  hepatic  plexus,  which 
passes  into  the  liver  at  the  porta  and,  running  in  the  portal  canal  with  the 
hepatic  artery  and  portal  vein,  is  distributed  to  various  parts  of  the  organ. 
This  plexus,  which  is  the  only  nerve  supplying  the  liver,  consists  partly  of 
medullated  and  partly  of  non-medullated  fibres,  and  is  an  extension  of  the 
great  solar  plexus,  already  often  mentioned.  Into  that  plexus,  as  we  have 
already  seen,  the  right  (posterior)  vagus  sends  the  greater  part  of  its  fibres, 
and  in  that  plexus  both  the  abdominal  splanchnic  nerves,  major  and  minor, 
end,  on  both  sides  of  the  body.  The  left  (anterior)  vagus  forms  slight  con- 
nections only  with  the  solar  plexus,  but  sends  off  a  very  distinct  branch 
directly  to  the  hepatic  plexus.  The  liver,  therefore,  has  nervous  connec- 
tion with  the  central  nervous  system  by  both  vagus  nerves  and  by  the 
(abdominal)  splanchnic  nerves.  Besides  this,  other  nerve-fibres  find  their 
way  through  the  splanchnic  sympathetic  chain,  or  possibly  otherwise,  to 
the  solar  plexus  from  the  spinal  cord  without  taking  part  in  either  of  the 
splanchnic  nerves ;  and  these  may,  perhaps,  join  the  hepatic  plexus. 

Concerning  the  destination  of  the  fibres  of  the  hepatic  plexus  within 
the  liver  we  know  little  or  nothing  definitely.  Some,  undoubtedly,  supply 
the  hepatic  artery  and  its  branches  ;  but  we  cannot  at  present  say  what 
proportion  of  the  whole  number  of  fibres  end  in  this  way.  Some,  again, 
are  destined  for  the  bile-ducts,  and  before  the  plexus  passes  into  the  liver 
it  sends  fibres  to  the  gall-bladder ;  these  probably  end  in  the  muscular 
coats  of  these  organs.  Whether  any  of  the  nerve-fibres  end  in  the  re- 
markably muscular  coats  of  the  portal  vein,  or  whether,  as  theoretical 
reasons  would,  perhaps,  lead  us  to  suppose,  some  are  connected  with  the 
hepatic  cells,  we  do  not  for  certain  know,  though  some  observers  have 
claimed  to  have  traced  nerve-fibres  directly  into  the  hepatic  cells. 

§  387.  With  regard  to  the  exact  nature  of  the  influence  started  by  the 
puncture  of  the  medulla,  and  the  path  by  which  that  influence  reaches  the 
liver,  our  information  is  at  present  very  imperfect.  One  thing  seems  clear, 
viz.,  that  the  influence  in  question  is  not  carried  down  by  the  main  vagus 
trunks ;  for  not  only  has  the  section  of  both  these  nerves  in  the  neck  no 
marked  effect  in  the  way  of  producing  diabetes,  but  the  "  diabetic  punc- 
ture "  of  the  medulla  oblongata  is  as  efficient  after  division  of  both  vagus 
nerves  as  before.  Seeing  how  close  to,  or  almost  indentical  with,  the  vaso- 
motor centre  is  the  diabetic  centre,  if  we  may  use  the  phrase,  it  seems  nat- 
ural to  suppose  that  the  undue  conversion  of  glycogen  into  sugar  which 
follows  the  puncture  is  the  result  of  some  vasomotor  disturbance  in  the 
liver,  for  instance,  dilatation  of  the  hepatic  artery.  But  we  have  no  clear 
proof  that  this  is  the  true  explanation. 

29 


450  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

§  388.  A  temporary  diabetes  may  he  brought  about  by  the  administra- 
tion of  the  substance  phloridzin.  This,  however,  is  a  glucoside,  and  part 
of  the  sugar  which  appears  in  the  urine,  after  a  dose  of  it,  may  come  direct 
from  the  drug  itself;  but  the  quantity  of  sugar  discharged  is  too  great  to 
be  accounted  for  in  this  way,  and  similar  diabetic  effects  are  produced  by 
the  administration  of  phloretin,  a  derivate  of  phloridzin,  not  a  glucoside, 
and  not  giving  rise  to  sugar  by  its  own  decomposition.  The  sugar  which 
appears  in  the  urine  after  a  dose  of  this  substance  seems  to  come  in  part 
at  least  from  the  hepatic  store  of  glycogen  when  that  is  present ;  but  the 
drug  will  give  rise  to  sugar  in  the  urine  of  starving  animals,  from  whose 
livers  (and  other  tissues)  glycogen  is  presumably  absent. 

Artificial  diabetes  is  also  a  prominent  symptom  of  urari  poisoning.  This 
is  not  due  to  the  artificial  respiration,  which  is  had  recourse  to  in  order  to 
keep  the  urarized  animals  alive ;  because,  though  disturbance  of  the  respir- 
atory functions  sufficient  to  interfere  with  the  hepatic  circulation  may  pro- 
duce sugar  in  the  urine,  artificial  respiration  may  with  care  be  carried  on 
without  any  sugar  making  its  appearance.  Moreover,  urari  causes  diabetes 
in  frogs,  although  in  these  animals  respiration  can  be  satisfactorily  carried 
on  without  any  pulmonary  respiratory  movements.  The  exact  way  in 
which  this  form  of  diabetes  is  brought  about  has  not  yet  been  clearly  made 
out. 

A  very  similar  diabetes  is  seen  in  carbonic-oxide  poisoning ;  and  is  one 
of  the  results  of  a  sufficient  dose  of  morphia,  of  amyl  nitrite  and  of  some 
other  drugs. 

There  can  be  no  doubt  that  in  diabetes,  arising  from  whatever  cause, 
the  sugar  appears  in  the  urine  because  the  blood  contains  more  sugar  than 
usual.  The  system  can  only  dispose  (either  by  oxidation,  or  as  seems  more 
probable  in  other  ways)  of  a  certain  quantity  of  sugar  in  a  certain  time. 
Sugar  injected  into  the  jugular  vein  reappears  in  the  urine  whenever  the 
injection  becomes  so  rapid  that  the  percentage  of  sugar  in  the  blood  reaches 
a  certain  (low)  limit.  Sugar  in  the  urine  means  an  excess  of  sugar  in  the 
blood.  How  in  natural  diabetes  that  excess  arises  has  not  at  present  been 
clearly  made  out.  It  may  be  that  some  forms  of  diabetes  resemble  the 
artificial  diabetes  just  described  as  resulting  from  puncture  of  the  medulla, 
and  arise  from  a  too  rapid  conversion  of  the  hepatic  glycogen,  or  from 
carbohydrate  material  failing  to  be  stored  up  as  glycogen,  or  from  an 
excessive  manufacture  of  carbohydrate  material  by  the  hepatic  cells.  All 
forms  of  diabetes,  however,  cannot  be  satisfactorily  explained  in  this  way  ; 
and  it  has  been  suggested,  though  adequate  proof  has  not  yet  been  supplied, 
that  the  sugar  of  diabetes  is  of  a  peculiar  nature  and  accumulates  in  the 
blood  because  it  is  unable  to  undergo  those  changes,  whatever  they  be, 
which  befall  the  normal  sugar  of  the  blood.  We  cannot  here  discuss  the 
subject  in  detail ;  but  there  is  much  to  be  said  in  favor  of  the  view  that  the 
sources  of  the  excess  of  sugar  in  the  blood  may  be  various,  and  hence  that 
several  distinct  varieties  of  diabetes  may  exist.  In  severe  cases  of  diabetes 
the  aberrant  nature  of  the  metabolism  which  is  going  on  in  some  or  other 
of  the  tissues  of  the  body  is  shown  by  the  appearance  of  abnormal  sub- 
stances in  the  urine.  Thus  acetone  is  frequently  present,  and  the  fatal  issue 
of  certain  cases  has  been  attributed  to  poisoning  by  that  substance ;  oxybu- 
tyric  acid  and  other  various  organic,  chiefly  volatile,  acids  are  also  some- 
times present.  But  in  respect  to  these  and  other  abnormal  bodies  we  are 
not  at  present  clear  whether  they  are,  like  the  sugar  itself,  the  products  of 
an  abnormal  metabolism  which  is  the  root  of  the  disease,  or  whether  they 
are  secondary  products,  that  is  to  say,  products  of  the  general  disordered 
metabolism  induced  by  the  constant  presence  in  the  blood  of  an  excess  of 


SPLEEN.  451 

sugar.  We  have  already,  iu  discussing  the  formation  of  glycogen,  called 
attention  to  the  fact  that  in  severe  cases  of  diabetes  the  sugar  must  have  a 
non-amylaceous  source ;  and  the  fact  that  the  urea  is  increased  (and  that 
too  in  some  cases  in  ratio  with  the  sugar)  in  diabetes,  suggests  that  the 
sugar  may  arise  from  proteids  which  have  been  split  up  into  a  nitrogenous 
(urea)  and  a  non-nitrogenous  moiety,  and  so  points  out  the  way  in  which 
proteids  may  be  a  source  of  glycogen. 

As  a  sort  of  converse  to  diabetes  we  may  mention  that  the  administra- 
tion of  arsenic  in  sufficient  doses  or  for  an  adequate  time  prevents  an  accu- 
mulation of  glycogen  in  the  liver  and  apparently  in  the  body  generally, 
whatever  be  the  diet  used.  The  presence  of  the  metal  in  the  hepatic  cell 
seems  to  prevent  the  cell  substance  from  manufacturing  glycogen  either 
from  carbohydrate  material  brought  to  it,  or  out  of  its  own  substance.  As 
another  kind  of  converse  we  may  also  state  that  the  administration  of 
glycerin,  especially  through  the  alimentary  canal,  diminishes  the  effect  of 
the  diabetic  puncture,  or  of  morphia  or  of  other  poisoning,  in  hurrying  on 
the  hepatic  store  of  glycogen  into  sugar,  and  thus  diminishes  the  sugar  in 
the  urine;  the  presence  of  the  glycerin  in  the  hepatic  cell  appears  to  be  in 
some  way  a  hindrance  to  the  conversion  of  the  glycogen  into  sugar.  Now 
glycerin  injected  into  the  alimentary  canal  of  a  normal  animal  leads  to  an 
increase  of  glycogen  in  the  liver;  and  the  view  very  naturally  suggests 
itself  that  this  increase  arising  from  the  glycerin  is  to  be  explained  by  the 
glycerin  inhibiting  in  some  way  a  normal  conversion  of  the  glycogen  store 
into  sugar  which  is  continually  going  on,  and  thus  increasing  for  the  time 
that  store. 

SPLEEN. 

§  389.  The  movements  of  the  spleen.  After  a  meal  the  spleen  increases 
in  size,  reaching  its  maximum  about  five  hours  after  the  taking  of  food ;  it 
remains  swollen  for  some  time,  and  then  returns  to  its  normal  bulk.  In 
certain  diseases,  such  as  the  pyrexia  attendant  on  certain  fevers  or  inflam- 
mations, and  more  especially  in  ague,  a  somewhat  similar  temporary  enlarge- 
ment takes  place.  In  prolonged  ague  a  permanent  hypertrophy  of  the 
spleen,  the  so-called  ague-cake,  occurs. 

The  turgesceuce  of  the  spleen  seems  to  be  due  to  a  relaxation  both  of  the 
small  arteries  and  of  the  muscular  tissue  of  the  capsule  and  of  the  trabeculse ; 
to  be,  in  fact,  a  vascular  dilatation  accompanied  by  a  local  inhibition  of  the 
tonic  contraction  of  the  other  plain  muscular  fibres  entering  into  the  struc- 
ture of  the  organ,  the  latter,  at  all  events  in  some  animals,  being  probably 
the  more  important  of  the  two.  And  the  condition  of  the  spleen,  like  that 
of  other  vascular  areas,  appears  to  be  regulated  by  the  central  nervous  sys- 
tem, the  digestive  turgescence  being  fairly  comparable  to  the  flushed  con- 
dition of  the  pancreas  and  of  the  gastric  membrane  during  their  phases  of 
activity. 

The  application  of  the  plethysmographic  method  to  the  spleen,  carried 
out  in  the  way  which  we  described  in  speaking  of  the  kidney  (§  346), 
enables  us  to  study  more  exactly  the  variations  in  volume  which  the  organ 
undergoes. 

A  "spleen  curve"  (Fig.  117)  taken  in  the  same  way  as  a  "  kidney  curve" 
does  not,  in  the  dog  at  all  events,  show  variations  in  the  volume  of  the  spleen 
corresponding  with  the  pulse  waves.  The  kidney  curve,  as  we  have  seen, 
(§  346),  gives  clear  indications  of  each  heart-beat,  but  the  spleen  curve 
shows,  besides  the  larger  waves  of  which  we  shall  speak  directly,  only  undu- 
lations due  to  the  respiratory  movements  ;  and  these,  always  very  slight,  are 


452  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

sometimes  not  visible.  In  other  words,  the  spleen  does  not  expand  with  the 
increase  of  blood-pressure  occurring  in  the  splenic  arteries  after  each  heart- 
beat; this  may  be  due  to  the  muscular  coat  resisting  expansion.  Moreover, 
when  the  supply  of  blood  to  the  spleen  is  wholly  and  suddenly  cut  off,  as  by 
clamping  the  aorta,  the  spleen  curve  sinks  very  slowly,  showing  that  the 
spleen  is  diminishing  in  volume  not  suddenly  but  very  slowly.  The  path- 
way of  the  blood  through  the  splenic  reticulum  is  peculiar;  and  increase  or 
decrease  in  the  volume  of  the  spleen  means  more  or  less  blood  held  in  the 
spleen-pulp,  not  necessarily  a  greater  or  less  flow  of  blood  through  the 
organ. 

Of  special  interest  are  the  large  slow  variations  of  volume  which,  besides 
the  respiratory  undulations,  the  spleen  curve  usually  shows,  as  seen  in  the 
figure.  Rhythmic  contractions  and  expansions,  though  not  always  present, 
frequently  make  their  appearance,  each  contraction  with  its  fellow  expansion 
lasting  in  the  cat  and  dog  about  a  minute,  and  recurring  with  great  regu- 

FIG.  117. 


IwfliyiiiiiuiiJiii  uf'iiiiiii 


Normal  Spleen  Curve  from  Dog.  (Roy.)  The  upper  curve  is  the  spleen  curve  showing  the 
rhythmic  contractions  and  expansions ;  the  smaller  waves  are  due  to  the  respiratory  movements. 
The  lower  curve  is  the  blood-pressure  curve,  and  the  point  a  of  the  spleen  curve  corresponds 
in  time  to  the  point  b  of  the  blood-pressure  curve.  The  marks  on  the  time-curve  below  indicate 
seconds. 

larity  for  a  long  time ;  and  besides  these  the  volume  varies  widely  from  time 
to  time.  There  can  be  little  doubt  but  that  the  rhythmic  variations  in 
volume  are  due  in  these  animals  to  rhythmic  contractions,  with  intervening 
relaxations,  of  the  muscular  trabeculse  and  capsule ;  the  slower  variations 
are  also  probably  due  to  the  same  cause.  In  many  animals  the  contractility 
of  the  splenic  tissue  is  shown  by  the  white  lines  of  constriction  which  appear 
when  the  electrodes  of  an  induction  machine  in  action  are  drawn  over  its 
surface;  and  similiar  lines  may  be  produced  by  mechanical  stimulation  with 
the  point  of  a  needle.  So  that  the  spleen  in  these  animals  may  be  considered 
as  a  muscular  organ,  now  expanding  to  receive  a  larger  quantity  of  blood 
and  now  contracting  to  drive  the  blood  on  to  the  liver.  When  the  muscular 
elements  are  scanty  in  or  absent  from  the  capsule  and  trabeculse,  the  expan- 
sion and  contraction  of  the  whole  organ  must  depend  alone  or  chiefly  on 
variations  in  the  width  of  the  supplying  arteries.  We  have  evidence,  more- 
over, that  the  muscular  activity  of  the  spleen,  whether  of  the  muscular  cap- 
sule and  trabeculse  and  arteries  combined,  or  of  the  latter  alone,  is  under  the 
dominion  of  the  nervous  system.  A  rapid  contraction  of  the  spleen  may  be 
brought  about  in  a  direct  manner  by  stimulation  of  the  splanchnic  or  vagus 
nerves,  or  in  a  reflex  manner  by  stimulation  of  the  central  end  of  a  sensory 
nerve ;  it  may  also  be  caused  by  stimulation  of  the  medulla  oblongata  with 


THE  FORMATION  OF  THE  CONSTITUENTS  OF  BILE.          453 

a  galvanic  current  or  by  means  of  asphyxia.  Though  the  matter  has  not 
yet  been  fully  worked  out,  we  have  already  sufficiently  clear  indications 
that  the  flow  of  blood  through  the  spleen  is,  through  the  agency  of  the 
nervous  system,  varied  to  meet  changing  needs.  At  one  time  a  small 
quantity  of  blood  is  passing  through  or  is  being  held  by  the  organ,  and  the 
metabolic  changes  which  it  undergoes  in  the  transit  are  comparatively 
slight.  At  another  time  a  larger  quantity  of  blood  enters  the  organ,  and  is 
let  loose,  so  to  speak,  into  the  splenic  pulp,  there  to  undergo  more  profound 
changes,  and  afterward  to  be  ejected  by  the  rhythmic  contractions  of  the 
muscular  trabeculae. 

It  is  further  obvious  that  these  changes  going  on  in  the  spleen  must  have 
an  important  influence  on  the  changes  going  on  in  the  liver;  it  cannot  be  of 
indifference  to  the  latter  organ,  whether  a  relatively  small  quantity  of  blood, 
relatively  little  changed,  reaches  it  from  the  spleen,  or  whether  it  receives  a 
relatively  large  quantity  of  blood,  profoundly  altered  by  the  changes  which 
it  has  undergone  in  the  spleen-pulp. 

§  390.  The  chemical  constituents  of  the  spleen.  Besides  the  chemical 
bodies  which  one  would  expect  to  find  in  a  vascular,  muscular  organ  full  of 
blood,  the  spleen  contains  bodies,  lodged  apparently  in  the  spleen-pulp,  which 
give  it  special  chemical  characters.  One  of  the  most  important  of  these  is 
a  special  proteid  of  the  nature  of  alkali-albumin,  holding  iron  in  some  way 
peculiarly  associated  with  it.  The  occurrence  of  this  ferruginous  proteid, 
accompanied  as  it  is  by  several  peculiar  but  at  present  little-understood  pig- 
ments, rich  in  carbon,  which  are  partly  present  in  the  cells  spoken  of  above 
and  partly  deposited  in  the  branched  cells  of  the  reticulum,  appears  to  be 
connected  with  the  changes  undergone  by  the  haemoglobin  which  we  shall 
presently  discuss.  The  inorganic  salts  of  the  spleen,  or  at  least  those  of  its 
ash,  are  remarkable  for  the  large  amount  of  both  soda  and  phosphates  and 
the  small  amount  of  potash  and  chlorides  which  they  contain,  thus  differing 
from  those  of  blood  corpuscles  on  the  one  hand,  and  from  those  of  blood- 
serum  on  the  other.  But  perhaps  the  most  striking  feature  of  the  spleen- 
pulp  is  its  richness  in  the  so-called  extractives.  Of  these  the  most  common 
and  plentiful  are  succinic,  formic,  acetic,  butyric,  and  lactic  acids,  inosit, 
leucin,  xanthin,  hypoxanthin,  and  uric  acid.  Tyrosin  apparently  is  not 
present  in  the  perfectly  fresh  spleen,  though  leucin  is ;  both  are  found  when 
decomposition  has  set  in.  The  constant  presence  of  uric  acid  is  remarkable, 
especially  since  it  has  been  found  even  in  the  spleen  of  animals,  such  as  the 
herbivora,  whose  urine  contains  none. 

The  richness  of  the  spleen  in  these  extractives  is  an  indication  of  the 
importance  of  the  metabolic  events  with  which  the  organ  has  to  do  ;  but  it 
will  be  more  profitable  to  discuss  what  goes  on  in  the  spleen  in  connection 
with  the  metabolic  changes  in  the  other  parts  of  the  body,  in  the  liver  for 
instance,  than  to  attempt  to  lay  down  any  so-called  "  functions "  of  the 
spleen.  When  we  confine  our  attention  to  the  spleen  itself  we  learn  very 
little ;  thus  the  whole  organ  may  be  successfully  removed  without  any  very 
obvious  changes  in  the  economy  resulting.  We  may  return,  therefore,  to 
the  discussion  of  the  formation  of  the  bilirubin  of  bile,  and  of  the  changes 
undergone  by  haemoglobin,  with  which,  as  we  shall  see,  the  spleen  is  con- 
nected, and  which,  moreover,  has  to  do  with  the  formation  of  other  pig- 
ments. 

THE  FORMATION  OF  THE  CONSTITUENTS  OF  BILE. 

§391.  Bile  pigments.  After  extirpation  of  the  liver  no  accumulation 
of  bile  pigment  or  bile  salts  takes  place  in  the  blood.  This  is  well  shown  in 


454  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

frogs,  which  survive  the  operation  for  some  considerable  time ;  but  the  same 
results  have  been  obtained  in  birds  (geese  and  ducks).  There  can  be  no 
doubt,  therefore,  that  these  substances  are  formed  in  the  liver,  arid  not  sim- 
ply withdrawn  from  the  blood  by  the  liver  in  some  such  way  as  we  have 
seen  reason  to  think  urea  is  withdrawn  from  the  blood  by  the  kidney. 

When  the  plasma  of  circulating  blood  is  made  to  contain  haemoglobin 
detached  from  the  corpuscles,  bile  pigment  frequently  makes  its  appearance 
in  the  urine.  The  presence  of  free  haemoglobin  may  be  obtained  by  inject- 
ing into  the  veins  a  solution  of  haemoglobin  or  blood  made  "  laky  "  by  freez- 
ing and  thawing  or  by  the  addition  of  a  small  quantity  of  bile  salts,  or  by 
simply  injecting  into  the  veins  a  quantity  of  distilled  water  or  a  small 
quantity  of  ether  or  chloroform  or  of  bile  salts,  all  of  which  tend  to  "  break 
up "  red  corpuscles  and  set  free  haemoglobin.  A  similar  result  occurs  in 
poisoning  by  certain  drugs,  such  as  toluylendiamine.  Under  these  circum- 
stances not  only  does  bile  pigment,  bilirubin,  make  its  appearance  in  the 
urine,  but  the  quantity  of  bilirubin  secreted  by  the  liver  is  increased. 
Obviously  the  presence  of  dissolved  haemoglobin  in  the  plasma  of  the  blood, 
and,  presumably  more  especially  of  the  blood  reaching  the  liver  by  the 
portal  vein,  leads  to  an  increased  formation  of  bilirubin,  which  take  place 
in  such  a  manner  that  the  whole  of  the  bilirubin  so  formed  does  not  pass 
into  the  bile  but  part  is  retained  in  or  thrown  back  into  the  circulation  and 
appears  in  the  urine. 

We  have  already  mentioned  the  chemical  connection  between  haemoglo- 
bin and  bilirubin.  Haemoglobin  after  the  detachment  of  its  proteids  com- 
ponent becomes  haematin  (C32H32N4FeO4).  By  treatment  with  sulphuric 
acid  or  otherwise  (§  294),  haematin  may  be  deprived  of  its  iron ;  and  this 
iron-free  haematin  (sometimes  called  haematoporphyrin)  is  said  to  have  the 
composition  C32H3.2N4O5,  differing  from  bilirubin  only  in  its  oxygen  and  hy- 
drogen (C32H32N4O5+2H2O— O^C^HggNA).1  Moreover,  in  old  blood-clots 
in  the  body  the  haemoglobin  of  the  clot  becomes  in  time  transformed  into 
an  iron-free  body  which  has  been  called  haematoidin,  but  which  both  in  com- 
position and  reactions  appears  to  be  identical  with  bilirubin. 

These  several  facts  lead  us  to  the  conclusion  that  the  bilirubin  of  the 
bile  is  simply  some  of  the  haemoglobin  of  the  blood  transformed  by  the 
throwing  off  of  its  proteid  and  its  iron  components.  It  is  natural  to  sup- 
pose that  the  transformation  takes  place  in,  and  is  effected  by,  the  agency  of 
the  hepatic  cells ;  and  this  view  is  supported  by  the  fact  that  the  hepatic 
cells  are  characterized  by  containing  certain  peculiar  iron  compounds. 
When  all  the  blood  is  carefully  washed  out  of  the  liver  by  injection  through 
the  bloodvessels,  by  which  means  the  remaining  bile  is  got  rid  of  at  the  same 
time,  the  hepatic  substance  is  found  to  contain  a  small  quantity  of  iron, 
sufficient  to  give  the  cells  a  diffused  dark  color  when  treated  with  ammo- 
nium sulphide ;  the  exact  amount  appears  to  vary  largely,  but  the  causes  of 
the  variation  have  not  been  determined.  That  this  iron  is  in  organic  com- 
bination is  indicated  by  the  fact  that  with  potassium  ferrocyanide  and  sul- 
phocyanide  the  blue  or  red  action  is  not  observed  until  after  treatment  with 
hydrochloric  acid.  Apparently  there  are  several  such  compounds,  of  a  pro- 
teid or  of  a  nuclein  (§  29)  nature,  from  some  of  which  the  iron  is  more  easily 
removed  than  others,  and  these  compounds  appear  to  be  present  in  both  the 
cell  substance  and  the  nucleus.  It  will  be  remembered  (§  213)  that  bile 
contains  a  distinct  quantity  of  iron,  which  probably  has  its  origin  in  the 
iron  thus  set  free  from  haemoglobin  and  retained  in  the  hepatic  cell ;  but  it 
does  not  follow  that  all  the  iron  thus  set  free  makes  its  way  into  the  bile ; 
and,  indeed,  the  quantity  of  iron  discharged  in  the  bile  in  twenty-four  hours 

1  Doubling  the  formula  for  bilirubin  given    in  $  214. 


THE  FORMATION  OF  THE  CONSTITUENTS  OF  BILE.          455 

is  much  smaller  than  the  quantity  calculated  to  be  set  free  in  the  formation 
out  of  the  haemoglobin  of  the  quantity  of  biliriibin  discharged  during  the 
same  period.  Apparently  the  iron  compounds  of  the  hepatic  cell  have  some 
other  work  than  the  simple  discharge  of  iron  into  the  bile. 

The  fact  mentioned  above,  that  the  presence  of  free  hemoglobin  in  the 
blood  leads  not  only  to  an  increase  of  bilirubin  in  the  bile,  but  also  to  its 
presence  in  the  urine,  offers  some  difficulties ;  for  if  the  bilirubin  be  formed 
out  of  hemoglobin  by  and  in  the  hepatic  cell,  one  would  expect  to  find  that 
the  whole  of  it  passed  into  the  bile,  and  that  it  could  not  appear  in  the 
blood  and  so  in  the  urine  unless  reabsorption  from  the  bile  passages,  due  to 
obstruction  took  place ;  and  there  is  no  evidence  of  any  sufficient  obstruc- 
tion occurring  in  these  cases.  Indeed  the  presence  of  bilirubin  in  the  urine 
in  these  cases  has  been  urged  by  some  as  an  argument  that  biiirubin  is 
formed  in  the  blood  or  at  least  elsewhere  than  in  the  liver,  and  is  simply 
excreted  by  the  liver.  Not  only,  however,  as  stated  above,  is  there  no  ac- 
cumulation of  bile  in  the  blood  after  extirpation  of  the  liver,  but  that  ope- 
ration prevents  the  appearance  of  bilirubin  in  the  urine  as  a  consequence  of 
the  presence  of  free  haemoglobin  in  the  blood.  The  phenomena  in  question, 
therefore,  do  not  disprove  that  the  biliruhin  is  formed  in  the  liver ;  they 
may  be  taken,  however,  to  show  that  that  formation,  viewed  as  a  secretory 
act,  is  peculiar,  since  the  hepatic  cell  appears  under  certain  circumstances  to 
discharge  its  product  of  secretion  into  the  blood  or  lymph  as  well  as  into 
the  bile  passages. 

§  392.  We  may  assume  then  that  the  hepatic  cell  has  the  power  of  split- 
ting up  the  haemoglobin  brought  to  it,  and  of  discharging  part  as  bilirubin 
while  it  retains  for  a  time  the  iron  component  in  some  organic  combination; 
and,  if  we  further  assume  that  it  works  upon  the  entire  haemoglobin  we  may 
presume  that  makes  some  subsequent  use  of  the  proteid  component.  But 
are  we  justified  in  assuming  that  the  whole  work  is  done  by  the  hepatic 
cells?  Are  we  to  conclude  that  bilirubin  is  manufactured  by  some  act  of 
the  hepatic  cells  which  includes  not  only  the  conversion  of  haemoglobin  into 
bilirubin,  but  also  the  extraction  of  the  haemoglobin  from  the  red  corpus- 
cles as  these  are  streaming  slowly  through  the  lobular  hepatic  capillaries  in 
close  contact  with  the  hepatic  cells  ?  Now,  as  far  as  we  know  at  present, 
haemoglobin  can  only  be  set  free  by  means  of  a  disintegration  of  the  corpus- 
cles ;  we  have  no  instances  of  a  corpuscle  parting  with  some  of  its  haemo- 
globin and  proceeding  on  its  way  otherwise  unchanged ;  and  we  have  no 
histological  evidence  of  any  disintegration  of  red  corpuscles  in  the  liver  cor- 
responding to  the  formation  of  bile.  Nor  can  we  draw  any  conclusion  from 
the  result  of  a  comparative  enumeration  of  red  corpuscles  in  the  portal  and 
hepatic  blood,  for  these  are  too  insecure  to  rest  any  conclusion  upon.  On 
the  other  hand,  as  we  have  just  seen,  the  presence  in  the  plasma  of  the  blood 
of  haemoglobin  in  a  free  condition  is  peculiarly  potent  in  exciting  the  forma- 
tion of  bilirubin.  The  evidence,  therefore,  is  very  strong  for  the  view  that 
as  far  as  the  formation  of  the  greater  part  at  least  of  the  bilirubin  is  con- 
cerned, the  action  of  the  hepatic  cell  is  limited  to  converting  into  bilirubin 
the  free  haemoglobin  offered  to  it  by  the  portal  blood. 

By  what  means,  under  normal  conditions,  is  the  presence  of  that  free 
haemoglobin  secured  ?  We  have  seen  reason  to  conclude  from  histological 
appearances  that  a  certain  number  of  red  corpuscles  undergo  change  in  the 
spleen-pulp  ;  and  it  seems  natural  to  infer  that  one  duty  of  the  spleen  is  to 
set  free  haemoglobin  from  the  corpuscles  and  thus,  through  the  splenic  veins 
and  so  the  portal  vein,  to  supply  the  liver  with  material  for  bilirubin.  But 
this  cannot  be  the  only  source,  since  the  secretion  of  bile  continues  after 
extirpation  of  the  spleen.  There  must,  therefore,  be  other  regions  of  the 


456  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

body  in  which  a  similar  change  of  red  corpuscles  is  going  on  ;  it  has  been 
suggested  that  the  red  marrow  of  bones  is  one  of  these ;  but  further  infor- 
mation on  these  points  is  needed. 

We  may  then  go  so  far  as  to  say  that  the  bilirubin  of  the  bile  is  derived 
from  the  haemoglobin  of  the  blood,  and  that  the  later  stages  of  the  trans- 
formation, including  the  discharge  of  the  iron  of  the  hsematin  component, 
take  place  in  and  by  means  of  the  hepatic  cell ;  but  much  beyond  this  is  at 
present  uncertain.  It  must  be  remembered  too  that,  though  after  extirpation 
of  the  liver  no  accumulation  of  bilirubin  takes  place,  showing  that  the 
bilirubin  is  formed  by  the  liver,  yet  the  whole  change  from  red  corpuscles 
to  bilirubin  may  occasionally  take  place  quite  apart  from  the  liver,  as 
shown  by  the  presence  of  hsernatoidin  in  old  blood-clots. 

§393.  The  formation  of  the  bile-acids.  About  this  we  know  still  less. 
Taking  glycocholic  and  taurocholic  acids  as  the  typical  bile-acids,  recognizing 
(§  215)  that  these  arise  from  the  union  of  cholalic  acid  with  glycin  and 
taurin  respectively,  and  remembering  that  taurin  is  found  in  several  tissues, 
and  that  glycin  (see  §  355)  though  not  an  actual  constituent  of  any  of  the 
tissues,  must  certainly  arise  in  tissue  metabolism,  we  may  conclude  that  the 
chief  work  in  this  respect  of  the  hepatic  cell  is  to  provide  cholalic  acid,  and 
to  effect  the  combination  with  glycin  and  taurin,  though  possibly  some 
amount  of  either  one  or  the  other  of  these  bodies  may  be  furnished  by  the 
hepatic  substance  itself.  As  to  how  cholalic  acid  arises  out  of  the  metabolism 
of  the  hepatic  cell,  we  know  no  more  than  we  do  about  the  formation  of 
kreatin  in  muscle  or  of  pepsin  in  a  gastric  cell.  We  are  equally  ignorant 
about  the  origin  of  glycin  and  taurin,  and  cannot  explain  why  in  one  animal 
glycocholic  and  in  another  taurocholic  acid  is  prominent  in  the  bile,  though 
the  two  bodies,  as  shown  especially  by  the  presence  of  sulphur  in  the  taurin, 
arc  widely  different.  It  has  been  observed  that  the  presence  of  bile  in  the 
intestines  seems  to  excite  the  liver  to  increased  biliary  action  j  since  the  bile- 
acids  are  rapidly  changed  in  the  intestine  and  the  cholalic  acid  speedily 
altered,  it  seems  probable  that  the  increased  biliary  activity  is  'due  to  the 
absorption  of  the  glycin  and  taurin  respectively.  From  which  we  may 
conclude  that  the  presence  of  these  bodies  stirs  up  the  hepatic  cell  to  an  in- 
creased formation  of  cholalic  acid. 

§  394.  As  a  general  rule,  the  formation  of  bile-acids  runs  parallel  with 
the  formation  of  bile-pigment,  an  increase  or  decrease  of  bile  meaning  an 
increase  or  decrease  of  both  constituents.  But  there  are  some  facts  which 
seem  to  show  that  the  two  actions  may  be  dissociated.  The  condition  or 
symptom  known  as  "jaundice"  is  essentially  an  excess  of  bilirubin  in  the 
blood,  whereby  the  tissues  such  as  the  skin  and  the  fluids  such  as  the  urine 
are  colored  with  the  yellow  pigment.  In  most  of  the  maladies  in  which 
jaundice  is  a  symptom,  there  is  evidence  of  an  obstruction  to  the  flow  of 
bile  through  the  bile  passages ;  and  the  presence  of  bile  in  the  blood,  and 
hence  in  the  tissues  at  large,  is  in  such  cases  due  to  the  fact  that  the  bile 
after  secretion  by  the  hepatic  cells  is  reabsorbed  from  the  bile-ducts  (see 
§  226). 

But  in  certain  cases  where  jaundice  is  a  prominent  symptom,  no  evidence 
of  any  obstruction  whatever  to  the  flow  of  bile  can  be  obtained.  This  is  the 
case  in  the  jaundice  of  yellow  fever  and  of  a  peculiar  allied  malady  known 
as  "  acute  yellow  atrophy  of  the  liver."  Now  in  these  cases  there  is  no  evi- 
dence of  an  accumulation  in  the  blood  or  elsewhere  of  bile-acids  as  there  is 
of  bile  pigment.  And  in  the  obscure  malady  known  as  simple  or  idiopathic 
jaundice,  in  which  though  the  anatomical  conditions  are  unknown  there  is 
nt  least  no  sign  of  obstruction,  the  urine  though  loaded  with  bile  pigment  is 
said  to  contain  no  bile-acids. 


ON   UREA   AND  ON   NITROGENOUS  METABOLISM.  457 

§  395.  The  question  may  be  asked,  Is  the  secretion  of  bile  independent 
of  or  in  some  way  or  other  connected  with  the  glycogenic  activity  of  the 
cells?  To  this  we  cannot  at  present  give  a  definite  answer.  In  some  of  the 
in  vertebra  ta  the  cells  in  the  organ,  called  a  liver,  which  manufacture  gly- 
cogen,  are  distinct  from  those  which  secrete  bile  or  other  digestive  juices; 
and  it  might  be  inferred  that  in  the  vertebrate  the  two  actions,  though 
taking  place,  as  they  certainly  do,  in  the  same  cell,  take  place  apart  and 
distinct.  There  are  facts  which  seem  to  indicate  that  the  two  are  intimately 
connected  ;  but  we  have  as  yet  no  exact  knowledge  concerning  the  matter. 
It  has  been  urged  that  the  portal  blood  is  chiefly  concerned  with  the  forma- 
tion of  glycogen,  and  the  blood  of  the  hepatic  artery  witli  the  secretion  of 
bile  ;  but  there  is  no  adequate  support  of  this  view.  It  must  be  remembered, 
moreover,  that,  in  addition  to  the  formation  of  glycogen  and  the  secretion 
of  bile,  other  metabolic  events,  especially  affecting  proteid  or  at  least  nitro- 
genous constituents  of  the  body,  are  also  taking  place ;  and  to  these  we 
must  now  turn. 

ON  UREA  AND  ON  NITROGENOUS  METABOLISM  IN  GENERAL. 

§  396.  We  have  seen  that  nitrogenous  proteid  material  in  some  form  or 
other  enters  into  the  composition  of  all  the  tissues  of  the  body,  and  we  have 
further  seen  that  it  is  so  conspicuously  and  constantly  present  wherever 
living  subtances  are  manifesting  vital  energies  as  to  justify  the  conclusion 
that  the  changes  which  it  undergoes  are  in  some  way  essential  to  the  mani- 
festation of  those  energies.  We  have  seen,  it  is  true,  reason  to  think  that  in 
some  tissues  at  least,  in  muscle  for  instance,  a  large  part  of  the  energy  set 
free  during  activity  preexisted  as  latent  energy  and  had  its  immediate  source 
not  in  proteid  (nitrogenous)  but  in  some  other  constituents  of  muscle ;  and 
indeed,  as  we  shall  see  later  on,  the  greater  part  of  the  whole  energy  of  the 
body  must  be  regarded  as  the  energy  of  carbon  compounds  and  not  of 
nitrogen  compounds;  but  this  is  quite  consistent  with  the  view  that  proteid 
material  in  some  way  or  other  essentially  intervenes  in,  we  may  perhaps  go 
so  far  as  to  say  directs,  the  changes  by  which  in  the  body  energy  is  set  free 
in  the  peculiar  way  which  we  speak  of  as  living. 

We  have  seen  that  at  all  events  the  greater  part  of  the  proteid  material 
of  the  food  enters  the  blood  as  proteid  material  either  as  peptone  or  in  some 
other  form,  and  is  carried  as  proteid  material  to  the  tissues. 

We  have  seen  that  the  nitrogen  of  proteid  material  leaves  the  body  so 
largely  in  the  form  of  urea,  that  the  other  nitrogenous  excretions  may  for 
the  time  be  left  out  of  consideration. 

And  lastly  we  have  seen  reason  to  think  that  this  urea  which  leaves  the 
body  in  urine  is  brought  to  the  kidney  as  urea  in  the  blood,  the  kidneys 
themselves  apparently  having  no  special  power  of  forming  urea  out  of  some- 
thing which  is  not  urea,  but  only  contributing  to  the  general  stock  of  urea 
by  virtue  of  their  own  proteid  metabolism.  We  have  now  to  study  the 
little  we  know  concerning  the  steps  by  which  the  proteid  material  of  the  food 
and  of  the  body  is  converted  into  this  urea  of  the  blood,  which  is  the  source 
of  the  urea  of  the  urine. 

§  397.  In  the  first  place  we  may  take  it  for  granted  that  the  urea  carried 
to  the  kidney  in  the  blood  had  an  antecedent  in  something  which  was  not 
urea.  We  can  hardly  suppose  that  the  proteid  constituent  of  living  sub- 
stance, when  in  the  course  of  its  metabolism  it  ceases  to  be  proteid,  breaks 
up  at  once  into  urea  and  into  non-nitrogenous  bodies.  All  we  have  learned 
goes  to  show  that  what  we  call  metabolism  is  not  a  single  abrupt  change, 
but  consists  essentially  in  a  series  of  changes ;  and  we  may  safely  conclude 


458  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

that  proteid  material  in  becoming  urea  passes  through  phases  in  which  the 
nitrogen  exists  in  chemical  combinations  distinct  from  proteid  material  on 
the  one  hand  and  urea  on  the  other. 

In  the  second  place  it  is  extremely  probable  that  the  series  of  changes 
by  which  proteid  material  becomes  urea  is  not  the  same  in  all  the  tissues 
and  on  all  occasions.  We  should  naturally  expect  to  find  the  proteid 
material  following  different  lines  of  metabolism  in  different  places  or  under 
different  circumstances,  the  different  lines  all  converging  to  the  same  body, 
urea,  because  for  some  reasons  or  other  urea  appears  to  be,  in  the  main,  the 
most  convenient  form  in  which  the  nitrogen  can  leave  the  blood  and  the 
body. 

We  should  accordingly  expect  to  find,  on  the  one  hand,  various  nitroge- 
nous bodies  resulting  from  proteid  metabolism  in  various  parts  of  the  body, 
and,  on  the  other  hand,  arrangements  by  means  of  which  these  various 
bodies  were  reduced  to  the  common  form  urea,  preparatory  to  their  dis- 
charge from  the  body  by  the  kidney.  And  actual  observation  as  far  as  it 
goes  supports  this  view,  though  our  knowledge  of  the  whole  matter  is  very 
imperfect. 

§  398.  We  may  turn  our  attention  first  to  the  metabolism  of  the  skeletal 
muscles,  since  these  represent,  as  far  as  mere  quantity  is  concerned,  by  far 
the  greater  part  of  the  proteid  capital  of  the  body.  We  may  safely  infer 
that  they  furnish  a  large  part  of  the  urea  of  the  urine;  though  undoubtedly 
a  small  mass  of  tissue  might  by  reason  of  its  more  rapid  metabolism  work 
over  a  greater  quantity  of  proteid  material  than  a  much  larger  mass  with  a 
slower  metabolism ;  yet  we  have  no  reason  to  think  that  the  proteid  metab- 
olism of  skeletal  muscle,  obscure  though  it  is  in  its  nature,  is  so  slow  as  to 
neutralize  the  probable  effect  of  the  great  bulk  of  muscle  existing  in  the 
body. 

In  dealing  with  the  chemistry  of  muscle  (§  62)  we  saw  that  urea,  save  in 
the  exceptional  instances  of  certain  cartilaginous  fishes,  was  conspicuous  by 
its  absence  from  the  extract  of  muscle,  whereas,  a  very  appreciable  quantity 
of  kreatin  was  invariably  present,  and,  indeed,  was  the  prominent  nitroge- 
nous crystalline  constituent  of  that  extract.  It  seems  difficult  to  resist  the 
conclusion  that  kreatin  is  the  main  normal  nitrogenous  product  of  the  metab- 
olism of  skeletal  muscles.  If  we  accept  this  view,  then,  upon  the  fact  of 
the  presence  of  kreatin  in,  and  the  absence  of  urea  from,  the  muscle  itself, 
we  may  base  the  conclusion  that  while  the  muscle  produces  kreatin  as  an 
antecedent  of  urea,  the  kreatin  so  produced  is  converted  into  urea  in  some 
part  of  the  body  other  than  the  muscle  itself.  Kreatin,  as  we  have  already 
seen,  may  be  easily  split  up,  and  we  may  probably  with  safety  assume  is  split 
up,  somewhere  in  the  body,  into  urea  and  sarcosin.  But  sarcosin  does  not 
appear  in  the  urine  as  such  ;  hence,  the  conversion  of  kreatin  into  (part  of) 
the  urea  of  the  urine  entails  as  well  the  further  conversion  of  sarcosin  into 
urea.  Now  sarcosin,  as  we  have  seen,  is  methyl-glycin  ;  we  may  regard  it 
for  our  present  purposes  as  simple  glycin,  and  hence  the  total  conversion  of 
kreatin  into  urea  entails  the  conversion  of  glycin  into  urea.  This,  however, 
does  not  offer  any  additional  difficulty,  since  we  know  from  direct  observa- 
tion that  glycin  introduced  into  the  alimentary  canal  does  not  reappear  as 
such  in  the  urine,  but  produces  a  corresponding  increase  in  the  urea  of  the 
urine ;  from  which  we  infer  that  glycin  absorbed  from  the  alimentary  canal 
is,  somewhere  in  the  body,  converted  into  urea.  We  shall  speak  of  this  con- 
version later  on,  and  shall  then  see  that,  as  far  as  urea  is  concerned,  glycin 
(amido-acetic  acid)  and  sarcosin  (methyl  glycin,  methyl-amido-acetic  acid) 
undergo  the  same  change,  the  amide  moiety  in  each  case  being  converted 
into  urea,  while  the  non-nitrogenous  moiety  is  oxidized  and  thro\vn  off. 


ON  UREA   AND  ON   NITROGENOUS  METABOLISM.  459 

Meanwhile,  we  may  state  the  conclusion  at  which  we  have  provisionally 
arrived,  namely,  that  the  nitrogenous  metabolism  of  muscle  probably 
gives  rise  to  kreatin,  which,  in  some  part  of  the  body  other  than  muscle, 
is  probably  split  up  into  urea,  ready  for  excretion,  and  into  sarcosin  which 
also,  somewhere  in  the  body,  is  further  converted  into  urea.  And  bearing 
in  mind  the  large  mass  of  the  skeletal  muscles,  we  may  further  conclude 
that  a  large  portion  of  the  urea  leaving  the  body  by  the  urine  is  formed  in 
this  way. 

§  399.  We  must  not,  however,  leave  this  statement  without  referring  to 
a  difficulty.  Kreatinin,  as  we  have  seen,  is  so  frequently  found  in  urine  as 
to  be  regarded  as  a  normal  constituent,  at  all  events,  of  human  urine ;  aftd 
kreatinin  is,  as  we  have  seen,  the  urinary  form,  so  to  speak,  of  kreatin ;  the 
one  body  easily  changes  into  the  other  by  the  assumption  or  removal  of 
H2O.  This  suggests  the  question,  Is  not  the  kreatinin  of  urine  the  repre- 
sentative of  the  kreatin  of  the  muscles,  which  is  thus  exerted  directly 
without  undergoing  the  change  into  urea  just  discussed  ?  In  answer  to 
this  we  may  say,  in  the  first  place,  that  the  quantity  of  kreatinin  in  the 
urine,  though  variable,  is  small ;  we  may  put  the  average  at  about  1  grm. 
in  twenty-four  hours.  Now,  muscle  contains  from  0.2  to  0.4  per  cent,  of 
kreatin  ;  and  this,  taking  the  total  muscle  of  the  body  (to  say  nothing  of 
other  sources  of  kreatin,  which  we  shall  mention  presently)  at  about  30 
kilos,  would  give  60  to  120  grms.  of  kreatin  as  present  in  the  muscles  of  the 
body  at  any  one  moment.  We  can  hardly  suppose  that  the  metabolism  of 
muscle  is  so  slow  as  out  of  this  stock  only  to  provide  the  1  grm.  of  kreat- 
inin in  twenty-four  hours.  Moreover,  the  kreatin  in  urine  vanishes  during 
starvation,  is  very  markedly  increased  by  a  diet  of  flesh  which  contains 
kreatin,  and  is  not  increased  either  by  muscular  exercise  (which,  however, 
would  only  indirectly  affect  nitrogenous  metabolism  of  muscle),  or  by  such 
conditions,  fever,  for  instance,  as  notably  increase  the  urea  of  urine  by  in- 
creasing the  nitrogenous  metabolism  of  muscle.  We  infer,  therefore,  that 
the  normal  presence  of  kreatinin  in  urine  is  due  to  the  direct  administration 
of  kreatin  present  in  a  (normal)  flesh  diet,  and  has  nothing  to  do  with  the 
muscular  metabolism  of  the  individual  who  is  secreting  the  kreatinin  in  his 
urine. 

The  fact,  however,  that  the  kreatin  present  in  the  muscle  of  the  food  and 
absorbed  from  the  alimentary  canal  does  not  undergo  a  change  into  urea,  but 
is  excreted  as  kreatinin,  that  is,  virtually  as  kreatin,  warns  us  to  be  careful 
in  adopting  the  conclusion  arrived  at  above,  that  the  kreatin  produced  by 
muscular  metabolism  in  the  living  body  is  a  conspicuous  antecedent  of  the 
urea  of  the  urine.  It  is  difficult  to  see  why  kreatin  passing  into  the  blood 
of  the  capillaries  of  the  muscle  should  be  changed  into  urea,  while  that  which 
passes  into  the  capillaries  of  the  portal  system  is  not ;  for  reasons  which  will 
be  apparent  presently,  we  should  rather  expect  that  the  latter  being  more 
directly  exposed  to  the  influence  of  the  liver  would  be  more  readily  and 
more  completely  converted  than  the  former.  Indeed,  the  question  forces 
itself  upon  us,  Is  kreatin,  after  all,  the  natural  main  product  of  the  nitro- 
genous metabolism  of  muscle?  It  is  possible  that  in  the  normal  metab- 
olism of  the  living  muscle  the  nitrogen  leaves  the  muscular  substance 
and  passes  into  the  blo.od  in  another  form,  as  some  substance  not  kreatin, 
and  that  it  is  as  the  muscle  dies  that  kreatin  is  formed,  just  as  the  solid 
myosin  is  unknown  to  living  fibre  but  makes  its  appearance  in  a  dying 
one?  We  have  no  positive  evidence,  however,  that  this  is  so,  and,  mean- 
while, may  continue  to  suppose  that  kreatin  is  formed,  and  that,  in  conse- 
quence, kreatin  is  a  conspicuous  antecedent  of  the  urea  of  the  urine ;  but 
we  must  not  regard  this  as  proved. 


460  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

§  400.  Our  knowledge  of  the  metabolism  of  the  nervous  tissues  is,  as 
we  have  seen,  very  imperfect  (§  70),  but  the  presence  of  the  kreatin  in  the 
central  nervous  system  leads  us  to  infer  that  the  nitrogenous  metabolism  of 
the  living  substance  of  nerve-cells  and  of  the  axis-cylinder  of  nerve-fibres 
is,  in  its  broad  features,  identical  with  that  of  muscle  substance.  The  mass, 
however,  of  the  nerve-cells  and  axis-cylinder  of  the  body,  all  put  together, 
is  small,  compared  with  the  mass  of  skeletal  muscle ;  moreover,  the  energy 
set  free  by  the  metabolism  of  a  mass  of  nervous  matter  though  "  higher  " 
in  quality  is  less  in  quantity  than  that  set  free  by  the  metabolism  of  an 
equal  mass  of  muscle,  or,  in  other  words,  its  metabolism  is  less  rapid. 
Hence,  we  may  probably  consider  the  metabolism  of  the  nervous  system 
as  a  mere  addition  to  that  of  the  muscular  system,  at  least  as  regards 
the  point  on  which  we  are  now  dwelling.  The  amount  of  nitrogenous 
metabolism  taking  place  in  connective  tissue,  cartilage,  bone,  and  the  skin 
is  probably  still  less,  and,  for  our  present  purposes,  needs  no  special  dis- 
cussion. 

§  401.  The  nitrogenous  metabolism  of  the  glands,  however,  more  par- 
ticularly that  of  the  liver,  does  deserve  special  consideration ;  and  we  may 
at  once  turn  to  a  quite  different  aspect  of  the  question  in  hand. 

When  the  rate  of  discharge  of  urea  from  the  body  is  observed  during  a 
period  of  some  length,  especially  under  varied  circumstances,  the  direct 
effect  of  nitrogenous  food  becomes  more  striking.  We  have  already  said, 
and  shall  again  return  to  the  point,  that  muscular  contraction  does  not  di- 
rectly increase  the  output  of  urea ;  the  discharge  of  urea,  for  instance,  is 
not  necessarily  increased  by  even  great  bodily  labor.  The  introduction, 
however,  of  even  a  small  quantity  of  proteid  material  into  the  alimentary 
canal  at  once  increases  the  urea  of  the  urine ;  and  in  the  curve  of  the  dis- 
charge of  urea  in  the  twenty-four  hours  each  meal  is  followed  by  a  con- 
spicuous rise.  The  absorption  of  proteid  material  from  the  alimentary 
canal  is  followed  by  an  immediate  proportionate  increase  in  the  quantity 
of  urea  which  is  secreted  by  the  kidneys,  and  that,  as  we  have  seen,  means 
an  increase  in  the  urea  brought  to  the  kidney  by  the  renal  artery.  What 
is  the  origin  of  this  additional  urea? 

Two  views  present  themselves.  On  the  one  hand,  since  some  portion  of 
the  proteid  material  of  every  meal,  at  all  events  of  every  necessary  meal, 
goes  to  repair  the  proteid  waste  continually  going  on  in  the  parts  of  the  body 
where  proteid  metabolism  is  taking  place,  we  may  suppose  that  the  presence 
of  an  extra  quantity  of  proteid  material  thrown  upon  the  blood  from  the 
food  acts  as  a  stimulus  to  the  tissues,  to  the  muscles,  for  instance,  as  well  as 
others,  stirs  them  up  to  increased  nitrogenous  metabolism  and  thus  produces 
an  increase  of  energy,  chiefly  if  not  exclusively  in  the  form  of  heat,  accom- 
panied by  an  increase  of  the  antecedents  of  urea  and  so  of  urea.  In  other 
words,  the  increase  of  urea  in  question  is  the  result  of  an  increase  in  the 
general  nitrogenous  metabolism  of  the  body. 

On  the  other  hand,  we  may  suppose  that  in  order  to  prevent  the  whole 
body  being  encumbered  with  it,  this  excess  of  proteid  food  material  is,  in 
some  special  part  of  the  body,  split  up  into  a  nitrogenous  and  a  non-nitro- 
genous moiety,  and  that,  while  the  latter  is  stored  up  as  fat  or  glycogen,  the 
former  is  at  once  converted  into  urea  and  got  rid  of.  We  have  already 
(§  218)  seen  that  a  step  in  this  direction  may  take  place  while  the  food  is  as 
yet  in  the  alimentary  canal ;  we  have  seen  that  pancreatic  juice  may  carry 
part  of  the  proteids  on  which  it  acts  beyond  the  stage  of  albumose  and  pep- 
tone, and  reduce  that  part  into  leucin,  tyrosin,  and  other  bodies.  We  do  not 
know,  as  we  have  already  said,  to  what  extent  this  more  profound  digestion 
by  pancreatic  juice  does  actually  take  place  in  the  living  body ;  it  may  take 


ON  UREA  AND  ON  NITROGENOUS  METABOLISM.  461 

place  to  a  very  slight  extent  and  it  may  under  certain  circumstances  take 
place  to  a  considerable  extent.  But  in  any  case  it  illustrates  the  way  in 
which  a  somewhat  similar  disruption  of  proteid  material,  a  disruption  which 
may  be  broadly  described  as  a  splitting  up  of  the  proteid  into  a  nitrogenous 
and  a  non-nitrogenous  moiety,  may  take  place  somewhere  in  the  body  and  so 
lead  to  the  sudden  formation  of  some  antecedent  of  urea.  The  antecedent 
may  be  leucin  or  may  be  some  other  body  or  bodies. 

In  support  of  this  view  maybe  urged  the  fact  that  such  bodies  as  leucin, 
glycin,  asparagin,  and  many  others  when  introduced  into  the  alimentary 
canal  are  transformed  into  urea.  When  these  bodies  are  administered  in 
not  too  great  quantities  they  do  not  reappear  in  the  urine,  but  the  urea  is 
proportionately  increased. 

§  402.  We  have  seen  reason  to  think  that  the  proteids  of  a  meal  are 
absorbed  not  by  the  lacteals  but  by  the  portal  bloodvessels,  and  such  bodies 
as  leucin  probably  take  the  same  course.  This  being  so,  all  these  bodies  pass 
through  the  liver  and  are  subjected  to  such  influences  as  maybe  exerted  by 
the  hepatic  cells.  Now,  we  have  no  positive  evidence  that  the  liver  does  or 
can  exert  such  an  action  on  proteid  material  itself  as  to  separate  a  relatively 
simple  nitrogen  compound  from  the  remaining  constituents,  leaving  these  to 
form  a  body  rich  in  carbon  ;  we  have  no  positive  proof  that  the  increase  of 
proteid  metabolism  just  spoken  of  as  leading  to  an  increase  of  urea  takes 
place  in  the  liver  rather  than  in  the  tissues  at  large ;  we  may  go  so  far  per- 
haps as  to  suspect  that  it  is  largely  or  wholly  confined  to  the  liver,  but  we 
have  no  convincing  demonstration.  We  have,  however,  a  convergence  of 
evidence  that  the  last  stage  of  the  process,  namely,  the  conversion  into 
urea  of  some  product  of  proteid  metabolism,  which  though  allied  to  is  not 
exactly  urea,  does  occur  in  the  liver.  In  the  first  place,  a  large  quantity 
of  urea  seems  to  be  present  in  the  liver  of  mammals  ;  in  this  respect  the  liver 
presents  a  strong  contrast  to  the  muscles ;  in  the  liver  of  birds  the  urea  is 
represented  by  u rates.  Moreover,  when  a  stream  of  fresh  blood  is  passed 
several  times  through  the  liver  of  an  animal  recently  killed,  the  percentage 
of  urea  in  the  blood  so  used  is  found  to  be  decidedly  increased.  This,  how- 
ever, does  not  prove  that  urea  is  formed  in  the  liver,  since  the  increased 
quantity  of  urea  in  the  blood  which  had  been  circulated  might  have  been 
simply  urea  which  had  been  washed  out  from  the  liver,  where  it  had  pre- 
viously been  staying.  Still  as  far  as  it  goes  it  is  suggestive.  In  the  second 
place,  in  certain  cases  of  a  form  of  disease  of  the  liver  known  as  acute  yellow 
atrophy  in  which  the  hepatic  cells  are  so  changed  that  their  functional 
activity  is  largely  diminished,  the  urea  of  the  urine  not  only  undergoes  a 
very  marked  decrease  but  appears  to  be  replaced  to  a  very  large  extent  by 
leucin.  This  fact  suggests  that  leucin  (and  not  for  instance  kreatin)  is  the 
chief  immediate  product  of  the  nitrogenous  metabolism  of  the  body,  and 
that  the  leucin  thus  produced  is  in  a  normal  state  of  things  converted  into 
urea  by  the  liver.  And  in  this  connection  it  may  be  remarked  that  not 
only  is  leucin  found  in  nearly  all  the  tissues  after  death,  especially  in  the 
glandular  tissues,  but  also  appears  with  striking  readiness  in  almost  all 
decompositions  of  proteids,  and  is,  moreover,  a  product  of  decomposition  of 
gelatiniferoue  substances.  Without  going,  however,  so  far  as  to  conclude 
that  leucin  is  the  chief  antecedent  of  urea,  we  may  take  the  above  observa- 
tion as  indicating  that  the  normal  liver  has,  in  some  way  or  other,  the  power 
of  converting  leucin  into  urea.  If  this  be  so  we  may  also  venture  to  suppose 
that  when  such  bodies  as  leucin,  glycin,  etc.,  introduced  into  the  alimentary 
canal  appear  in  the  urine  as  urea  the  transformation  has  taken  place  in  the 
liver.  The  body  tyrosin  which  so  often  accompanies  leucin,  belonging  as  it 
does  to  the  aromatic  series,  stands  on  a  different  footing  from  leucin  and  the  like. 


462  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

§  403.  The  transformation,  however,  of  leucin  into  urea  raises  a  new 
point  of  view.  Leucin,  as  we  know,  is  amido-caproic  acid  ;  and,  with  our 
present  chemical  knowledge,  we  can  conceive  of  no  other  way  in  which  leu- 
ciu  can  be  converted  into  urea  than  by  the  complete  reduction  of  the  former 
to  the  ammonia  condition  (the  caproic  acid  residue  being  either  elaborated 
into  a  fat  or  oxidized  into  carbonic  acid)  and  by  a  reconstruction  of  the  latter 
out  of  the  ammonia  so  formed.  We  have  a  somewhat  parallel  case  in  gly- 
cin,  which  is  amido-acetic  acid ;  here,  too,  a  reconstruction  of  urea  out  of 
an  ammonia  phase  must  take  place.  Moreover,  when  ammonium  chloride 
is  given  to  a  dog  a  very  large  portion  reappears  as  urea,  i.  e.,  there  is  an 
increase  in  the  urea  of  the  urine  corresponding  to  a  large  portion  of  the 
nitrogen  contained  in  the  ammonium  chloride.  And  in  the  case  of  other 
animals  also,  indeed  of  man  himself,  there  is  evidence  that  somewhere  in  the 
body  ammonia  may  be  converted  into  urea.  Hence  in  all  these  cases  where 
ammonia  or  ammonia  compounds  are  changed  into  urea,  the  last  step  at  all 
events  is  one  of  synthesis ;  and  this  suggests  the  possibility  that  in  the  ordi- 
nary proteid  metabolism  also,  the  downward  katabolic  series  of  changes 
may  finish  off  with  a  synthetic  effort,  the  last  stage  of  the  former  being  the 
appearance  of  an  ammonia  compound  which  is  subsequently  reconstructed 
into  urea. 

This  synthesis,  like  the  transformation  of  leucin  and  other  bodies,  prob- 
ably takes  place  in  the  liver;  and  in  support  of  this  view  we  have  a  certain 
amount  of  experimental  evidence.  Birds  maybe  kept  alive  after  total  extir- 
pation of  the  liver  for  a  longer  time  than  can  mammals ;  and  when  in  geese 
the  liver  is  removed  the  uric  acid  (representing  in  these  animals  the  urea  of 
the  mammal)  is  largely  decreased,  while  the  ammonia  of  the  urine  is  largely 
increased.  After  the  removal  of  the  liver  also,  leucin,  glycin,  and  other 
amides  or  amido-acids  administered  by  the  alimentary  canal  no  longer 
increase  the  uric  acid  of  the  urine,  as  they  do  in  the  intact  animal.  In  these 
animals,  the  synthesis  of  ammonia  compounds  into  uric  acid,  which  is  parallel 
to  the  synthesis  into  urea  occurring  in  the  mammal,  seems  to  take  place  in 
the  liver,  and  we  may  infer  is  in  some  way  or  other  effected  by  the  hepatic 
cells. 

As  to  the  exact  way  in  which  ammonia,  either  as  such  or  in  form  of  an 
amide  or  amido-acid  changes  into  urea,  we  have  no  certain  knowledge. 
Ammonium  carbonate,  we  know,  is  readily  formed  out  of  urea  by  simple 
hydration,  and  we  may  imagine  that  the  living  organism  can  carry  out  the 
reverse  process  and  dehydrate  ammonium  carbonate  into  urea.  There  is, 
however,  a  certain  amount  of  evidence  that  not  ammonium  carbonate  but 
ammonium  carbamate  is  the  immediate  antecedent  of  urea ;  and,  indeed,  out 
of  the  body,  by  electrolyzing  a  solution  of  ammonium  carbamate  with  alter- 
nating currents,  a  certain  amount  of  urea  may  be  artificially  produced.  But 
this  is  a  matter  too  obscure  to  be  discussed  here. 

§  404.  Uric  acid.  This,  like  urea,  is  a  normal  constituent  of  human 
urine,  and,  like  urea,  has  been  found  in  the  blood,  in  the  liver,  and  in  the 
spleen  ;  it  is  a  conspicuous  constituent  of  an  extract  of  the  latter  organ. 
In  some  animals,  such  as  birds  and  most  reptiles,  it  takes  the  place  of  urea. 
In  various  diseases  the  quantity  in  the  urine  is  increased ;  and  at  times,  as 
in  gout,  uric  acid  accumulates  in  the  blood,  and  a  deposit  of  urates  takes 
place  in  the  tissues.  Since  by  oxidation  a  molecule  of  uric  acid  can  be 
split  up  into  two  molecules  of  urea,  and  a  molecule  of  some  carbon  acid, 
uric  acid  is  commonly  spoken  of  as  a  less  oxidized  product  of  proteid  metab- 
olism than  urea.  But  there  is  no  evidence  whatever  to  show  that  the 
former  is  a  necessary  antecedent  of  the  latter  ;  on  the  contrary,  all  the  facts 
known  go  to  show  that  the  appearance  of  uric  acid  is  the  result  of  a  metab- 


ON  UREA  AND  ON  NITROGENOUS  METABOLISM.  463 

olism  slightly  diverging  from  that  leading  to  urea ;  indeed,  it  is  probable 
that  the  divergence  occurs  toward  the  end  of  the  series  of  changes,  for  urea 
given  by  the  mouth  to  birds  appears  in  the  urine  as  uric  acid,  and,  con- 
versely, uric  acid  given  to  mammals  appears  in  the  urine  as  area.  We 
have  no  evidence  to  prove  that  the  cause  of  the  divergence  lies  in  an  insuf- 
ficient supply  of  oxygen  to  the  organism  at  large;  on  the  contrary,  uric  acid 
occurs  in  the  rapidly  breathing  birds  as  well  as  in  the  more  torpid  reptiles. 
Nor  can  the  fact  that  in  the  frog,  again,  urea  replaces  uric  acid  be  explained 
by  reference  to  that  animal  having  so  large  a  cutaneous  in  addition  to  its 
pulmonary  respiration.  The  final  causes  of  the  divergence  are  to  be 
sought  rather  in  the  fact  that  urea  is  the  form  adapted  to  a  fluid,  and 
uric  acid  to  a  more  solid  excrement.  Nor  is  there  in  man  or  the  mam- 
mal any  satisfactory  physiological  or  clinical  evidence  that  an  increase  of 
uric  acid  is  the  result  of  deficient  oxidation.  The  absolute  amount  of  uric 
acid  discharged  by  man  and  its  proportion  to  the  urea  passed  at  the  same 
time  varies  a  good  deal.  There  is  no  positive  evidence  that  the  quantity 
excreted  is  necessarily  increased  by  nitrogenous  diet,  unless  some  disorder 
supervenes  ;  indeed,  it  is  asserted  that  both  absolutely  and  relatively  to  the 
urea  the  quantity  excreted  is  greater  upon  a  mixed  diet  than  upon  a  highly 
proteid  one.  Alkalies  in  the  food  seem  undoubtedly  to  diminish  it,  and 
alcohol,  at  least  in  excess,  to  increase  it. 

So  far  from  considering  uric  acid  as  a  less  oxidized  antecedent  of  urea, 
we  ought,  perhaps,  rather  to  regard  its  appearance  as  a  result  of  a  synthesis 
in  which  urea  or  some  allied  body  takes  part.  As  we  have  said,  uric  acid 
may  be  formed  synthetically  by  heating  together  urea  and  glycin  ;  and  it 
has  more  recently  been  similarly  prepared  from  various  allied  bodies.  As 
to  where  or  how  such  a  synthesis  is  effected  in  the  living  body,  we  know 
little  or  nothing  for  certain,  and  can  only  make  conjectures.  The  constant 
presence  of  uric  acid  in  the  spleen,  however,  and  the  frequently  noted  con- 
nection between  a  rise  and  fall  of  uric  acid  in  the  urine  and  variations  in 
the  volume  and  therefore  presumably  in  the  activity  of  the  spleen,  suggest 
that  the  change  may  be  brought  about  in  that  organ  ;  but  it  must  be  remem- 
bered that  in  birds  and  reptiles  the  formation  of  uric  acid  seems  to  be 
effected  in  the  same  organs  as  that  of  urea  and  in  an  analogous  manner  ;  and 
the  arguments  which  we  have  used  concerning  the  formation  of  urea  in  the 
liver  of  mammals,  maybe  applied  to  the  formation  of  uric  acid  in  the  livers 
of  birds  and  reptiles.  It  is  more  probable,  therefore,  that  in  the  mammal 
the  turn  to  uric  acid  rather  than  urea  is  given  in  the  liver,  the  spleen,  how- 
ever, possibly  playing  its  part  also  in  the  matter. 

§  405.  Of  the  meaning  of  the  appearance  in  the  tissues  of  such  bodies 
as  xanthin,  hypoxanthin,  guanin,  and  the  like,  and  of  the  exact  nature  of 
the  metabolism  which  gives  rise  to  them  or  which  they  themselves  undergo, 
we  know  little  or  nothing.  The  presence  of  these  several  bodies  may  be 
taken  as  illustrating  the  complex  and  varied  nature  of  proteid  metabolism 
to  which  we  referred  above.  Urea  is  the  chief  end-product  of  proteid 
metabolism,  but  that  end  is  probably  reached  in  several  ways ;  so  that  prob- 
ably a  very  large  number  of  nitrogenous  chemical  substances  make  a  mo- 
mentary appearance  in  the  body.  Some  of  these  fail  to  become  urea,  and 
either  without  or  after  further  change  make  their  appearance  in  the  urine. 
But  we  do  not  know  whether  their  appearance  is  accidental,  the  result  of 
imperfect  chemical  machinery,  or  whether  they,  though  small  in  quantity, 
serve  some  special  ends  in  the  economy.  Perhaps  sometimes  or  with  some 
of  them  it  is  the  one  case,  at  other  times  or  with  others  it  is  the  other  case. 

When  proteid  material  undergoes  outside  the  body,  either  by  the  action 
of  trypsin  or  as  the  result  of  decomposition  or  under  the  influence  of  chem- 


464  THE  METABOLIC  PKOCESSES  OF  THE  BODY. 

ical  agents,  that  change  by  which  it  is  converted  into  leucin,  the  leucin 
which  appears  in  some  considerable  quantities  is  accompanied  by  tyrosin, 
which  appears  in  smaller  quantities,  as  well  as  by  other  bodies.  The  almost 
constant  appearance  of  tyrosin  as  a  result  of  the  decomposition  of  proteid 
material  leads  one,  as  we  have  previously  said,  to  the  conception  that  some 
representative  of  the  aromatic  series  enters  into  the  constitution  of  proteid 
substance ;  and  it  is  possible  that  the  hippuric  acid  of  flesh-eating  animals 
derives  its  benzoic  acid  constituent  from  this  aromatic  radicle  of  proteid 
matter.  Tyrosin  itself  does  not  appear  in  the  body  as  a  normal  product  of 
proteid  metabolism,  and  we  are  therefore  led  to  infer  that  in  proteid  meta- 
bolism the  aromatic  radicle  takes  on  some  other  form.  Whether,  as  in 
tyrosin,  the  aromatic  (phenyl)  nucleus  is  associated  with  an  ammonia  repre- 
sentative or  no,  we  do  not  know.  But  if  it  is,  then,  since  neither  tyrosin  nor 
any  similar  body  is  a  constituent  of  normal  urine,  the  ammonia  constituent 
is  somewhere  dissociated  from  the  phenyl  one ;  and  while  the  former  con- 
tributes to  the  stock  of  urea,  the  latter  is  either  discharged  by  the  urine  as 
hippuric  acid,  having  as  we  have  seen  effected  in  the  kidney  a  new  associa- 
tion with  the  ammonia  representative,  glycin,  or  leaves  the  body  as  one  or 
other  of  the  urinary  phenyl  compounds,  or  possibly  may  be  oxidized  some- 
where into  carbonic  acid  and  water.  Our  knowledge  on  this  point  is 
limited,  but  we  have  ventured  to  refer  to  the  point  since  it  further  illus- 
trates the  complexity  of  proteid  metabolism. 

§  406.  In  speaking  of  urea  (§  337)  we  alluded  to  its  relations  to  the 
cyanogen  compounds.  Bearing  in  mind  the  peculiarly  large  amount  of 
energy  set  free  as  heat  during  the  isomeric  transformation  of  many  cyanogen 
compounds,  as  well  as  the  large  store  of  potential  energy  existing  in  cyanogen 
itself,  the  heat  of  combustion  of  which  is  very  large,  and  contrasting  these 
properties  with  those  of  ammonia  and  the  ammonia  compounds,  we  cannot 
help  being  tempted  toward  the  view  that  in  the  actual  living  structure  the 
nitrogen  exists  in  the  form  of  cyanogen  compounds,  and  that  in  the  passage 
to  dead  nitrogenous  waste,  during  which  energy  is  set  free,  the  cyanogen 
compound  changes  to  the  amide  or  other  ammonia  representative.  And  there 
are  several  facts  which  lend  support  to  such  a  view,  such  as  the  presence  of 
sulphocyanates  in  saliva  and  urine,  which  we  may  look  upon  as  a  sort  of 
leakage  of  cyanogen  factors,  the  artificial  production  of  kreatinin  out  of 
cyamide  and  sarcosin,  and  other  facts.  But  the  matter,  though  it  deserves 
to  be  borne  in  mind,  is  too  obscure  to  be  dwelt  on  here. 

§  407.  We  may  now  briefly  sum  up  the  varied  discussions  which  have 
occupied  us  in  the  present  section. 

Urea  is  the  main  end-product  of  proteid  metabolism.  Unlike  hippuric 
acid  and  some  other  constituents  of  urine,  urea  is  simply  excreted  by  the 
kidneys,  being  brought  to  them  in  the  blood,  they  apparently,  beyond  the 
simple  act  of  excretion,  doing  no  more  than  merely  contributing  to  the 
stock  of  urea  in  so  far  as  they  are  masses  of  proteid  material  undergoing 
proteid  metabolism  as  part  of  their  general  life.  What  are  the  immediate 
antecedents  of  urea  we  do  not  clearly  know  ;  but  it  is  probable  that  they 
are  not  one  but  several  and  indeed  possibly  many.  We  have  reason  to 
think  that  urea  may  be  formed  out  of  amides  or  amido-acids,  or  out  of 
ammonia  itself  by  a  synthetic  process;  and  we  have  indications  that  this 
synthesis  is  effected  in  the  liver  by  the  agency  of  the  hepatic  cells.  But  we 
do  not  know  whether  this  synthesis  bears  only  on  particular  nitrogen-hold- 
ing substances  of  food  or  of  the  body,  or  whether  it  comes  into  play  in  the 
normal  metabolism  of  proteid  material.  If  the  kreatin  which  is  so  con- 
spicuous a  constituent  of  muscular  and  nervous  structures  is  a  stage  in  the 
direct  line  to  urea,  then  the  synthesis  would  affect  only  the  sarcosin  which 


STRUCTURES  AND  PROCESSES  OF  OBSCURE  NATURE.        465 

the  kreatin  in  becoming  urea  sets  free.     But  it  is  by  no  means  clear  that 
kreatin  is  such  a  stage. 

The  evidence  as  far  as  it  goes  tends  to  show  that  the  metabolism  of  pro- 
teid  is  very  complex  and  varied,  that  a  large  number  of  nitrogen-holding 
substances  make  a  momentary  appearance  in  the  body,  taking  origin  at  this 
or  that  step  in  the  downward  stairs  of  katabolic  metabolism  and  changing 
into  something  else  at  the  next  step,  and  that  the  presence  in  various  parts 
of  the  body  and  even  in  the  urine,  in  small  quantities,  of  so  many  varied 
nitrogenous  crystalline  substances,  forming  a  large  part  of  what  are  known 
as  extractives,  has  to  do  with  this  varied  metabolism.  Possibly  the  transfor- 
mations by  which  nitrogen  thus  passes  downward  take  place  to  a  certain 
extent  in  such  organs  as  the  liver  and  the  spleen,  which  are  remarkably  rich 
in  these  extractives. 

ON  SOME  STRUCTURES  AND  PROCESSES  OF  OBSCURE  NATURE. 

§  408.  The  thyroid  body.  Certain  structures  of  obscure  nature,  but 
probably  connected  in  some  way  or  other  with  some  of  the  metabolic  pro- 
cesses in  the  body,  are  often  spoken  of  under  the  undesirable  name  of  "  duct- 
less glands."  Such  are  the  thyroid  body  or  gland,  the  pituitary  body,  the 
thymus,  and  the  suprarenal  capsules.  These  differ  from  each  other  so  essen- 
tially that  the  only  plea  which  can  be  urged  in  favor  of  considering  them 
together  is  convenience  and  our  ignorance  of  their  respective  functions. 

The  thyroid  body  is  the  one  of  the  group  most  deserving  to  be  called  a 
gland,  since  it,  like  the  lungs,  arises  as  a  two-lobed  diverticulum  from  the 
ventral  surface  of  the  anterior  part  of  the  alimentary  canal,  and  at  first, 
like  the  lungs  also,  behaves  as  if  it  were  about  to  become  a  double  racemose 
gland.  The  connection  with  the  throat,  however,  which  should  have  become 
a  duct,  is  soon  obliterated,  and  the  two  lobes,  united  with  each  other  by  an 
isthmus  across  the  trachea,  lose  all  traces  of  any  branching  ducts  within 
them,  and  become  transformed  into  masses  of  isolated  ductless  alveoli  bound 
together  with  connective  tissue. 

Hence,  when  a  section  is  taken  through  a  hardened  and  prepared  lobe 
of  an  adult  thyroid,  what  is  seen  is  a  limiting  capsule  of  connective  tissue 
sending  into  the  interior  numerous  septa,  which  surround  and  separate  from 
each  other  round  or  oval  spaces,  the  sections  of  the  isolated  alveoli.  These 
are  of  variable  size,  some  being  visible  to  the  naked  eye,  and  each  is  lined 
by  a  single  layer  of  low  columnar  or  cubical  nucleated  cells  resting  on  a 
basement  membrane,  leaving  a  large  cavity,  which  in  fresh  specimens  is 
filled  with  a  glairy  fluid.  The  cells  present  no  special  characters. 

The  septa  of  connective  tissue,  fairly  rich  in  elastic  elements,  but  remark- 
ably free  from  adipose  tissue,  contain  'numerous  bloodvessels  derived  from 
the  superior  and  inferior  thyroid  arteries,  the  branches  of  which,  relatively 
large  and  frequently  anastomosing,  end  for  the  most  part  in  capillary  net- 
works round  the  alveoli ;  from  these  capillaries  and  those  of  the  septa  the 
blood  is  gathered  into  veins  also  relatively  large,  which,  forming  plexuses 
on  the  surface  of  the  organ,  end  in  the  superior  middle  and  inferior  thy- 
roid veins.  The  thyroid  body  is  thus  furnished  with  an  abundant  supply 
of  blood. 

The  septa  also  contain  a  very  large  number  of  lymphatic  vessels,  which, 
both  on  the  surface  of  the  organ  and  along  the  septa,  are  arranged  in  plex- 
uses of  anastomosing  trunks  of  considerable  size.  Small  nodules  of  adenoid 
tissue  are  also  found  in  the  septa. 

The  nerves  of  the  thyroid  body  are  also  abundant.  They  are,  in  man, 
derived  chiefly  from  the  cervical  "sympathetic  nerve,  passing  off  from  the 

30 


460  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

middle  and  lower  cervical  ganglia;  their  exact  terminations  within  the 
organ  is  not  known.  Fine  filaments  are  also  said  to  be  given  off  to  it  from 
the  external  branch  of  the  superior  laryngeal  nerve. 

The  "  accessory  "  thyroid  bodies  often  found  are  of  the  same  nature  as 
the  main  body. 

Very  frequently,  so  frequently  in  the  adult  as  to  be  of  almost  normal 
occurrence,  the  alveoli  contain  not  simple  glairy  fluid  but  a  more  solid  clear 
material  called  "  colloid  ;"  this  generally  appears  in  the  centre  of  an  alveolus 
and  may  fill  up  the  whole  lumen ;  occasionally  more  or  less  changed  epi- 
thelial cells  may  be  seen  lying  between  it  and  the  layer  of  cells  resting  on 
the  basement  membrane.  Extravasations  of  blood  into  the  alveoli  are  also 
not  uncommon. 

The  thyroid  body  is  very  apt  to  become  enlarged,  sometimes  enor- 
mously so,  and  is  then  spoken  of  as  goitre.  The  enlargement  may  be  due 
simply  to  an  increase  in  the  number  of  otherwise  fairly  normal  alveoli  and 
septa.  But  very  often  a  number  of  alveoli  become  more  or  less  confluent, 
forming  a  cyst ;  and  at  times  the  whole  gland  appears  to  be  composed  of  a 
number  of  cysts  of  varying  size,  frequently  loaded  with  "colloid"  material. 
There  is  also  a  form  of  goitre  in  which  the  enlargement  is  chiefly  or  even 
exclusively  due  to  an  increase  in  the  vascular  supply,  the  bloodvessels  being 
abnormally  distended  ;  and  this  apparently  may  occur  without  any  structural 
changes  in  the  walls  of  the  bloodvessels.  Sometimes,  however,  the  arteries 
undergo  aneurismal  enlargements,  with  changes  in  their  coats. 

The  glairiness  of  the  fluid  contents  of  the  alveoli  has  generally  been 
attributed  to  the  presence  of  mucin,  and  this  body  has  also  been  said  to 
have  been  found  within  the  lymphatic  vessels  running  in  the  septa;  but 
some  observers  have  urged  that  the  material  in  question  is  not  true  mucin, 
but  a  peculiar  form  (or  forms)  of  proteid  substance.  The  "  colloid  "  material 
so  frequently  appearing  has  also  been  regarded  as  allied  to  mucin,  but  its 
exact  nature  has  not  as  yet  been  satisfactorily  determined.  Besides  these 
special  substances  the  alveoli  or  cysts  also  contain  serum-albumin  and 
globulin.  The  "  extractives  "  of  the  thyroid  appear  to  contain  kreatin  or 
kreatinin  in  not  inconsiderable  quantities,  xanthin,  and  lactic  (paralactic) 
acid ;  guanin  is  said  to  be  absent.  In  large  and  old  cysts  cholesterin  is 
sometimes  present ;  and  when,  as  often  happens,  extravasations  of  blood  into 
the  cysts  have  taken  place,  haemoglobin,  or  at  a  later  stage  hsematoidin 
(bilirubin)  has  been  found. 

§  409.  The  large  supply  of  blood  to  the  thyroid  suggests  the  idea  that 
the  organ  is  the  seat  of  some  of  the  subsidiary  metabolic  processes  to  which 
we  referred  in  the  last  section,  and  this  view  is  supported  by  the  presence  of 
the  extractives  just  mentioned  ;  but  we  have  no  detailed  knowledge  of  what 
actually  goes  on. 

The  presence  of  the  peculiar  mucin-like  body  in  the  alveoli,  and  the  ten- 
dency to  "  colloid  formation  "  further  suggest?  some  relation  of  the  organ  to 
the  formation  or  distribution  of  mucin  ;  and  this  view  has  derived  a  certain 
support  from  some  experimental  results,  but  these,  though  numerous,  have 
proved  neither  uniform  nor  accordant.  When  in  certain  animals  (monkeys, 
dogs,  and  other  carnivora,  and  the  same  has  been  observed  in  man)  the  gland 
is  extirpated,  even  with  the  greatest  care,  the  operation  is  frequently  followed 
by  the  occurrence  of  peculiar  nervous  symptoms,  such  as  muscular  twitchings 
and  tremors,  spasms,  and  even  tetanic  convulsions  (more  especially  observed 
in  young  animals),  accompanied  or  succeeded  by  irregularity  or  failure  of 
voluntary  movements ;  subsequently  there  may  ensue  varied  symptoms 
which  may  be  described  under  the  general  term  of  disordered  nutrition, 
ending  eventually  in  death.  In  a  certain  number  of  cases,  however,  in  the 


STRUCTURES  AND  PROCESSES  OF  OBSCURE  NATURE.        467 

above  kinds  of  animal,  no  serious  symptoms  follow,  even  the  total  extirpation 
of  the  organ  producing  no  marked  effect ;  and  in  rabbits  and  other  her- 
bivorous animals  removal  is  said  never  to  be  followed  by  any  of  the  above 
results.  It  has  been  urged  that  the  symptoms  when  seen  are  the  effects  not 
of  the  mere  absence  of  the  organ,  but  of  mischief  set  up  by  the  operation  in 
adjoining  structures,  more  especially  in  the  laryngeal  nerves  and  vagus 
trunks  ;  but  this  does  not  seem  a  valid  explanation.  If,  as  suggested  above, 
certain  metabolic  processes  are  normally  going  on  in  the  organ,  we  may  fairly 
suppose  that,  in  the  absence  of  the  organ,  the  interruption  of  the  normal 
sequence  of  chemical  change  would  throw  upon  the  circulation  certain 
strange  substances  which,  acting  like  a  poison,  might  produce  the  nervous 
symptoms,  throw  into  disorder  the  nutrition  of  various  tissues,  and  finally 
bring  about  death.  We  may  further  explain  the  cases  where  symptoms  are 
absent  by  supposing  that,  for  some  reason  or  other,  "  things  have  taken  a 
different  turn,"  the  particular  poisonous  substance  have  not  made  their 
appearance,  but  innocuous  ones  have  taken  their  place ;  and  we  know  how 
slight  a  change  in  chemical  composition  may  turn  a  poison  into  an  inert 
body.  This,  of  course,  remains  a  mere  supposition  until  we  can  state  what 
the  exact  metabolic  processes  are,  and  name  the  substances  which  work  the 
mischief;  but  it  seems  more  reasonable  to  accept  such  a  provisional  supposi- 
tion, than  to  conclude  that  the  thyroid  may  be  removed  without  producing 
any  effect  whatever  on  the  organism.  An  animal  without  a  thyroid  may 
appear  perfectly  well,  because  the  circumstances  to  which  it  is  exposed  do 
not  happen  to  test  the  imperfection  from  which  it  is  really  suffering,  just  as 
a  man's  inability  to  swim  may  not  be  apparent  until  he  happens  to  fall  into 
the  water.  The  animals  which  do  succumb  to  the  operation  of  removal  of 
the  organ  are,  for  some  reason  or  other,  put  to  the  test,  and  are  found  want- 
ing. The  very  discordance  of  the  experimental  results  points  the  physio- 
logical moral  that  the  phenomena  which  we  are  as  yet  able  to  observe  form, 
as  it  were,  a  mere  surface  covering  intricate  processes  at  present  wholly,  or 
nearly  wholly,  hidden  from  us. 

The  above  experimental  results  receive  additional  interest  and  at  the 
same  time  support  from  clinical  experience.  The  connection  between  goitre 
and  cretinism — the  latter  disease  being,  broadly  speaking,  a  result  of  dis- 
ordered nutrition  telling  largely  on  the  nervous  system — has  long  been 
recognized  ;  and  attention  has  also  been  called  to  some  tie  between  disease 
of  the  thyroid  and  a  morbid  condition,  known  as  myxoedema,  in  a  certain 
number  of  cases  of  which  mucin  or  a  mucin-like  body  has  been  found  in 
great  excess  in  the  skin  and  in  other  tissues.  In  monkeys  the  removal  of 
the  thyroid  has,  in  some  cases,  been  followed,  besides  the  symptoms  men- 
tioned above,  some  of  which  resemble  those  of  myxoedema,  by  an  accumu- 
lation of  mucin  or  a  mucin-like  body  in  the  skin  and  various  tissues.  It  is 
very  difficult  not  to  connect  this  with  the  formation  in  the  thyroid  of  colloid 
material  in  the  contents  of  the  alveoli.  But  we  know  so  little  about  the 
nature  of  mucin  and  its  allies,  about  their  real  relations  to  more  ordinary 
proteid  substances,  and  about  the  part  which  they  play  in  physiological  pro- 
cesses, that  any  views  as  to  the  exact  connection  between  the  presence  of 
mucin  in  the  tissues  at  large  and  changes  taking  place  in  the  thyroid  must 
be  at  present  to  a  large  extent  speculation. 

The  large  vascular  supply  of  the  thyroid,  and  the  phenomena  of  a 
disease  known  as  exophthalmic  goitre,  in  which  vascular  enlargement 
of  the  thyroid  is  associated  with  cardiac  symptoms,  and  other  vascular 
disturbances,  especially  of  the  head,  have  suggested  that,  apart  from 
metabolic  processes,  the  circulation  in  the  thyroid  may,  perhaps  in  a 
more  or  less  mechanical  way,  be  connected  with  and  influence  the  cir- 


468  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

dilation  in  the  brain.  But  the  exact  nature  of  this  influence  has  not 
been  made  clear. 

§  410.  The  pituitary  body.  The  lower,  posterior,  lobe  of  this  organ 
resembles  the  thyroid  body  (the  upper,  anterior,  lobe  is  of  quite  distinct 
nature,  being  really  a  part  of  the  central  nervous  system). 

Concerning  the  processes  which  take  place  in  these  alveoli,  and  the  pur- 
poses of  the  organ  as  a  whole,  we  know  absolutely  nothing. 

§  411.  The  supra-renal  bodies.  A  (mammalian)  supra-renal  body,  when 
cut  across,  is  seen  to  consist  of  two  distinct  parts,  an  outer  thicker  cortical 
part,  of  yellowish  color,  striated  radially,  and  an  inner  thicker  medullary 
part  of  dark  color.  At  the  depression  on  the  anterior  surface  called  the 
hilus,  whence  issues  the  comparatively  large  supra-renal  vein,  the  cortex 
thins  away  so  that  the  medulla  comes  to  the  surface.  These  two  parts,  cor- 
tex and  medulla,  are  not,  like  the  cortex  and  medulla  of  a  lymphatic  gland, 
different  arrangements  of  the  same  material,  but  are  of  essentially  different 
nature,  and,  indeed,  are  of  different  origin.  The  medulla  is  derived  from,  is 
a  modification  of,  sympathetic  ganglia,  while  the  cortex  is  derived  from 
masses  of  mesoblastic  cells  surrounding  the  great  bloodvessels ;  and  in  some 
animals  the  two  form  wholly  separate  bodies.  The  so-called  accessory  supra- 
renals  are  composed  of  cortex  alone. 

The  whole  organ  is  surrounded  by  a  capsule  of  connective  tissue,  free 
from  muscular  fibres,  and  not  very  rich  in  elastic  elements.  From  the  cap- 
sule septa  pass  inward  and  form  a  framework,  the  cavities  of  which  are  filled 
by  cells  or  groups  of  cells  differing  in  nature  and  differently  arranged  in 
the  cortex  and  in  the  medulla,  The  middle  larger  part  of  the  cortex  is 
composed  of  somewhat  long  solid  columns  of  polyhedral  cells,  lodged  in 
corresponding  meshes  of  the  framework.  The  columns,  which  are  three  or 
four  cells  thick  and  several  cells  in  length,  though  somewhat  irregular  and 
varying  in  size,  do  not  anastomose,  being  wholly  separated  from  each  other 
by  the  bars  of  connective  tissue,  and  possess  no  central  cavity  or  lumen. 
The  bloodvessels,  which  are  abundant  in  these  bars  of  connective  tissue,  do 
not  penetrate  the  columns.  The  cell  substance  of  the  cells  is  of  a  yellow- 
ish color,  often  containing  yellowish  oil  globules,  and  possesses  a  clear  round 
nucleus. 

In  the  outer  part  of  the  cortex  immediately  underneath  the  capsule  is  a 
thin  zone  in  which  the  groups  of  cells  are  not  columnar,  but  rounded  and 
irregular  ;  and  again  in  the  inner  part  of  the  cortex  abutting  on  the  medulla 
is  another  thin  zone,  in  which  the  columnar  arrangement  is  lost,  the  cells 
being  here  disposed  in  a  network  of  thin  cords,  and  the  individual  cells  to  a 
large  extent  separated  from  each  other  by  delicate  continuations  of  the 
coarser  connective-tissue  septa.  Hence,  the  main  median  part  of  the  cortex, 
which  from  the  prominent  columnar  arrangement  appears  striated  radially, 
is  often  called  the  zona  fasciculata,  the  thin  outer  part  the  zona  glomerulosa, 
and  the  thin  inner  part  the  zona  reticularis ;  but  as  far  as  the  essential  cha- 
racters of  the  cells  are  concerned,  all  the  three  zones  are  alike. 

The  medulla  also  consists  of  cells  or  groups  of  cells  lying  in  the  meshes 
of  a  connective-tissue  framework,  but  the  cells  are  of  a  different  nature  from 
those  of  the  cortex.  They  are  irregular  and  often  branched,  and  their  cell 
substance,  though  it  sometimes  contains  pigment,  is  generally  clear  and 
transparent.  The  medulla,  moreover,  is  further  distinguished  from  the 
cortex  by  the  abundant  supply  of  bloodvessels  and  of  nerves. 

The  cells  of  the  medulla  and  of  the  inner  zone  (zona  reticularis)  of  the 
cortex  are  very  apt  to  undergo  change  after  death,  and  to  become  diffluent. 

The  arteries  which  come  from  the  aorta  and  from  the  renal  and  phrenic 
arteries  pass  into  the  organ  on  the  surface,  and  traversing  the  cortex,  supply- 


STRUCTURES  AND  PROCESSES  OF  OBSCURE  NATURE.        469 

ing  as  they  go  both  capsule  and  cortex  with  a  moderate  number  of  vessels, 
end  in  the  medulla  the  connective-tissue  bars  of  which  bear  numerous  large 
venous  sinuses,  into  which  the  capillaries  pour  their  blood,  and  from  which 
the  blood  is  gathered  up  into  the  supra-renal  vein. 

A  large  number  of  nerves,  consisting  chiefly  of  medullated  fibres  from 
the  solar  plexus,  the  renal  plexus,  the  phrenic  nerve,  and  the  vagus,  pass 
into  the  supra-renal  body  at  the  hilus  and  on  the  under  surface,  and  forming 
numerous  plexuses,  coarse  and  fine,  some  carrying  small  groups  of  nerve 
cells,  end  chiefly  in  the  medulla,  though  some  pass  on  to  the  cortex.  The 
ultimate  endings  are  not  yet  known. 

The  lymphatics  are  fairly  numerous,  and  form  plexuses  in  the  capsule 
and  in  the  connective  tissue  of  the  framework ;  it  is  stated  that  the  lym- 
phatic vessels  surrounding  the  groups  of  cells  in  the  cortex  communicate 
with  spaces  between  the  cells. 

§  412.  Besides  the  ordinary  proteid  and  other  chemical  constituents,  the 
supra-renal  body  contains  some  substance  or  substances  possessing  striking 
color  reactions,  giving  a  dark-blue  or  dark-green  color  with  ferric  chloride, 
and  a  carmine-red  tint  with  various  oxidizing  agents.  This  substance  (whose 
nature  is  not  exactly  known,  and  which  is  confined  to  or  most  abundant  in 
the  medulla)  is  not  soluble  in  the  ordinary  solvents  of  pigments,  such  as 
alcohol,  ether,  chloroform,  etc.,  but  is  readily  soluble  in  dilute  acids. 

Among  the  extractives,  hippuric  and  benzoic  acid,  and  taurocholic  acid 
or  taurin  have  been  found,  but  it  is  not  certain  that  these  are  normal  con- 
stituents. 

§  413.  Some  of  the  histological  features  of  the  supra-renal  bodies, 
namely,  the  groups  of  cells  and  their  abundant  blood-supply,  suggest,  on 
the  one  hand,  that  important  metabolic  processes  take  place  in  them,  some 
of  which  are  probably  connected  with  the  history  of  the  pigments  of  the 
body  at  large.  On  the  other  hand,  the  unusually  large  nerve-supply,  and 
the  derivation  of  part  of  the  body  from  the  sympathetic  ganglia,  suggest 
peculiar  nervous  connections.  And  the  organ  has  often  served  as  a  starting- 
point  for  speculations  in  these  two  directions ;  but  our  exact  knowledge  con- 
cerning them  is  very  limited.  The  results  of  experiment  have  taught  us 
little ;  extirpation,  for  example,  has  been  often  followed  by  the  death  of  the 
animal  operated  upon,  but  the  cause  of  the  death  in  such  cases  is  by  no 
means  clear. 

One  fact,  gained  by  clinical  experience,  is  the  only  real  item  of  know- 
ledge which  we  possess.  Disease  of  the  supra-renal  bodies,  apparently 
tubercular  in  nature  and  beginning  in  the  medulla,  is  so  often  associated 
with  a  change  in  the  color  or  with  an  increase  of  the  pigment  of  the  skin — 
"  bronzed  skin,"  "  Addison's  disease  " — that  some  connection  between  the  two 
must  exist;  but  the  several  links  of  the  chain  are  as  yet  unknown.  It  is 
tempting  to  associate  the  increase  of  pigment  in  the  bronzed  skin  with  the 
chromogen  or  color-yielding  substance  spoken  of  above;  but  we  have  no 
warrant  for  doing  so,  such,  for  instance,  as  any  indication  of  ties  between 
the  supra-renal  bodies  and  changes  either  in  haemoglobin  itself  or  in  bilirubin, 
which  two  bodies  we  have  reason  to  regard  more  particularly  as  mothers  of 
pigment.  Moreover  the  bronzed  skin  is  only  one  of  the  symptoms  of  Addi- 
son's disease,  failure  of  nutrition  and  nervous  symptoms  being  also  present. 

§  414.  The  thymus.  This,  though  it  arises  in  the  embryo  as  a  paired 
outgrowth  from  the  epithelial  walls  of  a  pair  of  visceral  clefts,  and  thus 
begins  as  an  epithelial  structure  into  which  mesoblastic  elements  subse- 
quently intrude,  soon  puts  on  such  characters  as  to  appear  essentially  a 
lymphatic  structure,  and,  indeed,  might  be  regarded  as  a  part  of  the  lymph- 
atic system. 


470  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

It  consists  of  a  capsule  of  connective  tissue,  plain  muscular  fibres  being 
absent,  and  septa  or  trabeculse  of  the  same  nature  which  divide  the  organ 
into  a  number  of  irregular  more  or  less  cylindrical  anastomosing  follicles 
or  lobules,  and  send  finer  radiating  septa  into  the  interior  of  each  lobule. 
These  lobules  present  the  same  characters  throughout  the  whole  mass  of 
the  organ,  there  not  being,  as  in  a  lymphatic  gland,  any  distinction  between 
a  cortex  and  a  medulla  of  the  whole  body.  The  words  are,  however, 
applied  to  each  lobule,  to  distinguish  the  central  from  the  peripheral  part 
of  the  lobule  itself.  Both  the  central  medulla  and  the  peripheral  cortex 
of  each  lobule  consist  of  a  framework  of  reticular  connective  tissue,  which 
in  the  cortex  is  identical  with  or  closely  allied  to  adenoid  tissue,  but  in  the 
medulla  is  coarser  and  more  open  and  to  a  larger  extent  composed  of 
branched  anastomosing  epithelioid  cells.  The  meshes  of  the  cortex  are 
crowded  with  leucocytes,  but  these  are  much  less  abundant  in  and  more 
easily  fall  out  of  the  medulla,  so  that  in  sections  the  medulla  appears  more 
transparent  than  the  cortex.  It  will  be  observed  that  this  arrangement  is 
almost  the  reverse  of  that  obtaining  in  the  alveolus  of  a  lymphatic  gland, 
in  which  the  finer  gland  substance  with  its  adenoid  tissue  crowded  with  leu- 
cocytes is  placed  in  the  centre,  surrounded  by  the  more  open  network  of 
the  lymph  sinus. 

The  bloodvessels  of  the  thymus  running  along  the  septa  form  capillary 
networks  which,  though  closer  and  more  abundant  in  the  cortical  than  in 
the  medullary  portions  of  the  lobules,  have  no  such  special  arrangements 
as  obtains  in  lymphatic  glands. 

Lymphatic  vessels,  abundant  in  the  capsule  and  septa,  are  undoubtedly 
in  connection  with  the  substance  of  the  lobules. 

The  medullary  substance  frequently  contains  bodies,  known  as  "  con- 
centric capsules,"  nests  of  concentrically  disposed  nucleated  flattened  epi- 
thelial or  epithelioid  cells.  They  appear  to  arise  from  a  proliferation  of 
the  epithelioid  cells  lining  small  bloodvessels,  and  have  been  supposed  to 
be  connected  with  the  degenerative  changes  by  which,  with  obliteration  of 
the  vessels,  the  whole  organ  dwindles  away  soon  after  birth. 

§  415.  From  the  thymus  there  may  be  extracted  by  means  of  saline 
solution  a  form  of  globulin  or  a  proteid  allied  to  globulin  which,  like  the 
corresponding  bodies  from  lymphatic  glands  or  from  leucocytes,  seems  to 
have  some  special  relations  to  the  formation  of  fibrin.  Thus,  as  has  already 
been  said  (§  22),  a  solution  of  this  globulin-like  body  from  the  thymus, 
injected  into  the  veins  will  give  rise  to  extensive  intra-vascular  clotting. 

The  thymus,  like  the  other  bodies  on  which  we  are  now  dwelling,  is 
also  rich  in  extractives.  Thus  xanthin,  hypoxanthin,  leucin,  lactic,  suc- 
cinic  and  other  acids  have  been  found  in  it. 

But  of  what  really  takes  place  in  the  body  we  have  no  exact  knowledge. 
Since  the  thymus  is  best  developed  before  birth,  disappearing  after  birth  at 
a  rate  which  varies  much  in  different  individuals  and  still  more  in  different 
kinds  of  animals,  and  being  eventually  replaced  by  fat  and  connective 
tissue,  it  is  obvious  that  its  chief  functions  are  in  some  way  associated  with 
events  taking  place  before  birth  or  in  early  life. 

THE  HISTORY  OF  FAT.     ADIPOSE  TISSUE. 

§  416.  Globules  of  fat  of  various  sizes  make  their  appearance  in  the 
very  elements  of  most  of  the  tissues,  in  muscular  fibres,  in  epithelial  cells, 
in  nerve  cells,  in  leucocytes,  and  so  on  ;  and  the  medulla  of  medullated 
nerves  consists  largely  of  a  peculiar  fatty  material.  Besides  this  certain 
cells  of  connective  tissue  at  various  times,  and  in  various  places,  become  so 


THE  HISTORY  OF  FAT.      ADIPOSE  TISSUE.  471 

loaded  with  fat  that  groups  of  the  cells  become  practically  masses  of  fat. 
Connective  tissue  thus  loaded  with  fat  is  called  adipose  tissue ;  and  masses 
of  adipose  tissue  of  all  manner  of  sizes  and  of  shapes  adapted  to  the  several 
situations  are  found  in  various  parts  of  the  body.  Many  of  the  internal 
organs,  more  especially  the  kidneys,  are  wrapped  in  adipose  tissue ;  but  the 
largest  deposit  is  one  lying  in  the  subcutaneous  tissue,  sometimes  called  the 
"panniculus  adiposus";  and  a  "fat"  body  is  distinguished  from  a  "lean" 
body  chiefly,  though  by  no  means  exclusively,  by  the  amount  of  subcuta- 
neous adipose  tissue. 

Of  all  the  tissues  of  the  body  adipose  tissue  is  the  most  fluctuating  in 
bulk;  within  a  very  short  space  of  time  a  large  amount  of 'adipose  tissue 
may  disappear,  and  within  an  almost  equally  short  time  the  quantity 
present  in  a  body  may  be  several  times  multiplied.  When  too  much  or  too 
little  food  is  given  it  is  the  subsequent  adipose  tissue  which  first  and  most 
rapidly  increases  or  decreases  in  bulk. 

§  417.  A  small  piece  of  adipose  tissue,  examined  under  a  low  power, 
appears  to  be  made  up  almost  entirely  of  rounded  masses  of  highly  refrac- 
tive material,  closely  packed  together.  These  rounded  masses,  which  stain 
an  intense  black  with  osmic  acid  and  give  other  reactions  of  fat,  are 
arranged  in  irregular  lobules  ;  between  the  lobules,  and  between  the  indi- 
vidual rounded  masses,  may  be  seen  a  small  amount  of  fibrillated  connec- 
tive tissue  carrying  bloodvessels. 

When  the  tissue  has  been  hardened  and  stained,  and  the  fat  has  been 
removed  by  solvents,  what  was  previously  only  visible  as  a  rounded  mass 
of  fat  is  now  seen,  under  higher  powers,  to  be  a  cell,  but  a  cell  nearly  the 
whole  of  the  cell  substance  of  which  has  become  transformed  into  a  single 
large  vacuole.  Over  the  greater  part  of  the  circumference  of  the  cell  the 
cell  substance  is  reduced  to  a  mere  thin  shell  or  envelope,  or  cell  mem- 
brane, but  at  one  part  a  thicker  disc-like  remnant  is  seen,  and  in  this  is 
placed  a  rounded  or  oval,  often  flattened  nucleus.  Between  these  fat-cells 
may  be  seen  a  few  bundles  of  connective  tissue  forming  a  scanty  loose  net- 
work, the  rounded  meshes  of  which  are  occupied  by  the  fat-cells,  the  matrix 
of  the  bundles  appearing  at  places  continuous  with,  or  adherent  to,  the 
envelopes  of  the  cells ;  ordinary  connective-tissue  corpuscles  are  also  here 
and  there  present,  though  rarely  visible  between  the  larger,  50/Ji  to  130//, 
fat-cells.  In  injected  specimens  it  is  further  seen  that  the  connective-tissue 
meshwork  carries  small  bloodvessels,  which  form  capillary  networks  around 
the  groups  of  fat-cells  and  even  around  individual  cells.  After  death,  upon 
cooling,  the  fat  in  the  fat-cells  may  solidify  in  crystals. 

It  is  obvious  that  a  fat-cell  is  a  cell  belonging  to  connective  tissue,  in  the 
cell  substance  of  which  fat  has  been  collected  to  such  an  extent  that  the  cell, 
which  increases  largely  in  bulk  during  the  process,  is  almost  wholly  trans- 
formed into  a  large  vacuole  filled  with  fat,  the  cell  substance  being  reduced 
to  a  thin  envelope  of  the  vacuole,  thickened  at  one  part  where  the  nucleus, 
thrust  on  one  side  by  the  gathering  fat,  is  placed.  Adipose  tissue  is  a  col- 
lection of  such  fat-cells  held  together  by  a  meagre  quantity  of  vascular  con- 
nective tissue. 

By  studying  the  development  of  adipose  tissue  in  the  embryo  or  else- 
where, we  may  trace  out  the  steps  of  the  formation  of  the  fat-cells.  In  the 
embryo,  in  a  situation  where  adipose  tissue  is  about  to  be  formed,  the  con- 
nective tissue  is  seen  to  contain  a  number  of  small  nucleated  cells,  rounded 
or  somewhat  irregular  in  form,  the  cell  substance  of  which  at  first  presents 
no  special  characters,  and  contains  not  more  than  what  may  be  called  the 
ordinary  amount  of  fat  globules  or  spherules.  Very  soon,  however,  these 
minute  drops  or  specks  increase  in  number,  the  cell  substance  at  the  same 


472 


THE  METABOLIC  PKOCESSES  OF  THE  BODY. 


time  increasing  in  bulk  while  remaining  round  or  becoming  more  distinctly 
so,  and  the  smaller  drops  run  together  into  larger  ones  [Fig.  118].  This 
goes  on  ;  the  fat  increasing  in  quantity  coalesces  more  and  more,  and  the 
cell,  as  a  whole,  becomes  larger  and  larger,  the  cell  substance  at  first  keep- 
ing up  in  bulk  with  the  increasing  fat,  but  subsequently  ceasing  to  increase, 
being  apparently  used  up  in  the  formation  of  the  fat.  Thus  the  original 
small  "  protoplasmic  "  cell  is  at  last  transformed  into  the  larger  fat-cell,  all 
the  fat  having  run  together  into  a  vesicle  the  envelope  of  which,  thickened 
on  one  side  to  carry  the  nucleus,  is  furnished  by  the  remnant  of  the  cell 
substance.  In  some  cases,  the  nucleus  instead  of  being  pushed  early  on  one 
side,  remains  central  though  the  collection  of  fat  has  become  considerable ; 
it  is,  however,  eventually  displaced.  The  whole  process  appears  very  similar 
to  the  deposition  of  mucin  in  the  cells  of  a  mucous  gland  (§  205) ;  and  we 


J 


Deposition  of  Fat  in  Connective-tissue  Cells :  /,  a  cell  with  a  few  isolated  fat-droplets  in  its 
protoplasm ;  /,  a  cell  with  a  single  large  and  several  minute  drops  ;  /',  fusion  of  two  large  drops ; 
g,  granular  or  plasma  cell,  not  yet  exhibiting  any  fat  deposition  ;  c.  t.,  flat  connective-tissue  cor- 
puscles ;  c.  c.,  network  of  capillaries.] 

may  by  analogy  infer  that  the  fat-cell  becomes  a  fat-cell  by  the  cell  manu- 
facturing fat  in  some  way  or  other,  and  depositing  the  fat  so  formed  in 
the  interstices  of  its  substance.  The  most  striking  superficial  distinctions 
.seem  to  be  that  in  the  mucous  cell  the  granules  or  spherules  remain  discrete 
within  the  cell,  being  separated  by  bars  of  cell  substance,  whereas  in  the  fat- 
cell  the  globules,  as  they  form,  run  together  until  at  last  they  unite  into  a 
single  mass;  and  further  that  while  in  the  mucous  cell,  even  when  most 
heavily  loaded,  a  relatively  large  amount  of  active  cell-substance  still  re- 
mains, in  the  fat-cell  a  mere  remnant  is  left  and  that  chiefly  surrounding  the 
displaced  nucleus. 

Some  observers  are  of  opinion  that  the  cells  belonging  to  connective  tissue 
which  thus  become  fat-cells  of  adipose  tissue  belong  exclusively  to  the  kind 
which  we  spoke  of  as  plasma  cells,  but  this  is  doubtful.  Others  again,  while 
admitting  that  the  cells  which  become  fat-cells  resemble  in  appearance  or- 
dinary connective-tissue  corpuscles  and  may  like  them  be  branched,  believe 
them  nevertheless  to  constitute  a  special  kind  of  connective-tissue  corpuscle, 
being  led  to  this  view  by  the  fact,  that  though  adipose  tissue  is  very  generally 
distributed  throughout  the  connective  tissue  of  the  body,  it  is  apt  to  appear 
in  particular  situations,  rather  than  in  others,  and  in  some  tracts  of  connec- 
tive tissue  never  under  normal  circumstances  makes  its  appearance.  Others 
again  maintain  that,  under  favorable  circumstances,  any  connective  tissue 
corpuscle  may  become  a  fat-cell. 


THE   HISTORY  OF  FAT.      ADIPOSE  TISSUE.  473 

The  fat  in  the  interior  of  bones  forming  the  yellow  marrow  appears  to 
have  the  same  general  structure  and  to  be  formed  in  the  same  way  as  the 
rest  of  the  adipose  tissue. 

§  418.  The  fat  thus  deposited  in  a  fat-cell  sooner  or  later  disappears.  It 
is  not  injected  bodily  into  the  surrounding  lymph-spaces  of  the  connective 
tissue,  but  passes  away  either  into  the  blood  stream  or  into  the  lymphatics 
by  some  processes  not  as  yet  fully  understood.  The  shell  of  cell  substance 
which  forms  the  envelope  of  the  fat-cell  is  probably  of  a  differentiated  nature, 
and  may  have  properties  which  assist  the  escape  of  the  fat ;  but  on  this 
point  we  have  no  exact  knowledge.  The  disappearance  of  the  fat  appears 
to  take  place  in  two  different  ways.  On  the  one  hand,  and  this  perhaps  is 
the  more  ordinary  method,  the  fat  gradually  disappears,  little  by  little,  and 
the  rounded  distended  vesicle  gradually  assumes  the  characters  of  a  connec- 
tive-tissue corpuscle,  even  of  a  branched  one.  On  the  other  hand,  especially 
when  the  disappearance  is  rapid  and  total,  the  space  previously  occupied  by 
fat  becomes  filled  with  a  clear  fluid  resembling  lymph,  the  fat  vesicle  being 
transformed  into  a  lymph  vesicle.  This  condition,  however,  is  temporary 
only,  the  lymph  is  subsequently  absorbed  and  the  vesicle  shrinks.  At  times 
the  emptying  of  the  cell,  whether  by  the  one  method  or  the  other,  is  followed 
by  a  rejuvenescence  of  the  cell,  the  nucleus  by  division  gives  rise  to  several 
nuclei,  and  the  cell  divides  into  new  cells,  each  of  which  may,  under  appro- 
priate conditions,  develop  again  into  a  fat-cell. 

§  419.  The  fat  thus  lodged  in  adipose  tissue  varies  somewhat  in  composi- 
tion in  various  animals,  but  is  chiefly  composed  of  olein,  palmitin,  and  stearin 
in  varying  proportions,  with  small  quantities  of  the  glycerin  compounds  of 
such  fatty  acids  as  butyric,  capronic,  caprylic,  etc.,  together  with  a  little 
lecithin  and  cholesterin.  The  "  fat  "  of  one  animal,  that  is,  the  fat  thus  con- 
tained in  adipose  tissue,  differs  from  the  fat  of  another  animal  partly  by  the 
presence  of  more  or  less  of  one  or  more  of  these  less  abundant  fats,  but 
chiefly  by  the  proportion  in  which  the  three  main  fats,  olein,  palmitin,  and 
stearin,  are  respectively  present  in  the  mixed  fat.  The  melting-points  of 
these  three  fats  being  different,  the  melting-point  of  the  fat  of  the  body  will 
differ  according  to  the  relative  proportions  in  which  the  three  are  present. 
Thus  the  subcutaneous  fat  of  man  melts  at  from  15°  to  22°  or  higher,  the 
fat  round  the  kidney  being  firmer  and  not  melting  until  25°  ;  the  fat  of  the 
dog  melts  at  about  22°,  that  of  the  goose  at  about  25°,  of  the  ox  at  about 
40°,  and  of  the  sheep  at  50°,  the  less  resistant  fat  of  the  man  and  dog  con- 
taining relatively  more  olein  than  that  of  the  ox  or  of  the  sheep. 

§  420.  When  we  come  to  consider  the  question,  By  what  processes  does 
the  fat  make  its  appearance  in  the  fat-cell?  we  are  brought  face  to  face  with 
much  the  same  kind  of  problem  as  that  which  occupied  us  in  dealing  with 
glycogen.  On  the  one  hand  we  may  suppose  that  the  fat  is  brought  to  the 
fat-cell  as  fat  and  is  in  some  way  taken  up  by  the  cell  and  deposited  in  the 
cell  substance  with  little  or  no  change.  On  the  other  hand,  we  may  suppose 
that  the  fat  is  manufactured  by  the  fat-cell,  in  some  such  way  as  mucin  or 
pepsin  is  manufactured  by  a  mucous  or  a  gastric  cell,  out  of  and  by  means 
of  its  cell  substance,  and  that  the  process  of  fattening,  or  of  producing  fat  in 
fat-cells,  consists  essentially  in  feeding  and  so  building  up  the  cell  substance 
which  subsequently  breaks  down  into  fat,  and  does  not  consist  merely  in 
bringing  fat  within  reach  of  the  cell.  Which  of  these  views  is  the  true  one, 
or  how  far  are  both  these  operations  carried  on  in  the  animal  body? 

In  support  of  the  latter  view  it  may  be  urged  that,  not  only  the  more 
complex  living  substance,  but,  as  we  have  more  than  once  urged,  the  simpler 
proteid  constituent  of  living  substance,  obviously  contains  what  we  may  call 
a  fatty  radicle,  so  that  we  might  expect  fat  to  be  formed  out  of  its  metah- 


474  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

olism.  And  as  a  matter  of  fact  not  only  in  adipose  tissue,  but  in  every 
part  of  the  body,  living  substance  is  continuously  giving  rise  to  and  tempo- 
rarily depositing  in  itself  some  amount  of  fat,  and  in  what  is  known  as  fatty 
degeneration  there  seems  to  be  evidence  of  the  formation  of  fat  out  of 
proteid  material. 

On  the  other  hand,  we  have  traced  the  fats  taken  as  food,  and  found  that 
they  pass  with  comparatively  little  change  from  the  alimentary  canal,  chiefly 
through  the  intermediate  passage  of  the  lacteals,into  the  blood,  from  which 
they  rapidly  disappear  after  a  meal.  We  might  infer  from  this  that  an 
excess  of  fat  thus  entering  the  blood  would  naturally  be  disposed  of  by 
being  simply  stored  up  in  the  available  adipose  tissue  without  any  further 
change  ;  we  can  imagine  that  the  fat  not  immediately  wanted  by  the 
economy  passes  in  some  way  from  the  blood  to  the  connective  tissue  (the 
white  blood-corpuscles  which  appear  loaded  with  fat  after  a  meal  possibly 
acting  as  intermediaries),  and  that  the  connective-tissue  corpuscles  swallow 
the  fat  brought  to  them  after  the  fashion  of  an  amoeba,  not  digesting  it  but 
simply  keeping  it  in  store  until  it  is  wanted  elsewhere. 

What  do  experiments  teach  on  this  matter? 

In  the  first  place,  it  is  evident  that  in  an  animal  fattened  on  ordinary  fat- 
tening food,  only  a  small  fraction  of  the  fat  stored  up  in  the  body  can  possi- 
bly come  direct  from  the  fat  of  the  food.  Long  ago  in  opposition  to  the 
views  of  Dumas  and  his  school,  who  taught  that  all  construction  of  organic 
material,  that  all  actual  manufacture  of  living  substance  or  even  of  its 
organic  constituents,  was  confined  to  vegetables  and  unknown  in  animals, 
Liebig  showed  that  the  butter  present  in  the  milk  of  a  cow  was  much  greater 
than  could  be  accounted  for  by  the  scanty  fat  present  in  the  grass  or  other 
fodder  she  consumed.  He  also  urged  as  an  argument  in  the  same  direction, 
that  the  wax  produced  by  bees,  which  though  having  a  different  composition 
from  fat  may  be  used  as  an  analogy,  is  out  of  all  proportion  to  the  wax  or 
allied  bodies  contained  in  their  food,  consisting  as  this  does  chiefly  of  sugar. 
And  it  has  since  been  shown  in  many  ways  that,  in  fattening  animals,  the 
fat  accumulated  in  the  body  cannot  be  accounted  for  by  the  fat  which  has 
been  taken  in  the  food.  It  has  been  proved  by  direct  analysis.  Thus  of 
two  young  pigs,  as  much  alike  as  possible,  of  the  same  litter,  one  was  killed 
and  analyzed,  the  amount  of  fat  in  the  body  being  among  other  things  deter- 
mined. The  other  was  fattened  for  a  certain  length  of  time  on  food  whose 
composition  was  known,  and  then  killed  and  analyzed.  It  was  found  that 
for  every  100  parts  of  fat  in  the  food  472  parts  of  fat  were  stored  up  in  the 
body  during  the  fattening  period.  It  is  clear  that  fat  may  be  formed  in  the 
body  out  of  something  which  is  not  fat. 

§  421.  There  are  two  possible  sources  of  this  manufactured  fat.  The 
carbohydrates  of  the  food  form  one  source.  In  treating  of  digestion  (§  243), 
we  referred  to  the  possibility  of  carbohydrates  during  digestion  in  the  alimen- 
tary canal  becoming  by  fermentation  converted  into  butyric  acid ;  and  we 
suggested  that  higher  and  more  complex  members  of  the  same  fatty  acid  series 
might  be  obtained  out  of  carbohydrates  by  somewhat  analogous  changes, 
carried  on,  however,  not  in  the  alimentary  canal  by  means  of  foreign  organ- 
ized ferments,  but  in  the  tissues  through  the  activity  of  the  tissues  them- 
selves. We  cannot  as  yet  trace  out  the  steps  nor  can  we  definitely  point  to 
any  particular  tissues  other  than  the  fat-cells  themselves  as  the  seats  of  any 
such  changes.  But  there  can  be  no  doubt  that  the  carbohydrate  material 
does  in  some  way  or  other  give  rise  to  fat.  A  carbohydrate  diet  is  the  kind 
of  diet  most  efficacious  in  producing  an  accumulation  of  fat  in  the  body ; 
sugar  or  starch,  in  some  form  or  other,  is  always  a  large  constituent  of 
ordinary  fattening  foods. 


THE  HISTORY   OF  FAT.      ADIPOSE  TISSUE.  475 

Another  source  of  fat  is  to  be  found  in  the  proteids.  We  have  seen  that 
the  urea  of  the  urine  practically  represents  the  whole  of  the  nitrogen  which 
passes  through  the  body.  Now  in  any  given  quantity  of  urea,  the  amount 
of  carbon  is  far  less  than  that  found  in  the  quantity  of  proteid  containing 
the  same  amount  of  nitrogen.  Thus  the  percentage  composition  of  the  two 
being  respectively, 

Carbon.       Hydrogen.       Oxygen.          Nitrogen.        Sulphur. 

Urea 20.00  6.66  26.67  46.67 

Proteid 53  7.30  23.04  15.53  1.13 

100  grms.  of  urea  contain  about  as  much  nitrogen  as  300  grms.  of  proteid ; 
but  the  300  grms.  of  proteid  contain  139  grms.  (159  —  20)  more  carbon  than 
do  the  100  grms.  urea.  Hence  the  300  grms.  of  proteid  in  passing  through 
the  body  and  giving  rise  to  100  grms.  of  urea,  would  leave  behind  139  grms. 
of  carbon,  in  some  combination  or  other  ;  and  this  surplus  of  carbon,  if  the 
needs  of  the  economy  did  not  demand  that  it  should  be  immediately  con- 
verted into  carbonic  acid  and  thrown  off  from  the  body,  might  be  deposited 
somewhere  in  the  form  of  fat.  It  has  been  calculated  that  in  this  way  100 
grms.  of  proteid  food  might  furnish  24  grms.  of  fat.  We  have  already  seen, 
in  treating  of  the  action  of  the  pancreatic  juice  (§  218),  that  there  is  evi- 
dence of  a  fatty  element  (viz.,  leucin,  which  is  amido-caproic  acid,  and  so 
belongs  to  the  fatty  acid  series)  being  thrown  off  from  the  complex  proteid 
compound  in  the  very  process  of  digestion  ;  and  though,  as  we  have  said,  we 
have  no  proof  that  this  action  of  pancreatic  juice  takes  place  largely  in  the 
normal  body,  its  value  as  an  example  is  none  the  less  important. 

Some  observers  have  pushed  this  view  of  the  production  of  fat  out  of 
proteids  so  far  as  to  insist  that  all  the  fat  formed  in  the  body  arises  in  this 
way  out  of  proteid  material,  and  that  when  carbohydrate  food  gives  rise  to 
the  formation  of  fat  it  does  so  by  shielding  from  oxidation  the  carbon  moiety 
of  the  proteid  food  taken  at  the  same  time,  thus  permitting  it  to  be  stored 
up  as  fat.  The  carbohydrate  itself,  they  argue,  never  becomes  fat  but  its 
presence  allows  fat  to  be  formed  out  of  proteid  material.  This  view  has 
obviously  a  very  important  economical  bearing,  since,  if  it  be  true,  it  is  use- 
less to  increase  the  carbohydrate  material  of  food  for  the  purpose  of  fatten- 
ing, unless  a  sufficient  proportion  of  proteid  material  be  given  at  the  same 
time. 

The  view,  however,  has  been  proved  to  be  untenable  by  several  investiga- 
tions carried  out  on  different  animals.  It  has  been  shown  than  an  animal 
rapidly  fattened  on  a  diet  consisting  of  proteids  with  much  carbohydrate  will 
store  up  far  more  fat  than  can  possibly  be  accounted  for  by  the  proteids  of 
the  diet.  Thus  a  dog,  the  fat  in  whose  body  had  been  reduced  to  a  minimum 
by  starvation,  was  fed  for  a  period  on  measured  quantities  of  proteids  and 
carbohydrates,  and  killed.  The  amount  of  fat  found  after  death  in  his  body, 
making  full  allowance  for  the  fat  which  remained  after  the  starvation  and 
for  the  fat  accompanying  the  proteids  in  the  meat  given  as  food,  was  found 
to  be  far  more  than  could  be  supplied  by  the  carbon  in  the  proteids  of  the 
food,  even  supposing  that  every  jot  of  those  proteids  which  did  not  go  to 
make  up  the  increase  of  the  proteid  "  flesh  "  of  the  body  taking  place  during 
the  fattening  was  used  for  the  purpose  of  forming  fat.  Similar  experiments 
on  geese  and  pigs  have  led  to  similar  results ;  and  if  fat  be  formed  in  this 
way  in  the  bodies  of  carnivora  and  omnivora,  we  may  be  sure  that  the  same 
holds  good  for  the  bodies  of  herbivora.  We  may  therefore  conclude  that  fat 
can  be  constructed  in  the  body  on  the  one  hand  out  of  proteid  material  and 
on  the  other  hand  by  some  direct  conversion  of  carbohydrates. 


476  THE  METABOLIC  PKOCESSES  OF  THE  BODY. 

§  422.  It  is  clear  then  that  a  construction  of  fat  does  occur  in  the  body 
somewhere.  What  limits  can  we  place  on  the  degree  to  which  this  construc- 
tion is  carried  ?  When  the  food  contains  sufficient  actual  fat  to  account  for 
the  fat  stored  up  in  the  body,  does  any  construction  of  fat  take  place?  In 
the  first  place  we  find  that  when  the  food  contains  abnormal  fats  such  as  are 
not  present  in  the  body,  spermaceti  for  instance,  or  erucin  (from  rape-seed 
oil),  these  fats  are  not  to  be  found,  or  are  found  in  very  small  quantity,  in 
the  fat  which  is  stored  up  in  the  body  as  a  consequence  of  a  large  supply  of 
that  food.  In  the  second  place  we  may  call  to  mind  the  statement  previously 
made,  that  the  composition  of  fat  varies  in  different  animals.  The  fat  of  a 
man  differs  from  the  fat  of  a  dog,  even  if  both  feed  on  exactly  the  same  food, 
fatty  or  otherwise.  Were  the  fat  which  is  taken  as  food  stored  up  as  adi- 
pose tissue  directly  and  without  change,  recourse  being  had  to  other  sources 
of  food  for  the  construction  of  fat  only  in  cases  where  the  fat  in  the  food  was 
deficient,  we  should  expect  to  find  that  the  nature  of  the  fat  of  the  body 
would  vary  greatly  with  the  food.  So  far  from  this  being  the  case,  direct 
experiments  show  that  the  fat  of  the  dog  is,  as  far  as  composition  is  con- 
cerned, very  largely  independent  of  the  food,  that  the  normal  constituents 
of  fat  make  their  appearance  very  much  as  usual,  and  in  very  much  their 
appropriate  proportion,  though  their  proportion  in  the  food  may  largely  vary, 
and  though  some  of  them  may  be  wholly  absent.  Thus  in  one  experiment 
the  fat  of  the  body  contained  considerable  quantities  of  stearin  after  a  diet 
free  from  stearin,  and  in  another  preserved  the  normal  amount  of  olein  after 
a  diet  free  from  olein. 

Of  course  it  is  quite  possible  that  in  such  cases  as  these,  though  the 
stearin,  or  the  olein,  when  absent  from  the  food,  was  in  some  way  or  other 
constructed  anew,  yet  at  the  same  time  those  constituents  which  were  present 
were  simply  stored  up ;  and  the  small  quantity  of  erucin  present  in  the  fat 
of  the  body  after  feeding  on  erucin  must  have  been  directly  stored  up.  So 
also,  when  an  animal  is  rapidly  fattened  on  a  diet  consisting  of  a  small 
quantity  of  proteid  and  a  large  quantity  of  fat,  the  amount  of  fat  stored  up 
may  be  too  great  to  have  come  from  the  proteids  of  the  diet,  in  which  case  we 
may  infer  that  it  was  the  actual  fat  of  the  food  simply  deposited  in  the  fat-cells 
of  the  body.  But  even  in  this  case,  as  more  distinctly  in  the  others,  it  is  also 
open  for  us  to  suppose  that  all  the  fat  taken  as  food  was  in  some  way  or  other 
disposed  of,  and  that  all  the  new  fat  which  made  its  appearance  was  constructed 
anew.  And  the  latter  view  is  more  perhaps  in  harmony  with  the  histological 
facts  previously  mentioned,  as  well  as  supported  by  other  considerations. 

At  the  present,  however,  we  may  be  content  with  the  following  conclu- 
sions :  1.  Fat  is  actually  formed  in  the  animal  body,  and  the  fat  present  at 
any  moment  in  the  body  is  not  exclusively,  if  at  all,  fat  merely  stored  up 
from  the  fat  of  the  food.  2.  The  carbon  elements  of  the  newly-formed  fat 
may  be  supplied  either  from  carbohydrate  food,  or  from  the  carbon  surplus  of 
proteid  food,  or  from  fats  taken  as  food  which  are  not  the  natural  constitu- 
ents of  the  body-fat.  3.  The  fat  stored  up  appears  as  fat  granules  or  drops 
deposited  in  the  cell- substance  of  certain  cells,  and  the  increase  of  the  fat  in 
the  cells  is  accompanied  first  by  a  growth,  and  subsequently  by  a  consump- 
tion of  the  cell  substance ;  but,  as  in  the  analogous  case  of  glycogen,  there 
is  no  complete  evidence  to  show  whether  the  fat  granules  which  appear  are 
simply  deposited  by  the  cell  substance  in  a  more  or  less  mechanical  manner, 
without  their  forming  an  integral  portion  of  that  cell  substance,  the  chief 
stages  of  the  manufacture  of  the  fat  having  been  gone  through  elsewhere,  or 
whether  they  arise  from  a  breaking  up,  a  functional  metabolism  of  the  cell 
substance  of  the  fat-cell  itself;  the  latter  view  is  on  the  whole,  however,  the 
more  probable. 


THE  MAMMARY  GLAND.  477 

THE  MAMMARY  GLAND. 

§  423.  Since  milk  is  a  secretion,  and  indeed  an  excretion,  the  mammary 
gland  ought  not  to  be  classed  as  a  metabolic  tissue,  in  the  limited  meaning 
we  are  now  attaching  to  those  words.  Yet  the  metabolic  phenomena  giving 
rise  to  the  secretion  of  milk  are  so  marked  and  distinct,  have  so  many 
analogies  with  the  purely  metabolic  events  which  take  place  in  adipose 
tissue,  and  so  strikingly  illustrate  metabolic  events  in  general,  that  it  will 
be  more  convenient  to  consider  the  matter  here,  rather  than  in  any  other 
connection. 

The  mammary  gland,  formed  like  a  sweat  gland,  of  which  it  may  be  con- 
sidered an  extreme  development,  by  an  ingrowth  of  the  Malpighian  layer  of 
the  epidermis,  is  a  compound  racemose  gland,  constructed  after  the  general 
plan  of  such  a  gland  and  thus  composed  of  branching  ducts,  ending  in 
secreting  alveoli. 

§  424.  The  appearances  presented  by  the  alveoli  differ  widely  according 
as  the  gland  is  one  which  is  being  used  for  suckling,  or  is  one  in  a  resting  or 
dormant  condition,  that  is  to  say,  before  any  pregnancy  at  all  has  taken 
place  or  in  the  interval  between  two  suckling  periods.  In  the  suckling  gland 
each  alveolus  consists  of  a  basement  membrane,  presenting  the  usual  charac- 
ters, lined  with  a  single  layer  of  cells  leaving  a  wide  lumen  ;  but  the  appear- 
ances presented  by  the  cells  differ  from  time  to  time  according  to  circum- 
stances and  are  not  the  same  in  all  the  alveoli  at  the  same  time.  We  may, 
however,  distinguish  two  conditions  which,  since  they  seem  to  correspond  to 
the  loaded  and  discharged  conditions  of  an  ordinary  gland,  we  may  call  the 
loaded  and  the  discharged  phase  respectively,  conditions  intermediate  between 
the  two  being  met  with. 

In  the  discharged  phase  the  alveolus  is  lined  by  a  layer  of  low  cubical 
or  even  flattened  cells,  so  that  the  relatively  large  area  of  the  alveolus  is 
almost  wholly  occupied  by  the  lumen  in  which  some  of  the  constituents  of 
the  milk  may  still  be  retained.  Each  cell  consists  of  granular  cell  substance 
in  which  is  placed  a  rounded  or  oval  nucleus.  Sometimes  the  free  edge  of 
the  cell  is  jagged  and  uneven  as  if  a  portion  of  the  free  border  had  been 
torn  away. 

In  a  fully  loaded  phase  the  appearances  are  very  different.  The  alveolus 
is  now  lined  with  a  layer  of  tall  columnar  cells  projecting  unevenly  into  the 
lumen,  the  outline  of  which  is  correspondingly  irregular  and  the  area  of 
which  is  much  reduced.  While  the  broader  base  of  each  cell  rests  on  the 
basement  membrane,  the  other  end,  conical  or  irregular,  stretches  toward  the 
centre  of  the  lumen.  Instead  of  one  nucleus,  two  or  even  more  are  now 
present,  one  well  formed  and  normal  being  placed  nearer  the  base,  and  the 
others,  often  showing  signs  of  breaking  or  degeneration,  nearer  the  free  end. 
Sometimes  constrictions  are  seen  whereby  the  free  peripheral  portion  of  the 
cell,  including  one  or  more  of  the  nuclei,  is  apparently  being  separated  from 
the  basal  portion  in  which  the  remaining  nucleus  is  lodged  ;  and  occasionally 
portions  or  fragments  of  cells,  nucleated  or  nucleusless,  may  be  seen  lying 
in  the  cavity  of  the  alveolus.  In  the  cell  substance,,  especially  toward  the 
free  border  of  the  cell,  are  numerous  oil  globules  of  various  sizes  as  well  as 
granules  or  particles  of  other  nature ;  some  of  the  larger  oil  globules  may  be 
seen  projecting  from  the  surface  as  if  about  to  be  extruded  from  the  cell ; 
and  in  the  cavity  of  the  alveolus  oil  globules  with  a  thinner  or  thicker  coat- 
ing of  cell  substance  are  frequently  present. 

Between  such  a  fully  loaded  phase  and  a  completely  discharged  phase 
various  intermediate  conditions  may  be  observed,  the  cells  being  of  greater 
or  less  height,  containing  one  nucleus  only  or  more  than  one,  the  cell  sub- 


478  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

stance  occupied  with  a  few  or  with  many  oil  globules  and  other  granules, 
and  the  free  border  more  or  less  jagged. 

§  425.  The  dormant  resting  mammary  gland,  that  for  instance  of  an 
animal  which  has  never  been  pregnant,  is  much  smaller  than  a  suckling 
gland,  owing  to  the  alveoli  being  both  smaller  and  less  numerous.  Each 
alveolus,  moreover,  is  not  a  cavity  lined  with  a  single  layer  of  epithelium,  but 
a  solid  cylinder  or  mass  of  comparatively  small,  rounded  or  polyhedral 
cells.  So  long  as  pregnancy  does  not  occur  the  growth  of  these  is  exceed- 
ingly slow,  and  the  products  of  such  metabolism  as  goes  on  in  them  are 
carried  away  by  the  blood,  so  that  under  normal  circumstances  no  secretion 
takes  place. 

When  pregnancy  occurs  rapid  growth  of  the  mamma  takes  place,  numer- 
ous new  alveoli  being  formed  by  budding,  but  all  for  a  time  remaining  solid 
cylinders  of  cells.  At  the  approach  of  the  birth  of  the  offspring,  the 
central  cells  undergo  metabolic  changes,  especially  a  fatty  transformation, 
and  either  before  or  after  birth  are  cast  off,  leaving  a  single  layer  to  line  the 
alveoli  and  to  carry  on  the  work  of  secretion  as  described  above.  It  is 
generally  supposed  that  these  shed  cells  supply  the  so-called  "  colostrum  cor- 
puscles "  characteristic  of  the  first  rnilk,  of  which  we  shall  speak  presently. 
At  the  end  of  lactation  an  absorption  of  some  of  the  alveoli  takes  place, 
and  in  old  age  still  further  absorption  goes  on  with  great  diminution  of  the 
lamina. 

§  426.  The  connective  tissue,  joining  together  the  lobules  of  various 
sizes,  surrounding  the  lobules  and  running  in  between  the  projecting  blind 
ends  of  the  alveoli  within  the  lobules,  is  rich  in  bloodvessels,  which  form 
capillary  networks  round  the  alveoli ;  it  also  carries  a  considerable  number 
of  lymphatic  vessels  which  arise  in  lymph-spaces  around  the  alveoli  and  else- 
where. Leucocytes  are  numerous  in  the  spaces  of  this  connective  tissue, 
and  some  of  them  may  make  their  way  through  the  basement  membrane 
and  between  the  secreting  cells  into  the  cavities  of  the  alveoli  and  so  appear 
in  the  milk. 

§  427.  The  nature  of  milk.  Human  milk  has  a  specific  gravity  of  from 
1028  to  1034,  and  when  quite  fresh  possesses  a  slightly  alkaline  reaction. 
It  speedily  becomes  acid ;  and  cow's  milk,  even  when  quite  fresh,  is  some- 
times slightly  acid,  the  change  of  reaction  taking  place  during  the  stagna- 
tion of  the  milk  in  the  mammary  ducts. 

The  constituents  of  milk  are : 

1.  Proteids,  viz.,  casein,  and  an  albumin,  agreeing  in  its  general  features 
with  ordinary  serum-albumin,  but  which,  since  it  is  said  to  differ  somewhat 
in  its  solubilities  and  rotary  power  from  serum-albumin,  has  been  called 
lactalbumin.  The  casein,  as  we  have  seen  (§  193),  undergoes  through  the 
action  of  rennin  a  change  whereby  insoluble  casein  (tyrein)  makes  its  ap- 
pearance and  the  milk  is  curdled.  Casein  may,  however,  be  precipitated  in 
an  unchanged  form  by  saturating  milk  with  neutral  salts,  or  by  the  careful 
addition  of  acetic  acid  to  diluted  milk,  or  by  first  adding  to  the  diluted  milk 
a  slight  quantity  of  acetic  acid  and  then  passing  through  it  a  stream  of 
carbonic  acid.  In  the  filtrate  the  presence  of  the  lactalbumin,  which  occurs 
in  small  and  variable  quantities,  may  be  shown  by  coagulation  with  heat,  or 
by  precipitation  with  potassium  ferrocyanide,  etc.  In  the  process  of  curdling 
the  casein,  as  stated  in  §  193,  appears  to  be  not  simply  changed  into  tyrein, 
but  to  be  split  up  into  tyrein  and  into  another  proteid,  which  unlike  the 
lactalbumin  is  not  coagulated  by  heat  and  which  appears  to  be  allied  to 
peptone  or  alburnose.  This  or  a  similar  peptone-like  body  has  also  been 
found  in  small  quantities  even  in  milk  which  has  not  curdled  ;  it  has  been 
called  lactoprotein.  The  lactalbumin,  though  coagulated  by  heat  when 


THE  MAMMARY  GLAND.  479 

isolated,  is  not  so  coagulated  as  it  exists  in  the  natural  milk,  the  alkalinity 
of  the  milk,  which  is  increased  by  boiling,  preventing  this.  Similarly 
casein,  though  coagulated  by  heat  when  simply  suspended  in  water  after 
being  precipitated,  is  not  coagulated  by  heat  when  it  exists  in  a  natural  con- 
dition in  milk  ;  in  these  respects  casein  behaves  like  alkali-albumin,  which  it 
resembles  in  other  features  also.  Hence  milk  when  boiled  does  not  coagu- 
late as  a  whole,  though  in  the  superficial  layers  exposed  to  the  air  changes 
take  place  by  which  a  film  or  skin,  derived  chiefly  from  the  albumin  but 
partly  from  the  casein,  appears  on  the  surface ;  if  this  be  removed  a  fresh 
portion  undergoes  the  same  change. 

2.  Fats.    These  are,  in  the  main,  palmitin,  stearin,  and  olein  ;  but  other 
fats,  supplied  by  butyric  and  other  fatty  acids  in  combination  with  glycerin, 
accompany  the  above  in  small  quantities.     In  this  respect  the  fat  of  milk 
resembles  that  of  adipose  tissue.     Lecithin  and  cholesterin  are  also  present 
in  very  small  quantity,  as  well  as  a  yellow  coloring  matter.    The  fat  present 
in  milk  differs  in  different  animals  as  to  the  relative  proportion  of  olein, 
palmitin,  and  stearin,  and  as  to  the  kinds  and  relative  amount  of  the  other 
scantier  fats. 

The  mixture  of  these  fats,  fluid  at  ordinary  temperatures,  is  present  in 
natural  milk  in  the  form  of  globules  of  various  sizes  but  for  the  most  part 
exceedingly  small  (in  man  from  2  //  to  5  />.).  Milk  is  in  fact  a  typical  emul- 
sion, and  it  is  the  presence  of  the  casein  in  the  milk  which  brings  about  the 
emulsion. 

On  standing  a  great  deal  of  the  fat  collects  on  the  top  of  the  milk  in  the 
form  of  cream,  but  in  this,  as  in  the  butter  which  is  formed  from  it,  the 
globules  are  still  discrete,  so  long  at  least  as  the  butter  is  "  fresh."  By  the 
use  of  a  centrifugal  machine  nearly  the  whole  of  the  fat  may  be  separated 
from  the  plasma. 

3.  Milk  sugar  or  lactose.     This  is  very  apt  to  undergo  fermentation  into 
lactic  acid,  through  the  agency  of  an  organized  ferment ;  the  milk  thus 
becomes  sour,  and  the  casein  is  precipitated  in  a  flocculent  form  when  the 
acid  is  produced  in  sufficient  quantity.     Since  the  change  will  take  place 
even  when  every  care  is  taken  to  exclude  germs  from  the  atmosphere  having 
access  to  the  milk,  the  organized  ferments  must  be  present  in  the  milk  in  the 
ducts  of  the  gland. 

4.  Salts.    Though  traces  of  urea  and  kreatinin  have  been  noted  by  some 
observers,  the  extractives  of  milk,  beyond  the  lecithin  and  cholesterin  al- 
ready mentioned,  are  insignificant.      The  salts  are  of  more   importance ; 
these  are  chiefly  calcic  phosphate,  of  whose  function  in  the  process  of  curd- 
ling we  spoke  in   §  193,  and   potassic  and  sodic  chlorides,  with   a  small 
quantity  of  magnesic  phosphate.     Sulphates  appear  to  be  absent.     A  small 
quantity  of  an  iron  salt   is  present,   and    traces   of   sulphocyanide    have 
been  observed.     Besides  the  phosphorus  in  the  actual  form  of  phosphates, 
milk    contains  a  further  considerable  quantity  of  phosphorus  in  the  pro- 
teids  and  in  the  nuclein,  as  well   as  some  sulphur  in   the  former.     The 
inorganic  constituents  of  milk   may,  broadly  speaking,  be  said  to  differ 
distinctly  from  those  of  blood,  and  to  much  more  nearly  resemble  those 
of  the  entire  body. 

The  composition  of  milk  in  the  same  animal  varies  widely  from  time  to 
time,  and  besides  undergoes  marked  changes  during  the  period  of  lactation. 
The  relative  general  composition  of  human  milk  and  that  of  the  cow,  the 
mare,  and  the  bitch  may  perhaps  be  shown  by  the  following  table ;  but  it  is 
difficult  to  draw  an  average,  since  the  individual  analyses  given  differ  so 
much  ;  the  figures  given  for  casein  and  fat  in  the  milk  of  the  bitch  may  be 
unusually  high. 


480  THE  METABOLIC  PROCESSES  OF  THE  BODY. 

Average  Composition  of  Milk  in  Different  Animals. 

Woman.          Cow.          Mare.  Bitch. 


Casein    etc      .     -     .     . 

2 

4 

2  5 

10 

Fats    

2  75 

4 

2 

10 

Su^ar     .    .    .    ,    . 

.  5 

44 

5 

3  5 

Salts           •    . 

0  25 

0  6 

0  5 

0  5 

Total  Solids   .... 

10 

13 

10 

24 

Water    . 

90 

87 

90 

7fi 

The  quantity  of  milk  secreted  by  a  woman  in  twenty  hours  at  the  height 
of  lactation  has  been  calculated  at  700  to  800  c.c.  A  good  milch  cow  will 
yield  about  10  litres  of  milk  per  diem. 

§  428.  Colostrum.  This  is  the  name  given  to  the  milk  secreted  at  the 
beginning  of  a  period  of  lactation,  just  before  and  for  some  days  after  par- 
turition. This  milk  differs  from  the  subsequent  milk  in  microscopical  cha- 
racters and  in  chemical  composition. 

When  ordinary  milk  is  examined  under  the  microscope  hardly  anything 
is  seen  besides  the  fat  globules  except  a  very  few  imperfect  cells  or  portions 
of  cells,  consisting  of  cell  substance  more  or  less  loaded  with  fat  and  con- 
taining sometimes  a  more  or  less  altered  nucleus.  A  few  minute  granules, 
thought  by  some  to  be  particles  of  suspended  casein  or  nuclein,  are,  however, 
also  visible. 

Colostrum,  on  the  other  hand,  contains  a  large  number  of  cells  or  cor- 
puscles, which  have  been  called  "  colostrum  corpuscles."  Some  of  these 
closely  resemble  leucocytes,  others  are  either  cells  of  about  the  same  size, 
round  or  irregular,  and  possessing  a  nucleus,  often  misshapen,  or  are  merely 
portions  of  cell  substance  without  a  nucleus.  In  all  of  them  the  cell  sub- 
tance  may  be  loaded  with  fat  globules  or  may  be  fairly  free  from  fat.  Some 
of  these  cells  appear  to  be  undergoing  disintegration  ;  some  may  at  a  favor- 
able temperature  exhibit  slow  amo3boid  movements,  and  must  then  at  least 
be  regarded  as  living. 

Colostrum  also  differs  from  ordinary  milk  in  containing  not  only  a  large 
quantity  of  albumin  (lactalbumin),  but  also  a  decided  amount  of  globulin. 
In  consequence  of  this,  colostrum  differs  from  milk,  inasmuch  as  it  is  dis- 
tinctly coagulated  by  heat. 

As  stated  above,  during  the  rapid  growth  by  which  the  gland  is  enlarged 
preparatory  to  lactation,  the  alveoli  are  at  first  solid  masses  of  cells  with 
little  or  no  lumen,  and  a  lumen  is  established  subsequently  by  the  discharge 
of  the  central  cells.  It  is  usually  supposed  that  the  cells  so  discharged, 
some  undergoing  much,  others  comparatively  little,  change,  supply  the  colos- 
trum corpuscles  just  spoken  of,  and  at  the  same  time  furnish  the  globulin 
and  excess  of  albumin  also  characteristic  of  colostrum.  But  this  is  not  cer- 
tain. The  alveoli  at  this  time  contain  peculiar  cells  resembling  colostrum 
corpuscles  except  that  they  are  free  from  fat ;  and  it  is  suggested  that  these 
being  discharged  and  taking  up  fat  in  amoeboid  fashion  become  colostrum 
corpuscles.  Some  regard  the  colostrum  corpuscles  as  simply  leucocytes 
which  have  similarly  taken  up  fat. 

§  429.  The  mammary  gland  is  present  both  in  the  female  and  the  male 
child  at  birth  ;  and  in  both  sexes  at  and  for  a  few  days  after  birth  is  thrown, 
in  common  with  all  the  other  secreting  glands,  into  secretory  activity,  and  a 
small  quantity  of  milk,  the  "  witches'  milk,"  so  called  by  the  Germans,  is 
discharged  from  the  nipple.  The  milk  resembles  in  all  essential  features 
the  milk  of  lactation.  In  both  sexes  this  initial  activity  soon  passes  off,  the 
gland  in  the  female  further  developing  at  puberty,  but  in  the  male  remain- 
ing, save  in  exceptional  cases,  in  its  infantile  condition  or  somewhat  retro- 
grading. 


THE  MAMMARY  GLAND.  481 

§  430.  The  secretion  of  milk.  From  what  has  been  already  said,  it  is 
obvious  that  the  secretion  of  milk,  while  resembling  the  secretion  of  the 
other  secreting  glands  which  we  have  studied  in  being  essentially  an  activity 
of  the  epithelial  cells  lining  the  alveoli,  nevertheless  presents  certain  inter- 
esting features  special  to  itself.  If  the  account  given  in  §  424  be  a  true 
one,  morphological  changes  in  the  cells  are  more  prominent  than  in  the  case 
of  other  glands ;  and  we  may  interpret  the  appearances  there  related  some- 
what as  follows :  When  the  discharged  gland  with  its  low  epithelium  begins 
the  work  of  loading,  the  cells  distinctly  "  grow."  Their  cell  substance  in- 
creases in  bulk  and,  elongating,  projects  into  the  lumen  of  the  alveolus.  At 
the  same  time  the  nucleus  divides  as  if  the  cell  were  about  to  give  birth  to 
new  cells ;  but  at  first,  at  all  events,  no  division  of  the  cell  substance  takes 
place,  and  the  new  nuclei  lie  imbedded  in  a  common  cell  body.  The  cell 
substance  meanwhile  puts  on  secretory  activity  ;  it  deposits  in  itself  material 
to  form  milk.  The  deposit  of  fat  is  conspicuous  and  easily  recognized,  but 
we  may  fairly  infer  that  the  other  less  easily  distinguished  proteid  and  car- 
bohydrate materials  are  deposited  in  the  cell  substance  in  a  similar  fashion. 
Then  follows  the  ejection  of  the  prepared  material,  and  this  may  take  place 
in  one  of  two  ways.  The  oil  globules  of  fat  may  be  extruded  from  the  cell 
substance  much  in  the  same  way  that  an  amoeba  extrudes  its  excrement,  and 
possibly  other  constituents  of  milk  may  be  ejected  by  a  similar  method. 
But,  besides  this,  the  deferred  cell  division  now  takes  place  in  a  somewhat 
imperfect  fashion,  so  that  portions  of  the  old  cell  carrying  nuclei  with  them 
come  asunder  from  the  rest  of  the  cell  in  which  a  nucleus  is  left,  and  lie 
loose  in  the  lumen  of  the  alveolus ;  portions  of  cell  substance  free  from 
nuclei  appear  also  to  be  cast  off.  Here,  in  the  lumen  of  the  alveolus,  they 
rapidly  undergo  change  ;  the  cell  substance  is  altered  and  dissolved,  and  its 
load  of  prepared  material,  probably  undergoing  in  the  act  some  further 
change,  is  set  free,  the  nuclei  also  undergoing  change  and  becoming  ulti- 
mately broken  up.  Hence  the  constituents  of  milk  are  provided  for,  not 
only  as  in  other  glands  by  the  material  with  which  the  cell  loads  itself  and 
subsequently  discharges  into  the  lumen  of  the  alveolus,  but  also  by  the 
actual  substance  of  part  of  the  cell  itself.  The  characteristic  nuclein  of  the 
milk  has  thus  its  origin  in  all  probability  in  the  shed  nuclei  of  the  secreting 
cells,  and  we  may  perhaps  infer  that  the  still  more  characteristic  casein 
exists  in  milk  in  the  form  of  casein  and  not  of  some  other  proteid  in  conse- 
quence of  this  intervention  of  the  actual  cell  substance  in  the  formation  of 
the  milk. 

§  431.  The  secretion  of  milk  then  would  appear  to  illustrate,  even  more 
fully  and  clearly  than  do  other  glands,  the  truth  on  which  we  have  so  often 
insisted,  that  a  secretion  is  eminently  the  result  of  the  metabolic  activity  of 
the  secreting  cell.  The  blood  is  the  ultimate  source  of  milk,  but  it  becomes 
milk  only  through  the  activity  of  the  cell,  and  that  activity  consists  largely 
in  a  metabolic  manufacture  by  the  cell,  and  in  the  cell,  of  the  common  things 
brought  by  the  blood  into  the  special  things  present  in  the  milk.  Experi- 
mental results  tell  the  same  tale.  Thus  the  quantity  of  fat  present  in  milk 
is  largely  and  directly  increased  by  proteid,  but  not  increased — on  the  con- 
trary diminished — by  fatty  food.  This  effect  on  the  mammary  gland  in 
particular  is  in  accordance  with  what  we  shall  presently  learn  to  be  the 
general  effect  on  the  body  of  proteid  in  contrast  to  that  of  fatty  food  ; 
proteid  food  seems  to  increase  the  general  metabolic  activity  of  the  body, 
while  fatty  food  tends  to  lessen  it.  Moreover  the  proteid  food  seems  actually 
to  furnish  the  fat ;  and  we  have  already  suggested  a  manner  in  which  pro- 
teids  may  give  rise  to  fat.  That  the  fat  of  the  milk  need  not  necessarily 
come  from  the  fat  of  the  food  is  shown  by  the  following  experiment :  A.  bitch 

31 


482  NUTRITION. 

fed  on  meat  for  a  given  period  gave  off  more  fat  in  her  milk  than  she  could 
possibly  have  taken  in  her  food  ;  and  this  moreover  took  place  while  she  was 
gaining  in  weight  and  "  laying  on  fat,"  so  that  she  could  not  have  supplied 
the  mammary  gland  with  fat  by  simply  transferring  fat  from  the  store  pre- 
viously existing  in  the  adipose  tissue  of  her  body ;  she  apparently  obtained 
the  fat  ultimately  from  the  proteids  of  her  food.  And  the  histological  facts 
given  above  favor  the  view  that  the  formation  of  fat  out  of  proteids  in  such 
cases  takes  place  in  the  cells  of  the  alveoli.  The  experimental  then  as  well 
as  the  histological  evidence  goes  to  show  that  the  fat  of  milk  is  formed  in  the 
cell  and  by  the  cell,  and  is  not  simply  gathered  out  of  the  blood. 

The  casein  in  a  similar  way  seems  to  be  formed  by  the  action  of  the  cell. 
It  cannot  be  gathered  out  of  the  blood,  since  the  blood  contains  no  real 
casein  ;  it  must  be  formed  in  the  gland.  Some  observers  have  maintained 
that  when  milk  is  kept  at  35°,  the  casein  is  increased  through  some  ferment 
action  taking  place  in  the  milk  itself;  but  this  seems  not  to  be  the  case,  and 
the  formation  of  casein  must  be  regarded  as  the  result  of  the  action  of  the 
cell.  Even  the  albumin  present  appears  to  be  not  the  ordinary  serum- 
albumin  simply  passed  from  the  blood  through  the  cell  into  the  lumen  of 
the  alveolus,  but  the  slightly  different  lactalbumin.  We  may  perhaps  regard 
the  albumin  as  less  difficult  to  manufacture  than  the  casein ;  and  we  may 
explain  the  fact  that  relatively  to  the  albumin  the  casein  is  less  at  the  very 
beginning  and  especially  toward  the  end  of  lactation,  by  supposing  that  the 
cell  has  in  the  first  case  not  got  into  full  working  order,  and  in  the  second  case 
is  waning  in  power.  The  peptone-like  body  in  milk,  though  small  in  quantity, 
is  a  further  indication  of  the  proteid  metabolism  taking  place  in  the  cell. 

That  the  milk  sugar,  lactose,  also  is  formed  in  and  by  the  cell,  is  indicated 
by  the  facts  that  it  is  found  in  no  other  part  of  the  body,  and  that  its  pres- 
ence in  milk  is  not  dependent  on  carbohydrate  food,  for  it  is  maintained  in 
abundance  in  the  milk  of  carnivora  when  these  are  fed  exclusively  on  meat, 
as  free  as  possible  from  any  kind  of  sugar  or  glycogen.  A  glycogen-like 
body  has  moreover  been  described  as  existing  in  the  cells,  and  it  is  suggested 
that  this  body  is  the  antecedent  of  the  lactose. 

We  thus  have  evidence  in  the  mammary  gland  of  the  formation,  by  the 
metabolic  activity  of  the  secreting  cell,  of  the  representatives  of  the  three 
great  classes  of  food-stuffs,  proteids,  fats,  and  carbohydrates. 

§  432.  That  both  the  secretion  and  ejection  of  milk  are  under  the  con- 
trol of  the  nervous  system  is  shown  by  common  experience,  but  the  exact 
nervous  mechanism  has  not  yet  been  fully  worked  out.  While  the  erection 
of  the  nipple  ceases  when  the  spinal  nerves  which  supply  the  breast  are 
divided,  the  secretion  continues,  and  is  not  arrested  even  when  the  sym- 
pathetic as  well  as  the  spinal  nerves  are  cut. 


CHAPTEE  V. 

NUTRITION. 
THE  STATISTICS  OF  NUTRITION. 


§  433.  THE  preceding  chapter  has  shown  us  how  wholly  impossible  it  is 
at  present  to  master  the  metabolic  phenomena  of  the  body,  by  attempting  to 
trace  out,  forward  or  backward,  the  several  changes  undergone  by  the  indi- 
vidual constituents  of  the  food,  the  body,  or  the  waste  products.  Another 


THE  STATISTICS  OF  NUTRITION.  483 

method  is,  however,  open  to  us — the  statistical  method.  We  may  ascertain 
the  total  income  and  the  total  expenditure  of  the  body  during  a  given 
period,  and  by  comparing  the  two  maybe  able  to  draw  conclusions  concern- 
ing the  changes  which  must  have  taken  place  in  the  body  while  the  income 
was  being  converted  into  the  output.  Many  researches  have  been  carried 
•out  by  this  method  ;  but  valuable  as  are  the  results  which  have  been  thereby 
gained,  they  must  be  received  with  caution,  since  in  this  method  of  inquiry 
a  small  error  in  the  data  may,  in  the  process  of  calculation  and  inference, 
lead  to  wrong  conclusions.  The  great  use  of  such  inquiries  is  to  suggest 
ideas,  but  the  views  to  which  they  give  rise  need  to  be  verified  in  other 
ways  before  they  can  acquire  real  worth. 

Composition  of  the  animal  body.  The  first  datum  we  require  is  a  know- 
ledge of  the  composition  of  the  body,  as  far  as  the  relative  proportion  of  the 
various  tissues  is  concerned.  In  the  human  body  the  proportions  by  weight 
of  the  chief  tissues,  in  the  fresh  state,  are  probably  somewhat  as  follows : 

Adult  man.  Newborn  baby. 

Per  cent.  Per  cent. 

Skeleton 15.9  17.7 

Muscles 41.8  22.9 

Thoracic  viscera 1.7  3.0 

Abdominal  viscera 7.2  11.5 

Fat 18.2]  900 

Skin 6.9] 

Brain 1.9  15.8 

An  analysis  of  a  cat  has  given  the  following  result : 

Per  cent. 

Muscles  and  tendons , 45.0 

Bones 14.7 

Skin .  12.0 

Mesentery  and  adipose  tissue 3.8 

Liver 4.8 

Blood  (escaping  at  death) 6.0 

Other  organs  and  tissues 13.7 

One  point  of  importance  to  be  noticed  in  these  analyses  is  that  the 
skeletal  muscles  form  nearly  half  the  body ;  we  have  already  seen  (§  38) 
that  about  a  quarter  of  the  total  blood  in  the  body  is  contained  in  them,  and 
have  already  (§  398)  insisted  that  a  large  part  of  the  metabolism  of  the 
body  is  carried  on  in  the  muscles.  Next  to  the  muscles  we  must  place  the 
liver,  for  though  far  less  in  bulk  than  them,  it  is  subject  to  a  very  active 
metabolism  ;  this  is  suggested  by  the  fact  that  it  alone  may  hold  about  a 
quarter  of  the  whole  blood,  and  is  also  indicated  by  the  numerous  facts 
brought  before  us  in  the  preceding  chapter. 

§  434.  The  starving  body.  Before  attempting  to  study  the  influence  of 
food,  it  will  be  useful  to  ascertain  what  changes  occur  in  the  body  when  all 
food  is  withheld.  A  cat  of  known  weight  was  starved  for  thirteen  days. 
At  the  beginning  of  the  period  the  body  was  presumed  to  have  the  compo- 
sition above  given  ;  at  the  close  of  the  period  a  direct  analysis  of  the  body 
was  made.  From  this  it  appeared  that  during  the  hunger  period  the  cat 
hand  lost  734  grammes  of  solid  material,  of  which  248.8  were  fat  and  118.2 
muscle,  the  remainder  being  derived  from  the  other  tissues.  The  percent- 
ages of  dry  solid  matter  lost  by  the  more  important  tissues  during  the  period 
ivere  as  follows : 


484  NUTRITION. 

Per  cent. 

Adipose  tissue 97.0 

Spleen 63.1 

Liver 56.6 

Muscles 30.2 

Blood 17.6 

Brain  and  spinal  cord  .    . 0.0 

Thus,  the  loss  during  starvation  fell  most  heavily  on  the  fat,  indeed  nearly 
the  whole  of  this  disappeared.  Next  to  the  fat,  the  glandular  organs,  the 
tissues  which  we  have  seen  to  be  eminently  metabolic,  suffered  most.  Then 
come  the  muscles,  that  is  to  say,  the  skeletal  muscles,  for  the  loss  in  the 
heart  was  very  trifling ;  obviously  this  organ,  on  account  of  its  importance 
in  carrying  on  the  work  of  the  economy,  was  spared  as  much  as  possible ;  it 
was,  in  fact,  fed  on  the  rest  of  the  body.  The  same  remark  applies  to  the 
brain  and  spinal  cord;  in  order  that  life  might  be  prolonged  as  much  as  pos- 
sible, these  important  organs  were  nourished  by  material  drawn  from  less 
noble  organs  and  tissues.  The  blood  suffered  proportionately  to  the  general 
body-waste,  becoming  gradually  less  in  bulk,  but  retaining  the  same  specific 
gravity  ;  of  the  total  dry  proteid  constituents  of  the  body,  17.3  per  cent,  was 
lost,  which  agrees  very  closely  with  the  17.6  per  cent,  dry  material  (almost 
wholly  proteid)  lost  by  the  blood.  It  is  worthy  of  remark  that  the  tissues 
in  general  become  more  watery  than  in  health.  Similar  observations  on 
other  animals  have  led  to  similar  results,  the  chief  discordance  being  that 
in  some  cases  the  bones  have  suffered  considerable  loss,  in  others  compara- 
tively little.  We  might  be  inclined  to  infer  from  these  data  the  conclu- 
sions that  metabolism  is  most  active  in  the  adipose  tissue,  next  in  such 
metabolic  tissues  as  the  hepatic  cells  and  spleen-pulp,  then  in  the  muscles, 
and  so  on  ;  but  we  have  no  warrant  for  these  conclusions.  Because  the 
Joss  of  cardiac  and  nervous  tissue  was  so  small,  we  must  not  therefore, 
infer  that  their  metabolism  was  feeble ;  they  may  have  undergone  rapid 
metabolism,  and  yet  have  been  preserved  from  loss  of  substance  by 
their  drawing  upon  other  tissues  for  their  material.  The  great  loss  of 
adipose  tissue  is  obviously  to  be  explained  by  the  fact  that  that  tissue 
is  essentially  a  storehouse  of  material,  and  the  similarly  great,  though  less, 
loss  in  the  spleen  and  liver  indicates,  as,  indeed,  the  facts  recorded  in  the 
previous  chapter  suggest,  that  these  organs,  too,  serve  in  part  as  store- 
houses. 

During  the  starvation  period,  the  urine  contained,  in  the  form  of  urea 
(and  that  practically  represents  all  the  nitrogen  of  the  urine),  27.7  grammes 
of  nitrogen.  Now,  the  amount  of  muscle  which  was  lost  during  the  period 
contained  about  15.2  of  nitrogen.  Thus,  more  than  half  the  nitrogen  of 
the  output  during  the  starvation  period  must  have  come  ultimately  from  the 
metabolism  of  muscular  tissue.  This  fact  we  have  already  used  in  discus- 
sing the  history  of  urea,  and  shall  have  occasion  to  make  further  use  of  it 
hereafter.  The  amount  of  urea  excreted  per  diem  has  been  observed,  in 
some  cases,  to  fall  very  rapidly  during  the  first  day  or  two  of  starvation, 
and  then  to  diminish  gradually,  though  often  showing  considerable  irregu- 
larities. In  other  cases  no  such  large  initial  fall  has  been  observed.  It  is 
most  marked  in  animals  which  have  been  well  fed  before  the  beginning  of 
the  starvation,  especially  in  those  which  have  had  a  rich  nitrogenous  diet ; 
and  the  discharge,  in  these  cases,  of  an  extra  quantity  of  urea  in  the  first 
day  or  two  is  obviously  connected  with  that  immediate  effect  of  food  on 
the  excretion  of  urea,  to  which  we  have  already  (§  401)  referred,  and  to 
which  we  shall  have  to  return  in  speaking  of  what  is  known  as  "  luxus- 
consumption." 


THE  STATISTICS  OF  NUTRITION.  485 

Comparison  of  Income  and   Output  of  Material. 

§  435.  Method.  We  have  now  to  inquire  how  the  elements  of  food  are 
distributed  in  the  excreta,  in  order  that,  from  the  manner  of  the  distribu- 
tion, we  may  infer  the  nature  of  the  intermediate  stages  which  take  place 
within  the  body.  By  comparing  the  ingesta  with  the  excreta,  we  shall 
learn  what  elements  have  been  retained  in  the  body,  and  what  elements 
appear  in  the  excreta  which  were  not  present  in  the  food ;  from  these  we 
may  infer  the  changes  which  the  body  has  undergone  through  the  influence 
of  the  food. 

In  the  first  place,  the  real  income  must  be  distinguished  from  the  ap- 
parent one  by  the  subtraction  of  the  feces.  We  have  seen  that  by  far  the 
greater  part  of  the  feces  is  undigested  matter,  i.  e.t  food  which,  though 
placed  in  the  alimentary  canal,  has  not  really  entered  into  the  body.  The 
share  in  the  feces  taken  up  by  matter  which  has  been  excreted*  from  the 
blood  into  the  alimentary  canal,  is  so  small  that  it  may  be  neglected ;  cer- 
tainly, with  regard  to  nitrogen,  the  whole  quantity  of  this  element,  which 
is  present  in  the  feces,  may  be  regarded  as  indicating  simply  undigested 
nitrogenous  matter. 

The  income,  thus  corrected,  will  consist  of  so  much  nitrogen,  carbon,  hy- 
drogen, oxygen,  sulphur,  phosphorus,  saline  matters,  and  water,  contained  in 
the  proteids,  fats,  carbohydrates,  salts,  and  water  of  the  food,  together  with 
the  oxygen  absorbed  by  the  lungs,  skin,  and  alimentary  canal.  The  output 
may  be  regarded  as  consisting  of  (1)  the  respiratory  products  of  the  lungs, 
skin,  and  alimentary  canal,  consisting  chiefly  of  carbonic  acid  and  water, 
with  small  quantities  of  hydrogen  and  carburetted  hydrogen,  these  two  latter 
Doming  exclusively  from  the  alimentary  canal ;  (2)  of  perspiration,  consist- 
ing chiefly  of  water  and  salts,  for  the  dubious  excretion  (see  §  366)  of  urea 
by  the  skin  may  be  neglected,  and  the  other  organic  constituents  of  sweat 
amount  to  very  little ;  and  (3)  of  the  urine,  which  is  assumed  to  contain  all 
the  nitrogen  really  excreted  by  the  body,  besides  a  large  quantity  of  saline 
matters,  and  of  water.  Where  great  accuracy  is  required,  the  total  nitrogen 
of  the  urine  ought  to  be  determined  ;  it  is  maintained,  however,  that  no 
errors  of  serious  importance  arise  when  the  urea  alone,  as  determined  by 
Liebig's  method  (which  was  largely  used  in  the  researches  forming  the  basis 
of  the  present  discussion),  is  taken  as  the  measure  of  the  total  quantity  of 
nitrogen  in  the  urine,  since,  in  this  method,  other  nitrogenous  bodies  besides 
urea  are  precipitated,  and  so  contribute  to  the  quantitative  result.  It  has 
been,  and,  indeed,  still  is,  debated  whether  the  body  may  not  suffer  loss  of 
nitrogen  by  other  channels  than  by  the  urine  and  feces,  whether  nitrogen' 
may  not  leave  the  body  by  the  skin,  or,  indeed,  in  a  gaseous  state,  by  the 
lungs.  The  balance  of  the  conflicting  evidence  seems,  however,  in  favor 
of  the  view  that  no  such  loss  takes  place.  It  would  appear  that  though 
nitrogen,  the  pivot,  so  to  speak,  of  the  chemical  changes  of  living  beings, 
forms  so  large  a  portion  of  the  atmosphere,  and,  moreover,  is  physically 
diffused  through  the  bodies  of  both  plants  and  animals,  free  nitrogen  is  of 
no  chemical  use  to  either  of  them.  It  enters  into  and  remains  in  their 
bodies  as  an  inert  substance,  and  the  nitrogen  which  leaves  a  plant  or  ani- 
mal, in  a  gaseous  state,  is  simply  a  part  of  the  same  inert  supply,  and  does 
not  come  from  the  breaking  up  of  the  nitrogenous  substances  of  the  body 
or  of  the  food. 

Of  these  elements  of  the  income  and  output,  the  nitrogen,  the  carbon, 
and  the  free  oxygen  of  respiration  are  by  far  the  most  important.  Since 
water  is  of  use  to  the  body  for  merely  mechanical  purposes,  and  not  solely 
as  food  in  the  strict  sense  of  the  word,  the  hydrogen  element  becomes  a 


486  NUTRITION. 

dubious  one  ;  the  sulphur  of  the  proteids  and  the  phosphorus  of  the  fats  are 
insignificant  in  amount ;  while  the  saline  matters  stand  on  a  wholly  differ- 
ent footing  from  the  other  parts  of  food,  inasmuch  as  they  are  not  sources 
of  energy,  and  pass  through  the  body  with  comparatively  little  change. 
The  body-weight  must,  of  course,  be  carefully  ascertained  at  the  beginning 
and  at  the  end  of  the  period,  correction  being  made  where  possible  for  the 
feces. 

It  will  be  seen  that  the  labor  of  such  inquiries  is  considerable.  The 
urine,  which  must  be  carefully  kept  separate  from  the  feces,  requires  daily 
measurement  and  analysis.  Any  loss  by  the  skin,  either  in  the  form  of 
sweat,  or,  in  the  case  of  woolly  animals,  of  hair,  must  be  estimated  or  ac- 
counted for.  The  food  of  the  period  must  be,  as  far  as  possible,  uniform  in 
character,  in  order  that  the  analyses  of  specimens  may  serve  faithfully  for 
calculations  involving  the  whole  quantity  of  food  taken ;  and  this  is  espe- 
cially the  case  when  the  diet  is  a  meat  one,  since  portions  of  meat  differ  so 
much  from  each  other.  But  the  greatest  difficulty  of  all  lies  in  the  estima- 
tion of  the  carbonic  acid  produced  and  the  oxygen  consumed.  In  some  of 
the  earlier  researches  this  factor  was  neglected,  and  the  variations  occurring 
were  simply  guessed  at,  through  which  very  serious  errors  were  introduced. 
No  comparison  of  income  and  output  can  be  considered  satisfactory  unless 
at  least  the  carbonic  acid  produced  be  directly  measured  by  means  of  a  res- 
piration chamber.  And  in  order  that  the  comparison  should  be  really  com- 
plete, the  water  given  off  by  the  skin  and  lungs  must  be  directly  measured 
also  ;  but  this  seems  to  be  more  difficult  than  the  determination  of  the  car- 
bonic acid. 

In  the  plan  originally  adopted  by  Regnault  and  Reiset,  and  followed  by  some 
other  observers,  the  animal  experimented  on  is  allowed  to  breathe  a  limited  and 
measured  atmosphere.  The  carbonic  acid,  as  fast  as  it  is  formed,  is  fixed  and 
removed  by  a  strong  solution  of  caustic  potash,  and  the  normal  percentage  of 
oxygen  in  the  atmosphere  is  maintained  by  a  supply  of  this  gas  from  a  gas-holder. 
In  this  way  both  the  oxygen  consumed  and  the  carbonic  acid  produced  are  directly 
determined,  while  the  continual  supply  of  fresh  oxygen  prevents  any  evil  effects 
due  to  breathing  a  confined  portion  of  air.  In  order,  however,  to  avoid  all  possible 
errors  arising  from  a  too  restricted  atmosphere,  a  different  method  has  been  adopted 
by  Pettenkofer  and  Voit.  Their  apparatus  consists  essentially  of  a  large  chamber, 
capable  of  holding  a  man  comfortably.  By  means  of  a  steam-engine  a  current  of 
pure  air,  measured  by  a  gasometer,  is  drawn  through  the  chamber.  Measured 

Eortions  of  the  outgoing  air  are  from  time  to  time  withdrawn  and  analyzed  ;  and 
rom  the  data  afforded  by  these  analyses  the  amounts  of  carbonic  acid  (and  other 
gases)  and  of  water  given  off  by  the  occupant  of  the  chamber  during  a  given  time 
are  determined.  The  oxygen  consumed  is  not  determined  directly;  but  if  the 
total  amounts  of  carbonic  acid  and  of  water  given  out  by  the  lungs  and  skin  are 
ascertained,  and  the  amount  of  urine  and  feces  known,  the  quantity  of  oxygen 
consumed  may  be  arrived  at  by  a  simple  calculation.  For  evidently  the  difference 
between  the  terminal  weight  plus  all  the  egesta  and  the  initial  weight  plus  all  the 
ingesta  can  be  nothing  else  than  the  weight  of  the  oxygen  absorbed  during  the 
period.  This  method  in  turn,  however,  is  also  open  to  objections,  since  minute 
errors  in  the  analyses  of  the  small  samples  of  air  employed  for  the  determinations 
attain  considerable  dimensions  when  these  are  multiplied  so  as  to  give  the  changes 
in  the  whole  mass  of  air  passed  through  the  apparatus.  It  seems,  moreover,  unde- 
sirable to  leave  the  quantity  used  of  so  important  an  element  as  oxygen  to  be  deter- 
mined by  indirect  calculations. 

Let  us  imagine,  then,  an  experiment  of  this  kind  to  have  been  completely 
carried  out ;  that  the  animal's  initial  and  terminal  weights  have  been  accu- 
rately determined ;  the  composition  of  the  food  satisfactorily  known  to  con- 
sist of  so  much  proteid.  fat,  carbohydrates,  salts,  and  water,  and  to  contain 


THE  STATISTICS  OF  NUTRITION.  487 

so  much  nitrogen  and  carbon  ;  the  weight  of  the  feces  and  the  nitrogen  they 
contain  ascertained  ;  the  nitrogen  of  the  urine  determined  ;  the  carbonic 
acid  and  water  given  off  by  the  whole  body  carefully  measured,  and  the 
amount  of  oxygen  absorbed  calculated — what  interpretation  can  be  placed 
on  the  results? 

Let  us  suppose  that  the  animal  has  gained  w  in  weight  during  the  period. 
Of  what  does  w  consist  ?  Is  it  fat  or  proteid  material  which  has  been  laid 
on,  or  simply  water  which  has  been  retained,  or  some  of  one  and  some  of  the 
other?  Let  us  further  suppose  that  the  nitrogen  of  the  urine  passed  during 
the  period  is  less,  say  by  x  grammes,  than  the  nitrogen  in  the  food  taken, 
after  deduction,  of  course,  of  the  nitrogen  in  the  feces.  This  means  that 
x  grammes  of  nitrogen  have  been  retained  in  the  body ;  and  we  may  with 
reason  infer  that  they  have  been  retained  in  the  form  of  proteid  material. 
We  may  even  go  further,  and  say  that  they  are  retained  in  the  form  of  flesh, 
i.  e.,  of  muscle.  In  this  inference  we  are  going  somewhat  beyond  our  tether, 
for  the  nitrogen  might  be  stored  up  as  some  proteid  constituent  of  the  hepatic 
cells  or  of  some  other  tissue;  indeed,  it  might  be  for  the  while  retained  in 
the  form  of  some  nitrogenous  crystalline  body.  But  this  last  event  is 
unlikely  ;  and  if  we  used  the  word  "  flesh  "  to  mean  nitrogen-holding  living 
substance  (proteid)  of  any  kind,  we  may  without  fear  of  any  great  error  reckon 
the  deficiency  of  x  grammes  nitrogen  as  the  storing  up  of  a  grammes  flesh. 
There  still  remain  w  —  a  grammes  of  increase  to  be  accounted  for.  Let  us 
suppose  that  the  total  carbon  of  the  egesta  has  been  found  to  be  y  grammes 
less  than  that  of  the  ingesta ;  in  other  words,  that  y  grammes  of  carbon  have 
been  stored  up.  Some  carbon  has  been  stored  up  in  the  flesh  with  the 
nitrogen  just  considered ;  this  we  must  deduct  from  ?/,  and  we  shall  then 
have  yr  grammes  of  carbon  to  account  for.  Now  there  are  only  two  prin- 
cipal forms  in  which  carbon  can  be  stored  up  in  the  body — as  glycogen  or  as 
fat.  The  former  is,  even  in  most  favorable  cases,  inconsiderable,  and  we 
therefore  cannot  err  greatly  if  we  consider  the  retention  of  y'  grammes  carbon 
as  indicating  the  laying  on  of  b  grammes  fat.  If  a  +  b  are  found  equal  to 
iv,  then  the  whole  change  in  the  economy  is  known  ;  if  w —  (a  -\-  b}  leaves  a 
residue  c,  we  infer  that  in  addition  to  the  laying  on  of  flesh  and  fat  some 
water  has  been  retained  in  the  system.  If  w  —  (a  -f  b)  gives  a  negative 
quantity,  then  water  must  have  been  given  off  at  the  same  time  that  flesh 
and  fat  were  laid  on.  In  a  similar  way  the  nature  of  a  loss  of  weight  can  be 
ascertained,  whether  of  flesh  or  fat  or  of  water,  and  to  what  extent  of  each. 
The  careful  comparison,  the  debtor  and  creditor  account  of  income  and  out- 
put, enables  us,  with  the  cautions  rendered  necessary  by  the  assumptions  just 
now  mentioned,  to  infer  the  nature  and  extent  of  the  bodily  changes.  The 
results  thus  gained  ought,  of  course,  if  an  account  is  kept  of  the  water  taken 
in  and  given  out,  to  agree  with  the  amount  of  oxygen  consumed,  and  also  to 
tally  with  the  conclusions  arrived  at  concerning  the  retention  or  the  reverse 
of  water. 

Having  thus  studied  the  method,  and  seen  its  weaknesses  as  well  as  its 
strength,  we  may  briefly  review  the  results  which  have  been  obtained  by  its 
means. 

§  436.  Nitrogenous  metabolism.  When  a  meal  of  lean  meat,  as  free  as 
possible  from  fat,  is  given  to  a  dog  which  has  previously  been  deprived  of 
food  for  some  time,  and  whose  body  therefore,  is  greatly  deficient  in  flesh,  it 
might  be  expected  that  the  larger  part  of  the  food  would  be  at  once  stored 
up  to  supply  pressing  deficiencies,  and  that  only  the  smaller  part  would  be 
immediately  worked  off  as  urea  corresponding  to  the  nitrogenous  metabolism 
going  on  in  the  body  at  the  time,  increased  somewhat  by  the  labor  thrown 
on  the  economy  by  the  very  presence  of  the  food.  This,  however,  is  not  the 


488  NUTRITION. 

case  as  far  as  the  nitrogen  of  the  meal  is  concerned  ;  the  larger  portion 
passes  off  as  urea  at  once,  and  only  a  comparatively  small  quantity  is 
retained.  If  the  diet  be  continued,  and  we  are  supposing  the  meals  given 
to  be  large  ones,  the  proportion  of  the  nitrogen  which  is  given  off  in  the  form 
of  urea  goes  on  increasing  until  at  last  a  condition  is  established  in  which 
the  nitrogen  of  the  egesta  exactly  equals  that  of  the  ingesta.  This  condition, 
which  is  spoken  of  as  "  nitrogenous  equilibrium,"  is  attained  in  dogs  with  an 
exclusively  meat  diet  only  when  large  quantities  of  food  are  given,  and  it  is 
not  easily  maintained  for  any  length  of  time.  The  exact  quantity  of  meat 
required  to  attain  nitrogenous  equilibrium  varies  with  the  previous  condition 
of  the  dog ;  equilibrium  is  frequently  attained  when  1500  or  1800  grammes 
of  meat  are  given  daily. 

Thus  the  most  striking  effect  of  a  purely  nitrogenous  diet  is  largely  to 
increase  the  nitrogenous  metabolism  of  the  body ;  and  we  shall  see  later  on 
that  it  increases  the  metabolism  not  only  of  the  nitrogenous  but  also  of  the 
other  constituents  of  the  body. 

The  establishment  of  nitrogenous  equilibrium  does  not  mean  that  a  body- 
equilibrium  is  established,  that  the  body-weight  neither  increases  nor  dimin- 
ishes. On  the  contrary,  when  the  meal  necessary  to  balance  the  nitrogen  is 
a  large  one,  the  body  though  it  is  neither  gaining  nor  losing  nitrogen  may 
gain  in  total  weight ;  and  the  increase  is  proved  by  calculation  from  the 
income  and  output,  and  indeed  by  actual  examintion  of  the  body,  to  be  due 
to  the  laying  on  of  fat.  The  amount  so  stored  up  may  be  far  greater  than 
can  possibly  be  accounted  for  by  any  fat  still  adhering  to  the  meat  given  as 
food.  We  are  therefore  driven  to  the  conclusion  that  the  proteid  food  is 
split  into  a  urea  moiety  and  a  fatty  moiety,  that  the  urea  moiety  is  at  once 
discharged,  and  that  such  of  the  fatty  moiety  as  is  not  made  use  of  directly 
by  the  body  is  stored  up  as  adipose  tissue.  And  this  disruption  of  the  pro- 
teid, as  we  have  already  (§  401)  suggested,  explains  at  the  same  time  why 
the  meat  diet  so  largely  and  immediately  increases  the  urea  of  the  egesta. 

The  characteristic  effect  of  proteid  food  to  increase  the  metabolism  of  the 
body  is  shown  on  other  animals  besides  the  dog,  and  not  only  by  means  of 
calculations  of  what  is  supposed  to  take  place  in  the  body,  but  also  by  direct 
analysis.  Thus  the  analysis  of  the  body  of  a  pig,  which  had  been  fed  on  a 
known  diet,  compared  with  the  analysis  of  that  of  another  pig  of  the  same 
litter,  killed  at  the  time  when  the  first  was  put  on  the  fixed  diet,  gave  as  a 
result  that  of  the  dry  nitrogenous  material  of  the  food  only  about  7  per  cent, 
was  laid  up  as  dry  proteid  material  during  the  fattening  period,  though  the 
amount  of  proteid  food  was  low.  This  contrasts  strongly  with  the  amount 
of  fat  stored  up  during  the  same  period  (see  §  420).  Similar  observations 
carried  out  on  sheep  showed  that  in  these  animals  the  storing  up  of  nitro- 
genous material  was  even  less,  only  about  4  per  cent,  of  that  given  in  the 
food. 

Every  quantity  of  proteid  material  taken  into  the  alimentary  canal  thus 
appears  to  affect  proteid  metabolism  in  two  ways.  On  the  one  hand,  it  ex- 
cites a  rapid  proteid  metabolism  giving  rise  to  an  immediate,  and  generally 
large,  increase  of  urea ;  on  the  other  hand,  it  serves  to  maintain  the  more 
regular  normal  proteid  metabolism  continually  taking  place  in  the  body, 
and  so  contributes  to  the  normal  regular  discharge  of  urea.  It  seems  very 
natural  to  suppose  that  the  proteid  which  plays  the  first  of  these  two  parts  is 
not  really  built  up  into  the  tissues,  does  not  become  actual  living  substance, 
but  undergoes  the  changes  which  give  rise  to  urea  outside  the  actual  living 
substance  in  the  blood  or  elsewhere  ;  and  we  have  seen  that  under  the  influ- 
ence of  the  pancreatic  juice  some  of  the  proteid  food  may  undergo  the  greater 
part  of  such  a  change  while  it  is  as  yet  within  the  alimentary  canal.  Heiice 


THE  STATISTICS  OF  NUTRITION.  489 

has  arisen  the  very  natural  distinction  to  which  we  have  already  alluded 
between  "tissue  proteids  "  or  "  morphotic  proteids  "  which  are  actually  built 
up  into  the  living  substance  of  the  tissues  and  give  rise  to  urea  through  the 
metabolism  of  living  substance,  and  "  circulating  proteids  "  or  "  floating  pro- 
teids "  which  do  not  at  any  period  of  their  career  within  the  body  become 
an  integral  part  of  the  living  substance  and  by  their  metabolism  set  free 
energy  not  in  the  way  of  vital  manifestations,  but  in  the  form  of  heat  only. 
We  shall  latter  on  consider  what  is  the  exact  meaning  which  we  ought  to 
attach  to  the  words  "  becoming  part  of  the  living  substance  ;"  and  hence 
shall  defer  until  then  any  discussion  of  the  appropriateness  of  these  phrases 
and  of  the  validity  of  the  distinction  which  they  formulate. 

It  was  once  thought,  as  we  shall  presently  see,  erroneously,  that  the  ex- 
clusive purpose  of  proteid  food  was  to  supply  the  proteid  tissues,  and  that 
all  the  energy  set  free  in  the  body  in  vital  manifestations,  such  as  movement 
and  the  like  as  distinguished  from  heat,  had  its  origin  in  proteid  metabolism, 
the  metabolism  of  fats  and  carbohydrates  giving  rise  to  heat  only.  Hence 
when  it  first  became  known  that  a  certain  proportion  of  proteid  food  appar- 
ently underwent  a  metabolism  giving  rise  to  heat  only,  without  becoming 
part  of  the  tissues,  this  seemed  to  be  a  wasteful  expenditure  of  precious 
material ;  and  the  metabolism  of  this  portion  of  proteid  food  was  accordingly 
spoken  of  as  a  "  luxus-consumption,"  a  wasteful  consumption. 

Before  leaving  this  subject  we  may  call  attention  to  a  possible  analogy 
between  the  history  of  proteids  and  that  of  fats  and  carbohydrates.  The 
uniform  composition  of  the  blood,  which  the  body  seems  ever  striving  to 
maintain,  probably  applies  to  its  proteids  as  well  as  to  its  other  constituents. 
We  have  seen  that  a  surplus  of  non-nitrogenous  materials  in  the  blood  is 
withdrawn  from  the  circulation  and  stored  up  as  fat  or  glycogen,  and  it  is 
possible  that  an  excess  of  proteids  might  similarly  be  stored  up  in  some  tis- 
sue or  tissues,  in  the  hepatic  cells  for  instance,  though  from  the  facts  pre- 
viously mentioned  it  is  obvious  that  the  power  of  storage  is  far  less  than  in 
the  case  of  fats  and  carbohydrates.  Such  a  store  of  proteid  matter  would 
represent  a  sort  of  circulating  proteid,  but  nevertheless  for  its  final  metab- 
olism might  have  to  form  an  integral  part  of  some  living  tissue  unit. 

§437.  The  effects  of  fatty  and  of  carbohydrate  food.  Unlike  those  of 
proteid  food,  the  effects  of  fats  and  carbohydrates  cannot  be  studied  alone. 
When  an  animal  is  fed  simply  on  non-nitrogenous  food,  death  soon  takes 
place ;  the  food  rapidly  ceases  to  be  digested,  and  starvation  ensues.  We 
can  therefore  only  study  the  nutritive  effects  of  these  substances  when  they 
are  taken  together  with  proteid  material. 

When  a  small  quantity  of  fat  is  taken,  in  company  with  a  fixed  moderate 
quantity  of  proteid  material,  the  whole  of  the  carbon  of  the  food  reappears 
in  the  egesta.  No  fat  is  stored  up ;  some  even  of  the  previously  existing  fat 
of  the  body  may  be  consumed.  As  the  fat  of  the  meal  is  increased,  a  point 
is  soon  reached  at  which  carbon  is  retained  in  the  body  as  fat.  So  also  with 
starch  or  sugar;  when  the  quantity  of  this  is  small,  there  is  no  retention  of 
carbon  ;  as  soon,  however,  as  it  is  increased  beyond  a  certain  limit,  carbon  is 
stored  up  in  the  form  of  fat  or,  to  a  smaller  extent,  as  glycogen.  Fats  and 
•carbohydrates,  therefore,  differ  markedly  from  proteid  food  in  that  they  are 
not  so  distinctly  provocative  of  metabolism.  This  is  exceedingly  well  shown 
in  the  results  obtained  on  the  pig  previously  mentioned.  It  was  found  that 
472  units  of  fat  were  laid  on  for  every  100  units  of  fat  taken  as  such  in 
the  food  (which  consisting  of  barley-meal,  etc.  contained  a  very  small  amount 
of  actual  fat),  while  for  every  100  units  of  the  total  dry  non-nitrogenous  food 
including  fat,  starch,  cellulose,  etc.,  no  less  than  21  units  were  retained  in 
the  body  in  the  form  of  fat.  No  clearer  proof  than  this  could  be  afforded 


490  NUTRITION. 

that  fat  is  formed  in  the  body  out  of  something  which  is  not  fat.  In  §  421 
we  have  already  discussed  this  formation  of  fat  out  of  carbohydrates. 

As  one  might  imagine,  the  presence  of  fat  or  carbohydrates  in  the  food 
is  found  to  decrease  the  amount  of  proteid  material  necessary  to  establish 
nitrogenous  equilibrium.  For  instance,  with  a  diet  of  800  grms.  meat  and 
150  grms.  fat,  the  nitrogen  in  the  egesta  became  equal  to  that  in  the  ingesta 
in  a  dog,  in  whose  case  1800  grms.  meat  had  to  be  given  to  produce  the  same 
result  in  the  absence  of  fat  or  carbohydrates. 

On  the  other  hand,  it  was  found  that,  with  a  fixed  quantity  of  fatty  or 
carbohydrate  food,  an  increase  of  the  accompanying  proteid  led  not  to  a 
storing  up  of  the  surplus  carbon  contained  in  the  extra  quantity  of  proteid, 
but  to  an  increase  in  the  consumption  of  carbon.  Proteid  food  increases 
not  only  proteid  but  also  non-nitrogenous  metabolism.  This  explains  how 
an  excess  of  proteid  food  may,  by  the  increase  of  general  metabolism,  actu- 
ally reduce  the  fat  of  the  body. 

We  have  at  present  no  exact  information  concerning  the  nutritive  differ- 
ences between  fats  and  carbohydrates,  beyond  the  fact  that  in  the  final  com- 
bustion of  the  two,  while  carbohydrates  require  sufficient  oxygen  to  combine 
with  their  carbon  only,  there  being  already  sufficient  oxygen  in  the  carbo- 
hydrate itself  to  form  water  with  the  hydrogen  present,  fats  require  in  addi- 
tion oxygen  to  combine  with  some  of  their  hydrogen.  Hence  in  herbivora,. 
living  largely  on  carbohydrates,  a  larger  portion  of  the  oxygen  consumed 
reappears  in  the  carbonic  acid  of  the  egesta  than  in  carnivora,  in  which, 
animals,  living  chiefly  on  proteids  and  fats,  more  of  it  leaves  the  body  com- 
bined with  hydrogen  to  form  water.  This  relation  of  the  oxygen  to  the  car- 
bonic acid  is* often  expressed  as  the  quotient  of  the  volume  of  the  carbonic 
acid  expired  divided  by  the  volume  of  the  oxygen  consumed,  the  "  respira- 

CO 
tory  quotient,"     ™,  which  is  in  herbivora  about  0.9  and  in  carnivora  about 

O2 

0.6  or  0.7.  When  an  herbivorous  animal  starves,  it  feeds  on  its  own  fat,  and 
under  these  circumstances  the  respiratory  quotient  falls  to  the  carnivorous 
standard ;  and  indeed  many  circumstances  affect  this  respiratory  quotient. 
The  carbohydrates  are  notably  more  digestible  than  the  fats,  but  on  the  other 
hand  the  fats  contain  more  potential  energy  in  a  given  weight.  As  to  the 
nutritive  difference  between  starch  and  sugar,  we  know  nothing  very  definite; 
it  has  been  thought,  however,  that  cane-sugar  is  rather  more  fattening  than 
starch. 

§  438.  The  effects  of  gelatin  as  food.  It  is  a  matter  of  common  experi- 
ence that  gelatin  will  not  supply  the  place  of  proteids  as  a  constituent  of 
food.  Animals  fed  on  gelatin  together  with  fat  or  carbohydrates  die  very 
much  in  the  same  way  as  when  they  are  fed  on  non-nitrogenous  material 
alone.  Nevertheless  it  would  appear,  as  might  be  expected,  that  the  presence 
of  gelatin  in  food  is  not  without  effect.  This  nitrogenous  equilibrium  is 
established  at  a  lower  level  of  real  proteid  food  when  gelatin  is  added.  In 
a  dog,  moreover,  fed  on  a  diet  of  gelatin  and  fat,  the  excess  of  nitrogen  in 
the  excreta  over  that  in  the  ingesta  is  less  than  when  the  same  dog  is  fed  on 
a  diet  of  fat  alone  ;  that  is  to  say,  the  gelatin  has  sheltered  from  metabolism 
some  proteid  constituents  of  the  body ;  and  the  consumption  of  fat  seems 
also  to  be  lessened  by  the  presence  of  gelatin.  These  facts  become  intelli- 
gible if  we  suppose  that  gelatin  is  rapidly  split  up  into  a  urea  and  a  fat 
moiety  in  the  same  way  that  we  have  seen  a  certain  quantity  of  proteid 
material  to  be.  It  is  this  direct  destructive  metabolism  of  proteid  matter 
which  gelatin  can  take  up ;  it  seems,  however,  unable  to  imitate  the  other 
function  of  proteid  matter,  and  to  take  part  in  the  formation  of  living  sub- 
stance ;  or  in  the  phraseology  of  a  preceding  paragraph  (§  436),  it  can  take 


THE  STATISTICS  OF  NUTRITION.  491 

the  place  of  circulatiug  but  not  of  tissue  proteid.  What  is  the  cause  of  this 
difference  we  cannot  at  present  say. 

§  439.  Peptone  as  food.  Since  proteids  are  at  least  largely,  as  we  have 
seen  (§  262),  converted  into  and  absorbed  as  peptone,  and  since,  as  we  have 
also  seen,  the  peptone  appears  during  the  very  act  of  absorption  to  be  recon- 
verted into  some  other  form  of  proteid  matter,  possibly  serum-albumin,  it 
might  seem  natural  to  suppose  that  peptone  given  as  food  would,  as  far  as 
metabolism  is  concerned,  play  the  same  part  as  other  proteids.  Neverthe- 
less, some  observers  have  maintained  with  regard  to  both  peptones  and  the 
allied  albumoses  that,  like  gelatin,  these  bodies  "  can  take  the  place  of  cir- 
culating but  not  of  tissue  proteid."  On  the  whole,  however,  the  evidence 
goes  to  show  that  animals  can  "  lay  on  flesh  "  when  the  proteid  in  their  food 
consists  entirely  of  peptone  or  albumose.  A  difficulty  appertaining  to 
digestion  prevents  any  large  substitution  of  peptone  for  ordinary  proteids, 
since  as  might  be  expected  diarrhoea  is  apt  to  be  set  up. 

§  440,  The  effects  of  salts  as  food.  All  food  contains,  besides  the  sub- 
stances possessing  potential  energy,  which  we  have  just  studied,  certain  saline 
matters,  organic  and  inorganic,  having  in  themselves  little  or  no  such  poten- 
tial energy,  but  yet  either  absolutely  necessary  or  highly  beneficial  to  the 
body.  These  must  have  important  functions  in  directing  the  metabolism  of 
the  body  ;  the  striking  distribution  of  them  in  the  tissues,  the  preponderance 
of  sodium  and  chlorides  in  blood-serum  and  of  potassium  and  phosphates  in 
the  red  corpuscles,  for  instance,  must  have  some  meaning ;  but  at  present  we 
are  in  the  dark  concerning  it.  The  element  phosphorus  seems  no  less  im- 
portant, from  a  biological  point  of  view,  than  carbon  or  nitrogen  ;  it  is  as 
absolutely  essential  for  the  growth  of  a  lowly  being  like  penicillium  as  for 
man  himself.  We  find  it  probably  playing  an  important  part  as  the  con- 
spicuous constituent  of  lecithin  and  other  complex  fats  belonging  to  the 
nervous  system  ;  we  find  it  prominent  in  the  peculiar  body  nuclein  ;  we  find 
it  peculiarly  associated  with  the  proteids,  but  we  cannot  explain  its  role.  The 
element  sulphur,  again,  is  only  second  to  phosphorus,  and  we  find  it  as  a  con- 
stituent of  nearly  all  proteids ;  but  we  cannot  foretell  the  exact  changes 
which  would  take  place  in  the  economy  if  all  the  sulphur  of  the  food  were 
withdrawn.  In  the  kreatin  of  the  epidermis  and  its  appendages,  hairs,  etc., 
sulphur  is  probably  undergoing  excretion,  though  its  presence  in  kreatin  may 
have  to  do  with  the  peculiar  physical  characters  of  corneous  epithelium. 

AVe  know  that  the  various  saline  matters  are  essential  to  health  ;  that 
when  they  are  not  present  in  proper  proportions  nutrition  is  affected.  Dogs 
fed  on  food  freed  as  much  as  possible  from  all  saline  matters,  but  otherwise 
abundant,  with  a  proper  proportion  of  the  food-stuffs,  soon  exhibit  symptoms 
showing  that  the  metabolism  of  their  tissues,  especially  of  their  central 
nervous  system,  is  going  wrong ;  they  suffer  from  weakness,  soon  amounting 
to  paralysis,  and  are  often  carried  off  by  convulsions.  And  more  or  less 
similar  derangements  of  nutrition  follow  the  absence  or  a  deficiency  of  indi- 
vidual salts.  During  starvation  these  various  salts  continue  to  be  discharged 
from  the  body ;  in  some  way  or  other  they  are  carried  along  in  the  metabolic 
stream,  and  their  presence  is  in  some  way  essential  to  the  various  metabolic 
processes ;  hence,  they  need  to  be  always  present  in  daily  food.  In  what 
way  it  is  that  they  thus  direct  metabolism  we  do  not  know ;  we  are  aware 
that  the  properties  and  reactions  of  various  proteid  substances  are  closely 
dependent  on  the  presence  of  certain  salts,  but  beyond  this  we  know  very 
little.  The  inorganic  salts  are  those  the  nutritive  value  of  which  has  been 
chiefly  studied  by  experiment,  but  we  have  reason  to  believe  that  the  organic 
salts,  or  extractives,  which  are  present  in  greater  or  less  quantity  in  all  food 
of  both  vegetable  and  animal  origin,  are  no  less  essential  to  the  proper  meta- 


492  NUTRITION. 

bolic  activities  of  the  body.  The  undoubted  connection  of  scurvy  with  the 
lack  of  fresh  vegetable  food,  other  conditions  helping,  may  perhaps  turn  in 
part  on  this,  for  the  evidence  that  the  disease  is  due  to  the  deficiency  of 
potash  alone  is  not  conclusive. 

Lastly,  water  has  an  effect  on  metabolism,  as  shown,  among  other  things, 
by  the  fact  that  when  the  water  of  a  diet  is  increased,  the  urea  is  increased 
to  an  extent  beyond  that  which  can  be  explained  by  the  increase  of  fluid 
increasing  the  facilities  of  mere  excretion. 

THE  ENERGY  OF  THE  BODY. 

The  Income  of  Energy. 

§  441.  Broadly  speaking,  the  animal  body  is  a  machine  for  converting 
potential  into  actual  energy.  The  potential  energy  is  supplied  by  food ; 
this  the  metabolism  of  the  body  converts  into  the  actual  energy  of  heat 
and  mechanical  labor.  We  have  in  the  present  section  to  study  what  is 
known  of  the  laws  of  this  conversion,  and  of  the  distribution  of  the  energy 
set  free. 

Neglecting  all  subsidiary  and  unimportant  sources  of  energy,  we  may  say 
that  the  income  of  animal  energy  consists  in  the  oxidation  of  food  into  its 
waste  products — viz.,  the  oxidation  of  proteids,  fats,  and  carbohydrates  into 
urea,  carbonic  acid,  and  water.  A  principle  laid  down  by  the  chemist  teaches 
that  the  potential  energy  of  any  body,  considered  in  relation  to  any  chemical 
change  which  it  may  undergo,  is  the  same  when  the  final  result  is  the  same, 
whether  that  result  be  gained  at  one  leap  or  by  a  series  of  steps  ;  that,  for 
instance,  the  energy  set  free  by  the  oxidation  of  1  grm.  of  fat  into  carbonic 
acid  and  water  is  the  same,  whatever  the  changes  forward  or  backward  which 
the  fat  undergoes  before  it  finally  reaches  the  stage  of  carbonic  acid  and 
water;  and  similarly,  that  the  energy  available  for  the  body  in  1  grm.  of 
dry  proteid  is  the  energy  given  out  by  the  complete  combustion  of  that  1 
grm.,  less  the  energy  given  out  by  the  complete  combustion  of  that  quantity 
of  urea  to  which  the  1  grm.  of  proteid  gives  rise  in  the  body.  Taking  this 
as  our  guide,  we  can  readily  calculate  the  amount  of  potential  energy  con- 
tained in  an  average  twenty-four  hours'  diet,  and  thus  obtain  the  average 
daily  income  of  energy.  For  the  potential  energy  of  most  of  the  substances 
used  as  food  has  been  determined  by  direct  calorimetric  observations ;  and 
the  several  determinations,  though  they  vary  somewhat,  agree  sufficiently 
closely  to  serve  as  data  for  the  calculations  in  question. 

The  total  combustion  of  the  following  substances  has  given  for  one 
gramme  of  each  substance  the  following  results  expressed  in  calories — that 
is,  in  gramme-degree  units  of  heat : 

Meat,  free  from  fat,  5103  and  5324.  Fibrin,  5511.  Egg-albumin,  5579. 
Thus,  taking  round  numbers,  we  may  say  that  1  grm.  of  proteid  material 
contains  5000  or  5500  calories  of  potential  energy,  according  as  we  use  the 
lower  or  higher  determinations. 

Fat  beef  or  mutton,  9069,  9365,  9423.  Butter,  7267  or  9192.  Again, 
in  round  numbers,  we  may  say  that  1  grm.  of  fat  contains  about  9000 
calories. 

Arrowroot  (nearly  pure  starch),  3912.  Starch,  4123.  Cellulose,  4146. 
Dextrose,  3692.  Cane  sugar,  3866.  Here  again,  taking  round  numbers, 
we  shall  not  be  far  wrong  in  saying  that  the  potential  energy  of  1  grm.  of 
carbohydrate  material  is  about  4000  calories. 

The  combustion  of  1  grm.  of  urea  sets  free  an  amount  of  energy  which 
has  been  determined  by  one  observer  as  2206,  by  another  as  2465  calories. 


THE  ENERGY  OF  THE  BODY.  493 

We  have  seen  (§421)  that  1  grm.  of  proteid  gives  rise  in  the  body  to  J 
grm.  urea.  Hence,  to  obtain  the  energy  of  1  grm.  proteid  material  avail- 
able for  the  economy,  we  must  deduct  from  its  potential  energy  one-third 
the  potential  energy  of  1  grm.  urea — that  is,  in  round  numbers,  700  or  800 
calories.  This  will  give  us  5000  —  700,  or  5500  —  800,  that  is,  4300  or  4700 
calories,  according  as  we  take  the  lower  or  higher  data  ;  or  we  may  take  as 
a  mean  4500  calories.  The  data,  then,  so  far,  are  as  follows : 

1  gramme  proteid 4500  calories. 

1  gramme  fat 9000       u 

1  gramme  carbohydrate 4000       " 

The  average  diet  of  an  average  man — that  is,  the  average  amount  of 
each  food-stuff  respectively  taken  daily — may  be  determined  experimentally 
or  statistically.  Thus,  a  man  may  determine  by  a  series  of  trials  the  diet 
on  which,  while  neither  losing  or  gaining  weight  and  maintaining  "  nitro- 
genous equilibrium  "  (§  436),  he  enjoys  good  health.  Or  an  average  may 
be  struck  of  a  large  number  of  diets  used  by  various  people.  We  shall 
have  something  to  say  of  this  latter  statistical  method  when  we  come  to 
speak  of  diet.  For  the  present  purpose  we  may  use  one  arrived  at  experi- 
mentally, which  we  will  speak  of  as  Ranke's  diet,  since  it  was  determined 
by  a  physiologist  of  that  name  from  observations  on  himself.  It  was  com- 
posed of  1000  grms.  proteid,  100  grms.  fat,  240  grms.  carbohydrate.  Such 
a  diet  would  give 

100  grammes  proteid  (4500) 450,000  calories. 

100  grammes  fat  (9000) 900,000       " 

240  grammes  carbohydrate  (4000) 960,000       " 

2,310,000       " 

If  we  translate  the  units  of  heat  into  units  of  work,  the  2,310,000  gramme- 
degree,  or  2310  kilogramme-degree  calories  will  give  us  about  980,000,  or, 
in  round  numbers,  somewhere  about  one  million  kilogramme-metres. 

We  may,  in  passing,  call  attention  to  the  fact  that  the  proteids  supply  a 
relatively  small  part  of  the  total  energy,  and  that  the  share  contributed  by 
the  large  mass  of  carbohydrates  is  not  much  greater  than  that  belonging  to 
the  much  smaller  quantity  of  fat.  In  the  average  diet  obtained  by  the  sta- 
tistical method,  in  which  the  data  are  largely  drawn  from  public  institutions, 
the  (cheaper)  carbohydrates  are  still  further  increased  at  the  expense  of  the 
(dearer)  fats,  a  change  which  may  tend  to  reduce  somewhat  the  total  energy  ; 
but  this  does  not  materially  affect  the  broad  results  just  given. 

The  Expenditure. 

§  442.  There  are  two  ways  only  in  which  energy  is  set  free  from  the 
body :  mechanical  labor  and  heat.  The  body  loses  energy  in  producing 
muscular  work,  as  in  locomotion  and  in  other  kinds  of  labor,  in  the  move- 
ments of  the  air  in  respiration  and  speech,  and,  though  to  a  hardly  recog- 
nizable extent,  in  the  movements  of  the  air  or  contiguous  bodies  by  the 
pulsations  of  the  vascular  system.  The  body  loses  energy  in  the  form  of 
heat  by  conduction  and  radiation,  by  respiration  and  perspiration,  and  by 
the  warming  of  the  urine  and  feces.  All  the  internal  work  of  the  body,  all 
the  mechanical  labor  of  the  internal  muscular  mechanisms  with  their  accom- 
panying friction,  all  the  molecular  labor  of  the  nervous  and  other  tissues, 
is  converted  into  heat  before  it  leaves  the  body.  The  most  intense  mental 


494  NUTRITION. 

action,  unaccompanied  by  any  muscular  manifestations,  the  most  energetic 
action  of  the  heart  or  of  the  bowels,  with  the  slight  exceptions  mentioned 
above,  the  busiest  activity  of  the  secreting  or  metabolic  tissues,  all  these  end 
simply  in  augmenting  the  expenditure  in  the  form  of  heat. 

A  normal  daily  expenditure  in  the  way  of  mechanical  labor  can  be  easily 
determined  by  observation.  Whether  the  work  take  on  the  form  of  walk- 
ing, or  of  driving  a  machine,  or  of  any  kind  of  muscular  toil,  a  good  day's 
work  may  be  put  down  at  about  150,000  kilogramme-metres. 

The  normal  daily  expenditure  in  the  way  of  heat  cannot  be  so  readily 
determined.  Direct  calorimetric  observations  on  the  whole  body  are  attended 
with  so  many  difficulties,  except  in  the  case  of  small  animals,  that  their  value 
is  uncertain  ;  and  observations  made  by  placing  a  part  only  of  the  body,  an 
arm  or  leg  for  example,  in  the  calorimeter,  and  from  the  data  thus  gained 
calculating  the  heat  produced  by  the  whole  body,  are  subject  to  many  ad- 
ditional sources  of  error. 

The  calorimeters  usually  employed  in  chemical  operations,  in  measuring,  for 
instance,  the  heat  given  out  in  chemical  changes,  are  unsuitable  for  experiments 
on  living  animals.  Such  are  the  mercury  calorimeter,  in  which  the  chemical  action 
to  be  studied  is  made  to  take  place  in  the  midst  of  a  mass  of  mercury,  from  the 
consequent  expansion  of  which  through  the  heat  taken  up  the  amount  of  heat  given 
out  is  calculated,  or  the  ice  calorimeter  in  which  in  a  similar  way  the  heat  given  out 
is  calculated  by  the  amount  of  ice  melted.  The  latter  has  been  used  for  physio- 
logical purposes,  but  an  animal  surrounded  by  ice  is  under  such  abnormal  conditions 
that  the  results  are  of  little  value.  The  methods  usually  adopted  by  physiologists 
are  as  follows : 

In  one  method,  the  water  calorimeter,  the  animal  is  placed  in  a  metal  chamber 
surrounded  by  a  jacket  filled  with  water.  The  heat  given  out  by  the  animal  warms 
the  water  in  the  jacket,  and  the  amount  given  out  is  calculated  upon  the  increase 
of  the  temperature  of  the  water.  By  supplying  the  animal  with  air  through  a  long 
spiral  tube  passing  through  the  water-jacket,  the  heat  given  out  in  the  expired  air 
is  prevented  from  being  lost. 

This  method  may  be  employed  in  a  simpler  form,  when  the  heat  given  out  by  a 
part  of  the  body,  the  arm  or  leg  for  instance,  is  all  that  has  to  be  determined. 
The  part  is  then  merely  placed  in  a  bath  of  water,  from  the  changes  of  tempera- 
ture of  which  the  amount  given  out  is  calculated.  And  this  modification  or  the 
method  may  with  due  precautions  be  employed  for  the  whole  body. 

In  Rqsenthal's  calorimeter  the  chamber  in  which  the  body  or  part  of  the  body 
is  placed  is  surrounded  by,  not  a  water-jacket,  but  an  air-jacket,  which  thus  serves 
as  an  air  calorimeter.  The  instrument  consists  essentially  of  three  concentric 
copper  cylinders  ;  the  inner  one  contains  the  animal  (or  other  source  of  heat) ;  the 
other  one  serves  merely  as  a  casing  to  protect  those  inside  from  changes  of  temper- 
ature due  to  currents  of  air  and  the  like ;  and  the  middle  one  encloses  an  air-space 
between  itself  and  the  inner  one.  There  are  special  arrangements  for  closing  the 
cylinders  after  the  introduction  of  the  animal,  and  for  supplying  the  animal  with 
air  for  breathing  purposes.  With  the  air-jacket,  or  space  between  the  inner  or 
middle  cylinders,  are  connected  a  manometer  and  a  thermometer.  When  an  animal 
(or  other  source  of  heat)  is  placed  in  the  inner  cylinder,  the  temperature  and  the 
pressure  of  the  air  in  the  air-jacket  are  increased ;  and  from  the  amounts  of  increase 
measured  by  the  thermometer  and  the  manometer  the  amount  of  heat  given  out 
from  the  animal  is  calculated. 

The  calorimeters  of  D'Arsonval  and  Rubner  are  constructed  on  very  similar 
principles. 

Various  attempts  have  been  made  to  acertain  the  amount  of  heat  given 
out  by  the  body  in  an  indirect  manner,  as  for  instance  by  calculating  the 
heat  given  out  by  the  oxidation  of  the  food.  As  trustworthy  as  any  is  the 
plan  of  simply  subtracting  the  normal  daily  mechanical  expenditure  from 
the  normal  daily  income.  Thus  150,000  kilogramme-metres  subtracted  from 
one  million  kilogramme-metres  gives  850,000  kilogramme-metres  as  the  daily 


THE  ENERGY  OF  THE  BODY.  495 

expenditure  in  the  form  of  heat ;  i.  e.,  between  one-fifth  and  one-sixth  of  the 
total  income  is  expended  as  mechanical  labor,  the  remaining  four-fifths  or 
five-sixths  leaving  the  body  in  the  form  of  heat.  The  results  given  by  direct 
calorimetric  observations  and  by  other  calculations  give  somewhat  higher 
figures  than  these  ;  and  indeed  these  may  probably  be  taken  as  under  rather 
than  over  the  true  amount.  In  any  case  they  are  to  be  regarded  as  furnishing 
nothing  more  than  a  rough  average,  the  exact  amount  varying  according  to 
the  size,  the  weight,  and  the  condition  of  the  individual,  as  well  as  according 
to  variations  in  circumstances. 

§  443.  The  energy  of  mechanical  ivork.  We  have  already  in  treating  of 
muscle  and  elsewhere  partly  discussed  this  subject,  but  may  here  say  the  rest 
that  has  to  be  said. 

The  older  writers,  even  after  it  had  been  proved  that  the  animal  body  was 
constructive,  as  far  as  the  formation  of  fat  was  concerned,  still  held  to  the 
distinction  between  nitrogenous  or  plastic  and  non-nitrogenous  or  respiratory 
food.  Put  broadly,  this  view  was  that  all  the  nitrogenous  food  went  to  build 
up  the  proteid  tissues,  the  muscular  flesh  and  the  like,  and  that  the  nitro- 
genous egesta  arose  solely  from  the  functional  metabolism  of  these  tissues, 
while  the  non-nitrogenous  food  was  used  with  equal  exclusiveuess  for  respira- 
tory or  calorific  purposes,  being  either  directly  oxidized  in  the  blood,  or,  if 
present  in  excess,  stored  up  as  fatty  tissue.  According  to  this  view  the  two 
classes  of  income  corresponded  exactly  to  the  two  forms  of  expenditure. 
We  have  already  urged  several  objections  against  this  view.  We  have  seen 
that  in  the  blood  itself  very  little  oxidation  takes  place  ;  that  it  is  the  active 
tissue,  and  not  the  passive  blood-plasma,  which  is  the  seat  of  oxidation.  We 
have  further  seen  that  proteid  food  may  undoubtedly  be,  in  the  above  sense, 
respiratory  and  incidentally  give  rise  to  the  storing  up  of  fat.  One  division 
of  the  view  is  thereby  overthrown.  We  have  now  to  inquire  whether  the 
other  division  holds  good,  whether  muscle  and  the  other  proteid  tissues  are 
fed  exclusively  on  the  proteid  material  of  food,  and  whether  muscular 
energy  comes  exclusively  from  the  metabolism  of  the  proteid  constituents  of 
muscle.  We  have  already  seen  (§  63)  that  when  the  muscle  itself  is  exam- 
ined, we  find  no  proof  of  nitrogenous  waste,  but,  on  the  other  hand,  clear 
evidence  of  the  production  of  non-nitrogenous  bodies,  such  as  carbonic  acid. 
And  when  we  ask  the  question,  Does  muscular  exercise  proportionately 
increase  the  urea  given  off'  by  the  body  as  a  whole  ?  for  this  according  to  the 
theory  in  question  it  certainly  ought  to  do,  the  evidence  we  can  obtain, 
though  somewhat  varying,  gives  on  the  whole  a  decidedly  negative  answer. 

In  the  majority  of  observations  no  marked  change  at  all  in  the  amount 
was  met  with ;  indeed,  in  some  cases  there  was  a  distinct  decrease,  followed 
by  an  increase  on  the  following  days.  Some  observers,  however,  found  a 
very  marked  increase,  and  this  was  especially  the  case  when  the  subject 
under  observation  took  a  large  amount  of  food  and  performed  very  severe 
labor.  On  the  whole,  the  various  results  obtained  by  different  observers 
justify  the  conclusion  that  exercise  by  itself,  even  when  severe,  does  not 
necessarily  increase  the  amount  of  urea  excreted,  but  that  conditions  may 
obtain  in  which  such  an  increase  undeniably  occurs.  We  may  draw  the 
further  conclusion  that  experiments  of  this  kind  do  not  supply  the  right 
method  for  determining  the  point  at  issue.  It  must  be  remembered  that  it 
is  not  the  muscles  alone  which  feel  the  influence  of  the  labor  ;  the  circulation 
and  indeed  the  whole  body  are  affected  by  it.  If  we  suppose  a  large  part 
or  even  only  some  part  of  the  urea  to  come  from  other  than  muscular  metab- 
olism, from  changes  in  the  hepatic  cells  for  instance,  we  should  expect  that 
these  changes,  and  with  them  the  amount  of  urea  discharged,  would  be 
influenced  by  labor,  especially  by  severe  labor. 


496  NUTRITION. 

In  no  case  has  a  direct  relation  between  the  amount  of  labor  and 
amount  of  urea  been  observed.  More  than  this,  the  following  experience 
lands  us  in  an  absurdity,  if  we  suppose  the  whole  energy  of  muscular  work 
to  arise  from  proteid  metabolism.  Two  observers  performed  a  certain 
amount  of  work  (an  ascent  of  a  mountain)  on  a  non-nitrogenous  diet,  and 
estimated  the  amount  of  urea  passed  during  the  period.  Assuming  the 
urea  to  represent  the  oxidation  of  so  much  proteid  matter,  which  oxidation 
represented  in  turn  so  much  energy  set  free,  they  found  that,  whereas  the 
actual  work  done  amounted  to  129.026  and  148.656  kilogramme-kilometres 
for  each  observer  respectively,  the  total  energy  available  from  proteid 
metabolism  during  the  period  was  in  the  case  of  the  first  68.69,  and  of  the 
second  68.376  kilogramme-kilometres.  That  is  to  say,  the  energy  set  free 
by  the  proteid  metabolism  of  the  muscles  engaged  in  the  work  was  far  less 
than  the  amount  necessary  to  accomplish  the  work  actually  done,  to  say 
nothing  of  its  having  to  provide  as  well  for  the  movements  of  respiration 
and  circulation.  Their  muscular  energy,  therefore,  must  have  had  other 
sources  than  proteid  metabolism. 

That,  on  the  contrary,  the  production  of  carbonic  acid  is  at  once  and 
largely  increased  by  muscular  exercise  is  beyond  all  doubt.  One  hour's 
hard  labor  will  increase  fivefold  the  quantity  of  carbonic  acid  given  off 
within  the  hour.  And  in  an  experiment  directed  to  this  point  it  was 
found  that  a  man  in  twenty-four  hours  consumed  954  grammes  oxygen  and 
produced  1284  grammes  carbonic  acid  when  doing  work,  as  against  708 
grammes  oxygen  consumed  and  911  grammes  carbonic  acid  produced  when 
remaining  at  rest,  the  quantity  of  urea  secreted  being  in  the  first  case  37 
grammes,  in  the  second  37.2  grammes. 

It  is  evident  that  the  conclusions  arrived  at  by  the  statistical  method 
entirely  corroborate  those  gained  by  an  examination  of  muscle  itself,  viz., 
that  during  muscular  contraction  the  explosive  decomposition  which  takes 
place  bears  chiefly,  if  not  exclusively,  on  the  non-nitrogenous  constituents 
of  the  muscle,  and  that  it  is  the  non-nitrogenous  products  which  alone 
escape  from  the  muscle  and  from  the  body,  any  nitrogenous  products  which 
result  being  retained  within  the  muscle,  or  at  least  within  the  body.  We 
must,  therefore,  reject  the  second  as  well  as  the  first  division  of  the  views 
under  discussion  ;  not  only  is  the  muscle  not  fed  exclusively  on  proteid 
material,  but  also  its  energy  does  not  arise  from  an  exclusively  proteid 
metabolism. 

Animal  Heat. 

§  444.  The  sources  and  distribution  of  heat.  We  have  already  seen 
that  the  conception  of  the  non-nitrogenous  portions  of  food  being  solely 
calorifacient  or  respiratory  proves  to  be  unfounded  when  we  attempt  to 
trace  the  history  of  the  food  on  its  way  through  the  body.  The  same  view 
is  still  more  strikingly  shown  to  be  inadequate  when  we  study  the  manner 
in  which  the  heat  of  the  body  is  produced.  We  may,  indeed,  at  once 
affirm  that  the  heat  of  the  body  is  generated  by  the  chemical  changes, 
which  we  may  speak  of  generally  as  those  of  oxidation,  undergone  not  by 
any  particular  substances,  but  by  the  tissues  at  large.  Wherever  metab- 
olism is  going  on,  or,  to  be  more  exact,  wherever  destructive  metabolism, 
katabolism,  is  going  on,  heat  is  being  set  free.  In  growth  and  in  repair,  in 
the  deposition  of  new  material,  in  the  transformation  of  lifeless  pabulum 
into  living  tissue,  in  the  constructive  metabolism,  the  anabolism  of  the  body, 
and  in  the  smaller  synthetic  processes  of  which  we  spoke  in  dealing  with  urea 
(§  403),  heat  is  undoubtedly  to  a  certain  extent  being  absorbed  and  ren- 


THE  ENERGY  OF  THE  BODY.  497 

dered  latent ;  the  energy  of  the  construction  may  be,  in  part  at  least,  sup- 
plied by  the  heat  present.  But  all  this,  and  more  than  this,  viz.,  the  heat 
present  in  a  potential  form  in  the  substances  themselves  so  built  up  into  the 
tissue,  is  lost  to  the  tissue  during  its  destructive  metabolism  ;  so  that  the 
whole  metabolism,  the  whole  cycle  of  changes  from  the  lifeless  pabulum 
through  the  living  tissue  back  to  the  lifeless  products  of  vital  action,  is 
eminently  a  source  of  heat. 

Of  all  the  tissues  of  the  body  the  muscles,  not  only  from  their  bulk, 
forming  as  they  do  so  large  a  portion  of  the  whole  frame,  but  also  from  the 
characters  of  their  metabolism,  must  be  regarded  as  the  chief  sources  of 
heat. 

In  treating  (§  65)  of  the  thermal  changes  in  muscle  we  have  seen  that 
in  the  total  energy  expended  in  a  muscular  contraction,  the  ratio  of  that 
which  appears  as  heat  to  that  which  appears  as  external  work  is  variable. 
If  we  take  a  proportion  which  is  somewhat  higher  than  the  mean  of  the 
range  there  given  (one-fifth  to  one-twenty-fifth),  and  assume  that  the  energy 
involved  in  the  work  done  in  a  muscular  contraction  is  about  one-tenth 
of  the  total  energy  expended,  the  rest  going  out  as  heat,  then,  upon  the 
calculation  that  the  total  external  work  of  the  body  is  about  one-fifth  of 
the  total  energy  set  free  in  the  body,  it  is  clear  that  the  heat  given  out  by 
the  muscles,  even  if  we  consider  only  the  heat  given  out  when  they  are  con- 
tracting, must  form  a  very  large  part  of  the  total  heat  given  out  by  the 
body.  And  even  if,  as  recent  researches  indicate,  the  muscular  machine 
works  more  economically  than  we  have  hitherto  supposed,  the  amount  of 
heat  given  out  by  the  skeletal  muscles  must  still  remain  very  large.  More- 
over, to  the  skeletal  muscle  we  must  add  the  heart  which,  never  resting, 
does  in  the  twenty-four  hours,  as  we  have  seen  (§  127),  no  inconsiderable 
amount  of  work,  and  must  give  rise  to  no  inconsiderable  amount  of  heat. 
But  the  skeletal  muscles,  though  frequently,  are  not  continually  contract- 
ing ;  they  have  periods,  at  times  long  periods,  of  rest ;  and  during  these 
periods  of  rest,  metabolism,  of  a  subdued  kind  it  is  true,  but  still  a  metab- 
olism involving  an  expenditure  of  energy  is  going  on.  This  quiescent 
metabolism  must  also  give  rise  to  a  certain  amount  of  heat ;  and  if  we  add 
this  amount,  which  in  the  present  state  of  our  knowledge  we  cannot  exactly 
gauge,  to  that  given  out  during  the  movements  of  the  body,  it  is  very  clear, 
even  in  the  absence  of  exact  data,  that  the  metabolism  of  the  muscles  must 
supply  a  very  large  proportion  of  the  total  heat  of  the  body.  They  are 
par  excellence  the  thertnogenic  tissues. 

Next  to  the  muscles  in  importance  come  the  various  secreting  glands. 
In  these  the  secreting  elements,  at  the  periods  of  secretion  at  all  events,  are 
in  a  state  of  metabolic  activity,  which  activity  as  elsewhere  must  give  rise 
to  heat.  In  the  case  of  the  salivary  gland  of  the  dog  the  temperature  of 
the  saliva  secreted  during  stimulation  of  the  chorda  has  been  found  to  be 
as  much  as  1°  or  1.5°  higher  than  that  of  the  blood  in  the  carotid  artery  at 
the  same  time,  and  in  all  probability  the  investigation  of  other  secreting 
glands  would  lead  to  similar  results.  Of  all  these  various  glands  the  liver 
deserves  special  attention  on  account  of  its  size  and  large  supply  of  blood, 
and  because  it  appears  to  be  continually  at  work.  If  there  be  any  truth 
in  the  views  urged  in  the  preceding  chapter  touching  the  large  and  varied 
metabolic  work  of  the  liver,  we  must  conclude  that  a  very  large  amount  of 
heat  is  set  free  in  this  organ  ;  and  that  holds  good  even  if  we  make  a  large 
allowance  for  the  various  synthetic  anabolic  processes  which  may  take  place 
and  by  which  heat  would  be  absorbed  and  made  latent.  We  find,  indeed, 
that  the  blood  in  the  hepatic  vein  is  the  warmest  in  the  body.  Thus  in  the 
dog  a  temperature  of  40.73°  C.  has  been  observed  in  the  hepatic  vein, 

32 


498  NUTRITION. 

while  that  of  the  vena  cava  inferior  was  38.35°  to  39.58°,  and  that  of  the 
right  heart  37.7°.  The  fact  that  the  blood  of  the  hepatic  vein  is  warmer 
than  that  of  either  the  portal  vein  or  the  aorta,  shows  that  the  increased 
temperature  is  not  due  simply  to  the  liver  being  far  removed  from  the  sur- 
face of  the  body. 

The  brain,  too,  may  be  regarded  as  a  source  of  heat,  since  its  tempera- 
ture is  higher  than  that  of  the  arterial  blood  with  which  it  is  supplied ; 
though  from  the  smaller  quantity  of  blood  passing  through  its  vessels,  as 
well  as  from  the  changes  in  it  being  less  massive,  it  cannot,  in  this  respect, 
compare  with  either  the  liver  or  the  muscles  as  a  source  of  heat  to  the 
body. 

The  blood  itself  cannot  be  regarded  as  a  source  of  any  considerable 
amount  of  heat,  since,  as  we  have  so  frequently  urged,  the  oxidations  01 
other  metabolic  changes  taking  place  in  it  are  comparatively  slight.  The 
heat  evolved  by  the  indifferent  tissues,  such  as  bone,  cartilage,  and  connec- 
tive tissue,  may  be  passed  over  as  insignificant ;  and  we  cannot  even  regard 
the  adipose  tissue  as  a  seat  of  the  production  of  heat,  since  the  fat  of  the 
fat-cells  is  in  all  probability  not  oxidized  in  situ,  but  simply  carried  away 
from  its  place  of  storage  to  the  tissue  which  stands  in  need  of  it,  and  it  is  in 
the  tissue  that  it  undergoes  the  metabolism  by  which  its  latent  energy  is  set 
free.  Some  amount  of  heat  is  also  produced  by  the  changes  \vhich  the  food 
undergoes  in  the  alimentary  canal  before  it  really  enters  the  body. 

Hence,  taking  a  survey  of  the  whole  body,  we  may  conclude  that  since 
metabolism  is  going  on  to  a  greater  or  less  extent  everywhere,  heat  is  every- 
where being  generated  ;  but  that,  looked  at  from  a  quantitative  point  of  view, 
the  muscles  and  the  glandular  organs  must  be  regarded  as  the  main  sources 
of  the  heat  of  the  body,  the  muscles  being,  in  all  probability,  the  more  im- 
portant of  the  two. 

§  445.  But  heat,  while  being  thus  continually  produced,  is  as  continually 
being  lost,  by  the  skin,  the  lungs,  the  urine,  and  the  feces.  The  blood  pass- 
ing from  one  part  of  the  body  to  the  other,  and  carrying  warmth  from  the 
tissues  where  heat  is  being  rapidly  generated,  to  the  tissues  or  organs  where 
heat  is  being  lost  by  radiation,  conduction,  or  evaporation,  tends  to  equalize 
the  temperature  of  the  various  parts,  and  thus  maintains  a  "  constant  bodily 
temperature." 

When  the  production  of  heat  is  not  great  as  compared  with  the  loss  there 
is  no  great  accumulation  of  heat  within  the  body,  the  temperature  of  which 
consequently  is  but  slightly  raised  above  that  of  surrounding  objects.  Thus 
the  temperature  of  the  frog,  for  instance,  is  rarely  more  than  0.04°  to  0.05° 
above  that  of  the  atmosphere,  though  in  the  breeding  season  the  difference 
may  amount  to  1°.  Such  animals,  and  they  comprise  all  classes  except  birds 
and  mammals,  are  spoken  of  as  cold-blooded  ;  they  have  been  also  called 
poikilothermic,  that  is,  of  varied  temperature.  Exceptions  among  them  are 
not  uncommon.  Some  fish,  such  as  the  tunny,  are  warmer  than  the  water 
in  which  they  live,  and  in  a  species  of  python  (P.  bivittatus)  a  difference  of 
as  much  as  12°  has  been  observed.  In  a  beehive  the  temperature  may  rise 
at  times  as  much  as  to  40°.  In  the  so-called  warm-blooded  animals,  birds 
and  mammals,  the  loss  and  production  of  heat  are  so  balanced  that  the  tem- 
perature of  the  body  remains  constant  at,  in  round  numbers,  35°  or  40°, 
whatever  be  the  temperature  of  the  air  ;  hence  these  have  been  called  homoio- 
thermic,  of  constant  temperature.  The  temperature  of  man  is  about  37°  ; 
in  some  birds  it  is  as  high  as  44°  (Hirundo),  and  in  the  wolf  it  is  said  to  be 
as  low  as  35.24°. 

This  temperature  is  with  slight  variations  maintained  throughout  life. 
After  death  the  generation  of  heat  rapidly  diminishes,  and  the  body 


THE  ENERGY  OF  THE  BODY.  499 

speedily  becomes  cold  ;  but  for  some  short  time  immediately  following  upon 
systemic  death,  a  rise  of  temperature  may  be  observed,  due  to  the  fact  that, 
while  the  metabolism  of  the  tissue  is  still  going  on,  the  loss  of  heat  is  some- 
what checked  by  the  cessation  of  the  circulation.  The  onset  of  pronounced 
rigor  mortis  causes  a  marked  accession  of  heat,  and  when  occurring  after 
certain  diseases  may  give  rise  to  a  very  considerable  elevation  of  tempera- 
ture. 

This  mean  bodily  temperature  of  warm-blooded  animals  is,  during  health, 
maintained,  with  slight  variations  of  which  we  shall  presently  speak,  within 
a  very  narrow  margin,  a  rise,  or  indeed  a  fall  of  much  more  than  a  degree 
#bove  or  below  the  limit  given  above  being  indicative  of  some  failure  in  the 
organism,  or  of  some  unusual  influence  being  at  work.  It  is  evident,  there- 
fore, that  the  mechanisms  which  coordinate  the  loss  with  the  production  of 
heat  must  be  exceedingly  sensitive.  It  is  obvious,  moreover,  that  the  mech- 
anisms may  act  when  the  bodily  temperature  is  tending  to  rise,  by  either, 
checking  the  production  or  by  augmenting  the  loss  of  heat ;  conversely  when 
the  bodily  temperature  is  tending  to  fall,  they  may  act  by  either  increasing 
the  production  or  by  diminishing  the  loss  of  heat.  As  the  regulation  of  tem- 
perature by  variations  in  the  loss  of  heat  is  better  known  than  regulation  by 
variations  in  production,  it  will  be  best  to  consider  the  former  first. 

§  446.  Regulation  by  variations  in  loss.  Heat  is  lost  to  the  body  by  the 
warming  of  the  feces  and  of  the  urine,  by  the  warming  of  the  expired  air,  by 
the  evaporation  of  the  water  of  respiration,  by  conduction  and  radiation 
from  the  skin,  and  by  the  evaporation  of  the  water  of  perspiration.  It  has 
been  calculated  that  the  relative  amounts  of  the  loss  by  these  several  chan- 
nels are  as  follows :  In  warming  the  feces  and  urine  about  3,  or,  according 
to  others,  6  per  cent.  By  respiration  about  20,  or,  according  to  others, 
about  only  9  per  cent.,  leaving  77,  or  alternately  85,  per  cent,  for  conduction 
and  radiation  and  evaporation  by  the  skin. 

The  two  chief  means  of  loss,  then,  which  are  at  all  susceptible  of  any 
great  amount  of  variation,  and  which  can  be  used  to  regulate  the  tempera- 
ture of  the  body,  are  the  skin  and  the  lungs. 

The  more  air  passes  in  and  out  of  the  lungs  in  a  given  time,  the  greater 
will  be  the  loss  in  warming  the  expired  air,  and  in  evaporating  the  water  of 
respiration.  In  such  animals  as  the  dog,  which  do  not  perspire  freely  by  the 
skin,  respiration  is  a  most  important  means  of  regulating  the  temperature  ; 
and  in  the  dog  a  very  close  connection  may  be  observed  between  the  produc- 
tion of  heat  and  respiratory  activity.  The  changes  which  give  rise  to  this 
loss  take  place  before  the  inspired  air  reaches  the  pulmonary  alveoli ;  both 
the  warming  and  the  evaporation  are  effected  in  the  nasal  and  pharyngeal, 
and  to  some  extent  in  the  bronchial  passages.  Some  observers  have  main- 
tained that  the  left  side  of  the  heart  is  warmer  than  the  right,  and  hence 
have  argued  that  chemical  changes  leading  to  a  considerable  development  of 
heat  take  place  in  the  pulmonary  capillaries.  It  would  appear,  however, 
that  the  right  ventricle,  owing  to  its  lying  nearer  to  the  liver,  the  high  tem- 
perature of  which  has  already  been  mentioned,  is,  in  reality,  rather  hotter 
than  the  left.  And,  indeed,  we  have  no  satisfactory  evidence  of  any  large 
amount  of  heat  being  produced  by  any  pulmonary  metabolism. 

The  great  regulator,  however,  is  undoubtedly  the  skin ;  and  this  has  a 
more  or  less  double  action.  In  the  first  place,  it  regulates  the  loss  of  heat 
by  means  of  the  vasomotor  mechanism.  The  more  blood  passes  through 
the  skin  the  greater  will  be  the  loss  of  heat  by  conduction,  radiation,  and 
evaporation.  Hence  any  action  of  the  vasomotor  mechanism  which,  by 
causing  dilatation  of  the  cutaneous  vascular  areas,  leads  to  a  large  flow 
of  blood  through  the  skin,  will  tend  to  cool  the  body  ;  and,  conversely,  any 


500  NUTRITION. 

vasomotor  action  which,  by  constricting  the  cutaneous  vascular  areas,  or  by 
dilating  the  splanchnic  vascular  areas,  causes  a  smaller  flow  through  the  skin,, 
and  a  larger  flow  of  blood  through  the  abdominal  viscera,  will  tend  to  heat 
the  body.  In  the  second  place,  besides  this,  the  special  nerves  of  perspiration 
will  act  directly  as  regulators  of  temperature,  increasing  the  loss  of  heat 
when  they  promote,  and  lessening  the  loss  when  they  cease  to  promote,  the 
secretion  of  the  skin.  The  working  of  this  heat-regulating  mechanism  is 
well  seen  in  the  case  of  exercise.  Since  every  muscular  contraction  gives 
rise  to  heat,  exercise  must  increase  for  the  time  being  the  production  of  heat; 
yet  the  bodily  temperature  rarely  rises  so  much  as  a  degree  centigrade,  if  at 
all.  By  exercise  the  respiration  is  quickened,  and  the  loss  of  heat  by  the 
lungs  increased.  The  circulation  of  blood  is  also  quickened,  and  the  cuta- 
neous vascular  areas  becoming  dilated,  a  larger  amount  of  blood  passes 
through  the  skin.  Added  to  this,  the  skin  perspires  freely.  Thus  a  large 
amount  of  heat  is  lost  to  the  body,  sufficient  to  neutralize  the  addition 
caused  by  the  muscular  contraction,  the  increase  which  the  more  rapid  flow 
of  blood  through  the  abdominal  organs  might  tend  to  bring  about  being 
more  than  sufficiently  counteracted  by  their  smaller  supply  for  the  time. 
The  sense  of  warmth  which  is  felt  during  exercise  in  consequence  of  the 
flushing  of  the  skin,  is,  in  itself,  a  token  that  a  regulative  cooling  is  being 
carried  on.  In  a  similar  way  the  application  of  external  cold  or  heat  defeats 
its  own  ends,  either  partially  or  completely.  Under  the  influence  of  external 
cold,  the  cutaneous  vessels  are  constricted,  and  the  splanchnic  vascular  areas 
dilated,  so  that  the  blood  is  withdrawn  from  the  colder  and  cooler  regions  to 
the  hotter  and  heat-producing  organs.  This  vascular  change  may  be  used 
to  explain  the  fact  that  stripping  naked  in  a  cold  atmosphere  often  gives  rise 
to  a  distinct  increase  in  the  mean  temperature  of  the  blood,  as  indicated  by 
a  thermometer  placed  in  the  mouth,  though  possibly  the  effect  may  be  partly 
due  to  an  actual  increase  of  the  production  of  heat.  Under  the  influence  of 
external  warmth,  on  the  other  hand,  the  cutaneous  vessels  are  dilated,  a  rapid 
discharge  of  heat  takes  place ;  and  if  the  circumstances  be  such  that  the  body 
can  perspire  freely,  and  the  perspiration  be  readily  evaporated,  the  tempera- 
ture of  the  body  may  remain  very  near  to  the  normal,  even  in  an  excessively 
hot  atmosphere.  Thus,  more  than  a  century  ago,  two  observers  were  able 
to  remain  with  impunity  in  a  chamber  heated  even  to  127°  C.  (260°  Fahr.), 
and  with  ease  in  one  so  hot  that  it  became  painful  for  them  to  touch  the 
metal  buttons  of  their  clothing.  It  is  unnecessary  to  give  any  more  exam- 
ples of  this  regulation  of  temperature  by  variations  in  the  loss  of  heat ;  they 
all  readily  explain  themselves. 

§  447.  The  production  of  heat,  its  variations  and  regulation.  As  we 
have  already  said,  the  exact  determination  of  the  amount  of  heat  produced 
in  the  living  body  is  attended  with  great  difficulties ;  still,  certain  conclu- 
sions have  been  arrived  at  based  partly  on  direct  calorimetric  observations, 
the  more  recent  ones  with  improved  calorimeters  being  especially  valuable, 
and  partly  on  what  seem  to  be  trustworthy  deductions  from  observed  chemi- 
cal changes. 

The  rate  of  production  of  heat  in  a  living  body  is  determined  by  a 
variety  of  circumstances.  In  the  first  place,  what  may  be  called  the  general 
rate  of  metabolism,  and  so  of  the  production  of  heat,  varies  in  different 
kinds  of  animals.  Of  two  animals  of  the  same  bulk  and  weight  placed  under 
the  same  circumstances,  one  "  living  faster  "  than  the  other,  metabolizes  its 
living  substance  more  rapidly,  and  so  produces  heat  more  rapidly.  Thus 
direct  calorimetric  observations,  as  far  as  they  at  present  go,  show  that  a 
man,  on  the  average,  produces  more  heat,  per  kilo,  per  hour,  than  does  a 
dog,  and  a  dog  more  than  a  rabbit.  Probably  every  species  has  what  may/ 


THE  ENERGY  OF  THE  BODY.  501 

be  called  its  specific  coefficient,  and  every  individual  his  personal  coefficient 
of  heat-production,  the  coefficient  being  the  expression  of  the  inborn  qualities 
proper  to  the  living  substance  of  the  species  and  of  the  individual. 

A  larger  living  body  will  naturally  produce  more  heat  than  a  smaller 
living  body  of  the  same  nature,  since  the  larger  body  possesses,  so  to  speak, 
a  greater  number  of  heat-producing  units.  But  this  is  neutralized  by  an 
opposing  tendency.  The  smaller  body,  having  relatively  to  its  bulk  a 
larger  amount  of  surface,  loses  heat  at  a  more  rapid  rate  than  does  the 
larger  body ;  and,  therefore,  to  maintain  the  balance  between  loss  and  pro- 
duction, so"  as  to  secure  the  same  constant  bodily  temperature  (and,  as  we 
have  just  seen,  the  bodily  temperature  of  warm-blooded  animals  is  remarka- 
bly uniform),  it  must  produce  heat,  per  unit  of  its  body,  at  a  more  rapid 
rate.  As  a  rule,  the  greater  loss  of  heat  owing  to  the  relatively  greater 
.surface  is  so  marked  that  of  two  animals  having  the  same  constant  bodily 
temperature,  of  two  species  of  mammals,  or  of  two  individuals  of  the  same 
race,  we  should  expect  the  smaller  one  to  produce  a  relatively  larger  amount 
of  heat.  And  direct  calorimetric  observations  show  that  this  is  so.  The 
struggle  for  existence  has  raised  what  we  have  just  called  the  specific  or 
personal  coefficient  of  the  smaller  animal. 

From  what  we  have  seen  concerning  the  immediate  effects  of  a  meal,  we 
should  be  inclined  to  expect  that  food  would  temporarily  increase  the  pro- 
duction of  heat ;  and  not  only  is  this  view  confirmed  by  common  experience 
and  by  our  own  sensations,  but  direct  calorimetric  observations  afford  ex- 
perimental proof  of  its  truth.  In  the  dog  it  has  been  found  that  the  rate  of 
production  increases  after  a  meal,  reaching  its  maximum  from  the  sixth  to 
the  ninth  hour,  and  then  declining  to  a  level  which  may  be  regarded  as  that 
secured  by  the  general  metabolism  of  the  body,  and  which  appears  to  be 
maintained  with  remarkable  constancy  until  after  long  starvation  the 
economy  begins  to  break  down. 

Labor,  muscular  work,  has  a  powerful  influence  in  increasing  the  pro- 
duction of  heat.  As  we  have  seen,  of  the  total  heat  produced  in  the  body, 
a  certain  portion  must  always  be  attributed  to  muscular  contractions,  which 
^ven  in  the  most  quiet  body  are  always  going  on  ;  in  an  ordinary  active 
body  a  considerable  quantity  of  heat  must  be  thus  generated.  Hence,  the 
more  active  the  body  the  greater  the  production  of  heat.  As  we  stated 
before  (§  85),  in  a  contraction  the  proportion  of  the  energy  set  free  to  do 
work  to  that  set  free  as  heat  appears  to  vary  under  different  circumstances  ; 
and  the  increase  of  heat  due  to  labor  probably  varies  in  a  corresponding 
way.  The  details  of  this  relation  have  yet  to  be  worked  out,  but  we  may  at 
least  conclude  that,  when  a  man  pushes  his  daily  labor  beyond  the  150,000 
kilogramme-metres,  the  additional  energy  thus  leaving  his  body  as  work  done 
is  not  taken  out  of  the  850,000  kilogramme-metres  given  in  §  442  as  the 
average  daily  output  of  heat,  but  the  total  setting  free  of  energy  and  the 
total  production  of  heat  is  at  the  same  time  increased.  And  it  need  hardly 
be  said  that  the  figures  in  question  give  only  an  average  estimate  for  a  man 
of  average  build  and  weight,  taking  an  average  amount  of  average  food, 
and  doing  an  average  amount  of  work. 

§  448.  The  production  of  heat  thus  determined  by  these  several  influ- 
ences, some  of  which  are  themselves  regulated  by  the  nervous  system,  is 
further  regulated  in  a  remarkable  manner.  For  it  is  not  solely  by  variations 
in  the  loss  of  heat  that  the  constant  temperature  of  the  warm-blooded  animal 
is  maintained.  Variations  in  the  amount  of  heat  actually  generated  in  the 
body  constitute  an  important  factor,  not  only  in  the  maintenance  of  the 
normal  temperature,  but  also  in  the  production  of  the  abnormally  high  or 
low  temperatures  of  various  diseases.  Many  considerations  have  long  led 


502  NUTRITION. 

physiologists  to  suspect  the  existence  of  a  nervous  mechanism,  by  which 
afferent  impulses  arising  in  the  skin  or  elsewhere  might,  through  the  central 
nervous  system,  originate  efferent  impulses,  whose  effect  would  be  to  in- 
crease or  to  diminish  the  metabolism  of  the  muscles  or  other  organs,  and 
thus  to  increase  or  diminish  the  amount  of  heat  generated  for  the  time 
being  in  the  body.  The  existence,  in  fact,  of  a  metabolic  or  thermogenic 
nervous  mechanism,  comparable  in  many  respects  to  the  vasomotor  mechan- 
ism or  to  the  various  secreting  nervous  mechanisms,  seems  in  itself  a  priori 
probable.  And  we  have  experimental  evidence  that  such  a  mechanism  does 
really  exist. 

The  warm-blooded  animal  is  distinguished  from  the  cold-blooded  animal 
by  the  fact  that  when  it  is  exposed  to  cold  or  heat  it  does  not,  like  the  latter, 
become  colder  or  hotter,  as  the  case  may  be,  but,  within  certain  limits,  main- 
tains its  normal  temperature.  If  the  maintenance  of  the  temperature  of  the 
warm-blooded  animal  during  exposure  to  cold  is  assisted  by  an  increased 
production  of  heat,  and  is  not  due  simply  to  a  diminished  loss,  there  should  be 
evidence  of  an  increased  metabolism  during  that  exposure.  We  ought  to  find,, 
under  these  circumstances,  an  increased  production  of  carbonic  acid  and  an 
increased  consumption  of  oxygen,  since  it  is  to  these  products,  rather  than 
to  the  nitrogenous  factors,  on  the  peculiarities  of  which  as  uncertain  signs- 
of  metabolism  we  have  already  insisted,  we  must  look  for  indications  of  the 
rise  or  fall  of  metabolic  activity. 

Taking  the  consumption  of  oxygen,  and,  though  with  less  confidence,, 
the  production  of  carbonic  acid,  as  a  measure  of  metabolic  activity  and  so- 
of  heat-production,  it  has  been  shown  that  a  marked  contrast  in  this  respect 
exists  between  cold-blooded  and  warm-blooded  animals  exposed  to  changes 
of  temperature.  In  the  cold-blooded  animal,  cold  diminishes  and  heat  in- 
creases the  metabolic  activity  of  the  body;  as  the  temperature  to  which  the 
animal  is  subjected  rises  or  falls,  so  the  consumption  of  oxygen  and  produc- 
tion of  carbonic  acid  is  increased  or  lessened.  The  body  of  a  cold-blooded 
animal  behaves  in  this  respect  like  a  mixture  of  dead  substances  in  a  chem- 
ist's retort ;  heat  promotes  and  cold  retards  chemical  action  in  both  cases. 
Very  different  is  the  behavior  of  a  warm-blooded  animal.  In  this  case,  within 
a  lower  and  a  higher  limit,  cold  increases  and  heat  diminishes  the  bodily 
metabolism,  as  shown  by  the  increased  or  diminished  consumption  of  oxygen 
and  production  of  carbonic  acid  as  the  temperature  falls  or  rises.  In  these 
animals  there  is  obviously  a  mechanism  of  some  kind,  counteracting,  and 
indeed  overcoming,  the  more  direct  effects  which  alone  obtain  in  cold- 
blooded animals.  And  that  this  mechanism  is  of  a  nervous  nature  is  indi- 
cated by  the  following  facts  : 

When  a  warrn-blooded  animal  is  poisoned  byurari,the  temperature  falls 
and  the  metabolism,  measured  by  the  consumption  of  oxygen  and  the  pro- 
duction of  carbonic  acid,  sinks  also ;  and  that  the  latter  is  the  cause,  not  the 
effect,  of  the  former  is  shown  by  the  fact  that  the  metabolism  continues  ta 
fall  though  loss  of  heat  be  prevented  by  surrounding  the  animals  with  wrap- 
pings of  cotton-wool.  In  such  a  urarized  animal,  exposure  to  higher  tem- 
peratures augments  and  exposure  to  lower  temperatures  diminishes  metab- 
olism; the  urarized  warm-blooded  animal,  in  fact,  behaves  like  a  cold-blooded 
animal.  Similar,  but  perhaps  not  such  striking  or  so  constant  results,  are 
gained  by  division  of  the  medulla  oblongata.  After  this  operation  the  tem- 
perature of  the  body  sinks,  and  the  fall,  though  partly  due  to  increased  loss 
of  heat  by  the  skin,  caused  by  the  dilated  condition  of  the  cutaneous  vessels, 
is  also  accompanied  by  diminished  metabolism,  and  is,  therefore,  in  part  due 
to  diminished  production  of  heat.  And  when  an  animal  is  in  this  condition, 
exposure  to  higher  temperatures  increases  and  exposure  to  lower  tempera- 


THE  ENERGY  OF  THE  BODY.  503 

tures  diminishes  the  bodily  metabolism.  We  can  best  explain  these  results 
by  supposing  that,  under  normal  conditions,  the  muscles,  which  as  we  have 
seen  contribute  so  largely  to  the  total  heat  of  the  body,  are  placed,  by  means 
of  their  motor  nerves  and  the  central  nervous  system,  in  some  special  con- 
nection with  the  skin,  so  that  a  lowering  of  the  temperature  of  the  skin  leads 
to  an  increase,  while  a  heightening  of  the  temperature  of  the  skin  leads  to  a 
decrease  of  the  muscular  metabolism.  Further,  the  centre  of  this  thermo- 
taxic  reflex  mechanism  appears  to  be  placed  somewhere  in  the  nervous  system 
above  the  spinal  cord.  When  urari  is  given,  the  reflex  chain  is  broken  at 
its  muscular  end ;  when  the  spinal  cord  is  divided,  the  break  is  nearer  the 
centre. 

We  may  add  that  the  muscular  metabolism  which  thus  helps  to  regulate 
temperature  need  not  involve  visible  muscular  contractions.  At  the  same 
time,  the  heat  given  out  by  the  muscles  will  be  temporarily  increased  at 
every  contraction  which  may  occur.  Thus,  the  shivering  which  follows 
exposure  to  cold  distinctly  helps  to  warm  the  body ;  indeed,  some  observers 
have  been  led  to  think  that,  in  man,  this  visible  effect  of  cold  plays  a  more 
important  part  in  his  heat  regulation  than  the  invisible  actions  which  we 
have  just  described.  We  may  also  add  that  the  regulative  nervous  mechan- 
ism may  apparently  be  overborne  by  an  exposure  to  too  great  heat  or  cold. 
When,  for  instance,  the  cold  to  which  the  animal  is  exposed  becomes  exces- 
sive, the  reaction  of  the  thermotaxic  nervous  system  is  powerless  against  the 
direct  action  on  the  tissues  of  the  depressing  influences,  and  the  metabolism, 
together  with  the  temperature,  sinks. 

§  449.  In  a  number  of  experiments  it  has  been  shown  that  injuries  to, 
such  as  those  caused  by  puncture  or  galvanic  cautery,  or  electrical  stimula- 
tion of,  limited  portions  of  the  more  central  portions  of  the  brain  may  give 
rise  to  a  great  increase  of  the  temperature  of  the  body  without  producing 
any  other  marked  symptom.  The  increase  is  shown  by  the  increase  of  met- 
abolism, increased  production  of  carbonic  acid,  and  increased  consumption 
of  oxygen,  as  well  as  by  direct  calorimetric  observations,  to  be  due  to  an 
increased  production  of  heat.  This  naturally  suggests  that  the  portions  of 
the  brain  in  question  contain  the  hypothetical  heat  centre  just  mentioned, 
the  lesion  on  stimulation  exciting  the  centre  to  activity  by  direct  action  on 
it,  instead  of  in  the  usual  reflex  manner.  The  matter  has  not,  however,  as 
yet  been  clearly  worked  out ;  and  indeed  observers  are  not  agreed  as  to  the 
exact  parts  of  the  brain  injury  to  which,  or  stimulation  of  which,  produces 
the  effect. 

§  450.  By  regulative  mechanisms  of  the  kind  just  discussed  the  tem- 
perature of  the  warm-blooded  animal  is  maintained  within  very  narrow 
limits.  In  ordinary  health  the  temperature  of  a  man  varies  between  36° 
and  38°,  the  narrower  limits  being  36.25°  and  37.5°,  when  the  thermometer 
is  placed  in  the  axilla.  In  the  mouth  the  reading  of  the  thermometer  is 
somewhat  (0.25°  to  1.5°)  higher ;  in  the  rectum  it  is  still  higher  (about 
0.9°)  than  in  the  mouth.  The  temperature  of  infants  and  children  is 
slightly  higher  and  much  more  susceptible  of  variation  than  that  of  adults, 
and  after  forty  years  of  age  the  average  maximum  temperature  (of  health) 
is  somewhat  lower  than  before  that  epoch.  A  diurnal  variation,  independent 
of  food  or  other  circumstances,  has  been  observed,  the  maximum  ranging 
from  9  A.M.  to  6  P.M.  and  the  minimum  from  11  P.M.  to  3  A.M.  Meals  cause 
sometimes  a  slight  elevation,  sometimes  a  slight  depression,  the  direction  of 
the  influence  depending  on  the  nature  of  the  food — alcohol  seems  always  to 
produce  a  fall.  Exercise  and  variations  of  external  temperature,  within 
ordinary  limits,  cause  a  very  slight  change,  on  account  of  the  compensating 
influences  which  have  been  discussed  above.  The  rise  from  even  active 


504  NUTRITION. 

exercise  does  not  amount  to  1°  ;  when  labor  is  carried  to  exhaustion  a  de- 
pression of  temperature  may  be  observed.  In  travelling  from  very  cold  to 
very  hot  regions  a  variation  of  less  than  a  degree  occurs,  and  the  tempera- 
ture of  inhabitants  of  the  tropics  is  practically  the  same  as  of  those  dwell- 
ing in  arctic  regions. 

§  451.  Many  of  the  maladies  of  the  body  are  characterized  by  an  in- 
crease of  the  bodily  temperature  known  as  "  fever "  or  "  pyrexia,"  the 
thermometer  very  frequently  rising  to  39°  or  40°,  not  unfrequently  to  41°, 
and  at  times  reaching  43°  or  even  44°  ;  but  these  higher  temperatures  can- 
not long  be  borne  without  the  organism  failing.  And,  as  we  have  said,  any 
increase  in  man  of  the  bodily  temperature  beyond  38°,  or  even  beyond  37.5°, 
indicates  some  disturbance.  In  most  cases  the  rise  of  temperature  has  a 
definite  objective  cause,  some  local  inflammation  or  suppuration,  or,  as  in 
specific  fevers,  the  presence  in  the  economy  of  some  "  materies  morbi,"  of 
the  nature  of  an  organized  germ  or  of  some  other  nature.  We  cannot  here 
discuss  the  connection  between  local  inflammation  or  the  specific  poison  and 
the  high  temperature,  but  we  have  increasing  evidence  that  the  high  tem- 
perature of  fever  is  due,  not  merely  to  a  diminution  of  the  loss  of  heat, 
though  this  may  be  a  factor,  but  also,  and  indeed  chiefly,  to  an  increased 
production  of  heat.  In  fever  the  production  of  carbonic  acid  and  the  con- 
sumption of  oxygen,  that  is  to  say,  the  metabolic  changes  of  the  tissues,  are 
increased.  The  urea  also  is  increased,  and  that  in  such  a  way  as  to  confirm 
the  view  already  expressed  that  much  of  the  heat  comes  from  such  a  metab- 
olism of  the  skeletal  muscles  as,  unlike  an  ordinary  contraction,  directly 
involves  the  nitrogenous  elements.  The  inordinate  metabolism  of  the  body 
at  large  thus  characteristic  of  fever  is  shown  by  the  wasting  which  it  entails. 
Calorimetric  observations  also  show  in  a  direct  manner  that  the  production 
of  heat  is  increased.  Of  course,  mere  increased  production  alone  would  be 
insufficient  to  raise  the  temperature  of  the  body,  for  it  might  be  met,  up  to 
a  very  high  limit,  by  a  compensating  increase  of  loss  of  heat ;  but  in  fever 
this  compensation  is  wanting,  and  it  is  perhaps  this  absence  of  due  regu- 
lation which  is  most  characteristic  of  the  febrile  condition. 

In  some  maladies  the  bodily  temperature  falls  distinctly  below  the  normal 
average,  reaching  for  instance  35°,  or  even  lower.  In  such  cases  there  can 
be  little  doubt  that  the  condition  is  due  to  diminished  metabolism  and 
diminished  heat  production. 

One  of  the  most  marked  phenomena  of  starvation  is  the  fall  of  tempera- 
ture, which  becomes  very  rapid  during  the  last  days  of  life  The  lowered 
metabolism  diminishes  the  production  of  heat,  and  the  lowered  temperature 
in  turn  still  further  diminishes  the  metabolism.  Indeed,  the  low  tempera- 
ture is  a  powerful  factor  in  bringing  about  death,  for  life  may  be  much 
prolonged  by  wrapping  a  starving  animal  in  some  bad  conductor,  so  as  to 
economize  the  bodily  heat. 

§  452.  Effects  of  great  heat.  As  we  said  above,  the  regulative  heat 
mechanism  is  unable  to  withstand  the  strain  of  too  great  an  external  heat  or 
too  prolonged  an  exposure  to  a  great  but  less  degree  of  heat.  The  tempera- 
ture of  the  body  then  rises  above  the  normal ;  and  it  has  been  observed  that 
the  temperature  is  more  easily  raised  by  warmth  than  depressed  by  cold,  at 
least  when  neither  is  very  intense.  When  either  in  this  way  by  external 
warmth  or  through  pyrexia  the  temperature  of  the  body  is  raised  some  6° 
or  7°  above  the  normal,  to  45°  or  thereabouts,  death  speedily  ensues.  The 
chain  of  events  thus  leading  to  death  has  not  been  as  yet  clearly  made  out, 
and  most  likely  the  events  do  not  take  exactly  the  same  course  in  all  cases ; 
but  we  shall  probably  not  go  far  wrong  in  attributing  death  to  the  fact  that 
the  high  temperature  hurries  on  the  metabolism  of  the  several  tissues,  of 


THE  ENERGY   OF  THE    BODY.  505 

some  more  than  others,  at  such  a  spendthrift  rate  that  their  capital  is  soon 
exhausted.  We  have  seen  (§  314)  that  too  warm  blood  produces  dyspnoea 
and  soon  exhausts  the  metabolic  capital  of  the  respiratory  centre.  Too 
warm  blood  similarly  hurries  on  the  beats  of  the  heart ;  an  explosion  of  the 
contractile  substance  is  each  time  prematurely  brought  on  before  a  sufficient 
quantity  of  explosive  substance  is  accumulated,  each  stroke  becomes  more 
.and  more  feeble  as  the  rate  is  quickened,  the  beats  become  irregular  and 
finally  cease.  Either  of  these  two  events  alone  and  certainly  both  together 
are  enough  to  bring  the  working  of  the  bodily  mechanism  to  an  end  ;  but 
other  tissues  beside  the  heart  and  the  respiratory  centre  are  suffering  in  the 
same  way,  notably  the  rest  of  the  central  nervous  system.  This,  too,  is 
being  hurried  on  unduly  in  its  inner  changes,  so  that  not  only  consciousness 
is  lost  and  other  objective  manifestations  of  nervous  action  go  wrong  or  fail, 
but  the  regulative  grasp  of  the  central  nervous  system  on  the  tissues  of  the 
body  at  large  is  loosened,  and  tumult  takes  the  place  of  order.  Whether 
this  or  that  sign  of  disorder  comes  to  the  front,  whether,  for  instance, 
•convulsions  take  place,  would  appear  to  depend  upon  the  exact  turn 
taken  by  the  abnormal  events.  In  heat-stroke,  more  commonly  known  as 
sun-stroke,  the  essential  condition  of  which  seems  to  be  a  rapid  rise  of  the 
temperature  of  the  body,  owing  to  a  sudden  failure  of  the  thermotaxic 
mechanism,  the  symptoms  vary.  Sometimes  the  heart  suddenly  gives 
way,  at  other  times  the  respiratory  centre  seems  to  be  more  directly 
affected ;  sometimes  convulsions  make  their  appearance,  but  more  com- 
monly death  takes  place  through  a  comatose  condition  of  the  brain,  an 
initial  phase  of  excitement  of  the  central  nervous  system  being  not  uufre- 
quently  witnessed. 

Mammalian  muscle,  it  will  be  remembered  (§  82),  becomes  rigid  at 
about  50°  ;  but  death  probably  always  occurs  before  that  higher  tempera- 
ture is  reached  by  the  blood,  so  that  a  sudden  rigor  mortis  from  heat 
(rigor  caloris)  cannot  be  regarded  as  a  factor  in  death  from  exposure  to 
too  great  heat. 

§  453.  Effects  of  great  cold.  The  effects  of  a  too  great  lowering  of  the 
temperature  of  the  body,  which  is,  generally,  the  result  of  too  great  ex- 
ternal cold,  and  rarely,  if  ever,  arises  from  internal  causes  lowering  the 
metabolism  and  thus  the  production  of  heat,  are,  in  their  origin,  the  reverse 
of  those  of  a  too  high  temperature.  The  metabolism  of  the  tissues  is  low- 
ered ;  and  not  only  are  the  katabolic  changes,  which  lead  to  the  setting  free 
of  energy  thus  affected,  but  the  anabolic  changes  also  share  in  the  depres- 
sion. Thus  "  living  substance  "  falls  to  pieces  less  readily,  but  is  also  made 
up  less  readily ;  and  could  this  slackening  of  metabolism  be  carried  on  in 
the  several  tissues  at  a  rate  proportionate  to  the  rate  at  which  each  tissue 
lives,  life  might  thus  be  brought  to  a  peaceful  end  by  gradual  arrest  of  the 
life  of  each  part  of  the  whole  body.  And,  indeed,  in  some  cases,  where  the 
lowering  of  the  temperature  takes  place  gradually,  something  like  this  does 
occur  even  in  warm-blooded  animals.  The  diminished  metabolism  tells  first 
and  chiefly  on  the  central  nervous  system,  especially  on  the  brain,  and  more 
particularly  on  those  parts  of  that  organ  which  are  concerned  in  con- 
sciousness. The  intrinsic  lowering  of  the  cerebral  metabolism  is  further 
assisted  by  a  slowing  of  the  heart-beat  and  of  the  breathing ;  drowsiness  is 
succeeded  by  a  condition  very  like  to,  if  not  identical  with,  that  known 
as  sleep,  which  we  shall  study  later  on,  but  by  a  sleep  which  insensibly 
passes  into  the  sleep  of  death.  In  sonie  cases,  however,  especially  those 
in  which  the  lowering  of  the  temperature  is  sudden  and  rapid,  disorders  of 
the  nervous  system  intervene  and  convulsions  like  those  of  asphyxia  are 
produced'. 


506  NUTRITION. 

ON  NUTRITION  IN  GENERAL. 

§  454.  It  may  now  be  profitable  to  take  a  brief  survey  of  the  various 
conclusions  at  which  we  have  arrived  concerning  the  problems  of  nutrition. 

We  have  seen  that  the  several  tissues,  using  lymph  as  a  medium,  live 
upon  the  blood,  taking  up  from  the  blood  the  materials  for,  and  returning 
to  the  blood  the  products  of,  their  metabolism.  The  blood  itself  we  have 
also  seen  to  be  replenished  with  food  from  the  alimentary  canal  and  with 
oxygen  from  the  lungs,  and  to  be  freed  from  waste  products  by  means  of 
the  excretory  organs.  In  this  double  action  the  raw  material  of  the  food,, 
on  the  one  hand,  undergoes  between  its  being  placed  in  the  mouth  and 
its  taking  part  in  the  metabolism  of  the  tissue  which  ultimately  uses 
it,  many  intermediate  changes  carried  on  in  various  parts  of  the  body, 
and  the  waste  products  similarly  undergo  intermediate  changes  between 
leaving  the  tissue  and  appealing  in  the  urine,  the  sweat,  or  the  expired 
air. 

We  have  further  seen  reason  to  think  that  the  metabolic  events  of  the 
body  take  place,  in  the  main,  in  the  tissues,  not  in  the  blood  stream  on  it& 
way  between  the  heart  and  the  tissues.  Changes  proper  to  the  blood 
itself  take  place  in  the  blood ;  the  corpuscles,  red  and  white,  with  the 
plasma  undergo,  like  the  rest  of  the  body,  their  proper  metabolic  cycles, 
and  in  this  sense  blood  may  be  called  a  tissue,  if  there  is  any  advantage 
in  using  the  phrase ;  but,  apart  from  these  intrinsic  blood  changes,  as  far 
as  we  can  see  at  present  the  metabolism  undergone,  during  their  transit 
along  the  blood  channels,  by  the  substances  which  are  merely  carried  in 
the  blood  from  place  to  place,  is  an  insignificant  part  of  the  total  metab- 
olism of  the  body. 

By  metabolism  of  a  tissue  we  understand  the  total  chemical  changes 
taking  place  in  the  tissue ;  and  we  divide  these  changes  into  those  which 
either  directly  or  indirectly  are  concerned  in  the  building  up  (anabolic) r 
and  those  which  are,  in  like  manner,  concerned  in  the  breaking  down 
(katabolic)  of  the  living  substance.  We  shall  explain  presently  wha.t  we 
mean  by  the  words  "  directly  "  and  "  indirectly,"  used  in  this  connection. 
And  we  may  here  repeat  the  caution  (§  30)  that  though,  for  convenience 
sake,  we  use  the  phrase  "  living  substance,"  what  is  really  meant  by  the 
words  is  not  a  thing  or  body  of  a  particular  chemical  composition,  but 
matter  undergoing  a  series  of  changes. 

§  455.  Since  the  several  tissues  originate  through  a  differentiation  of 
the  simpler  primordial  protoplasm,  we  may  infer  that  we  have  a  right  to 
speak  of  a  general  plan  of  metabolism  common  to  all  the  tissues,  modified 
in  various  particulars  in  various  tissues.  It  is  more  reasonable,  for  instance, 
to  suppose  that  there  is  such  a  general  plan  common  to  both  muscle  and 
gland,  than  to  suppose  that  the  metabolism  of  the  one  differs  wholly  from 
or  only  accidentally  resembles  that  of  the  other.  And  we  may  profitably 
take  the  nutrition  of  muscle  as  exemplifying,  in  the  midst  of  the  features 
special  to  the  muscle,  the  general  plan  of  vital  metabolism.  The  muscle,  in 
a  normal  state  of  things,  lives  ultimately  on  the  proteids,  fats,  carbohydrates, 
salts,  and  water  of  the  food,  and  on  the  oxygen  of  the  inspired  air,  but 
lives  directly  on  the  blood  which  brings  these  things  to  it.  Taking  the  pro- 
teids first,  we  may  ask  the  question,  How  does  the  blood  supply  the  muscle 
with  proteids? 

The  blood  contains  three  classes  of  proteids :  1,  serum-albumin  ;  2,  para- 

flobulin,  and  3,  fibrinogen.     With  regard  to  the  function   of  these  three 
inds  of  proteids  in  the  nutrition  of  muscle,  our  only  conclusions  at  present 
are  indirect  onces,  based  chiefly  on  the  results  of  experiments  as  to  the  rel- 


ON  NUTRITION  IN  GENERAL.  507 

ative  value  of  these  substances  in  maintaining  or  restoring  the  irritability 
of  muscle.  It  is  found  that  when  the  washed-out  frog's  heart  (§  148)  is  fed 
with  defibrinated  blood,  the  restoration  is  as  good  as  with  whole  blood  ;  and 
that  while  the  effects  of  globulin  are  uncertain,  and  while  peptone  and  albu- 
rnose  appear  to  act  in  an  injurious  manner,  the  restorative  effects  of  serum- 
albumin  are  marked.  From  these  results  we  may  provisionally  infer  that 
the  muscle  in  its  (total)  anabolic  changes  takes  up  and  so  lives  upon  the 
serum-albumin  of  the  blood.  But  this  conclusion  must  be  regarded  as  pro- 
visional only,  and  indeed  uncertain.  For  we  must  remember  that  the  blood 
supplies  not  only  the  food  (including  oxygen)  for  the  muscle,  but  also  the 
conditions  under  which  the  muscle  can  live  and  avail  itself  of  the  food 
offered  to  it.  The  complex  actions  through  which  a  certain  quantity  of 
proteid  and  other  material  is  built  up  into  living  muscular  substance,  need 
for  their  execution  a  favorable  medium,  need  certain  physical  and  chemical 
conditions ;  and  it  may  be  that  the  favorable  influence  of  serum-albumin  is 
simply  due  to  its  presence  in  some  way  assisting  the  transformation  into 
living  substance  of  raw  material  still  remaining  in  the  muscular  fibres  and 
not  to  its  supplying  new  raw  material. 

Dextrose  is,  as  we  have  repeatedly  said,  always  present  in  the  blood  in 
small  quantity,  and  appears  to  be  the  only  carbohydrate  contituent  of  blood- 
plasma.  Experiments  carried  out  on  a  large  animal,  such  as  the  horse  or 
cow,  have  shown  that  the  venous  blood  coming  from  a  muscle  contains  less 
dextrose  than  the  arterial  blood  going  to  the  muscle,  and  that  the  difference 
is  much  increased  by  throwing  the  muscle  into  contraction.  From  this  we 
may  provisionally  conclude  that  dextrose  is  an  essential  part  of  the  food  of 
the  muscle. 

The  blood,  as  we  have  seen,  also  contains  a  certain  amount  of  fat ;  and 
if  we  push  the  analogy  between  the  whole  body  and  the  muscle,  we  may 
infer  that  the  muscle  takes  up  fat  as  food  for  itself  from  the  blood.  But 
we  have  no  experimental  evidence  in  favor  of  this.  Moreover,  we  have 
seen  that  fat  and  carbohydrate  are,  in  the  animal  body,  more  or  less  trans- 
ferable. We  have  distinct  proof  that  the  body  can  transform  carbohydrate 
into  fat ;  and  it  is  very  probable  that  it  can  transform  fat  into  carbohydrate. 
Seeing  how  much  more  easily  a  soluble  diffusible  carbohydrate  like  sugar 
can  be  carried  from  place  to  place  by  the  fluids  of  the  body  than  can  im- 
miscible fats,  it  seems  reasonable  to  suppose  that  when  the  body  has  to  draw 
upon  its  store  of  fat  in  the  cells  of  adipose  tissue,  the  fat,  on  leaving  the  fat- 
cell,  is  transformed  into  sugar,  its  carbon,  so  to  speak,  being  dealt  out  to  the 
tissues  in  the  form  of  dextrose.  Indeed,  we  may,  perhaps,  dwelling  on  the 
fact  that  a  muscle,  though  itself  essentially  of  proteid  build,  turns  over 
(§  85)  in  its  daily  work  so  much  more  carbon  than  nitrogen,  entertain  the 
view  that  what  muscle  wants  as  food  is  a  certain  amount  of  proteid  plus  an 
additional  quantity  of  carbon  in  some  form  or  other,  and  that  dextrose  is  a 
convenient  form  in  which  the  additional  carbon  can  be  supplied.  And  we 
may  hold  this  view  without  prejudice  to  any  opinion  that  the  carbon  so 
brought,  while  being  built  up  into  the  living  substance,  may  be  again  ar- 
ranged as  fat,  and  in  the  course  of  the  metabolism  of  the  muscle  may  be 
later  on  separated  from  the  living  substance  and  deposited  in  the  fibre  as 
globules  of  fat.  But  our  knowledge  is  at  present  insufficient  to  decide 
whether  this  view  is  true  or  not. 

The  various  salts  brought  to  the  muscle  by  the  plasma,  though  they  supply 
no  energy,  are  as  essential  to  the  life  of  muscle  as  the  energy-holding  proteid 
or  carbon  compound  ;  and  experiments  made  with  regard  to  some  of  them, 
calcic  salts,  for  instance,  show  that  their  presence  or  absence  materially  affects 
the  maintenance  or  restoration  of  irritability.  Some  of  these  probably  play 


508  NUTRITION. 

the  part  only  of  securing  by  their  presence  favorable  conditions  for  the  due 
metabolic  processes,  somewhat  after  the  way  in  which  the  presence  of  calcic 
phosphate  determines  the  curdling  of  milk  ;  but  some  we  probably  ought  to 
regard  as  actually  entering  into  the  processes  themselves.  Of  these  matters,, 
however,  we  know  very  little. 

§  456.  The  end-products  of  muscular  metabolism  are,  as  we  have  seen, 
carbonic  acid,  lactic  acid,  and  kreatin,or  some  other  nitrogenous  bodies,  and 
we  have  already  (§  85)  said  all  we  have  to  say  concerning  the  formation  of 
these  products.  We  may,  however,  briefly  consider  here  the  question,  What 
is  the  relation  of  these  various  metabolic  processes  to  the  structural  elements 
of  the  tissue?  When  we  say  that  the  muscular  fibre  is  continually  under- 
going metabolism  do  we  mean  that  very  jot  and  tittle  of  the  fibre  is  under- 
going change  and  that  at  the  same  rate?  We  can  hardly  suppose  this.  It 
seems  unlikely,  for  instance,  that  the  metabolism  of  the  fibrillar  substance  is 
identical  with  that  of  the  interfibrillar  substance,  whatever  be  the  view  we 
take  as  to  the  properties  or  meaning  of  the  two  substances.  Further,  if  we 
accept  the  suggestion  made  in  §  85  as  to  a  contractile  substance,  which, 
though  having  peculiar  qualities,  being  peculiarly  related  to  and  having 
peculiar  connections  with  the  rest  of  the  fibre,  may  in  a  broad  way  be  com- 
pared with  the  glycogen  of  an  hepatic  cell,  we  can  conceive  that  this  contrac- 
tile substance  may  be  manufactured  without  the  whole  of  it  at  least  having 
been  at  any  time  an  integral  part  of  what  we  may  in  a  stricter  sense  call  the 
real  living  substance  of" the  fibre.  We  should  thus  be  led  to  regard  the 
metabolic  events  occurring  in  muscle  as  falling  into  two  classes  at  least: 
those  taking  place  in  the  living  more  permanent  framework,  and  those  bear- 
ing on  the  formation  and  destruction  of  the  contractile  substance  lodged  in 
that  living  framework.  Further,  if  we  suppose  that  the  metabolism  by 
which  the  muscles  supply  so  much  of  the  heat  of  the  body,  and  which,  as  we 
have  seen,  may  and  does  go  on  independently  of  contractions,  is  not  a  me- 
tabolism of  the  same  contractile  substance  differing  from  the  metabolism  of 
a  contraction  in  being  so  ordered  that  all  the  energy  goes  out  as  heat,  none 
being  employed  to  effect  a  change  of  form,  but  is  a  metabolism  of  some  other 
"  thermogenic  "  substance,  we  should  have  to  add  a  third  class  to  the  other 
two.  These,  of  course,  are  at  present  matters  of  speculation  ;  but  on  the 
whole,  the  evidence  we  can  gather  tends,  and  perhaps  increasingly  tends,  to 
show  that  in  muscle  there  does  exist  such  a  framework  of  what  we  may  call 
more  distinctly  living  substance  which  rules  the  histological  features  of  the 
fibre,  and  whose  metabolism  though  high  in  quality  does  not  give  rise  to 
massive  discharges  of  energy,  and  that  the  interstices,  so  to  speak,  of  this 
framework  are  occupied  by  various  kinds  of  material  related  in  different 
degrees  to  the  framework  and  therefore  deserving  to  be  spoken  of  as  more 
or  less  living,  the  chief  part  of  the  energy  set  free  by  muscle  coming  directly 
from  the  metabolism  of  some  or  other  of  this  material.  And  the  same  view 
may  be  extended  to  other  tissues.  Both  the  framework  and  the  intercalated 
material  undergo  metabolism,  and  have,  in  different  degrees,  their  anabolic 
and  katabolic  changes  ;  both  are  concerned  in  the  life  of  the  living  substance 
but  one  more  directly  than  the  other,  and  this  is  what  was  meant  by  the 
terms  "  directly  "  and  "  indirectly,"  used  in  §  454.  Such  a  mode  of  expres- 
sion seems  preferable  to  the  more  common  one,  based  on  the  analogy  of  a 
firearm,  of  the  muscle  fibre  firing  off  the  contractile  material ;  in  the  firearm 
there  are  no  such  connections  between  the  machine  and  the  charges  as  obtain 
in  the  living  mechanism.  We  may  perhaps  further  be  led  by  this  to  dis- 
tinguish between  growth  as  bearing  on  the  framework,  and  mere  temporary 
nutrition  as  bearing  on  the  accumulation  and  expenditure  of  the  lodged 
material.  We  mav  add  that  since  some  of  the  material  so  lodged  in  the 


ON  NUTRITION  IN  GENERAL.  509 

framework  will  consist  of  substances  which  have  not  yet  undergone  metabo- 
lism, but  are  either  about  to  be  worked  up  into  the  framework  itself,  or  are 
about  to  be  transformed  in  a  more  direct  way  into  some  product  of  metabo- 
lism, or  are  substances  whose  presence  is  in  some  way  necessary  for  the  car- 
rying on  of  metabolic  processes  in  which  they  themselves  take  no  bodily 
part,  we  must  recognize  a  continuity  without  any  sharp  break  between  this 
material  which  we  regard  as  part  of  the  tissue,  and  the  lymph  which  simply 
bathes  the  tissue  and  flows  through  the  interstices.  Hence  such  phrases  as 
"  tissue  proteid  "  and  "  floating  proteid  "  (§  436),  are  undesirable  if  they  are 
understood  to  imply  a  sharp  line  of  demarcation  between  the  "  tissue  "  and 
the  blood  or  lymph,  though  useful  as  indicating  two  different  lines  or  degrees 
of  metabolism. 

§  457.  The  products  of  muscular  metabolism  pass  into  the  lymph  bath- 
ing the  fibre,  and  so,  either  by  a  direct  path  into  the  capillaries  or  by  a  more 
circuitous  course  through  the  general  lymphatic  system,  into  the  blood.  The 
fate  of  the  carbonic  acid  we  have  fully  treated  of  in  dealing  with  respiration  ; 
the  little  we  know  concerning  the  nitrogenous  product  or  products  has  been 
stated  in  dealing  with  urea ;  the  third  recognized  product  is  lactic  acid, 
sarcolactic  acid.  Did  any  considerable  amount  of  oxidation  take  place  in 
the  blood  stream  while  the  blood  is  flowing  along  the  larger  channels,  sub- 
ject only  to  the  influence  of  the  vascular  walls,  we  might  fairly  expect  that 
the  lactic  acid  discharge  from  the  muscles  would  be  subjected  to  oxidizing 
influences  while  still  within  the  blood  stream  of  the  larger  channels.  We 
have,  however,  no  satisfactory  evidence  of  any  lactic  acid  being  oxidized  in 
this  way.  On  the  contrary,  there  is  a  certain  amount  of  experimental  and 
other  evidence  that  lactic  acid  present  in  the  blood  is  somehow  or  other  dis- 
posed of  by  the  liver ;  and  that  if  the  liver  fail  to  do  its  duty,  lactic  acid 
may  appear  in  the  urine.  It  is  tempting  to  suppose  that  it  might  there  by 
a  synthetic  effort  be  converted  into  glycogen,  the  liver  thus  utilizing  some 
of  the  muscular  waste  product,  but  the  experimental  and  other  evidence  is 
all  against  this  view.  In  the  absence  of  actual  knowledge  we  infer  that  it 
is  in  the  liver  oxidized  into  carbonic  acid  and  water,  thus  adding  its  con- 
tribution to  the  supply  of  heat,  or  prepared  in  some  way  for  oxidation  else- 
where. Probably  such  a  change  is  not  confined  to  the  liver,  but  takes  place 
in  other  organs,  such  as  the  spleen.  Thus  the  kind  of  action  on  which  we 
dwelt  in  treating  of  urea,  namely,  that  the  products  of  the  metabolism  of 
one  organ  are  carried  to  other  organs  for  further  elaboration  and  possible 
utilization,  applies  to  the  non-nitrogenous  as  well  as  to  the  nitrogenous  pro- 
ducts of  muscular  metabolism  ;  and  if  a  muscle  gives  rise  to  other  non-nitro- 
genous products  than  carbonic  and  lactic  acid  these  are  probably  disposed  of 
in  some  such  way  as  the  lactic  acid.  In  speaking  of  glycogen  in  the  winter 
frog  (§  380)  we  said  that  possibly  the  glycogen  so  stored  up  might  arise 
from  sugar  brought  to  the  liver  from  other  tissues.  If  that  be  so,  we  should 
further  expect  that  some  at  least  of  that  sugar,  either  as  such  or  as  some 
allied  substance,  would  come  from  the  skeletal  muscles  which  form  so  large 
a  part  of  the  body  of  the  frog ;  and  if  so,  we  must  conclude  that  under  the 
special  circumstances  obtaining  in  the  winter  frog  the  muscles  discharge  into 
the  blood  a  non-nitrogenous  product  not  in  the  form  either  of  carbonic  or 
lactic  acid.  It  is  perhaps,  however,  more  probable  that  the  sugar  in  ques- 
tion comes  from  a  metabolism  of  the  fat  stored  up  in  the  "  fatty  bodies  "  and 
elsewhere. 

§  458.  As  far  as  we  can  see  at  present  the  plan  of  nutrition  thus  briefly 
sketched  out  for  muscle  holds  good  for  the  other  tissues  as  well,  the  chief  or 
at  least  the  most  conspicuous  differences  bearing  on  the  nature  and  proper- 
ties of  and  the  changes  undergone  by  the  material  formed  by  and  held  by 


510  NUTRITION. 

the  more  distinctly  structural  work.  Thus  the  rnucin  of  the  salivary  mucous 
cell  finds  its  analogue  either  in  the  contractile  substance  itself,  or  more  prob- 
ably in  some  early  nitrogenous  product  of  the  explosion  of  the  contractile 
substance,  such  as  may  correspond  to  the  myosin  of  rigid  muscle.  The 
metabolism  of  the  hepatic  cell  seems,  as  we  have  seen,  to  be  especially  cha- 
racterized by  its  returning  to  the  blood  a  body,  viz.,  sugar,  still  containing  a 
considerable  amount  of  energy,  available  for  use  in  other  parts  of  the  body. 
And  this  suggests  the  question  whether  in  the  normal  metabolism  of  mus- 
cular substance  a  similar  something,  still  holding  a  considerable  quantity 
of  energy,  some  proteid  substance  for  instance,  may  not  be  returned  to  the 
blood ;  so  that  the  metabolism  of  muscle  is  imperfectly  described  in  saying 
that  the  results  are  carbonic  and  lactic  acids  and  an  antecedent  of  urea.  If 
this  be  so,  then  muscles  may  be  of  other  use  to  the  body  at  large  than  as 
mere  contractile  machines,  just  as  the  liver  has  other  uses  than  the  produc- 
tion of  bile.  And  the  same  considerations  may  be  applied  to  the  other 
tissues  as  well. 

§  459.  Whether  the  chief  product  of  the  metabolism  of  any  tissue  be  a 
proteid  substance,  or  a  fat,  or  a  carbohydrate,  proteid  substance  is  the  pivot, 
so  to  speak,  of  the  metabolism,  and  nitrogenous  bodies  always  appear  as  the 
products  of  metabolism.  This  is  strikingly  seen  in  the  nutrition  of  plants 
where,  as  far  as  mere  bulk  or  weight  is  concerned,  the  active  metabolizing 
tissue  is  insignificant  compared  with  the  mass  of  products  of  metabolism 
heaped  up  in  the  form  of  starch  or  cellulose  or  some  allied  carbohydrate. 
The  protoplasm  of  a  vegetable  cell  soon  becomes  a  mere  film  bearing  a  heavy 
burden  of  heaped-up  metabolic  products  and  eventually  disappears ;  and 
of  that  film  only  a  part  corresponds  to  what  we  spoke  of  above  as  the  living 
framework  of  the  muscle.  Yet  that  scanty  proteid-built  framework  is  more 
or  less  directly  concerned  in  the  production  of  the  carbohydrate  material 
and  the  various  conversions  which  that  material  undergoes.  Proteid,  nitro- 
gen, changes  are  entangled  with  the  carbon  changes  ;  and  since  the  products 
of  metabolism  in  the  plant  are  not  as  in  the  animal  cast  out  of  the  organism, 
but  for  the  most  part  heaped  up  within  it,  we  find  the  plant  storing  up  in 
parts,  where  if  they  serve  no  useful  purpose  they  at  least  do  no  harm,  nitro- 
genous products  of  metabolism,  such  as  those  known  as  vegetable  alkaloids, 
many  of  which  by  their  amide  nature  betray  their  kinship  to  the  animal 
nitrogenous  product  urea. 

§  460.  The  rate  at  which  in  the  adult,  leaving  aside  for  the  present  the 
special  nutrition  of  the  young,  nutrition  is  carried  on,  and  the  characters  of 
the  nutrition,  are  dependent  on  a  variety  of  circumstances.  Each  tissue  has 
of  course  a  line  of  nutrition  of  its  own  which  circumstances  may  favor  or 
hinder,  but  cannot  change  in  nature  ;  the  nutrition  of  the  hepatic  cell  cannot 
be  altered  to  that  of  the  muscular  fibre.  The  same  tissue,  moreover,  has  in 
different  races  and  different  individuals  specific  and  individual  characters  of 
nutrition  ;  the  flesh  of  a  dog  is  not  the  same  as  that  of  a  man,  the  muscle  of 
one  man  lives  differently  from  that  of  another,  the  metabolism  per  unit  of 
body  weight  is,  as  we  have  seen,  greater  in  the  smaller  organism,  and  so  on. 

Within  the  limits  and  subject  to  the  conditions,  however,  thus  fixed  by 
race  and  personality,  general  influences  produce  general  variations  in  nutri- 
tion. The  rate  of  nutrition  of  a  tissue,  for  instance,  is  dependent  on  the  food, 
on  the  amount  and  nature  of  the  food  material  brought  to  the  tissue  by  the 
blood.  We  have  seen  that  proteid  food,  in  contrast  to  carbon  food,  markedly 
increases  the  metabolism  of  the  body.  Since  this  increase  tells  not  only  on 
the  nitrogenous  but  also  on  the  carbon  metabolism  (§  437),  it  cannot  be  the 
result  of  a  mere  luxus  consumption  of  the  proteid  food  itself;  and  unless  we 
suppose  that  the  presence  of  the  excess  of  proteid  material  either  in  the  ali- 


ON  NUTRITION   IN  GENERAL.  511 

inentary  canal,  or  while  passing  through  the  capillaries  of  some  organ,  such 
as  the  liver,  acts  as  a  stimulus  to  some  reflex  nervous  machinery  through 
whose  action  the  metabolism  of  certain  or  of  all  the  tissues  is  hurried  on,  we 
must  conclude  that  it  is  the  direct  access  of  proteid  material  to  the  tissues 
themselves  which  stirs  them  up  to  increased  metabolic  activity.  That  pro- 
teid food  should  do  this,  and  not  carbohydrate  or  fat,  seems  to  be  connected 
with  the  fact  just  dwelt  on  that  proteid  material  is  the  pivot  of  metabolism. 

§  461.  In  the  preceding  chapters  of  this  work  we  have  had  abundant 
evidence  that  the  metabolism  of  the  tissues  is  subject  to  the  government  of 
the  central  nervous  system ;  the  contraction  of  a  muscle,  the  secretory  activity 
of  a  gland,  the  increased  or  diminished  production  of  heat,  all  afford  in- 
stances of  nervous  impulses  affecting  metabolism.  In  most  of  these  instances 
the  changes  induced  fall  within  the  downward,  katabolic  phase  and  have  a 
downward  character ;  thus,  when  a  muscle  contracts,  the  result  is  a  conver- 
sion of  more  complex  bodies  into  simpler  bodies ;  and  the  same,  as  far  as  we 
can  see,  is  true  of  most  other  cases.  But  it  is  open  for  us  to  suppose  that 
nervous  impulses  might  affect  the  upward,  anabolic  phase  and  have  a  con- 
structive influence.  There  are  no  reasons  for  regarding  such  an  action  as 
impossible ;  and  indeed  some  phenomena,  such  as  those  of  inhibitory  nerves 
and  the  antagonism  between  these  and  augmentor  nerves,  pointedly  suggest 
some  such  view.  Thus,  we  may  suppose  that  an  inhibitory  impulse  produces 
such  changes  in  the  cardiac  muscular  substance  that  the  upward  constructive 
processes  are  assisted  and  the  downward  disruptive  processes  checked,  whereby 
the  setting  free  of  energy  is  checked,  and  so  the  beats  hindered  or  stopped, 
the  inhibitory  effect  being  followed  by  a  period  of  rebound  in  which  the 
savings  of  the  inhibited  period  are  spent  in  increased  action.  Conversely  we 
may  suppose  that  an  augmentor  impulse  hinders  the  anabolic  and  assists  the 
katabolic  changes,  and  conversely  also,  when  it  has  done  its  work,  leaves  the 
tissue  with  diminished  capital  manifested  by  feebler  beats  or  by  the  absence 
of  the  power  to  beat.  And  similarly  in  the  case  of  the  respiratory  centre 
and  other  tissues.  When  we  have  to  study  the  origination  of  visual  im- 
pulses in  the  retina  we  shall  come  upon  a  view  that  a  wave  of  light  may 
affect  what  we  shall  call  a  visual  substance  either  by  promoting  anabolic 
constructive  changes  or  by  increasing  katabolic  destructive  changes  accord- 
ing to  its  wave  length.  There  is  then  evidence,  to  a  certain  extent,  for  the 
view  on  which  we  are  dwelling  ;  but,  without  discussing  the  matter  any  fur- 
ther, we  may  say  that  the  conception,  though  suggestive,  has  not  yet  been 
demonstrated,  and  so  far  can  only  be  spoken  of  as  probable. 

§  462.  One  value  perhaps  of  such  a  view  lies  in  the  fact  that  it  warns  us 
against  assuming  that  a  nervous  impulse  can  only  produce  disruptive  kata- 
bolic changes  such  as  are  seen  in  muscular  contraction  or  in  secretion.  The 
effects  of  stimulating  a  nerve  going  to  a  muscle  or  a  salivary  gland  are  strik- 
ing and  obvious,  and  the  behavior  of  a  muscle  or  a  gland  as  far  as  contraction 
and  secretion  are  concerned  is,  within  certain  limits,  under  experimental  con- 
trol. But  there  are  certain  phenomena,  seen  chiefly  in  the  course  of  disease, 
and  lying,  to  a  very  small  extent  only,  within  the  control  of  experiment, 
which  seem  to  show  that  the  central  nervous  system  governs  the  metabolic 
changes,  the  nutrition,  not  only  of  muscle  and  gland,  but  of  various  other 
tissues,  in  a  deeper  and  more  general  way  than  that  of  simply  promoting  (or 
hindering)  contraction  or  secretion.  Thus,  as  we  have  seen  (§  81),  when  the 
connection  between  a  muscle  and  the  central  nervous  system  is  severed,  the 
muscle  eventually  wastes  and  loses  its  vitality ;  when  all  the  nerves  going  to 
the  submaxillary  gland  are  severed,  the  gland,  instead  of  being,  as  in  the 
normal  condition,  intermittingly  active  and  quiescent,  pours  forth  a  continu- 
ous "paralytic"  secretion  and  eventually  degenerates  and  wastes.  When  in 


512  NUTRITION. 

a  rabbit  the  fifth  nerve  is  divided  in  the  skull  the  loss  of  sensation  in  those 
parts  of  the  face  of  which  it  is  the  sensory  nerve  is  followed  by  nutritive 
changes.  Very  soon,  within  twenty-four  hours,  the  cornea  becomes  cloudy : 
and  this  is  the  precursor  of  an  inflammation  which  may  involve  the  whole 
eye  and  end  in  its  total  disorganization.  At  the  same  time  the  nasal  cham- 
bers of  the  side  operated  on  are  inflamed,  and  very  frequently  ulcers  make 
their  appearance  on  the  lips  and  gums.  And  similar  results  have  been  seen 
in  other  animals,  including  man.  If  the  operation  be  conducted  in  a  young 
animal,  which  subsequently  lives  to  maturity,  the  head  may  become  bilater- 
ally unsym metrical,  as  shown  especially  by  the  skull.  Again,  division  of 
both  vagus  nerves  is  very  apt  to  be  followed  by  inflammation  of  both  lungs, 
by  fatty  degeneration  of  the  heart,  and  so  by  death. 

In  several  of  these  instances  the  effect  is  a  mixed  one,  and  the  problem 
complicated.  Thus,  in  the  case  of  division  of  the  fifth  nerve,  seeing  how 
delicate  a  structure  the  eye  is,  and  how  carefully  it  is  protected  by  the  mech- 
anisms of  the  eyelids  and  tears,  it  seems  reasonable  to  suppose  that  the  in- 
flammation in  question  might  simply  be  the  result  of  the  irritation  caused 
by  dust  and  contact  with  foreign  bodies,  to  which  the  eye,  no  longer  guided 
and  protected  by  sensations,  these  being  destroyed  by  the  section  of  the  nerve, 
became  subject.  In  the  same  way  the  ulcers  on  the  lips  and  gums  might  be 
explained  as  injuries  inflicted  by  the  teeth  on  those  structures  in  their  insen- 
sitive condition.  And  some  observers  maintain  that  the  inflammation  of  the 
eye  may  be  greatly  lessened  or  altogether  prevented  if  the  organ  be  carefully 
covered  up,  and  in  all  possible  ways  protected  from  the  irritating  influences 
of  foreign  bodies.  Other  observers,  however,  have  failed  to  prevent  the 
inflammation  in  spite  of  every  care.  So,  also  the  inflammation  of  the  lungs 
following  upon  division  of  both  vagus  nerves  seems  to  be  due,  not  to  any 
direct  nutritive  action  of  the  pulmonary  branches  of  the  vagus  on  the  pul- 
monary tissue,  but  to  food  accumulating  in  the  pharynx,  owing  to  the  paral- 
ysis of  the  oesophagus  and  larynx,  and  then  passing  into  the  air  passages, 
and  so  setting  up  inflammation.  Death  in  these  cases  is,  moreover,  often  the 
simple  result  of  inanition  caused  by  the  paralysis  of  the  oesophagus  allowing 
no  food  to  reach  the  stomach.  The  phenomena  of  the  paralytic  secretion  of 
saliva  are  also  of  a  complicated  nature. 

But  even  without  insisting  on  such  instances  as  the  above,  various  other 
phenomena  of  disease  seem  to  indicate  such  an  influence  of  the  nervous 
system  on  nutrition  as  we  are  discussing.  As  examples  we  might  mention 
the  rapid  and  peculiar  degeneration  of  and  loss  of  contractility  in  the  skeletal 
muscles  in  certain  affections  of  the  spinal  cord,  the  changes  in  the  muscles 
being  more  rapid  and  profound  than  in  the  nerves ;  the  phenomena  of  bed- 
sores, especially  the  so-called  acute  bedsores  of  cerebral  apoplexy;  some  at 
least  of  the  cases  of  vesical  affections  attendant  on  spinal  cerebral  diseases 
or  injuries  ;  the  more  rapid  atrophy  and  loss  of  contractility  in  muscles  which 
follow  upon  contusions  of  nerves  as  compared  with  the  effects  of  simple  sec- 
tion of  nerves ;  the  occurrence  of  certain  eruptions,  such  as  lichen,  zona, 
ecthyma,  etc.,  in  various  spinal  or  cerebral  diseases,  and  indeed  the  general 
phenomena,  and  especially  the  topography  of  the  eruption,  of  a  large  number 
of  cutaneous  diseases.  Lastly,  but  not  least,  we  might  quote  the  general 
process  of  inflammation.  These  are  examples  of  disordered  nutrition.  To 
them  we  might  add  as  instances  of  altered  but  yet  orderly  nutrition  the 
remarkable  connections  observed  between  changes  in  the  form  of  the  fingers 
and  growth  of  the  nails  and  hairs,  and  certain  internal  maladies,  such,  for 
instance,  as  the  "  clubbed  fingers"  of  phthisical  and  other  patients,  and  the 
like.  We  might  also  call  attention  to  the  influence  of  light  on  the  nutrition 
of  animals.  The  experience  of  blind  people  and  blind  animals  indicates 


ON   DIET.  513 

some  special  connection  between  visual  sensations  and  the  nutrition  of  the 
skin  ;  and  this  can  hardly  be  other  than  a  nervous  connection.  The  effects 
of  prolonged  darkness  on  nutrition  in  general  and  the  experimental  results 
which  show  that  the  total  metabolism  of  the  body  is  influenced  by  light,  also 
suggest  some  nervous  action.  The  influence  of  cold,  again,  in  determining 
the  growth  of  hair  points  in  the  same  direction. 

Making  every  allowance  for  the  intervention  in  the  production  of  the 
phenomena  quoted  above  of  such  factors  as  common  actions  of  the  nervous 
system  already  well  known  to  us,  such  as  vasomotor  changes,  making  every 
allowance  for  the  consequences  of  the  failure  or  bluntness  of  sensation  and 
the  absence  of  those  beneficial  after-results  of  muscular  activity  which  we 
pointed  out  in  §  86,  recognizing,  moreover,  that  changes  in  one  organ  may 
affect  the  condition  of  other  distant  organs  by  changes  induced  in  the  com- 
position or  qualities  of  the  blood,  there  still  remains  a  residue  which  seems 
distinctly  to  point  to  the  conclusion  that  the  influence  of  the  nervous  system 
is  not  limited  to  such  changes  of  the  muscles  as  belong  to  the  production  of 
contractions  or  the  generation  of  heat,  but  bears  on  the  whole  nutrition  of 
the  muscle.  Similar  considerations  lead  us  also  to  conclude  that  the  influ- 
ence of  the  nervous  system  bears  on  the  whole  nutrition  of  the  glands,  of  the 
bloodvessels,  of  the  skin,  and  of  the  connective  tissue  in  general,  in  fact  of 
nearly  the  whole  body. 

ON  DIET. 

§  463.  An  ordinary  man  living  an  ordinary  life  will  need  for  the  main- 
tenance of  vigorous  health  a  certain  amount  of  food  of  a  certain  kind  ;  this 
we  may  take  as  a  normal  diet. 

Presuming  that  the  experience  of  man  has  led  him  to  adopt  what  is  good 
for  him,  we  may  ascertain  approximately  the  normal  diet  by  means  of  the 
statistical  method,  by  examining  the  nature  and  amount  of  the  daily  food  of 
a  very  large  number  of  individuals.  The  most  valuable  data  for  this  pur- 
pose are  those  gained  by  inquiries  among  persons  who  choose  their  own  food  ; 
the  results  gained  from  the  diets  used  in  prisons  or  other  institutions,  or 
among  bodies  of  men  such  as  the  army,  though  more  readily  arrived  at,  are 
open  to  the  objection  that  the  diets  in  question  are  determined  in  part  by  the 
theoretical  opinions  of  those  whose  duty  it  is  to  fix  the  diet.  Putting  together 
the  various  statistical  results  thus  obtained,  and  selecting  the  quantities  which 
seem  to  be  most  commonly  used  rather  than  attempting  to  strike  a  strict 
average  or  take  a  strict  mean,  we  find  that  in  an  ordinary  diet  for  the  twenty- 
four  hours  the  several  food-stuffs  are : 

Proteids , from  100  to  130  grammes. 

Fats "      40  to    80 

Carbohydrates "    450  to  550 

to  these  we  must  add 

Salts 30  grammes. 

Water 2800        " 

The  total  (available)  potential  energy  of  the  lower  estimate  is  2610,  of  the 
higher  3505  (kilogramme-degree)  calories,  calculated,  in  round  numbers,  on 
the  data  of  §  441.  With  such  a  statistical  diet  we  may  compare  an  experi- 
mental diet,  that  is  to  say  a  diet  arrived  at  through  a  series  of  trials  on  an 
individual  man  whose  body  might  be  taken  to  be  an  average  one,  that  diet 
being  considered  a  normal  one  in  which  the  body,  maintaining  vigorous 

33 


514  NUTRITION. 

health,  neither  gained  nor  lost  in  weight,  and  remained,  moreover,  in  nitro- 
genous equilibrium  with  the  nitrogen  of  the  egesta  equal  to  that  of  the  ingesta. 
To  make  sure  that  under  such  a  diet  the  body  was  remaining  of  the  same 
composition,  there  ought  to  be  evidence  of  a  carbon  equilibrium  also,  other- 
wise during  the  period  of  the  experiment  fat  might  be  replaced  by  water 
(see  §  435;  ;  but  this  is  unlikely,  and  we  may  therefore  accept  the  method 
as  a  fair  one.  It  has  given  in  the  hands  of  two  different  observers  the 
following  somewhat  different  results,  the  diet  A  being  that  already  quoted 
in  §  441 : 

A  B 

Proteids 100  grammes  118 

Fats 100        "          56 

Carbohydrates 240        "        500 

Salts 25 

Water     2600 

The  total  (available)  potential  energy  is  respectively  2310  and  3035  calories. 

On  the  whole,  the  diets  gained  by  the  two  methods  agree  very  largely. 
To  put  down  a  single  column  of  figures  as  "  the  normal  diet"  would  be  to 
affect  a  vain  and  delusive  accuracy.  If  we  desire,  for  theoretical  purposes, 
to  select  some  one  set  of  figures  rather  than  others,  we  might  be  influenced  by 
the  considerations  that  the  lower  amount  of  proteids  in  the  experimental 
diet  was  nearer  the  mark  than  the  higher  amount  of  some  of  the  statistical 
diets,  and  further  that,  where  cost  is  not  of  moment,  the  substitution  of  fat 
for  an  excess  of  carbohydrates  is  desirable.  We  should  be  thus  led  to  take 
the  experimental  diet  A  as  on  the  whole  the  best  or  most  "  normal "  one,  and 
that  is  the  one  which  we  employed  in  the  calculations  of  §  441.  It  will  be 
observed  that  the  potential  energy  of  this  diet  is  less  than  that  of  any  of  the 
others,  and,  as  we  said  while  then  speaking  of  it,  may  be  considered  low ; 
but  there  was  no  evidence  that  it  was  insufficient.  Still  it  must  be  remem- 
bered that  neither  it  nor  any  of  the  others  is  to  be  regarded  as  distinctly 
proved  to  be  the  real  normal  diet.  Against  the  experimental  diet  we  may 
urge  that  the  number  of  experiments  have  been  few,  and  conducted  on  a  few 
individuals  only  at  most,  and  that  a  larger  number  of  experiments,  with  a 
variety  of  combinations  of  different  amounts  of  the  several  food-stuffs,  might 
lead  to  a  different  result ;  that,  for  instance,  with  certain  amounts  of  fats  and 
carbohydrates,  the  amount  of  proteid  needed  to  maintain  healthy  bodily 
equilibrium,  including  nitrogenous  equilibrium,  might  be  reduced  much  be- 
low the  100  grammes,  especially  if  particular  kinds  of  proteids,  fat,  or  carbo- 
hydrates were  used,  and  especial  attention  (see  §  440)  were  paid  to  the  salts. 
And,  indeed,  a  considerable  number  of  observations  have  been  made  tending 
to  show  that  a  man  of  average  size  and  weight  may  continue  in  nitrogenous 
equilibrium  and  in  good  health  with  a  daily  ration  of  much  less  than  100 
grammes  proteid,  with  as  little  as  40  grammes  for  example.  To  this  we 
shall  have  to  refer  in  speaking  of  a  vegetable  diet.  Against  the  statistical 
diet,  on  the  other  hand,  we  may  urge  that  instinct  is  not  an  unerring  guide, 
and  that  the  choice  of  a  diet  is  determined  by  many  other  circumstances 
than  the  physiological  value  of  the  food. 

§  464.  Taking,  however,  some  such  diet  as  the  above  to  be  the  approxi- 
mately true  normal  diet,  we  may  call  attention  to  the  fact  that  the  normal 
diet  is  made  up  of  each  of  the  three  great  food-stuffs,  carbohydrates  being  in 
excess.  W7e  may  here  remark  incidentally  that  the  diets  of  both  the  car- 
nivora  and  herbivora  agree  with  that  of  omnivora  in  containing  all  three 
food-stuffs ;  they  differ  from  each  other  as  to  the  relative  proportions  only. 
As  we  have  seen,  the  body  may  be  maintained  in  equilibrium  on  proteid  food 


ON  DIET.  515 

alone ;  but  an  exclusively  proteid  diet  is  not  only  bought  dearly  in  the 
market,  but  also  paid  for  dearly  within  the  economy ;  we  are,  of  course, 
now  speaking  of  man.  To  obtain  the  necessary  carbon  out  of  the  carbon 
moiety  of  proteid  unnecessary  labor  is  thrown  on  the  economy,  and  the  system 
tends  to  become  blocked  with  the  amides  and  other  nitrogenous  waste  arising 
out  of  the  nitrogen  moiety  simply  thrown  off  to  secure  the  carbon. 

Fats  and  carbohydrates  are  much  more  akin  to  each  other  than  is  either 
to  proteid ;  and  if,  on  the  one  hand,  as  (§  455)  seems  possible  or  even  prob- 
able, the  fat  of  the  food  and  of  the  body  is  converted  into  sugar  either  on 
its  way  to  become  built  up  into  the  tissue,  or  in  the  course  of  the  changes 
taking  place  outside  the  real  living  framework  of  the  tissue  by  which  it  is 
reduced  to  carbonic  acid,  and  that,  on  the  other  hand,  carbohydrates  can 
furnish  the  fat  whose  presence  in  the  body  is  necessary,  we  might  expect  that 
carbohydrate  alone  without  fat  might  with  proteid  form  a  normal  diet.  But 
on  this  point  experience  is  probably  to  be  trusted  ;  and  we  may  infer  that  in 
every  normal  diet  some  fat  at  least  must  be  added  to  the  starches  and  the 
sugars. 

The  advantage  of  this  mixture  is  probably  felt  while  the  food  is  as  yet 
within  the  alimentary  canal.  What  we  have  learned  concerning  digestion 
leads  us  to  regard  it  as  a  complicated  process,  and  we  cannot  readily  imagine 
that  the  proteolytic,  amylolytic,  and  adipolytic  changes  run  their  several 
courses,  especially  in  the  small  and  large  intestine,  apart  from  and  irrespec- 
tive of  each  other.  We  are  rather  led  to  suppose  that  the  accompaniment 
of  one  set  of  changes,  in  some  indirect  manner,  favors  the  others ;  and  it  is 
for  that  reason  probably  that  we  take  our  food-stuffs  not  separately,  but 
mixed  in  the  same  meal,  often  on  the  same  plate,  and  even  in  the  same  mouth- 
ful. But  apart  from  this  the  two  food-stuffs,  fats  and  carbohydrates,  must 
play  different  parts  in  the  economy,  so  that  the  one  cannot  be  wholly  substi- 
tuted for  the  other ;  and  though,  beyond  the  fact  that  the  one  seems  to  be  a 
source  of  energy  and  the  other  not,  we  do  not  as  yet  know  the  true  physio- 
logical function  of  the  hydrogen  of  the  fat  as  compared  with  that  of  the 
differently  disposed  hydrogen  of  the  carbohydrate,  we  may  perhaps  infer 
that  the  difference  of  use  within  the  body  of  the  two  kinds  of  food-stuffs 
bears  not  so  much  on  their  ultimate  consumption  to  supply  energy,  as  on 
the  various  complicated  processes  which  they  undergo  and  arrangements  in 
which  they  take  part  before  the  end  of  their  work  is  reached.  We  have 
had  a  hint  that  the  carbohydrate  more  rapidly  supplies  the  heat-giving 
metabolism  than  does  the  fat ;  and  this  suggests  an  advantage  to  the  economy 
in  receiving  daily  a  certain  portion  of  the  more  tardy  material,  while  at  the 
same  time  it  may  be  taken  to  mean  that  the  fat  before  it  is  used  to  give  rise 
to  energy  has  first  to  be  converted  into  sugar,  and  so  takes  more  time  in  its 
work. 

The  main  carbohydrate  of  every  diet  is  starch,  and  as  far  as  we  can  learn 
at  present,  the  starch; which  is  so  large  a  part  of  the  cereals  and  vegetables 
consumed  by  man  is  the  same  body  in  all  of  them  ;  for  the  use  of  such  bodies 
as  inulin  is  so  insignificant  that  it  may  be  neglected.  Man,  however,  con- 
sumes no  inconsiderable  quantity  of  sugar,  chiefly  cane  sugar.  Since  the 
starch  of  a  meal  does  not  become  available  for  the  economy  until  it  has  been 
converted  into  sugar,  we  might  be  inclined  to  infer  that  it  was  a  matter  of 
indifference  whether  the  carbohydrate  of  a  diet  were  supplied  as  starch  or 
as  sugar.  But  besides  the  fact  that  any  large  deficit  of  starch  in  a  diet 
might  seriously  interfere  with  the  general  course  of  digestion,  especially  if  as 
urged  above  the  several  digestive  processes  are  more  or  less  dependent  on 
each  other,  it  must  be  remembered  that  the  sugar  into  which  starch  is 
changed  by  digestion  is  maltose,  while  cane  sugar  appears  to  be  either 


516  NUTRITION. 

absorbed  as  cane  sugar  or  at  most  only  inverted.  Moreover,  if  our  labora- 
tory experiments  truly  represent  the  digestion  taking  place  in  the  living 
body,  only  part  of  the  starch  (§  194)  is  changed  into  maltose,  while  part 
becomes  some  variety  of  dextrine  or  of  starch.  Our  knowledge  of  sugars 
and  of  their  fate  in  the  economy  is  too  imperfect  for  us  to  be  able  to  state 
the  effects  on  the  body  of  digested  starch  as  compared  with  those  of  cane 
sugar  or  milk  sugar ;  but  that  these  are  or  may  be  different  is  shown  by  the 
experience  of  medical  practice.  In  many  cases  the  total  effect  on  the  body 
of  a  diet  from  which  cane  sugar  is  as  much  as  possible  eliminated,  though 
starch  be  allowed,  is  very  different  from  that  of  one  of  which  cane  sugar 
forms  an  appreciable  part. 

Concerning  cellulose,  which  in  herbivora  appears  certainly  to  serve  as 
a  source  of  energy  and  to  be  a  real  food-stuff,  our  knowledge  will  not  allow 
us  to  decide  whether  it  has  any  special  uses  of  its  own,  or  whether  the  body 
is  simply  led  to  utilize  and  make  the  best  of  what  is  a  necessary  accompani- 
ment of  the  starch  of  vegetable  food. 

Concerning  the  salts  present  in  a  diet,  we  need  only  repeat  what  was  said 
in  §  440,  that  these,  though  affording  of  themselves  little  or  no  energy,  are 
as  essential  a  part  of  a  diet  as  the  energy-giving  food-stuffs,  inasmuch  as  they 
in  some  way  or  other  direct  metabolism  and  the  distribution  of  energy. 
And  this  is  true  not  only  of  the  inorganic  salines,  such  as  chlorides  and 
phosphates,  but  also  of  the  so-called  extractives.  As  we  have  seen,  the 
presence  of  these  bodies,  both  the  simpler  inorganic  and  the  more  complex 
organic  salts,  in  the  blood  or  in  the  extra-vascular  juices  or  lymph  of  the 
tissues  is  essential  to  or  directs  or  modifies  the  metabolic  activity  of  the  sev- 
eral tissues.  The  beneficial  effects,  as  components  of  special  diets,  of  such 
things  as  beef-tea  and  meat  extract,  which  consist  chiefly  of  salts  and  ex- 
tractives, with  a  very  small  quantity  of  albumose  or  other  forms  of  proteid, 
and  the  effects  either  beneficial  or  deleterious  of  drugs,  both  turn  in  common 
upon  their  taking  a  part  of  some  kind  or  other  in,  it  may  be  upon  their 
interference  with,  metabolic  processes.  The  salts  and  extractives  of  a  diet 
may  be  looked  upon  as  necessary  daily  medicines,  and  a  medicine  as  a  more 
or  less  extraordinary  variation  in  these  elements  of  a  diet. 

Alcohol,  to  the  use  of  which  as  a  component  of  an  ordinary  diet  special 
interest  for  various  reasons  attaches,  comes  in  this  class.  For  though 
observations  show  that  the  greater  part  of  a  moderate  dose  of  alcohol  is 
oxidized  within  the  body,  and  so  serves  as  a  source  of  energy,  man  ha& 
recourse  to  alcohol  not  for  the  minute  quantity  of  energy  which  is  supplied 
by  itself,  but  for  its  powerful  influence  on  the  distribution  of  the  energy 
furnished  by  other  things.  That  influence  is  a  very  complex  one  and  can- 
not be  fully  discussed  here.  It  is  stated  that  moderate  or  small  doses  of 
alcohol  diminish  the  consumption  of  oxygen  and  production  of  carbonic 
acid,  that  is  to  say,  diminish  the  total  result  of  the  metabolism  of  the  body, 
while  larger  but  still  not  intoxicating  doses  have  a  contrary  effect  and  in- 
crease the  total  metabolism.  But  such  a  statement  affords  no  sound  basis 
for  any  conclusion  as  to  the  general  physiological  effect  of  alcohol,  or  as  to 
its  usefulness  as  part  of  an  ordinary  diet ;  it  does  not  justify  such  a  conclu- 
sion, for  example,  as  that  alcoholic  drinks,  taken  in  moderation,  by  diminish- 
ing metabolism  economize  the  resources  of  the  body.  The  prominent  physio- 
logical problem  of  dietetics  is  not  either  to  increase  or  diminish  the  metab- 
olism of  the  body,  but  to  direct  that  metabolism  into  proper  channels ;  and 
whether  in  each  particular  case  a  given  dose  of  alcohol  gives  a  right  or  a 
wrong  turn  to  the  physiological  processes  of  the  body  depends  on  the  partic- 
ular circumstances  of  the  case.  For  the  action  of  all  these  bodies  of  which 
we  are  now  speaking,  in  contrast  with  the  actions  of  the  food-stuffs  proper,, 


ON  DIET.  517 

is  not  only  complex  but  variable ;  so  complex  and  variable  that  simple  ex- 
perience is  at  present  a  more  trustworthy  guide  than  speculative  physiology. 
We  may  add  that  the  physiological  action  of  alcoholic  drinks  is  still  further 
complicated  by  the  fact  that  most  such  drinks  contain,  beside  ethylic  alcohol, 
various  other  allied  substances,  whose  action  is  even  more  potent  than  that 
of  the  ethylic  alcohol  itself,  and  whose  presence  very  markedly  determines 
the  total  effect  of  the  drink.  Such  articles  of  diet  as  tea  and  coffee  stand 
upon  very  much  the  same  footing  as  alcohol. 

The  quantity  of  fluid  which  a  man  drinks  or  should  drink  daily,  or 
more  correctly  the  quantity  of  water  which  he  should  daily  add  to  the 
dry  solids  of  his  diet,  must  vary  widely  according  to  circumstances.  It 
will  differ  according  as  he  is  perspiring  freely  or  not,  according  to  the 
nature  of  the  dry  solids  of  the  diet,  whether  largely  carbohydrate  or  not, 
and  so  on.  A  lower  limit,  below  which  excretion  is 'impeded,  and  a  higher 
limit,  above  which  digestion  and  metabolism  are  injuriously  affected,  prob- 
ably exist ;  but  we  have  as  yet  no  adequate  data  which  will  enable  us  to  fix 
either  of  them. 

§  465.  In  the  selection  of  articles  of  food  to  supply  the  food-stuffs  and 
other  constituents  of  a  normal  diet,  regard  must,  of  course,  be  had  in  the 
first  place  to  the  amount  of  potential  energy  present  in  the  material.  The 
articles  chosen  for  the  daily  fare  must  contain  between  them  so  much  pro- 
teid,  fat,  and  carbohydrate,  representing  so  much  available  energy.  But 
it  is  no  less  important  that  the  potential  energy  in  the  material  should  be 
really  available  for  the  economy.  The  material  must  have  such  qualities 
that  it  is  digested  within  the  alimentary  canal,  and  further  that  its  digestion 
and  absorption  do  not  give  rise  to  trouble  either  in  the  alimentary  canal  or 
in  that  secondary  digestion  carried  on  by  means  of  the  various  metabolic 
events  which  we  have  discussed  in  preceding  sections.  A  really  nutritious 
substance  is  one  which  not  only  contains  in  itself  an  adequate  supply  of 
energy,  but  is  of  such  a  nature  that  its  energy  can  be  appropriated  by  the 
economy  with  ease,  or  at  least  with  as  little  trouble  as  possible.  We  have 
approximate  data  for  determining  how  far  an  estimate  of  the  relative  use- 
fulness of  various  articles  of  food  must  be  corrected,  by  allowing  for  the  pro- 
portion of  each  which  after  an  ordinary  meal  merely  passes  through  the 
alimentary  canal,  and  the  energy  of  which  is  not  in  any  way  available  for 
the  body's  use.  Thus,  a  number  of  observations  carried  out  on  healthy 
individuals  gave,  in  the  case  of  the  following  articles  of  food,  the  following 
figures  as  the  percentage,  reckoned  in  each  case  on  dry  material,  which 
could  be  recovered  from  the  feces,  and  was,  therefore,  not  digested  and  not 
used  by  the  body  :  Meat,  5  per  cent. ;  eggs,  5  per  cent. ;  milk,  9  per  cent. ; 
bread  (white),  4  per  cent. ;  black  bread,  15  per  cent. ;  rice,  4  per  cent. ; 
maccaroni,  4  per  cent. ;  maize,  7  per  cent. ;  peas,  9  per  cent. ;  potatoes,  11 
per  cent.  It  must,  however,  be  remembered  that  the  actual  correction  to  be 
made  in  any  case  will  depend  on  the  mode  of  cooking  of  the  material,  on 
the  character  of  the  meal  of  which  it  forms  part,  and  on  the  individual 
capabilities  of  the  consumer,  the  latter,  too,  varying  under  different  circum- 
stances. 

The  above  refers  to  what  may  be  called  rough  digestibility,  but  besides 
this  there  are  other  circumstances  to  be  considered.  The  same  food-stuff 
in  two  articles  of  food,  though  actually  digested,  that  is  to  say  taken  up  by 
the  alimentary  canal,  may,  even  while  still  within  the  alimentary  canal, 
undergo  changes  in  the  one  case  differing  from  those  in  the  other.  A  pro- 
teid  may  for  instance  in  one  case  tend  to  be  entirely  converted  into  peptone, 
or  to  break  up  into  leucin,  etc.,  or  in  other  cases  to  undergo  other  changes ; 
and  a  carbohydrate  may  in  one  case  be  absorbed  as  maltose,  and  in  another 


518  NUTRITION. 

give  rise  to  lactic  acid.  Indeed,  when  we  speak  of  the  digestibility  or  the 
indigestibility  of  this  or  that  article  of  food,  we  do  not  in  many  cases  so 
much  mean  the  relative  amount  of  the  substance  taken  up  in  some  way  or 
other  by  the  alimentary  canal  as  the  characters  advantageous  or  otherwise 
of  the  changes  which  it  undergoes  in  being  so  taken  up. 

Hence  the  purely  chemical  statement  of  the  amount  of  potential  energy 
present  in  an  article  of  food  is  no  safe  guide  of  the  physiological  value  of 
the  substance.  A  chunk  of  cheese  stands  very  high  on,  generally  at  the 
top  of,  a  table  of  the  nutritive  value  of  articles  of  food  drawn  up  on  ex- 
clusively chemical  principles — according  to  the  units  of  energy  present  in 
a  unit  of  the  material — but  it  is  very  low  down  in  a  corresponding  physio- 
logical table.  And  similarly  a  dish  of  old  peas  has  a  very  different  physio- 
logical function  from  a  plate  of  fresh  meat,  even  when  both  contain  the 
same  amount  of  nitrogen. 

In  thus  correcting  for  digestion  the  nutritive  value  of  a  diet  it  must 
also  be  borne  in  mind  that  the  alimentary  canal,  while  chiefly  a  receptive 
organ,  is  also  to  some  extent  (§  245)  an  excretory  organ  ;  a  free  passage 
through  the  canal  is  needed  not  only  for  carrying  off  undigested  matter  but 
also  for  getting  rid  of  excreted  matter ;  and  the  presence  of  the  former,  up 
to  certain  limits,  assists  the  discharge  of  the  latter.  Were  it  possible  to 
prepare  a  diet  every  jot  and  tittle  of  which  could  be  digested  and  absorbed, 
the  use  of  such  a  diet  would  probably  bring  about  disorder  in  the  economy, 
through  the  absence  of  a  sufficiently  rapid  discharge  of  the  matters  ex- 
creted into  the  alimentary  canal.  Hence  cellulose  and  like  substances, 
even  when  unutilized  through  absorption,  are  not  without  their  use,  and 
experience  shows  that  digestion  may  be  promoted  by  eating  undigestible 
things. 

§  466.  The  several  food-stuffs  of  a  diet  may  be  drawn  from  the  animal 
or  from  the  vegetable  kingdom.  Vegetable  proteids  appear  to  undergo  the 
same  changes  in  the  alimentary  canal  as  do  animal  proteids,  and  the  main 
effects  on  the  body  of  proteids  from  the  two  sources  seem  to  be  the  same. 
Our  knowledge  at  present,  however,  is  too  imperfect  to  enable  us  to  decide 
whether  the  functions  of  the  two  are  exactly  the  same,  whether  the  body 
behaves  exactly  the  same  upon  a  diet  in  which  the  proteids  are  exclusively 
of  vegetable  origin,  as  upon  a  diet  in  which,  otherwise  the  same,  the  pro- 
teids are  partly  of  animal  origin  also.  Nor  have  we  much  better  know- 
ledge of  the  relative  nutritive  value  of  vegetable  and  animal  fats.  And  as 
we  have  already  said,  we  possess  little  or  no  exact  knowledge  as  to  the  part 
played  by  those  extractives  in  respect  to  the  amount  and  nature  of  which 
animal  food  strikingly  differs  from  vegetable  food.  In  attempting,  there- 
fore, a  judgment  from  a  purely  physiological  point  of  view  as  to  the  value 
of  an  exclusively  vegetarian  diet  compared  with  a  diet  of  both  animal  and 
vegetable  origin,  we  can  do  little  more  at  present  than  inquire  whether  the 
former  supplies  the  several  food-stuffs  in  adequate  quantity,  in  proper  pro- 
portion, and  in  such  a  form  as  to  be  economically  utilized  by  the  body. 

The  careful  examination  during  three  separate  periods  of  several  days 
each  of  the  ingesta  and  egesta  of  a  man,  28  years  old,  weighing  57  kilos, 
who  had  for  three  years  lived  on  an  exclusively  vegetable  diet,  viz.,  bread, 
fruit,  and  oil,  gave  the  following  results  : 

The  daily  diet  consisted  on  the  average  of  719  grms.  solid  matter  and 
1084  grms.  water.  It  contained 

Proteids 54  grammes,  containing  8.4  N. 

Fats 22  grammes. 

Carbohydrates 557  grammes  (about  J  sugar  and  \  starch). 

(Cellulose) 16  grammes. 


ON   DIET.  519 

The  daily  feces  weighed,  when  fresh,  333  grms.  containing  75  grms.  solid 
matter,  and  were  therefore  both  bulky  and  watery.  There  were  present  in 
the  feces,  fat  7  grms.,  starch  17  grms.,  cellulose  9  grms.,  showing  that  30 
per  cent,  of  the  fat,  6  per  cent  of  the  starch,  and  56  per  cent,  of  the  cellu- 
lose had  not  been  utilized  by  the  body.  The  subject  had  really  lived  on 
fat,  15  grms.,  carbohydrates "540  grms.  (and  cellulose  7  grms.).  The  feces 
contained  no  less  than  3.46  nitrogen.  If  we  reckon  the  whole  of  this  as 
proteid,  this  would  give  22  grms.  of  undigested  proteid,  so  that  there  has 
been  a  waste  of  41  per  cent,  of  the  proteids,  leaving  only  32  grms.  avail- 
able for  real  use  in  the  body ;  and,  indeed,  a  very  small  portion  only  of  this 
nitrogen  can  be  regarded  a3  really  discharged  from  the  body  itself.  The 
total  solids  of  the  feces  must  be  reckoned  as  partly  excreta  but  chiefly  un- 
digested food.  If  we  regard  the  75  grms.  of  solid  feces  as  entirely  undi- 
gested food,  the  whole  solid  food  available  for  the  body  must  be  reduced 
from  719  grms.  to  644  grms. 

The  urine  of  the  day  contained  5.33  grms.  nitrogen  ;  this  added  to  the 
3.46  grms.  nitrogen  in  the  feces  gives  8.79  grms.  nitrogen  in  the  total  egesta 
as  compared  with  the  8.4  grms.  nitrogen  of  the  food,  indicating  a  slight 
loss  of  nitrogenous  material  from  the  body ;  but  if  we  suppose  that  all  the 
nitrogen  in  the  feces  was  not  in  the  form  of  undigested  food  we  may  neglect 
this ;  and  indeed  the  subject  of  the  observation  was  in  apparently  good 
health  and  stationary  weight. 

Compared  with  either  of  the  normal  diets  given  in  §  463,  the  above  diet 
is  striking  for  the  low  amount  of  proteids  and  of  fats  and  the  relative  ex- 
cess of  carbohydrates.  But  though  such  a  diet  may  be  taken  as  perhaps 
fairly  typical  of  the  daily  food  of  a  rigid  vegetarian,  a  much  more  richly 
proteid  diet  may  be  obtained  from  sources  still  strictly  vegetable.  Thus 
the  diet,  entirely  vegetable  in  nature,  of  an  average  Japanese  laborer  of 
about  the  same  weight  as  the  individual  whose  data  we  have  just  given  has 
been  estimated  to  consist  of  proteids  102  grms.,  fat  17  grms.,  carbohydrates 
578  grms.  And  the  diet  of  a  Roumanian  peasant,  living  chiefly  on  beans 
and  maize  with  the  addition  of  fat  of  some  kind,  has  been  calculated  to 
furnish  no  less  than  proteids  182  grms.,  fat  93  grms.,  carbohydrates  968 
grms. ;  but  the  real  nutritive  value  of  such  a  diet  must  need  very  large  cor- 
rection indeed.  (  Cf.  §  465.) 

The  examination  of  the  diet  of  an  individual  living  with  a  fair  nitro- 
genous equilibrium  and  apparently  good  health  on  .a  modified  vegetable 
diet — that  is  to  say,  one  which  included  milk  and  eggs — gave  the  follow- 
ing :  Proteids,  74  grms. ;  fat,  58  grms. ;  carbohydrates,  490  grms.,  a  diet 
which  differs  from  the  normal  diet  almost  solely  in  the  lesser  amount  of 
proteids,  one-third  of  which,  by  the  by,  was  supplied  by  the  animal  mate- 
rial, eggs  and  milk.  In  another  instance,  nitrogenous  equilibrium  and 
fairly  good  health  were  secured,  for  some  weeks  at  all  events,  on  a  vege- 
table diet  yielding  proteids,  about  100  grms. ;  fat,  70  grms. ;  carbohydrates, 
400  grms. ;  but  in  this  nearly  the  whole  of  the  fat  was  furnished  by  the 
animal  product  butter,  and  Liebig's  extract  was  freely  used. 

Confining  ourselves,  however,  to  the  more  strictly  vegetarian  diet,  we 
may  conclude  in  the  first  place  that,  unless  the  daily  food  be  very  large  in 
amount,  the  proteid  element  of  such  a  diet  falls  considerably  below  the  100 
or  more  grms.  given  in  the  normal  diet.  But  we  cannot  authoritatively  say 
that  such  a  reduction  is  necessarily  an  evil ;  for,  as  we  stated  above  (§  463), 
our  knowledge  will  not,  at  present,  permit  us  to  make  an  authoritative 
exact  statement  as  to  the  extent  to  which  the  proteid  may  be  reduced  with- 
out disadvantage  to  the  body  when  accompanied  by  adequate  provision  of 
the  other  elements  of  food  ;  and  this  statement  holds  good  whether  the  body 


520  NUTRITION. 

be  undertaking  a  small  or  large  amount  of  labor.  A  second  feature  of  such 
a  diet  is  the  marked  reduction  of  the  fat  and  its  replacement  by  carbohy- 
drates. Although  here  again  we  cannot  make  a  distinctly  authoritative 
statement,  the  evidence  which  we  possess  bears  clearly  in  the  direction  that 
such  a  reduction  is  a  marked  disadvantage.  A  third  and  very  characteristic 
feature  of  the  strictly  vegetarian  diet  is  the  relatively  large  amount  of  un- 
digested food  lost  to  the  body  and  discharged  as  feces.  Even  when  the  diet 
is  scanty,  so  that  the  proteid  element  is  low,  the  amount  of  feces  relatively 
to  the  total  food  is  high ;  and  when  a  more  normal  proteid  contribution  is 
secured  by  ample  meals  the  feces  becomes  exceedingly  voluminous.  Indeed 
when,  leaving  man,  we  compare  the  herbivorous  with  the  carnivorous  mam- 
mal, we  find  that  the  former  is  almost  as  clearly  distinguished  from  the  latter 
by  its  frequent  and  abundant  feces  as  by  the  anatomical  features  of  its  or- 
ganization. We  have  already  urged  that,  since  the  feces  serve  as  a  means 
of  excretion  of  the  real  waste  products  of  metabolism,  a  certain  amount  of 
vehicle  to  carry  these  away  is  of  advantage  or  even  necessary  ;  but  there  are 
no  facts  at  present  known  to  us  which  show  that  the  larger  intestinal  cur- 
rent of  the  purely  vegetable  diet  effects  any  such  good  as  can  compensate 
for  the  obvious  waste  of  labor  incurred  in  the  transport  and  management, 
to  say  nothing  of  the  opportunities  of  mischief  offered  by  a  mass  of  mate- 
rial more  subject  to  the  dominion  of  foreign  organisms  than  even  to  that  of 
the  body  itself,  though  these  opportunities  are  less  than  with  a  correspond- 
ing mass  of  animal  origin.  With  respect  to  these  three  features,  then,  the 
strictly  vegetarian  diet  seems,  on  physiological  grounds,  inferior  to  one  of 
a  mixed  nature.  There  are,  as  we  said,  other  aspects,  still  of  a  physiologi- 
cal kind,  to  be  considered,  such  as  the  relative  digestibility  of  vegetable 
articles  of  food,  the  relative  metabolic  value  of  the  food-stuffs  of  vegetable 
origin,  and  the  influence  of  animal  extractives  ;  but  any  fuller  discussion  of 
these  points  would  be  out  of  place  here. 

§  467.  We  have  treated  the  diet  discussed  above  as  a  normal  diet,  suit- 
able for  man  under  ordinary  or  general  circumstances.  Ought  such  a  diet 
to  be  modified  for  the  various  exigencies  of  life,  such  as  labor,  age,  climate, 
and  the  like  ? 

We  shall  discuss  the  influence  of  age  in  the  concluding  portions  of  this 
work. 

We  may  be  inclined,  at  first  sight,  to  assume  that  the  total  amount  of 
the  diet  should  vary  with  the  weight,  that  is,  the  size,  of  the  individual; 
and,  indeed,  in  discussions  on  nutrition,  statements  concerning  metabolism 
and  amounts  of  food  are  often  given  in  terms  of  per  kilo  of  body  weight. 
In  a  broad  sense,  it  may  be  true  that  a  small  man  needs  less  food  than 
a  large  one ;  but  it  must  be  remembered  that,  as  we  saw  in  speaking  of 
animal  heat,  the  smaller  organism,  having  the  relatively  larger  surface, 
carries  on  a  more  rapid  metabolism  per  unit  of  body  weight,  and  so  needs 
relatively  more  food.  And,  moreover,  the  influence  of  size  is  probably  far 
less  than  the  influence  exerted  by  the  inborn  individual  characters  of  the 
organism,  giving  rise  to  what  we  may  call  the  personal  equation  of  metab- 
olism. The  smaller  metabolism  of  woman,  leading  to  the  use  of  scantier  diet, 
as  compared  with  that  of  man,  is  to  be  regarded  in  this  light  rather  than 
with  reference  to  the  average  lesser  weight  of  woman.  The  relative  metab- 
olism of  the  two  sexes  may  be  illustrated  by  the  case  of  an  active  man  and 
his  wife,  both  of  about  the  same  age  and  weight,  the  man  being  rather  the 
heavier  and  the  woman  rather  the  older,  who  in  carrying  out  together  an 
experiment  on  the  relative  values  of  vegetable  and  animal  food,  both  lived 
for  some  time  on  the  same  kind  of  diet,  and  found  that  nutritive  equilibrium 
was,  in  the  one  case  and  in  the  other,  maintained  when 


ON   DIET.  521 

Proteids.  Fats.  Carbohydrates. 

The  man  consumed  daily  about  .    .    .    100  70  400 

The  wife         'k  tk      .    .    .     60  67  340 

The  most  striking  difference  is  in  the  proteids. 

§  468.  With  regard  to  climate,  the  chief  considerations  attach  to  tem- 
perature. When  the  body  is  exposed  to  a  low  temperature  the  general  metab- 
olism of  the  body  is  increased  owing  to  a  regulative  action  of  the  nervous 
system  (§  448).  We  might  infer  from  this  that  more  food  is  necessary  in 
cold  climates  ;  and,  since  the  increase  in  the  metabolism  appears  to  manifest 
itself  chiefly  in  a  greater  discharge  of  carbonic  acid,  and  therefore  to  be  espe- 
cially a  carbon  metabolism,  we  might  infer  that  the  carbon  elements  of 
food  should  be  especially  increased.  When  the  body  is  exposed  to  high  tem- 
peratures, the  same  reflex  mechanism  tends  to  lower  the  metabolism  ;  but  the 
effects  in  this  direction  are  much  less  clear  than  those  of  cold,  and  soon 
reach  their  limits;  the  bodily  temperature  is  maintained  constant  under  the 
influence  of  surrounding  warmth  not  so  much  by  diminished  production  as 
by  increased  loss.  We  may  infer  from  this  that  in  warm  climates  not  less, 
but,  if  anything  rather  more,  food  than  in  temperate  climates  is  necessary  in 
order  to  supply  the  perspiration  needed  for  the  greater  evaporation  and  dis- 
charge of  heat  by  the  skin. 

In  both  cold  and  warm  climates,  however,  man  trusts  much  more  to  vari- 
ations in  his  clothing  and  immediate  surroundings  to  protect  him  against 
cold  or  to  guard  him  from  heat  than  to  any  marked  variations  in  his  normal 
diet.  In  the  former  he  may,  perhaps,  be  expected  to  eat  somewhat  more, 
since,  in  spite  of  wrappings,  the  skin  still  feels  in  part  the  cold,  and  thus  the 
nervous  mechanism  for  the  increase  of  metabolism  is,  to  a  certain  extent,  set 
to  work.  And  since  the  metabolism  thus  increased  appears  to  affect  espe- 
cially the  carbon  of  the  body,  he  may  further  be  expected  to  increase  the  fats 
rather  than  the  carbohydrates  of  his  food,  seeing  that  the  former  supply 
him  with  the  most  energy  for  their  weight.  But  it  is  very  doubtful  whether 
what  he  might  thus  be  expected  to  gain  over  a  corresponding  increase  in 
carbohydrates  is  not  more  than  counterbalanced  by  the  increased  labor  of 
digestion ;  and  the  habits  of  the  dwellers  in  arctic  climates  cannot  safely  be 
taken  as  guides  in  this  matter,  for  their  reputed  love  of  fat  is  probably  the 
result  of  that  being  their  most  available  form  of  carbon.  Indeed,  the  evi- 
dence that  the  increase  of  metabolism  provoked  by  cold  bears  exclusively 
on  carbon  constituents  is  so  uncertain  that  it  may  be  doubted  whether  any 
change  in  the  normal  diet,  beyond  some  increase  in  the  whole,  should  be  made 
to  meet  a  cold  climate.  Similar  reasons  would  lead  one  to  infer  that  man 
in  the  warmer  climate  would  maintain,  on  the  whole,  the  same  normal  diet, 
the  only  change  being,  perhaps,  to  increase  it  slightly,  possibly  throwing 
the  increase  chiefly  on  the  carbohydrates  with  the  special  view  of  further- 
ing perspiration. 

§  469.  A  special  diet  for  the  purpose  of  fattening — that  is  to  say,  for  the 
accumulation  of  adipose  tissue  out  of  proportion  to  the  rest  of  the  body — is 
not  needed  in  the  case  of  man.  The  power  to  store  up  fat  in  adipose  tissue 
is  much  more  dependent  on  certain  inborn  qualities  of  the  organism  which 
we  cannot  at  present  define  than  on  the  kind  of  food  ;  of  two  bodies  living 
on  the  same  diet,  and  under  the  same  circumstances,  one  will  become  fat 
while  the  other  will  remain  lean  ;  and  it  is  an  object  of  the  agriculturist  to 
develop  by  breeding  and  selection  a  "  constitution  "  which  will  store  up  the 
most  fat  on  the  cheapest  diet.  In  fattening  animals  the  chief  care,  when  the 
selection  of  the  kind  of  animal  has  been  made,  is  to  provide  adequate  carbo- 
hydrate food,  which,  as  we  have  seen,  is  the  chief  fattener  ;  and  the  object  of 
the  farmer  in  rearing  stock  for  the  butcher  is  mainly  to  convert  cheap  vege- 


522  NUTRITION. 

table  carbohydrate  into  dear  animal  fat.  Further  aids  in  fattening  may  be 
found  in  providing  repose  for  the  body  of  such  a  kind  that,  while  sufficient 
energy  is  expended  to  secure  adequate  digestion  and  absorption  of  food,  all 
causes  leading  to  an  increase  of  metabolism  by  which  energy  is  set  free  and 
leaves  the  body  are  avoided  as  much  as  possible. 

To  avoid  fat  rather  than  to  increase  it  is  often  an  object  of  human  care. 
This  may  be  effected  by  diminishing  fats  and  carbohydrates,  but  also,  in  a 
very  marked  manner,  by  relatively  increasing  the  proteids.  Proteid  food, 
as  we  have  seen,  augments  the  whole  metabolism  of  the  body,  hurrying  on 
the  destruction  not  only  of  proteid  but  of  carbon  food ;  and  a  tendency  to 
corpulency  may  be  counteracted  by  a  diet  in  which  fats  and  carbohydrates 
are  much  restricted,  and  proteids  are  largely  increased.  When,  as  in  what 
is  known  as  the  Banting  method,  the  diet  is  almost  exclusively  proteid,  the 
nitrogenous  overwork  entails  dangers  on  organisms  which  do  not  possess  the 
power  of  ridding  themselves  freely  of  the  large  amount  of  nitrogenous  waste 
which  such  a  diet  produces.  A  less  severe  method  in  which  the  fats  and 
carbohydrates  are  diminished  only,  not  entirely  done  away  with,  and  the 
proteids  only  moderately  increased,  is  less  open  to  objection  ;  and  such  a  diet, 
assisted  by  other  hygienic  conditions,  has  proved  successful. 

An  increase  of  daily  food,  largely  proteid  in  nature,  given  under  cir- 
cumstances such  as  a  large  amount  of  passive  exercise  and  skin  stimulation, 
known  as  "  massage,"  which  will  not  only  favor  digestion  but  also  promote 
metabolism  in  general,  may  be  given  with  favorable  results.  In  this  way 
an  enormous  metabolism  may  be  excited,  and  yet  so  carried  on  that  the 
body  gains  both  in  flesh  and  in  fat.  Thus,  in  one  case,  the  patient  with  an 
initial  weight  of  45  kilos,  and  a  daily  nitrogenous  metabolism  calculated  as 
28  grms.  proteid,  reached  in  the  course  of  about  fifty  days  a  weight  of  60 
kilos,  the  daily  nitrogenous  metabolism  being  raised  on  one  occasion  to  182 
grms.  proteid/ with  an  average  on  the  whole  period  of  150  grms.  During 
the  treatment  no  less  than  8420  grms.  of  proteid  were  taken  as  food. 

§  470.  With  regard  to  labor,  since,  as  we  have  seen,  the  energy  expended 
as  work  done  is  not  taken  out  of  and  away  from  the  amount  set  free  as  heat, 
the  two  forms  of  energy  being  so  related  that  an  increase  of  work  done  is 
accompanied  by  a  greater  or  less  increase  of  heat  set  free,  it  is  obvious  that 
a  man  who  is  doing  a  hard  day's  muscular  work  needs  a  larger  income  of 
energy  for  the  day  than  does  an  idle  man.  What  we  have  learnt  concerning 
muscular  metabolism  further  shows  us  that  the  additional  energy  needed  is 
not  necessarily  to  be  supplied  by  an  increase  in  the  proteid  components  of 
the  diet ;  the  energy  of  muscular  contraction  does  not  come,  as  was  once 
thought,  from  proteid  metabolism  (§  443).  The  fact  that  it  is  the  carbon 
metabolism  which  is  augmented  in  muscular  work  may  suggest  that  the 
extra  food  for  extra  work  should  be  exclusively  carbon  compounds ;  and  if, 
as  seems  probable,  the  carbohydrates  are  more  readily  and  directly  available 
for  the  functional  metabolism  of  muscle  than  are  the  fats,  we  might  be  further 
led  to  recommend  an  increase  in  carbohydrates  to  form  a  diet  especially  suited 
for  labor.  But  several  considerations  should  make  us  hesitate  before  we  come 
to  such  a  conclusion.  A  muscle  is  not  a  machine  within  the  body  which  can 
be  loaded  and  fired  off  irrespective  of  the  rest  of  the  body.  In  the  perform- 
ance of  muscular  labor,  the  condition  of  the  muscle,  the  amount  of  energy 
available  in  the  muscle  itself,  is  of  course  of  prime  importance ;  but,  and 
this  perhaps  especially  holds  good  in  severe  labor,  of  great  importance  also, 
we  might  almost  say  of  no  less  importance,  is,  as  we  have  urged  (§  333),  the 
power  of  the  body  as  a  whole  to  avail  itself  of  the  energy  latent  in  the  mus- 
cle. The  power  of  doing  work  hangs  not  on  the  muscle  alone,  but  on  the 
heart,  the  lunjis,  the  nervous  system,  and,  indeed,  on  the  whole  body.  It  is 


ON  DIET.  523 

very  doubtful  whether  we  ever,  even  in  supreme  efforts,  draw  upon  more 
than  a  portion  of  the  capital  of  energy  lodged  in  the  muscle  itself;  fatigue 
is  far  more  a  nervous  than  a  muscular  condition,  and  even  the  distinctly 
muscular  fatigue  is  as  we  have  seen  (§  86),  partly  at  least  the  result  of  the 
accumulation  of  products  and  not  alone  the  using  up  of  available  energy. 
In  choosing  a  diet  for  muscular  labor  we  must  have  in  view  not  the  muscle 
itself  but  the  whole  organism.  And  though  it  is  possible  that  future  research 
may  suggest  minor  changes  in  the  various  components  of  a  normal  diet  such 
as  would  lessen  the  strain  during  labor  on  this  or  that  part  of  the  body,  on 
the  muscles  as  well  as  on  other  organs,  our  present  knowledge  would  rather 
lead  us  to  conclude  that  what  is  good  for  the  organism  in  comparative  rest 
is  good  also  for  the  organism  in  arduous  work,  that  the  diet,  normal  for  the 
former  condition,  would  need  for  the  latter  a  limited  total  increase  but  no 
striking  change  in  its  composition.  In  preparing  the  body  for  some  com- 
ing arduous  labor,  in  "  training,"  as  it  is  called,  an  increase  of  proteid  food, 
for  the  purpose  of  hurrying  on  the  general  metabolism  of  the  body,  and 
thus  of  making  "  new  flesh  "  and  renovating  the  body,  so  to  speak,  in  view 
of  the  strain  to  be  put  upon  it,  may  perhaps  suggest  itself;  but  even  this  is 
doubtful. 

The  principles  of  such  a  conclusion  with  regard  to  muscular  work  may 
be  applied  with  still  greater  confidence  to  nervous  or  mental  work.  The 
actual  expenditure  of  energy  in  nervous  work  is  relatively  small,  but  the 
indirect  influence  on  the  economy  is  very  great.  The  closeness  and  intri- 
cacies of  the  ties  which  bind  all  parts  of  the  body  together  are  very  clearly 
shown  by  the  well-known  tendencies  of  so-called  brain  work  to  derange  the 
digestive  and  metabolic  activities  of  the  body ;  and  if  there  be  any  diet 
especially  suited  for  intellectual  labor  it  is  one  directed  not  in  any  way 
toward  the  brain,  but  entirely  toward  lightening  the  labors  of  and  smoothing 
the  way  for  such  parts  of  the  body  as  the  stomach  and  the  liver. 


BOOK  III. 

THE  CENTRAL  NERVOUS  SYSTEM  AND  ITS  INSTRUMENTS, 


CHAPTER    I. 

THE  SPINAL   CORD. 

ON  SOME  FEATURES  OF  THE  SPINAL  NERVES. 

§  471.  WE  have  called  the  muscular  and  nervous  tissues  the  master 
tissues  of  the  body ;  but  a  special  part  of  the  nervous  system,  that  which  we 
know  as  the  central  nervous  system,  the  brain  and  spinal  cord,  is  su- 
preme among  the  nervous  tissues  and  is  master  of  the  skeletal  muscles  as 
well  as  of  the  rest  of  the  body.  We  have  already  (Book  I.,  Chapter  III.) 
touched  on  some  of  the  general  features  of  the  nervous  system,  and  have 
now  to  study  in  detail  the  working  of  the  brain  and  spinal  cord.  We  have 
to  inquire  what  we  know  concerning  the  laws  which  regulate  the  discharge 
of  efferent  impulses  from  the  brain  or  from  the  cord,  and  to  learn  how  that 
discharge  is  determined,  on  the  one  hand,  by  intrinsic  changes  originating, 
apparently,  in  the  substance  of  the  brain  or  of  the  cord,  and,  on  the  other 
hand,  by  the  nature  and  amount  of  the  afferent  impulses  which  reach  them 
along  afferent  nerves. 

As  we  shall  see,  the  study  of  the  spinal  cord  cannot  be  wholly  separated 
from  that  of  the  brain,  the  two  being  very  closely  related.  Nevertheless,  it 
will  be  of  advantage  to  deal  with  the  spinal  cord  by  itself  as  far  as  we  can. 
The  medulla  oblongata  or  spinal  bulb1  we  shall  consider  as  part  of  the  brain. 
But  before  we  speak  of  the  spinal  cord  itself,  it  will  be  desirable  to  say  a 
few  words  concerning  the  spinal  nerves,  that  is  to  say,  the  nerves  which 
issue  from  the  spinal  cord. 

We  have  already  seen  (§  92)  that  each  of  the  spinal  nerves  arises  by  two 
roots,  an  anterior  root  attached  to  the  ventral  or  anterior  surface,  and  a  pos- 
terior root  attached  to  the  dorsal  or  posterior  surface  of  the  cord.  We  have 
further  seen  that  the  latter  bears  a  ganglion,  a  "  ganglion  of  the  posterior 
root"  or  "spinal  ganglion,"  as  we  have  (§  93)  studied  the  structure  of  this 
ganglion. 

We  stated  at  the  same  time  that  while  the  trunk  of  a  spinal  nerve  con- 
tained both  efferent  and  afferent  fibres,  the  efferent  fibres  were  gathered  up 
into  the  anterior  root  and  the  afferent  fibres  into  the  posterior  root ;  but  we 
gave  no  proof  of  this  statement. 

§  472.  Before  we  proceed  to  do  so,  it  will  be  as  well  to  say  a  few  words 
on  the  terms  "  efferent "  and  "  afferent."  By  efferent  nerve-fibres  we  mean 
nerve-fibres  which  in  the  body  usually  carry  impulses  from  the  central  ner- 

1  The  term  medulla  oblongata  is  not  only  long,  but  presents  difficulties,  since  the  word 
medulla  is  now  rarely  used  to  denote  the  whole  spinal  cord  (medulla  spinalis)  but  is  gen- 
erally used  to  denote  the  peculiar  coat  of  a  nerve-fibre,  the  white  substance  of  Schwann. 
In  using  instead  the  word  bulb  or,  if  necessary,  spinal  bulb  there  is  little  fear  of  confu- 
sion with  any  other  kind  of  bulb.  The  adjective  is  in  not  uncommon  use,  in  such  phrases 
as  "  bulbar  paralysis." 

525 


526  THE  SPINAL  CORD. 

vous  system  to  peripheral  organs.  Most  efferent  nerve-fibres  carry  impulses 
to  muscles,  striated  or  plain,  and  the  impulses  passing  along  them  give  rise 
to  movements  ;  hence  they  are  frequently  spoken  of  as  "  motor  "  fibres.  But 
all  efferent  fibres  do  not  end  in  or  carry  impulses  to  muscular  fibres ;  we  have 
seen  for  instance  that  some  efferent  fibres  are  secretory.  Moreover,  all  the 
nerve-fibres  going  to  muscular  fibres  do  not  serve  to  produce  movement ; 
some  of  them,  as  in  the  case  of  certain  vagus  fibres  going  to  the  heart,  are 
inhibitory  and  may  serve  to  stop  movement. 

By  "  afferent "  nerve-fibres  we  mean  nerve  fibres  which  in  the  body 
usually  carry  impulses  from  peripheral  organs  to  the  central  nervous  sys- 
tem. A  very  common  effect  of  the  arrival  at  the  central  nervous  system  of 
impulses  passing  along  afferent  fibres  is  that  change  in  consciousness  which 
we  call  a  "  sensation ;"  hence  afferent  fibres  or  impulses  are  often  called 
"  sensory  "  fibres  or  impulses.  But  as  we  have  already  in  part  seen,  and  as 
we  shall  shortly  see  in  greater  detail,  the  central  nervous  system  may  be 
affected  by  afferent  impulses,  and  that  in  several  ways,  quite  apart  from  the 
development  of  any  such  change  of  consciousness  as  may  be  fairly  called  a 
sensation.  We  shall  see  reason  for  thinking  that  afferent  impulses  reaching 
the  spinal  cord,  and,  indeed,  other  parts  of  the  central  nervous  system,  may 
modify  reflex  or  automatic  or  other  activity  without  necessarily  giving  rise 
to  a  "  sensation."  Hence  it  is  advisable  to  reserve  the  terms  "  efferent "  and 
"  afferent "  as  more  general  modes  of  expression  than  "  motor  "  or  "  sensory." 

We  have  seen  in  treating  of  muscle  and  nerve,  that  the  changes  pro- 
duced in  the  muscle  serve  as  our  best  guide  for  determining  the  changes 
taking  place  in  a  motor  nerve ;  when  a  motor  nerve  is  separated  from  its 
muscle  (§  70)  the  only  change  which  we  can  appreciate  in  it  is  an  electrical 
change.  Similarly  in  the  case  of  an  afferent  nerve,  the  central  system  is  our 
chief  teacher  ;  in  a  bundle  of  afferent  fibres  isolated  from  the  central  nervous 
system,  in  a  posterior  root  of  a  spinal  nerve  for  instance,  the  only  change 
which  we  can  appreciate  is  an  electrical  change.  To  learn  the  characters  of 
afferent  impulses  we  must  employ  the  central  nervous  system.  But  in  this 
we  meet  with  difficulties.  In  studying  the  phenomena  of  motor  nerves  we 
are  greatly  assisted  by  two  facts.  First,  the  muscular  contraction  by  which 
we  judge  of  what  is  going  on  in  the  nerve  is  a  comparatively  simple  thing, 
one  contraction  differing  from  another  only  by  such  features  as  extent  or 
amount,  duration,  frequency  of  repetition  and  the  like,  and  all  such  differ- 
ences are  capable  of  exact  measurement.  Secondly,  when  we  apply  a  stim- 
ulus directly  to  the  nerve  itself,  the  effects  differ  in  degree  only  from  those 
which  result  when  the  nerve  is  set  in  action  by  natural  stimuli,  such  as  the 
will.  When  we  come,  on  the  other  hand,  to  investigate  the  phenomena  of 
afferent  nerves,  our  labors  are  for  the  time  rendered  heavier,  but  in  the  end 
more  fruitful,  by  the  following  circumstances  :  First,  when  we  judge  of  what 
is  going  on  in  an  afferent  nerve  by  the  effects  which  stimulation  of  the  nerve 
produces  in  some  central  nervous  organ,  in  the  way  of  exciting  or  modify- 
ing reflex  action,  or  modifying  automatic  action,  or  affecting  consciousness, 
we  are  met  on  the  very  threshold  of  every  inquiry  by  the  difficulty  of  clearly 
distinguishing  the  events  which  belong  exclusively  to  the  afferent  nerve  from 
those  which  belong  to  the  central  organ.  Secondly,  the  effects  of  applying 
a  stimulus  to  the  peripheral  end-organ  of  an  afferent  nerve  are  very  different 
from  those  of  applying  the  same  stimulus  directly  to  the  nerve  trunk.  This 
may  be  shown  by  the  simple  experience  of  comparing  the  sensation  caused 
by  bringing  any  sharp  body  into  contact  with  a  nerve  laid  bare  in  a  wound 
with  that  caused  by  contact  of  an  intact  skin  with  the  same  body.  These 
and  like  differences  reveal  to  us  a  complexity  of  impulses,  of  which  the 
phenomena  of  motor  nerves  gave  us  hardly  a  hint. 


ON  SOME  FEATURES  OF  THE  SPINAL  NERVES.  527 

We  shall  further  see  in  detail  later  on  that  our  consciousness  may  be 
affected  in  many  different  ways  by  afferent  impulses ;  we  must  distinguish 
not  only  sensory  from  other  afferent  impulses,  but  also  different  kinds  of 
sensory  impulses  from  each  other.  Certain  afferent  nerves  are  spoken  of  as 
nerves  of  special  sense,  and  the  nature  of  the  afferent  impulses  passing  along 
these  special  nerves,  together  with  the  modifications  of  consciousness  caused 
by  arrival  of  these  impulses  at  the  central  nervous  system,  constitute  by  them- 
selves a  complex  and  difficult  branch  of  study.  In  some  of  the  problems 
connected  with  the  central  nervous  system  we  shall  have  to  appeal  to  the 
results  of  a  study  of  these  special  senses ;  but,  on  the  other  hand,  a  knowl- 
edge of  the  central  nervous  system  is  necessary  to  a  proper  understanding 
of  the  special  senses ;  and  on  the  whole  it  will  be  more  convenient  to  study 
the  former  before  the  latter. 

§  473.  The  proof  that  the  afferent  and  efferent  fibres  which  are  both 
present  in  the  trunk  of  a  spinal  nerve  are  parted  at  the  roots,  the  efferent 
fibres  running  exclusively  in  the  ventral  or  anterior  root,  and  the  afferent 
fibres  exclusively  in  the  dorsal  or  posterior  root,  is  as  follows : 

When  the  anterior  root  is  divided  the  muscles  supplied  by  the  nerve 
cease  to  be  thrown  into  contractions,  either  by  the  will  or  by  reflex  action, 
while  the  structures  to  which  the  nerve  is  distributed  retain  their  sensibil- 
ity. During  the  section  of  the  root,  or  when  the  proximal  stump  connected 
with  the  spinal  cord  is  stimulated,  no  sensory  effects  are  produced.  When 
the  distal  stump  is  stimulated  the  muscles  supplied  by  the  nerve  are  thrown 
into  contractions.  When  the  posterior  root  is  divided  the  muscles  supplied 
by  the  nerve  continue  to  be  thrown  into  action  by  an  exercise  of  the  will,  or 
as  part  of  a  reflex  action,  but  the  structures  to  which  the  nerve  is  distributed 
lose  the  sensibility  which  they  previously  possessed.  During  the  section  of 
the  root,  and  when  the  approximal  stump  is  stimulated,  the  sensory  effects 
are  produced.  When  the  distal  stump  is  stimulated  no  movements  are 
called  forth.  These  facts  demonstrate  that  sensory  impulses  pass  exclusively 
by  the  posterior  root  from  the  peripheral  to  the  central  organs,  and  that 
motor  impulses  pass  exclusively  by  the  anterior  root  from  the  central  to  the 
peripheral  organs ;  and  as  far  as  our  knowledge  goes  the  same  holds  good 
not  only  for  sensory  and  motor  but  also  for  afferent  and  efferent  impulses. 

An  exception  must  be  made  to  the  above  general  statement,  an  account 
of  the  so-called  "  recurrent  sensibility  "  which  is  witnessed  in  conscious  mam- 
mals under  certain  circumstances.  It  sometimes  happens  that  when  the 
distal  stump  of  the  divided  anterior  root  is  stimulated,  signs  of  pain  are  wit- 
nessed. These  are  not  caused  by  the  concurrent  muscular  contractions  or 
cramp  which  the  stimulation  occasions,  for  they  persist  after  the  whole  trunk 
of  the  nerve  has  been  divided  some  little  way  below  the  union  of  the  roots 
above  the  origins  of  the  muscular  branches,  so  that  no  contractions  take 
place.  They  disappear  when  the  posterior  root  is  subsequently  divided,  and 
they  are  not  seen  if  the  mixed  nerve-trunk  be  divided  close  to  the  union  of 
the  roots.  The  phenomena  are  probably  due  to  the  fact  that  bundles  of 
sensory  fibres  of  the  posterior  root,  after  running  a  short  distance  down  the 
mixed  trunk,  turn  back  and  run  upward  in  the  anterior  root  (being  distrib- 
uted probably  to  the  pia  mater)  and  by  this  recurrent  course  give  rise  to 
the  recurrent  sensibility. 

§  474.  Concerning  the  ganglion  on  the  posterior  root,  we  may  say  de- 
finitely that  we  have  no  evidence  that  it  can  act  as  a  centre  of  reflex  action, 
nor  have  we  any  evidence  that  it  can  spontaneously  give  origin  to  efferent 
impulses  and  thus  act  as  an  automatic  centre,  as  can  the  central  nervous 
system  itself.  The  bodies  of  the  nerve-cells  behave  somewhat  differently 
from  the  axis-cylinders  at  some  distance  from  the  cells,  though,  as  we  have 


528  THE  SPINAL  CORD. 

seen,  these  are  in  reality  processes  of  the  nerve-cells  ;  thus  the  nerve-cells  in 
the  ganglion  appear  to  be  more  sensitive  to  certain  poisons  than  are  the 
nerve-fibres  of  the  nerve-trunk.  But  beyond  this,  our  knowledge  concern- 
ing the  function  of  the  ganglion  is  almost  limited  to  the  fact  that  it  is  in 
some  way  intimately  connected  with  the  nutrition  of  the  nerve.  As  we  have 
already  (§  81)  said,  when  a  mixed  nerve-trunk  is  divided  the  peripheral 
portion  degenerates  from  the  point  of  section  downward  toward  the  periph- 
ery. The  central  portion  does  not  so  degenerate,  and  if  the  length  of  nerve 
removed  be  not  too  great,  the  central  portion  may  grow  downward  along  the 
course  of  the  degenerating  peripheral  portion,  and  thus  regenerate  the  nerve. 
This  degeneration  is  observed  when  the  mixed  trunk  is  divided  in  any  part 
of  its  course  from  the  periphery  to  close  up  to  the  ganglion.  When  the  pos- 
terior root  is  divided  between  the  ganglion  and  the  spinal  cord,  the  portion 
attached  to  the  spinal  cord  degenerates,  but  that  attached  to  the  ganglion 
remains  intact.  When  the  anterior  root  is  divided,  the  proximal  portion  in 
connection  with  the  spinal  cord  remains  intact,  but  the  distal  portion  between 
the  section  and  the  junction  with  the  other  root  degenerates ;  and  in  the 
mixed  nerve-trunk  many  degenerated  fibres  are  seen,  which,  if  they  be  care- 
fully traced  out,  are  found  to  be  motor  (efferent)  fibres.  If  the*  posterior 
root  be  divided  carefully  between  the  ganglion  and  the  junction  with  the 
anterior  root,  the  small  portion  of  the  posterior  root  left  attached  to  the 
peripheral  side  of  the  ganglion  above  the  section  remains  intact,  as  does  also 
the  rest  of  the  root  from  the  ganglion  to  the  spinal  cord,  but  in  the  mixed 
nerve-trunk  are  seen  numerous  degenerated  fibres,  which  when  examined  are 
found  to  have  the  distribution  of  sensory  (afferent)  fibres.  Lastly,  if  the 
posterior  ganglion  be  excised,  the  whole  posterior  root  degenerates,  as  do  also 
the  sensory  (afferent)  fibres  of  the  mixed  nerve-trunk.  Putting  all  these 
facts  together,  it  would  seem  that  the  growth  of  the  efferent  and  afferent 
fibres  takes  places  in  opposition  directions,  and  starts  from  different  nutri- 
tive or  "  trophic  "  centres.  The  afferent  fibres  grow  away  from  the  ganglion 
either  toward  the  periphery  or  toward  the  spinal  cord.  The  efferent  fibres 
grow  outward  from  the  spinal  cord  toward  the  periphery.  This  difference 
in  their  mode  of  nutrition  is  frequently  of  great  help  in  investigating  the 
relative  distribution  of  efferent  and  afferent  fibres.  When  a  posterior  root 
is  cut  beyond  the  ganglion,  or  the  ganglion  excised,  all  the  afferent  nerves 
degenerate,  and  in  the  mixed  nerve-branches  these  afferent  fibres,  by  their 
altered  condition,  can  readily  be  traced.  Conversely,  when  the  anterior 
roots  are  cut,  the  efferent  fibres  alone  degenerate,  and  can  be  similarly  recog- 
nized in  a  mixed  nerve-tract.  When  the  anterior  is  divided  some  few  fibres 
in  it  do  not,  like  the  rest,  degenerate,  and  when  the  posterior  root  is  divided, 
a  few  fibres  in  the  anterior  root  are  seen  to  degenerate  like  those  of  the  pos- 
terior root ;  these  appear  to  be  the  fibres  which  give  to  the  anterior  root  its 
"  recurrent  sensibility."  In  the  case  of  certain  spinal  nerves  at  all  events, 
it  has  also  been  ascertained  that  when  the  posterior  root  is  divided,  while 
most  of  the  fibres  in  the  part  of  the  root  thus  cut  off  from  the  ganglion  but 
left  attached  to  the  cord  degenerate,  some  few  do  not.  These  few  appear  to 
have  their  trophic  centre  not  in  the  ganglion,  but  in  some  part  of  the  spinal 
cord  itself;  we  shall  refer  to  these  later  on. 

This  method  of  distinguishing  nerve-fibres  by  the  features  of  their  de- 
generation, called  the  "degeneration  method,"  or  sometimes,  from  the  name 
of  the  physiologist  who  introduced  it,  the  "  Wallerian  method,"  has  proved 
of  great  utility.  Thus  in  the  vagus  nerve,  which  is  composed  not  only  of 
fibres  which  spring  from  the  real  vagus  root,  but  also  of  fibres  proceeding 
from  the  spinal  accessory  roots,  the  two  may  be  distinguished  by  section  of 
the  vagus  and  spinal  accessory  roots  respectively.  We  shall  presently  see 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  529 

that  this  method  may  be  applied  to  the  differentiation  of  tracts  of  fibres  in 
the  brain  and  spinal  cord. 

THE  STRUCTURE  OF  THE  SPINAL  CORD. 

§  475.  Lying  within  the  vertebral  canal  the  spinal  cord  is  protected  by 
its  "  membranes,"  the  dura  mater,  the  arachnoid  membrane  and  the  pia 
mater.  The  consideration  of  the  arrangement  of  these  membranes  and  of 
the  structure  of  the  dura  mater  and  arachnoid  we  will  leave  until  we  come 
to  speak  of  the  vascular  and  lymphatic  supplies  of  the  central  nervous  sys- 
tem ;  the  histology  of  the  pia  mater  may  more  fitly  come  with  that  of  the 
spinal  cord  itself. 

.  Along  its  whole  length  from  its  junction  with  the  bulb  at  its  termination 
in  the  filum  terminals  the  spinal  cord,  while  possessing  certain  general  fea- 
tures, is  continually  changing  as  to  special  features.  It  will  be  convenient 
to  study  first  the  general  structure  of  some  particular  part,  for  instance  the 
middle  of  the  thoracic  (dorsal)1  region,  and  afterward  to  point  out  the  special 
features  which  obtain  in  the  several  regions. 

A  transverse  vertical  section  of  either  a  fresh  or  a  hardened  and  pre- 
pared spinal  cord  at  the  thoracic  region  possesses  an  outline  which  is,  roughly 
speaking,  circular.  In  the  middle  of  the  anterior  or  ventral  surface  is  a  ver- 
tical fissure,  the  ventral  or  anterior  fissure  (Fig.  119,  A.  F.\  running  some 
way  across  the  thickness  of  the  cord  from  the  ventral  toward  the  dorsal  sur- 
face. Opposite  to  it  on  the  posterior  or  dorsal  surface  is  a  corresponding, 
deeper  but  narrower,  dorsal  or  posterior  fissure  (Fig.  119,  P.  F.)  which,  how- 
ever, as  we  shall  see,  differs  materially  in  nature  from  the  anterior  fissure, 
and  ought  to  be  called  a  septum  rather  than  a  fissure.  Between  the  two  fis- 
sures the  substance  of  the  cord  is  reduced  to  a  narrow  isthmus  uniting  the 
two  lateral  halves,  which  in  a  normal  cord  are  like  each  other  in  every 
respect.  In  the  middle  of  the  isthmus  lies  the  section  of  a  small  canal,  the 
central  canal  (Fig.  119,  c.  c.),  which  is  all  that  remains  of  the  relatively  wide 
neural  canal  of  the  embryo. 

Each  lateral  half  consists  of  an  outer  zone  of  white  matter  surrounding, 
except  at  the  isthmus,  an  inner  more  or  less  crescentic,  or  comma-shaped 
mass  of  gray  matter.  The  convexity  of  each  crescent  is  turned  toward  the 
median  line  of  the  cord,  the  two  crescents  being  placed  back  and  back  and 
joined  together  by  the  isthmus  just  spoken  of.  The  somewhat  broader  ante- 
rior extremity  of  the  crescent,  or  head  of  the  comma,  is  called  the  anterior 
cornu  or  horn;  and  the  narrower  posterior  extremity  of  the  crescent,  or  tail 
of  the  comma,  is  called  the  posterior  cornu  or  horn.  The  part  by  which  each 
horn  is  joined  on  to  the  middle  part  of  the  crescent  is  called  the  cervix,  ante- 
rior and  posterior  respectively.  The  isthmus  joining  the  backs  of  the  two 
crescents,  like  the  crescents  themselves,  consists,  for  the  most  part,  of  gray 
matter,  the  band  running  posterior  or  dorsal  to  the  central  canal  being  called 
the  posterior  gray  commissure  (Fig.  119,  p.  g.  c.),and  the  band  running  ante- 
rior or  ventral  to  the  canal  being  called  the  anterior  gray  commissure  (Fig. 
119,  a.  g.  c.).  The  posterior  fissure  touches  the  posterior  gray  commissure, 

1  It  is  very  desirable  to  use  the  terms  "  dorsal  "  and  "  ventral  "  for  the  parts  of  the 
cerebro-spinal  axis  which  lie  respectively  near  the  dorsal  or  back  part,  arid  the  ventral  or 
belly  part  of  the  body,  instead  of  the  terms  posterior  and  anterior;  but  if  this  is  done 
the  use  of  the  word  dorsal  to  denote  the  region  of  the  cord  between  the  lumbar  and  cer- 
vical regions  is  apt  to  lead  to  confusion  ;  hence  the  introduction  of  the  word  thoracic.  If 
this  use  of  dorsal  and  ventral  be  adhered  to,  before  and  behind,  above  and  below,  may 
conveniently  be  used  to  denote  nearer  the  head  and  nearer  the  tail  (or  coccyx)  respect- 
ively; anterior  and  posterior  may  also  be  used  in  the  same  sense  except  in  the  case  of 
anterior  and  posterior  fissure  and  horn,  which  terms  seem  too  much  honored  by  time  to 
be  thrown  aside. 

34 


530 


THE  SPINAL  CORD. 


but  the  anterior  gray  commissure  is  separated  from  the  bottom  of  the  ante- 
rior fissure  by  a  band  of  white  matter,  called  the  anterior  white  commissure, 


FIG.  119. 


P.F 


A  Transverse  Dorso-ventral  Section  of  the  Spinal  Cord  (Human)  at  the  Level  of  the  Sixth 
Thoracic  (Dorsal)  Nerve.  (Sherrington.)1  Magnified  15  times.  One  lateral  half  only  is  shown. 
The  large  conspicuous  nerve-cells  (drawn  from  actual  specimens)  are  shaded  black  to  render 
their  relative  size,  shape  and  position  more  obvious;  the  outline  of  the  gray  matter  has  been 
made  thick  and  dark  in  order  to  render  it  conspicuous.  A.  F.,  antetror  fissure;  P.  -F.,  posterior 
fissure ;  c.  c.,  central  canal ;  c.  g.  s.,  central  gelatinous  substance ;  A.  r.,  anterior  root :  P.  ?'.,  lateral 
(or  intermediate)  bundle;  P.  ?•'.,  median  bundle  or  posterior  root  of  spinal  nerve;  p' p"  fibres  of 
posterior  root  passing,  p'  indirectly  through  the  substance  of  Rolando,  p"  directly  into  gray  mat- 
ter;  a.  g.  c.,  anterior  gray  commissure ;  p.  g.  c.,  posterior  gray  commissure ;  a.  c.,  anterior  white 
commissure;  ant.  col,  anterior  column  ;  lat.  col.,  lateral  column  ;  post,  col.,  posterior  column;  s. g., 
the  substance  of  Rolando  ;  s.,  septum  marking  out  the  external  posterior  column  or  column  of 
Burdach,  c.  p.,  from  the  median  posterior  column,  or  column  of  Goll,  m.p.  1,  cells  of  the  anterior 
horn  ;  3,  posterior  column,  or  vesicular  cylinder,  or  column  of  Clarke,  the  area  of  the  cylinder 
defined  by  a  dotted  line ;  4,  cells  of  the  intermedio-lateral  tract  or  lateral  tract  or  lateral  horn ;  6, 
cells  of  the  posterior  horn  ;  7,  cells  of  the  anterior  cervix ;  y,  a  tract  of  fibres  passing  from  the 
vesicular  cylinder  to  the  lateral  column. 


1  For  this  and  many  succeeding  figures  1  am  deeply  indebted  to  my  friend  and  former 
pupil,  Dr.  Sherrington,  who  has  kindly  prepared  the  figures  for  me  from  his  original 
drawings. 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  531 

or  more  simply,  the  ivhite  commissure,  or  sometimes  the  anterior  commissure 
(Fig.  119,  a.  c.). 

If  the  section  be  taken  at  the  level  of  the  origin  of  a  pair  of  spinal 
nerves,  it  will  be  seen  that  the  anterior  or  ventral  root,  piercing  the  white 
matter  opposite  the  head  of  the  comma  in  several  distinct  bundles  (Fig. 
119,  A.  r.),  plunges  into  the  anterior  cornu,  while  the  posterior  or  dorsal 
root  (Fig.  119,  P.  r.,  P.  r'.),  having  the  appearance  of  a  single  undivided 
bundle,  passes,  in  part  at  least,  into  the  posterior  horn.  Both  roots  are  dis- 
persed lengthways  along  the  cord,  the  hinder  roots  of  one  nerve  being  close 
to  the  foremost  roots  of  the  nerve  below,  but  it  is  only  the  anterior  roots 
which  are  dispersed  sideways.  The  compact  bundle  of  the  posterior  root 
divides,  with  tolerable  sharpness,  the  white  matter  in  each  lateral  half  of 
the  cord  into  (1)  a  posterior  portion  lying  between  the  posterior  fissure  and  the 
posterior  root,  which  portion  since,  as  we  shall  see,  runs  in  the  form  of  a  col- 
umn along  the  length  of  the  cord,  is  called  the  posterior  column,  and  (2)  into 
a  portion  lying  to  the  outside  of  the  posterior  root  between  it  and  the  ante- 
rior fissure,  called  the  antero-lateral  column.  This  latter  may  be  considered 
as  further  divided,  by  the  entrance  of  the  anterior  roots  into  a  lateral  column 
{Fig.  119,  fat.  col.)  between  the  posterior  root  and  the  most  external  bundle 
of  the  anterior  root,  and  into  an  anterior  column  (Fig.  119,  ant.  col.),  between 
the  anterior  fissure  and  the  most  external  bundle  of  the  anterior  root.  The 
part  traversed  by  the  bundles  of  the  anterior  root,  as  they  make  for  the 
anterior  horn,  accordingly  belongs  to  the  anterior  column  ;  but  some  writers 
speak  of  the  anterior  column  as  lying  between  the  anterior  fissure  and  the 
nearest  bundle  of  the  anterior  root,  thus  making  the  region  of  the  anterior 
root  belong  to  neither  anterior  nor  lateral  column.  And  indeed  the  dis- 
tinction between  the  anterior  and  the  lateral  column  is.  to  a  great  extent, 
artificial. 

§  476.  The  "  white  matter "  consists  exclusively  of  medullated  fibres 
supported  partly  by  connective  tissue  and  partly  by  a  peculiar  tissue  known 
as  neuroglia,  of  which  we  shall  presently  speak.  The  fibres  are  of  various 
sizes,  but  many  of  them  are  large,  and  in  all  of  them  the  medulla  is  conspic- 
uous. They  run  for  the  most  part  longitudinally,  so  that  in  transverse  sec- 
tions of  the  cord  nearly  the  whole  of  the  white  matter  appears  under  the 
microscope  to  be  composed  of  minute  circles,  the  tranverse  sections  of  the 
longitudinally-disposed  fibres,  imbedded  in  the  supporting  structures.  The 
""  gray  matter "  also  contains  medullated  fibres,  but  these  are  for  the  most 
part  exceedingly  fine  fibres  possessing  a  medulla  which  appears  to  differ 
from  that  of  an  ordinary  nerve-fibre,  since  it  does  not  stain  readily  with 
osmic  acid,  but  is  rendered  visible  by  special  modes  of  preparation  such  as 
that  known  as  Weigert's.  Hence  these  fine  fibres  are  not  apparent  in  ordi- 
nary carmine  or  other  specimens,  and  indeed  their  presence  was  for  a  long 
time  overlooked.  Besides  these  fine  medullated  fibres,  if  we  may  call  them 
such,  the  gray  matter  contains  what  the  white  matter  does  not,  nerve-cells 
with  branching  processes,  naked  axis-cylinders,  and  delicate  filaments  aris- 
ing from  the  division  of  axis-cylinders  or  from  the  branching  of  nerve-cells, 
all  these  various  structures  being  imbedded  in  neuroglia.  Owing  to  the  re- 
lative abundance  of  the  white  refractive  medulla,  the  white  matter  possesses 
in  fresh  specimens  a  characteristic,  opaque  white  color  ;  hence  the  name. 
The  gray  matter  from  the  relative  scantiness  of  medulla  has  no  such  opaque 
whiteness,  is  much  more  translucent,  and  in  fresh  specimens  has  a  gray  or 
rather  pinkish-gray  color,  the  reddish  tint  being  due  to  the  presence  partly 
of  pigment  and  partly  of  blood,  for  the  bloodvessels  are  much  more  abun- 
dant in  the  gray  matter  than  in  the  white. 

The  pia  mater  which  closely  invests  the  cord  all  around  consists  of  con- 


532  THE  SPINAL  CORD. 

nective  tissue  fairly  rich  in  elastic  elements  and  abundantly  supplied  with 
bloodvessels  ;  it  is  indeed  essentially  a  vascular  membrane  and  furnishes  the 
nervous  elements  of  the  cord  with  their  chief  supply  of  blood.  It  sends  in 
at  intervals  partitions  or  septa  of  the  same  nature  as  itself  radiating  toward 
the  central  gray  matter.  The  narrow  posterior  fissure  is  completely  filled  up 
by  a  large  septum  of  this  kind,  indeed,  as  we  have  said,  is  in  reality  not  a 
fissure  but  a  large  septum  ;  but  the  anterior  fissure  is  too  wide  for  such  an 
arrangement ;  the  whole  membrane  dips  down  into  this  fissure,  following  the 
surface  of  the  cord  and  being  reflected  at  the  bottom.  From  these  primary 
septa,  secondary  finer  septa  still  composed  of  ordinary  fibrillated  connective 
tissue,  carrying  bloodvessels,  branch  off;  but  these  are  soon  merged  into  the 
peculiar  supporting  tissue,  called,  as  we  have  said,  neuroglia.  This  consists 
in  the  first  place  of  small  branching  cells,  lying  in  various  planes.  The 
branching  is  excessive,  so  that  the  body  of  the  cell  is  reduced  to  very  small 
dimensions,  indeed  at  times  almost  obliterated,  the  nucleus  disappearing 
while  the  numerous  branches  are  continued  as  long,  fine  filaments  or  fibres 
pursuing  a  devious  but  for  the  most  part  a  longitudinal  course.  In  the 
second  place  these  cells  and  fibres  or  filaments  are  imbedded  in  a  homo- 
geneous ground  substance.  Relatively  to  the  fibres  and  ground  substance 
the  bodies  of  the  cells  (which  are  called  Deiter's  cells),  especially  bodies  such 
as  bear  obvious  nuclei,  are  very  scanty ;  hence  in  sections,  especially  in 
transverse  sections,  of  the  cord  the  neuroglia  has  often  a  dotted  or  punctated 
appearance,  the  dots  being  the  transverse  sections  of  the  fine  longitudinally- 
disposed  fibres  imbedded  in  the  ground  substance.  Examined  chemically, 
the  neuroglia  is  found  to  be  composed  not  like  connective  tissue  of  gelatin, 
but  of  a  substance  which  appears  to  be  closely  allied  to  keratin,  the  chief 
constituent  of  horny  epidermis,  hairs  and  the  like,  and  which  has  therefore 
been  called  neurokeratin  (see  also  §  68).  And  indeed  this  neuroglia,  though 
like  connective  tissue  a  supporting  structure,  is  not,  like  connective  tissue, 
of  mesoblastic,  but  of  epiblastic  origin.  The  walls  of  the  neural  canal  of 
the  embryo  which  are  transformed  into  the  spinal  cord  of  the  adult  consist 
at  first  of  epithelial,  epiblastic  cells  ;  and  while  some  of  these  cells  become 
nervous  elements,  others  become  neuroglia.  The  epithelial  cells  which  are 
destined  to  form  neuroglia  become  exceedingly  branched,  while  their  orig- 
inally protoplasmic  cell  substance  becomes  transformed  to  a  large  extent 
into  neurokeratin. 

The  neuroglia  fills  up  the  spaces  between  the  radiating  larger  septal  pro- 
longations of  the  pia  mater  and  the  finer  branched  septa  which  starting  from 
the  larger  ones  carry  minute  bloodvessels  into  the  interior  of  the  white 
matter.  In  these  spaces  it  is  so  arranged  as  to  form  delicate  tubular  canals, 
of  very  variable  size,  running  for  the  most  part  in  a  longitudinal  direction. 
Each  of  these  tubular  canals  is  occupied  by  and  wholly  filled  up  with 
medullated  nerve-fibre  of  corresponding  size.  A  medullated  nerve-fibre  of 
the  white  matter  of  the  spinal  cord  resembles  a  medullated  nerve-fibre  of  a 
nerve  (§  68)  in  being  composed  of  an  axis-cylinder  and  a  medulla ;  but  it 
possesses  no  primitive  sheath  or  neurilemma.  This  is  absent  and  indeed  is 
not  wanted  ;  the  tubular  sheath  of  neuroglia  affords  in  the  spinal  cord  (and, 
as  we  shall  see,  in  the  central  nervous  system  generally)  the  support  which  in 
a  nerve  is  afforded  by  the  neurilemma.  Nodes  are,  according  to  most  authors, 
absent,  but  some  say  they  are  present. 

The  white  matter  of  the  cord  consists  then  of  a  more  or  less  solid  mass  of 
neuroglia,  having  the  structure  just  described,  which  is  permeated  by  minute 
canals,  some  exceedingly  fine  and  carrying  very  fine  2  /j.  fibres,  others  larger 
and  carrying  fibres  up  to  the  size  of  15  /-/.  This  mass  is  further  broken  up 
into  areas  by  the  smaller  and  larger  vascular  connective-tissue  septa  with  the 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  533 

edges  and  endings  of  which  the  neuroglia  is  continuous.  Most  of  the  nerve- 
fibres,  as  we  have  said,  run  longitudinally  and  in  a  transverse  section  of  the 
cord  are  cut  transversely ;  but,  as  we  shall  see,  fibres  are  continually  passing 
into  and  out  of  the  white  matter,  and  in  so  doing  take  a  more  or  less  trans- 
verse course ;  these,  however,  are  few  compared  with  those  which  run  in  a 
longitudinal  direction.  On  the  outside  of  the  cord  below  the  pia  mater  the 
neuroglia  is  developed  into  a  layer  of  some  thickness  from  which  nerve-fibres 
are  absent ;  this  is  often  spoken  of  as  an  inner  layer  of  the  pia  mater ;  but 
being  neuroglia  and  not  connective  tissue  is  of  a  different  nature  from  the 
pia  mater  proper.  A  layer  of  this  superficial  neuroglia  also  accompanies 
the  larger  septa,  and  a  considerable  quantity  is  present  in  the  large  septum 
called  the  posterior  fissure. 

The  pia  mater  carries  not  only  bloodvessels  but  also  lymphatics ;  of  these, 
however,  we  shall  speak  when  we  come  to  deal  with  the  vascular  arrange- 
ments of  the  whole  of  the  central  nervous  system. 

§  477.  In  the  gray  matter  we  may  distinguish  the  larger,  more  conspicu- 
ous nerve-cells  and  the  rest  of  the  gray  matter  in  which  these  cells  lie.  We 
have  already  (§  95)  described  the  general  features  of  these  larger  nerve-cells, 
and  shall  have  presently  to  speak  of  their  special  characters  and  grouping. 
Meanwhile  the  most  important  point  to  remember  about  them  besides  the 
fact  that  they  vary  largely  in  form  and  size  is  that  while  one  process  may  or 
does  become  an  axis-cylinder  of  a  nerve-fibre,  the  others  rapidly  branch,  and 
breaking  up  into  fine  nerve-filaments  are  lost  to  view  in  the  rest  of  the  gray 
matter. 

These  larger  nerve-cells  form,  however,  a  part  only,  and  in  most  regions 
of  the  cord  the  smaller  part,  of  the  whole  gray  matter.  In  a  transverse  sec- 
tion from  the  thoracic  region  (Fig.  119)  a  few  only  of  these  larger  nerve-cells 
are  seen  in  the  whole  section,  and  though  they  appear  more  numerous  in  sec- 
tions from  the  cervical  and  especially  from  the  lumbar  regions  (Figs.  121, 
122),  yet  in  all  cases  they  occupy  the  smaller  part  of  the  area  of  the  gray 
matter.  The  larger  part  of  the  gray  matter  consists,  besides  a  neuroglia 
supporting  the  nervous  elements,  of  nerve-filaments  running  in  various  direc- 
tions and  forming,  not  a  plexus  properly  so  called,  but  an  interlacement  of 
extreme  complexity.  These  filaments  are,  on  the  one  hand,  the  fine  medul- 
lated  fibres  spoken  of  above  as  being  recognized  with  difficulty,  and,  on  the 
other  hand,  non-medullated  filaments  ranging  from  fairly  wide  and  con- 
spicuous naked  axis-cylinders  down  to  fibrils  of  extreme  tenuity,  the  latter 
arising  apparently  either  from  the  division  of  axis-cylinders  and  nerve-fibres 
passing  into  or  out  of  the  gray  matter  or  from  the  continued  branching  of 
processes  of  nerve-cells.  By  the  modes  of  preparation  now  available  it  has 
been  shown  that  the  fine  medullated  fibres,  so  far  from  being  rare,  are  in 
certain  parts  of  the  gray  matter  so  abundant  as  even  to  preponderate  over 
the  non-medullated  fibres  or  fibrils.  Lastly,  besides  the  conspicuous  nerve- 
cells  spoken  of  above,  which,  though  of  various  sizes,  may  all  perhaps  be 
spoken  of  as  large,  a  very  large  number  of  other  cells  of  small  size,  some  of 
which  at  all  events  must  be  regarded  as  true  nerve-cells,  are  present  in  the 
gray  matter. 

The  neuroglia  in  which  all  these  structures,  nerve-cells,  fine  medullated 
nerve-fibres,  naked  axis-cylinders,  and  fine  filaments  are  imbedded,  is  identical 
in  its  general  characters  with  that  of  the  white  matter,  but,  as  naturally  fol- 
lows from  the  nature  of  the  nervous  elements  which  it  supports,  is  differently 
arranged.  Instead  of  forming  a  system  of  tubular  channels  it  takes  on  the 
form  of  a  sponge-work  with  large  spaces  for  the  larger  nerve-cells  and  fine 
passages  for  the  nervous  filaments.  At  the  junction  of  the  gray  matter  with 
the  white  matter,  the  neuroglia  of  the  one  is  continuous  with  that  of  the 


534  THE  SPINAL  CORD. 

other,  and  the  connective-tissue  septa  of  the  latter  run  right  into  the  former  • 
the  outline  of  the  gray  matter  is  not  smooth  and  even,  but  broken  by  tooth- 
like  processes  due  to  the  septa.  Since,  as  we  have  just  said,  some  of  the  true 
nerve-cells  are  very  small,  and  since  the  nerve-filaments  like  the  neuroglia 
fibres  are  very  fine  and  take  like  them  an  irregular  course,  it  often  becomes 
very  difficult  in  a  section  to  determine  exactly  which  is  neuroglia  and  which 
are  nervous  elements.  The  neuroglic  cells  may,  however,  be  distinguished 
perhaps  from  the  smaller  nerve-cells  by  their  nuclei  not  being  so  conspicuous 
or  so  relatively  large  as  in  a  nerve-cell,  and  by  their  staining  differently. 

The  gray  matter,  then,  may  be  broadly  described  as  a  bed  of  neuroglia, 
containing  a  certain  number  of  branching  nerve-cells,  for  the  most  part 
though  not  exclusively  large  and  conspicuous,  but  chiefly  occupied  by  what 
is  not  so  much  a  plexus  as  an  intricate  interweaving  of  nerve-filaments 
running  apparently  in  all  directions.  Some  of  these  filaments  are  fairly  con- 
spicuous naked  axis  cylinders,  and  a  few  are  easily  recognized  medullated 
fibres  of  ordinary  size  ;  but  by  far  the  greater  number  are  either  exceedingly 
fine  medullated  fibres,  whose  medulla  is  only  made  evident  by  special  modes 
of  preparation,  or  delicate  fibrils  devoid  of  medulla.  With  the  nervous  web 
formed  by  these  filaments  the  branching  processes  of  the  nerve- cells,  on  the 
one  hand,  and  the  divisions  of  nerve-fibres  passing  into  or  out  of  the  gray 
matter  on  the  other  hand,  appear  to  be  continuous.  It  may  be  added  that 
the  gray  matter  is  well  supplied  with  bloodvessels,  these  being  in  it,  as  stated 
above,  relatively  much  more  numerous  than  in  the  white  matter. 

§  478.  The  central  canal  is  lined  by  a  single  layer  of  columnar  epithelial 
cells,  which  are  generally  described  as  bearing  cilia ;  but  it  is  not  certain 
that  the  processes  which  may  be  seen  projecting  from  the  surfaces  of  the 
cells  are  really  cilia.  These  epithelial  cells  rest  not  on  a  distinct  basement 
membrane,  but  on  a  bed  of  neuroglia,  free  apparently,  or  nearly  so,  from 
nervous  elements  which  surrounds  the  central  canal  and  is  sometimes  spoken 
of  as  the  substantia  gelatinosa  centralis  (Fig.  119,  c.  g.  ».).  The  attached 
basis  of  the  epithelial  cells  are  branched  or  taper  to  a  filament,  and  become 
continuous  with  the  branched  cells  or  fibres  of  the  neuroglia  below.  As  we 
said  above,  the  neuroglia  elements  are  transformed  epithelial  cells  ;  and  the 
continuity  of  the  cells,  which  retaining  the  characters  of  epithelial  cells  form 
a  lining  to  the  canal,  with  the  cells  which  have  become  branched  and  lost 
their  epithelial  characters,  indicates  the  epithelial  origin  of  the  latter. 

The  central  canal  with  the  surrounding  area  of  neuroglia  forms  the 
central  part  of  the  isthmus  uniting  the  two  lateral  halves  of  the  cord.  Pos- 
terior (dorsal)  to  this  central  mass  lies  the  posterior  gray  commissure  (Figs. 
119,  121,  122,  p.  g.  c.)  composed  chiefly  of  fine  filaments  running  trans- 
versely, and  anterior  (ventral)  to  it  lies  first  the  thinner  anterior  gray  com- 
missure (Figs.  119,  121,  122  a.  g.  c.)  of  a  similar  nature,  and  then  the  rela- 
tively thick  white  commissure  (Figs.  119,  121,  122,  a.  c.)  which  is  formed  by 
medullated  fibres  crossing  over  from  one  side  of  the  cord  to  the  other,  and 
thus  constitutes  a  decussation  of  fibres  along  the  whole  length  of  the  cord. 
On  each  side  the  central  mass  of  neuroglia  of  which  we  are  speaking 
gradually  merges  into  the  central  gray  matter  of  the  corresponding  lateral 
half. 

The  end  or  head  (caput),  as  it  is  frequently  called,  of  the  posterior  horn 
is  occupied  not  by  ordinary  gray  matter,  but  by  a  peculiar  tissue,  the  sub- 
stantia gelatinosa  of  Rolando,  which  forms  a  sort  of  cap  to  the  more  ordinary 
gray  matter,  but  differs  in  size  and  shape  in  different  regions  of  the  cord. 
(Cf.  Figs.  119,  120,  121,  s.  g.)  In  carmine  and  some  other  modes  of  prepa- 
ration it  is  frequently  stained  more  deeply  than  is  the  ordinary  gray  matter, 
and  in  such  preparations  is  very  conspicuous.  It  may  be  described  as  con- 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  535 

sisting  of  a  somewhat  peculiar  neuroglia  traversed  by  fibres  of  the  posterior 
root,  and  containing  a  large  number  of  cells  which,  for  the  most  part  small, 
the  cell-bodies  being-  small  relatively  to  the  nuclei,  are  not  all  alike,  some 
being  probably  nervous  and  others  not.  It  takes  origin  from  the  cells  form- 
ing the  immediate  walls  of  the  embryonic  medullary  canal.  In  the  embryo 
this  canal  is  relatively  wide,  though  compressed  from  side  to  side,  and  in 
transverse  sections  of  the  medullary  tube  appears  at  a  certain  stage  as  a 
narrow  oval  slit  placed  vertically  and  reaching  almost  from  the  dorsal  to  the 
ventral  surface.  The  dorsal  part  of  this  long  slit  is  later  on  closed  up  by 
the  coming  together  of  the  walls  and  the  obliteration  of  the  greater  part  of 
the  cavity,  leaving  the  ventral  part  to  form  a  circular  canal,  which  by  the 
development  of  the  anterior  columns  assumes  the  central  position.  During 
this  closure  of  the  dorsal  part  of  the  canal  a  mass  of  the  cells  lining  the 
canal  is  cut  from  the  rest  on  each  side,  and  during  the  subsequent  growth 
takes  up  a  position  at  the  end  of  the  posterior  horn.  Hence,  though  it  never 
apparently  contains  any  cavity,  the  substance  of  Rolando  may  be  regarded 
as  an  isolated  portion  of  the  walls  of  the  medullary  canal,  which  has  under- 
gone a  development  somewhat  different  from  that  of  the  portion  which 
remains  as  the  lining  of  the  central  canal.  Traces  of  this  origin  may  be 
seen  even  in  the  adult.  Thus,  in  the  lower  end  of  the  cord,  in  what  we  shall 
speak  of  presently  as  the  conns  medullaris,  the  central  canal  widens  out  dor- 
sally,  and  in  section  (Fig.  120,  A)  presents  on  each  side  a  bay  x  stretching 

FIG.  120. 


Diagram  to  illustrate  the  Nature  of  the  Substance  of  Rolando.  The  figures  are  purely  dia- 
grammatic and  are  not  drawn  to  the  same  scale.  In  all  three  figures  the  gray  matter  is  shaded 
with  fine  lines  and  the  white  matter  with  dots.  A,  transverse  section  of  the  lower  end  of  the 
conus  medullaris  in  man  ;  e,  epithelium  lining  the  medullary  canal ;  x,  lateral  expansion  of  the 
canal;  B,  transverse  section  of  the  spinal  cord  of  the  calf  in  the  lower  thoracic  region;  r,  sub- 
stance of  Rolando :  c,  central  canal ;  C,  transverse  section  through  mid-thoracic  region  of  cord  in 
man. 

out  toward  the  position  of  the  posterior  horn.  At  this  region  of  the  cord, 
though  both  white  and  gray  matter  are  developed  on  the  ventral  surface,  the 
posterior  columns  do  not  meet  on  the  dorsal  surface,  but  leave  the  central 
canal  covered  only  by  tissue  which  perhaps  may  be  called  neuroglia,  but  is 
of  peculiar  nature  and  origin.  In  the  calf,  in  a  part  of  the  dorsal  region  the 
substance  of  Rolando  is  not  confined  to  the  tip  of  the  posterior  horn,  but  is 
continued  to  meet  its  fellow  in  the  middle  line.  (Fig.  120,  B.}  If  we  imagine 
the  dorsal  portion  of  the  canal  of  A  to  be  cut  off  from  the  ventral  portion, 
its  cavity  to  be  obliterated,  and  the  lining  epithelium  with  some  of  the  sur- 
rounding elements  to  undergo  a  special  development,  the  condition  in  B  is 
reached  by  the  growth  of  the  posterior  columns.  From  B  the  transition  to 
the  normal  state  of  things  as  in  Fig.  120,  c,  is  a  very  slight  one.  The  extreme 
dorsal  tip  of  the  horn,  being  of  a  more  open  texture  than  the  substance  of 
Rolando,  is  sometimes  called  the  zona  spongiosa. 

§  479.   The  grouping  of  the  nerve-cells.  The  nerve-cells,  at  all  events  the 
cells  which  are  large  enough  to  to  be  easily  and  without  doubt  recognized  to 


536  THE  SPINAL  CORD. 

be  nerve-cells,  form,  as  we  have  seen,  only  a  part  of  the  gray  matter,  and  in 
some  parts  of  the  cord,  in  the  thoracic  region  for  instance,  are  so  sparse  that 
in  a  section  of  the  spinal  cord  iu  this  region  thin  enough  to  show  its  histo- 
logical  features  satisfactorily,  the  bodies  of  a  few  only  of  such  cells  are 
visible  (Fig.  119)  ;  the  greater  part  of  the  gray  matter  consists  not  of  the 
bodies  of  conspicuous  nerve-cells,  but  of  a  mass  of  fibres  and  fibrils  passing 
apparently  in  all  directions.  In  the  cervical  (Fig.  121),  and  especially  in 
the  lumbar  (Fig.  122)  regions,  the  nerve-cells  are  both  absolutely  and  rela- 
tively more  abundant ;  but  even  in  a  section  taken  from  the  lumbar  region 
the  nerve-ce'lls,  all  put  together,  form  the  smaller  part  of  the  whole  area  of 
gray  matter.  Moreover,  in  respect  of  the  number  of  cells,  all  the  sections 
of  even  the  same  region  of  the  cord  are  not  alike.  Seeing  that  the  cord  may 
be  considered  as  growing  out  of  the  fusion  of  a  series  of  paired  ganglia,  each 
ganglion  corresponding  to  a  nerve  (c/.  §  92),  we  may  fairly  expect  to  find  the 
fusion  not  complete,  so  that  the  nerve-cells  would  appear  more  numerous 
opposite  a  nerve  than  in  the  middle  between  two  nerves.  In  some  of  the 
lower  animals  this  arrangement  is  most  obvious,  and  there  are  some  reasons 
for  thinking  that  even  in  man  the  nerve-cells  are  metamerically  increased  at 
the  level  of  each  nerve. 

Even  when  casually  observed,  it  is  obvious  that  the  nerve-cells  are  not 
scattered  in  a  wholly  irregular  manner  throughout  the  gray  matter,  being, 
for  instance,  much  more  conspicuous  in  the  anterior  horn  than  elsewhere ; 
and  more  careful  observation  allows  us  to  arrange  them  to  a  certain  extent 
in  groups. 

The  cells  of  the  anterior  horn  are  for  the  most  part  large  and  conspicuous, 
67/J-  to  135/x  in  diameter,  branch  out  in  various  directions,  and  present  an 
irregular  outline  in  sections  taken  in  different  planes.  We  have  reason  to 
think  that  every  one  of  them  possesses  an  axis-cylinder  process,  which,  in 
the  case  at  all  events  of  most  of  the  cells,  passing  out  of  the  gray  matter 
becomes  a  fibre  of  the  adjacent  anterior  root.  They  are  obvious  and  con- 
spicuous in  all  regions  of  the  cord,  though  much  more  numerous  and  indi- 
vidually larger  in  the  cervical  and  lumbar  enlargements  than  in  the  thoracic 
region.  We  may  further,  with  greater  or  less  success,  divide  them  into 
separate  groups. 

In  the  cervical  and  lumbar  regions  a  fairly  distinct  group  of  cells  is  seen 
lying  on  the  median  side  of  the  gray  matter  close  to  the  anterior  column 
(Figs.  121,  122,  1).  This  may  be  called  the  median  group.  It  appears  also 
in  the  thoracic  region  (Fig.  119,  1)  ;  indeed,  the  question  arises  whether  all 
the  cells  of  the  anterior  horn  in  this  region  do  not  belong  to  this  group.  The 
other  cells  so  conspicuous  in  the  lumbar  and  cervical  enlargements,  and  there- 
fore probably  in  some  way  associated  with  the  limbs,  may  be  spoken  of  as 
forming  altogether  a  lateral  group ;  but  we  may,  though  with  some  uncer- 
tainty, subdivide  them  into  two  or  three  groups.  Thus  in  the  lumbar  region 
a  group  of  cells  (Fig.  122,  2  ^)  lying  near  the  lateral  margin  of  the  more 
dorsal  part  or  base  of  the  horn  may  be  distinguished,  as  a  lateral  sub-group, 
from  the  cells  occupying  the  ventral  lateral  corner  of  the  horn  and  forming 
a  ventral  or  anterior  sub-group  (Fig.  122,  2  oc) ;  and  the  same  distinction, 
though  with  less  success,  may  be  made  in  the  cervical  region  (Fig.  121). 
Further,  we  may  perhaps  in  both  regions  distinguish  a  group  of  cells  placed 
more  in  the  very  middle  of  the  horn  as  a  central  sub -group  (Figs.  121,  122, 
2  /?).  But,  in  all  cases,  the  separation  of  these  cells,  which  we  have  spoken 
of  as  a  whole  as  lateral  cells,  into  minor  groups,  is  far  less  distinct  than  the 
separation  of  the  median  group  from  these  lateral  cells,  especially  if  we 
admit  that  in  the  thoracic  region  the  median  group  is  alone  clearly  repre- 
sented. 


THE  STRUCTURE  OF  THE  SPINAL  CORD. 
FIG.  121. 


537 


Rr' 


Rr. 


CT 


C.P.T 


Transverse  Dorso-ventral  Section  of  Spinal  Cord  (Human)  at  the  Level  of  the  Sixth  Cervical 
Nerve.  (Sherrington.)  This  is  drawn  on  the  same  scale  as  Fig.  119,  that  is,  magnified  fifteen  times 
r.f .  L,  lateral  reticular  formation  ;  r.f.  p.,  posterior  reticular  formation;  p',  fine  fibres  of  lateral 
bundle  of  the  posterior  root ;  p",  p'",  fibres  of  median  bundle  of  posterior  root  entering  gray  matter 
from  external  posterior  column  ;  x,  gray  matter  of  posterior  horn  ;  Sp.  a.,  bundles  of  fibres  belong- 
ing to  the  spinal  accessory  nerve :  in  the  lateral  reticular  formation  they  are  seen  cut  transversely  ; 
b,  is  the  natural  septum  of  connective  tissue  marking  out  the  cerebellar  tract  C.  T.  from  the  crossed 
pyramidal  tract  C.  P.  T. ;  z.  s.,  zona  spongiosa ;  2  «. ,  /3,  y,  lateral  cells  of  the  anterior  horn  ;  5,  cells 
in  the  region  of  the  lateral  reticular  formation.  The  other  letters  of  reference  are  the  same  as  in 
Fig.  119. 


538  THE  SPINAL  COED. 

In  the  thoracic  region  a  group  of  rather  smaller  cells  is  seen  at  the  base 
of  the  anterior  horn,  near  to  the  junction  with  the  isthmus  (Fig.  119,  7). 
In  the  cervical  and  lumbar  region  these  cells  are  very  scanty  (Figs  121 
122,  7). 

Tfie  cells  of  the  posterior  horn  contrast  strongly  with  those  of  the  ante- 
rior horn  in  being  few,  and  for  the  most  part  small.  They  are  branched  ; 
and  though  we  have  reason  to  believe  that,  like  the  cells  of  the  anterior 
horn,  they  possess  each  an  axis-cylinder  process,  this  is  not  easily  deter- 
mined by  actual  observation ;  the  processes  do  not  run  out  to  join  the  poste- 
rior root,  as  do  the  corresponding  processes  in  the  anterior  horn,  and  there- 
fore are  not  so  readily  seen.  These  cells  occur  in  all  regions  of  the  cord, 
and  appear  to  be  arranged  in  two  or  more  groups.  The  lateral  margin  of 
the  posterior  horn,  at  about  the  middle  or  neck  of  the  horn,  is  along  the 
whole  length  of  the  cord,  but  especially  in  the  cervical  region,  much 
broken  up  by  bundles  of  fibres  passing  in  various  directions  and  forming 
an  open  network,  called  the  lateral  retieular  formation  (Figs.  121,  122,  r.f. 
lat.).  In  all  regions  of  the  cord  a  number  of  cells  are  found  associated 
with  this  retieular  formation,  forming  the  group  of  the  lateral  retieular  for- 
mation (Figs.  121,  122,  5).  In  all  regions  of  the  cord,  also  a  group  of  cells 
(Figs.  119,  121,  122,  6)  is  found  in  that  part  of  the  horn  where,  a  little 
ventral  to  the  substance  of  Rolando,  the  uniform  field  of  gray  matter  is 
broken  up  into  a  kind  of  network  by  a  number  of  bundles  of  white  fibres 
running  in  various  directions.  The  network  has  also  been  called  a  retieu- 
lar formation,  and  has  received  the  name  of  posterior  retieular  formation 
(Figs.  121,  122,  r.f.  p.)  to  distinguish  it  from  the  lateral  retieular  forma- 
tion just  mentioned  ;  the  two,  however,  in  some  regions  (see  Fig.  119)  join 
each  other,  and  thus  cut  off  a  ventral  portion  of  the  posterior  horn  con- 
taining nerve-cells  from  a  dorsal  portion,  x  in  Figs.  121,  122,  in  which  no 
obvious  or  conspicuous  nerve-cells  are  present. 

The  groups  of  cells  just  mentioned,  with  the  restrictions  and  modifications 
spoken  of,  occur  along  the  whole  length  of  the  cord ;  but  the  group  of  cells 
to  which  we  must  now  call  attention  is  almost  confined  to  a  special  region 
of  the  cord,  or  at  least  is  but  feebly  represented  elsewhere.  In  the  thoracic 
region,  especially  in  the  lower  thoracic  region  (we  shall  return  to  the  limits 
of  the  group  later  on),  at  the  base  of  the  posterior  horn  (Fig.  119,  3),  just 
ventral  to  the  curve  formed  by  the  posterior  gray  commissure  as  this  bends 
dorsally  to  join  the  posterior  horn,  is  seen  on  each  side  of  the  cord  a  con- 
spicuous group  of  cells  known  as  Clarke's  column,  or  the  posterior  vesicular 
column  or  vesicular  cylinder.  The  cells  composing  this  group,  though  vary- 
ing in  size  at  different  levels,  are  rather  large  cells,  and  are  for  the  most 
part  fusiform,  with  their  long  axis  placed  lengthways  along  the  cord,  so 
that  in  transverse  sections  they  often  appear  to  have  a  rather  small  round 
body.  They  are  surrounded  by,  and  as  it  were  imbedded  in,  a  mass  of  fine 
fibres,  the  area  of  which  is  indicated  by  a  dotted  line  in  Fig.  118. 

Also  conspicuous  in  the  thoracic  region  is  another  group  of  cells  lying 
on  the  outer  side  of  the  middle  of  the  gray  matter  at  about  the  junction  of 
the  anterior  and  posterior  horns.  This  is  known  as  the  intermedio-lateral 
tract,  and  is  sometimes  called  the  lateral  horn  (Fig.  119,  4).  The  cells  com- 
posing it  are  somewhat  small  spindle-shaped  cells  with  their  long  axis 
placed  transversely.  The  group  is  conspicuous,  as  we  have  said,  in  the 
thoracic  region ;  it  may  be  recognized  in  the  lumbar  region  (Fig.  121,  4), 
but  in  the  cervical  region  becomes  confused  with  the  most  dorsally  placed 
or  lateral  sub-group  of  the  anterior  horn.  We  shall,  however,  have  to  re- 
turn to  these  groups  of  cells  when  we  come  to  speak  of  the  differences 
between  the  spinal  regions  of  the  cord. 


THE  STRUCTURE  OF  THE  SPINAL  CORD. 


539 


§  480.  The  tracts  of  white  matter.  At  first  sight  the  white  matter  of  the 
cord  appears  to  be  of  uniform  nature.  We  can  use  the  nerve-roots  to 
delimitate  the  anterior,  posterior,  and  lateral  columns,  but  \ve  appear  to 


FIG.  122. 


Transverse  Dorso-ventral  Section  of  the  Spinal  Cord  (Human)  at  the  Level  of  the  Third  Lum- 
bar Nerve.  (Sherrington.)  This  is  drawn  to  the  same  scale  as  Figs.  119, 120,  and  in  the  same 
way,  except  that  the  outline  of  the  gray  matter  is  not  exaggerated,  Pr'.  median;  Pr.  interme- 
diate ;  Pr".  lateral  bundles  of  posterior  roots.  The  region  comprised  under  m.t.  is  the  marginal 
zone  of  Lissauer's  zone.  The  other  letters  of  reference  are  the  same  as  in  Figs.  119, 121.  The 
three  figures,  119, 121, 122,  are  intended  to  illustrate  the  main  differential  features  of  the  cervical, 
thoracic,  and  lumbar  cord. 

have  no  criteria  to  distinguish  parts  in  each  column.  In  the  cervical  and 
upper  thoracic  regions  of  the  cord,  a  septum  (Fig.  119,  «.)  in  the  posterior 
column,  somewhat  more  conspicuous  than  the  other  septa,  has  enabled 
anatomists  to  distinguish  an  inner  median  portion,  the  median  posterior 


540  THE  SPINAL  CORD. 

column,  commonly  called  the  postero-median  column  or  column  of  Goll  (Fig. 
119.  m.p.\  from  an  outer  lateral  portion,  the  external  posterior  column,  com- 
monly called  the  postero-external  column  or  column  of  Burdach  (Fig.  119, 
e.  p."),  the  lateral  part  of  which,  nearer  the  gray  matter,  has,  for  reasons 
which  we  shall  see  later  on,  been  called  the  posterior  root-zone.  But  beyond 
this  neither  the  irregular  septa  nor  other  features  will  enable  us  to  distin- 
guish one  part  of  the  white  matter  as  different  in  nature  from  another. 
Nor  have  we  better  success  when  with  the  scalpel  we  attempt  to  dissect 
out  the  white  matter  into  separate  strands.  Nevertheless  we  have  convin- 
cing evidence  that  the  white  matter  is  arranged  in  strands,  or  tracts,  or 
columns,  which  have  different  connections  at  their  respective  ends,  which 
behave  differently  under  different  circumstances,  which  we  have  every 
reason  to  believe  carry  out  different  functions,  but  which  cannot  be  sepa- 
rated by  the  scalpel,  because  each  of  them  is  more  or  less  mixed  with  fibres 
of  a  different  nature  and  origin.  The  evidence  for  the  existence  of  these 
tracts  is  twofold. 

One  kind  of  evidence  is  embryological  in  nature.  When  a  nerve-fibre 
is  being  formed  in  the  embryo,  either  in  the  spinal  cord  or  elsewhere,  the 
essential  axis-cylinder  is  formed  first  and  the  less  essential  medulla  is 
formed  later.  Now  when  the  developmental  history  of  the  spinal  cord  is 
studied  it  is  found  that,  in  the  several  regions  of  the  cord,  all  the  fibres  of 
the  white  matter  do  not  put  on  the  medulla  at  the  same  time.  On  the  con- 
trary, in  certain  tracts,  the  medulla  of  the  fibres  makes  its  appearance  early, 
in  others  later.  By  this  method  it  becomes  possible  to  distinguish  certain 
tracts  from  others. 

Another  kind  of  evidence  is  supplied  by  facts  relating  to  the  degenera- 
tion of  the  fibres  of  the  white  matter.  We  have  seen  (§  474)  that  the 
degeneration  of  a  nerve-fibre  is  the  result  of  the  separation  of  the  fibre 
from  its  trophic  centre,  and  that  while  the  trophic  centre  of  the  afferent 
fibres  is  in  the  ganglion  on  the  posterior  root,  that  of  the  efferent  fibres  is 
in  some  part  of  the  spinal  cord.  In  the  case  of  the  efferent  fibres  the 
degeneration  might  be  spoken  of  as  descending  from  the  spinal  cord  to  the 
muscles  or  other  peripheral  organs.  In  the  case  of  the  afferent  fibres  of 
the  trunk  of  the  nerve,  the  degeneration  is  also  one  descending  from  the 
ganglion  down  to  the  skin  or  other  peripheral  organ.  When,  however,  the 
section  is  carried  through  the  posterior  root  of  a  spinal  nerve,  the  degener- 
ation takes  place  in  the  part  of  the  nerve  between  the  section  and  the  spinal 
cord ;  it  runs  up  from  the  section  to  and  into  the  spinal  cord,  and  may, 
therefore,  be  called  an  ascending  degeneration.  Thus  we  may  say  that 
when  a  nerve-trunk  or  when  a  nerve-root  is  cut  completely  across,  all  the 
fibres,  which  are  thereby  separated  from  their  trophic  centres,  degenerate. 
When  the  nerve-trunk  is  divided,  all  the  fibres  below  the  section  undergo 
descending  degeneration.  If  the  anterior  root  be  cut  across,  all  the  fibres 
of  the  root  below  the  section  undergo  descending  degeneration.  If  the 
posterior  root  be  cut  across,  all  the  fibres  of  the  root  above  the  section  un- 
dergo ascending  degeneration  with  the  exception  of  certain  fibres  which  do 
not  degenerate  at  all,  and  of  which  we  shall  speak  later  on. 

When  the  spinal  cord  is  cut  across,  for  instance  in  the  dorsal  region,  all 
the  fibres  of  the  white  matter  do  not  degenerate  either  in  the  part  of  the 
cord  above  the  section  or  in  the  part  below.  Some  fibres,  and  indeed  some 
tracts  of  fibres  degenerate,  and  some  do  not.  Further,  some  tracts  degen- 
erate in  the  cord  above  the  section,  and  thus  undergo  what  has  been  called 
an  ascending  degeneration ;  other  tracts  degenerate  in  the  cord  below  the 
section,  and  thus  undergo  what  has  been  called  a  descending  degeneration. 
These  terms  must,  however,  be  used  witk  caution.  When  a  nerve-trunk  is 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  541 

cut  across,  the  degeneration  actually  descends,  in  the  sense  that  the  progress 
of  the  degenerative  changes  may  be  traced  downward ;  they  begin  at  the 
section  and  travel  downward  at  a  rate  sufficiently  slow  to  permit  a  difference 
being  observed  between  the  progress  of  degeneration  at  a  spot  near  the  sec- 
tion and  that  of  one  further  off.  After  section  of  or  injury  to  the  spinal  cord, 
however,  it  is  not  possible  to  trace  any  such  progress  either  upward  or  down- 
ward ;  in  the  tracts  both  above  and  below  the  section  or  injury,  degenera- 
tion either  begins  simultaneously  along  the  whole  length  of  the  degenerating 
tract,  or  progresses  along  the  tract  so  rapidly  that  no  differences  can  be 
observed,  as  far  as  the  stage  of  degeneration  is  concerned,  between  parts  near 
to  and  those  far  from  the  section  and  injury.  When,  for  instance,  the  cord 
is  divided  in  the  cervical  region,  subsequent  examination  of  the  tracts  of 
the  so-called  descending  degeneration  shows  that  the  degeneration  is  as  far 
advanced  in  the  lumbar  region  far  away  from  the  section  as  in  the  cervical 
region  just  below  the  section.  Applied  to  the  spinal  cord,  therefore,  the 
term  descending  degeneration  means  simply  degeneration  below  the  seat  of 
injury  or  disease,  ascending  degeneration  means  simply  degeneration  above 
the  seat  of  injury  or  disease.  We  may  add  that  the  histological  features  of 
the  degeneration  of  fibres  in  the  spinal  cord  are  not  wholly  identical  with 
those  of  the  degeneration  of  fibres  in  a  nerve-trunk.  Thus,  the  neurilemma 
with  its  nuclei  being  absent  from  the  fibres  of  the  cord,  no  proliferation  of 
nuclei  takes  place ;  the  axis-cylinder  and  medulla  simply  break  up,  are  ab- 
sorbed, and  disappear. 

Similar  degenerations,  ascending  or  descending,  or  both,  are  seen  when 
the  section  is  not  carried  right  through  the  whole  cord,  but  particular  parts 
of  the  cord  are  cut  through  or  deeply  injured.  And  similar  degenerations 
occur  as  the  consequences  of  disease  set  up  in  parts  of  the  cord. 

In  this  way  the  results  of  sections  of  or  of  other  injuries  to  or  of  diseases 
of  the  spinal  cord,  have  enabled  us  to  mark  out  certain  tracts  of  the  white 
matter  as  undergoing  degeneration  and  others  as  not,  and,  moreover,  certain 
tracts  as  undergoing  descending  and  others  as  undergoing  ascending  degen- 
eration. Further,  the  delimitation  of  tracts  of  white  matter  by  the  process 
of  degeneration  agrees  so  well  with  the  results  of  the  embryological  method 
as  to  leave  no  doubt  that  the  white  matter  does  consist  of  tracts  which  differ 
from  each  other  in  nature  and  in  function. 

The  several  tracts  thus  indicated  vary  in  different  regions  of  the  cord. 
They  may  be  broadly  described  as  follows : 

I.  Descending  tracts,  that  is  to  say,  tracts  which  undergo  a  descending 
degeneration  in  the  sense  noted  above. 

The  most  important  and  conspicuous  is  a  large  tract  (Fig.  123,  cr.P.) 
occupying  the  posterior  part  of  the  lateral  column,  coming  close  upon  the 
outer  margin  of  the  posterior  horn,  and  for  the  most  part  not  reaching  the 
surface  of  the  cord.  We  shall  have  to  return  to  this  tract  more  than  once, 
and  may  here  simply  say  that  it  is  most  distinctly  marked  out  by  both  the 
embryological  and  the  degeneration  methods,  that  it  may  be  traced  along 
the  whole  length  of  the  cord  from  the  top  of  the  cervical  region  to  the  end 
of  the  sacral  region,  and  that  it  enters  the  cord  from  the  brain  through  the 
structures  called  the  pyramids  of  the  bulb,  which  we  shall  study  later  on. 
These  pyramids  cross  over  or  decussate  as  they  are  about  to  pass  into  the 
cord,  forming  what  is  known  as  the  decussation  of  the  pyramids,  and  the 
tract  of  fibres  in  question  shares  in  this  decussation.  Hence  this  tract  is 
called  the  crossed  pyramidal  tract  or  more  simply  the  pyramidal  tract. 

A  smaller,  less  conspicuous  descending  tract  occupies  the  median  portion 
of  the  anterior  column  (Fig.  123,  d.P.}.  This  is  not  only  much  smaller  but 
also  much  more  variable  than  the  crossed  pyramidal  tract,  is  not  present  in 


542 


THE  SPINAL  CORD. 


the  lower  animals,  being  found  in  man  and  the  monkey  only  and  being  better 
developed  in  man  than  in  the  monkey,  and  reaches  a  certain  way  only  down 
the  spinal  cord,  generally  coming  to  an  end  in  the  thoracic  region,  "it,  too, 
comes  down  from  the  pyramid,  and  is  a  continuation  of  that  part  of  the  pyra- 
mid which,  unlike  the  rest,  does  not  decussate  in  the  bulb;  thus  the  tract 
which  coming  down  from  the  left  side  of  the  brain  runs  in  the  left  pyramid 

FIG.  123. 


cr.P. 


asc.  a.  I 


d.P 


Diagram  to  illustrate  the  General  Arrangement  of  the  Several  Tracts  of  White  Matter  in  the 
Spinal  Cord.  (Sherrington.)  The  section  is  taken  at  the  level  of  the  fifth  cervical  nerve.  The 
relations  of  the  tracts  in  different  regions  of  the  cord  are  shown  in  Fig.  127.  The  ascending  tracts, 
tracts  of  ascending  degeneration,  are  shaded  with  dots,  the  descending  tracts,  tracts  of  descending 
degeneration,  are  shaded  with  lines;  the  shading  in  each  case  put  on  one  side  of  the  cord,  only 
the  reference  letters  being  placed  on  the  other  side.  cr.P.  crossed  pyramidal  tract,  or  more 
shortly,  pyramidal  tract;  d.P.  direct  pyramidal  tract  shaded  on  the  side  opposite  to  that  on 
which  cr.P.  is  shaded,  in  order  to  indicate  the  difference  of  the  two  as  to  crossing;  C.b.  cerebellar 
tract ;  s.lr.  and  c.r.  together  indicate  the  median  posterior  tract,  or  tract  of  fibres  of  the  posterior 
roots,  cr.  representing,  as  is  explained  more  fully  in  the  text,  the  cervical  and  s.lr.  the  sacral, 
lumbar,  and  dorsal  roots;  asc.a.l.  the  antero-lateral  ascending  tract;  desc.l.  the  antero-lateral  de- 
scending tract.  The  area,  not  shaded,  marked  x,  is  the  small  descending  tract  or  rather  patch 
mentioned  in  the  text  as  observed  in  certain  regions  of  the  cord,  in  the  external  posterior  column 
r.z.  The  small  area  at  the  tip  of  the  posterior  horn,  marked  L,  is  the  posterior  marginal  zone  of 
Lissauer's  zone. 

in  the  bulb,  passes  down  into  the  left  anterior  column  of  the  cord.  Hence 
this  smaller  tract  is  called  the  direct  pyramidal  tract 

These  two  are  the  most  conspicuous  and  important  descending  tracts,  but 
names  have  been  given  to  two  other  descending  tracts.  One,  known  as  the 
antero-lateral  descending  tract,  is  a  large  tract  placed  in  the  antero-lateral 
column,  and  seen  in  section  (Fig.  123,  desc.  I.)  as  an  elongated  area  stretch- 
ing from  the  pyramidal  tract  toward  the  anterior  column  and  reaching  at 
times  as  far  as  the  anterior  fissure.  The  area  is  large,  however,  because  the 
tract  is  very  diffuse,  that  is  to  say,  the  fibres  with  descending  degeneration, 
or  fibres  which  degenerate  below  the  section  or  injury,  are  very  largely 
mixed  up  with  fibres  which  do  not  degenerate ;  in  this  respect  this  tract 
contrasts  with  the  pyramidal  tract,  which  is  to  a  much  greater  extent  com- 
posed of  fibres  with  descending  degeneration,  though  even  in  it  there  are 
a  considerable  number  of  fibres  which  do  not  degenerate.  Indeed,  this 
antero-lateral  descending  tract  is  so  diffuse  that  it  hardly  deserves  to  be 
called  a  tract. 

The  other  is  a  small,  narrow,  comma-shaped  tract  (Fig.  123,  «),  situated 
in  the  middle  of  the  external  posterior  column  which  has  been  observed  in 
the  cervical  and  upper  thoracic  regions,  and  has  been  called  the  "  descend- 
ing "  comma  tract.  But  the  degeneration  reaches  a  short  way  only  below 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  543 

the  section  or  injury,  and  the  group  of  fibres  thus  degenerating  can  hardly 
be  considered  as  forming  a  tract  comparable  to  the  other  tracts.  .  The  area 
probably  represents  fibres  of  the  posterior  root  which  take  a  descending  course 
soon  after  their  entrance  into  the  cord. 

II.  Ascending  tracts,  that  is  to  say,  tracts  in  which  the  degeneration  takes 
place  above  the  section  or  injury. 

A  conspicuous  ascending  tract  of  a  curved  shape  (Fig.  123,  C.b.)  occu- 
pies the  outer  dorsal  part  of  the  lateral  column  lying  to  the  outside  of  the 
crossed  pyramidal  tract,  between  it  and  the  surface  of  the  cord.  It  appears 
to  begin  in  the  upper  lumbar  region,  being  said  to  be  absent  from  the  lower 
lumbar  and  sacral  cord,  and  may  be  traced  upward  increasing  in  size  through 
the  thoracic  and  cervical  cord  to  the  bulb.  In  the  bulb  it  may  be  traced 
into  the  restiform  body  or  inferior  peduncle  of  the  cerebellum,  and  so  to  the 
cerebellum  ;  for  the  restiform  body  serves,  as  we  shall  see,  in  each  lateral 
half  of  the  brain,  as  the  main  connection  of  the  cerebellum  with  the  bulb 
and  spinal  cord.  Hence  this  tract  is  called  the  cerebellar  tract. 

A  second  important  ascending  tract  occupies  the  median  portion  of  the 
posterior  columns  (Fig.  123,  cr.,  s.lr.),  and  so  far  coincides  with  what  we 
described  above  as  the  median  posterior  column,  in  the  upper  regions  of  the 
cord,  that  it  may  be  called  the  median  posterior  tract ;  it  extends  along  the 
whole  length  of  the  spinal  cord,  varying  at  different  levels  in  a  manner  which 
we  shall  presently  study,  and  ending  above  in  the  bulb. 

A  third  ascending  tract,  called  the  ascending  antero-lateral  tract,  or  tract 
of  Gowers,  occupies  (Fig.  1 23,  asc.  a.  /.)  the  outer  ventral  part  of  the  lateral 
column.  It  has  somewhat  the  form  of  a  comma,  with  the  head  filling  up 
the  angle  left  between  projecting  portions  of  the  cerebellar  and  pyramidal 
tracts,  and  the  tail  stretching  away  ventrally  along  the  outer  margin  of  the 
lateral  column  outside  the  antero-lateral  descending  column,  the  end  of  the 
tail  often  reaching  to  the  anterior  roots.  It  may  be  traced  along  the  whole 
length  of  the  cord,  but  it  is  not  so  distinct  and  compact  a  tract  as  the  two 
ascending  tracts  just  mentioned  ;  the  fibres  with  ascending  degeneration,  that 
is  to  say,  the  fibres  degenerating  above  the  section  or  seat  of  injury,  are  very 
largely  mixed  with  fibres  of  a  different  nature  and  origin. 

We  may  further  remark  that  these  several  tracts  differ  from  each  other, 
in  some  cases  markedly,  as  to  the  diameter  of  their  constituent  fibres.  Thus 
the  cerebellar  tract  is  composed  almost  exclusively  of  remarkably  coarse 
fibres.  The  median  posterior  tract,  on  the  contrary,  is  made  up  of  fine  fibres 
of  very  equable  size,  while  the  fibres  of  the  antero-lateral  ascending  tract 
are  of  a  size  intermediate  between  the  other  two.  The  pyramidal  tract,  on 
the  other  hand,  is  made  up  of  fibres  of  almost  all  sizes  mixed  together. 

The  tracts  then  which  are  thus  marked  out  are,  as  descending  tracts,  the 
crossed  and  the  direct  pyramidal  tracts,  with  the  less  distinct  or  'important 
antero-lateral  descending  tract ;  and,  as  ascending  tracts,  the  cerebellar  tract, 
the  median  posterior  tract,  and  the  less  distinct  antero-lateral  ascending 
tract.  If  we  suppose  all  these  tracts  taken  away  there  is  still  left  a  consider- 
able area  of  white  matter,  namely,  nearly  the  whole  of  the  external  posterior 
column,  the  external  anterior  column,  including  the  region  traversed  by  the 
bundles  of  the  anterior  roots,  and  that  part  of  the  lateral  column  which  lies 
between  the  antero-lateral  descending  tract  and  the  crossed  pyramidal  tract 
on  the  outside  and  the  gray  matter  on  the  inside.  From  this  area  of  white 
matter  we  may  put  on  one  side  at  present  the  external  posterior  column, 
because,  as  we  shall  see,  this  column  is  largely  composed  of  the  fibres  of 
the  posterior  root  which  pass  through  this  column,  especially  through  the 
lateral  part  of  it  near  the  gray  matter,  on  their  way  to  their  ultimate  desti- 
nation ;  hence  the  alternative  name  of  posterior  root-zone.  We  may  simi- 


544  THE  SPINAL  CORD. 

larly  leave  for  the  present  the  small  zone  of  white  matter  composed  of  very 
fine  fibres  known  as  the  posterior  marginal  zone  or  Lissauer's  zone  (Fig.  123, 
L.),  lying  dorsal  to  the  tip  of  the  posterior  horn  and  in  the  lower  regions 
reaching  to  the  outside  of  the  cord  ;  for  this,  too,  belongs  to  the  fibres  of  the 
posterior  root.  Leaving  these  parts  out  of  consideration  we  may  say  as 
regards  the  rest  of  the  white  matter  that  the  present  state  of  our  knowledge 
will  not  allow  us  to  divide  it  into  special  tracts.  All  this  area  is  largely 
composed  of  fibres  which  do  not  undergo  either  ascending  or  descending 
degeneration  as  the  result  of  section,  injury,  or  disease.  It  has  been  sug- 
gested that  these  fibres  either  have  no  trophic  centre  at  all  or  have  double 
ones,  one  above  and  one  below,  on  either  of  which  they  can  in  case  of  need 
lean  ;  so  that  when  the  fibre  is  divided  at  any  level,  the  upper  portion  is  still 
nourished  from  some  centre  above,  and  the  lower  from  some  centre  below. 
At  all  events,  whether  this  be  the  true  explanation  or  not,  the  fibres  in  this 
part  of  the  white  matter  cannot  be  differentiated  into  tracts  by  a  study  of 
their  degeneration.  Fibres  of  this  kind,  which  we  can  speak  of  neither  as 
ascending  nor  as  descending,  also  occur  in  the  external  posterior  column 
mingled  with  the  fibres  of  the  posterior  root.  And  we  may  repeat  the  cau- 
tion, that  even  in  the  several  ascending  and  descending  tracts  just  described, 
especially  in  those  which  we  spoke  of  as  less  distinct  or  as  more  diffuse,  many 
fibres  are  present  which  undergo  neither  ascending  nor  descending  degen- 
eration. 

§  481.  It  may  be  as  well  perhaps  to  insist  here  once  more,  that  when 
these  several  tracts  or  the  fibres  running  in  the  tracts  are  spoken  of  as  ascend- 
ing or  descending,  what  is  meant  is  that  the  degeneration  takes  place  above 
the  section  or  seat  of  injury  or  disease  in  the  one  case,  and  takes  place  below 
in  the  other.  It  has  been  supposed  by  many  that  the  nervous  impulses  which 
these  fibres  severally  carry,  travel  in  the  same  direction  as  that  taken  by  the 
degeneration,  that  the  ascending  tracts  carry  impulses  from  below  upward, 
that  is  to  say,  carry  impulses  which  arising  from  peripheral  organs  pass  to 
various  part  of  the  spinal  cord  or  of  the  brain,  that  they  are,  in  other  words, 
channels  of  afferent  impulses,  and  that  conversely  the  descending  tracts  carry 
efferent  impulses.  To  this  view  is  often  added  as  a  corollary,  that  the  tracts 
which  do  not  degenerate  at  all  carry  impulses  both  ways,  and  hence  cannot 
be  considered  as  either  afferent  or  efferent  channels,  but  simply  as  communi- 
cating channels.  Upon  this  it  may  be  remarked  that  impulses  do  not  neces- 
sarily travel  in  the  same  direction  as  the  degeneration;  when  a  spinal 
nerve-trunk  is  divided  the  afferent  fibres  as  well  as  the  efferent  fibres  both 
degenerate  in  a  descending  direction  toward  the  periphery,  though  the 
former  carry  impulses  in  the  other  direction.  Hence  the  direction  of  degen- 
eration is  no  proof  of  the  direction  in  which  impulses  travel ;  moreover,  as 
we  have  seen,  degeneration  does  not  actually  travel  along  the  fibres  of  the 
spinal  cord  in  the  same  way  that  it  does  along  the  fibres  of  a  nerve-trunk. 
It  may  be  that  the  descending  tracts  do  carry  impulses  in  a  descending  di- 
rection, that  is,  efferent  impulses,  and  that  the  ascending  tracts  serve  to  carry 
afferent  impulses ;  but  the  proof  that  they  do  thus  respectively  act  must  be 
supplied  from  other  facts  than  those  of  degeneration.  Moreover,  we  shall 
have  to  return  to  these  ascending  and  descending  tracts  and  to  study  their 
behavior  along  the  length  of  the  cord  before  we  can  use  the  facts  concerning 
them  as  a  basis  for  any  discussion  as  to  their  functions. 

§  482.  The  connections  of  the  nerve-roots.  If  we  regard  the  spinal  cord, 
and  apparently  we  have  a  right  to  do  so,  as  resulting  from  the  fusion  of  a 
series  of  segments  or  metameres,  each  segment,  represented  by  a  pair  of  spinal 
nerves,  being  a  ganglionic  mass,  that  is  to  say,  a  mass  containing  nerve-cells 
with  which  nerve-fibres  are  connected,  we  should  expect  to  find  that  the 


THE  STRUCTUEE  OF  THE  SPINAL  COED.  545 

fibres  of  a  spinal  nerve  soon  after  entering  in  or  before  issuing  from  the 
spinal  cord  are  connected  with  nerve-cells  lying  in  the  neighborhood  of  the 
attachment  of  the  nerve  to  the  cord.  We  should,  we  say,  expect  to  find 
this ;  but  owing  to  the  difficulty  of  tracing  individual  nerve-fibres  through 
the  tangled  mass  of  the  substance  of  the  cord,  our  actual  knowledge  of  the 
termination  of  the  fibres  of  the  posterior  root  and  origin  of  the  fibres  of  the 
anterior  root  is  at  present  far  from  complete. 

With  regard  to  the  anterior  root,  there  can  be  no  doubt  that  a  very  large 
proportion  of  the  fibres  in  the  root  are  continuations  of  the  axis-cylinders  of 
cells  in  the  anterior  horn.  The  fibres  which  can  thus  be  traced  are  of  large 
diameter  and  appear  to  be  chiefly  if  not  exclusively  motor  fibres  for  the 
skeletal  muscles.  In  the  frog  a  laborious  enumeration,  on  the  one  hand,  of 
the  number  of  fibres  in  the  anterior  roots,  and,  on  the  other  hand,  of  the 
number  of  cells  of  the  anterior  horn  in  the  areas  corresponding  to  the 
nerve-roots  has,  it  is  true,  shown  a  very  remarkable  agreement  in  the  number 
between  the  two.  We  might  be  inclined  from  this  to  conclude  that  all  the 
fibres  of  an  anterior  root  start  directly  from  cells  in  the  anterior  horn,  and 
that  all  the  cells  in  the  anterior  horn  end  in  fibres  of  the  nearest  anterior 
root,  But  several  considerations  prevent  us  from  trusting  too  much  to  this 
observation  especially  in  the  case  of  the  higher  animals.  The  anterior  root 
contains  other  fibres  than  motor  fibres  for  the  skeletal  muscles,  vasomotor 
fibres  for  instance,  secretory  fibres  and  others  ;  and  it  is  a  priori  unlikely 
that  these  should  have  origin  from  the  same  cells  as  the  motor  fibres  of  the 
skeletal  muscles.  Moreover,  as  a  matter  of  fact,  some  of  the  fibres  have 
been  traced  through  the  anterior  horn,  on  the  one  hand,  toward  the  posterior 
horn,  and,  on  the  other  hand,  toward  the  lateral  column  ;  others  again  are 
found  to  pass  through  the  anterior  horn  of  their  own  side  to  the  bottom  of 
the  anterior  fissure  where,  crossing  over  to  the  other  side  and  thus  forming 
part  of  the  anterior  white  commissure,  they  appear  to  ascend  to  the  anterior 
horn  of  the  other  side.  We  cannot  at  present  make  any  positive  statement 
as  to  the  real  origin  and  exact  nature  of  these  fibres  which  thus  upon  enter- 
ing the  cord  pass  by  the  cells  in  the  anterior  horn  without  joining  them, 
though  those  which  cross  by  the  anterior  white  commissure  are  supposed  to 
take  origin  in  the  cells  of  the  anterior  horn  of  the  other  side ;  it  is  sufficient 
for  our  present  purposes  to  remember  that  while  a  large  number  of  the  fibres 
of  the  anterior  root,  presumably  those  supplying  the  skeletal  muscles,  take 
origin  in  the  cells  of  the  anterior  horn,  shortly  before  they  issue  from  the 
cord,  others  have  some  other  origin.  -And  similarly  we  have  reason  to  think 
that  all  the  cells  in  the  anterior  horn  do  not  send  out  axis-cylinder  processes 
to  join  the  anterior  roots  of  the  same  side.  We  may,  however,  regard  a 
large  number  at  all  events  of  the  cells  of  the  anterior  horn,  at  the  level  of 
as  well  as  a  little  below  and  a  little  above  the  level  of  the  exit  of  any  par- 
ticular anterior  root,  as  constituting  a  sort  of  nucleus  of  origin  for  the  larger 
number  of  the  fibres,  and  those  most  probably  the  skeletal  motor  fibres,  of 
that  anterior  root. 

The  posterior  root  enters  the  cord  not  in  several  bundles  laterally  scat- 
tered as  does  the  anterior  root,  but  in  a  more  compact  mass.  This  mass, 
however,  consists  of  at  least  two  distinct  bundles,  which  upon  their  entrance 
into  the  cord,  take  different  courses.  One  bundle,  the  larger  one,  lying  to 
the  inner  or  median  side  of  the  other,  consisting  of  relatively  coarse  fibres, 
and  called  the  median  bundle  (Fig.  121,  JV),  passes  obliquely  into  the  lateral 
part  of  the  external  posterior  column,  which,  as  we  have  said,  is  in  conse- 
quence often  spoken  of  as  the  posterior  root-zone.  Here  the  fibres  changing 
their  direction  run  longitudinally  for  some  distance  upward  (some,  however, 
certainly  in  the  upper  cervical  region,  and  probably  in  other  regions,  run  a 

35 


546  THE  SPINAL  CORD. 

short  distance  downward),  but  eventually  either  go,  as  we  shall  see,  to  form 
the  median  posterior  tract  or  make  their  way  back  into  the  gray  matter  at 
the  base  of  the  posterior  horn  and  thus  join  the  vesicular  cylinder,  though 
some  are  said  to  be  continued  on  through  the  gray  matter  into  the  anterior 
horn.  The  other  smaller  bundle  placed  to  the  outside  of  the  former,  and 
called  the  lateral  bundle  (Fig.  121,  Pr),  may  be  again  divided  into  an  inter- 
mediate bundle  (Fig.  122,  Pr)  lying  next  to  the  median  bundle,  and  into  a 
still  more  lateral  bundle  (Fig.  122,  Pr").  The  former,  consisting  also  of 
coarse  fibres,  plunges  directly  through  the  substance  of  Rolando  at  the  ex- 
tremity of,  and  so  into  the  gray  matter  of  the  horn,  where  the  fibres  chang- 
ing their  direction  run  in  part  at  least  longitudinally  in  the  gray  matter  in 
bundles  known  as  "  the  longitudinal  bundles  of  the  posterior  horn  "  (Figs. 
121,  122,  r.f.  p.),  some  of  which  appear  to  pass  on  to  the  anterior  horn. 
The  small,  most  external  or  lateral  portion  of  the  lateral  bundle,  consisting  of 
fine  fibres  and  sometimes  spoken  of  as  the  lateral  bundle,  on  entering  the  cord 
at  once  ascends  for  some  distance,  and  thus  forms  the  thin  layer  of  fine  fibres, 
the  posterior  marginal  zone  or  Lissauer's  zone,  indicated  in  Fig.  122  by  m.  t., 
which  lies  between  the  actual  extremity  of  the  horn  and  the  surface  of  the 
cord,  and  in  the  upper  regions  of  the  cord  (c/.  Fig.  121,  p')  runs  some  way 
upward  on  the  lateral  margin  of  the  horn  between  the  gray  matter  and  the 
crossed  pyramidal  tract.  As  it  ascends  this  layer  continually  gives  off  fibres 
to  the  gray  matter  of  the  posterior  horn  in  the  cells  of  which  they  appear 
to  end. 

Thus,  while  part  of  the  median  bundle  does  not  join  the  gray  matter  at 
all  but  goes  to  form  the  median  posterior  tract,  the  rest  of  that  bundle  and 
all  the  other  fibres  of  the  root,  sooner  or  later,  join  the  gray  matter  either 
of  the  posterior  horn  or  of  some  other  part. 

§  483.  The  special  features  of  the  several  regions  of  the  spinal  cord.  The 
cord  begins  below  in  the  slender  filament  called  ihefilum  terminate,  which 
lying  in  the  vertebral  canal,  in  the  midst  of  the  mass  of  nerve-roots  called 
the  cauda  equina,  rapidly  enlarges  at  about  the  level  of  the  first  lumbar  ver- 
tebra into  the  conus  medullaris.  This  may  be  regarded  as  the  beginning  of 
the  lower  portion  of  a  fusiform  enlargement  of  the  cord  known  as  the  lumbar 
swelling,  which  reaches  as  high  as  about  the  attachment  of  the  roots  of  the 
twelfth  or  eleventh  thoracic  nerve  at  the  level  of  the  eighth  thoracic  ver- 
tebra, the  broadest  part  of  the  swelling  being  about  opposite  the  third  lumbar 
nerve.  Above  the  lumbar  swelling,  through  the  thoracic  region  the  some- 
what narrowed  cord  retains  about  the  same  diameter  until  it  reaches  the 
level  of  the  first  or  second  thoracic  nerve  opposite  the  seventh  cervical  ver- 
tebra where  a  second  fusiform  enlargement,  the  cervical  swelling ',  broader  and 
longer  than  the  lumbar  swelling,  begins.  The  broadest  part  of  the  cervical 
swelling  is  about  opposite  to  the  fifth  or  sixth  cervical  nerve ;  from  thence 
the  diameter  of  the  cord  becomes  gradually  somewhat  less  until  it  begins  to 
expand  into  the  bulb,  but  even  in  the  highest  part  is  greater  than  in  the 
thoracic  region.  The  sectional  area  of  the  cord  increases  therefore  from 
below  upward,  but  not  regularly,  the  irregularity  being  due  to  the  lumbar 
and  cervical  swellings.  The  extremity  of  the  filum  terminale  is  said  to  con- 
sist entirely  of  neuroglia  closely  invested  by  the  membranes,  even  the  central 
canal  being  absent.  A  little  higher  up  the  central  canal  begins,  and  nerve- 
cells  with  nerve-fibres  make  their  appearance  in  the  neuroglia ;  thus  a  kind 
of  gray  matter  covered  by  a  thin  superficial  layer  of  white  matter  is  estab- 
lished. We  have  already  referred  to  the  peculiar  features  of  the  lower  end 
of  the  conus  (§  477) ;  but  higher  up  the  canal  becomes  central  and  small, 
the  posterior  columns  are  developed,  and  the  gray  matter  contains  more 
nervous  elements  and  relatively  less  neuroglia,  becomes  in  fact  ordinary  gray 


THE  STRUCTURE  OF  THE  SPINAL  CORD. 


547 


matter.  From  thence  onward  to  very  near  the  junction  with  the  bulb,  where 
transitional  features  begin  to  come  in,  the  spinal  cord  may  be  said  to  have 
the  general  structure  previously  described. 

The  sectional  area  of  the  white  matter  increases  in  absolute  size  and  on 
the  whole  in  a  steady  manner  from  below  upward.  In  other  words,  in  a 
section  at  any  level,  the  number  of  longitudinal  fibres  forming  the  white 
matter  is  greater  than  the  number  at  a  lower  level,  and  less  than  the  number 
at  a  higher  level ;  for  any  difference  which  may  exist  in  the  diameter  of  the 
individual  fibres  is  insufficient  to  explain  the  differences  in  the  total  sectional 

FIG.  124. 


XII    XI     X     IX  VIII  VII     VI     V      IV    III    II       I     VIII  VII    VI     V    IV    III     II      I 


Diagram  showing  the  United  Sectional  Areas  of  the  Spinal  Nerves,  proceeding  from  Below 
Upward.  In  this,  as  in  the  succeeding  figures,  125,  126,  127, 128,  129,  all  of  which  refer  to  man,  the 
left-hand  side  represents  the  bottom  of  the  cord  and  the  right-hand  the  top  of  the  cord,  the  nu- 
merals indicating  successively  the  sacral,  lumbar,  thoracic,  and  cervical  nerves.  The  several 
figures  are  not  drawn  to  the  same  scale. 

area  of  the  white  matter.  If  we  were  to  measure  in  man  the  sectional  area 
of  each  of  the  spinal  nerves  as  it  joins  the  cord,  and  to  add  them  together, 
passing  along  the  cord  from  below  upward,  the  results  put  in  the  form  of  a 
curve  would  give  us  some  such  figure  as  that  shown  in  Fig.  124 ;  the  area 
gained  by  adding  together  the  sectional  areas  of  the  nerves  increases  in  a 
fairly  steady  manner  from  below  upward.  The  curve  of  the  sectional  area 
of  the  white  matter  of  the  cord  taken  from  below  upward  would  be  very 
similar,  but  if  anything  more  regular.  It  must  be  understood,  however, 

FIG.  125. 


V    IV     III    II       I      V     IV     III 


I     XII     XI     X      X    VIII  VII  VI      V     IV 


VIII  VII    VI    V     IV     III     II 


Diagram  showing  the  Variations  in  the  Sectional  Area  of  the  Gray  Matter  of  the  Spinal  Cord 

along  its  Length. 


that  the  dimensions  of  the  areas  would  not  be  the  same  in  the  two  cases. 
The  sectional  area  of  the  white  matter  at  the  top  of  the  cervical  region,  though 
greater  than  anywhere  lower  down,  is  far  less  than  the  united  sectional  area 
of  all  the  nerves  below  that  level.  The  white  matter  is  not  formed  by  all 
the  fibres  from  the  nerves  which  join  the  spinal  cord  continuing  to  run  along 
the  cord  up  to  the  brain  ;  as  we  have  seen,  some  at  least  of  the  fibres  end  in 
the  gray  matter.  Nevertheless,  the  white  matter  in  passing  up  the  cord  ap- 


548  THE  SPINAL  CORD. 

pears  to  receive  a  permanent  addition  at  the  entrance  of  each  nerve.  We 
may  infer  that  each  nerve  has  a  representative  of  itself  starting  from  the 
level  of  its  entrance  and  running  up  to  some  part  of  the  brain.  Whether 
the  fibres  thus  representative  of  the  nerve  are  continuations  of  the  very 
fibres  of  the  nerve  itself,  or  are  new  fibres  starting  from  some  relay  of 
gray  matter,  with  which  the  fibres  of  the  nerve  are  also  connected,  is  another 
question. 

§  484.  The  gray  matter  in  contrast  to  the  white  matter  shows  great  varia- 
tions in  area  along  the  length  of  the  cord  (Fig.  125).  From  the  entrance 
of  the  coccygeal  nerve  upward  the  area  increases  very  rapidly,^  reaching  a 
maximum  at  about  the  level  of  the  fifth  lumbar  nerve.  It  then  rapidly  de- 
creases to  about  the  level  of  the  eleventh  thoracic  nerve,  maintains  about  the 
same  dimensions  all  through  the  thoracic  region,  and  begins  to  increase  again 
at  about  the  level  of  the  second  thoracic  nerve.  Its  second  maximum  is 
reached  at  about  the  level  of  the  fifth  or  sixth  cervical  nerve,  after  which 
the  area  again  becomes  smaller,  remaining,  however,  at  the  upper  cervical 
region  much  larger  than  in  the  thoracic  region. 

FIG.  126. 
10 


I      V    IV      III     II      I      XII    XI     X     IX    VIII  VII     VI     V    IV     III     II       I     VIII    VII  VI     V     IV     III    II       I 


Diagram  showing  the  Relative  Sectional  Areas  of  the  Spinal  Nerves  as  they  Join  the  Spinal 

Cord. 

The  meaning  of  these  variations  becomes  clear  when  we  turn  to  Fig.  126, 
which  shows  in  a  similar  diagrammatic  manner  the  sectional  areas  of  the 
several  spinal  nerves.  It  will  be  observed  that  the  increase  and  decrease  of 
the  sectional  area  of  the  gray  matter  follow  very  closely  the  increase  and 
decrease  of  the  quantity  of  nerve,  that  is  to  say,  neglecting  differences  in  the 
diameter  of  the  fibres,  in  the  number  of  nerve-fibres  passing  into  the  cord. 
The  sectional  areas  of  the  first  and  second  sacral,  fourth  and  fifth  lumbar 
nerves  are  very  large,  and  opposite  to  these  the  sectional  area  of  the  gray 
matter  of  the  cord  is  very  large  also  ;  the  enlargement  of  gray  matter  which 
is  the  essential  cause  of  the  lumbar  swelling  is  correlated  to  the  large  number 
of  fibres  which  enter  and  leave  the  cord  at  this  region  to  supply  chiefly  the 
lower  limbs.  Similarly  the  enlargement  of  gray  matter  which  is  the  essen- 
tial cause  of  the  cervical  swelling  is  correlated  to  the  large  number  of  fibres 
which  enter  and  leave  this  region  of  the  cord  to  supply  chiefly  the  upper 
limbs.  In  the  thoracic  region,  where  the  number  of  fibres  entering  and 
leaving  the  cord  is  relatively  less,  the  sectional  area  of  the  gray  matter  is 
also  less.  Since  the  attachments  of  the  several  spinal  nerves  are  not  exactly 
equidistant  from  each  other  along  the  length  of  the  cord,  the  sectional  area 
is  not  an  exact  measure  of  bulk  ;  the  total  bulk  of  gray  matter,  for  instance, 
belonging  to  two  nerves  which  enter  the  cord  close  together  is  less  than  that 
of  two  nerves  giving  rise  to  the  same  sectional  area  of  gray  matter  as  the 
former  two  but  entering  the  cord  far  apart  from  each  other.  Still  the  error 
which  may  be  introduced  by  taking  sectional  area  to  mean  bulk  is,  for  pres- 
ent purposes  at  all  events,  so  small  that  we  may  permit  ourselves  to  say  that 
in  the  successive  regions  of  the  spinal  cord  the  bulk  of  gray  matter  in  any 
segment  is  greater  or  less  according  to  the  size  of  the  nerve  (or  pair  of 
nerves,  right  and  left)  belonging  to  that  segment. 

From  this  anatomical  fact  we  appear  justified  in  drawing  the  conclusion 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  549 

that  at  all  events  a  great  deal  of  the  gray  matter  of  the  spinal  cord  may  be 
considered  as  furnishing  a  nervous  mechanism,  with  which  the  efferent  fibres 
of  each  spinal  nerve  just  before  they  leave  the  cord,  and  the  afferent  fibres 
soon  after  they  join  the  cord,  are  more  immediately  connected.  It  may  be 
that  the  whole  of  the  gray  matter  is  thus  directly  connected  with  and  thus 
rises  and  falls  with  the  fibres  of  the  nerves ;  or  it  may  be  that  there  is  a  sort 
ofv  cord  of  gray  matter,  which  maintains  a  uniform  bulk  along  the  whole 
length  of  the  cord  and  serves  as  a  basis  which  is  here  more  and  there  less 
swollen  by  the  addition  of  the  gray  matter  more  immediately  connected  with 
the  fibres  of  the  nerves.  This  question  the  method  which  we  are  now  using 
cannot  settle. 

§  485.  Owing  to  these  different  rates  of  increase  of  the  gray  and  white 
matter  respectively  along  the  length  of  the  cord,  we  find  that  in  sections  of 
the  cord  taken  at  different  levels  the  appearances  presented  vary  in  a  very 
distinct  manner.  This  is  strikingly  shown  by  comparing  Figs.  119,  121  and 
122.  At  the  level  of  the  third  lumbar  nerve  (Fig.  122)  the  gray  matter  is 
very  large,  reaching,  as  we  have  seen,  its  maximal  sectional  area  at  about 
this  point,  so  that  although  the  area  of  white  matter  is  not  very  great  the 
whole  area  of  the  cord  is  considerable. 

At  the  level  of  the  sixth  thoracic  nerve  (Fig.  119),  in  spite  of  the  white 
matter  having  very  decidedly  increased,  the  gray  matter  has  shrunk  to  such 
very  small  dimensions  that  the  total  sectional  area  of  the  cord  has  markedly 
diminished. 

At  the  level  of  the  sixth  cervical  (Fig.  121)  the  gray  matter  has  again 
increased,  reaching  here,  as  we  have  seen,  its  second  maximum ;  the  white 
matter  has  also  further  increased,  and  that  indeed  very  considerably,  so  that 
the  total  area  of  the  cord  is  much  greater  than  in  any  of  the  lower  regions. 

Further  details  of  the  varying  size  of  the  white  matter  and  of  the  gray 
matter  at  different  levels  are  also  shown  in  the  series  given  in  Fig.  127.  In 
these,  combined  with  the  three  figures  just  referred  to,  it  will  be  observed 
that  the  serial  increase  and  decrease  of  the  gray  matter  does  not  affect  all 
parts  of  the  gray  matter  alike,  so  that  the  outline  of  the  gray  matter  changes 
very  markedly  in  passing  from  below  upward.  In  the  coccygeal  region  each 
lateral  half  is  a  somewhat  irregular  oval,  and  in  the  sacral  region  (Fig.  127, 
Sac.)  the  differentiation  into  anterior  and  posterior  horns  is  still  very  indis- 
tinct. In  the  lumbar  region  the  two  horns  are  sharply  marked  out,  though 
both  the  posterior  and  anterior  horns  are  broad  and  more  or  less  quadrate. 
Iii  the  thoracic  region  the  decrease  of  gray  matter  has  affected  both  horns, 
so  that  both  are  pointed  and  slender,  while  the  junction  between  them  has 
not  undergone  so  much  diminution,  so  that  what  has  been  called  the  lateral 
horn  is  relatively  conspicuous.  In  the  cervical  region  the  returning  increase 
bears  much  more  on  the  anterior  horn,  which  again  becomes  large  and  broad, 
than  on  the  posterior  horn,  which  still  remains  slender  and  pointed.  Taking 
the  form  of  the  gray  matter  in  the  thoracic  region  as  the  more  typical  form 
of  the  gray  matter  we  may  say  that  while  the  increase  in  the  lumbar  swell- 
ing bears  equally  on  the  anterior  and  posterior  horns,  that  in  the  cervical 
region  bears  chiefly  on  the  anterior  horns. 

Now  we  have  no  reason  to  suppose  that  either  afferent  impulses  reach  the 
lumbar  spinal  cord  in  greater  numbers  from  the  lower  limbs,  or  along  any 
of  the  nerves  joining  this  part  of  the  cord,  or  that  those  which  do  reach  it 
are  of  a  more  complex  nature  than  is  the  case  with  the  afferent  impulses 
reaching  the  cervical  cord  along  the  nerves  of  the  upper  limbs.  The  increase 
of  gray  matter  in  the  posterior  horns  is  therefore  not  correlated  to  any  in- 
crease in  the  number  or  complexity  of  the  afferent  impulses  reaching  the 
cord ;  and  we  may  provisionally  conclude  that  at  least  a  large  part  of  the 


550 


THE  SPINAL  CORD. 


gray  matter  in  the  posterior  horn  is  not  specially  concerned  in  any  elabo- 
ration or  transformation  of  afferent  impulses  immediately  upon  their  arrival 
at  the  cord.  Indeed,  we  have  seen  that  while  there  is  ample  evidence  to 
connect  the  nerve-cells,  and  therefore  presumably  the  gray  matter  in  general 
of  the  anterior  horn  with  the  efferent  motor  fibres  of  the  anterior  root,  there 
is  no  corresponding  evidence  as  to  any  large  immediate  connection  of  the 
afferent  fibres  of  the  posterior  root  with  the  nerve-cells,  or  indeed  any  other 
part  of  the  gray  matter  of  the  posterior  horn.  We  may  add  that,  as  we 
shall  point  out  later  on,  so  essential  is  the  concurrence  of  appropriate  affer- 
ent impulses  to  the  due  carrying  out  of  complex  coordinate  motor  or  effer- 
ent impulses,  that  we  can  scarcely  expect  to  find  any  increase  in  the  nervous 
mechanisms  devoted  to  the  purely  motor  function  of  carrying  out  motor 
impulses  without  a  corresponding  increase  in  the  nervous  mechanisms 
belonging  to  the  afferent  impulses,  by  means  of  which  these  motor  impulses 

FIG.  127. 


C2 


s.lr 


C.b 


THE  STRUCTURE  OF  THE  SPINAL  CORD. 


551 


sr  ir  dr 


Sac. 


Diagram  illustrating  some  of  the  Features  of  the  Spinal  Cord  at  Different  Levels.  (Sherring- 
ton.)  All  the  figures  are  drawn  to  scale,  and  represent  the  cord  magnified  four  times.  They  show 
the  difference  at  different  levels  in  the  shape  and  size  of  the  cord,  in  the  outline  of  the  gray 
matter,  and  in  the  relative  position  of  the  anterior  and  posterior  fissures,  and  also  show  the  varia- 
tions at  different  levels  of  the  several  "  tracts"  of  the  white  matter. 

Q>  at  the  level  of  the  second  cervical  nerve,  C5  of  the  fifth  cervical,  C8  of  the  eighth  cervical. 
Do  of  the  second  thoracic,  Z>5  of  the  fifth  thoracic,  LI  of  the  first  lumbar,  L5  of  the  fifth  lumbar 
and  Sac.  of  the  second  sacral  nerve. 

The  shading  of  the  tracts  is  the  same  as  in  Fig.  123;  but  in  the  median  posterior  column  of 
/>2  the  areas  of  fibres  coming  from  the  sacral  nerves  s.r.  and  lumbar  nerves  l.r.  are  distinguished 
from  the  area  rf.r.  of  fibres  belonging  to  the  thoracic  nerves.  In  f?8  no  distinction  is  made  between 
any  of  these  sets  of  fibres ;  in  L5  only  fibres  of  sacral  nerves  are  represented  ;  in  L\  Ds  D5  the 
more  dorsal  small  portion  corresponds  in  sacral  fibres  and  the  next  to  lumbar,  or  lumbar  thoracic 
nerves. 


are  guided  and  coordinated.  Hence,  were  the  nervous  mechanisms  re- 
stricted to  the  posterior  horns,  we  should  expect  to  find  a  greater  parallel- 
ism than  does  actually  exist  between  them  and  the  anterior  horns. 

§  486.  The  changes  in  the  area  of  gray  matter  illustrated  by  the  state- 
ments and  diagrams  given  above  refer  to  the  gray  matter  as  a  whole — that 
is,  not  only  to  nerve-cells,  but  also  to  strands  and  networks  of  nerve-fibres 
and  nerve-fibrils,  and  indeed  include  to  a  certain  extent  neuroglia.  We 
have  seen  (§  479)  that  we  are  able  to  distinguish  certain  large  and  con- 
spicuous nerve-cells  in  the  gray  matter,  and  to  arrange  these  into  groups. 


552  THE  SPINAL  CORD. 

The  gray  matter  contains  many  other  small  nerve-cells,  which  we  are  not 
able  at  present  to  name  or  arrange,  but  whose  existence  must  always  be 
borne  in  mind.  Confining  ourselves  now,  however,  to  the  groups  of  larger, 
more  conspicuous  nerve-cells,  we  find  that,  broadly  speaking,  the  chief  dif- 
ferences which  can  be  observed  in  the  cells  of  the  anterior  horn  along  the 
length  of  the  cord  are  that  in  the  thoracic  region  the  nerve-cells  of  the  ante- 
rior horn  are  few  and  relatively  small,  while  in  the  cervical  and  lumbar 
region,  especially  in  the  latter,  they  are  numerous  and  large.  It  is  not  easy, 
even  if  possible,  to  distinguish  in  the  thoracic  region  the  several  groups  of 
cells  marked  in  Figs.  12l"and  122  as  2  oc,  [1,  y  •  the  median  group  (Figs.  121, 
122,  1),  indeed,  seems  to  be  the  only  group  present  in  the  mid-thoracic 
region  (Fig.  119,  1).  The  group  of  the  posterior  horn  (Figs.  119,  121,  122, 
6)  appears  to  be  about  the  same  in  all  regions. 

With  two  other  groups  of  nerve-cells  striking  differences  are  seen  in  dif- 
ferent regions.  The  vesicular  cylinder,  for  instance  (Fig.  119,  3),  is  most 
conspicuous  in  the  thoracic  region.  It  may  be  said  to  reach  from  the 
seventh  or  eighth  cervical  nerve  to  the  third  lumbar  nerve,  being  perhaps 
most  developed  in  the  lower  thoracic  and  upper  lumbar  region.  It  is 
absent  in  the  cervical  region  above  the  seventh  or  eighth  cervical  nerve,  and 
in  the  lumbar  region  below  the  third  lumbar  nerve ;  but  a  similar  group  of 
cells  is  present  opposite  the  second  and  third  cervical  nerves ;  a  group  of 
more  doubtful  likeness  is  seen  in  the  sacral  region  below,  and  the  column  is 
said  to  have  a  representative  in  the  bulb  above  the  spinal  cord  proper.  It 
seems  natural  to  infer  that  the  cells  forming  this  vesicular  cylinder  are 
connected  neither  with  the  ordinary  somatic  motor  fibres  governing  the 
skeletal  muscles,  nor  with  the  ordinary  afferent  sensory  somatic  fibres 
coming  from  the  skin  and  elsewhere,  but  in  some  way  with  some  special 
sets  of  fibres ;  on  this  point,  however,  no  authoritative  statement  can  as  yet 
be  made. 

The  lateral  horn  or  intermedio-lateral  tract  (Fig.  119,  4)  is  also  most  con- 
spicuous in  the  thoracic  region.  In  the  lumbar  region  it  is  lost  or  traced 
with  great  difficulty,  and  in  the  cervical  region  seems  to  be  merged  into  the 
most  dorsally  placed  division  of  the  lateral  group  of  cells  of  the  anterior 
horn.  It  is  possible  that  this  group  represents  in  the  limbless  thoracic 
region  the  cells  which  are  developed  into  the  great  lateral  group  of  the  ante- 
rior horn  in  the  regions  of  the  limbs. 

§  487.  The  white  matter,  as  we  have  seen,  increases  in  sectional  area 
with  considerable  regularity  from  below  upward.  If,  instead  of  a  diagram 
of  the  increase  of  the  whole  white  matter  we  construct  in  a  similar  way 
diagrams  of  the  anterior,  posterior,  and  lateral  columns  respectively,  we 
find  that  while  the  sectional  area  of  the  lateral  column  (Fig.  128)  increases 
with  some  considerable  regularity  from  below  upward,  though  not  so  regu- 
larly as  does  the  whole  area  of  white,  matter,  both  the  anterior  (Fig.  129) 
and  the  posterior  (Fig.  130)  columns  agree  to  a  certain  extent  with  the  gray 
matter  in  showing  a  decided  increase  in  both  the  lumbar  and  the  cervical 
swellings.  We  may,  provisionally  at  least,  infer  from  this  that,  while  con- 
siderable portions  of  both  the  anterior  and  the  posterior  columns  are,  like 
the  adjoining  gray  matter,  in  some  way  or  other  concerned  in  the  exit  and 
entrance  of  efferent  and  afferent  fibres,  the  larger  portion  of  the  lateral 
column  is  concerned  in  the  transmission  of  impulses  to  and  fro,  between  the 
local  mechanisms  below,  immediately  connected  with  the  several  spinal 
nerves,  and  the  brain  above.  This  conclusion  seems  incidentally  confirmed 
(though  these  diagrams  must  not  be  strained  to  carry  detailed  inferences)  by 
the  sudden  increase  of  the  lateral  column  above  the  lumbar  swelling,  as  if 
the  large  mass  of  nervous  mechanism  for  the  lower  limbs  concentrated  in 


THE  STRUCTURE  OF  THE  SPINAL  CORD. 


553 


this  region  demanded  a  sudden  increase  in  the  number  of  fibres  connecting 
it  with  the  brain  above. 

This  more  or  less  continuous  increase  of  the  lateral  column  partly  ex- 
plains the  change  of  form  in  the  general  outline  of  the  transverse  section 
of  the  cord  which  is  observed  in  passing  upward  from  the  lower  to  the 
higher  regions.  In  the  coccygeal,  sacral,  and  lumbar  regions  the  outline, 
though  varying  somewhat,  chiefly  owing  to  the  disposition  of  the  gray 
matter,  is  on  the  whole  circular.  In  the  thoracic  region,  especially  in  the 
upper  part,  the  increase  of  the  lateral  columns  increases  the  side-to-side 


FIG.  128. 


y      (V    HI    ||       I      V     IV    III     II      I      XII    XI    X     IX  VIII  VII  VI     V     IV    III     II       I    VIII    VII    VI      V     IV    III 

Diagram  showing  the  Variations  in  the  Sectional  Area  of  the  Lateral  Columns  of  the  Spinal 

Cord  along  its  Length. 

FIG.  129. 


V     IV     III      II       I      XII    XI     X     IX  VIII  VII    VI      V     IV     III 


VIII    VII   VI      V     IV 


Diagram  showing  the  Variations  in  the  Sectional  Area  of  the  Anterior  Columns  of  the  Spinal 

Cord  along  its  Length. 


FIG.  130. 


I     XII    XI    X      IX   VIII  VII    VI     V    IV 


Diagram  showing  the  Variations  in  the  Sectional  Area  of  the  Posterior  Columns  of  the  Spinal 

Cord  along  its  Length. 

diameter  so  much  that  the  section  becomes  oval,  and  in  the  cervical  region 
this  increase  of  the  side-to-side  diameter  out  of  proportion  to  the  dorso-ven- 
tral  diameter  is  very  marked.  The  actual  outline  of  the  whole  transverse 
section  is,  however,  determined  also  to  a  certain  extent  by  the  changes  of 
form  of  the  gray  matter. 

The  cord,  moreover,  undergoes  along  its  length  a  change  which  is  not 
very  clearly  indicated  in  the  diagrams  (Figs.  129,  130).  By  comparing  the 
series  of  transverse  sections  given  in  Fig.  127,  it  will  be  seen  that  the  rela- 
tive position  of  the  central  canal  shifts  along  the  length  of  the  cord.  In  the 
sacral  and  lumbar  regions  the  central  canal  is  nearly  at  the  centre  of  the 
circle  of  outline,  and  the  posterior  and  anterior  fissures  are  nearly  of  equal 
depth.  Even  in  the  upper  lumbar  region,  and  still  more  in  the  thoracic  re- 
gion, the  position  of  the  central  canal  is  shifted  nearer  to  the  ventral  sur- 
face, so  that  the  posterior  fissure  becomes  relatively  longer,  deeper  than  the 


554  THE  SPINAL  CORD. 

anterior.  This  shifting  goes  on  through  the  cervical  region  up  to  about  the 
level  of  the  second  cervical  nerve,  where  it  is  arrested  by  the  beginning  of 
the  changes  through  which  the  spinal  cord  is  transformed  into  the  far  more 
complicated  bulb. 

This  lengthening  of  the  posterior  fissure  indicates  an  increase  in  the 
dorso-ventral  diameter  of  the  posterior  columns,  and  this,  not  being  accom- 
panied by  a  compensating  diminution  of  the  side-to-side  diameter,  shows  in 
turn  that  the  posterior  columns  undergo  an  increase  in  passing  upward. 
From  this  we  may  add  to  the  provisional  conclusion  just  arrived  at  with 
regard  to  the  lateral  columns,  the  further  conclusion  that  some  part  of  the 
posterior  columns  also  is  concerned  in  transmitting  impulses,  in  a  more  or 
less  direct  manner,  between  the  various  regions  of  the  cord  below  and  the 
brain  above.  The  anterior  columns  do  not  increase  in  the  same  marked 
manner,  though  over  and  above  the  increase  due  to  the  lumbar  and  cervical 
swellings  a  continued  increase  may  be  observed,  especially  in  the  upper  cer- 
vical region ;  it  is  in  this  upper  region  that  the  direct  pyramidal  tract  is 
best  developed. 

§  488.  The  provisional  conclusions  at  which  we  have  arrived  are  further, 
to  a  certain  extent  at  least,  confirmed  and  extended  by  a  study  of  the 
behavior  at  the  several  regions  of  the  cord  of  the  special  tracts  of  white 
matter  described  in  §  479. 

The  pyramidal  tract,  that  is  to  say,  the  crossed  pyramidal  tract  entering 
the  spinal  cord  above  from  the  pyramid,  is  very  large  in  the  cervical  region, 
having  the  form  and  situation  shown  in  Fig.  1 27,  C2  C3  C8.  From  thence 
downward  it  diminishes  in  size,  the  diminution  being  especially  rapid  in  the 
lumbar  swelling  (Fig.  127,  A),  where  the  tract,  being  no  longer  covered  in 
by  the  cerebellar  tract,  comes  to  the  surface  of  the  cord  ;  but  it  may  be 
traced  by  the  degeneration  method  down  as  far  as  the  coccygeal  region,  and 
indeed  appears  to  be  coexistent  with  the  entrance  of  spinal  nerves  into  the 
cord.  Diminution  of  the  tract  means  a  lessening  of  the  number  of  fibres; 
and  since  we  cannot  suppose  that  any  of  the  fibres  come  suddenly  to  an  end 
in  the  tract  itself,  we  are  led  to  infer  that  along  the  cord,  from  above  down- 
ward, fibres  are  successively  leaving  the  tract  and  passing  to  some  other  part 
of  the  cord.  We  seem  further  justified  in  concluding  that  the  fibres  which 
thus  successively  leave  the  tract  go  to  join  the  series  of  local  nervous  mech- 
anisms with  which  the  spinal  nerves  communicate,  as  we  have  seen  reason 
to  believe,  upon  their  entrance  into  the  cord.  Indeed,  as  we  shall  see  later 
on,  we  have  reason  to  think  that  the  nervous  mechanisms  which  the  fibres  in 
question  join  are  those  belonging  to  the  motor  fibres  of  the  anterior  roots. 
This  pyramidal  tract  does  not  begin  in  the  pyramid,  but  may  be  traced 
through  the  lower  parts  of  the  brain  right  up  to  special  areas  in  the  cortex 
or  surface  of  the  cerebral  hemispheres ;  and  very  strong  reasons  may  be 
brought  forward  in  support  of  the  view  that  the  fibres  of  this  tract  are 
fibres  which  carry  impulses  from  the  cortex  to  successive  portions  of  the 
spinal  cord  and  there  give  rise  to  efferent  impulses  which  pass  to  appro- 
priate skeletal  muscles.  The  tract,  therefore,  is  not  only  a  descending 
tract  by  virtue  of  the  mode  of  degeneration,  but  may  be  spoken  of  in  a 
broad  sense  as  a  tract  of  efferent  impulses  descending  from  the  cerebral 
cortex ;  and,  indeed,  it  is  maintained  that  it  is  the  channel  of  the  partic- 
ular kind  of  efferent  impulses  which  we  shall  speak  of  as  voluntary  or  voli- 
tional impulses.  We  may  add  that  as  the  tract  passes  along  a  path,  which 
we  shall  subsequently  describe,  from  the  cerebral  cortex  through  the  lower 
parts  of  the  brain  to  the  pyramid,  it  gives  off  fibres  to  mechanisms  con- 
nected with  several  of  the  cranial  nerves,  much  in  the  same  way  that  it 
gives  off  fibres  to  the  spinal  nerves. 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  555 

We  may,  therefore,  picture  to  ourselves  this  pyramidal  tract  as  starting 
in  the  form^f  a  broad  sheaf  of  fibres  from  a  certain  district  on  the  surface 
of  one  of  the  cerebral  hemispheres.  Putting  aside  for  the  present  any  pos- 
sible increase  of  the  number  of  fibres  by  division  of  fibres  (though  we  have 
reason  to  think  that  this  does,  to  a  certain  extent,  occur),  we  may  regard 
the  tract  as  being  at  its  maximum  at  its  beginning  in  the  cortex.  As  it 
descends  to  the  decussation  of  the  pyramids  in  the  bulb  it  loses  a  certain 
number  of  fibres,  which  pass  off  to  the  cranial  nerves.  Having  crossed  and 
entered  into  the  lateral  column  of  the  cord  it  continues  to  give  off  fibres  to 
the  spinal  nerves,  probably  to  the  anterior  root  of  each  in  succession,  and 
so  goes  on  its  way  down  the  cord  continually  diminishing  until  the  last 
remaining  fibres  are  given  off  to  the  last  coccygeal  nerve. 

When  degeneration  is  set  up  along  this  tract,  as  may  be  done  by  in- 
juries to  particular  areas  of  the  cerebral  cortex,  the  main  mass  of  degene- 
rated fibres,  after  crossing  over  from  one  side  of  the  cerebro-spinal  axis  to 
the  other  in  the  decussation  of  the  pyramids  at  the  lower  end  of  the  bulb, 
during  its  further  progress  down  the  spinal  cord,  keeps  to  the  side  to  which 
it  has  crossed  right  down  to  the  end.  Hence,  as  we  have  said,  it  is  called 
the  crossed  pyramidal  tract.  The  main  mass  of  fibres,  the  degeneration  of 
which  has  been  started  by  injury  to  the  left  side  of  the  brain,  crosses  over  to 
the  right  side  of  the  spinal  cord  and  runs  down  the  lateral  column  of  the 
right  side  to  the  end  of  the  cord.  Nevertheless,  some  fibres  appear  to  cross 
over  again  in  the  spinal  cord  and  then  to  run  along  the  same  side  as  the  side 
of  the  brain  injured — along  the  left  side  in  the  case  just  mentioned.  Such 
fibres  are  spoken  of  as  "  recrossed  fibres." 

The  direct  pyramidal  tract  (Fig.  127,  dP),  except  that  it  does  not  cross 
at  the  decussation  of  the  pyramids,  is  otherwise  similar  to  the  crossed  pyra- 
midal tract,  and,  indeed,  is  a  part  of  the  same  strand  to  which  the  crossed 
tract  belongs.  When  degeneration  in  this  tract  is  started  by  injury  to  par- 
ticular areas  of  the  cerebral  cortex,  say  on  the  left  half  of  the  brain,  the 
degeneration  may  be  traced  through  the  left  anterior  pyramid,  and  so  to  the 
left  median  anterior  column  of  the  spinal  cord.  The  direct  tract  is  never  so 
extensive  or  marked  as  the  crossed  tract,  does  not  reach  so  far  down,  is  much 
more  variable  both  in  length  and  in  sectional  area,  and,  as  we  have  said,  is 
almost  confined  to  man.  Diminishing  as  it  descends  it  may  be  said  to  cease 
in  the  middle  thoracic  region  (Fig.  127,  A  A)-  Taking  an  average,  we  may 
say  that,  of  the  whole  strand  running  in  the  pyramids  above  the  decussa- 
tion, about  three-fourths  of  the  fibres  go  to  form  the  crossed  and  about  one- 
fourth  to  form  the  direct  tract.  We  shall  see  later  on  that  the  impulses 
coming  down  along  the  united  tract  in  the  brain  may,  broadly  speaking, 
be  said  to  cross  over  wholly  from  one  side  to  the  other  before  they  reach 
the  skeletal  muscles,  so  that  the  impulses  passing  along  the  fibres  in,  say, 
the  left  pyramid,  reach  the  muscles  of  the  right  limbs  and  right  side  of  the 
body,  whether  the  fibres  cross  over  at  the  decussation  to  form  the  crossed 
or  remain  on  the  same  side  to  form  the  direct  pyramidal  tract.  We  are, 
therefore,  led  to  infer  that  the  fibres  in  the  direct  tract,  as  they  pass  down 
the  cord,  cross  over  in  the  cord  itself  before  they  make  connections  with  the 
fibres  of  the  anterior  roots.  Probably  the  crossing  is  effected  by  means  of 
some  of  the  decussating  fibres  which  form  the  anterior  white  commissure. 
A  part  only,  indeed  a  small  part,  of  the  commissure  can  serve  this  purpose  ; 
most  of  the  fibres  of  the  commissure,  and  in  the  lower  regions  of  the  cord, 
where  the  direct  tract  no  longer  exists,  all  the  fibres,  must  have  some  other 
functions.  Some  of  the  fibres  of  this  great  pyramidal  tract  leave  the  tract, 
as  we  have  said,  to  join  some  of  the  cranial  nerves  before  the  pyramids  of 
the  bulb  are  reached  ;  and  the  impulses  passing  along  these  fibres  also  cross 


556  THE  SPINAL  CORD. 

over  to  the  opposite  side  before  they  issue  along  the  cranial  nerves.  Hence 
we  infer  that  these  fibres  decussate  above  the  decussation  of  the  pyramids 
just  as  those  of  the  direct  tract  decussate  below  it.  So  that  of  the  whole 
strand  as  it  leaves  the  cerebral  cortex,  while  the  main  mass  of  fibres  crosses 
over  at  the  decussation  of  the  pyramids,  the  rest  of  the  fibres  cross  the 
middle  line  in  succession  from  the  level  of  the  third  cranial  nerve  to  the 
level  of  the  lower  limit  of  the  direct  tract ;  below  the  decussation  of  the 
pyramids  the  crossing  takes  place  by  means  of  the  anterior  commissure  of 
the  cord,  above  the  decussation  by  means  of  what  we  shall  later  on  learn  to 
speak  of  as  the  raphe  of  the  bulb,  or  by  structures  corresponding  to  this 
higher  up. 

§  489.  The  cerebellar  tract  (Fig.  127,  C.b.)  is,  as  we  have  seen,  a  tract  of 
ascending  degeneration  ;  the  degeneration  in  it  makes  its  appearance  above 
the  section  or  seat  of  other  injury  of  the  cord.  It  begins  somewhat  sud- 
denly at  the  level  of  the  second  lumbar  nerve  region,  being  absent  at  least 
as  a  distinct  tract  below ;  injury  of  the  cord  at  the  level  of  the  middle  and 
lower  lumbar  nerves  leads  to  no  marked  tract  of  degeneration  (though  pos- 
sibly scattered  single  fibres  may  degenerate),  while  injury  higher  up  does. 
The  tract  lies,  as  we  have  said,  close  to  the. surface  of  the  cord  in  the  poste- 
rior part  of  the  lateral  column  just  outside  the  crossed  pyramidal  tract, 
and  while  varying  somewhat  in  the  shape  of  its  section  from  level  to  level, 
remains  throughout  a  somewhat  narrow  crescentic  patch.  At  the  top  of 
the  spinal  cord,  it  passes,  as  we  have  said,  from  the  lateral  columns  into 
the  restiform  bodies  of  the  bulb,  and  so  to  certain  parts  of  the  cere- 
bellum. 

When  the  section  or  lesion  is  limited  to  one  side  of  the  cord,  the  degen- 
eration is  similarly  limited  to  the  same  side,  and  that  along  its  whole  course 
up  to  the  cerebellum  ;  there  is  no  evidence  of  any  of  the  fibres  decussating 
in  the  cord. 

The  area  of  the  tract  increases  from  below  upward.  This  has  been  deter- 
mined by  the  embryological  method,  by  noting  the  appearance  of  the  me- 
dulla in  the  fibres,  as  well  as  by  comparing  the  extent  of  the  degeneration 
following  upon  a  section  high  up  in  the  cord  with  that  following  upon  a  sec- 
tion lower  down.  From  this  we  infer  that  the  fibres  composing  the  tract 
must  start  successively  from  other  parts  of  the  cord  along  its  length — that 
is  to  say,  the  tract  must  be  fed  by  fibres  coming  from  other  structures  in  the 
cord.  On  the  other  hand,  it  is  found  that  the  degenerated  area  following 
upon  a  section  or  injury  diminishes  as  it  is  traced  upward  ;  when,  for  instance, 
a  section  is  made  in  the  mid-thoracic  region,  the  area  of  degeneration  in  the 
tract  is  greater  immediately  above  the  section  than  it  is  higher  up,  say  in 
the  cervical  region.  From  this  we  are  led  to  infer  that  though  the  tract  is 
successively  fed  along  its  course  by  fibres  coming  from  other  parts  of  the 
cord,  some  of  the  fibres  entering  the  tract,  though  like  their  companions 
undergoing  an  ascending  degeneration,  do  not  like  them  continue  in  the 
tract  right  up  to  the  cerebellum,  but  pass  off  to  other  parts  of  the  cord 
on  their  way  upward.  This,  however,  is  equivalent  to  saying  that  the 
tract  is  not  a  pure  or  homogeneous  one,  but  consists  of  at  least  two  sets  of 
fibres,  only  one  of  which  is  continued  on  to  the  cerebellum  and  strictly 
deserves  the  name  of  "  cerebellar."  It  may,  perhaps,  here  be  mentioned 
that  while  the  fibres  composing  the  tract  are,  as  a  whole,  conspicuously 
coarse,  large  fibres,  with  these  there  are  mingled,  especially  in  the  thoracic 
region,  a  number  of  much  finer  fibres  ;  but  these  apparently  undergo  a 
descending,  not  an  ascending,  degeneration,  and  do  not,  therefore,  really 
belong  to  the  tract ;  they  may  be  fibres  which  have  strayed  from  the  pyram- 
idal tract. 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  557 

We  have  as  yet  no  very  clear  evidence  as  to  the  origin  of  the  fibres  which 
compose  the  tract.  Unlike  the  case  of  the  median  posterior  tract  of  which 
we  have  next  to  speak,  no  degeneration,  at  least  in  the  lumbar  and  thoracic 
regions,  appears  in  the  tract  after  section  merely  of  the  roots  of  the  nerves ; 
to  produce  the  degeneration  the  cord  itself  must  be  injured.  From  this  we 
may  infer  that  the  tract  is  not  fed  directly  by  the  fibres  of  the  posterior  roots. 
Some  observers  maintain  that  the  tract  is  fed  by  fibres  coming  from  the 
vesicular  cylinder  and  point  out  that  both  the  tract  and  the  column  begin  at 
the  same  level  somewhat  suddenly  ;  but  the  want  of  parallelism  between  the 
course  of  the  tract  and  that  of  the  cylinder  along  the  length  of  the  cord,  the 
latter  being  as  we  said  conspicuous  in  the  thoracic  region  while  the  tract 
steadily  increases  upward,  is  distinctly  opposed  to  such  a  view.  From  the 
fact  that  the  degeneration  taking  place  in  it  is  an  ascending  one,  it  is  sup- 
posed that  the  tract  is  the  channel  for  ascending,  that  is  to  say,  in  a  broad 
sense,  afferent  impulses.  And  considerable  interest  attaches  to  the  fact  that 
these  impulses  should  be  carried,  not  to  the  cerebrum  but  to  the  cerebellum. 
Our  knowledge  on  this  point,  however,  is  very  imperfect,  and  what  can  be 
said  in  the  matter  had  better  be  said  later  on. 

§  490.  The  median  posterior  tract  is  the  other  conspicuous  tract  of  ascend- 
ing degeneration ;  it  also  is  supposed  to  be  a  channel  for  ascending  afferent 
impulses;  and  this  view  is  rendered  almost  certain  by  the  intimate  relations 
of  the  tract  to  the  fibres  of  the  posterior  roots. 

In  dealing  so  far  with  the  tracts  of  degeneration  in  the  spinal  cord  we 
have  always  spoken  of  the  degeneration  as  being  the  result  of  lesions  of  the 
spinal  cord  itself.  Experiments  on  animals,  however,  and  clinical  experience 
have  shown  that  division  or  injury  of  the  fibres  of  the  posterior  roots  is  fol- 
lowed by  tracts  of  degeneration  in  the  spinal  cord,  though  no  damage  what- 
ever may  have  been  done  to  the  substance  of  the  cord  itself.  These  tracts 
make  their  appearance  in  the  median  posterior  columns,  the  exact  path  and 
limits  of  the  degeneration  differing  with  the  different  spinal  nerves.  The 
results  of  the  division  of  different  groups  of  nerves  are  so  instructive  that  we 
may  dwell  upon  them  in  detail. 

If  the  posterior  roots  of  two  or  three  lumbar  nerves  (on  one  side)  be 
divided,  an  examination  of  the  cord,  after  an  interval  long  enough  to  allow 
degeneration  to  be  well  established,  will  bring  to  light  the  following  features  : 
The  divided  roots  will  be  found  to  have  degenerated  right  up  to  their  entrance 
into  the  cord.  A  section  of  the  cord  opposite  the  entrance  of  the  lowest 
divided  root  will  show  no  degeneration  of  the  cord  beyond  that  of  the  bundles 
of  fibres  passing  in.  A  little  higher  up  degeneration  will  be  observed  in  the 
external  posterior  column  close  to  the  posterior  horn ;  and  as  we  ascend  we 
find  that  this  degeneration  first  spreads  over  a  large  portion  of  the  external 
posterior  column,  and  then  invades  the  median  posterior  column ;  the  de- 
generation does  not  affect  the  whole  of  the  median  posterior  column  but 
leaves  intact  a  small  dorsal  portion,  roughly  triangular  in  shape,  at  the  angle 
between  the  fissure  and  the  dorsal  surface  of  the  cord,  as  well  as  some  portion 
of  the  more  ventral  part  of  the  column  nearest  the  gray  commissure.  Still 
a  little  higher  up  we  should  find  that  degenerated  fibres  had  disappeared 
from  the  external  portion  of  the  external  posterior  column  close  to  the  gray 
matter,  though  still  existing  in  the  more  median  part  of  that  column  as  well 
as  in  the  median  posterior  column  to  the  extent  just  indicated.  Still  a  little 
higher  up  the  whole  of  the  degeneration  would  have  disappeared  from  the 
external  posterior  column,  but  the  tract  of  degeneration  in  the  median  pos- 
terior column  would  remain,  the  extent  of  degeneration  being  dependent  on 
the  number  of  roots  which  had  been  divided.  Lastly,  by  carrying  the  sec- 
tions still  higher  up  the  cord  we  should  be  able  to  trace  this  tract  in  the 


558  THE  SPINAL  COED. 

median  posterior  column  right  up  to  the  bulb,  where  it  would  come  to  an 
end. 

If  we  divided  some  of  the  thoracic  nerves  instead  of  the  lumbar  we  should 
obtain  very  similar  results :  a  degeneration  of  the  external  posterior  columns 
a  little  above  the  entrance  of  the  roots,  spreading  across  the  column  toward 
the  median  line,  and  wholly  disappearing  at  a  certain  height  above,  accom- 
panied by  a  degeneration  of  a  part  of  the  median  posterior  column,  reaching 
from  a  little  distance  above  the  entrance  of  the  divided  nerve-roots  right  up 
to  the  bulb.  This  latter  tract  of  degeneration  would,  however,  not  occupy 
the  same  position  as  that  consequent  upon  division  of  the  lumbar  nerves ;  its 
position  would  be  more  ventral,  nearer  the  gray  commissure,  and  rather  more 
lateral.  Compare  Fig.  127,  Z)2,  where  Ir.  indicates  the  degeneration  due  to 
section  of  the  lumbar  nerves,  and  dr.  that  of  the  thoracic  nerves.  If  we 
divided  some  of  the  cervical  posterior  roots  we  should  get  similar  results, 
with  the  difference  that  the  tract  of  degeneration  in  the  median  posterior 
columns  would  occupy  a  position  still  more  ventral  and  still  more  lateral 
(Fig.  127,  C5  c.  r.),  while  if  we  divided  the  sacral  nerves  the  tract  of  degen- 
eration would  be  dorsal  and  median  to  the  tract  belonging  to  the  lumbar 
nerves,  and  would  occupy  more  or  less  of  the  triangle  left  below  that  tract 
(Fig.  127,  D2  s.-r.).  The  degeneration,  it  will  be  understood,  is  in  all  cases 
confined  to  the  same  side  of  the  cord  as  that  of  the  divided  roots.  We  may 
add,  in  order  to  complete  the  story  of  the  effects  of  division  of  the  posterior 
roots,  that  the  section  leads  to  degeneration  of  the  marginal  zone  (Lissauer's 
tract),  but  this  degeneration  reaches  for  a  certain  distance  only  up  the  cord 
and  then  disappears.  It  will  be  remembered  that  this  zone  is  fed  by  fibres 
(of  fine  calibre)  belonging  to  the  external  or  lateral  bundle  of  the  posterior 
roots. 

These  results  may  be  interpreted  as  follows  :  The  (great  majority  of  the) 
fibres  of  the  posterior  root,  cut  off  from  their  ganglion  by  the  division, 
degenerate  centripetally  toward  the  spinal  cord.  We  have  previously  seen 
that  many  of  the  fibres  of  the  root  pass  into  the  external  posterior  column 
and  run  up  in  that  column  for  some  distance.  The  degeneration  observed  in 
this  column  for  some  distance  above  the  entrance  of  the  divided  roots  shows 
that  the  fibres  run  lengthways  for  some  distance  is  this  column,  while  the 
disappearance  of  the  degeneration  a  little  higher  up  similarly  shows  that  the 
fibres  eventually  leave  the  column.  The  appearance  of  degeneration  in  the 
median  posterior  column  shows  that  some  of  these  fibres  have  passed  into 
that  column  from  the  external  posterior  column,  and  the  continuation  of  that 
degeneration  right  up  to  the  bulb  indicates  that  these  fibres  pursue  an 
unbroken  course  in  that  column  along  the  whole  length  of  the  cord.  The 
area  of  degeneration,  or  more  exactly  the  number  of  degenerated  fibres  in 
the  continued  tract  of  degeneration  in  the  median  posterior  column  is  much 
less  than  that  in  the  temporary  or  short  tract  of  degeneration  in  the  ex- 
ternal posterior  column.  This  shows  that  some  only  of  the  fibres  passing 
into  the  external  posterior  column  go  on  to  join  the  median  posterior  column 
and  so  reach  the  bulb ;  the  rest  obviously  take  another  path,  and  we  have 
already  seen  reason  to  think  that  many  of  these  end  in  the  gray  matter  of 
the  cord.  Hence  of  all  the  fibres  joining  the  cord  in  a  posterior  root,  while 
some,  and  these  we  may  add  are  chiefly  fine  fibres,  entering  the  gray  matter 
directly  or  passing  into  the  posterior  marginal  zone,  soon  make  such  con- 
nections that  the  degeneration  due  to  the  section  of  the  roots  spreads  no 
further,  a  large  number,  and  these  chiefly  coarse  fibres,  before  they  make  any 
such  connection  pass  into  and  occupy  for  some  length  of  the  cord  the  external 
posterior  column.  We  may  here  remark  that  though  these  fibres  are  spread 
over  the  greater  part  of  this  column,  they  do  not  form  the  whole  of  the 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  559 

column ;  they  are  mixed  up  with  fibres  of  a  different  nature  and  origin.  Of 
these  fibres  of  the  posterior  root  which  thus  run  in  the  external  posterior 
column  while  still  dependent  for  their  nutritive  activity  on  the  ganglion  of 
the  root,  some,  indeed  the  greater  part,  leave  the  tract  and  make  such  con- 
nections in  the  gray  matter  that  their  degeneration  ceases ;  others,  forming 
the  smaller  part,  pass  into  the  median  posterior  column,  and  taking  up  a 
definite  position  in  that  column  pursue  an  unbroken  course  to  the  bulb. 

All  the  fibres,  therefore,  of  the  posterior  roots  do  not  end  in  the  gray 
matter  soon  after  their  entrance  into  the  cord.  A  representative  of  each  root 
is  carried  right  up  to  the  bulb  by  means  of  the  median  posterior  column  ;  of 
the  axis-cylinders  which  leave  the  ganglion  on  the  root,  a  certain  relatively 
small  number  pursue  an  unbroken  course  for  some  little  distance  through 
the  external  posterior  column,  and  for  the  rest  of  their  way  through  the 
median  posterior  column,  along  the  whole  length  of  the  cord  above  the 
entrance  of  the  root  until  they  find  an  ending  in  the  gray  matter  of  the  bulb. 
Further,  each  spinal  nerve  has  this  representative  of  its  posterior  root  placed 
in  a  definite  position  in  the  posterior  median  column,  the  arrangement  being 
such,  as  shown  in  Fig.  127,  that  the  lower  (sacral)  nerves  find  their  place  in 
the 'more  dorsal  and  median  part  of  the  column,  while  the  nerves  above  are 
successively  placed  in  positions  more  arid  more  ventral  and  external. 

As  far  as  our  knowledge  goes  at  present  we  are  led  to  believe  that  this 
median  posterior  tract  is  very  largely  made  up  of  fibres  having  this  origin. 
It  affords  a  channel  by  which  afferent  impulses  are  carried  straight  up  the 
cord  from  the  nerve-trunk  without  making  connections  on  the  way.  We 
may  repeat  that  the  path  is  confined  to  the  same  side  of  the  cord  along  its 
whole  length  ;  there  is  no  crossing  over  to  the  other  side. 

In  the  above  description  we  have  spoken  only  of  the  result  following  sec- 
tion of  the  posterior  roots  outside  the  cord ;  but  it  will  be  understood  that 
similar  results  follow  upon  section  or  of  injury  to  or  disease  of  the  cord  itself 
affecting  the  posterior  columns  or  the  bundles  of  the  roots  as  they  enter  the 
cord.  When  such  a  lesion  occurs  there  may  be  observed  in  the  region  of  the 
cord  above  the  lesion  a  degeneration  of  the  external  posterior  column,  reach- 
ing some  little  distance  up,  and  a  more  limited  degeneration  of  a  part  of  the 
median  posterior  column  stretching  right  up  to  the  bulb.  The  position  and 
form  of  the  tract  of  the  degeneration  in  the  median  posterior  column  will 
depend  on  the  level  of  the  lesion  along  the  length  of  the  cord,  according  as 
it  interrupts  the  ascending  representatives  of  the  sacral  nerves  only,  or  of 
the  lumbar  and  sacral  nerves,  or  of  the  dorsal  and  cervical  nerves  as  well. 
A  complete  section  or  hemi-section  of  the  cord  will  produce  results  corre- 
sponding to  the  division  on  both  sides  or  on  one  side  of  all  the  nerves  below 
the  section. 

We  may  add  that  while,  according  to  some  observers,  the  strand  of  fibres 
belonging  to  a  particular  root  or  group  of  roots  having  once  taken  up  its 
position  in  the  median  posterior  column  remains  unchanged  until  it  reaches 
the  bulb,  according  to  others  it  diminishes  in  area,  some  of  its  fibres  making 
connections  in  the  cord  itself. 

§  491.  The  antero-lateral  ascending  tract  (Fig.  127,  asc.  a.  /.)  is  less  well 
known  than  either  of  the  two  preceding ;  it  is  also  more  diffuse,  that  is  to 
say,  the  fibres  undergoing  degeneration  are  more  largely  mixed  with  fibres 
of  a  different  nature  and  origin.  It  appears  to  extend  down  the  cord  to  a 
lower  level  than  the  cerebellar  tract,  but  its  lower  limit  has  not  yet  been 
accurately  determined.  Since  the  degeneration  taking  place  in  it  is  an 
ascending  one,  it  has  been  inferred  that  it  serves  as  the  path  for  afferent 
and  indeed  for  sensory  impulses.  Degeneration  in  it  is  seen  only  after 
section  or  injury  of  the  substance  of  the  cord  itself,  not  after  division  of  the 


560  THE  SPINAL  COED. 

posterior  roots.  If,  then,  it  is  to  be  regarded  as  a  channel  of  afferent  im- 
pulses passing  into  it  from  the  posterior  roots,  those  impulses  must  pass  into 
it  along  those  fibres  of  the  posterior  root  which  find  secondary  trophic  cen- 
tres in  some  part  of  the  gray  matter ;  in  this  respect  this  tract  resembles  the 
cerebellar  tract,  and  differs  from  the  median  posterior  tract.  The  latter 
is  the  direct  continuation  up  the  cord  to  the  bulb  of  such  fibres  as  are  still 
trusting  for  their  nutritive  activity  to  the  cells  of  the  ganglion  on  the  pos- 
terior root ;  the  fibres  of  both  the  former  trust  for  their  nutritive  activity  to 
some  part  of  the  gray  matter  of  the  cord,  and  presumably  to  the  nerve- 
cells  of  that  gray  matter.  A  further  resemblance  between  the  antero-lateral 
ascending  and  cerebellar  tracts  must  be  admitted,  if  future  researches  con- 
firm the  opinion  of  those  who  hold  that  the  former  like  the  latter,  at  the  top 
of  the  cord,  pass  along  the  restiform  body  to  the  cerebellum.  Indeed,  under 
such  a  view  it  would  appear  probable  that  the  antero-lateral  tract  is  simply 
a  more  diffuse  and  outlying  part  of  the  cerebellar  tract. 

§  492.  We  may  now  briefly  pass  in  review,  somewhat  as  follows,  the  chief 
facts  which  we  have  learned  concerning  the  structure  of  the  spinal  cord, 
always  keeping  in  view  their  physiological  meaning. 

The  important  feature  of  the  spinal  cord  is  the  presence  of  what  we  have 
called  "  gray  matter,"  and  all  our  knowledge  goes  to  show  that  the  important 
powers  of  the  spinal  cord,  by  which  it  differs  from  a  thick  multiple  nerve, 
and  by  virtue  of  which  we  speak  of  it  as  a  nervous  centre  or  series  of  centres, 
are  in  some  way  or  other  associated  with  this  gray  matter. 

With  this  gray  matter  the  fibres  of  the  spinal  nerves  are  connected.  The 
greater  part  of  the  fibres  of  the  anterior  root  certainly  end  in  or  rather  take 
origin  from  the  gray  matter  close  to  the  attachment  of  the  root,  and  the  rest 
most  probably  join  the  gray  matter  at  no  great  distance.  The  fibres  of  the 
posterior  root  run,  as  we  have  seen,  for  some  little  distance  in  the  white 
matter,  but  if  we  except  the  special  bundle  which  runs  in  the  median  pos- 
terior tract  right  up  the  cord  to  the  bulb, without  joining  the  spinal  gray 
matter  at  all,  we  may  say  that  the  fibres  of  the  posterior  root  also  join  the 
gray  matter  not  far  from  the  attachment  of  the  root. 

Morphological  reasons  lead  us,  as  we  have  seen,  to  regard  the  spinal  cord 
as  a  series  of  segments,  each  segment  corresponding  to  a  pair  of  nerves  ; 
and  even  in  the  spinal  cord  of  man  we  may  recognize  a  segmental  ground- 
work, obscured  though  this  is  by  fusion  and  overlaid  by  the  several  commis- 
sural  tracts.  Each  segment  of  this  groundwork  we  may  conceive  of  as  a 
central  mass  of  gray  matter,  connected  on  each  side  with  an  anterior  and  a 
posterior  root,  thus  constituting  a  segmental  nervous  mechanism  capable  of 
carrying  out  certain  functions. 

Such  a  segment  has  been  compared  to  a  ganglion,  but  it  differs  strikingly 
from  a  ganglion,  whether  of  the  posterior  root  or  of  the  splanchnic  system, 
both  in  structure  and  in  function.  A  ganglion  and  the  gray  matter  of  a 
spinal  segment  both  contain  nerve-cells,  and  so  far  resemble  each  other ;  but 
there  the  resemblance  for  the  most  part  ends.  In  a  ganglion  the  constituent 
nerve-cell  is  a  development  of  the  axis-cylinder  of  a  fibre  into  a  nucleated 
cell-body  which  lies  on  the  course  of  the  fibre,  and  may,  as  in  a  splanchnic 
ganglion,  be  placed  just  where  one  fibre  divides  into  two  or  more.  We  have 
clear  evidence  that  the  cell,  that  is  to  say,  the  nucleus  with  the  adjacent  cell 
substance,  exercises  an  important  influence  on  the  nutrition,  and  so  on  the 
functional  activity  of  the  nerve-fibre,  it  acts,  as  we  have  seen,  as  a  "trophic 
centre."  There  are  also  reasons  for  thinking  that  the  cell  substance  is  more 
sensitive,  more  readily  responsive  to  changes  in  its  circumstances  than  is  the 
axis-cylinder  at  some  distance  from  the  cell.  But  we  have  no  satisfactory 
evidence  that  the  cell  can  automatically  originate  nervous  impulses  in  itself 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  561 

as  the  outcome  of  its  own  intrinsic  changes.  Nor  have  \ve  any  evidence 
that  the  cell  can  exert  any  marked  transforming  power  over  the  impulses 
passing  along  the  fibre ;  the  impulses  which  travel  away  from  the  cell  do  not 
appear  to  differ  markedly  from  those  which  travel  toward  it.  The  several 
instances  in  which  there  seemed  to  be  evidence  that  splanchnic  ganglia  acted 
as  centres  either  of  reflex  or  of  automatic  action,  have,  as  we  have  seen, 
broken  down  ;  and  it  is  not  even  suggested  that  the  ganglia  of  the  posterior 
roots  possess  any  such  powers.  The  gray  matter  of  the  spinal  cord,  on  the 
other  hand,  as  we  have  already  seen,  and  as  we  shall  see  more  in  detail,  is 
especially  characterized  by  the  possession  of  reflex  and  automatic  as  well  as 
of  other  powers. 

In  structure,  moreover,  such  a  spinal  segment  differs  strikingly  from  a 
ganglion  and  exhibits  features  unknown  in  ganglia.  In  a  ganglion  the 
nerve-fibres  may  divide,  and  in  a  small  peripheral  ganglion  the  division  may 
give  rise  to  very  delicate  fibrils ;  but  the  fibres  or  fibrils  resulting  from  the 
division  leave  the  ganglion  to  follow  their  appropriate  courses ;  the  division 
serves  for  dispersion  only.  In  the  spinal  cord,  on  the  other  hand,  both 
efferent  and  afferent  fibres  divide  in  such  a  way  that  their  divisions  are  lost 
to  view  in  the  gray  matter ;  division  here  seems  to  serve  the  purpose  of  union. 
The  efferent  fibres  of  the  anterior  root  may  be  traced  back  as  a  process  of  a 
cell  in  the  anterior  horn.  That  cell  gives  off  other  processes,  but  no  one  of 
these  processes  is  continued  as  an  axis-cylinder  process  stretching  across  the 
gray  matter  until  it  becomes  a  fibre  of  the  posterior  root,  or  as  anything  like 
such  an  axis-cylinder  process.  On  the  contrary,  all  the  processes,  except 
the  axis-cylinder  process,  divide  into  branches,  and  appear  to  end  in  ner- 
vous fibrils  lost  to  view  in  the  gray  matter.  Conversely,  though  our  know- 
ledge of  the  junction  of  the  posterior  fibres  with  the  gray  matter  is  much  more 
imperfect  than  that  of  the  junction  of  the  anterior  fibres,  what  we  do  know 
leads  us  to  believe  that  the  fibres  of  the  posterior  root,  either  by  the  media- 
tion of  cells,  or  by  direct  division  of  the  axis-cylinder  without  the  media- 
tion of  cells,  similarly  break  up  into  fibrils  and  are  similarly  lost  in  the 
gray  matter.  All  the  evidence  goes  to  show  that  the  anterior  and  posterior 
roots  are  functionally  continuous ;  this  functional  continuity  is,  however, 
effected  not  by  a  gross  continuity  of  axis-cylinders,  but  in  a  peculiar  man- 
ner through  the  division  of  branches  of  nerve-cells  or  of  axis-cylinders  into 
the  nervous  tangle  which  forms  such  a  special  feature  of  the  gray  matter  of 
the  cord.  We  may,  perhaps,  venture  to  regard  the  gray  matter  of  the  seg- 
mental  groundwork,  of  which  we  are  now  alone  speaking,  as  constituting  a 
nervous  network  or  web,  formed  certainly  in  part  by  the  rapidly  dividing 
branches  of  nerve-cells,  and  probably  in  part  by  the  divisions  of  directly 
dividing  nerve-fibres. 

In  any  ordinary  section  of  the  spinal  cord  the  gray  matter  presents  to 
view  much  more  than  this  nervous  groundwork.  To  say  nothing  of  the  in- 
dubitable neuroglia  and  the  obscure  structures,  including  small  cells,  which 
are  claimed  now  to  be  neuroglia,  now  to  be  nervous  in  nature,  the  gray 
matter  in  every  section  shows  numerous  distinct  nerve-fibres  crossing  it  in 
various  directions  ;  of  these  fibres  a  few  are  ordinary  medullated  fibres,  some 
are  non-medullated  fibres,  that  is  to  say,  are  naked  axis-cylinders,  while 
others,  and  these  the  more  numerous,  are  the  peculiar  medullated  fibres  of 
small  diameter  spoken  of  in  §  476.  A  large  number  of  these  fibres,  indeed 
all  the  larger  ones,  though  they  go  to  make  up  what  we  call  gray  matter, 
are  not  continuous  with,  and  do  not  belong  to,  the  groundwork  or  nervous 
web,  at  all  events  do  not  form  part  of  the  groundwork  seen  in  the  same 
section  as  themselves.  They  are  simply  fibres  traversing  the  groundwork,  in 
spaces  of  the  neuroglia  bed,  on  their  way  up  or  down  the  cord,  or  across  the 

36 


562  THE  SPINAL  CORD. 

cord  from  one  part  to  another.  It  may  be  that  some  of  the  finer  medullated 
fibres  do  really  enter  into  the  groundwork,  and  so  contribute  to  the  nervous 
web ;  but  our  knowledge  is  too  imperfect  to  afford  a  clear  decision  on  this 
point.  Our  inability  to  define  its  exact  limits  need  not,  however,  prevent 
our  recognizing  the  existence  of  the  groundwork. 

The  prominence  in  this  groundwork  of  the  larger  nerve-cells  has  led  to 
the  conception  that  the  powers  of  the  spinal  segment  are  exercised  by  these 
nerve-cells  to  the  exclusion  of  the  other  elements  of  the  nervous  web.  But 
such  a  view  has  not  been  adequately  proved.  What  we  do  know  is  that  the 
nuclei  and  cell-bodies  of  the  cells  of  the  anterior  horn  exercise  an  important 
influence  on  the  nutrition  of  the  fibres  of  the  anterior  root  which  proceed 
from  them,  and  possibly  also  influence  the  nutrition  of  the  other  branches 
of  the  cells  forming  part  of  the  groundwork  ;  and  these  cells  are  probably 
so  conspicuous  a  feature  of  every  section  of  the  spinal  cord  because  of  the 
important  task  intrusted  to  them  of  maintaining  in  due  order  the  nutrition 
of  the  long  stretch  of  motor  fibres  reaching  from  them  to  the  muscular 
fibres  or  other  peripheral  organs.  The  fibres  of  the  posterior  root  are  not  so 
obviously  connected  with  the  conspicuous  cells  of  the  gray  matter ;  indeed, 
as  we  have  said,  it  may  be  doubted,  though  the  view  is  maintained  by  some, 
whether  any  cell  intervenes  to  secure  the  continuity  of  a  posterior  fibre  with 
the  groundwork,  a  division  of  the  axis-cylinder  serving  this  purpose  ;  and 
this  becomes  intelligible  when  we  bear  in  mind  that  the  posterior  fibres  are 
governed  as  far  as  their  nutrition  is  concerned  by  the  nerve-cells  of  the  gan- 
glion on  the  posterior  root,  which  ought  probably  to  be  considered  as  much 
a  part  of  the  spinal  cord  as  the  cells  of  the  anterior  horn.  The  nerve-cell 
of  the  ganglion  is  adequate  to  secure  the  due  nutrition  of  the  nerve-fibre 
until  it  joins  the  groundwork,  and  probably  helps  to  maintain  the  nutrition 
of  the  groundwork  itself. 

Hence  we  may  perhaps,  until  fresh  evidence  to  the  contrary  is  brought 
forward,  incline  to  the  view  that  the  powers  of  the  gray  matter  do  not 
depend  on  the  conspicuous  cells  alone  or  even  chiefly,  but  on  the  peculiar 
molecular  constitution  and  nature  of  the  whole  groundwork.  The  nuclei  of 
the  cells  of  the  anterior  horn  with  the  cell  substance  adjacent  to  each  and 
the  cells  of  the  ganglia  on  the  posterior  root  probably  govern  the  nutrition, 
and  so  the  functional  activity  of  the  groundwork  as  well  as  of  the  issuing 
and  entering  fibres ;  but  there  appears  to  be  as  yet  no  convincing  evidence 
of  any  other  peculiar  powers  confined  to  the  cells  and  absent  from  other 
parts  of  the  groundwork.  We  may  add  that,  in  accordance  with  this  view7, 
the  other  cells  of  the  gray  matter,  such  as  those  of  the  vesicular  cylinder, 
are  to  be  regarded  as  of  importance  for  governing  the  nutrition  of  fibres, 
commissural  and  others,  starting  from  the  spinal  segment,  and  of  the  part 
of  the  groundwork  from  which  by  their  mediation  the  fibres  start,  rather 
than  for  determining  the  functions  of  the  groundwork  of  the  segment  or  of 
the  fibres  receiving  impulses  from  it. 

§  493.  The  segmental  groundwork  of  gray  matter  belonging  to  each  pair 
of  spinal  nerves  is  so  fused  with  that  of  all  the  other  pairs  as  to  form  along 
the  whole  length  of  the  cord  a  mass  of  gray  matter  which  appears,  under 
certain  circumstances  at  all  events,  to  be  continuous  in  the  sense  that  impulses 
may  pass  in  all  directions  along  it.  But  each  spinal  segment  is  in  addition 
connected  by  means  of  tracts  of  white  matter  with  parts  more  or  less  distant. 
The  crossed  pyramidal  tract  is  such  a  longitudinal  commissural  tract,  con- 
necting apparently  each  spinal  segment  in  succession  with  a  certain  part  of 
the  cortex  of  the  cerebrum.  We  have  reason  to  think,  as  we  shall  see  later 
on,  that  impulses  descending  this  or  that  fibre  or  group  of  fibres  of  this  tract 
give  rise  to  the  issue  of  motor  impulses  along  this  or  that  fibre  or  group  of 


THE  STRUCTURE  OF  THE  SPINAL  CORD.  563 

fibres  of  an  anterior  root.  We  do  not  at  present  know  what  is  the  exact 
manner  by  which  the  fibre  in  the  pyramidal  tract  is  connected  with  the  fibre 
of  the  anterior  root.  It  seems  certain,  however,  that  the  connection  is  not 
in  the  form  of  a  fibre  isolated  from  the  rest  of  the  gray  matter,  containing, 
so  to  speak,  the  pyramidal  fibre  into  a  cell  of  the  anterior  horn  whence  the 
fibre  of  the  anterior  root  issues.  Most  probably  the  pyramidal  fibre  makes 
connections  with  the  segmental  groundwork  spoken  of  above,  whether  with 
or  without  the  intervention  of  a  cell  we  cannot  at  present  tell.  The  direct 
pyramidal  tract  is  a  like  tract  of  less  extent  downward,  and  the  less  known 
antero-lateral  descending  tract  is  probably  of  a  similar  nature. 

The  cerebellar  and  antero-lateral  ascending  tracts  are  in  like  manner  to 
be  regarded  as  longitudinal  commissures  between  the  successive  spinal  seg- 
ment below  and  some  part  of  the  brain  above.  We  have  reason  to  think 
that  these  tracts  convey  upward  impulses  of  a  nature  which  may  be  called 
afferent,  and  are,  therefore,  in  some  way  probably  connected  with  the  pos- 
terior roots.  We  do  not  know  as  yet  the  exact  nature  of  the  connection ; 
but  probably  in  those  cases  also  the  commissural  fibres  are  united  not  directly 
to  the  posterior  fibres,  but  indirectly  by  means  of  the  segmental  groundwork. 
And  since  these  tracts  do  not  degenerate  after  section  of  the  posterior  roots, 
but  only  after  section  or  other  lesion  of  the  cord  itself,  we  may  infer  that 
their  junction  with  the  groundwork  is  effected  by  means  of  trophic  cells,  by 
means  of  some  or  other  of  the  cells  spoken  of  a  little  while  before. 

The  median  posterior  tract  seems  to  be  a  commissural  tract  of  a  nature 
different  from  any  of  the  above.  Through  it  a  certain  part  of  each  posterior 
root  is  brought  into  connection,  not  with  its  own  spinal  segment,  but  with  the 
bulb  above,  and  so  with  the  brain,  which  thus  receives  direct  representatives 
of  each  afferent  spinal  nerve.  If,  however,  as  some  maintain,  the  bundle  in 
this  tract  starting  from  a  spinal  nerve  below  diminishes  as  it  proceeds  upward, 
throwing  off  fibres  to  pass  elsewhere,  though  always  carrying  some  fibres 
right  up  to  the  bulb,  we  must  add  to  the  above  the  further  view  that  this 
tract  connects  also  each  posterior  root,  not  with  its  own  segment,  but  with 
other  more  or  less  distant  segments. 

§  494.  All  the  evidence  which  we  possess  goes  to  show  that  each  strand  of 
each  of  these  tracts  runs  isolated,  that  is  to  say,  makes  no  connections  with 
adjoining  structures  at  any  part  of  its  course,  from  its  beginning  or  end  in  the 
brain  and  its  end  or  beginning  in  its  appropriate  spinal  segment,  or  in  the 
case  of  the  median  posterior  tract  from  its  beginning  in  the  ganglion  of  a 
posterior  root  and  its  end  in  the  bulb  or  in  some  distant  spinal  segment.  In 
the  crossed  pyramidal  tract,  for  instance,  we  have  reason  to  think  that  one 
or  more  fibres  run  a  quite  unbroken  and  isolated  course  from  the  cortex 
of  the  cerebrum  through  various  parts  of  the  brain,  along  the  whole  length 
of  the  cord  until  they  reach  the  lowermost  spinal  segmental  mechanism. 
These  tracts  serve  in  no  way  to  connect  one  segmental  mechanism  with 
another.  The  segmental  mechanisms  are,  however,  connected  together ;  and 
the  connections  between  them  seem  to  be  of  two  kinds.  In  the  first  place, 
as  we  have  already  suggested,  the  segmental  pieces  of  gray  matter  are  so 
fused  together  as  to  form  what  appears  to  be  a  continuity  of  gray  matter  from 
one  end  of  the  cord  to  the  other.  Though  we  cannot  actually  track  our 
way  histologically  through,  and  are  still  less  aware  of  the  physiological  na- 
ture of  the  labyrinth  of  nerve-cells,  fibres,  and  fibrils  which  make  up  what 
we  have  called  the  groundwork,  we  may  with  considerable  probability  assume 
that  the  passage  of  nervous  impulses  along  it  is  determined  as  much  by  the 
condition  of  the  material  as  by  its  anatomical  disposition  ;  that,  for  instance, 
the  restrictions  to  the  flow  of  an  impulse  are  brought  about  much  more  fre- 
quently by  the  refusal  of  the  molecules  of  nervous  matter  to  take  up  the 


564  THE  SPINAL  COED. 

molecular  disturbance  which  is  the  essence  of  the  impulse ;  that  is  to  say,  by 
molecular  resistance  than  by  actual  breaks  of  continuity  in  the  nervous 
matter.  Indeed,  we  have  some  reasons  for  thinking  that  actual  structural 
continuity  of  nervous  material  is  not  essential  to  functional  continuity ;  that 
a  nerve-fibril,  for  instance,  may  produce  its  due  effect  on  another  nerve-fibril 
or  on  a  nerve-cell,  if  sufficiently  in  contact  with  it,  though  the  microscope 
fails  to  demonstrate  actual  continuity. 

But  besides  the  gray  matter  there  are  areas  of  white  matter  which  do  not 
belong  either  to  the  nerve-roots  as  these  are  making  their  way  into  the  gray 
matter,  or  to  any  of  the  tracts  which  we  have  mentioned.  These  compris'e 
the  strands  of  fibres  which  do  not  undergo  either  ascending  or  descending 
degeneration  when  parts  of  the  spinal  cord  are  injured  or  diseased.  The 
area  of  white  matter  left  when  all  the  various  tracts  of  ascending  and  de- 
scending degeneration  detailed  above  are  taken  out,  seems  at  all  events  in  the 
higher  parts  of  the  cord  (Fig.  127),  relatively  small,  and  future  observations 
may  continue  to  still  further  reduce  it ;  but  it  must  be  remembered  that  none 
of  the  above  mentioned  tracts  are  "  pure; "  they  are  all  more  or  less  mixed 
up,  and  some  largely  mixed  up,  with  fibres  which  do  not  degenerate.  Our 
knowledge  is  at  present  too  scanty  to  allow  us  to  make  any  statement  with 
confidence  concerning  the  function  either  of  the  fibres  forming  the  white 
matter  not  yet  marked  out  into  tracts,  or  of  the  fibres  scattered  among  the 
acknowledged  tracts.  But  we  may,  at  all  events  provisionally,  assume  that 
these  fibres  serve  in  the  main  as  commissures  connecting  the  successive  seg- 
mental  mechanisms  with  each  other ;  we  may  conclude  that  changes  taking 
place  in  one  segmental  mechanism  can  by  means  of  these  fibres  produce 
correlated  changes  in  some  other  distant  segmental  mechanism,  without 
calling  into  action  any  of  the  gray  matter  of  the  intervening  segmental 
mechanisms. 

The  commissures  which  we  may  suppose  to  be  thus  furnished  by  white 
matter  are  longitudinal  commissures  connecting  the  segmental  mechanisms 
of  the  same  lateral  half  of  the  spinal  cord  with  each  other.  A  transverse 
connection  between  the  two  lateral  halves  is  afforded  in  some  measure  by  the 
anterior  white  commissure.  We  shall  see,  however,  later  on,  reasons  for 
thinking  that  many  impulses  besides  those  passing  along  the  anterior  com- 
missure cross  from  one  side  of  the  cord  to  the  other  ;  and  these,  whether  they 
pass  along  distinct  fibres  or  along  the  general  groundwork,  must  travel  by 
the  gray  matter  of  the  isthmus  forming  the  anterior  and  posterior  gray  com- 
missures. 

Thus,  as  far  as  we  can  see  at  present,  the  spinal  cord  consists  of  a  series 
of  segmental  mechanisms  with  their  respective  afferent  and  efferent  roots  (the 
gray  matter  of  the  several  segments  being  continuous  along  the  cord),  of 
encephalic  ties  of  white  matter  between  the  several  segments  and  the  brain, 
of  longitudinal  commissural  tracts  connecting  together  the  several  segmental 
mechanisms,  and  of  transverse  commissures  running  largely  in  the  gray 
matter. 

THE  REFLEX  ACTIONS  OF  THE  SPINAL  CORD. 

§  495.  In  the  preceding  portions  of  this  work  we  have  repeatedly  seen 
that  though  we  can  learn  much  concerning  the  working  of  an  organ  or  tissue 
or  part  of  the  body  by  studying  its  behavior  when  isolated  from  the  rest  of 
the  body,  all  the  conclusions  thus  gained  have  to  be  checked  by  a  study  of 
the  behavior  of  the  same  organ  or  part  while  it  is  still  an  integral  part  of  the 
intact  body.  All  the  several  organs  and  tissues  are  so  bound  together  by 
various  ties  that  the  actions  of  each  depend  on  the  actions  of  the  rest ;  and  to 


THE  REFLEX  ACTIONS  OF  THE  SPINAL  CORD.  565 

say  that  the  life  of  each  part  is  a  function  of  the  life  of  the  whole,  is  no  less 
true  than  to  say  that  the  life  of  the  whole  is  a  function  of  the  life  of  each 
part.  This  is  especially  borne  in  upon  us  when  we  come  to  study  the  actions 
of  the  central  nervous  system.  We  may,  on  anatomical  grounds,  separate 
the  spinal  cord  from  the  brain  ;  but  when  we  come  to  consider  the  respective 
functions  of  the  two,  we  are  brought  face  to  face  with  the  fact  that  in  actual 
life  a  large  part  of  the  work  of  the  brain  is  carried  out  by  means  of  the 
spinal  cord,  and  conversely  the  spinal  cord  does  its  work  habitually  under 
the  influence  of,  if  not  at  the  direct  bidding  of,  the  brain.  We  may  gain 
certain  conclusions  by  studying  the  behavior  of  the  spinal  cord  isolated  from 
the  brain,  or  of  parts  of  the  spinal  cord  isolated  from  each  other ;  but  we 
must  be  even  more  cautious  than  when  we  were  dealing  with  other  parts  of 
the  body,  and  must  greatly  hesitate  to  take  it  for  granted  that  the  work 
which  we  can  make  the  spinal  cord  or  a  part  of  the  spinal  cord  do,  when 
isolated  from  the  brain,  is  the  work  which  is  actually  done  in  the  intact  body 
when  the  brain  and  spinal  cord  form  an  unbroken  whole.  Moreover,  this 
caution  becomes  increasingly  necessary  when  in  our  studies  we  pass  from  the 
simpler  nervous  system  of  one  animal  to  the  more  complex  nervous  system 
of  another ;  for  it  is  by  the  complexity  of  their  central  nervous  systems, 
more  than  by  anything  else,  that  the  "  highest  "  animals  are  differentiated 
from  those  "  below"  them.  When  we  compare  a  rabbit,  a  dog,  a  monkey, 
and  a  man,  the  differences  in  the  vascular,  digestive,  and  respiratory  systems 
of  the  four,  striking  as  they  may  appear,  sink  into  insignificance  compared 
with  the  differences  exhibited  by  their  respective  central  nervous  systems. 
We  need  caution  when  from  the  results  of  experiments  on  dogs  or  rabbits 
we  draw  conclusions  as  to  the  digestion  or  circulation  of  man,  but  we  need 
far  greater  caution  when  from  the  behavior  of  the  isolated  spinal  cord  of 
one  of  these  animals  we  infer  the  behavior  of  the  intact  spinal  cord  of  man. 

A  further  difficulty  meets  us  when  an  experimental  investigation  entails 
operative  interference  with  the  central  nervous  system.  Removal  or  section 
of,  or  other  injury  to  parts  of  the  brain  or  spinal  cord  is  very  apt  to  give  rise 
in  varying  degree  to  what  is  known  as  "  shock."  The  cutting  or  tearing  or 
other  lesion  of  any  considerable  mass  of  nervous  substance  affects  the  activity, 
not  only  of  the  structures  immediately  injured,  but  of  other,  it  may  be  far 
distant  structures.  The  nature  of  "shock"  is  not  as  yet  thoroughly  under- 
stood, but  may  perhaps,  in  part  at  all  events,  be  explained  by  regarding  the 
lesion  as  a  very  powerful  stimulus,  which,  partly  by  way  of  inhibition  but 
still  more  by  way  of  exhaustion,  depresses  or  suspends  for  a  while  normal 
functions,  and  thus  gives  rise  to  temporary  diminution  or  loss  of  conscious- 
ness, of  volition,  of  reflex  movements,  and  other  nervous  actions.  Thus  a 
section  through  the  spinal  cord,  even  when  made  with  the  sharpest  instru- 
ment and  with  the  utmost  skill,  so  as  to  avoid  all  bruising  as  much  as  possi- 
ble, may  for  a  while  suspend  all  reflex  activity  of  the  cord,  or  indeed  all  the 
obvious  activities  of  the  whole  central  nervous  system.  We  may  add  that 
such  a  "shock"  of  the  central  nervous  system  may  also  be  produced  by 
sudden  lesions  not  bearing  directly  on  the  central  nervous  system,  as,  for 
instance,  by  extensive  injury  to  a  limb. 

Moreover,  in  many  cases  in  which  the  effects  of  experimental  interference 
have  been  watched  for  some  considerable  time,  days,  months,  or  years  after 
the  operation,  it  has  been  observed,  on  the  one  hand,  that  phenomena  which 
are  conspicuous  in  the  early  period  may  eventually  disappear,  and,  on  the 
other  hand,  that  activities  which  are  at  first  absent  may  later  on  make  their 
appearance ;  movements,  for  instance,  which  are  at  first  frequent  after  a  while 
die  away,  and  conversely,  movements  which  at  first  seemed  impossible  are 
later  on  easily  achieved.  We  have  to  distinguish  or  to  attempt  to  distin- 


566  THE  SPINAL  CORD. 

guish  between  the  temporary  and  the  lasting  effects  of  the  operation,  includ- 
ing among  the  former  not  only  those  of  ordinary  "  shock,"  but  others  of 
slower  development  or  longer  duration.  In  many  instances  where  a  part  of 
the  central  nervous  system  is  by  section  or  otherwise  suddenly  separated 
from  the  rest,  the  phenomena  suggest  that  the  separated  part  is  at  first  pro- 
foundly influenced  as  to  its  activities  by  the  withdrawal  of  various  influences 
which  previously  were  being  exerted  upon  it  by  the  rest  of  the  system,  but 
later  on  accommodates  itself  to  the  new  conditions,  and  learns,  so  to  speak, 
to  act  without  the  help  of  those  influences.  And  indeed  it  is  possible  that 
some  of  the  effects  of  even  immediate  "shock  "  may  be  due,  not  as  suggested 
above,  to  the  action  of  an  inhibitory  or  exhausting  stimulus,  but  to  the 
sudden  cessation  of  habitual  influences. 

Still,  in  spite  of  all  these  difficulties,  it  is  possible  not  only  to  ascertain 
the  working  of  an  isolated  portion  of  the  central  nervous  system,  but  even  to 
infer  from  the  results  some  conclusions  as  to  the  share  taken  by  that  portion 
in  the  working  of  the  entire  and  intact  system.  There  can  be  no  doubt,  for 
instance,  that  the  spinal  cord  can,  quite  apart  from  the  brain,  carry  out 
various  reflex  actions,  and,  moreover,  it  does  carry  out  actions  of  this  kind 
when  in  the  intact  organism  it  is  working  in  contact  with  the  brain.  Indeed, 
the  carrying  out  of  various  reflex  actions  seems  to  be  one  of  the  most  impor- 
tant functions  of  the  spinal  cord,  so  much  so  that  though  the  brain  or,  at 
least,  parts  of  the  brain  can  also  and  do  develop  reflex  actions,  the  spinal 
cord  offers  the  best  field  for  the  study  of  these  actions.  We  have  already 
(§  97)  touched  on  the  general  features  of  reflex  actions,  and  elsewhere  have 
incidentally  dwelt  on  particular  instances ;  we  may  therefore  confine  our- 
selves now  to  certain  points  of  special  interest. 

§  496.  Reflex  movements  are  perhaps  best  studied  in  the  frog  and  other 
cold-blooded  animals,  since  in  these  the  actions  of  the  cord  are  less  dependent 
on,  and  hence  less  obscured  by  the  working  of,  the  other  parts  of  the  central 
nervous  system.  They  obtain,  however,  in  the  warm-blooded  mammal  also,  but 
in  these  special  preparations  are  necessary  to  secure  their  full  development. 
In  the  frog  the  shock,  which,  as  we  have  said,  follows  upon  division  of  the 
spinal  cord  and  for  awhile  suspends  reflex  activity,  soon  passes  away ;  within 
a  very  short  time  after  the  bulb,  for  instance,  has  been  divided  the  most  com- 
plicated reflex  movements  can  be  carried  on  by  the  frog's  spinal  cord  when 
the  appropriate  stimuli  are  applied.  With  the  mammal  the  case  is  very 
different.  For  days  even  after  division  of  the  spinal  cord  the  parts  of  the 
body  supplied  by  nerves  springing  from  the  cord  below  the  section  may  ex- 
hibit very  feeble  reactions  only.  In  the  dog,  for  instance,  after  division  of 
the  spinal  cord  in  the  lower  dorsal  region,  the  hind  limbs  hang  flaccid  and 
motionless,  and  pinching  the  hind  foot  evokes  as  a  response  either  slight 
irregular  movements  or  none  at  all.  Indeed,  were  our  observations  limited 
to  this  period  we  might  infer  that  the  reflex  actions  of  the  spinal  cord  in  the 
mammal  were  but  feeble  and  insignificant.  If,  however,  the  animal  be  kept 
alive  for  a  longer  period,  for  Weeks,  or  better  still  for  months,  though  no 
union  or  regeneration  of  the  spinal  cord  takes  place,  reflex  movements  of  a 
powerful,  varied,  and  complex  character  manifest  themselves  in  the  hind 
limbs  and  hinder  parts  of  the  body;  a  very  feeble  stimulus  applied  to  the 
skin  of  these  regions  promptly  gives  rise  to  extensive  and  ^ yet  coordinate 
movements.  Indeed,  the  more  the  matter  is  studied,  the  stronger  is  the  evi- 
dence that  the  reflex  movements  carried  out  by  isolated  portions  of  the 
spinal  cord  of  the  mammal  are  hardly  less  definite,  complete,  and  purpose- 
ful than  those  witnessed  in  the  frog.  It  is  worthy  of  attention,  as  bearing 
out  the  remarks  made  above  on  the  great  differentiation  of  the  central  ner- 
vous system  in  the  higher  animals,  that  the  reflex  phenomena  in  mammals 


THE  REFLEX   ACTIONS  OF  THE  SPINAL  CORD.  567 

vary  very  much  not  only  in  different  species  but  also  in  different  individuals 
and  in  the  same  individual  under  different  circumstances.  Race,  age,  and 
previous  training  seem  to  have  a  marked  effect  in  determining  the  extent 
and  character  of  the  reflex  actions  which  the  spinal  cord  is  capable  of 
carrying  out ;  and  these  seem  also  to  be  largely  influenced  by  passing  cir- 
cumstances, such  as  whether  food  has  been  recently  taken  or  not.  It  has 
been  asserted  that  the  isolated  spinal  cord  of  the  rabbit,  which  has  been 
the  subject  of  so  many  experiments,  is,  as  compared  with  that  of  the  dog 
and  many  other  mammals,  singularly  deficient  in  the  power  of  carrying  out 
complex  reflex  movements. 

In  studying  reflex  actions  in  man  we  are  met  with  the  difficulty  that  we 
never  have  to  deal  with  a  portion  of  the  spinal  cord  separated  from  the  rest 
of  the  central  nervous  system  under  the  favorable  circumstances  of  experi- 
mental investigation.  In  man,  we  must  be  content  to  examine  reflex  actions 
either  while  the  whole  nervous  system  is  intact,  or  when  a  portion  of  the  cord 
has  been  wholly  or  partially  separated  by  some  more  or  less  diffuse  disease 
or  by  some  accident  involving  more  or  less  crushing  of  the  nervous  struc- 
tures. Hence,  the  caution  already  given,  as  to  drawing  inferences  concern- 
ing man  from  the  results  of  experiments  on  animals,  acquires  still  greater 
force. 

§  497.  Confining  ourselves  at  first  to  the  results  of  experiments  on 
animals  we  may  say  that  in  both  cold-blooded  and  warm-blooded  animals 
the  salient  feature  of  ordinary  reflex  actions  is  their  purposeful  character, 
though  every  variety  of  movement  may  be  witnessed,  from  a  simple  spasm 
to  a  most  complex  manoeuvre.  And  in  all  reflex  movements,  both  simple 
and  complex,  we  can  recognize  certain  determining  influences  which  more 
or  less  directly  contribute  to  the  shaping  of  this  purposeful  character. 

Thus  the  features  of  any  movement  taking  place  as  part  of  a  reflex  action 
are  in  part  determined  by  the  characters  of  the  afferent  impulses.  Simple 
nervous  impulses  generated  by  the  direct  stimulation  of  afferent  nerve-fibres 
generally  evoke  as  reflex  movements  merely  irregular  spasms  in  a  few 
muscles ;  whereas  the  more  complicated  differentiated  sensory  impulses  gen- 
erated by  the  application  of  the  stimulus  to  the  skin,  readily  give  rise  to 
large  and  purposeful  movements.  It  is  easier  to  produce  a  complex  reflex 
action  by  a  slight  pressure  on  or  other  stimulation  of  the  skin  than  by  even 
strong  induction-shocks  applied  directly  to  a  nerve-trunk.  If  in  a  brainless 
frog,  the  area  of  skin  supplied  by  one  of  the  dorsal  cutaneous  nerves  be  sepa- 
rated by  section  from  the  rest  of  the  skin  of  the  back,  the  nerve  being  left 
attached  to  the  piece  of  skin  and  carefully  protected  from  injury,  it  will  be 
found  that  slight  stimuli  applied  to  the  surface  of  the  piece  of  skin  easily 
evoke  reflex  actions,  whereas  the  trunk  of  the  nerve  may  be  stimulated  with 
even  strong  currents  without  producing  anything  more  than  irregular  move- 
ments. In  ordinary  mechanical  and  chemical  stimulation  of  the  skin  it  is 
not  a  single  impulse  but  a  series  of  impulses  which  passes  upward  along  the 
sensory  nerve,  the  changes  in  which  may  be  compared  to  the  changes  in  a 
motor  nerve  during  tetanus.  In  every  reflex  action,  in  fact,  the  central 
mechanism  may  be  looked  upon  as  being  thrown  into  activity  through  a 
summation  of  the  afferent  impulses  reaching  it.  Hence  while  a  reflex  action 
is  readily  called  forth  by  even  feeble  induction-shocks  applied  to  the  skin 
if  they  be  repeated  sufficiently  rapidly,  a  solitary  induction-shock  is  inef- 
fectual unless  it  be  strong  enough  to  cause  in  the  skin  or  nerves  changes 
of  an  electrolytic  nature  sufficient  to  give  rise  of  themselves  to  a  series  of 
impulses. 

§  498.  When  a  muscle  is  thrown  into  contraction  in  a  reflex  action,  the 
pitch  of  the  sound  which  it  gives  forth  does  not  vary  with  the  stimulus,  but 


568  THE  SPINAL  CORD. 

is  constant,  being  the  same  as  that  given  forth  by  a  muscle  thrown  into  con- 
traction by  the  will.  From  which  we  infer,  even  bearing  in  mind  the  dis- 
cussion in  §  78  concerning  the  nature  of  the  muscular  sound,  that  in  a  re- 
flex action  the  afferent  impulses  do  not  simply  pass  through  the  centre  in 
the  same  way  that  they  pass  along  afferent  nerves,  but  are  profoundly  modi- 
fied. And  in  accordance  with  this  we  find,  as  we  shall  see,  that  a  reflex 
action  takes  up  an  amount  of  time,  the  greater  part  of  which  is  spent  in  the 
carrying  out  of  the  central  changes,  and  which  though  variable  is  always 
much  longer,  and  may  be  very  much  longer,  than  that  taken  up  by  the  mere 
passage  of  a  nervous  impulse  along  a  corresponding  length  of  nerve-fibre. 
The  term  reflex  action  is  therefore  an  unsuitable  one.  The  afferent  impulse 
is  not  simply  reflected  or  turned  aside  into  an  efferent  channel ;  on  its  arrival 
at  the  centre,  it  starts  changes  of  a  different  nature  from  and  more  complex 
than  its  own ;  and  the  issue  of  efferent  impulse  is  the  result  of  those  more 
complex  changes,  not  the  mere  continuation  of  the  simpler  afferent  impulse. 
In  other  words,  the  interval  between  the  advent  at  the  central  organ  of 
afferent,  and  the  exit  from  it  of  efferent  impulses,  is  a  busy  time  for  the  ner- 
vous substance  of  that  organ  ;  during  it  many  processes,  of  which  we  have 
at  present  very  little  exact  knowledge,  are  being  carried  on. 

§  499.  The  character  of  the  movement  forming  part  of  a  reflex  action 
is  also  influenced  by  the  intensity  of  the  stimulus.  A  slight  stimulus,  such 
as  gentle  contact  of  the  skin  with  some  body,  will  produce  one  kind  of 
movement ;  and  a  strong  stimulus,  such  as  a  sharp  prick  applied  to  the  same 
spot  of  skin,  will  call  forth  quite  a  different  movement.  When  a  decapi- 
tated snake  or  newt  is  suspended  and  the  skin  of  the  tail  slightly  touched 
with  the  finger,  the  tail  bends  toward  the  finger  ;  when  the  skin  is  pricked 
or  burnt,  the  tail  is  turned  away  from  the  offending  object.  And  so  in 
many  other  instances.  It  must  be  remembered,  of  course,  that  a  difference 
in  the  intensity  of  the  stimulus  entails  a  difference  in  the  characters  of  the 
efferent  impulses ;  gentle  contact  gives  rise  to  what  we  call  a  sensation  of 
touch,  while  a  sharp  prick  gives  rise  to  pain,  consciousness  being  differently 
affected  in  the  two  cases  because  the  afferent  impulses  are  different.  Hence 
the  instances  in  question  are  in  reality  fuller  illustrations  of  the  dependence, 
to  which  we  called  attention  above,  of  the  characters  of  a  reflex  movement 
on  the  characters  of  the  afferent  impulses. 

Further,  as  we  have  already  pointed  out  (§  97),  while  the  motor  impulses 
started  by  a  weak  stimulus  applied  to  an  afferent  nerve  are  transmitted  along 
a  few,  those  started  by  a  strong  stimulus  may  spread  to  many  efferent  nerves. 
Granting  that  any  particular  afferent  nerve  is  more  especially  associated  with 
certain  efferent  nerves  than  with  any  others,  so  that  the  reflex  impulses  gen- 
erated by  afferent  impulses  entering  the  cord  by  the  former  pass  with  the 
least  resistance  down  the  latter,  we  must  evidently  admit  further  that  other 
efferent  nerves  are  also,  though  less  directly,  connected  with  the  same  affer- 
ent nerve,  the  passage  into  the  second  efferent  nerve  meeting  with  a  greater 
but  not  an  insuperable  resistance.  When  a  frog  is  poisoned  with  strychnine, 
a  slight  touch  on  any  part  of  the  skin  may  cause  convulsions  of  the  whole 
body ;  that  is  to  say,  the  afferent  impulses  passing  along  any  single  afferent 
nerve  may  give  rise  to  the  discharge  of  efferent  impulses  along  any  or  all 
of  the  efferent  nerves.  This  proves  that  a  physiological,  if  not  an  ana- 
tomical, continuity  obtains  between  all  parts  of  the  spinal  cord  which  are 
concerned  in  reflex  action,  that  the  nervous  network  intervening  between 
the  afferent  and  efferent  fibres  forms  along  the  whole  length  of  the  cord  a 
functionally  continuous  field.  This  continuous  network,  however,  we  must 
suppose  to  be  marked  out  into  tracts  presenting  greater  or  less  resistance  to 
the  progress  of  the  impulses  into  which  afferent  impulses,  coming  along  this 


THE  KEFLEX   ACTIONS  OF  THE  SPINAL   CORD.  569 

or  that  afferent  nerve,  are  transformed  on  their  advent  at  the  network  ;  and 
accordingly  the  path  of  any  series  of  impulses  in  the  network  will  be  deter- 
mined largely  by  the  energy  of  the  afferent  impulses.  And  the  action  of 
strychnine  may  be,  in  part,  explained  by  supposing  that  it  reduces  and 
equalizes  the  normal  resistance  of  this  network,  so  that  even  weak  impulses 
travel  over  all  its  tracts  with  great  ease. 

§  500.  Further,  the  movement,  forming  part  of  a  reflex  action,  varies  in 
character  according  to  the  particular  part  of  the  body  to  which  the  stimulus 
is  applied.  The  reflex  actions  developed  by  stimulation  of  the  internal  vis- 
cera are  different  from  those  excited  by  stimulation  of  the  skin.  We  have 
reason  to  think  that  the  contraction  of,  or  other  changes  in  a  skeletal  muscle 
may  produce,  by  reflex  action,  contractions  of  other  muscles  ;  and  such 
reflex  actions  also  differ  from  those  started  by  stimulation  of  the  skin.  In 
reflex  actions  started  by  applying  a  stimulus  to  the  skin  the  movements  vary 
largely,  according  to  the  particular  area  of  the  skin  which  is  affected.  Thus, 
pinching  the  folds  of  skin  surrounding  the  anus  of  the  frog  produces  differ- 
ent effects  from  those  witnessed  when  the  flank  or  toe  is  pinched  ;  and, 
speaking  generally,  the  stimulation  of  a  particular  spot  calls  forth  partic- 
ular movements.  In  the  case  of  the  simple  reflex  movements,  it  appears  to 
be  a  general  rule  that  a  movement  started  by  the  stimulation  of  a  sensory 
surface  or  region  on  one  side  of  the  body  is  developed  on  the  same  side  of 
the  body,  and  if  it  spreads  to  the  other  side,  still  remains  most  intense  on  the 
same  side ;  the  movement  on  the  other  side,  moreover,  is  symmetrical  with 
that  on  the  same  side.  It  has  been  maintained  that  "  crossed  "  or  diagonal 
reflex  movements,  as  where  stimulation  of  one  fore-foot  leads  to  movements 
of  the  opposite  hind  limb,  do  not  occur  unless  some  portion  of  the  bulb  be 
left  attached  to  the  spinal  cord.  Seeing  that  locomotion  of  four-footed 
animals  is  largely  effected  by  diagonal  movements  of  the  limbs,  one  would 
rather  have  expected  to  find  the  spinal  cord  itself  provided  with  mechan- 
isms to  assist  in  carrying  them  out ;  and,  indeed,  it  is  affirmed  that  in  the 
case  of  cold-blooded  animals  and  of  many  young  mammals,  after  a  division 
of  the  spinal  cord  below  the  bulb,  a  gentle  stimulation  will  provoke  a  diagonal 
movement,  slight  pressure  on  one  fore  foot,  for  example,  giving  rise  to  move- 
ments in  the  opposite  hind  leg  ;  a  strong  stimulus,  however,  will  produce  an 
ordinary  one-sided  movement.  Again,  when  in  a  dog  the  cord  has  been 
divided  in  the  lower  thoracic  region  so  that  the  hind  limbs  depend  on  the 
lumbar  cord  alone,  a  rhythmically  repeated  drawing  up  and  letting  down 
of  the  hind  limbs  is  witnessed  when  these  are  allowed  to  hang  down  ;  and 
these  movements,  which  appear  to  be  of  a  reflex  nature  excited  by  the 
pendent  position  of  the  limbs,  are  often  seen  to  alternate  regularly  in  the 
two  limbs,  the  right  leg  being  extended  while  the  left  leg  is  being  drawn  up 
and  vice  versa.  It  may  further  be  observed  that  if  the  foot  of  one  pendent 
limb  be  pinched  while  the  other  limb  is  passively  flexed  the  flexion  of  the 
limb  which  is  pinched  is  accompanied  by  an  extension  of  the  other  limb. 
In  these  respects,  however,  different  animals,  as  already  urged,  differ  from 
each  other. 

§  501.  From  these  and  similar  phenomena  we  may  infer  that  the  nervous 
network  spoken  of  above  is,  so  to  speak,  mapped  out  into  nervous  mechan- 
isms by  the  establishment  of  lines  of  greater  or  less  resistance,  so  that  the 
disturbances  in  it  generated  by  certain  afferent  impulses  are  directed  into 
certain  efferent  channels.  It  may  be  added  that  though  conspicuously  pur- 
poseful movements  seem  to  need  the  concurrent  action  of  several  segments  of 
the  cord,  and  as  a  rule,  the  greater  the  length  of  the  cord  involved  the  more 
complex  and  the  more  distinctly  purposeful  the  movement,  still  the  move- 
ments evoked  by  even  a  segment  of  the  cord  may  be  purposeful  in  character ; 


570  THE  SPINAL   CORD. 

hence  we  must  conclude  that  every  segment  of  the  nervous  network  is 
mapped  out  into  mechanisms.  But  the  arrangement  of  these  mechanisms, 
especially  of  the  more  complex  ones,  is  not  a  fixed  and  rigid  one.  We  can- 
not always  predict  exactly  the  nature  of  the  movement  which  will  result 
from  the  stimulation  of  any  particular  spot,  because  the  result  will  vary 
according  to  the  condition  of  the  spinal  cord,  especially  in  relation  to  the 
strength  and  character  of  the  stimulus.  Moreover,  under  a  change  of  cir- 
cumstances a  movement  quite  different  from  the  normal  one  may  make  its 
appearance.  Thus  when  a  drop  of  acid  is  placed  on  the  right  flank  of  a 
brainless  frog,  the  right  foot  is  almost  invariably  used  to  rub  off  the  acid  ; 
in  this  there  appears  nothing  more  than  a  mere  "  mechanical"  reflex  action. 
If,  however,  the  right  leg  be  cut  off,  or  the  right  foot  be  otherwise  hindered 
from  rubbing  off  the  acid,  the  left  foot  is,  under  the  exceptional  circum- 
stances, used  for  the  purpose.  This  at  first  sight  looks  like  an  intelligent 
choice.  A  choice  it  evidently  is ;  and  were  there  many  instances  of  choice, 
and  were  there  any  evidence  of  a  variable  automatism,  like  that  which  we 
call  "  volition,"  being  manifested  by  the  spinal  cord  of  the  frog,  we  should 
be  justified  in  supposing  that  the  choice  was  determined  by  an  intelligence. 
But,  as  we  shall  have  occasion  later  on  to  point  out,  a  frog,  deprived  of  its 
brain  so  that  the  spinal  cord  only  is  left,  makes  no  spontaneous  movements 
at  all.  Such  an  entire  absence  of  spontaneity  is  wholly  inconsistent  with  the 
possession  of  intelligence.  Then  again  the  above  experiment,  if  not  the  only 
instance,  is,  at  all  events,  by  far  the  most  striking  instance  of  choice  on  the 
part  of  a  brainless  frog.  We  are,  therefore,  led  to  conclude  that  the  phe- 
nomena must  be  explained  in  some  other  way  than  by  being  referred  to  the 
working  of  an  intelligence.  Moreover,  this  conclusion  is  supported  by  the 
behavior  of  other  animals.  Thus  similar  vicarious  reflex  movements  may 
be  witnessed  in  mammals,  though  not  perhaps  to  such  a  striking  extent  as 
in  frogs.  In  dogs,  in  which  partial  removal  of  the  cerebral  hemispheres  has 
apparently  heightened  the  reflex  excitability  of  the  spinal  cord,  the  remark- 
able scratching  movements  of  the  hind  leg  which  are  called  forth  by  stimu- 
lating a  particular  spot  on  the  loins  or  side  of  the  body,  are  executed  by 
the  leg  of  the  opposite  side,  if  the  leg  of  the  same  side  be  gently  held. 
In  this  case  the  vicarious  movements  are  effectual,  the  leg  not  being,  as 
in  the  case  of  the  frog,  crossed  over  so  as  to  bear  on  the  spot  stimu- 
lated, and  cannot  be  considered  as  betokening  intelligence.  Again,  the 
"  mechanical "  nature  of  reflex  actions  is  well  illustrated  by  the  behavior 
of  a  decapitated  snake.  When  the  body  of  the  animal  in  this  condition 
is  brought  into  contact  at  several  places  at  once  with  an  arm  or  a  stick, 
complex  reflex  movements  are  excited,  the  obvious  purpose  as  well  as 
effect  of  which  is  to  twine  the  body  round  the  object.  A  decapitated 
snake  will,  however,  with  equal  and  fatal  readiness  twine  itself  round  a 
red-hot  bar  of  iron,  which  is  made  to  touch  its  skin  in  several  places  at 
the  same  time. 

§  502.  In  considering  the  nature  of  the  events  in  the  spinal  cord  which 
determine  the  behavior  of  the  frog  in  the  instance  just  mentioned  we  must 
bear  in  mind  that  the  movements  in  question  are  "  coordinated  ; "  that  is  to 
say,  not  only  are  many  distinct  muscles  brought  into  play,  but  certain  rela- 
tions are  maintained  between  the  amount,  duration,  and  exact  time  of  occur- 
rence of  the  contraction  of  each  muscle  and  those  of  the  contractions  of  its 
fellow  muscles  sharing  in  the  movement.  In  the  absence  of  such  coordina- 
tion the  movement  would  become  irregular  and  ineffectual.  We  shall  have 
occasion  later  on  in  dealing  with  voluntary  movements  to  point  out  that 
the  coordination  and  hence  the  due  accomplishment  of  a  voluntary  move- 
ment is  dependent  on  certain  afferent  impulses  passing  up  from  the  con- 


THE  REFLEX  ACTIONS  OF  THE  SPINAL  CORD.  571 

tracting  muscles  to  the  central  nervous  system,  and  guiding  the  discharge 
of  the  efferent  impulses  which  call  forth  the  contractions.  When  these 
afferent  impulses  affect  consciousness  we  speak  of  them  as  constituting  a 
"  muscular  sense  ;  "  it  is,  as  we  shall  see,  by  the  "  muscular  sense"  that  we 
become  aware  of  and  can  appreciate  the  condition  of  our  muscles.  But  we 
have  reason  to  think  that  the  afferent  impulses  which  constitute  the  basis  of 
the  muscular  sense,  whatever  be  their  exact  nature,  in  order  to  play  their 
part  in  bringing  about  the  coordination  of  a  voluntary  movement  need  not 
pass  right  up  to  the  brain  and  develop  a  distinct  muscular  "sense,"  but  may 
produce  their  effect  by  working  on  the  nervous  mechanisms  of  the  spinal 
cord  with  which  the  motor  fibres  carrying  out  the  movement  are  connected. 
In  other  words,  the  coordination  of  a  voluntary  movement  takes  place  in 
the  part  of  the  spinal  cord  which  carries  out  the  movement,  and  not  in  the 
brain,  though  the  latter  may  be  conscious  of  the  whole  movement  including 
its  coordination. 

But  if  the  spinal  cord  possesses  mechanisms  for  carrying  out  coordinated 
movements,  which  in  the  case  of  voluntary  movements  are  discharged  by 
nervous  impulses  descending  from  the  brain,  we  may  infer  that  in  reflex 
actions  the  same  mechanisms  are  brought  into  action  though  they  are  dis- 
charged by  afferent  impulses  coming  along  afferent  nerves  instead  of  by 
impulses  descending  from  the  brain.  The  movements  of  reflex  origin,  in  all 
their  features  except  their  exciting  cause,  appear  identical  with  voluntary 
movements  ;  the  two  can  only  be  distinguished  from  each  other  by  a  know- 
ledge of  the  exciting  cause.  And  it  seems  unreasonable  to  suppose  that  the 
spinal  cord  should  possess  two  sets  of  mechanisms  in  all  respects  identical, 
save  that  the  one  is  discharged  by  volitional  impulses  from  the  brain  and  the 
other  by  afferent  impulses  from  afferent  nerves. 

We  are  led  therefore  to  the  conclusion  that  in  a  reflex  action  two  kinds 
of  afferent  impulses  are  concerned  :  the  ordinary  afferent  impulses  which  dis- 
charge the  nervous  mechanism  within  the  cord  and  so  provoke  the  move- 
ment, and  the  afferent  impulses  which  connect  that  nervous  mechanism  with 
the  muscles  about  to  be  called  into  play,  and  which  take  part  in  the  coordi- 
nation of  the  movement  provoked.  The  nature  of  these  latter  afferent  im- 
pulses is  at  present  obscure  ;  but  if  we  admit,  as  we  seem  compelled  to  do,  that 
the  character  of  a  reflex  action  is  determined  by  them  as  well  as  by  the  affer- 
ent impulses  which  actually  discharge  the  mechanism,  it  seems  possible  that 
a  fuller  knowledge  of  these  coordinating  afferent  impulses  may  afford  an 
adequate  explanation  of  the  fact  that  when,  as  in  the  case  of  the  frog  in 
question,  the  usual  set  of  muscles  cannot  be  employed  by  the  nervous 
mechanism,  recourse  is  had  to  another. 

We  have  avoided  the  introduction  of  the  word  "  consciousness  "  as  un- 
necessarily complicating  the  question  ;  and  it  would  be  out  of  place  to  discuss 
psychological  problems  here.  We  may  remark,  however,  that  since  we  have 
no  objective  proofs  of  consciousness  outside  ourselves,  and  only  infer  by 
analogy  that  such  and  such  an  act  is  an  outcome  of  consciousness  on  account 
of  its  likeness  to  acts  which  are  the  outcome  of  our  own  consciousness,  we 
conclude  that  the  brainless  frog  possesses  no  active  consciousness  like  our 
own,  because  absence  of  spontaneous  movements  seems  to  be  irreconcilable 
with  the  existence  of  an  active  consciousness  whose  very  essence  is  a  series 
of  changes.  Consciousness,  as  we  recognize  it,  seems  to  be  necessarily  ope- 
rating as,  or  to  be  indissolubly  associated  with  the  presence  of,  an  incessantly 
repeated  internal  stimulus ;  and  we  cannot  conceive  of  that  stimulus  failing 
to  excite  mechanisms  of  movement  which,  as  in  the  case  of  the  brainless 
frog,  are  confessedly  present.  We  may,  however,  distinguish  between  an 
active  continuous  consciousness,  such  as  we  usually  understand  by  the  term, 


572  THE  SPINAL  CORD. 

and  a  passing  and  momentary  condition,  which  we  may  speak  of  as  con- 
sciousness, but  which  is  wholly  discontinuous  from  an  antecedent  or  from  a 
subsequent  similar  momentary  condition  ;  and  indeed  we  may  suppose  that 
the  complete  consciousness  of  ourselves,  and  the  similarly  complete  con- 
sciousness which  we  infer  to  exist  in  many  animals,  has  been  gradually 
evolved  out  of  such  a  rudimentary  consciousness.  We  may,  on  this  view, 
suppose  that  every  nervous  action  of  a  certain  intensity  or  character  is  ac- 
companied by  some  amount  of  consciousness,  which  we  may,  in  a  way,  com- 
pare to  the  light  emitted  when  a  combustion,  previously  giving  rise  to 
invisible  heat,  waxes  fiercer.  We  may  thus  infer  that  when  the  brainless 
frog  is  stirred  by  some  stimulus  to  a  reflex  act,  the  spinal  cord  is  lit  up  by  a 
momentary  flash  of  consciousness  coming  out  of  darkness  and  dying  away 
into  darkness  again  ;  and  we  may  perhaps  further  infer  that  such  a  passing 
consciousness  is  the  better  developed  the  larger  the  portion  of  the  cord  in- 
volved in  the  reflex  act  and  the  more  complex  the  movement.  But  such  a 
momentary  flash,  even  if  we  admit  its  existence,  is  something  very  different 
from  consciousness  as  ordinarily  understood,  is  far  removed  from  intelli- 
gence, and  cannot  be  appealed  to  as  explaining  the  "  choice  "  spoken  of 
above. 

§  503.  Lastly,  the  characters  of  a  reflex  movement  are,  as  we  need 
hardly  say,  dependent  on  the  intrinsic  condition  of  the  cord.  The  action 
of  strychnine  just  alluded  to  is  an  instance  of  an  apparent  augmentation 
of  reflex  action  best  explained  by  supposing  that  the  resistances  in  the  cord 
are  lessened.  There  are  probably,  however,  cases  in  which  the  explosive 
energy  of  the  nervous  substance  is  positively  increased  above  the  normal. 
Conversely,  by  various  influences  of  a  depressing  character,  as  by  various 
anaesthetics  or  other  poisons,  reflex  action  may  be  lessened  or  prevented ; 
and  this  again  may  arise  either  from  an  increase  of  resistance  or  from  a 
diminution  in  the  actual  discharge  of  energy.  So  also  various  diseases  may 
so  affect  the  spinal  cord  as  to  produce,  on  the  one  hand,  increased  reflex  ex- 
citability, so  that  a  mere  touch  may  produce  a  violent  movement,  and,  on  the 
other  hand,  diminished  reflex  excitability,  so  that  it  becomes  difficult  or  im- 
possible to  call  forth  reflex  action. 

§  504.  When  we  come  to  study  the  reflex  actions  of  man  we  should  at 
first  perhaps  be  inclined  to  infer  that,  since  in  him  the  spinal  cord  is  so 
largely  used  as  the  instrument  of  the  brain,  the  independent  reflex  actions  of 
the  cord,  at  least  such  as  affect  skeletal  muscles,  are  in  him  of  much  less  im- 
portance than  they  appear  to  be  in  animals,  and  experience  seems  to  sup- 
port this  view.  But  it  must  be  remembered  that  in  his  case,  as  we  have 
already  stated  (§  496),  we  lack  the  guidance  of  experimental  results ;  we 
are  obliged  to  trust  to  the  entangled  phenomena  of  disease  or  to  a  study  of 
the  behavior  of  the  cord  while  it  is  still  a  part  of  an  intact  nervous  system  ; 
and  each  of  these  methods  presents  difficulties  of  its  own.  The  movements, 
which  in  the  intact  human  body  we  can  recognize  as  indubitable  reflex 
actions,  are,  as  a  rule,  simple  and  unimportant.  They  are,  in  by  far  the 
greater  number  of  instances,  occasioned  by  stimulation  of  the  skin  or  of  the 
mucous  membrane,  for  the  most  part  involve  a  few  muscles  only,  and 
rarely  indicate  any  very  complex  coordination.  The  flexion,  followed  by 
extension,  of  the  leg,  which  is  called  forth  by  tickling  the  sole  of  the  foot, 
or  the  winking  of  the  eye  when  the  cornea  or  conjunctiva  is  touched,  may 
perhaps  be  regarded  as  the  type  of  these  movements.  A  very  common 
form  of  reflex  action  is  that  in  which  a  muscle  or  group  of  muscles  is 
thrown  into  contraction  by  stimulation  of  the  overlying  or  neighboring 
skin,  as  when  the  abdominal  muscles  contract  upon  stroking  the  skin  of 
the  abdomen  or  the  testicle  is  retracted  upon  stroking  the  inside  of  the 


THE  REFLEX  ACTIONS  OF  THE  SPINAL  COKD.  573 

thigh.  A  reflex  movement  may  occur  as  the  result  of  stimulation  of  an 
organ  of  special  sense,  parts  of  the  central  nervous  system  other  than  the 
spinal  cord  serving  as  the  centre.  A  sound  or  a  flash  of  light  readily  pro- 
duces a  start,  a  bright  light  makes  the  eye  wink  and  may  cause  the  person 
to  sneeze  (the  greater  coordination  manifest  in  this  act  being  due  to  the 
fact  that  the  complex  respiratory  mechanism  is  brought  into  play  (§  334), 
and  reflex  movement  may  result  from  a  taste  or  smell.  A  special  form  of 
reflex  action,  or  at  least  an  action  resembling  a  reflex  action,  is  called  forth 
by  sharply  striking  certain  tendons;  for  instance,  striking  the  tendon  below 
the  patella  gives  rise  to  a  sudden  extension  of  the  leg,  known  as  the  "knee- 
jerk";  but  it  will  be  best  to  discuss  these  "  tendon  reflexes,"  or  "  muscle 
reflexes,"  as  they  are  called,  later  on  in  another  connection. 

On  the  whole  the  reflex  movements  carried  out  by  the  intact  nervous 
system  of  man  are,  we  repeat,  scanty  and  comparatively  simple ;  but  we 
are  not  justified  in  inferring  from  this  that  the  human  spinal  cord,  left  to 
itself,  is  incapable  of  doing  more ;  that  owing  to  the  predominant  activity 
of  the  brain  it  has  lost  the  powers  possessed  by  the  spinal  cord  in  the  lower 
animals.  For  it  may  be  that  the  cord,  when  joined  to  the  brain,  is  through 
various  influences  proceeding  from  the  latter  in  a  different  condition  from 
that  in  which  it  is  when  separated  from  the  brain ;  indeed,  we  have  reason 
to  think  that  this  is  so ;  and  we  may  here  remark  that  in  the  lower  animals, 
as  in  man,  the  development  of  reflex  movements  is  difficult  and  uncertain 
in  the  presence  of  the  brain. 

When  we  turn  to  the  teaching  of  disease,  however,  we  again  find  that 
reflex  movements  carried  out  by  the  cord  or  by  parts  of  the  cord  are,  on 
the  whole,  scanty  and  simple. 

In  some  stages  of  certain  diseases  of  the  spinal  cord  extensive  reflex 
movements  are  witnessed ;  but  these  are  not  purposeful,  coordinated  move- 
ments, such  as  have  been  described  above  as  occurring  in  frogs  and  mam- 
mals after  experimental  interference,  but  rather  mere  exaggerations  of  the 
simpler  reflex  movements  witnessed  when  the  nervous  system  is  intact.  In 
cases  of  paraplegia  (such  being  the  term  generally  used  when  disease  or 
injury  has  cut  off  the  cord,  generally  the  lower  part  of  the  cord,  from  the 
brain,  so  that  the  will  cannot  bring  about  movements  in,  and  the  mind 
derives  no  sensation  from,  the  parts  below  the  lesion,  the  legs  for  instance), 
it  sometimes  happens  that  contact  with  the  bedclothes  or  other  external 
objects  sets  up  from  time  to  time  rhythmically  repeated  movements,  the 
legs  being  alternately  drawn  up  and  thrust  out  again.  And  an  exaggera- 
tion of  the  "  knee-jerk  "  or  other  "  tendon  reflexes  "  is  a  very  common  symp- 
tom in  certain  spinal  diseases.  It  is  rarely,  if  ever,  that  reflex  movements 
of  a  really  complicated  character  are  observed.  Moreover,  clinical  experi- 
ence shows  that  in  man,  when  a  portion  of  the  cord  is  isolated,  reflex 
actions  carried  out  by  means  of  that  portion,  so  far  from  being  exagger- 
ated, are  much  more  commonly  exceedingly  feeble  or  absent  altogether.  In 
the  cases  in  which  the  physiological  continuity  of  the  lower  with  the  upper 
part  of  the  cord  has  been  broken  by  disease,  by  some  growth  invading  the 
nervous  structures,  or  by  some  changes  of  the  nervous  structures  themselves, 
we  may  attempt  to  explain  the  absence  from  the  lower  part  or  coordinate 
reflex  activity,  such  as  is  seen  in  the  lower  animal,  as  due  to  the  disease  not 
only  affecting  the  powers  of  the  actually  diseased  part,  but  influencing  the 
whole  cord  below,  and  either  by  inhibition,  of  which  we  shall  speak  pres- 
ently, or  in  some  other  way  depressing  its  functions.  But  the  same  absence 
of  complex  reflex  movements  is  also  often  observed  in  cases  in  which  the 
cord  has  been  severed  by  accident,  and,  indeed,  though  accidental  injuries  to 
the  human  cord  generally  produce  more  profound  and  extensive  mischief 


574  THE  SPINAL  CORD. 

than  that  which  results  in  animals  from  experimental  interference,  clinical 
experience  tends,  on  the  whole,  to  support  the  view  that  in  man  the  more 
complete  subordination  of  the  spinal  cord  to  the  brain  has  led  to  the  dying 
out  of  the  complex  reflex  actions  which  are  so  conspicuous  in  the  lower 
animals.  This,  however,  cannot  be  regarded  as  distinctly  proved. 

When  we  come  to  study  voluntary  movements,  we  shall  see  reason  to 
think  that  in  man,  as  in  the  lower  animals,  the  will  in  carrying  out  these 
movements  makes  use  of  complex  nervous  mechanisms  situated  in  the 
spinal  cord — nervous  mechanisms  into  the  working  of  which,  as  urged 
above,  afferent  impulses  enter  largely ;  and  it  seems  improbable  that  these 
spinal  mechanisms  should  be  capable  of  being  thrown  into  action  by  the 
will  only.  In  the  act  of  walking,  for  instance,  it  is  highly  probable  that 
the  movements  of  the  legs  are  the  direct  results  of  the  action  of  nervous 
mechanisms  in  the  lumbar  cord  brought  into  play  by  the  will,  being  thus, 
in  an  indirect  manner  only,  the  products  of  volitional  impulses ;  and  even 
in  man,  though  clinical  experience  only  affords  us  instances  of  this  machin- 
ery working  apart  from  the  brain  in  a  damaged  condition  and  under  unfa- 
vorable circumstances,  so  that  the  resemblance  of  the  movements  observed 
to  the  complete  act  of  walking  is  but  feeble,  still  it  seems  similarly  probable 
that  under  more  favorable  circumstances  the  lumbar  cord  separated  from 
the  brain  might  as  part  of  a  reflex  act  carry  out  the  movements  in  a  more 
complete  and  coordinate  manner. 

§  505.  We  have  dwelt  above  chiefly  on  reflex  actions,  in  which  the 
efferent  impulses  cause  contractions  of  skeletal  muscles,  since  these  are 
undoubtedly  the  most  common  and  the  most  prominent  forms  of  reflex 
action  ;  but  it  must  not  be  forgotten  that  the  efferent  impulses  of  reflex 
origin  may  produce  contractions  of  other  muscles,  as  well  as  other  effects, 
such  as  secretion,  for  instance.  On  several  of  these  we  have  dwelt,  from 
time  to  time  in  previous  parts  of  this  work,  and  it  will  be  unnecessary  to 
repeat  them  here.  But  it  may  be  worth  while  to  point  out  that  the  spinal 
cord,  by  serving  as  a  reflex  centre  for  innumerable  ties  which  correlate  the 
nutritive  or  metabolic  activities  of  the  several  tissues  to  events  taking  place 
in  other  parts  of  the  body,  plays  a  conspicuous  part  in  securing  the  welfare 
of  the  whole  body.  In  dealing  (§  462)  with  the  general  problems  of  nutri- 
tion, we  stated  that  an  orderly  nutrition  appears  to  be  in  some  way  depend- 
ent on  nervous  influences.  Many  of  these  nervous  influences  appear  to 
issue  from  the  spinal  cord,  either  as  parts  of  a  reflex  act  or  as  the  outcome 
of  some  automatic  processes.  When  in  a  dog  the  lumbar  cord  is  wholly 
separated  from  the  rest  of  the  cord  by  section,  the  nutrition  of  the  hind 
limbs  and  the  general  health  of  the  animal  may,  with  care,  be  maintained 
in  a  very  satisfactory  condition  ;  but  if  that  small  separated  piece  of  the 
cord  be  destroyed,  death  inevitably  ensues  before  long,  in  spite  of  every  care 
and  precaution,  being  brought  about  apparently  by  the  disordered  nutrition 
of  the  hind  limbs  and  other  parts  supplied  by  nerves  coming  from  the  lum- 
bar cord.  In  man  extensive  injuries  to  the  spinal  cord  are  followed  by  bed- 
sores and  other  results  of  impaired  nutrition  ;  and,  indeed,  death  is  gen- 
erally brought  about  in  this  way  in  cases  of  paraplegia  caused  by  accidental 
crushing  or  severance  of  the  cord. 

§  506.  Inhibition  of  reflex  action.  The  reflex  actions  of  the  spinal  cord, 
like  other  nervous  actions,  may  be  totally  or  partially  inhibited,  that  is  to 
say,  may  be  arrested  or  hindered  in  their  development  by  impulses  reaching 
the  centre  while  it  is  already  in  action.  Thus,  if  the  body  of  a  decapitated 
snake  be  allowed  to  hang  down,  slow  rhythmic  pendulous  movements,  which 
appear  to  be  reflex  in  nature,  soon  make  their  appearance ;  all  these  may 
be  for  a  while  arrested  by  slight  stimulation,  as  by  gently  stroking  the  tail. 


THE  KEFLEX  ACTIONS  OF  THE  SPINAL   CORD.  575 

We  have  already  seen  that  the  action  of  such  nervous  centres  as  the  respi- 
ratory and  vasomotor  centres,  which  frequently,  at  all  events,  is  of  a  reflex 
nature,  may  be  either  inhibited  or  augmented  by  afferent  impulses.  The 
micturition* centre  in  the  mammal,  which  is  also  largely  a  reflex  centre,  may 
be  easily  inhibited  by  impulses  passing  downward  to  the  lumbar  cord  from 
the  brain,  or  upward  along  the  sciatic  nerves.  In  the  case  of  dogs,  whose 
spinal  cord  has  been  divided  in  the  thoracic  region,  micturition  set  up  as  a 
reflex  act  by  simple  pressure  on  the  abdomen  or  by  sponging  the  anus  is  at 
once  stopped  by  sharply  pinching  the  skin  of  the  leg.  And  it  is  a  matter 
of  common  experience  that  in  man  micturition  maybe  suddenly  checked  by 
an  emotion  or  other  cerebral  event.  The  erection  centre  in  the  lumbar 
cord,  also  in  large  measure  a  reflex  centre,  is  similarly  susceptible  of  being 
inhibited  by  impulses  reaching  it  from  various  sources.  And,  indeed, 
many  similar  instances  of  the  inhibition  of  reflex  movements  might  readily 
be  quoted. 

Several  apparent  instances  of  the  inhibition  of  reflex  acts  are  not  really 
such  ;  in  these  cases  all  the  nervous  processes  of  the  act  may  take  place  in 
their  entirety  and  yet  fail  to  produce  their  effect  on  account  of  a  failure  in 
the  muscular  part  of  the  act.  Thus,  when  we  ourselves  by  an  effort  of  the 
will  stop  the  reflex  movements  which  otherwise  would  be  produced  by 
tickling  the  soles  of  the  feet,  we  achieve  this  to  a  large  extent  by  throwing 
voluntarily  into  action  certain  muscles,  the  contractions  of  which  antagonize 
the  action  of  the  muscles  engaged  in  carrying  out  the  reflex  movements. 
But  it  may  be  doubted,  even  in  these  cases,  whether  inhibition  is  always  or 
wholly  to  be  explained  in  this  way ;  and  certainly  in  very  many  instances 
of  reflex  inhibition  no  such  muscular  antagonism  is  present,  and  the  reflex 
act  is  checked  at  its  nervous  centre. 

When  the  brain  of  a  frog  is  removed,  and  the  effects  of  shock  have 
passed  away,  reflex  actions  are  developed  much  more  readily  and  to  a  much 
greater  degree  than  in  the  entire  animal,  and  in  mammals  also  reflex  excita- 
bility has  been  observed  to  be  increased  by  removal  of  the  cerebral  hemi- 
spheres. This  suggests  the  idea  that  in  the  intact  nervous  system  the  brain 
is  habitually  exerting  some  influence  on  the  spinal  cord,  tending  to  prevent 
the  normal  development  of  the  spinal  reflex  actions.  And  we  learn  by  ex- 
periment that  stimulation  of  certain  parts  of  the  brain  has  a  remarkable 
effect  on  reflex  action.  If  a  frog,  from  which  the  cerebral  hemispheres  have 
been  removed  (the  optic  lobes,  bulb,  and  spinal  cord  being  left  intact),  be 
suspended  by  the  jaw,  and  the  toes  of  the  pendent  leg  be  from  time  to  time 
dipped  into  very  dilute  sulphuric  acid,  a  certain  average  time  will  be  found 
to  elapse  between  the  dipping  of  the  toe  and  the  resulting  withdrawal  of  the 
foot.  If,  however,  the  optic  lobes  or  optic  thalami  be  stimulated,  as  by 
putting  a  crystal  of  sodium  chloride  on  them,  it  will  be  found  on  repeating 
the  experiment,  while  these  structures  are  still  under  the  influence  of  the 
stimulation,  that  the  time  intervening  between  the  action  of  the  acid  on  the 
toe  and  the  withdrawal  of  the  foot  is  very  much  prolonged.  That  is  to  say, 
the  stimulation  of  the  optic  lobes  has  caused  impulses  to  descend  to  the  cord, 
which  have  there  so  interfered  with  the  nervous  processes  engaged  in  carry- 
ing.out  reflex  actions  as  greatly  to  retard  the  generation  of  efferent  impulses, 
or,  in  other  words,  has  inhibited  the  reflex  action  of  the  cord.  And  similar 
results  may  be  obtained  in  mammals  by  stimulating  certain  parts  of  the 
corpora  quadrigemina,  which  bodies  are  homologous  to  the  optic  lobes  of 
frogs.  From  this  it  has  been  inferred  that  there  is  present  in  this  part  of 
the  brain  a  special  mechanism  for  inhibiting  the  reflex  actions  of  the  spinal 
cord,  the  impulses  descending  from  this  mechanism  to  the  various  centres  of 
reflex  action  being  of  a  specific  inhibitory  nature.  But,  as  we  have  already 


576  THE  SPINAL  CORD. 

seen,  impulses  of  an  ordinary  kind,  passing  along  ordinary  sensory  nerves, 
may  inhibit  reflex  action.  We  have  quoted  instances  where  a  slight  stim- 
ulus, as  in  the  pendulous  movements  of  the  snake,  and  where  a  stronger 
stimulus,  as  in  the  case  of  the  micturition  of  the  dog,  may  produce  an  in- 
hibitory result;  we  may  add  that  in  the  frog  adequately  strong  stimuli 
applied  to  an  afferent  nerve  will  inhibit,  i.  e.,  will  retard  or  even  wholly 
prevent,  reflex  action.  If  the  toes  of  one  foot  are  dipped  into  dilute  sul- 
phuric acid  at  a  time  when  the  sciatic  of  the  other  leg  is  being  powerfully 
stimulated  with  an  interrupted  current,  the  period  of  incubation  of  the  reflex 
act  will  be  found  to  be  much  prolonged,  and  in  some  cases  the  reflex  with- 
drawal of  the  foot  will  not  take  place  at  all.  And  this  holds  good,  not  only 
in  the  complete  absence  of  the  optic  lobes  and  bulb,  but  also  when  only  a 
portion  of  the  spinal  cord,  sufficient  to  carry  out  the  reflex  action  in  the 
usual  way,  is  left.  There  can  be  no  question  here  of  any  specific  inhibitory 
centres,  such  as  have  been  supposed  to  exist  in  the  optic  lobes.  But  if  it  is 
clear  that  inhibition  of  reflex  action  may  be  brought  about  by  impulses 
which  are  not  in  themselves  of  a  specific  inhibitory  nature,  we  may  hesitate 
to  accept  the  view  that  a  special  inhibitory  mechanism  in  the  sense  of  one 
giving  rise  to  nothing  but  inhibitory  impulses  is  present  in  the  optic  lobes 
of  frogs,  and  after  removal  of  the  brain  that  the  exaltation  of  reflex  ac- 
tions which  is  manifest  is  due  to  the  withdrawal  of  such  a  specific  inhibitory 
mechanism. 

The  presence  of  the  brain  does  obviously  produce  an  effect  which  may 
be  broadly  spoken  of  as  inhibitory,  and  a  specific  action  of  the  brain,  in  an 
effort  of  the  will,  may  stop  or  inhibit  a  specific  reflex  action  ;  but  we  must 
not  in  these  matters  be  led  too  much  away  by  the  analogy  of  the  special 
and  limited  cardiac  inhibitory  mechanism.  There  we  have  apparently  to 
deal  with  fibres,  whose  exclusive  duty  it  is  to  convey  inhibitory  impulses 
from  the  bulb  to  the  cardiac  muscle,  and  inhibition  of  the  heart,  at  least 
through  nervous  influences,  is  exclusively  carried  out  by  them.  But  already, 
in  studying  the  nervous  mechanism  of  respiration,  we  have  seen  reason  to 
think  that  afferent  impulses  passing  along  the  same  nerves  and  probably 
along  the  same  fibres  may,  according  to  circumstances,  now  inhibit,  now 
augment  the  respiratory  centre,  and  have  thus  been  led  to  speak  of  in- 
hibitory impulses,  that  is,  impulses  producing  an  inhibitory  effect,  apart 
from  specific  inhibitory  fibres.  In  the  complex  working  of  the  central 
nervous  system  we  may  still  more  expect  to  come  across  similar  instances  of 
the  same  channels  serving  as  the  path  either  of  inhibition  or  of  augmenta- 
tion. In  all  probability  actions  or  processes  which  we  may  speak  of  as 
inhibitory  do  play,  as  indeed  we  shall  see,  an  important  part  in  the  whole 
work  of  the  central  nervous  system ;  in  all  probability  many  of  the  phe- 
nomena of  nervous  life  are  the  outcome  of  a  contest  between  what  we  may 
call  inhibitory  and  exciting  or  augmenting  forces ;  but  in  all  probability, 
also,  we  ought  rather  to  seek  for  the  explanation  of  how  vagus  impulses 
inhibit  the  beat  of  the  heart  by  reference  to  the  inhibitory  phenomena  of 
the  central  nervous  system,  than  to  attempt  to  explain  the  latter  by  the 
little  we  know  of  the  former.  At  present,  however,  we  must  be  content  with 
the  fact  that  experiments  on  animals  show  that  the  brain  not  only  by  some 
action  or  other  may  inhibit  particular  spinal  reflex  movements,  but  also 
habitually  exercises  a  restraining  influence  on  the  reflex  activity  of  the 
whole  cord,  though  we  are  unable  to  state  clearly  how  this  inhibition  is 
carried  out. 

We  say  "  experiments  on  animals,"  because  though  we  know,  as  stated 
above,  by  an  appeal  to  our  own  consciousness,  that  an  action  of  the  brain, 
an  effort  of  the  will,  may  stop  a  particular  reflex  act,  we  have  no  evidence 


THE  KEFLEX  ACTIONS  OF  THE  SPINAL  CORD.  577 

that  in  man  separation  of  the  cord  from  the  brain  leads,  as  in  animals,  to 
heightened  reflex  activity.  In  diseases,  or  injuries  to  the  cord,  reflex  actions 
are,  as  we  have  said,  sometimes  exaggerated,  but  it  is  possible,  and  indeed 
probable,  that  the  increase  is  due  to  the  morbid  processes  producing  a 
greater  irritability  of  the  cord  itself,  and  not  to  the  withdrawal  of  any  in- 
hibitory influences.  In  many  cases,  in  perhaps  the  greater  number,  no 
exaggeration  but  a  diminution  or  even  absence  of  reflex  activity  is  observed  ; 
so  much  so  that  could  we  trust  explicitly  to  clinical  experience,  we  should 
be  inclined  to  conclude  that  the  scantiness  of  spinal  reflex  action  in  man 
was  due  not  to  any  preoccupation  of  the  cord  by  influences  proceeding  from 
a  dominant  brain,  but  to  an  inherent  paucity  of  spinal  reflex  mechanisms. 
But  we  have  already  said  all  we  have  at  present  to  say  on  this  point. 

§  507.  The  time  required  for  reflex  actions.  When  one  eyelid  is  stimulated 
with  a  sharp  electrical  shock,  both  eyelids  blink.  Hence,  if  the  length  of 
time  intervening  between  the  stimulation  of  the  right  eyelid  and  the  move- 
ment of  the  left  eyelid  be  measured,  this  will  give  the  total  time  required 
for  the  various  processes  which  make  up  a  reflex  action.  It  has  been  found 
to  be  from  0.0662  to  0.0578  second.  Deducting  from  these  figures  the  time 
required  for  the  passage  of  afferent  and  efferent  impulses  along  the  fifth  and 
facial  nerves  to  and  from  the  bulb,  and  for  the  latent  period  of  the  con- 
traction of  the  orbicularis  muscle,  there  would  remain  0.0555  to  0.0471 
second  for  the  time  consumed  in  the  central  operations  of  the  reflex  act. 
The  calculations,  however,  necessary  for  this  reduction,  it  need  not  be  said, 
are  open  to  sources  of  error ;  moreover,  the  reflex  act  in  question  is  carried 
out  by  the  bulb  and  not  by  the  spinal  cord  proper.  Blinking  thus  produced 
is  a  reflex  act  of  the  very  simplest  kind ;  but,  as  we  have  seen  in  the  pre- 
ceding pages,  reflex  acts  differ  very  widely  in  nature  and  character ;  and  we 
accordingly  find,  as  indeed  we  have  incidentally  mentioned,  that  the  time 
taken  up  by  a  reflex  movement  varies  very  largely.  This,  indeed,  is  seen  in 
blinking  itself.  When  the  blinking  is  caused  not  by  an  electric  shock 
applied  to  the  eyelid,  but  by  a  flash  of  light  falling  on  the  retina,  in  which 
case  complex  visual  processes  are  involved,  the  time  is  distinctly  prolonged ; 
moreover,  the  results  in  different  experiments  in  which  light  serves  as  the 
stimulus  are  not  nearly  so  uniform  as  when  the  blinking  is  caused  by  stimu- 
lation of  the  eyelid. 

In  general  it  maybe  said  that  the  time  required  for  any  reflex  act  varies 
very  considerably  with  the  strength  of  the  stimulus  employed,  being  less  for 
the  stronger  stimuli ;  this  we  should  expect,  seeing  that  the  efferent  impulses 
of  the  reflex  act  are  not  simply  afferent  impulses  transmitted  through  the 
central  organ,  but  result  from  internal  changes  in  the  central  organ  started 
by  the  afferent  impulse  or  impulses ;  and  these  internal  changes  will  nat- 
urally be  more  intense  and  more  rapidly  effected  when  the  afferent  impulses 
are  strong.  It  is  stated  that  when  the  movement  induced  is  on  the  same  side 
of  the  body  as  the  surface  stimulation  of  which  starts  the  act,  the  time 
taken  up  is  less  than  when  the  movement  is  on  the  other  side  of  the  body, 
allowance  being  made  for  the  length  of  central  nervous  matter  involved  in 
the  two  cases ;  that  is  to  say,  the  central  operations  of  a  reflex  act  are  prop- 
agated more  rapidly  along  the  cord  than  across  the  cord.  The  rapidity  of 
the  act  varies,  of  course,  with  the  condition  of  the  spinal  cord,  the  act  being 
greatly  prolonged  when  the  cord  becomes  exhausted ;  and  a  similar  delay 
has  been  observed  in  cases  of  disease.  The  time  thus  occupied  by  purely 
reflex  actions  must  not  be  confounded  with  the  interval  required  when  the 
changes  taking  place  in  the  central  nervous  system  are  of  a  more  complicated 
nature,  and  more  or  less  distinctly  involve  mental  operations ;  of  the  latter 
we  shall  speak  later  on. 
37 


578  THE  SPINAL  CORD. 

THE  AUTOMATIC  ACTIONS  OF  THE  SPINAL  CORD. 

§  508.  We  speak  of  an  action  of  an  organ  or  of  a  living  body  as  being 
spontaneous  or  automatic  when  it  appears  to  be  not  immediately  due  to  any 
changes  in  the  circumstances  in  which  the  organ  or  body  is  placed,  but  to 
be  the  result  of  changes  arising  in  the  organ  or  body  itself  and  determined 
by  causes  other  than  the  influences  of  the  circumstances  of  the  moment. 
Some  automatic  actions  are  of  a  continued  character ;  others,  like  the  beat 
of  the  heart,  are  repeated  in  regular  rhythm ;  but  the  most  striking  auto- 
matic actions  of  the  living  body,  those  which  we  attribute  to  the  working  of 
the  will  and  which  we  call  voluntary  or  volitional,  are  characterized  by  their 
apparent  irregularity  and  variableness.  Such  variable  automatic  actions 
form  the  most  striking  features  of  an  intact  nervous  system,  but  are  conspic- 
uously absent  from  a  spinal  cord  when  the  brain  has  been  removed. 

A  brainless  frog  placed  in  a  condition  of  complete  equilibrium  in  which 
no  stimulus  is  brought  to  bear  on  it,  protected,  for  instance,  from  sudden 
passing  changes  in  temperature,  from  a  too  rapid  evaporation  by  the  skin 
and  the  like,  remains  perfectly  motionless  until  it  dies.  Such  apparently 
spontaneous  movements  as  are  occasionally  witnessed  are  so  few  and  seldom, 
that  we  can  hardly  do  otherwise  than  attribute  them  to  some  stimulus,  inter- 
nal or  external,  which  has  escaped  observation.  In  the  mammal  (dog)  after 
division  of  the  spinal  cord  in  the  dorsal  region  regular  and  apparently  spon- 
taneous movements  may  be  observed  in  the  parts  governed  by  the  lumbar 
cord.  When  the  animal  has  thoroughly  recovered  from  the  operation  the 
hind  limbs  rarely  remain  quiet  for  any  long  period  ;  they  move  restlessly  in 
various  ways ;  and  when  the  animal  is  suspended  by  the  upper  part  of  the 
body,  the  pendent  hind  limbs  are  continually  being  drawn  up  and  let  down 
again  with  a  monotonous  rhythmic  regularity,  suggestive  of  automatic 
rhythmic  discharges  from  the  central  mechanisms  of  the  cord.  In  the 
newly  born  mammal  too,  after  removal  of  the  brain,  movements  apparently 
spontaneous  in  nature  are  frequently  observed.  But  all  these  movements, 
even  when  most  highly  developed,  are  very  different  from  the  movements, 
irregular  and  variable  in  their  occurrence  though  orderly  and  purposeful  in 
their  character,  which  we  recognize  as  distinctly  voluntary.  Even  admit- 
ting that  some  of  the  movements  of  the  brainless  mammal  may  resemble 
voluntary  movements  in  so  far  as  they  are  due  to  changes  taking  place  in 
the  spinal  cord  itself  independent  of  the  immediate  influence  of  any  stimulus, 
we  are  not  thereby  justified  in  speaking  of  the  spinal  cord  as  developing  a 
will  in  the  sense  that  we  attribute  a  will  to  the  brain. 

§  509.  In  the  case  of  the  beat  of  the  heart,  the  automatic  rhythmic  dis- 
charge of  energy  appears  to  be  exclusively  the  outcome  of  the  molecular 
nutritive  changes  taking  place  in  the  cardiac  substance.  The  beat  may  be 
modified,  as  we  have  seen,  by  nervous  impulses  reaching  the  cardiac  sub- 
stance along  certain  nerves  ;  but  the  actual  existence  of  the  beat  is  wholly 
independent  of  these  extraneous  influences ;  the  rhythmic  discharge  con- 
tinues when  they  are  entirely  absent.  The  automatic  rhythmic  discharge  of 
respiratory  impulses  from  the  respiratory  centre  is  also  dependent  on  the 
intrinsic  molecular  changes  of  the  centre,  these  being,  as  we  have  seen, 
largely  determined  by  the  character  of  the  blood  streaming  through  it ;  but 
in  this  case  extrinsic  nervous  impulses,  reaching  the  centre  along  the  vagus 
and  other  nerves,  play  a  much  more  important  part  than  do  similar  impulses 
in  the  case  of  the  heart.  They  act  so  continually  on  the  centre  and  enter  so 
largely  into  its  working,  that  we  are  compelled  to  regard  the  activity  of  the 
centre  as  fed,  if  we  may  use  the  word,  not  only  by  the  intrinsic  molecular 
nutritive  processes  of  the  centre  itself,  but  also  by  the  extrinsic  nervous  in- 


THE  AUTOMATIC  ACTIONS  OF  THE  SPINAL   CORD.  579 

fluences  which  flow  into  the  centre  from  without.  The  automatism  of  the 
spinal  cord  as  a  whole  resembles,  in  this  respect,  that  of  the  respiratory  cen- 
tre rather  than  that  of  the  heart.  It  has  for  its  basis  doubtless  the  intrinsic 
molecular  changes  of  the  gray  matter,  on  whose  remarkable  constitution  we 
dwelt  in  a  previous  section  ;  the  metabolic  events  of  this  substance  are  so 
ordered  as  to  give  rise  to  discharges  of  energy ;  but  the  discharge  appears  to 
be  also  intimately  dependent  on  the  inflow  into  the  gray  matter  of  afferent 
impulses  and  influences.  The  normal  discharges  of  efferent  impulses  from 
the  cord  undoubtedly  take  place  under  the  influences  of  these  incoming  im- 
pulses ;  and  it  may  be  doubted  whether  the  gray  matter  of  the  cord  would 
be  able  in  the  absence  of  all  afferent  impulses,  to  generate  any  sustained 
series  of  discharges  out  of  its  merely  nutritive  intrinsic  changes.  The  auto- 
matic activity  of  the  cord  is  fed  not  only  by  intrinsic  nutritive  events,  but 
also  by  extrinsic  influences. 

In  this  feature  we  may,  however,  find  perhaps  the  reason  why  the  auto- 
matic activity  of  the  spinal  cord  is  so  limited  as  compared  with  that  of  the 
brain.  In  spite  of  certain  striking  but  superficial  characters  of  which  we 
shall  speak  later  on,  the  gray  matter  of  the  brain  presents  no  histological 
features  so  different  from  those  of  the  gray  matter  of  the  cord  as  to  justify 
us  in  concluding  that  the  one  is  capable  and  the  other  incapable  of  devel- 
oping the  impulses  which  we  call  volitional  out  of  the  molecular  nutritive 
changes  of  its  substance.  We  are,  therefore,  led  to  the  conclusion  that  the 
fuller  automatic  activity  of  the  brain  is  due  to  the  intrinsic  changes  of  its 
.substance  being  so  much  more  largely  assisted  by  the  influx  of  various 
afferent  impulses  and  influences,  notably  those  of  the  special  senses.  To  this 
question,  however,  we  shall  have  to  return  later  on. 

§  510.  In  treating  of  the  vascular  system  we  saw  that  the  central  ner- 
vous system  exercised  through  the  vasomotor  nerves  such  an  influence  on  the 
muscular  coats  of  the  bloodvessels  as  to  maintain  what  we  spoke  of  as 
"  tone,"  section  of  vaso-constrictor  fibres  leading  to  "  loss  of  tone."  We 
saw  further  that  arterial  tone,  though  normally  dependent  on  the  general 
vasomotor  centre  in  the  bulb,  could  be  kept  up  by  the  cord  itself,  that,  for 
instance,  a  tone  of  the  bloodvessels  of  the  hind  limbs  could  be  maintained 
by  the  isolated  dorso-lumbar  cord.  This  maintenance  of  arterial  tone  may 
be  spoken  of  as  one  of  the  "  automatic  "  functions  of  the  spinal  cord.  We 
have  also  seen  that  plain  muscular  fibres,  other  than  those  of  the  arteries, 
notably  the  fibres  forming  sphincters,  such  as  the  cardiac  and  pyloric  sphinc- 
ters of  the  stomach,  the  sphincter  of  the  bladder,  and  especially  the  sphinc- 
ter of  the  anus,  also  possess  tone,  and  that  the  tone  of  these  sphincters  is 
also  dependent  on  the  spinal  cord,  or  on  some  part  of  the  central  nervous 
system.  We  need  not  repeat  the  discussions  concerning  these  mechanisms 
and  other  instances  of  the  spinal  cord  exercising  an  automatic  influence 
over  various  viscera ;  we  have  referred  to  them  here,  since  they  serve  as 
an  introduction  to  a  question  which  been  much  debated,  and  which  has 
many  collateral  and  important  bearings,  namely,  the  question  whether  the 
spinal  cord  exercises  an  automatic  function  in  maintaining  a  tone  of  the 
skeletal  muscles. 

The  question  is  not  one  which  can  be  settled  off-hand  by  a  simple 
experiment.  Most  observers  agree  that  the  section  of  a  motor  nerve  does 
not  produce  any  clearly  recognizable  immediate  lengthening  of  a  muscle 
supplied  by  the  nerve,  in  the  same  way  that  section  of  a  vaso-constrictor 
nerve  undoubtedly  gives  rise  to  a  relaxation  of  the  muscular  fibres  in  the 
arteries  governed  by  it ;  and  it  has  been  inferred  from  this  that  skeletal 
tone  does  not  exist.  But  there  are  several  facts  to  be  taken  into  con- 
sideration before  we  can  come  to  a  just  decision. 


580  THE  SPINAL  COED. 

The  skeletal  muscles  have  been  described  as  being  placed  "  on  the  stretch  " 
in. the  living  body.  If  a  muscle  be  cut  away  from  its  attachments  at  each 
end,  it  shortens  ;  if  it  be  cut  across,  it  gapes.  In  other  words,  the-  muscle  in 
the  living  body  possesses  a  latent  tendency  to  shorten,  which  is  continually 
being  counteracted  by  its  disposition  and  attachments.  In  studying  mus- 
cular contraction  we  saw  (§  85)  that  the  shortening  of  a  contraction  is  fol- 
lowed by  a  relaxation  or  return  to  the  former  length,  both  the  contraction 
and  relaxation  being  the  result  of  molecular  changes  in  the  living  muscular 
substance.  We  have  now  to  extend  our  view  and  to  recognize  that,  apart 
from  the  occurrence  of  ordinary  contractions,  molecular  changes  are  by  means 
of  nutritive  processes  continually  going  on  in  the  muscle  in  such  a  way  that 
the  muscle,  though  continually  on  the  stretch,  does  not  permanently  lengthen, 
but  retains  the  power  to  shorten  upon  removal  or  lessening  of  the  stretch, 
and  conversely  though  possessing  this  power  of  shortening  permits  itself  to 
lengthen  when  the  stretch  is  increased.  In  this  way  the  muscle  is  able  to 
accommodate  itself  to  variations  in  the  amount  of  stretch  to  which  it  is  from 
time  to  time  subjected.  When  a  flexor  muscle,  for  instance,  contracts,  the 
antagonistic  extensor  muscle  is  put  on  an  increased  stretch  and  is  correspond- 
ingly lengthened  ;  when  the  contraction  of  the  flexor  passes  off  the  extensor 
returns  to  its  previous  length  ;  and  so  in  other  instances.  Thus  by  virtue  of 
certain  changes  within  itself  a  muscle  maintains  what  may  be  called  its  nat- 
ural length  in  the  body,  always  returning  to  that  natural  length  both  after 
being  shortened  and  after  being  stretched.  In  this  the  muscle  does  no  more 
than  do  the  other  tissues  of  the  body  which,  within  limits,  retain  their 
natural  form  under  the  varied  stress  and  strain  of  life ;  but  the  property  is 
conspicuous  in  the  muscle  ;  and  its  effects  in  skeletal  muscles  correspond  so 
closely  to  those  of  arterial  tone,  that  we  may  venture  to  speak  of  it  as  a  skel- 
etal tone.  Indeed,  the  molecular  changes  at  the  bottom  of  both  are  probably 
the  same. 

These  changes  are  an  expression  of  the  life  of  the  muscle ;  they  disappear 
when  the  muscle  dies  and  enters  into  rigor  mortis  ;  and,  moreover,  during 
life  they  vary  in  intensity  so  that  the  "  tone  "  varies  in  amount  according  to 
the  nutritive  changes  going  on.  We  have  seen  reason  to  believe  that  the 
nutrition  of  a  muscle,  as  of  other  tissues,  is  governed  in  some  way  by  the 
central  nervous  system.  We  saw,  in  treating  of  muscle  and  nerve  (§  81), 
that  the  irritability  of  a  muscle  is  markedly  affected  by  the  section  of  its 
nerve,  i.  e.t  by  severance  from  the  central  nervous  system  ;  and  again  (§  462), 
in  speaking  of  the  so-called  trophic  action  of  the  nervous  system,  we  referred 
to  changes  in  the  nutrition  of  muscles  occasioned  by  diseases  of  the  nervous 
system.  And  experience,  especially  clinical  experience,  shows  that  the  nu- 
tritive changes  which  determine  tone  are  very  closely  dependent  on  a  due 
action  of  the  central  nervous  system.  When  we  handle  the  limb  of  a  healthy 
man,  we  find  that  it  offers  a  certain  amount  of  resistance  to  passive  move- 
ments. This  resistance,  which  is  quite  independent  of,  that  is  to  say,  which 
may  be  clearly  recognized  in  the  absence  of  all  distinct  muscular  contrac- 
tions of  volitional  or  other  origin,  is  an  expression  of  muscular  tone,  of  the 
effort  of  the  various  muscles  to  maintain  their  "natural"  length.  In  many 
cases  of  disease  this  resistance  is  felt  to  be  obviously  less  than  normal ;  the 
limb  is  spoken  of  as  "  limp  "  or  "  flabby,"  or  as  having  "  a  want  of  tone."  In 
other  cases  of  disease,  on  the  other  hand,  this  resistance  is  markedly  increased ; 
the  limb  is  felt  to  be  stiff  or  rigid ;  more  or  less  force  is  needed  to  change  it 
from  a  flexed  to  an  extended,  or  from  an  extended  to  a  flexed  condition  ;  and 
in  the  range  of  disease  we  may  meet  with  very  varying  amounts  of  increased 
resistance,  from  a  condition  which  is  only  slightly  above  the  normal  to  one 
of  extreme  rigidity.  In  some  cases  the  condition  of  the  muscle  is  such  as  at 


THE  AUTOMATIC  ACTIONS  OF  THE   SPINAL  CORD.  581 

first  sight  seems  much  more  comparable  to  a  permanent  ordinary  contraction 
than  to  a  mere  exaggeration  of  normal  tone  ;  but  all  intermediate  stages  are 
met  with,  and  indeed  these  extreme  cases  may  be  taken  as  indicating  that 
the  molecular  processes  which  maintain  what  we  are  now  calling  tone  are  at 
bottom  of  the  same  nature  as  those  which  carry  out  a  contraction  ;  they 
serve  to  show  the  fundamental  identity  of  the  skeletal  tone  with  the  more 
obvious  arterial  tone. 

Clinical  experience  then  shows  that  the  central  nervous  system  does 
exert  on  the  skeletal  muscles  such  an  influence  as  to  give  rise  to  what  we  may 
speak  of  as  skeletal  tone — changes  in  the  central  nervous  system,  leading  in 
some  cases  to  diminution  or  loss  of  tone,  in  other  cases  to  exaggeration  of 
tone,  manifested  often  as  conspicuous  rigidity.  The  question  why  the 
changes  take  one  direction  in  one  case  and  another  in  another  is  one  of  great 
difficulty  (the  occurrence  of  extreme  rigidity  being  especially  obscure),  and 
cannot  be  discussed  here.  We  have  called  attention  to  the  facts  simply 
because  they  show  the  existence  of  skeletal  tone  and  its  dependence  on  the 
central  nervous  system.  This  conclusion  is  confirmed  by  experiments  on 
animals,  and  these  also  afford  proof  that  in  animals  the  spinal  cord  can  by 
itself,  apart  from  the  brain,  maintain  the  existence  of  such  a  tone.  In  a 
frog,  after  division  of  the  cord  below  the  brain,  the  limbs  during  the  period 
of  shock  are  flabby  and  toneless  ;  but  after  a  while,  as  the  shock  passes  off, 
tone  returns  to  the  muscles,  and  the  limbs  offer  when  handled  a  resistance 
like  that  of  the  limbs  of  an  entire  frog.  When  the  animal  is  suspended  the 
hind  limbs  do  not  hang  perfectly  limp  and  helpless,  but  assume  a  definite 
position  ;  and  that  this  position  is  due  to  some  influence  proceeding  from  the 
spinal  cord  is  shown  by  dividing  the  sciatic  nerve  on  one  side  ;  the  hind  limb 
on  that  side  now  hangs  quite  helpless.  This  more  pendent  position  shows 
that  some  of  the  flexors  have  lengthened  in  consequence  of  the  section  of 
the  nerve,  and  this  result  may  be  taken  as  refuting  the  argument,  quoted 
above  against  the  existence  of  tone,  which  is  based  on  the  statement  that  a 
muscle  cannot  be  observed  to  lengthen  after  section  of  its  nerve.  It  may  be 
here  remarked  that  if  the  brainless  frog,  whose  hind  limbs  are  more  or  less 
pendent  when  the  body  is  suspended,  be  placed  on  its  belly,  the  hind  limbs 
are  brought  into  a  flexed  position  under  the  body  by  means  of  obvious  mus- 
cular contraction  ;  and  from  this  it  might  be  inferred  that  the  maintenance 
of  the  position  of  the  pendent  limb  was  also  the  result  of  a  feeble  contrac- 
tion. But  no  obvious  contractions  can  be  observed  in  the  latter  case,  as  in 
the  former ;  and  when  in  the  former  the  limb  has  once  been  brought  into  the 
flexed  position,  that  position,  like  the  pendent  position,  is  maintained  without 
obvious  contractions.  As  we  said  above,  "tone"  may  pass  into  something 
which  appears  to  be  identical  with  a  contraction,  but  where  no  obvious  con- 
tractions are  observed  it  seems  preferable  to  speak  of  the  state  of  the 
muscle  as  one  of  tone. 

In  the  dog,  after  division  of  the  cord  in  the  thoracic  region,  the  hind 
limbs  during  the  period  of  shock  are  limp  and  toneless.  In  the  warm- 
blooded animal,  as  we  have  said,  the  effects  of  shock  are  much  more  lasting 
than  in  the  cold-blooded  animal ;  and  in  the  dog  the  tone  of  the  skeletal 
muscle  returns  much  more  slowly  than  in  the  frog.  Indeed,  when  the  divis- 
ion of  the  cord  has  taken  place  low  down  the  skeletal  tone  returns  very 
slowly,  and  may  be  manifested  very  feebly,  or  even  be  absent  altogether. 
But  under  favorable  circumstances,  when  a  sufficient  length  of  cord  has  been 
left,  a  fairly  normal  tone  is  reestablished.  In  man,  in  accordance  with  the 
facts  previously  mentioned  (§  504),  skeletal  tone,  which  has  been  lost  through 
the  continuity  of  the  cord  beino;  broken  by  disease  or  accident,  appears 
rarely  if  ever  to  return  fully  in  the  regions  below  the  lesion. 


582  THE  SPINAL  CORD. 

We  may  therefore  on  the  whole  of  the  evidence  conclude  that  the  main- 
tenance of  skeletal  tone  is  one  of  the  functions  of  the  cord  ;  but  we  may  here 
repeat  that  the  condition  of  the  cord,  on  which  depends  the  issue  from  the 
cord  along  the  efferent  cords  of  the  influences,  whatever  their  nature,  which 
produce  tone  in  the  muscle,  may  be,  and  indeed  is,  in  its  turn  dependent  on 
afferent  impulses.  In  the  case  of  the  frog  quoted  above  the  tone  of  the  pen- 
dent limbs  disappears  or  is  greatly  lessened  when  the  posterior  roots  of  the 
sciatic  nerves  are  divided,  though  the  anterior  roots  be  left  intact.  In  the 
absence  of  the  usual  stream  of  afferent  impulses  passing  into  it,  the  cord 
ceases  to  send  forth  the  influences  which  maintain  the  tone.  Hence  the 
maintenance  of  tone  presents  many  analogies  with  a  reflex  action,  especially 
when  we  remember  that,  as  stated  above,  tone  passes  insensibly  into  con- 
traction ;  and  it  may  be  a  mere  matter  of  words  whether  we  speak  of  the 
maintenance  of  tone  as  an  automatic  or  as  a  reflex  action  of  the  cord. 
We  may,  however,  distinguish  the  part  played  by  the  afferent  impulses  in 
assisting  the  cord  to  a  condition  in  which  it  is  capable  of  maintaining  tone 
from  the  part  played  by  an  afferent  impulse  in  causing  a  reflex  action  ;  in 
the  former  the  action  of  the  afferent  impulses  seems  analogous  to  that  of  a 
supply  of  arterial  blood  in  maintaining  an  adequate  irritability  of  the  ner- 
vous substance,  in  the  latter  the  afferent  impulses  lead  directly  to  a  discharge 
of  energy.  And  it  is  convenient  to  distinguish  the  two  things  by  different 
names. 

§  511.  The  close  connection  between  tone  and  reflex  action  is  illustrated 
by  the  so-called  "tendon  phenomena,"  which,  on  the  one  hand,  are  considered 
as  cases  of  ordinary  reflex  action,  and,  on  the  other  hand,  have  been  regarded 
as  exemplifying  a  special  influence  of  the  spinal  cord  on  the  irritability  of 
the  muscle.  It  is  well  known  that  when  the  leg  is  placed  in  an  easy  posi- 
tion, resting  for  instance  on  the  other  leg,  a  sharp  blow  on  the  patellar  tendon 
will  cause  a  sudden  jerk  forward  of  the  leg  brought  about  by  a  contraction 
of  the  quadriceps  femoris ;  it  is  necessary  or  at  least  desirable  for  a  good 
development  of  the  jerk,  that  the  tendon  (and  muscle)  should  be  somewhat 
on  the  stretch.  Similarly  the  muscles  of  the  calf  may  be  thrown  into  action 
by  tapping  the  tendo  Achillis,  put  somewhat  on  the  stretch  by  flexion  of  the 
foot ;  and  in  some  cases  the  same  muscles  may  be  made  to  execute  a  series  of 
regular  rhythmatic  contractions,  called  "  clonic  "  contractions,  by  suddenly 
pressing  back  the  sole  of  the  foot,  so  as  to  put  them  on  the  stretch.  These, 
and  other  instances  of  a  like  kind,  at  first  sight  appear  to  be,  and  indeed  are 
by  many  observers  maintained  to  be,  cases  of  reflex  action,  due  to  afferent 
impulses  started  in  the  tendon  ;  hence  they  have  been  frequently  spoken  of 
as  "  tendon-reflex."  Other  observers  maintain  that  they  are  not  reflex,  but 
due  to  direct  stimulation  of  the  muscles,  the  vibrations  set  up  in  the  more  or 
less  tense  tendon  being  transmitted  to  the  muscles,  and  so  throwing  the  latter 
into  contractions.  The  chief  arguments  against  their  being  reflex  are  that 
the  interval  between  the  tap  and  the  contraction  is  very  short  (0.03  or  0.04 
second),  shorter  than  the  ordinary  interval  of  a  reflex  action  (§  507),  and 
that  the  movement  persists  after  section  of  the  nerves  of  the  tendon.  The 
first  argument  is  perhaps  not  a  very  strong  one,  and  the  second  may  be  met 
by  supposing  that  in  such  a  case  at  least,  if  not  always,  the  reflex  act  really 
begins  in  the  muscle,  being  started  in  it  by  the  vibrations  transmitted  to  it 
along  the  tendon. 

But  even  if  we  admit  that  the  movements  are  purely  muscular,  started 
and  carried  out  in  the  muscle  without  the  help  of  the  usual  reflex  chain  of 
afferent  impulses,  spinal  centre,  and  efferent  impulses,  we  must  at  the  same 
time  admit  that  they  are  closely  dependent  on  the  integrity  of  the  spinal 
cord  and  of  the  connections  between  the  cord  and  the  muscle.  In  the  case 


THE  AUTOMATIC  ACTIONS  OF  THE  SPINAL  CORD.  583 

of  animals  they  disappear  when  the  spinal  cord  is  destroyed,  or  the  nerves 
going  to  the  muscles  are  severed,  or  even  when  the  posterior  roots  only  are 
divided.  The  measure  of  their  development  both  in  animals  and  in  man  is 
also  closely  dependent  on  the  condition  of  the  spinal  cord  and  of  the  central 
nervous  system  generally.  They  may  be  increased  or  diminished,  augmented 
or  inhibited  by  a  coincident  voluntary  effort  directed  toward  some  other  end, 
or  by  the  coincident  development  of  a  sufficiently  distinct  sensation.  In 
general  it  may  be  said  that  whatever  favors  the  activity  of  the  spinal  cord 
tends  to  increase  them,  and  whatever  depresses  the  activity  of  the  spinal  cord 
tends  to  diminish  them.  They  are  diminished  or  wanting  in  certain  diseases 
of  the  spinal  card  (<?.  </.,  locomotor  ataxia)  and  exaggerated  in  others  ;  so  much 
so  indeed  that  they  have  become  of  practical  clinical  importance  as  a  means 
of  diagnosis.  Whether  we  regard  them  as  instances  of  ordinary  reflex 
action,  or  consider  that  they  are  carried  out  by  the  muscle  itself  and  that 
the  cord  intervenes  only  so  far  as  to  increase,  maintain,  or  diminish  the  irri- 
tability of  the  muscular  substance,  it  remains  good  that  they  are  prominent 
whenever  the  conditions  increase  the  reflex  or  other  excitability  of  the  cord, 
and  dimmish  or  disappear  when  the  conditions  lower  or  abolish  that  excita- 
bility. 

§  512.  Disease  in  man  reveals  other  actions  of  the  spinal  cord  which 
bear  features  different  from  those  of  an  ordinary  reflex  movement,  and  yet 
have  been  described  as  reflex  in  nature.  For  instance,  certain  affections  of 
the  cord  are  characterized  by  the  legs  becoming  rigid  in  extreme  extension, 
the  rigidity  of  the  straightened  limbs  being  often  so  great  that  when  a  by- 
stander lifts  up  one  leg  from  the  bed  the  other  leg  is  raised  at  the  same 
time.  The  rigidity  is  due  to  the  extensor  muscles  being  thrown  into  a  state 
of  contraction,  which  is  so  uniform  and  long  continued  that  it  may  be 
spoken  of  as  a  "tonic"  contraction;  such  a  tonic  rigidity  may,  however, be 
replaced  by  a  series  of  rhythmic,  "  clonic  "  contractions.  It  has  sometimes 
been  observed  that  the  limbs  when  flexed  are  supple  and  free  from  rigidity, 
but  that  rigidity  sets  in  so  soon  as  they  are  brought  into  the  position  of  ex- 
tension, the  leg  becoming  suddenly  fixed  and  straight  somewhat  in  the  way 
that  a  clasp-knife  springs  back  when  opened  It  seems  clear  that  the  pecu- 
liar contraction  is  carried  out  by  means  of  the  spinal  cord,  but  the  whole 
action,  though  it  is  often  spoken  of  as  a  "  muscle-reflex,"  is  very  unlike  an 
ordinary  reflex  movement.  In  an  ordinary  movement  an  extensor  is 
brought  into  action  when  a  limb  is  flexed — not  when  it  is  already  ex- 
tended ;  and  if  in  a  reflex  act  the  condition  of  the  muscle  about  to  be 
thrown  into  action  determines  in  any  way  the  discharge  of  impulses  from 
the  reflex  centre,  we  should  expect  that  the  stretching  of  an  extensor 
muscle  by  flexion — not  its  relaxation  by  extension — would  determine  the 
discharge  of  extensor  impulses.  In  the  case  of  the  diseases  in  question 
just  the  opposite  seems  to  take  place  ;  the  position  which  appears  to  deter- 
mine the  development  of  the  remarkable  contraction  is  precisely  that  in 
which  the  strain  upon  the  extensors  is  at  its  minimum.  It  may  be  doubted, 
therefore,  whether  the  word  reflex  should  be  used  to  denote  such  phenom- 
ena ;  but  the  phenomena  themselves  deserve  attention,  especially,  perhaps, 
as  showing  how  in  the  disorders  of  the  gray  matter  of  the  cord  due  to  dis- 
ease impulses  or  influences  which  are  latent  only  in  health  become  actual 
and  effective. 

It  remains  for  us  to  speak  of  the  part  played  by  the  spinal  cord,  as  the 
instrument  of  the  brain,  in  the  execution  of  voluntary  movements  and  in  the 
development  of  conscious  sensations  ;  but  it  will  be  best  to  consider  these 
matters  in  connection  with  the  brain  itself,  to  the  study  of  which  we  must 
now  turn. 


584  THE   BRAIN. 

CHAPTER    II. 

THE  BRAIN. 
ON  SOME  GENERAL  FEATURES  OF  THE  STRUCTURE  OF  THE  BRAIN. 

§  513.  IT  would  be  out  of  place  to  attempt  to.  give  here  a  complete  de- 
scription of  the  structure  of  the  brain ;  but  certain  features  must  be  kept 
fresh  in  the  mind  as  a  basis  for  physiological  discussion  ;  and  to  these  we 
must  now  turn  our  attention,  a  general  acquaintance  with  the  topographical 
anatomy  of  the  brain  being  presupposed.1 

Like  the  spinal  cord,  the  brain  consists  of  "  white  matter,"  in  which  the 
nervous  elements  are  almost  exclusively  medullated  fibres,  and  of  "  gray 
matter,"  in  which  nerve-cells  and  other  nervous  elements  are  also  present ; 
but  the  gray  matter  of  the  brain  is  much  more  variable  in  structure  than 
that  of  the  spinal  cord,  and  possesses  features  peculiar  to  itself;  these  we 
shall  study  later  on. 

For  physiological  purposes  the  brain  may  be  conveniently  divided  into 
parts  corresponding  to  the  divisions  which  appear  in  it  in  the  embryo.  At 
an  early  stage  in  the  life  of  the  embryo,  that  part  of  the  medullary  tube 
which  is  about  to  become  the  brain  differs  from  that  which  is  about  to 
become  the  spinal  cord,  in  that  the  central  canal,  which  in  the  latter  is  of 
fairly  uniform  bore  along  its  whole  length,  is  in  the  former  alternately 
widened  and  narrowed,  so  that  the  tube  forms  a  series  of  vesicles,  the  cerebral 
vesicles,  succeeding  each  other  lengthways.  At  first  these  vesicles  are  three 
in  number,  called  respectively  fore-brain,  mid-brain,  and  hind-brain  ;  but  the 
fore-brain,  after  having  developed  on  each  side  a  lateral  vesicle,  the  optic 
vesicle,  subsequently  transformed  into  the  retina  and  optic  nerve,  gives  rise 
in  front  of  itself  to  a  pair  of  vesicles  placed  side  by  side,  or  rather  to  a  single 
vesicle  with  a  deep  median  furrow,  the  vesicle  of  the  cerebrum,  containing  a 
cavity  divided  by  a  median  partition  into  two  cavities,  lying  side  by  side, 
which  open  into  the  cavity  of  the  original  fore-brain  by  a  Y-shaped  opening. 
This  embryonic  chain  of  vesicles  is  developed  into  the  adult  brain  by  unequal 
growth  of  the  walls  and  unequal  expansion  of  the  cavities,  certain  features 
being  also  impressed  upon  it  by  the  bend  on  the  longitudinal  axis,  which 
takes  place  in  the  region  of  the  mid-brain  and  is  known  as  the  cranial 
flexure. 

§  514.  In  the  hind  part  of  the  hinder  vesicle  or  hind-brain,  the  ventral, 
basal  portion  or  floor  is  thickened  to  form  the  bulb,  while  the  greater  part  of 
the  dorsal  portion  or  roof  does  not  thicken  at  all,  is  not  transformed  into 
nervous  elements,  but  remains  as  a  single  layer  of  epithelium,  adherent  to 
the  pia  mater  overlying  it,  and  so  forms  a  thin  covering  to  the  lozenge- 
shaped  cavity  of  the  vesicle,  now  known  as  the  fourth  ventricle. 

In  the  front  part  of  the  same  hind-brain,  on  the  contrary,  the  roof  and 
sides  are  enormously  developed  into  the  conspicuous  cerebellum  overhanging 
the  front  part  of  the  fourth  ventricle,  while  the  floor  is  also  thickened  into 
the  pons  Varolii. 

This  thickening  of  the  pons  is  largely  made  up,  on  the  one  hand,  of  hori- 
zontal nerve-fibres,  which  run  transversely  from  each  side  of  the  cerebellum 
into  the  pons,  or  from  one  side  of  the  cerebellum  to  the  other,  and,  on  the 
other  hand,  of  longitudinal  fibres,  which  run  forward  from  the  bulb  and  are 

1  Figs.  131  and  140.  which  will  be  found  in  succeeding  sections,  may  with  advantage 
be  consulted  in  reading  this  section,  though  not  specially  referred  to  in  the  text. 


STRUCTURE  OF  THE  BRAIN.  585 

wrapped  round  by  and  interlaced  with  the  others.  At  the  front  margin  of 
the  pons  these  longitudinal  fibres,  augmented  in  number,  appear  as  two  thick 
strands,  the  crura  cerebri,  forming  the  floor  of  the  mid-brain,  the  roof  of 
which  is  thickened  into  the  corpora  qiiadrigemina,  and  the  cavity  of  which 
is  reduced  to  a  narrow  tubular  passage,  the  aqueduct  of  Sylvius,  or  iter  a  tertio 
ad  quartum  ventriculum. 

At  the  level  of  the  fore-brain  the  crura  cerebri,  diverging  rapidly  from 
each  other  as  they  pass  forward,  leave  the  median  portion  of  the  floor  of  the 
vesicle  now  known  as  the  third  ventricle  very  thin,  but  form,  especially  behind 
and  ventrally,  thick  lateral  walls,  which  are  further  increased  in  thickness 
by  the  development  on  each  side  of  a  mass  largely  composed  of  gray  matter, 
known  as  the  optic  thalamus.  The  roof  of  the  third  ventricle,  like  that  of 
the  fourth  ventricle,  is  not  developed  into  nervous  elements,  but  remains 
extremely  thin,  and  consists  of  nothing  more  than  a  single  layer  of  epi- 
thelium. 

§  515.  In  front  of  the  third  ventricle  each  diverging  crus  cerebri  spreads 
out  in  a  small  radial  fashion  into  the  corresponding  half  of  the  paired  vesicle 
of  the  cerebellum  now  developed  into  the  preponderant  cerebral  hemispheres, 
the  two  cavities  of  which  are  now  known  as  the  lateral  ventricles.  The 
growth  of  the  cerebral  hemispheres  is  not  only  much  greater  than  that  of  the 
rest  of  the  brain,  but  also  takes  place  in  a  special  manner.  At  their  first 
appearance  the  cerebral  hemispheres  lie  wholly  in  front  of  the  fore-brain  or 
vesicle  of  the  third  ventricle,  but  in  their  subsequent  growth,  while  expand- 
ing in  nearly  all  directions,  they  extend  especially  backward.  Thus,  in  the 
adult  brain,  on  the  dorsal  surface  they  not  only  completely  cover  up  the 
third  ventricle  but  also  overlap  the  mid-brain,  reaching  so  far  back  as  to 
cover  the  front  border  of  the  cerebellum,  while  on  the  ventricle  surface, 
though  in  the  middle  line  they  leave  exposed  the  floor  or  ventral  portions 
of  the  walls  of  the  third  ventricle,  at  the  sides  they  are  seen  to  reach  as  far 
backward  as  on  the  dorsal  surface.  The  median  furrow  on  the  dorsal  sur- 
face which  separates  each  hemisphere  from  its  fellow  is  at  first  shallow,  but 
rapidly  deepens,  so  that  as  the  hemispheres  grow  they  become  separated 
from  each  other  by  a  narrow,  deep  longitudinal  fissure,  into  which,  as  we 
shall  see,  a  fold  of  the  dura  mater  dips.  This  fissure  is  not  only  deep  ver- 
tically— i.  e.,  from  the  dorsal  surface  ventrally — but  at  the  front  of  the 
brain  runs  backward  in  the  middle  line  almost  as  far  as  the  level  of  the 
third  ventricle,  so  as  completely  to  separate  from  each  other  the  anterior 
parts  of  each  hemisphere,  known  as  the  anterior  lobes ;  at  the  back  of  the 
brain  also  it  similarly  runs  forward  in  the  middle  line  for  a  considerable 
distance,  so  as  to  separate  from  each  other  the  posterior  lobes.  Hence  the 
two  great  masses  of  the  cerebral  hemisphere  are  united  with  each  other,  not 
along  their  whole  length,  but  for  about  a  third  of  that  length,  the  isthmus 
or  bridge  thus  connecting  them  lying  at  some  depth  below  the  dorsal  sur- 
face at  the  bottom  of  the  longitudinal  fissure,  in  about  the  middle  third  of 
its  length. 

At  its  first  appearance  each  lateral  ventricle  is  of  a  more  or  less  oval 
form,  its  walls  are  of  uniform  thickness,  and  it  lies  in  front  of  the  third  ven- 
tricle. During  the  growth  of  the  hemispheres  it  acquires  a  peculiar  shape 
and  becomes  divided  into  an  anterior  cornu  or  horn  stretching  into  the  anterior 
portion,  a  posterior  horn  stretching  into  the  posterior  portion,  and  a  descend- 
ing horn,  which  curves  laterally  and  ventrally  into  the  middle  portion  of  the 
hemisphere ;  owing  to  the  great  backward  extension  of  the  hemispheres  the 
lateral  ventricles  come  to  lie  not  only  in  front  of  but  also  at  the  side  of,  and 
indeed,  to  a  certain  extent,  above  or  dorsal  to  the  third  ventricle ;  and  dur- 
ing the  growth  of  the  parts  the  originally  wide  Y-shaped  opening  which 


586  THE  BRAIN. 

placed  the  hind  ends  of  the  two  lateral  ventricles  in  communication  with  the 
front  of  the  third  ventricle  becomes  narrowed  into  a  slit-like  passage  of  sim- 
ilar form,  the  foramen  of  Monro,  which  still  opening  into  the  front  of  the 
third  ventricle,  now  leads  on  each  side  from  a  point  rather  in  front  of  the 
middle  of  the  lateral  ventricle. 

As  the  hemisphere  enlarges,  the  growth  of  the  walls  of  the  vesicle  is  not 
uniform  in  all  parts.  At  an  early  period  there  may  be  observed  in  the  ven- 
tral wall  or  floor  of  the  vesicle  a  thickening,  which  assuming  a  special,  more 
or  less  semilunar,  form  and  projecting  into  the  cavity  becomes  the  body 
known  as  the  corpus  striatum.  As  development  proceeds  the  corpus  striatum 
on  each  side  becomes  attached  to  the  optic  thalamus,  lying  behind  and  to  the 
median  side  of  itself,  the  radiating  fibres  of  the  cms  cerebri  passing  between 
the  two,  and  also  as  we  shall  see  dividing  the  corpus  striatum  into  two  bodies, 
called  the  nucleus  caudatas  and  nucleus  lenticular  is.  A  notable  result  of  this 
growth  and  change  of  position  of  the  hemispheres  and  of  the  coalescence  of 
the  corpus  striatum  with  the  optic  thalamus  is  that  the  latter  body,  though 
really  belonging  to  the  third  ventricle,  comes  to  project  somewhat  into  the 
lateral  ventricle ;  a  strip  of  the  upper  surface  of  the  optic  thalamus,  along 
its  outer,  lateral  edge,  forms  a  portion  of  the  floor  of  the  lateral  ventricle  in 
the  median  region  on  each  side  of  the  third  ventricle.  Besides  this  special 
development  of  the  corpus  striatum,  the  walls  of  each  vesicle,  with  the 
exception  of  the  median  part  by  which  the  two  vesicles  coalesce  with  each 
other,  become  (we  are  now  speaking  of  the  higher  mammals)  thickened 
much  in  the  same  way  all  over,  the  surface  being  folded  so  as  to  give  rise  to 
convolutions  or  gyri  separated  by  furrows  or  sulci;  and  the  thickening 
taking  place  in  such  a  way  as  to  give  the  ventricle  its  peculiar  shape.  The 
median  coalesced  part  undergoes  a  different  and  peculiar  change.  This  part, 
which  at  first  lies  in  front  of  the  third  ventricle,  through  the  changes 
brought  about  by  the  growth  of  the  hemispheres  so  shifts  its  position  as  to 
lie  immediately  over,  dorsal  to  third  ventricle,  very  much  as  if  this  part  of 
the  cerebral  vesicles  had  been  folded  back  over  the  fore-brain.  In  the 
junction  itself  we  may  distinguish  a  dorsal  and  a  ventral  portion.  The 
dorsal  portion  is  developed  into  a  system  of  transverse  commissural  fibres 
passing  across  from  one  hemisphere  to  the  other.  In  the  median  region  these 
fibres  form  a  thick  compact  band,  called  the  corpus  callosum,  which  may  be 
exposed  to  view  at  the  bottom  of  the  longitudinal  fissure,  while  on  each  side 
they  spread  away  in  all  directions  to  nearly  all  parts  of  the  surface  of  the 
hemispheres,  passing  over  and  helping  to  form  the  roof  of  the  lateral  ven- 
tricles. The  band  is  not  flat  but  curved  ventralward ;  hence  in  a  longitudi- 
nal vertical  section  of  the  brain  taken  in  the  middle  line  it  presents  a  curved 
form  with  the  concavity  directed  ventralward.  While  this  dorsal  portion  of 
the  junction  is  developed  at  the  sides  as  well  as  in  the  middle  line,  the  ven- 
tral portion  is  developed  in  the  median  region  only,  and  that  in  a  special 
way,  so  that  it  forms  below,  ventral  to,  the  corpus  callosum  an  arched  plate, 
in  the  shape  of  a  triangle  with  the  apex  directed  forward,  called  the  fornix, 
which  lies  immediately  above  the  thin  epithelial  roof  of  the  third  ventricle. 
In  front,  the  narrower  apical  portion  of  the  fornix  lies  at  some  little  distance 
below,  ventral  to,  the  corpus  callosum,  and  here  the  junction  between  the 
two  vesicles  is  reduced  to  a  thin  sheet,  the  septum  lucidum;  but  behind,  the 
broader  basal  portion  of  the  fornix  is  arched  up  so  as  to  lie  immediately 
under  and  touch  the  corpus  callosum.  Hence  the  septum  lucidum  has  the 
form  of  a  more  or  less  triangular  vertical  sheet,  broad  in  front  and  narrow- 
ing behind,  separating  the  two  lateral  ventricles.  The  sheet  may  be  con- 
ceived of  as  being  double  and  formed  by  the  apposition  of  two  layers,  one 
belonging  to  each  ventricle  ;  between  these  two  layers  is  developed  a  narrow 


STRUCTURE  OF  THE  BRAIN.  587 

closed  cavity  containing  fluid,  called  the  fifth  ventricle.  But  while  the  lateral 
ventricles  open  by  the  foramen  of  Monro  into  the  third  ventricle,  and  the 
third  ventricle  is  continuous  by  means  of  the  aqueduct  with  the  fourth  ven- 
tricle, which  again  passes  into  the  central  canal  of  the  spinal  cord,  the  whole 
series  being  developed  out  of  the  same  embryonic  neural  canal,  the  fifth  ven- 
tricle communicates  with  none  of  them  ;  it  is  a  cavity  of  different  origin. 

The  corpus  callosum  or  dorsal  portion  of  the  junction  between  the  vesi- 
cles spreads  out,  as  we  have  said,  laterally  along  its  whole  length,  and  thus 
forms  a  broad  band  joining  the  two  hemispheres  together ;  the  middle  por- 
tion spreads  out  in  a  more  or  less  straight  direction,  though  curving  over  the 
ventricle  upward  and  downward  to  reach  various  parts  of  the  hemisphere, 
while  the  front  and  hind  ends  bend  round  on  each  side  forward  and  back- 
ward to  reach  the  anterior  and  posterior  parts.  Thus  through  the  corpus 
callosum  the  thick  \\all  of  one  ventricle  is  made  continuous  with  that  of  the 
other.  The  disposition  of  the  fornix  or  ventral  portion  of  the  junction  is 
very  different.  At  its  apex  in  front  the  fornix  bifurcates  into  two  bands, 
known  as  the  pillars  of  the  fornix,  which  on  each  side  become  continuous 
with,  and  take  a  peculiar  course  in  the  walls  of  the  third  ventricle.  In  like 
manner  behind,  the  angles  of  the  base  of  the  fornix  are  continuous  with  the 
walls  of  the  lateral  ventricles,  that  is  to  say,  with  the  thick  mass  of  the 
hemispheres,  being  also  prolonged  as  two  special  strands  of  fibres  called  the 
crura  of  the  fornix.  But  along  each  side  of  the  triangle,  between  the  attach- 
ments in  front  and  behind,  the  substance  of  the  fornix  is  not  continued  into 
the  substance  of  the  corresponding  hemisphere ;  the  edge  of  the  fornix 
appears  on  each  side  to  lie  loose  on  the  dorsal  surface  of  the  optic  thalamus, 
which  here  forms  the  median  portion  of  the  floor  of  the  lateral  ventricle ; 
between  the  optic  thalamus  below  and  the  fornix  above  there  seems  to  be  a 
narrow  slit  by  which  the  cavity  of  the  lateral  ventricle  communicates  with 
parts  outside  itself.  In  reality,  however,  there  is  no  actual  breach  of  con- 
tinuity though  there  is  a  breach  of  nervous  substance.  The  slit  is  bridged 
over  by  a  layer  of  epithelium,  by  means  of  which  the  edge  of  the  fornix  is 
made  continuous  with  the  upper  surface  of  the  optic  thalamus,  and  the 
median  wall  of  the  lateral  ventricle  made  complete.  But  this  layer  of 
epithelium  has  the  following  peculiar  relations  to  the  pia  mater  covering  the 
brain : 

We  have  said  that  the  roof  of  the  third  ventricle,  like  that  of  the  fourth 
ventricle,  consists  only  of  a  layer  of  epithelium  devoid  of  nervous  elements. 
We  have  further  seen  that  the  fornix  and  the  hind  part  of  the  corpus  cal- 
losum with  which  it  is  continuous  overlie  the  third  ventricle,  the  free  base 
of  the  fornix  with  the  rounded  hind  end  of  the  corpus  callosum  above  form- 
ing together  the  hind  border  of  the  junction  or  bridge  between  the  two  hemi- 
spheres. The  pia  matter  covering  the  dorsal  surface  of  the  brain,  passing 
forward  under  this  curved  border,  spreads  over  the  top  of  the  third  ventricle, 
becoming  adherent  to  the  layer  of  epithelium  just  referred  to,  and  thus 
forms  a  vascular  sheet  called  the  velum  interposition,  which  serves  as  the 
actual  roof  of  the  third  ventricle,  immediately  below,  ventral  to,  the  fornix  ; 
it  cannot  be  seen  without  previously  removing  the  fornix.  At  the  lateral 
edge  of  the  fornix,  on  each  side,  this  same  vascular  sheet  of  pia  mater  pro- 
jects from  beneath  the  fornix  into  the  lateral  ventricle,  carrying  with  it  the 
layer  of  epithelium  which,  as  we  said,  made  the  edge  of  the  fornix  actually 
continuous  with  the  rest  of  the  walls  of  the  lateral  ventricle;  the  part  of  the 
pia  mater  thus  seen  projecting  beyond  the  edge  of  the  fornix  when  the 
lateral  ventricle  is  laid  open  is  called  the  choroid  plexus.  To  this  peculiar 
intrusion  of  the  pia  mater,  by  which  the  nutrition  of  the  brain  is  assisted, 
we  shall  return  when  we  come  to  speak  of  the  vascular  arrangements  of  the 


588  THE  BRAIN. 

brain.  Meanwhile  we  may  point  out,  that  while  this  vascular  ingrowth  seems 
to  make  the  cavity  of  the  third  ventricle  continuous  with  that  of  the  lateral 
ventricle  on  each  side,  and  all  three  with  the  exterior  of  the  brain,  it  really 
does  not  do  so.  The  cavity  of  the  third  ventricle  is  made  complete  by  the 
layer  of  epithelium  forming  its  roof,  and  the  cavity  of  the  lateral  ventricle 
is  made  complete  by  the  layer  of  epithelium  passing  from  the  lateral  edge  of 
the  fornix  over  the  choroid  plexus  to  the  other  parts  of  the  wall  of  the 
ventricle.  To  pass  along  this  line  from  the  actual  cavity  of  the  lateral  into 
that  of  the  third  ventricle  one  must  first  pierce  the  epithelium  covering  the 
choroid  plexus,  thus  gaining  access  to.  the  pia  mater  of  the  plexus  and  of  the 
velum,  and  then  again  pierce  the  epithelium  coating  the  under  surface  of 
the  vellum  and  forming  the  roof  of  the  third  ventricle  It  is  only  by  the 
foramen  of  Munro  that  a  real  communication  exists  between  the  cavity  of 
the  lateral  and  that  of  the  third  ventricle. 

Thus  by  the  large  growth  and  backward  extension  of  the  cerebral  hemi- 
spheres, the  third  ventricle  comes  to  form,  as  it  were  the  front  end  of  the 
cerebro-spinal  axis,  the  crura  cerebri  expanding  on  each  side  of  the  third 
ventricle  into  the  cerebral  hemispheres  which  cover  up  the  ventricle  on  the 
dorsal  surface,  but  leave  its  wall  exposed  on  the  ventral  surface.  Attached 
to  the  dorsal  surface  of  the  third  ventricle  at  its  hind  end,  ventral  to  and 
somewhat  projecting  beyond  the  base  of  the  fornix,  lies  the  pineal  gland 
with  its  attachments,  the  remnants  of  a  once-important  median  organ  ;  and 
attached  to  the  ventral  surface  of  the  ventricle,  at  the  apex  of  a  funnel- 
shaped  projection,  the  infundibulum,  lies  the  pituitary  body,  also  a  remnant 
of  important  ancestral  structures. 

§  516.  We  may  then  divide  the  whole  brain  into  a  series  of  parts  corre- 
sponding to  the  main  divisions  of  the  embryonic  brain.  At  the  front  lie 
the  cerebral  hemispheres,  with  the  lateral  ventricles,  developed  out  of  the 
cerebral  vesicles ;  and  with  these  are  associated  the  corpora  striata,  the  term 
cerebral  hemisphere  being  sometimes  used  so  as  to  include  these  bodies,  and 
sometimes  so  as  to  exclude  them.  Next  come,  corresponding  to  the  original 
fore-brain,  the  parts  forming  the  walls  of  the  third  ventricle,  conspicuous 
among  which  are  the  optic  thalami ;  for  these  bodies,  though  they  appear 
to  intrude  into  the  lateral  ventricles,  belong  properly  to  the  third  ventricle. 
In  the  mid-brain  which  follows,  the  cavity,  now  the  tubular  passage  of  the 
aqueduct,  is  roofed  in  by  the  two  pairs,  anterior  and  posterior,  or  corpora 
quadrigemina,  the  dimensions  of  which  are  not  very  great ;  but  a  thick 
floor  is  furnished  by  the  crura  cerebri.  In  each  crus  we  must  distinguish 
between  a  dorsal  portion  called  the  tegmentum,  in  which  a  large  quantity 
of  gray  matter  is  present,  and  in  which  a  great  complexity  in  the  arrange- 
ment of  fibres  exists,  and  a  ventral  portion,  the  pes  or  crusta,  which  is  a 
much  more  uniform  mass  of  longitudinally  disposed  fibres.  As  the  crura 
passing  forward  diverge  into  the  cerebral  hemisphere  on  each  side,  the  teg- 
mentum ceases  at  the  hinder  end  and  ventral  parts  of  the  optic  thalamus ; 
it  is  the  pes  which  supplies  the  mass  of  fibres  radiating  into  each  cerebral 
hemisphere.  In  a  view  of  the  ventral  surface  of  the  brain,  the  base  of  the 
brain  as  it  is  frequently  called,  the  crura  may  be  seen  emerging  from  the 
anterior  border  of  the  pons.  This  we  have  spoken  of  as  the  thickened  floor 
of  the  front  part  of  the  hind-brain,  but  in  reality  it  encroaches  a  little  on 
the  mid-brain,  the  hind  part  of  the  corpora  quadrigemina  being  in  the  same 
dorso-ventral  plane  as  the  front  part  of  the  pons.  (See  Fig.  131.)  In  the 
main,  however,  the  pons  belongs  to  the  fore  part  of  the  hind-brain,  the  roof 
and  sides  of  which  are  developed,  as  we  have  said,  into  the  cerebellum. 
This  superficially  resembles  the  cerebral  hemispheres  in  its  large  size,  and 
in  the  special  development  of  its  surface,  which  is  formed  of  gray  matter 


THE  BULB.  589 

folded  in  a  remarkable  manner  and  often  spoken  of  as  cortex.  The  cere- 
bellum, though  the  lateral  portions,  called  the  hemispheres,  project  above 
the  median  portion,  called  the  vermis,  is,  unlike  the  cerebrum,  a  single 
mass ;  each  lateral  half,  however,  sends  down  ventrally  a  mass  of  fibres 
which,  running  transversely,  partly  end  in  thepons  and  partly  are  continued 
across  the  pons  into  the  other  lateral  half;  this  mass  of  fibres,  thus  con- 
stituting, as  we  have  said,  a  considerable  part  of  the  pons,  forms  on  each 
side,  just  as  it  leaves  the  cerebellum  to  enter  the  pons,  a  thick  strand, 
called  the  middle  peduncle  of  the  cerebellum.  From  the  cerebellum  there 
also  proceeds  backward  into  the  bulb  on  each  side  a  thick  strand  of  fibres, 
the  inferior  peduncle  of  the  cerebellum  or  restiform  body ;  and  a  third 
strand,  the  superior  peduncle  of  the  cerebellum,  passes  forward  on  each 
side  into  the  region  of  the  corpora  quadrigemina.  As  the  cerebellar 
peduncles  converge  behind  the  corpora  quadrigemina  the  angle  between 
them  is  filled  up  by  a  thin  sheet  of  nervous  matter,  the  valve  of  Vieussens, 
which  thus  for  a  little  distance  backward  forms  a  roof  for  the  front  part  of 
the  fourth  ventricle,  just  where  the  lozenge-shaped  cavity  is  narrowing  to 
become  the  aqueduct.  Behind  the  cerebellum  and  pons  comes  the  bulb, 
which,  as  we  have  said,  is  the  thickened  floor  of  the  hind  part  of  the  hind- 
brain,  the  roof  of  the  cavity  being  here  practically  wanting. 

Of  these  several  divisions  the  first  division,  that  of  the  cerebral  hemi- 
spheres, including  the  corpora  striata,  stands  apart  from  the  rest  by  reason 
both  of  its  origin  and  the  character  of  its  development.  As  we  shall  see, 
this  anatomical  distinction  corresponds  to  a  physiological  difference. 

Of  the  other  parts  of  the  brain  the  crura  cerebri  deserve  special  atten- 
tion. We  may  regard  these  as  starting  in  the  cord,  but  largely  augmented 
in  the  bulb;  they  traverse  the  pons,  where  they  are  still  further  increased, 
and  passing  beneath  the  corpora  quadrigemina,  with  which  as  well  as  with 
the  cerebellum  they  make  connections,  end  partly  in  the  region  of  the  optic 
thalami  arid  walls  of  the  third  ventricle,  but  to  a  great  extent  in  the  cere- 
bral hemispheres.  We  may,  in  a  certain  sense,  consider  the  rest  of  the 
brain  as  built  upon  and  attached  to  these  fundamental  basal  or  ventral 
strands. 

§  517.  Connected  with  the  brain  are  a  series  of  paired  nerves,  the 
cranial  nerves.  The  first  and  second  pair,  the  olfactory  nerves  and  the 
optic  nerves,  differ  in  their  origin  and  mode  of  development  from  all  the 
rest  so  fundamentally  as  to  cause  regret  that  they  are  included  in  the  same 
category.  We  shall  consider  these  by  themselves  in  due  course.  The  re- 
maining pairs,  from  the  third  pair  to  the  twelfth,  forming  a  much  more 
homogeneous  category,  we  shall  also  consider  in  their  proper  place.  We 
must  now  turn  to  study  in  greater  detail  some  of  the  structural  features  of 
the  brain,  and  we  may  with  advantage  begin  with  the  bulb. 

THE  BULB. 

§  518.  The  spinal  cord,  as  it  ascends  to  the  brain,  becomes  changed 
into  the  more  complex  bulb,  partly  by  a  shifting  of  the  course  of  the  tracts 
of  white  fibres,  partly  by  an  opening  up  of  the  narrow  central  canal  into 
the  wide  and  superficial  fourth  ventricle,  but  chiefly  by  the  development  of 
new  gray  matter. 

When  the  anterior  ventral  aspect  of  the  bulb  is  examined  (Fig.  131,  C.), 
it  will  be  seen  that  the  anterior  columns  of  the  cord  are  interrupted  for 
some  distance  in  the  median  line  by  bundles  of  fibres  (Py.  dec.)  which, 
appearing  to  rise  up  from  deeper  parts,  cross  over  from  side  to  side  and  so 
confuse  the  line  of  the  anterior  fissure.  This  is  the  decussation  of  the  pyra- 


590 


THE  BRAIN. 


8          S  =  § 


g       £  S  sr  *  -S 

>>  ^    0>    C      •  .-    M 

S3  «  *  c,  a  c 

O 


11  3|~:1*II1 

I     "  rUo'SS.S*i'5to 

I     c*  ^5=      •**  o  § 

*  «^o      «»^<?"t^'^'ai 


iivii^i 

o  « -o  g  5  *  § 

•2  a  ffi  «  .2  I 
M  q  u  ^2  f-.  a. 


THE  BULB.  591 

mlds%  above  which  the  place  of  the  anterior  columns  of  the  spinal  cord  is 
taken  by  two  larger,  more  prominent  columns,  the  pyramids  of  the  bulb 
(Py.),  which  are  continued  forward  to  the  hind  margins  of  the  pons.  On 
the  other  side  of,  lateral  to,  each  pyramid  lies  a  projecting  oval  mass,  the 
olivary  body  or  inferior  olive  (ol.)  separating  the  pyramid  from  a  column  of 
white  matter,  the  restiform  body  (JR),  which,  occupying  the  lateral  region  of 
the  bulb,  when  traced  backward  appears  to  continue  the  line  of  the  lateral 
column  of  the  cord,  and  when  traced  forward  is  seen  to  run  up  to  the  cere- 
bellum as  the  inferior  peduncle  of  that  organ.  On  the  posterior  dorsal 
aspect  no  such  decussation  is  seen.  The  two  posterior  columns  of  the  cord 
diverge  from  each  other,  leaving  between  them  a  triangular  space,  the 
calamus  scriptorius,  which  is  the  hind  part  of  the  lozenge-shaped  shallow 
cavity  of  the  fourth  ventricle.  As  the  cord  passes  into  the  bulb,  the  poste- 
rior column,  as  a  whole,  grows  broader,  and  the  division  into  a  median  pos- 
terior and  an  external  posterior  column  becomes  very  obvious  and  distinct 
by  the  appearance  of  a  conspicuous  furrow  separating  the  two.  At  some 
distance,  however,  in  front  of  the  point  of  divergence  of  the  columns  or 
apex  of  the  calamus  scriptorius,  the  furrow  becomes  less  marked,  and  it 
eventually  fades  away.  In  its  course  the  furrow  takes  such  a  line  that  the 
median  posterior  column,  forming  the  immediate  lateral  boundary  of  the 
fourth  ventricle,  has  the  appearance  of  a  strand  broad  behind  but  thinning 
away  in  front,  while  the  external  posterior  column,  also  broadening  as  it 
advances  forward,  seems  to  be  wedged  in  between  the  median  posterior 
column  on  its  median  edge  and  the  restiform  body  on  its  lateral  edge ; 
hence  the  former  is  here  called  the  fasciculus  (or  funiculus)  gracilis  (in.  p.\ 
and  the  latter  the  fasciculus  (or  funiculus)  cuneatus  (e.  p.).  Further  forward 
both  columns  seem  to  merge  with  each  other  and  with  fibres  which  curve 
round  to  form  part  of  the  restiform  body ;  the  relations,  however,  of  these 
two  columns  to  each  other  and  to  the  other  parts  of  the  bulb,  as  well  as 
the  nature  of  the  other  several  changes  by  which  the  cord  is  transformed 
into  the  bulb,  are  disclosed  by  transverse  vertical  (dorso-ventral)  sections, 
to  the  study  of  which  we  must  now  turn. 

A  section  (Fig.  132,  1)  taken  at  the  hind  margin  of  the  decussation,  at 
which  level  the  first  cervical  nerve  takes  origin,  when  compared  with  a  sec- 
tion of  the  cord  at  the  level  of  the  second  cervical  nerve  (cf.  Fig.  127,  C2), 
shows  that  certain  changes  are  already  taking  place  in  the  gray  matter.  The 
anterior  horns  are  not  much  altered,  but  the  posterior  horns  are,  as  it  were, 
pushed  out  laterally  and  dorsally  so  that  the  posterior  columns,  which  as  yet 
retain  their  previous  great  depth,  become  very  much  broader  than  they  are 
lower  down,  encroaching,  so  to  speak,  on  the  lateral  columns.  At  the  same 
time  the  substance  of  Rolando  («.  g.),  forming  the  head  or  caput  of  the  horn, 
has  enlarged  into  a  more  or  less  globular  form,  and  lies  near  the  surface  of 
the  cord  though  separated  from  it  by  a  compact  tract  of  longitudinal  fibres 
(  V.  a.},  which,  as  we  shall  see,  belongs  to  the  fifth  cranial  nerve.  A  con- 
siderable development  of  the  reticular  formation  (/.  ret.)  at  the  side  of  the 
gray  matter  ventral  to  the  posterior  horn  has  also  taken  place,  and  this  with 
the  shifting  of  the  position  of  the  posterior  horn  has  driven  the  lateral  horn 
(/.  h.)  nearer  to  the  anterior  horn.  From  this  lateral  horn  a  root  of  the 
eleventh  spinal  accessory  cranial  nerve  (XL)  may  be  seen  taking  origin. 
Further,  a  great  increase  of  gray  matter  round  the  central  canal  may  also 
be  observed. 

These  changes,  however,  are  of  degree  only ;  what  seems  to  be  an  abso- 
lutely new  feature  is  the  presence  of  bundles  of  fibres  (Py.  dec.)  which  start- 
ing from  the  anterior  column  of  one  side  cross  over  to  and  are  apparently 
lost  in  the  gray  matter  of  the  neck  of  the  anterior  horn  of  the  other  side ;  in 


592  THE  BRAIN. 

so  crossing  the  fibres  push  aside  the  bottom  of  the  anterior  fissure.  When 
the  course  of  these  fibres  is  investigated,  either  by  simple  microscopic  obser- 
vation, or  still  better  by  the  method  of  degeneration,  it  is  found  that  they 
may  be  traced  from  the  anterior  column  of  one  side,  across  the  anterior 
commissure,  through  the  neck  of  the  anterior  horn  to  the  lateral  column  of 
the  opposite  side,  and  to  that  part  of  the  lateral  column  which  we  have  pre- 
viously described  as  the  crossed  pyramidal  tract. 

In  a  section  a  little  higher  up  (Fig.  132,  2),  these  decussating  fibres  form 
on  each  side  a  large  strand  which  starts  from  a  part  of  the  anterior  column, 
now  becoming  distinctly  marked  off  as  the  pyramid  (Py.\  an(^  *s  apparently 
lost  in  the  reticular  formation,  but  in  reality  passes  on  to  the  crossed  pyram- 
idal tract  of  the  lateral  column.  This  strand,  as  it  crosses  over,  completely 
cuts  off  the  head  of  the  anterior  horn  from  the  more  central  gray  matter, 
and  forms  with  its  fellow  a  large  area  of  decussating  fibres  between  the 
bottom  of  the  anterior  fissure  and  the  central  gray  matter.  When  a  surface 
view  of  the  bulb  is  examined  the  decussation  is  seen  to  be  effected  by  alter- 
nate bundles,  passing  now  from  right  to  left,  now  from  left  to  right ;  and  in 
transverse  sections  we  find  correspondingly  that  the  anterior  fissure  appears 
bent  now  to  the  left  and  now  to  the  right,  according  as  the  section  .cuts 
through  a  bundle  passing  from  left  to  right  or  from  right  to  left. 

In  sections  still  higher  up  (Fig.  132,  3  and  4)  this  conspicuous  strand  of 
fibres  crossing  obliquely  from  side  to  side  will  be  no  longer  seen  ;  decussating 
fibres  are  seen  dorsal  to  the  anterior  fissure,  but  these,  of  which  we  shall  speak 
presently,  are  of  different  nature  and  origin.  The  fibres  which  in  sections 
below  were  seen  in  the  act  of  crossing  are  now  gathered  into  masses  of  longi- 
tudinal fibres,  the  pyramids  (Py.)  one  on  each  side  of  the  anterior  fissure, 
each  with  a  sectional  area  of  a  rounded  triangular  form  clearly  marked  out 
from  the  surrounding  structures ;  the  section  is  taken  above  the  decussation 
of  the  pyramids.  Or,  tracing  the  changes  from  below  upward,  we  may  say 
that  the  decussation  is  now  complete ;  on  each  side  the  whole  of  the  crossed 
pyramidal  tract  of  the  spinal  cord  has,  in  the  region  of  the  bulb  below  the 
level  of  the  present  sections,  crossed  over  to  the  other  side,  and  joining  with 
the  direct  pyramidal  tract  of  the  anterior  column  of  the  cord  of  the  same 
side  has  become  the  pyramid  of  the  bulb.  In  other  words,  the  decussation 
of  the  pyramids  is,  as  we  have  already  hinted,  the  passing  off  from  each 
pyramid  and  the  crossing  over  to  the  opposite  side  of  the  cord  of  those  fibres 
which  are  destined  to  become  the  crossed  pyramidal  tract  of  the  spinal  cord 
of  the  opposite  side,  while  the  rest  of  the  pyramid  pursues  its.  course  on  the 
same  side  as  the  direct  pyramidal  tract. 

§  519.  In  the  spinal  cord  the  bottom  of  the  anterior  fissure  is  separated 
from  the  central  canal  by  nothing  more  than  the  anterior  white  commissure 
and  a  narrow  band  of  gray  matter,  composed  of  the  anterior  gray  commissure 
and  of  part  of  the  central  gelatinous  substance.  During  the  decussation  of 
the  pyramids,  the  decussating  fibres  push,  as  it  were,  the  central  canal  with 
its  surrounding  gray  matter  to  some  distance  from  the  bottom  of  the  anterior 
fissure.  In  sections  above  the  decussation  the  bottom  of  the  fissure  does  not 
again  approach  the  central  canal,  but  continues  to  be  removed  to  some  dis- 
tance from  it,  and,  as  we  pass  upward,  to  an  increasing  distance,  by  the 
interposition  of  tissue  which  consists  largely  of  decussating  fibres.  These, 
however,  though  they  seem  to  continue  on  the  decussation  of  the  pyramids, 
are  shown  by  the  embryological  and  degeneration  methods  to  have  no  con- 
nection with  the  pyramids,  but  belong  to  another  system  of  decussation.  As 
we  have  seen  (§  477)  the  anterior  commissure  along  the  whole  length  of  the 
cord  contains  decussating  fibres.  Some  of  these  in  the  upper  part  of  the  cord 
are  fibres  crossing  from  the  direct  pyramidal  tract  of  one  side  to  the  gray 


THE  BULB. 
FIG.  132. 


593 


ar.C.i 


Py.doc. 


--J— fret. 


XII 


Py.dec. 


a.l.n. 


XII 


ol.a 


fa.i.' 


Transverse  Dorso-ventral  Sections  of  the  Bulb  (Man)  at  Different  Levels.  (Sherrington.)  Fig. 
132  and  Figs.  133-137  form  a  series  of  transverse  dorso-ventral  sections  of  the  brain  taken  at  dif- 
ferent levels  from  the  hind  end  of  the  bulb  to  the  front  of  the  third  ventricle;  the  several  levels 
are  shown  by  the  lines  drawn  in  Fig.  131.  They  are  all  magnified  twice.  The  details  are  shown, 
for  the  sake  of  simplicity,  in  diagrammatic  fashion  ;  the  white  matter  is  left  unshaded,  the  course 
of  the  fibres  being  indicated  in  a  few  important  instances  only;  the  gray  matter  is  shaded  for- 
mally, the  nerve-cells  being  indicated  in  the  case  only  of  the  nuclei  of  the  cranial  nerves.  The 
want  of  complete  bilateral  symmetry  which  is  often  met  with  in  such  sections  is  indicated  in 
several  of  the  figures. 

1.  At  the  hind  limit  of  the  decussation  of  the  pyramids ;  2.  In  the  middle  of  the  decussation; 
3.  At  the  upper  end  of  the  decussation ;  4.  Just  below  the  point  of  the  calamus  scriptorius ;  5.  Just 
above  the  point;  6.  Through  the  middle  of  the  ala  cinerea. 

Py.  Pyramids ;  Py.  dec.  decussation  of  the  pyramids ;  Supra  Py.  dec.  superior  decussation ;  /.  a.  i, 
internal  arcuate  fibres ;  /.  a.  e.  external  arcuate  fibres  ;  Cb.  position  of  cerebellar  tract ;  R.  resti- 
38 


594  THE  BRAIN. 

matter  of  the  other  side,  and  so  may  be  regarded  as  part  of  the  whole  pyram- 
idal tract ;  but  others  are  of  different  origin  ;  and  even  in  the  region  of 
the  actual  decussation  of  the  pyramids  some  of  the  fibres  which  cross  over 
do  not  belong  to  the  pyramidal  tract.  This  system  of  decussating  fibres 
becomes  increasingly  prominent  above  the  decussation  of  the  pyramids,  and 
through  it  the  ventral  area  of  the  bulb  between  the  central  canal  and  the 
anterior  fissure  is  much  increased.  The  fibres  as  they  cross  form  a  middle 
line  of  partition,  the  raphe  (Fig.  132,  4,  5,  r)  which  increases  in  depth  in  the 
upper  parts  of  the  bulb,  and  on  each  side  of  the  raphe  help  to  break  up  the 
gray  matter  (which  previously  formed  the  anterior  horns)  into  what  is 
called  the  reticular  formation.  We  shall  return  to  this  presently,  but  may 
here  call  attention  to  a  special  development  of  these  decussating  fibres 
which  is  seen  just  above  the  decussation  of  the  pyramids.  In  a  section  at 
this  level  (Fig.  132,  3)  a  strand  of  fibres  (Supra  Py.  dec.}  may  be  seen  to 
start  chiefly  from  the  gracile  nucleus  but  also  to  some  extent  from  the  cune- 
ate  nucleus,  to  sweep  round  the  central  gray  matter,  and  to  decussate  ven- 
tral to  this  between  it  and  the  bottom  of  the  anterior  fissure.  This  is  called 
the  superior  decussation,  or,  for  reasons  which  we  shall  see  later  on,  the  sen- 
sory decussation. 

§  520.  We  must  now  turn  to  the  posterior  fissure  and  its  relations  to 
the  fourth  ventricle.  We  saw  that  at  the  beginning  of  the  pyramidal  de- 
cussation, the  posterior  horns  had  been  thrown  backward  and  outward  so  as 
to  increase  the  posterior  columns.  The  posterior  fissure  is  still  of  great 
depth,  so  that  by  the  increase  of  depth  and  maintenance  of  depth  the  poste- 
rior column,  the  lateral  limit  of  which  is  still  sharply  marked  out  by  the 
swollen  head  of  the  posterior  horn  as  well  as  by  the  highest  posterior  root- 
lets of  the  first  cervical  nerve,  acquires  at  this  level  its  maximum  of  bulk. 

From  this  point  forward  the  depth  of  the  posterior  fissure  and  the  dorso- 
ventral  diameter  of  the  posterior  columns  diminishes.  The  head  of  the 
horn  (Fig.  132,  2)  is  thrown  still  further  outward  into  the  lateral  regions; 
developments  of  gray  matter  at  the  base  and  to  some  extent  at  the  neck  of 
the  horn  (of  these  we  shall  speak  presently)  encroach  (Fig.  132,  3)  dorsally 
on  the  white  matter  of  the  columns ;  and  the  central  gray  matter  appears 
to  rise  dorsally  at  the  expense  of  the  posterior  fissure,  in  coincidence  with 
the  development  described  above  as  taking  place  on  the  ventral  side  of  the 
canal. 

Still  a  little  further  forward,  in  a  section,  for  instance  (Fig.  132,  4),  a 
little  way  behind  the  apex  of  the  calamus  scriptorius,  the  central  gray  matter, 
which  still  forms  a  rounded  mass  around  the  central  canal,  is  brought  yet 
nearer  to  the  posterior  fissure. 

In  a  section  yet  a  little  further  forward  (Fig.  132,  5)  carried  through  the 
hinder  narrower  part  of  the  fourth  ventricle  itself,  it  is  seen  that  the  central 
canal  has  opened  out  on  the  dorsal  surface,  and  that  the  gray  matter,  which 

form  body  or  inferior  peduncle  of  the  cerebellum ;  e.  p.  external  posterior  column,  fasciculus 
cuneatus  ;  m.  p.  median  posterior  column,  fasciculus  gracilis;  r.  raphe  ;  1.  h.  lateral  horn  ;  m.  p.  n. 
nucleus  of  the  median  posterior  column  or  gracile  nucleus;  e.p.  n.  nucleus  of  the  external  pos- 
terior column  or  cuneate  nucleus  ;  e.  p.  n.  (m)  median  division  and  e.p.  n.  (I)  lateral  division  of 
the  same;  ol.  olivary  body  ;  ol.  a.  median  accessory,  and  ol.  e.  lateral  accessory  olive;  in.ol.  inter- 
olivary  layer;  a.  1.  n.  lateral  (antero-lateral)  nucleus;  n. a. arcuate  nucleus;  a.  c.  remnant  of  an- 
terior horn  ;  /.  ret.  reticular  formation  ;  s.  g.  substance  of  Rolando  ;  o.  r.  c.  I,  anterior  root,  and  p.  r. 
c.  I.  posterior  root  of  first  cervical  nerve;  XI.  root  of  spinal  accessory  nerve;  XII.  twelfth  or  hy- 
poglossal  nerve;  n.  XII.  nucleus  of  the  same  in  6;  the  nucleus  may  be  traced,  however,  through 
2,  3,  4,  5,  in  connection  with  the  fibres  of  the  nerve ;  s.  X.  sensory  or  main  part  of  the  glosso- 
pharyngeal-vago-accessory  nucleus;  X.  m.  motor  nucleus  of  the  vagus,  or  nucleus  ambiguus;  IX. 
«.  ascending  root  of  the  glosso-pharyngeal  nucleus;  V.  a.  ascending  root  of  the  fifth  nerve;  4th. 
fourth  ventricle  ;  the  ependyma  or  lining  is  indicated  by  a  thick  dark  line ;  and  in  5  and  6,  the 
tooth-like  section  of  the  projecting  obex  is  shown. 


THE  BULB.  595 

in  previous  sections  surrounded  it,  is  now  exposed  to  the  surface  on  the 
floor  of  the  ventricle,  the  median  posterior  columns  being  thrust  aside.  In 
a  still  more  forward  section  (Fig.  132,  6)  this  gray  matter  in  correspondence 
with  the  increasing  width  of  the  ventricle  occupies  a  still  wider  area,  thrust- 
ing still  further  aside  the  narrowing  upper  ends  of  the  two  posterior 
columns. 

During  these  successive  changes  the  large,  wide  posterior  (both  external 
posterior  and  median  posterior)  columns  of  the  cervical  spinal  cord  and 
beginning  bulb,  are  reduced  to  small  dimensions  and  in  the  end  disappear ; 
l)ii  t  before  we  speak  of  the  course  and  fate  of  the  tracts  of  fibres  constitu- 
ting these  columns  we  must  turn  to  the  important  changes  of  the  gray 
matter. 

§  521.  A  transverse  section  through  the  lower  end  of  the  decussation 
(Fig.  132,  1)  shows,  as  we  have  said,  few  differences  as  regards  the  gray 
matter  from  one  taken  at  the  level  of  the  second  cervical  nerve.  The 
changes  noticeable  are  mainly  the  changes  in  position  of  the  posterior  horns, 
the  increase  of  central  gray  matter  around  the  central  canal,  the  approach 
of  the  lateral  horn,  from  which  spring  the  roots  of  the  spinal  accessory 
nerve,  to  the  anterior  horn,  and  an  increase  of  the  reticular  formation  in  the 
bay  ventral  to  the  posterior  horn. 

In  the  middle  of  the  decussation  (Fig.  132,  2)  the  decussating  fibres  are 
cutting  the  head  of  the  anterior  horn  away  from  the  base  of  the  horn  and 
the  central  gray  substance,  and  the  isolated  head  is  diminished  in  size,  being 
separated  from  the  surface  of  the  cord  by  an  increasing  thickness  of  white 
matter.  The  lateral  horn  and  origin  of  the  spinal  accessory  root  do  not 
share  in  this  isolation,  but  are  driven  back  again  dorsally  toward  the  poste- 
rior root  to  join  the  reticular  formation  which  is  increasing  in  area,  while 
the  lateral  column  of  white  matter  is  diminishing  in  bulk  by  the  withdrawal 
of  the  pyramidal  tract. 

Still  a  little  further  forward,  the  anterior  horn  seems  at  first  sight  to 
have  wholly  disappeared  (Fig.  132,  3  and  4),  but  its  disappearance  is  coin- 
cident with  an  increase  of  the  reticular  formation  in  the  position  of  the 
lateral  columns,  as  well  as  with  the  growth  of  tissue  mentioned  above  between 
the  anterior  fissure  and  the  central  gray  matter.  In  fact,  between  the  ante- 
rior pyramids  on  the  ventral  side  and  the  largely  increased  and  laterally  ex- 
panded gray  matter  on  the  dorsal  side,  a  large  area  of  peculiar  tissue  now 
extends  on  each  side  for  a  considerable  distance  from  the  middle  line  of  the 
raphe,  encroaching  on  what  was  the  lateral  column  of  white  matter ;  and  a 
corresponding  area  of  similar  tissue  may  be  traced  from  this  level  through 
the  higher  parts  of  the  bulb  up  into  the  pons  and  crura  cerebri.  The  tissue 
consists  of  nerve-fibres  running  transversely,  longitudinally,  and  in  other 
directions,  so  as  to  form  a  network, the  bars  of  which  are  often  curved  ;  and 
with  these  fibres  are  found  branched  nerve-cells  in  considerable  number, 
some  of  them  small,  both  fibres  and  cells  being  as  elsewhere  imbedded  in 
neuroglia.  Though  differing  from  the  ordinary  gray  matter  of  the  cord  by 
the  more  open  character  of  its  network,  it  may  be  considered  as  a  form  of 
gray  matter.  We  may  consider  it  as  being  in  reality  the  gray  matter  of  the 
apparently  lost  anterior  horn  broken  up  and  dispersed  by  the  passage  of  a 
large  number  of  fibres  and  bundles  of  fibres,  especially  of  the  decussating 
fibres  spoken  of  in  §  518,  which  since  they  curve  through  this  area  from  the 
middle  line  laterally  are  called  arcuate  or  arciform  fibres,  internal  arcuate 
fibres  (Fig.  132,  6,  /.  a.  i.)  to  distinguish  them  from  the  external  arcuate 
fibres  (/.  a.  e.)  of  which  we  shall  speak  presently,  Fragments  of  more 
compact  gray  matter  also  belonging  probably  to  the  anterior  horn  are  seen 
at  intervals  in  this  area  (Fig.  132,  6,  ac.)  and  elsewhere.  We  have  seen  that 


596  THE  BKAIN. 

nearly  all  the  way  along  the  cord  the  gray  matter  of  the  neck  of  the  poste- 
rior horn  is  similarly  broken  up  by  bundles  of  fibres  into  what  we  there  called 
the  reticular  formation  (Figs.  121,  122,  r.f.  p.  and  r.f.  L) ;  and  this  area  in 
the  bulb  though  it  possesses  characters  of  its  own  is  also  called  the  reticular 
formation,  in  the  more  lateral  portion  of  this  formation,  the  network  is 
more  open  and  irregular,  the  fibres  are  finer,  and  the  nerve-cells  are  more 
abundant  than  in  the  median  portion  where  the  nerve-cells,  except  in  the 
immediate  neighborhood  of  the  raphe,  are  less  numerous  or  even  absent,  and 
the  fibres  are  coarser.  These  two  parts  are  sometimes  distinguished  as  the 
outer  or  lateral  and  the  inner  or  median  formation.  In  the  middle  line  the 
fibres  distinctly  interlace  and  decussate  in  an  oblique  manner,  some  running 
nearly  vertically  in  the  dorso-ventral  plane,  thus  constituting,  as  we  have 
said,  a  thick  raphe,  which,  however,  at  its  edges  gradually  mergesin  to  the 
more  open  network. 

§  522.  Within  the  area,  bounded  by  the  pyramids  ventrally,  the  ex- 
panded gray  matter  dorsally,  the  raphe  in  the  middle  line,  and  the  white 
matter  laterally,  certain  distinct  compact  masses  of  gray  matter  make  their 
appearance,  as  we  pass  upward  toward  the  pons. 

One  of  the  most  important  of  these  gives  rise  to  the  olivary  body,  or  in- 
ferior olive,  which,  as  we  have  seen,  projects  as  an  oval  mass  (Fig.  131,  ol.) 
on  each  side  of  the  pyramids,  reaching  from  a  level  which  is  somewhat 
higher  up  than  the  lower  limit  of  the  pyramids,  almost  but  not  quite  to  the 
pons.  The  olivary  body,  as  a  whole,  consists  partly  of  white  matter,  that  is,  of 
fibres,  and  partly  of  gray  matter,  sometimes  called  the  olivary  nucleus.  This 
latter  is  disposed  in  the  form  of  a  hollow  flask  or  curved  bowl,  with  deeply 
folded  or  plaited  walls,  having  a  wide  open  mouth  directed  inward  toward 
the  middle  line,  and  forward  toward  the  pons  (Fig.  132,  4,  5,  6,  ol.*).  The 
flask  is  filled  within  by  white  matter,  and  covered  up  on  its  outside  with 
white  matter  as  well  as  traversed  by  fibres.  The  gray  matter  thus  forming 
this  flask-shaped  nucleus  consists  of  small  rounded  nerve-cells,  lying  in  a 
bed  of  tissue  which  is  partly  ordinary  neuroglia,  and  partly  a  fine  nervous 
network. 

Lying  to  the  median  side  of  the  olivary  body,  immediately  dorsal  to  the 
anterior  pyramid  is  another  small  mass  of  gray  matter,  in  the  form  of  a  disc, 
appearing  in  transverse  sections  as  a  thick  bent  rod,  in  some  sections  consist- 
ing of  two  parts  (Fig.  132,  4,  ol.  a.).  This  is  the  accessory  olivary  nucleus.  A 
very  similar  body  lies  dorsal  to  the  olivary  nucleus,  in  the  lateral  reticular 
formation  ;  this  is  also  called  an  accessory  olivary  nucleus,  being  distinguished 
(Fig.  132,  6,  ol.  e.)  by  the  name  outer  accessory  nucleus  from  the  above-men- 
tioned inner  accessory  nucleus.  It  will  be  observed  in  these  transverse  sec- 
tions that  the  inner  accessory  nucleus  is  separated  from  the  olivary  nucleus 
by  a  bundle  of  white  fibres  (Fig.  132,  4,  5,  6,  XII.)  which,  running  ventrally 
from  the  gray  matter  in  the  dorsal  region,  comes  to  the  surface  between  the 
anterior  pyramids  and  the  olivary  body.  This  is  the  hypoglossal  or  twelfth 
cranial  nerve. 

On  the  surface  of  the  anterior  pyramid  itself  is  seen  on  each  side  a  small 
mass  of  gray  matter  (Fig.  132,  5,  6,  n.  a.),  which  since  it  appears  to  be  con- 
nected with  a  system  of  superficial  transverse  fibres,  which  we  shall  describe 
directly  as  the  external  arcuate  fibres  (Fig.  132,  3,  4,  5,  6,/.  a.  e.),  is  called 
the  arcuate  nucleus.  It  seems  to  belong  to  the  same  group  as  the  accessory 
olives. 

Lastly,  a  small  somewhat  diffuse  collection  of  gray  matter  is  seen  in  sec- 
tions as  a  rounded  mass  of  irregular  form  placed  laterally  to  the  reticular 
formation  (Fig.  132,  4,  5,  6,  a.  I.  ?i.).  This,  which  at  its  first  appearance 
seems  to  be  budded  off  from  the  general  mass  of  gray  matter  (Fig.  132,  3, 


THE  BULB.  597 

a.  1. 7i.)  and  which  is  probably  a  detached  portion  of  the  base  of  the  anterior 
horn  or  of  the  lateral  region  of  the  gray  matter,  is  called  the  lateral  or 
antero-lateral  nucleus. 

Hence,  besides  the  diffuse  reticular  formation,  this  ventral  part  of  the 
bulb  contains  more  sharply  defined  collections  of  gray  matter  in  the  olivary 
nucleus,  and  the  other  bodies  just  mentioned. 

§  523.  We  must  now  turn  to  the  dorsal  part  of  the  bulb.  Here  in  the 
first  place  we  must  distinguish  between  the  portions  of  gray  matter  which 
are  more  immediately  connected  with  the  cranial  nerves  taking  origin  from 
this  part  of  the  bulb,  and  the  portions  which  have  no  such  obvious  connec- 
tion. In  the  spinal  cord,  the  anterior  horns  supply,  as  we  have  seen,  the 
origins  of  the  successive  anterior  motor  nerves  ;  but  in  the  transformation  of 
the  cord  into  the  bulb  the  anterior  horns  have  been  broken  up  or  displaced  ; 
and  the  parts  of  the  anterior  horns  serving  as  the  nuclei  of  origin  for  motor 
nerves  have  been  translated  from  the  ventral  to  the  more  dorsal  regions. 
Hence,  it  is  in  the  more  dorsal  part  of  the  gray  matter  that  we  have  to  seek 
for  the  nuclei  of  origin  not  only  of  afferent  but  also  of  motor  cranial  nerves. 
It  will  be  convenient  to  consider  all  these  nuclei  of  origin  of  cranial  nerves 
by  themselves,  and  we  may  here  confine  ourselves  to  the  gray  matter  of  other 
nature.  We  may,  however,  say  that  these  nuclei  from  that  of  the  third 
nerve  backward  are  more  or  less  closely  associated  with  the  gray  matter 
immediately  surrounding  the  central  canal.  This  central  gray  matter,  in  the 
narrow  sense  of  the  term,  is  marked  out  somewhat  low  down  (Fig.  132,  3) 
by  the  fibres  of  the  sensory  decussation  which  sweep  round  it ;  it  appears  in 
sections  higher  up  as  a  fairly  distinct  region  (Fig.  132,  4)  ;  and  it  is  this 
part  of  the  gray  matter  which  is  exposed  on  the  floor  of  the  fourth  ventricle 
when  the  central  canal  (Fig.  132,  5,  6)  opens  out  into  that  space.  We  say 
exposed  ;  but  in  reality  the  true  gray  matter  is  covered  by  a  superficial  layer 
of  tissue  of  a  peculiar  nature  (indicated  in  Fig.  132,  5,  6,  by  a  thick  black 
line)  similar  to  that  which  is  found  at  the  hind  end  of  the  conus  medullaris 
in  the  spinal  cord. 

We  saw  that  at  the  level  of  the  first  cervical  nerve  coincident  with  the 
horizontal  flattening  out  of  the  posterior  horns  the  posterior  columns  assumed 
very  large  dimensions.  In  this  region  (Fig.  132, 1)  they  consist  entirely  of 
white  matter — that  is,  of  longitudinal  fibres. 

At  a  little  higher  level,  however,  at  the  level  of  the  middle  of  the  decus- 
sation for  example,  an  islet  of  gray  matter  (Fig.  132,  2,  m.  p.  n.)  makes  its 
appearance  in  the  median  posterior  column.  A  little  further  forward,  at  the 
level  of  the  established  pyramids,  it  will  be  seen  (Fig.  132,  3)  that  this  islet 
is  the  hind  end  of  an  invasion  from  the  more  centrally  placed  gray  matter, 
and  that  at  the  same  time  there  has  taken  place  a  similar  inroad  of  gray 
matter  into  the  external  posterior  column  (Fig.  132,  3,  e.  p.  n.)  ;  indeed,  a 
slight  extension  of  gray  matter  into  the  external  posterior  column  may  be 
seen  even  before  this  (Fig.  132,  2,  e.p.  n.).  It  will  further  be  observed  that 
these  gray  masses  have  so  largely  encroached  on  the  white  matter  that  both 
the  median  posterior  or  fasciculus  gracilis  and  the  external  posterior  column 
or  fasciculus  cuneatus,  instead  of  being  simply  tracts  of  white  fibres,  as  they 
were  in  the  hinder  part  of  the  bulb  and  in  the  cord,  have  now  become 
columns  of  gray  matter  covered  by  a  relatively  thin  layer  of  white  fibres. 
These  columns  of  gray  matter  are  now  called  respectively  the  median  poste- 
rior nucleus  or  nucleus  fasciculi  gracilis,  or,  more  shortly,  the  gracile  nucleus ; 
and  the  external  posterior  nucleus,  or  nucleus  fasciculi  cuneati,  or  the  cuneate 
nucleus.  From  the  ventral  aspect  of  these  nuclei  a  larger  number  of  fibres 
pass  ventrally,  with  a  more  or  less  curved  course,  to  form,  as  we  have  seen 
(§  518),  the  superior  decussation  and  to  pursue  certain  paths  through  the 


598  THE  BRAIN. 

reticular  formation,  of  which  we  shall  speak  later  on.  It  is  at  this  level  and 
for  some  little  distance  above  (Fig.  131,  4,  5)  that  these  nuclei  acquire  their 
greatest  development.  Further  forward  (Fig.  132,  6),  when  the  fourth  ven- 
tricle has  opened  out  and  the  nuclei  of  the  cranial  nerves  are  becoming  con- 
spicuous, and  the  posterior  columns  have  been  thrust  aside  laterally,  both 
these  nuclei  have  diminished  in  size;  still  further  forward  they  become  still 
smaller,  and  toward  the  pons  they  gradually  disappear. 

The  mass  of  gelatinous  substance,  forming  at  the  level  of  the  first  cervical 
nerve  the  swollen  caput  of  the  horn  close  to  the  surface  but  separated  from 
it  by  a  band  of  fibres  (  Fa.)  of  fine  calibre,  to  which  we  have  already  referred 
as  belonging  to  the  fifth  cranial  nerve,  increases  in  bulk  at  a  somewhat 
higher  level  (Fig.  132,  2,  3,  s.  g.)  and  forms  on  the  surface  a  slight  projec- 
tion, called  the  tubercle  of  Rolando.  It  soon,  however,  becomes  thrust  ven- 
trally  by  the  divergence  of  the  posterior  columns,  and  more  and  more  covered 
up  by  the  fibres  which  are  going  to  form  the  increasing  restiform  body  (Fig. 
132,  4,  5,  6,  R).  Retaining  this  position  the  islet  of  gelatinous  substance 
diminishes  in  size  further  forward  (Fig.  133,  s.  g.),  and  eventually  disappears. 

§  524.  The  fibres  of  the  bulb.  It  is  obvious,  from  what  has  already  been 
said,  that  the  arrangement  into  posterior,  lateral,  and  anterior  columns,  so 
clear  and  definite  in  the  spinal  cord,  becomes  broken  up  in  the  bulb  ;  indeed 
it  will  be  best,  in  treating  of  the  bulb,  not  to  attempt  to  trace  out  these 


f!eui. 

Through  the  Bulb  just  Behind  the  Pons.  vSherrington.)  Taken  in  the  line  110,  Fig.  131.  Pt/. 
pyramids ;  R.  restiform  body  ;  Cbm.  cerebellum ;  F.  fillet ;  /.  a.  e.  external,/,  a.  i.  internal  arcuate 
fibres  ;  t.  bundle  of  fibres  from  olive  to  the  lenticular  nucleus  ;  I.  posterior  longitudinal  bundles; 
n.f.  t.  nucleus  of  the  fasciculus  teres;  s.  o.  superior  olive;  n.  c.  e.  nucleus  centralis  (the  marks 
•within  it  are  sections  of  bundles  of  fibres  by  which  it  is  traversed);  s.  g.  substance  of  Rolando; 
V.  a.  ascending  root  of  fifth  nerve;  VII.  a.  nucleus  of  the  seventh  nerve;  VIII.  auditory  nerve, 
chiefly  the  dorsal  or  cochlear  root;  VIII.  <*  median  nucleus;  VIII.  ft.  lateral  nucleus;  VIII.  y. 
accessory  nucleus  of  auditory  nerve ;  IX.  fibres  of  root  of  ninth  nerve  passing  through  ascending 
root  of  fifth  nerve. 

columns,  but  to  speak  of  the  course  of  the  several  tracts  into  which  these 
columns  may  be  divided. 

The  direct  and  cross  pyramidal  tracts  of  the  cord  unite  to  form,  as  we 
have  seen,  the  pyramid  of  "the  bulb,  and  so  pass  on  to  the  pons.  We  need 
say  nothing  more  at  present  concerning  this  important  pyramidal  strand, 
except  that,  as  we  trace  it  down  from  the  pons  to  the  spinal  cord,  it  gives  off 
to  the  bulb  itself  fibres  which  make  connections  with  the  motor  fibres  of  the 
cranial  nerves  proceeding  from  this  region. 

Concerning  the  course  taken  by  the  other  less  conspicuous  "  descending  " 


THE  BULB.  599 

tract,  the  antero-lateral  descending  tract,  our  knowledge  is  very  imperfect ; 
nothing  definite  can  be  said  at  present. 

The  cerebellar  tract,  occupying  near  to  the  surface  a  position  which  in  the 
series  of  sections  (Fig.  132,  (76.)  appears  now  rather  more  ventral,  now  more 
dorsal,  eventually  passes  into  the  restiform  body,  of  which  it  forms  a  large 
part,  and  thus  reaches  the  cerebellum.  The  antero-lateral  ascending  tract 
possibly  also  takes  the  same  course,  but  this  is  not  as  yet  certain. 

The  median  posterior  tract  or  column,  becoming  the  fasciculus  gracilis, 
ends  in  the  gracile  nucleus  ;  and  in  a  similar  manner  the  external  posterior 
column,  or  fasciculus  cuneatus,  ends  in  the  median  and  lateral  masses  of  the 
cuneate  nucleus.  As  we  have  seen,  the  white  matter  of  these  columns  dim- 
inishes as  the  nuclei  increase;  and  the  nuclei  after  absorbing,  so  to  speak,  the 
white  matter  diminish  in  turn  ;  the  ascending  degeneration  observed  in  these 
columns  stops  at  these  nuclei.  It  is  a  suggestive  fact  that  as  these  nuclei 
diminish  forward  the  restiform  body  increases  in  bulk. 

The  remaining  fibres  of  the  cord,  belonging  partly  to  the  anterior  column 
and  partly  to  the  lateral  column,  not  gathered  into  any  of  the  above-men- 
tioned tracts,  appear  to  end,  chiefly  at  all  events,  in  the  reticular  formation 
of  the  bulb  itself,  though  some  are  carried  on  to  the  higher  parts  of  the 
brain. 

§  525.  Thus  of  the  various  tracts  or  strands  of  the  spinal  cord  two  only 
are  known  definitely  and  certainly  to  pass  as  conspicuous  unbroken  strands 
through  the  bulb  to  or  from  higher  parts ;  namely,  the  pyramidal  tract  to 
the  cerebrum  and  the  cerebellar  tract  to  the  cerebellum.  All,  or  nearly  all, 
the  rest  of  the  longitudinal  fibres  of  the  cord  reaching  the  bulb  end,  as  far 
as  we  know  at  present,  in  some  part  or  other  of  the  bulb ;  and  we  may  infer 
that  some  or  other  nerve-cells  of  the  bulb  serve  as  relays  to  connect  these 
fibres  of  the  cord  with  other  parts  of  the  brain. 

The  gracile  and  cuneate  nuclei  stand  out  conspicuously  as  relays  of  this 
kind,  and  through  them  the  posterior  columns  of  the  cord  make  secondary 
connections  on  the  one  hand  with  the  cerebellum,  and  on  the  other  hand 
with  various  regions  of  the  cerebrum.  We  have  said  (§  519)  that  fibres 
passing  ventrally  from  the  gracile  and  cuneate  nuclei  sweep  in  a  curved 
course  through  the  reticular  formation  as  the  internal  arcuate  fibres  (Fig. 
132, /.  a.  i.~).  The  hindmost  of  these  form  the  superior  decussation  already 
referred  to,  as  seen  in  sections  of  the  fore  part  of  and  in  front  of  the  pyram- 
idal decussation  (Fig.  132,  3,  Supra  Py.  dec.}.  After  decussating  ventral 
to  the  central  canal,  these  fibres  form  an  area  called  the  inter-olivary  layer 
(Fig.  132,  4,  in.  oL),  lying  dorsal  to  the  pyramids  between  two  of  the  olivary 
nuclei.  This  layer  may  be  regarded  as  the  hind  end  or  beginning  on  each 
side  of  a  remarkable  longitudinal  strand  called  the  fillet  (Figs.  131,  B.  F., 
133,  F.),  of  the  connections  of  which  in  the  front  part  of  the  brain  we  shall 
speak  hereafter.  Thus  these  two  nuclei  are  the  source  of  fibres  which  cross 
to  the  other  side  of  the  bulb,  and  reaching  the  inter-olivary  layer  dorsal  to 
the  pyramids  run  up  to  higher  parts  of  the  brain  by  the  fillet.  We  may 
add  that  the  formation  of  the  fillet  is  also  probably  assisted  by  fibres  from  a 
tract  which  lies  just  dorsal  to  the  inter-olivary  layer,  and  is  derived  from  the 
anterior  columns  of  the  cord.  Besides  its  fibres  of  descending  degeneration 
the  anterior  column  contains  fibres  of  ascending  degeneration,  and  these  rise 
dorsally  in  the  bulb  to  form  the  tract  in  question.  Though  the  whole  tract 
is  of  some  length,  the  component  fibres  appear  to  be  short. 

The  gracile  and  cuneate  nuclei  give  rise  also  to  other  fibres  which,  though 
also  sweeping  ventrally  and  crossing  to  the  other  side,  do  not,  when  they 
reach  the  inter-olivary  region,  assume  a  longitudinal  direction,  as  do  the 
fibres  forming  the  fillet,  but  as  external  arcuate  fibres  (Fig.  132,  /.  a.  e.) 


600  THE  BRAIN. 

pursue  a  course  which  is  at  first  ventral  along  the  side  of  the  anterior  fissure; 
and  then  lateral  over  the  ventral  surface  of  the  pyramid  and  olivary  nucleus, 
by  which  path  they  reach  the  lateral  surface  of  the  bulb,  and  so  the  restiform 
body  and  cerebellum.  In  this  way  the  two  nuclei  in  question  contribute  to 
the  restiform  body  of  the  opposite  side  of  the  bulb.  These  external  arcuate 
fibres,  which  as  they  sweep  round  the  ventral  surface  of  the  pyramid  traverse 
the  arcuate  nucleus,  though  they  vary  much  in  individual  brains,  form  a 
considerable  portion  of  the  white  matter  seen  on  the  ventral  and  lateral  sur- 
faces of  the  bulb ;  it  is  by  them  that  the  olivary  nucleus  is  covered  up. 

The  cuneate  and  gracile  nuclei,  besides  this  crossed  and  somewhat  round- 
about connection  with  the  restiform  body  of  the  opposite  side,  are  believed 
to  have  more  direct  connection  with  the  restiform  body  of  the  same  side  by 
means  of  fibres  which  pass  by  a  more  or  less  direct  lateral  path  from  them  to 
it.  Accepting  this  view,  we  may  say  that  the  two  nuclei  are  connected  with 
the  opposite  side  of  the  cerebellum  by  external  arcuate  fibres,  and  with  the 
same  side  of  the  cerebellum  by  the  other  fibres  just  mentioned.  In  any  case, 
the  connection  between  the  two  nuclei  and  the  cerebellum  is  large  and  im- 
portant. 

Thus  the  important  strand  of  fibres  which  is  called  in  the  bulb  the  resti- 
form body,  and  higher  up  the  inferior  peduncle  of  the  cerebellum,  is  con- 
nected with  the  spinal  cord  in  two  chief  ways ;  directly  by  means  of  the 
cerebellar  tract  and  indirectly  by  means  of  the  cuneate  and  gracile  nuclei, 
which,  as  we  have  said,  diminish  in  bulk  forward  as  the  restiform  body 
increases.  By  the  relay  of  the  gracile  nucleus  it  is  brought  into  connection 
with  the  median  posterior  column  along  the  whole  length  of  the  cord,  and  so 
with  that  division  of  the  posterior  roots  which  (§  490)  in  each  of  the  several 
spinal  nerves  goes  to  form  that  column.  By  the  relay  of  the  cuneate  nucleus 
it  is  brought  into  connection  with  such  parts  of  the  external  posterior  column 
as  end  in  that  nucleus,  and  thus  probably  with  other  fibres  of  the  posterior 
roots  of  the  upper  spinal  nerves.  And  if  we  admit  that  the  cerebellar  tract 
is  connected,  by  the  relay  of  the  vesicular  cylinder  or  by  other  nerve-cells, 
with  the  rest  of  the  posterior  roots  of  the  spinal  nerves,  we  may  conclude 
that  the  restiform  body  is,  by  means  of  these  relays,  a  continuation  of  the 
spinal  posterior  roots. 

The  restiform  body  and  so  the  cerebellum  is  also  specially  connected  with 
the  olivary  body  of  the  opposite  side  ;  for  when  in  young  animals  one  side 
of  the  cerebellum  is  removed,  the  olivary  body  of  the  opposite  side  atrophies. 
The  course  of  the  fibres  maintaining  this  connection  is  not  as  yet  accurately 
known,  but  they  probably  pass  from  the  olivary  nucleus  of  one  side  through 
the  inter-olivary  layer,  and  so  laterally  through  the  reticular  formation  of 
the  other  side.  Lastly,  we  may  add  that  a  tract  which  is  sometimes  included 
in  the  restiform  body  as  its  median  or  inner  division  has  quite  a  different 
origin  from  any  of  the  above  ;  the  fibres  which  compose  it  come,  as  we  shall 
see,  from  the  auditory  nerve. 

The  further  connections  of  the  bulb  with  the  cerebrum  it  will  be  best  to 
leave  until  we  come  to  deal  with  the  structural  arrangement  of  the  rest  of 
the  brain. 

Meanwhile  enough  has  been  said  to  show  that  the  bulb  differs  very  mate- 
rially in  structure  from  the  spinal  cord.  The  gray  matter  of  the  bulb  is  far 
more  complex  in  its  nature  than  is  that  of  any  part  of  the  cord  ;  and  the 
arrangement  of  the  several  strands  and  tracts  of  fibres  is  far  more  intricate. 
The  structural  features  on  the  whole  perhaps  suggest  that  the  main  functions 
of  the  bulb  are  twofold :  on  the  one  hand,  it  seems  fitted  to  serve  as  a  head- 
centre  governing  the  spinal  cord,  the  various  reins  of  which,  with  the  ex- 
ceptions noted,  it  holds,  as  it  were,  in  its  hands ;  on  the  other  hand,  it  ap- 


THE  GRAY  MATTER.  601 

pears  no  less  adapted  to  act  as  a  middleman  between  parts  of  the  spinal  cord 
below  and  various  regions  of  the  brain  above.  As  we  shall  see,  experiment 
and  observation  give  support  to  these  suggestions. 

THE  DISPOSITION  AND  CONNECTIONS  OF  THE  GRAY  AND  WHITE 
MATTER  OF  THE  BRAIN. 

THE   GRAY    MATTER. 

§  526.  As  we  pass  up  from  the  bulb  to  the  higher  parts  of  the  brain,  the 
differentiation  of  the  gray  matter  into  more  or  less  separate  masses,  which 
we  have  seen  begin  in  the  bulb,  becomes  still  more  striking.  We  have  to 
distinguish  a  large  number  of  areas  or  collections  of  gray  matter,  more  or 
less  regular  in  form  and  more  or  less  sharply  defined,  from  the  surrounding 
white  matter ;  to  such  collections  the  several  terms  corpus,  locus,  nucleus, 
and  the  like,  have  from  time  to  time  been  given.  These  areas  or  collections 
vary  greatly  in  size,  in  form,  and  in  histological  characters  ;  they  differ  from 
each  other  in  the  form,  size,  features,  and  arrangement  of  the  nerve-cells,  in 
the  characters  of  the  nervous  network  of  which  the  nerve-cells  form  a  part, 
and  especially  perhaps  in  the  extent  to  which  the  more  distinctly  gray  mat- 
ter is  traversed  and  broken  up  by  bundles  of  white  fibres.  Guided  by  the 
analogy  of  the  spinal  cord,  as  well  as  by  the  results  of  experiments  and 
observations  directed  to  the  brain  itself,  we  are  led  to  believe  that  the  com- 
plex functions  of  the  brain  are  intimately  associated  with  this  gray  matter ; 
and  a  full  knowledge  of  the  working  of  the  brain  will  carry  with  it  a  know- 
ledge of  the  nature  and  meaning  of  the  intricate  arrangement  of  the  cerebral 
gray  matter.  At  present,  however,  our  ignorance  as  to  these  things  is  great ; 
and,  though  various  theoretical  classifications  of  the  several  collections  of 
gray  matter  have  been  proposed,  it  will  perhaps  be  wisest  to  content  our- 
selves here  with  a  very  broad  and  simple  arrangement.  We  will  divide  the 
whole  gray  matter  of  the  brain  into  four  categories  only :  1.  The  central 
gray  matter  lining  the  neural  canal ;  and  this  we  may  consider  the  nuclei 
of  the  cranial  nerves,  some  of  which  are  closely  associated  with  it.  2.  The 
superficial  gray  matter  of  the  roof  of  some  of  the  main  divisions  of  the  brain, 
such  as  that  of  the  cerebral  hemispheres,  and  of  the  cerebellum.  3.  The 
intermediate  gray  matter  more  or  less  closely  connected  with  the  crura 
cerebri.  4.  Other  collections  and  areas  of  gray  matter.  We  will,  more- 
over, confine  ourselves  at  present  for  the  most  part  to  their  general  features 
and  topography,  reserving  what  we  have  to  say  concerning  their  histological 
characters  for  another  occasion. 

1.   The  Central  Gray  Matter  and  the  Nuclei  of  the  Cranial  Nerves. 

§  527.  The  ventricles  of  the  brain,  like  the  central  canal  of  the  spinal 
cord,  of  which  they  are  a  continuation,  are  lined  by  an  epithelium  which  is 
in  general  a  single  layer  of  columnar  cells  said  to  be  ciliated  throughout, 
though  it  is  often  difficult  to  demonstrate  the  cilia.  Beneath  this  epithelium 
lies  a  layer  of  somewhat  peculiar  neuroglia,  forming  with  the  epithelium,  as 
we  have  said  (§  523),  the  ependyma,  which,  well  developed  in  the  floor  of  the 
fourth  ventricle  and  in  the  walls  of  the  third  ventricle  and  of  the  aqueduct, 
is  thin  and  scanty  in  the  lateral  ventricles.  Beneath,  and  more  or  less  con- 
nected with  the  ependyma  in  the  sides  and  floor  of  the  third  ventricle,  is  a 
fairly  conspicuous  layer  of  gray  matter,  which  is  well  developed  in  the  parts 
of  the  floor  exposed  on  the  ventral  surface  of  the  brain,  and  known  as  the 
lamina  terminalis,  the  anterior  and  posterior  perforated  spaces,  the  tuber 


602  THE  BRAIN. 

cinereum,  etc.  This  layer  is  not  continued  forward  into  the  lateral  ventri- 
cles of  the  cerebral  hemispheres,  but  it  is  well  developed  backward  along  the 
aqueduct  (Figs.  136,  137),  and  in  the  floor  of  the  fourth  ventricle,  and  through 
the  bulb  becomes,  as  we  have  seen  (§  523),  continuous  with  the  central  gray 
matter  of  the  cord.  The  nerve-cells  of  this  gray  matter  are  on  the  whole 
small  and  in  many  places  scant. 

§  528.  The  several  roots  of  the  cranial  nerves  from  the  third  nerve 
backward  may  be  traced  within  the  brain  substance  to  special  collections  of 
gray  matter,  called  the  nuclei  of  the  cranial  nerves,  some  of  which  lie  close 
upon  the  central  gray  matter,  while  others  are  placed  at  some  distance  from 
it.  The  optic  nerve  and  what  is  sometimes  called  the  olfactory  nerve,  namely, 
the  olfactory  bulb  and  tract,  may  advantageously  be  dealt  with  apart,  since 
these  two  nerves  are  not,  like  the  other  cranial  nerves,  simple  outgrowths 
from  the  walls  of  the  original  neural  canal,  but  are  in  reality  elongated 
vesicles,  budded  off  from  the  neural  canal,  the  cavities  of  which  have  been 
obliterated.  We  may  add  that  part  of  the  retina,  and  of  the  gray  matter 
of  the  olfactory  tract,  may  perhaps  be  considered  as  corresponding  to  the 
nuclei  of  which  we  are  speaking,  the  retinal  and  proper  olfactory  fibres  being 
connected  with  them  very  much  as  the  fibres  of  the  remaining  cranial  nerves 
are  connected  with  their  respective  nuclei.  In  the  brain  the  segmental  reg- 
ularity of  the  nerve-roots  so  conspicuous  in  the  spinal  cord  is  very  greatly 
obscured.  We  shall  have  something  to  say  on  this  point  later  on  ;  but  at 
present  we  may  be  content  to  treat  the  several  nerves  in  a  simple  topographical 
manner.  They  may  be  seen  in  a  ventral  view  of  the  brain  (Fig.  131,  (7.), 
leaving  the  brain  at  various  levels  by  what  is  called  their  "  superficial 
origin  ;"  the  third  nerve  issuing  in  front  of  the  pons,  and  the  last  or  hypo- 
glossal  stretching  back  to  the  hind  end  of  the  bulb.  Part,  indeed,  of  the 
eleventh  nerve,  the  spinal  accessory  nerve  properly  so-called,  makes  con- 
nections with  the  spinal  cord  below  the  bulb  as  far  back  as  the  sixth  or 
seventh  cervical  nerve,  or  even  lower ;  but  this  part  may  by  these  connec- 
tions be  distinguished  from  the  remaining  part  of  the  nerve,  as  well  as  from 
all  other  cranial  nerves.  The  nuclei  to  which  the  nerve-roots  may  be  traced 
within  the  brain  substance,  sometimes  spoken  of  as  the  "  deep  origin,"  range 
in  position  from  the  hinder  part  of  the  bulb  to  the  hind  end  of  the  third 
ventricle.  The  nucleus  of  the  hypoglossal  nerve  begins  in  the  bulb  just 
above  the  decussation  of  the  pyramids,  the  nucleus  of  the  third  nerve  ends 
beneath  the  hind  end  of  the  floor  of  the  third  ventricle ;  and  all  the  rest  of 
the  nuclei  may  be  broadly  described  as  placed  between  these  limits  in  various 
parts  of  the  floor  of  the  central  canal  or  in  adjoining  structures,  though  part 
of  one  nucleus,  namely,  that  of  the  fifth  nerve,  can  be  traced,  as  we  shall  see, 
back  into  the  spinal  cord  as  far  as  the  second  cervical  nerve,  and  probably 
extends  still  further.  Fig.  138  is  a  diagram  showing  in  a  roughly  approxi- 
mate manner  the  nuclei  of  the  several  nerves  as  they  would  appear  in  a  bird's- 
eye  view  of  the  floor  of  the  aqueduct  and  fourth  ventricle  looked  at  on  the 
dorsal  aspect. 

§  529.  The  twelfth  or  hypoglossal  nerve.  The  nucleus  of  this  nerve,  which 
it  will  be  convenient  to  take  first  (Fig.  138,  XII.),  is  a  long  column  of  gray 
matter  lying  in  the  bulb  parallel  to,  and  very  close  to,  the  median  line.  It 
reaches  from  the  hinder  part  of  the  fourth  ventricle  at  about  the  level  of  the 
hind  end  of  the  auditory  nucleus,  as  far  back  as  beyond  the  hind  end  of 
the  olivary  body.  At  its  extreme  hind  end  or  beginning  (Fig.  132,  2) 
it  occupies  a  ventral  position  and  is  a  part  of  the  anterior  horn ;  thence 
it  gradually  rises  dorsally  (Fig.  132,  3,  4,  5),  but  so  long  as  the  Central 
canal  remains  closed,  continues  to  occupy  a  distinctly  ventral  position  in 
reference  to  the  central  canal ;  in  its  front  part  it  is,  by  the  opening  up 


THE  GRAY  MATTER.  603 

of  the  fourth  ventricle,  brought  into  an  apparently  more  dorsal  position 
(Fig.  132,  6). 

The  nucleus  consists  mainly  of  large  nerve-cells  with  distinct  axis-cylinder 
processes,  which  though  pursuing  a  somewhat  irregular  course  may  be  traced 
into  the  fibres  of  the  nerve.  These,  starting  from  the  ventral  surface  of  the 
nucleus  along  its  length,  run  ventrally  through  the  reticular  formation,  and 
making  their  way  in  a  series  of  bundles,  between  the  olivary  nucleus  on  the 
lateral  side  and  the  pyramid  and  median  accessory  olive  on  the  median  side, 
gain  the  surface  along  the  groove  which  separates  the  pyramid  from  the 
olivary  body. 

§  530.  The  ninth  or  glosso-pharyngeal,  tenth  or  vagus,  and  eleventh  or 
spinal  accessory  nerves.  It  will  be  advantageous  to  consider  these  three 
nerves  together. 

In  the  spinal  accessory  nerves  we  must  distinguish,  as  we  have  said,  two 
parts:  the  "spinal  accessory"  proper,  formed  by  the  roots  which  come  off 
from  the  cervical  spinal  cord,  reaching  as  far  down  as  the  sixth  or  seventh 
cervical  nerve,  and  the  "  bulbar  accessory,"  whose  roots  come  off  from  the 
bulb  just  below  the  vagus. 

The  spinal  accessory  proper  takes  origin  in  the  group  of  cells  lying  in  the 
extreme  lateral  margin  of  the  anterior  horn,  from  whence  the  fibres  proceed 
directly  outward  through  the  lateral  column,  and  issue  from  the  cord  along 
a  line  immediate  between  the  anterior  and  posterior  roots ;  the  upper  roots 
undergo,  with  the  portion  of  the  lateral  horn  from  which  they  spring,  the 
shifting  spoken  of  in  §  518. 

The  bulbar  accessory  starts  from  an  elongated  nucleus  in  the  bulb  which 
is  common  to  it,  to  the  vagus,  and  to  the  glosso-pharyngeal ;  hence  we  have 
taken  these  three  nerves  together.  This  (Fig.  138)  stretches  further  for- 
ward than  the  hypoglossal  nucleus,  reaching  the  level  of  the  transverse  fibres 
called  striae  acusticse  (str.),  but  does  not  extend  so  far  behind. 

In  transverse  sections  of  the  bulb,  which  pass  a  little  below  and  a  little 
above  the  point  of  the  calamus  scriptorius  (Fig.  132,  4,  5),  two  nuclei  or 
collections  of  cells  are  seen  in  the  gray  matter  round  the  central  canal. 
The  more  ventral  one  is  the  hypoglossal  nucleus,  the  more  dorsal  one  the 
beginning  or  hind  part  of  the  combined  accessory-vago-glosso-pharyngeal 
nucleus. 

When  a  little  further  forward  the  central  canal  opens  out  into  the  fourth 
ventricle  (by  which  change  the  hypoglossal  nucleus  (Fig.  132,  6  n.  xii.)  is 
brought  nearer  to  the  dorsal  surface  in  the  floor  of  the  fourth  ventricle)  this 
combined  nucleus,  increasing  in  breadth,  is  thrown  to  the  side  and  assumes  a 
more  lateral  position,  lying  now  on  the  side  of,  but  still  somewhat  dorsal  to, 
the  hypoglossal  nucleus,  between  it  and  the  now  diminishing  gracile  nucleus. 
In  this  position  the  nucleus  appears  to  consist  of  two  parts,  a  median  and 
lateral,  the  median  part  having  conspicuous  nerve-cells  of  moderate  size,  the 
lateral  part  having  but  few  cells  and  those  of  small  size.  From  this  level 
the  nucleus  runs  forward,  maintaining  nearly  the  same  position  in  the  floor 
of  the  fourth  ventricle,  but  gradually  becoming  thinner,  and  ends,  as  we  have 
said,  at  about  the  level  of  the  striae  acusticse  on  the  dorsal  surface  correspond- 
ing on  the  ventral  surface  to  a  level  a  little  behind  the  hind  margin  of  the 
pons. 

From  this  combined  nucleus,  but  chiefly  from  the  median  part,  fibres 
sweep  in  a  ventral  and  lateral  direction  through  the  dorsal  part  of  the  retic- 
ular formation,  pass  ventral  to,  or  in  some  cases  through,  the  gelatinous  sub- 
stance and  the  strand  of  fibres  connected  with  the  fifth  nerve  (Fig.  132,  V. 
a),  and  reach  the  surface  of  the  bulb  on  its  lateral  aspect  in  a  line  between 
the  olivary  and  restiform  bodies  (Fig.  131,  C).  Along  this  line  may  be  seen 


004  THE  BRAIN. 

(Fig.  131,  (7.),  a  series  of  roots  ;  of  these  the  lowest,  the  accessory  roots,  spring 
from  the  hind  part,  the  highest,  the  glosso-pharyngeal  roots,  from  the  front 
part  (and  it  is  these  especially  which  pierce  the  gelatinous  substance — Fig. 
133,  IX.  a),  and  the  intermediate,  the  vagus  roots,  from  the  middle  part  of 
the  combined  nucleus.  Hence  we  may  speak  of  the  hind  part  of  the  whole 
nucleus  as  being  the  accessory  nucleus,  the  middle  part  as  the  vagus  nucleus, 
and  the  front  part  as  the  glosso-pharyngeal  nucleus. 

All  the  fibres,  however,  of  the  roots  of  these  three  nerves  do  not  take 
origin  from  the  nucleus  in  question ;  some  of  the  fibres  start  in  a  different 
way.  In  sections  of  the  bulb  above  the  decussation  of  the  pyramid  a 
patch  of  gray  matter  is  seen  lying  in  the  lateral  part  of  the  reticular 
formation  (Fig.  132,  X.  m.),  about  midway  between  ventral  and  dorsal  sur- 
faces. What  is  thus  disclosed  by  sections  is  a  column  of  gray  matter,  the 
"nucleus  ambiguus"  (Fig.  138,  wa.),  stretching  about  as  far  forward  and 
backward  as  the  combined  accessory-vago-glosso-pharyngeal  nucleus,  but 
placed  distinctly  more  ventrally  and  somewhat  more  laterally.  (In  Fig. 
138,  it  and  the  combined  nucleus  are  represented  on  different  sides  of 
the  diagram  to  avoid  confusion  through  the  overlapping  of  the  shad- 
ing.) From  it  fibres  curve  round  (Fig.  132,  X.  m.)  to  join  the  accessory- 
vago-glosso-pharyngeal  roots,  but  especially  the  vagus  roots.  It  may, 
therefore,  be  considered  as  a  second  nucleus  of  the  vagus  (and  possibly 
of  the  other)  roots. 

But  there  is  yet  a  third  source  of  some  of  the  fibres  of  the  nerves  of  which 
we  are  speaking.  In  sections  through  the  bulb  there  may  be  seen  just  ven- 
tral to  and  a  little  lateral  to  the  combined  nucleus  (Fig.  132,  4,  5,  6,  IX.  a.) 
the  circular  section  of  a  longitudinal  bundle  of  fibres.  In  the  hinder  sections 
(Fig.  132,  4)  the  bundle  is  a  very  thin  one,  and  still  further  back  it  is  lost  to 
view,  though  there  are  reasons  for  thinking  that  some  of  the  fibres  are  con- 
tinued back  into  the  cervical  cord,  as  far  as  the  origin  of  the  fourth  cervical 
nerve  or  even  beyond  ;  in  the  more  forward  sections  (Fig.  132,  5  and  6)  it 
increases  in  diameter  and  may  be  traced  forward  to  the  front  end  of  the  com- 
bined nucleus  into  which  it  merges.  It  is  a  bundle  of  fibres  which,  starting 
successively  in  the  lateral  gray  matter  of  the  cervical  cord  and  higher  up  in 
the  reticular  formation  of  the  bulb,  run  longitudinally  forward  ;  the  bundle 
at  first  increases  in  size  by  the  addition  of  fresh  fibres  at  each  step ;  but  fur- 
ther forward  the  fibres  leave  the  bundle  to  pass  into  the  roots  of  the  nerves 
of  which  we  are  speaking,  especially  of  the  glosso-pharyngeal,  and  the  bundle 
eventually  ends  in  front  by  passing  into  the  glosso-pharyngeal  roots.  The 
gray  matter  from  which  these  fibres  take  origin  does  not  form  a  defined  com- 
pact area,  is  not,  therefore,  a  nucleus  in  the  sense  in  which  we  are  now  using 
the  term,  but  is  diffused  among  the  rest  of  the  gray  matter  along  a  consider- 
able length.  The  fibres  are,  nevertheless,  fibres  of  nerve-roots,  and  the  bundle 
is  called  the  ascending  root  of  the  glosso-pharyngeal,  the  term  ascending  being 
used  since  it  is  customary  to  trace  such  structures  from  below  upward,  that 
is,  from  behind  forward  ;  though  since  the  fibres  in  question  are  probably 
afferent  fibres  carrying  impulses  backward  from  the  nerves  to  the  gray 
matter,  "  descending  "  would  be  the  more  appropriate  word.  The  bundle 
has  also  been  called  the  fasciculus  solitarius ;  and,  since  its  position  has 
been  supposed  to  correspond  to  that  of  the  area  marked  out  experimentally 
as  the  respiratory  centre  (§  304),  it  has  been  spoken  of  as  the  respiratory 
bundle. 

The  roots  of  these  three  nerves,  then,  the  bulbar  accessory,  the  vagus, 
and  the  glosso-pharyngeal — all  leaving  the  surface  of  the  brain  along  the 
line  between  the  olive  and  the  restiform  body,  and  all  so  far  alike  that  it 
is  impossible  upon  mere  inspection  to  say  where  in  the  series  the  fibres  of 


THE  GRAY  MATTER.  605 

the  middle  nerve,  the  vagus,  begin  and  end — spring  from  three  sources,  the 
combined  nucleus,  the  nucleus  ambiguus,  and  the  ascending  root. 

§  531.  The  eighth  or  auditory  nerve.  This  nerve  differs  from  the  other 
nerves  which  we  are  now  considering,  in  being  a  nerve  of  special  sense  ;  its 
arrangements  are  complicated.  In  a  view  of  the  base  of  the  brain-  (Fig. 
131,  (?)  the  nerve  is  seen  to  leave  the  surface  of  the  brain  from  the  ventral 
surface  of  the  fore  part  of  the  restifbrm  body  at  the  hind  margin  of  the  pons 
as  two  strands  or  roots,  one  of  which  winds  round  the  restiform  body  so  as  to 
reach  its  dorsal  surface,  while  the  other  appears  to  sink  into  the  substance  of 
the  bulb  to  the  median  side  of  the  restiform  body  ;  and  in  a  transverse  sec- 
tion of  the  bulb  (Fig.  133),  just  behind  the  pons,  the  two  roots  may  be  seen 
embracing  the  restiform  body,  one  passing  on  its  dorsal  and  the  other  on  its 
ventral  side.  The  former  is  called  the  dorsal  root  (Fig.  133),  or  sometimes 
the  lateral  root,  or,  since  it  reaches  back  or  lower  down  than  the  other,  the 
posterior  or  inferior  root;  the  latter  is  called  the  ventral  root  (Fig.  135),  or 
sometimes  the  median  root,  or,  since  it  reaches  further  forward  or  higher  up 
than  the  other,  the  anterior  or  superior  root.  When  we  come  to  study  the 
ear  we  shall  find  that  one  division  of  the  auditory  nerve  is  distributed  to  the 
cochlea  alone,  and  is  called  the  nervus  cochlearis,  the  rest  of  the  nerve  being 
distributed  to  the  utricle,  saccule  and  semicircular  canals  as  the  nervus  ves- 
tibularis.  As  we  shall  see,  there  are  reasons  for  thinking  that  the  vestibular 
nerve  carries  up  to  the  brain  from  the  semicircular  canals  impulses  other 
than  those,  or  besides  those,  which  give  rise  to  sensations  of  sound,  whereas, 
the  cochlear  nerve  appears  to  be  exclusively  concerned  in  hearing ;  and  in 
some  structural  details  these  two  divisions  of  the  auditory  nerve  differ  from 
each  other.  Hence  it  is  important  to  note  that  according  to  careful  inves- 
tigations the  cochlear  nerve  is  the  continuation  of  the  dorsal  root  and  the 
vestibular  nerve  the  continuation  of  the  ventral  root.  With  these  roots  of 
the  auditory  nerve  proper  also  issues,  a  little  in  front  of  the  ventral  root,  the 
small  nerve  called  the  portio  intermedia  Wrisbergi,  which  goes  to  join  the 
facial  nerve. 

The  auditory  nucleus,  as  a  whole,  is  a  broad  mass,  having  in  transverse 
sections  of  the  bulb  a  somewhat  triangular  form,  lying  in  the  lateral  parts 
of  the  floor  of  the  fourth  ventricle,  reaching  in  front  somewhat  beyond  the 
level  of  the  strise  acusticse,  and  overlapping  behind  the  front  parts  of  the 
nucleus  ambiguus  and  the  combined  accessory-vago-glosso-pharyngeal  nu- 
cleus :  it  extends  laterally  some  distance  outside  the  former  nucleus. 

The  nucleus,  however,  consists  of  two  distinct  parts,  a  median  or  inner 
nucleus  (Fig.  138,  VIII.,  m.),  characterized  by  the  presence  of  small  cells, 
and  a  lateral  or  outer  nucleus  (Fig.  138,  VIII.,  I.),  the  cells  of  which  are 
much  larger,  some  of  them  being  very  large.  The  lateral  nucleus  is  placed 
somewhat  deeper  than,  ventral  to,  the  median  nucleus  ;  it  also  extends  farther 
forward  (Figs.  133  and  134,  VIIL,  /?),  so  that  the  front  end  of  the  whole 
nucleus  is  furnished  by  the  lateral  nucleus  alone,  which  at  its  front  end 
occupies  a  more  dorsal  position  than  at  its  hind  end. 

Moreover,  this  auditory  nucleus  thus  placed  in  the  floor  of  the  fourth 
ventricle  is  not  the  whole  of  the  nucleus  of  the  auditory  nerve.  At  the 
convergence  of  the  dorsal  and  ventral  roots  on  the  ventral  surface  of  the 
restiform  body  is  placed  a  group  of  cells,  forming  a  swelling  which,  in  its 
general  appearance  and  in  the  characters  of  its  cells  is  not  unlike  a  gan- 
glion on  the  posterior  root  of  a  spinal  nerve.  This  is  called  the  accessory 
nucleus. 

When  we  trace  the  fibres  of  the  nerve  centralward  into  the  brain,  we  find 
that  a  large  number,  at  least,  of  the  fibres  of  the  dorsal  root,  cochlear  nerve 
(Fig.  133),  end,  according  to  most  observers,  in  the  cells  of  the  accessory 


606 


THE  BRAIN. 


nucleus  or  in  nerve-cells  lying  dorsal  to  the  accessory  nucleus,  and  especially 
in  a  group  of  cells  giving  rise  to  the  tuberculum  acusticum,  which,  small  in 
man,  is  conspicuous  in  some  animals.  Hence,  the  farther  part  of  this  dorsal 
root,  as  it  winds  round  the  lateral  and  dorsal  surface  of  the  restiform  body, 
consists  largely,  if  not  wholly,  of  fibres  which  are  derived  not  directly  from 
the  trunk  of  the  nerve,  but  indirectly  through  the  relay  of  the  accessory 
nucleus  or  of  other  cells.  Reaching  the  dorsal  surface  of  the  restiform  body, 
these  fibres  appear  on  the  floor  of  the  fourth  ventricle  as  the  strice  acusticce 
(Fig.  131,  s£r.),  and  end  partly  in  the  median  nucleus,  partly  in  other  regions 
of  the  bulb.  The  exact  determination,  however,  of  the  endings  of  this  root 

FIG.  134. 


Through  the  widest  part  of  the  Fourth  Ventricle.  (Sherrington.)  Taken  in  the  line  111,  Fig. 
131.  Py.,  pyramidal  fibres  cut  transversely ;  tr.  P.,  the  superficial  (ventral)  transverse  fibres  of  the 
pons.  The  shaded  part  of  the  pons  (gr.  P.)  indicates  gray  matter  mingled  with  the  deeper  trans- 
verse fibres.  -F.,  the  fillet ;  Tp.,  the  trapezium  ;  C.  R.,  the  restiform  body  or  inferior  peduncle  of 
the  cerebellum,  cut  across  obliquely  :  -S.  P.,  the  superior  peduncles  of  the  cerebellum  ;  r.,  raphe  ; 
s.  o.,  superior  olive  ;  C.  D.,  corpus  dentatum  of  the  cerebellum  ;  Iff.  n.,  the  nucleus  of  the  roof; 
s.  g.t  tubercle  of  Rolando  ;  V.  S.,  section  through  sulcus  in  the  vermis  superior  of  the  cerebellum  , 
t.,  bundle  from  the  olive  to  the  lenticular  nucleus  •  VIII.,  the  eighth  or  auditory  nerve,  its  ventral 
or  vestibular  root  proceeding  from  VIII.  ft.  the  front  part  of  the  lateral  auditory  nucleus  ;  VII.  n., 
the  nucleus  of  the  seventh  or  facial  nerve ;  VI.,  the  nucleus  of  the  sixth  nerve  ;  VII.  g.,  fibres  of 
the  seventh  nerve  cut  across  as  they  sweep  round  the  nucleus  of  the  sixth  before  issuing  from 
the  pons  at  VII.;  4th,  the  fourth  ventricle,  here  roofed  in  by  the  cerebellum;  the  shading  of 
the  central  gray  matter  immediately  surrounding  the  ventricle  is,  for  the  sake  of  simplicity 
omitted. 

is  a  matter  of  considerable  difficulty ;  some  observers  regard  the  accessory 
nucleus  as  homologous,  not  with  the  Gasserian  and  with  the  spinal  ganglia, 


THE  GRAY   MATTER.  607 

but  with  the  other,  true,  cranial  nuclei ;  and  in  any  case  we  must  probably 
consider  the  median  division  of  the  auditory  nucleus,  not  as  a  nucleus  in  the 
sense  in  which  we  are  now  using  it,  but  rather  as  a  secondary  connection 
within  the  bulb. 

When  we  trace  the  ventral  root,  vestibular  nerve  (Fig.  134),  inward,  we 
find  that  it  makes,  according  to  most  observers,  no  connections  at  all  with  the 
accessory  nucleus ;  and  that  it  passes  (Fig.  134,  F///.)  to  the  median  side  of  the 
restiform  body,  between  it  and  the  ascending  root  of  the  fifth  nerve,  and  so 
reaches  the  lateral  division  of  the  nucleus,  in  the  large  cells  of  which  most 
at  least  of  its  fibres  are  said  to  end,  and  which,  therefore,  may  be  regarded 
as  the  nucleus  of  the  ventral  root.  On  this  point,  however,  all  authors  are 
not  agreed.  The  lateral  auditory  nucleus,  with  the  fibres  proceeding  to  and 
from  it,  lying  as  they  do  to  the  median  or  inner  side  of  the  restiform  body 
proper,  are  sometimes  spoken  of  as  the  median  or  inner  division  of  the  resti- 
form body;  and  from  the  nucleus  a  considerable  number  of  fibres  pass  up 
with  the  restiform  body  into  the  cerebellum  as  a  continuation  of  this 
"  median  division  of  the  restiform  body."  Some  authors  maintain  that  these 
fibres  are  continued  straight  on  from  the  nerve  to  the  cerebellum ;  but  the 
more  recent  investigations  seem  to  show  that  they  all  make  connections  with 
the  nerve-cells  of  the  lateral  nucleus  on  their  way.  These  fibres  constitute 
a  connection  between  the  auditory  (vestibular)  nerve  and  the  cerebellum, 
the  physiological  significance  of  which  we  shall  see  later  on  ;  we  may,  per- 
haps, compare  it  to  the  connection  between  the  posterior  roots  of  the  spinal 
nerves  and  the  cerebellum  through  (the  vesicular  cylinder  and)  the  cere- 
bellar  tract. 

The  other  central  connections  of  the  lateral  nucleus  are,  like  those  of 
the  accessory  and  of  the  median  nucleus,  complicated  and  obscure.  But  we 
may  call  attention  to  a  set  of  fibres  which,  starting  apparently  in  the  acces- 
sory nucleus,  run  directly  transverse  in  the  ventral  region  of  the  tegmentum 
just  dorsal  to  the  transverse  fibres  of  the  pons,  forming  what  is  called  the 
trapezium  (Fig.  134,  Tp.). 

Lastly,  we  may  add  that  the  fibres  of  the  peculiar  portio  intermedia 
appear  to  take  origin  from  the  accessory  nucleus. 

§  532.  The  seventh  or  facial  nerve.  The  nucleus  (Fig.  138,  VII.,  and 
Figs.  133,  134,  VII.  7i.)  of  this  nerve  (it  being  borne  in  mind  that  the  motor 
fibres  for  the  orbital  region  (the  orbicular  muscle,  etc.),  though  they  run  in 
the  trunk  of  this  nerve,  really  belong  to  the  third  nerve  and  take  origin 
from  the  hind  part  of  the  nucleus  of  the  third  nerve)  is  narrower  in  front 
than  behind,  reaches  from  the  level  of  the  striae  acusticse  some  distance  into 
the  region  of  the  pons,  and  occupies  in  the  midst  of  the  reticular  formation, 
a  little  dorsal  of  the  patch  of  gray  matter  called  the  upper  olive,  a  position 
corresponding  closely  to  that  of  the  nucleus  ambiguus.  The  cells  of  the 
nucleus  are  large,  and  possess  well-marked  axis-cylinder  processes,  which 
are  gathered  up  at  the  dorsal  surface  of  the  nucleus  to  form  the  root.  This, 
rising  up  dorsally,  describes  a  loop  (Fig.  134,  VII.  #.)  round  the  nucleus  of 
the  sixth  or  abducens  nerve,  running  forward  for  some  little  distance  dorsal 
to  that  nucleus,  and  then  descends  again  ventrally,  passing  to  the  lateral 
side  of  its  own  nucleus,  between  it  and  the  ascending  root  of  the  fifth  (  V.a.)  ; 
it  thus  gains  the  surface  of  the  brain  at  the  hinder  margin  of  the  pons, 
lateral  to  the  abducens,  opposite  the  front  end  of  the  groove  between  the 
olivary  body  and  the  restiform  body.  As  it  thus  encircles  the  nucleus  of 
the  abducens,  it  looks  as  if  it  were  receiving  fibres  from  that  body ;  but  the 
evidence  goes  to  show  that  these  fibres  simply  pass  through  the  nucleus,  and 
do  not  take  origin  from  any  of  its  cells. 

§  533.    The  sixth  or  abducens  nerve.  This  nerve  starts  from  a  compact  oval 


608 


THE  BRAIN. 


nucleus  (Fig.  138,  VI.),  lying  at  the  level  of  the  hinder  part  of  the  pons, 
and  therefore  of  the  front  part  of  the  fourth  ventricle,  in  the  central  gray 
matter  of  the  floor  of  the  ventricle,  or  rather  just  between  it  and  the  reticular 
formation,  a  little  on  one  side  of  the  median  line  (Fig.  134,  VI.).  A  slight 
swelling  of  the  floor  of  the  fourth  ventricle,  eminentia  teres,  marks  its  posi- 
tion (Fig.  138,  e.  t.).  The  nucleus  contains  fairly  large  nerve-cells,  with  dis- 
tinct axis-cylinder  processes.  These  are  gathered  at  the  median  side  of  the 
nucleus  to  form  the  thin  root,  which,  passing  ventrally  and  laterally,  at  some 
little  distance  from  the  median  raphe,  through  the  reticular  formation,  runs 
backward  above  the  pyramidal  bundles  of  the  pons,  and  finally  comes  to 
the  surface  at  the  hinder  edge  of  the  pons,  opposite  the  front  end  of  the 
pyramid  (Fig.  131,  C.). 


ar.P 


Through  the  Pons  at  the  Exit  of  the  Fifth  Nerve.  (Sherrington.)  In  the  line  112,  Fig.  131. 
C.  R.  remains  of  restiform  body  ;  S.  P.  superior  peduncle  of  the  cerebellum  ;  F.  m.  median  ;  F.  I. 
lateral  fillet ;  T.  R.,  tegmental  reticular  formation ;  tr.  P.  superficial  transverse  fibres  of  the  pons  ; 
I.  posterior  longitudinal  bundles  ;  V.  s.  superior  vermis  (sections  of  three  folia  are  shown,  one 
being  detached ;  between  them  the  intervening  sulci  laid  open  by  the  section  are  seen)  :  IVu. 
valve  of  Vieussens  or  anterior  velum  :  r.  raphe;  Py.  pyramidal  fibres  ;  gr.  P.  gray  matter  of  the 
pons  ;  s.  o.,  siiperior  olive  ;  t.,  placed  on  the  left  side  indicates  the  position  of  a  bundle  of  longi- 
tudinal fibres  which  may  be  traced  forward  into  the  subthalamic  regions  ;  V.  m.  motor  nucleus; 
V.  s.,  sensory  nucleus;  and  V.  roots  of  the  fifth  nerve.  4th,  fourth  ventricle;  shading  of  cen- 
tral gray  matter  omitted  as  in  Fig.  134. 

§  534.  The  fifth  or  trigeminal  nerve.  This  nerve,  as  it  comes  to  the  sur- 
face on  the  ventral  aspect  of  the  pons  (Fig.  131,  (7.),  near  the  front  edge, 
at  some  distance  from  the  median  line,  consists  of  two  parts,  a  smaller  motor 


THE  GRAY   MATTER, 


609 


root  and  a  larger  sensory  root,  the  latter  bearing  the  large  ganglion  of  Gas- 
ser ;  and  the  origin  of  the  nerve  is  in  many  ways  complex.  Both  roots 
may  be  traced  in  an  oblique  direction  (Fig.  135,  F.),  inward  and  toward 
the  dorsal  surface,  through  the  pons  to  the  reticular  formation  beneath  the 
floor  of  the  front  part  of  the  fourth  ventricle,  the  smaller  motor  root  taking 
up  a  position  median  to  the  larger  sensory  root. 

Here  the  motor  root  comes  into  connection  with  a  collection  of  nerve- 
cells  (Figs.  138  and  135,  V.  m.),  which  may  be  regarded  as  its  nucleus ;  but 
this  is  not  the  whole  nucleus  of  the  motor  root.  From  the  level  of  the 
nucleus  there  stretches  forward  as  far  as  the  level  of  the  anterior  corpora 
quadrigemina  a  bundle  of  longitudinal  fibres  which,  since  it  is  usually  traced 
from  the  front  backward  until  it  passes  into  the  root  of  the  nerve,  is  spoken 
of  as  the  descending  root  of  the  fifth  nerve. 

This  descending  root  begins  as  a  few  scattered  bundles  of  fibres  at  the 
level  of  the  anterior  corpora  quadrigemina,  in  the  peripheral  lateral  part 
of  the  central  gray  matter  surrounding  the  aqueduct,  dorsal,  and  lateral 
(Fig.  137,  V.  d),  to  the  nucleus  of  the  third  nerve  (Fig.  137,  III.  n.). 
From  thence  the  fibres  pass  backward,  augmenting  in  number,  and  soon 
form  a  compact  bundle,  semilunar  in  transverse  section,  lying  lateral  to  the 
fourth  nerve  as  this  is  rising  dorsally  (Fig.  136,  V.  d.~)  ;  still  increasing  in 


Through  the  Fore  Part  of  the  Pons.  (Sherrington.)  In  the  line  113,  Fig.  131.  Py.  pyramidal 
fibres  ;  F.  C.  fibres  from  the  frontal  cortex  ;  8.  P.  superior  peduncle  of  the  cerebellum ;  Fm.  me- 
dian portion;  Fl.  lateral  portion  of  the  fillet;  1.  posterior  longitudinal  bundles;  P.  C.  Q.  posterior 
corpora  quadrigemina ;  y.  fibres  which  become  detached  from  the  fillet,  and  further  forward  form 
(the  innermost)  part  of  the  pes  of  the  crus ;  1.  c.  locus  cseruleus ;  n.  P.  Q.  nucleus  of  the  posterior 
corpora  quadrigemina— the  outline  is  made  too  sharp;  IV.  bundles  of  the  fourth  nerve  decussat- 
ing, IV.  n.  its  nucleus  ;  V.  d.  descending  root  of  the  fifth  nerve ;  Aq.  the  aqueduct ;  c.  g.  the  region 
of  central  gray  matter. 

number  in  their  course  backward,  they  gradually  assume  a  more  ventral 
position  as  the  aqueduct  opens  into  the  fourth  ventricle.  All  along  its 
course  this  descending  root  has  attached  to  it  large  (70  A*  or  more  in  diam- 
eter), sparse  spheroidal  nerve-cells  of  striking  appearance ;  these,  however, 
seem  too  few  to  give  origin  to  all  of  the  fibres,  and  there  are  some  reasons 

39 


610 


THE  BRAIN. 


for  connecting  this  root  with  the  collection  of  gray  matter  called  "  locus  csd- 
ruleus."  (Fig.  136,  I.  c.). 

We  may  probably  regard  this  descending  root  as  belonging  to  the  motor 
division  of  the  nerve  ;  but  it  is  stated  that  many  of  the  fibres  of  this  root 
pass  into  the  sensory  root,  eventually  finding  their  way,  according  to  some 
observers,  into  the  ophthalmic  branch. 

The  sensory  root  may  be  similarly  traced  into  a  nucleus,  the  sensory 
nucleus  (Figs.  138  and  135,  V.  s.)  lying  lateral  to  the  motor  nucleus,  and 
connected  with  this  is  the  striking  tract  of  fibres,  to  which  already  we  have 
so  frequently  alluded,  and  which  is  called  the  ascending  root  of  the  fifth 


nerve. 


This  ascending  root  begins  as  a  bundle  or  bundles  of  few  fibres  which 
may  be  traced  backward  as  far,  at  least,  as  the  level  of  the  second  cervical 
nerve,  and  is  soon  conspicuous  in  transverse  sections  (Figs.  132  et  seq.,  V.  a.) 
as  a  semilunar  patch  of  white  matter  forming  a  sort  of  cap  on  the  outside  of 
the  swollen  caput  of  the  posterior  horn,  between  this  structure  and  the 
longitudinal  fibres  which  are  beginning  to  form  the  restiform  body  on  the 


Through  the  Crus  and  Anterior  Corpora  Quadrigemina.  (Sherrington).  One-half  only  is  shown 
in  the  line  114,  Fig.  131.  Py.  the  pyramidal  portion  of  the  pes  ;  jFV.the  region  of  the  pes  occupied 
by  fibres  from  the  frontal  portion  of  the  cortex;  Pr.  0.  the  region  occupied  by  fibres  coming  from 
the  occipital  portion  of  the  cortex  ;  y.  fibres  coming  from  the  fillet ;  Op.  the  optic  tract ;  F.  the 
fillet,  1.  the  lateral  portion,  m.  the  median  portion;  I.  the  posterior  longitudinal  bundle;  B.  a.  the 
brachium  of  the  anterior  corpus  quadrigeminum  ;  x.  fibres  from  the  posterior  commissure  of  the 
cerebrum ;  r.  raphe  ;  S.  n.  substantia  nigra ;  R.  n.  red  nucleus  ;  C.  g.  1.  lateral,  and  C.  g.  m.  median 
corpus  geniculatum  ;  P?;r.  pulvinar  of  optic  thalamus  ;  A.  Q.  n.  nucleus  or  gray  matter  of  anterior 
corpus  quadrigeminum  ;  III.  n.  nucleus  of  III.  third  nerve  ;  III/  rootlets  from  the  dorsal  part  of 
III.  n.  the  nucleus  of  the  third  nerve,  which  cross  the  median  line  to  emerge  with  rootlets  derived 
from  the  nucleus  of  the  opposite  side;  s.  m.  superficial  layer  of  fibres  of  the  ant.  corp.  quad.;  d. 
m.  deep  layer;  Aq.  aqueduct  surrounded  by  cerebral  gray  matter. 

surface.  Passing  upward,  and  continually  augmenting  in  bulk,  the  root 
clings,  as  it  were,  to  the  gelatinous  substance  of  the  caput  of  the  posterior 
horn,  and  sinks  with  it  inwardly  and  ventrally  as  this  becomes  covered  up 
first  by  the  restiform  body  and  subsequently  by  the  issuing  trunk  of  the 
great  eighth  or  auditory  nerve  (Figs.  133,  134).  Passing  still  forward,  be- 
yond the  disappearing  gelatinous  substance,  the  root,  still  growing  larger 
and  divided  into  several  distinct  bundles,  runs  into  the  reticular  formation 
of  the  pons  and,  reaching  the  level  of  the  sensory  nucleus,  suddenly  bends 
rounds  and  joins  the  sensory  root. 


THE  GKAY   MATTER. 
FIG.  138. 


611 


Diagram  to  illustrate  the  Position  of  the  Nuclei  of  the  Cranial  Nerves.    (Sherrington.)    The 
brain  is  supposed  to  be  viewed  from  the  dorsal  aspect,  the  cerebral  hemispheres  and  cerebellum 


612  THE  BRAIN. 

This  ascending  root  differs  from  the  descending  root  in  not  having  con- 
spicuously attached  to  it  any  collection  of  nerve-cells ;  in  this  respect  it- 
resembles  the  ascending  root  of  the  glosso-pharyngeal,  and  we  may  add  part 
of  the  posterior  root  of  an  ordinary  spinal  nerve,  the  fibres  of  which,  as  we 
have  seen,  pass  into  the  gray  matter  without  being  obviously  connected  with 
nerve-cells.  In  its  lower  part  at  least  it  consists  of  extremely  fine  fibres,  and 
indeed  looks  very  much  like  a  continuation  of  the  bulb  of  the  marginal 
(Lissauer's)  zone  of  the  spinal  cord. 

§  535.  The  fourth  or  trochlear  nerve.  The  nucleus  of  this  nerve  (Fig. 
138,  IV.)  is  a  column  of  somewhat  large  multipolar  cells  on  each  side  of  the 
median  line  below  the  aqueduct  (Fig.  136,  IV.  n.),  reaching  from  the  level 
of  the  junction  of  the  anterior  and  posterior  corpora  quadrigemina  to  the 
hinder  level  of  the  latter  body. 

The  root,  starting  from  the  lateral  surface  of  the  nucleus,  does  not  take 
at  first  a  ventral  direction,  but  sweeps  laterally  and  dorsally  in  the  outer 
layers  of  the  central  gray  matter  (Fig.  136),  and  so  curving  round  to  the 
dorsal  surfaces  reach  the  valve  of  Vieussens,  where  in  the  median  line  it 
decussates  with  its  fellow  in  the  substance  of  the  valve;  such  a  decussatiori 
at  a  distance  from  the  nucleus  of  origin  is  exceptional  in  the  cranial  nerves. 
Leaving  the  surface  of  the  brain  in  the  valve,  it  takes  a  superficial  course, 
curving  (Fig.  131,  B)  laterally  and  ventrally,  and  makes  its  appearance  in 
a  ventral  view  of  the  brain  at  the  front  edge  of  the  pons,  on  the  lateral  edge 
of  the  crus  (Fig.  131,  C). 

§  536.  The  third  or  oeulo-motor  nerve.  The  nucleus  of  this  nerve  (Fig. 
138,  ///.,  137,  III.  n.)  is  a  column  of,  for  the  most  part,  fairly  large  multip- 
olar cells  lying  on  each  side  close  to  the  median  line,  in  the  gray  matter  of 
the  central  canal,  just  dorsal  to  a  bundle  of  fibres  which  we  shall  speak  of 
as  the  longitudinal  posterior  bundle;  it  reaches  from  the  level  of  the  pos- 
terior commissure  in  the  third  ventricle  to  the  level  of  the  junction  of  the 
anterior  and  posterior  corpora  quadrigemina.  In  a  section  taken  through 
its  middle  (Fig.  137)  the  nucleus  is  seen  to  give  off  fibres  which  run  verti- 
cally toward  the  ventral  surface,  traversing  the  tegmentum  and  a  body  (En.) 

having  been  cut  away.  The  nuclei  are  represented  as  if  seen  through  transparent  material.  On 
the  right  side,  the  corpus  striatum  and  optic  thalamus  have  been  cut  away  horizontally  to  some 
little  depth  in  order  to  show  their  internal  structure.  L.  lateral,  E.  P.  external  posterior,  and  M. 
P.  median  posterior  column  of  the  cord;  I.  P.  inferior  peduncle,  S.  P.  superior  peduncle,  and  P. 
middle  peduncle  of  the  cerebellum,  all  cut  across.  The  dotted  curved  lines,  upper  and  lower,  on 
the  right  half  of  the  figure  to  which  the  dotted  line  P.  V.  outside  the  figure  points,  mark  the 
upper  and  lower  boundaries  of  the  pons  on  the  ventral  aspect.  The  outline  of  the  fourth  ven- 
tricle is  shown  by  a  bold  thick  line.  In  the  floor  of  the  ventricle  are  shown,  on  the  right  half: 
fp.  fovea  posterior ;  Th.  trigonum  hypoglossi ;  T.  ac.  trigonum  acusticum  ;  e.  t.  eminentia  teres : 
«.  m.  stride  medullares  on  acusticee ;  /.  a.  fovea  anterior ;  1.  c.  locus  cseruleus;  I.  g.  valve  of  Vieus- 
sens; Qp.  posterior,  and  Qa.  anterior  corpus  quadrigeminum  ;  Pg.  pineal  gland  ;  Nr.  the  outline 
of  the  red  nucleus;  3,  the  third  ventricle,  in  which  C.  indicates  the  middle  or  soft  commissure; 
F.  p.  a.  the  pillars  of  the  fornix,  behind  which  is  indicated  in  the  cavity  of  the  third  ventricle  the 
hollow  of  the  infundibulum  ;  C.  C.  g.  the  genu  of  the  corpus  callosum,  between  which  and  the 
fornix  the  cavity  often  called  the  fifth  ventricle  is  indicated;  F.  portion  of  convolution  of  frontal 
hemisphere  cut  across.  On  the  left  side  are  shown  :  C.  S.  corpus  striatum  ;  0.  T.  optic  thalamus ; 
Pv  pulvinar-  T  a.  tuberculum  anterius ;  ch.  s.  choroidal  sulcus  marking  the  place  of  reflection 
of  the  choroidal  plexus.  On  the  right  side  are  exposed  :  JV.  C.  head  of,  Nc.  end  of  tail  of  nucleus 
caudatus  ;  Cip'.,  dp",  the  two  parts  of  the  globus  pallidus,  and  Pt.  putamen  of  the  nucleus  le: 
cularis;  N.  a.  anterior  nucleus ;  N.  med.  medium  nucleus,  N.  lat.  lateral  nucleus,  and  Pv'.  pulvinar 
of  the  optic  thalamus;  da.  front  limb,  Cig.  knee  or  genu,  Cip.  hind  limb  of  internal  capsule ; 
external  capsule  ;  Cl.  claustrum.  The  numerals  III.  to  XII.  indicate  the  nuclei  of  the  respect 
cranial  nerves,  all  shown  on  the  left  side  with  the  exception  of  the  accessory  vago-glosso-pharyn- 
geal  IX..  X.,  IX.,  which  to  avoid  confusion  is  placed  on  the  right  side.  V.  is  the  motor  nuc 
of  the  fifth  nerve  with  the  descending  root,F.  a.  the  sensory  nucleus  of  the  same  with  the  long 
ascending  root  VIII.  m.  median  nucleus,  VIII.  I.  lateral  nucleus  of  the  auditory  nerve  :  n.  a.  nu- 
cleus ambiguus.  The  ascending  root  of  the  ninth  nerve  is  seen  at  the  hind  end  of  the  combined 
nucleus  of  IX., X.,  XI. 


THE  GRAY  MATTER.  613 

which  we  shall  presently  speak  of  as  the  "  red  nucleus,"  but  apparently 
making  no  connections  with  these  structures,  and  pierce  the  median  edge  of 
the  pes,  emerging  (Fig.  131,  (?.)  on  the  surface  to  the  median  side  of  each 
crus.  As  we  shall  see  later  on,  this  nerve  is  now  exclusively  efferent,  what- 
ever it  may  have  been  in  more  primitive  beings.  We  shall  also  see  later  on 
that  impulses  starting  from  the  cerebrum  of  one  side  pass  to  the  nerve  of  the 
other  side,  that  is  to  say,  decussate  ;  and  this  is  also  the  case  with  the  other 
efferent  cranial  nerves.  The  fibres  which  appear  to  take  origin  from  the 
nerve-cells  of  the  nucleus  do  not  cross  over  after  emerging  from  the  nucleus, 
but  keep  to  the  same  side  ;  there  is  no  distant  decussation  as  in  the  case  just 
noted  of  the  fourth  nerve.  There  are,  however,  fibres  (Fig.  137,  III'.)  which 
leaving  the  nucleus  cross  the  median  raphe  from  one  side  to  the  other,  and 
these  possibly  are  the  paths  for  the  decussation  of  the  impulses ;  but  they 
may  be  fibres  passing  from  the  crus  across  the  raphe  to  the  nucleus.  This 
nerve  has  special  relations  with  the  optic  tract,  but  of  these  we  shall  speak 
when  we  come  to  deal  with  the  functions  of  the  nerves. 

§  537.  In  attempting  to  understand  the  nature  and  relations  of  these 
cranial  nerves,  it  must  be  borne  in  mind  that,  while  morphological  studies 
lead  us  to  believe  that,  as  the  vertebrate  body  has  been  developed  out  of  an 
invertebrate  ancestry,  so  the  brain  of  the  vertebrate  has  arisen  by  a  series 
of  modifications  from  the  nervous  structures  placed  at  the  head  and  around 
the  mouth  of  an  invertebrate,  the  same  studies  teach  us  that  such  an  evo- 
lution has  been  accomplished  by  means  of  profound  changes.  We  have,  for 
instance,  reason  to  think  that  the  mouth  of  the  vertebrate  does  not  corre- 
spond to  the  mouth  of  the  invertebrate,  but  is  a  new  structure,  whose  appear- 
ance has  been  accompanied  by  a  considerable  dislocation  of  parts.  We  must 
accordingly  expect  to  find  the  indications  of  a  segmental  arrangement  greatly 
obscured  on  the  one  hand  by  transposition,  and  on  the  other  by  fusion. 

The  twelfth  or  hypoglossal  nerve  is  one  whose  nature  seems  fairly  simple. 
It  is  in  function  exclusively  an  efferent  nerve.  The  large  cells,  with  con- 
spicuous axis-cylinder  processes,  which  characterize  its  nucleus,  are  exactly 
like  those  of  the  anterior  horn  of  the  spinal  cord  which  give  origin  to  the 
fibres  of  an  anterior  root.  The  nucleus,  moreover,  in  its  position  corresponds 
to  part  of  the  anterior  horn  of  the  spinal  cord,  if  we  take  into  account  the 
shifting  involved  in  the  decussation  of  the  pyramids,  and  in  the  new  develop- 
ments of  the  bulb.  If  we  compare  Fig.  132  with  any  section  of  the  cord,  we 
see  that  the  hypoglossal  nerve  corresponds  to  an  anterior  root  of  the  spinal 
cord,  but  that  the  fibres,  after  leaving  the  cells  from  which  they  take  their 
origin,  traverse  in  the  former  a  large  tract  and  in  the  latter  case  a  small 
tract  of  tissue.  Whether  the  whole  nerve  corresponds  to  the  fibres  of  several 
segments  fused  together  or  to  those  of  one  segment  spread  out  longitudinally 
is,  for  our  present  purposes,  of  secondary  importance. 

Recognizing  the  hypoglossal  nerve  as  the  homologue  of  a  spinal  anterior 
root,  we  may  go  on  to  claim  the  nuclei  of  the  third  and  fourth  nerves  as 
similar  groups  of  cells  of  the  anterior  horn,  giving  rise  to  anterior  roots. 
The  position  of  the  nuclei,  the  character  of  the  cells,  the  function  of  the 
fibres,  all  support  this  view.  The  case  is  perhaps  not  so  clear  as  that  of  the 
hypoglossal  nerve,  since  there  are  reasons  for  thinking  that  these  nerves 
have  undergone  in  the  course  of  evolution  greater  changes  than  has  the 
hypoglossal  nerve ;  still  these  reasons  do  not  oppose  the  above  conclusion. 

The  nucleus  of  the  exclusively  motor  sixth  nerve  does  not  exactly  corre- 
spond to  those  of  the  third  and  fourth  in  position ;  but  we  may  probably 
place  it  in  the  same  series  with  them.  Thus  we  have  in  succession  the  third, 
fourth,  sixth,  and  twelfth  nerves,  with  their  respective  nuclei,  as  the  anterior 
roots  of  nerves  of  their  several  segments. 


614  THE   BRAIN. 

In  the  fifth  nerve  the  dislocation  and  fusion  spoken  of  above  has  intro- 
duced difficulties.  The  motor  nucleus,  with  the  fibres  of  the  motor  root  to 
which  it  gives  origin,  has  by  some  been  considered  as  homologous  to  the 
series  just  described  ;  but  it  is  at  once  obvious  that  we  cannot  look  upon  this 
great  fifth  nerve  as  corresponding  to  one  spinal  nerve,  with  its  anterior  and 
posterior  root,  great  as  the  superficial  resemblance  seems  to  be.  The  features 
of  the  remarkable  ascending  root  forbid  this.  The  fibres  of  this  root  may  be 
traced  back,  as  we  have  said,  to  the  very  beginning  of  the  bulb,  and,  indeed, 
into  the  spinal  cord  beyond  ;  as  far  as  can  be  ascertained,  they  are  not  in  an 
obvious  and  direct  manner  connected  with  nerve-cells  along  their  course  ; 
but  the  bundle  of  fibres  clings,  as  we  have  seen,  to  the  gelatinous  substance 
of  the  posterior  horn  of  the  spinal  cord  and  to  the  continuation  of  this  along 
the  bulb,  and  the  fibres  are  lost  in  this  structure.  The  root,  therefore,  as  we 
have  said,  corresponds  very  closely  to  part  at  least  of  the  posterior  root  of  a 
spinal  nerve,  and  though  the  matter  has  not  yet  been  experimentally  proved, 
we  may  infer  that  the  trophic  centres  of  these  fibres  are  to  be  found  in  the 
cells  of  the  Gasserian  ganglion. 

But  if  the  ascending  root  be  of  the  nature  of  a  posterior  root  (and  we 
may  incidentally  remark  that  the  term  ascending  has  been  unhappily 
chosen,  since,  if  it  be  an  afferent  root,  the  direction  of  the  impulses  which 
it  carries  will  be  a  descending  one,  namely,  from  the  entrance  in  the  pons 
toward  the  hinder  parts),  we  can  hardly  suppose  that  it  belongs  to  a  single 
segment,  or  is  the  complement  of  the  motor  root  alone  ;  in  it,  most  prob- 
ably, the  posterior  fibres  of  several  segments  are  blended  together.  Further, 
we  may,  perhaps,  infer  that  the  other  fibres  of  the  sensory  root  which  end 
directly  in  what  we  have  called  the  sensory  nucleus,  are  in  nature  quite  dis- 
tinct from  the  fibres  of  the  ascending  root ;  and,  if  so,  difficulties  arise  as  to 
the  nature  and  homologies  of  the  nucleus  in  question.  These,  however,  we 
must  not  discuss  here,  nor  can  we  enter  into  the  question  of  the  nature  of 
the  descending  root,  concerning  the  fibres  of  which,  as  we  have  said,  author- 
ities differ  as  to  whether  they  pass  into  the  motor  or  sensory  root.  We  have 
said  enough  to  show  that  this  fifth  nerve  is  extremely  complex,  and  that  its 
apparent  conformity  to  a  simple  spinal  nerve  is  in  reality  misleading. 

The  fibres  of  the  vagus,  glosso-pharyngeal,  and  bulbar  accessory,  taken 
together,  are  partly  efferent,  partly  afferent.  The  combined  nucleus  of  these 
three  nerves,  the  cells  of  which  are  small  and  devoid  of  conspicuous  axis- 
cylinder  processes,  is  usually  regarded  as  a  sensory  nucleus,  and  in  the  dia- 
gram (Fig.  138)  is  shaded  accordingly.  It  may,  perhaps,  be  compared  to 
the  sensory  nucleus  of  the  fifth.  Thus,  the  ascending  root,  or  fasciculus  soli- 
tarius,  presents  many  analogies  with  the  ascending  root  of  the  fifth,  and  we 
are  led  to  regard  this  as,  like  it,  a  gathering  of  certain  afferent  fibres  of  the 
posterior  roots  of  several  segments ;  in  its  case  also  the  term  ascending  is 
misleading.  But  there  are  many  difficulties  in  connection  with  this  nucleus, 
as  with  the  fifth.  We  must  not  enter  into  a  detailed  discussion  concerning 
them,  but  may  remark  that  we  have  here  perhaps  to  deal  with  complexities 
due  to  the  fact  that  certainly  many  vagus  and  glosso-pharyngeal  fibres,  and 
probably  some  of  those  of  the  fifth,  are  splanchnic  in  function. 

The  nucleus  ambiguus  contains  large  conspicuous  cells  and  we  may  prob- 
ably regard  it  as  a  motor  nucleus,  especially  of  the  vagus  fibres.  We  may 
also,  perhaps,  place  it  and  the  nucleus  of  the  seventh  nerve  in  the  same  cate- 
gory, and  further  class  with  them  the  motor  nucleus  of  the  fifth,  looking 
upon  all  three  as  so  many  detached  portions  of  gray  matter,  corresponding 
to  some  part  of  the  anterior  horn  of  the  spinal  cord.  Whether  they  are 
exactly  homologous  to  the  hypoglossal  nucleus,  and  their  fibres  to  simple 
anterior  roots,  is  not  so  clear. 


THE  GRAY  MATTER.  615 

Lastly,  the  auditory  nerve,  both  from  its  character  as  a  nerve  of  special 
sense  and  from  the  remarkable  features  of  its  nuclei,  is  even  more  difficult. 
Most  probably  it  results  from  the  fusion  of  more  roots  than  one ;  but  it  is 
impossible  at  present  to  obtain  a  clear  conception  of  the  nature  of  the  whole 
nerve. 

2.    The  Superficial  Gray  Matter. 

§  538.  The  whole  of  the  surface  of  each  cerebral  hemisphere  for  some 
little  depth  inward  consists  of  gray  matter,  possessing  special  characters; 
this  is  called  the  cortical  gray  matter,  or  the  cortex  cerebri,  or  shortly  and 
simply  the  cortex.  As  we  shall  see,  by  its  histological  and  still  more  by  its 
physiological  features,  it  stands  apart  from  all  other  kinds  of  gray  matter. 

The  whole  of  the  surface  of  the  cerebellum  is  also  covered  with  gray 
matter,  which,  while  possessing  features  of  its  own,  so  far  resemble  the  cere- 
bral cortex,  in  its  histological  characters  that  it  too  has  been  spoken  of  as 
cortex,  as  the  cortex  cerebelli.  By  its  functional  manifestations,  however, 
it  differs  widely  from  the  cerebral  cortex ;  and  since  there  are  many  advan- 
tages in  being  able  to  use  the  word  cortex  in  connection  with  the  cerebrum 
only,  it  is  desirable  not  to  speak  of  a  cerebellar  cortex  but  to  employ  the 
term  "  superficial  gray  matter  of  the  cerebellum." 

The  third  ventricle  and  the  hinder  part  of  the  fourth  ventricle  are  not 
roofed  in  by  nervous  material,  and  possess  no  superficial  gray  matter  at  all. 
In  the  corpora  quadrigemina,  which  form  the  roof  of  the  aqueduct  or  cavity 
of  the  mid-brain,  gray  matter  is  present  and  possesses,  in  the  case  of  the 
anterior  corpora  quadrigemina  at  least,  characters  to  a  certain  extent  analo- 
gous to  those  of  the  cortex  and  to  the  cerebellar  superficial  gray  matter ; 
but  it  will  be  best  to  consider  the  gray  matter  of  these  bodies  as  belonging 
to  another  category. 

3.    The  Intermediate  Gray  Matter  of  the   Crural  System. 

§  539.  We  have  seen  (§  516)  that  the  crura  cerebri  form  the  promi- 
nent part  of  a  system  of  longitudinal  fibres  stretching  from  each  cerebral 
hemisphere  to  the  bulb  and  to  the  spinal  cord.  This  system  of  fibres,  upon 
which  we  may  consider  the  various  parts  of  the  brain  to  be,  as  it  were, 
founded,  we  may  speak  of  as  the  crural  system.  It  is,  it  is  true,  not  one 
continuous  strand,  but  a  number  of  different  strands,  having  different  begin- 
nings and  endings  ;  but  these  all  contribute  to  the  crura  and  are  so  far  alike 
as  to  justify  us  in  considering  them  as  a  system.  The  cortical  gray  matter 
of  each  hemisphere  is,  as  we  shall  see,  connected  with  various  parts  of  this 
system,  and  in  one  sense  we  may  regard  this  system  as  beginning  in  the  cor- 
tex of  each  hemisphere,  and  ending  in  the  spinal  cord.  But  certain  masses 
of  gray  matter  in  the  hemisphere  not  strictly  cortical,  and  several  important 
masses  and  areas  of  gray  matter  lying  between  the  hemisphere  and  the  cord, 
are  connected  with  the  system ;  and  these  we  may  speak  of  as  the  "  inter- 
mediate gray  matter  of  the  crural  system." 

Corpus  striatum  and  optic  thalamus.  Of  all  these  several  collections  of 
gray  matter,  the  largest,  most  conspicuous,  and  perhaps  the  most  important 
are  the  two  masses  in  the  front  part  of  the  system  known  as  the  corpus  stria- 
tum and  optic  thalamus.  The  former  is,  as  we  have  seen  (§  515),  a  develop- 
ment of  the  wall  of  the  cerebral  vesicle,  the  latter  a  development  of  the  wall 
of  the  vesicle  of  the  third  ventricle.  They  are,  therefore,  of  different  origin ; 
although  in  the  course  of  the  growth  of  the  brain  they  become  closely  attached 
to  each  other,  they  are  at  the  outset  quite  separate  and  distinct.  Moreover, 
as  we  shall  see,  they  differ  from  each  other  so  essentially,  in  their  nature  and 
relations,  that  they  cannot  be  considered  as  homologous  bodies  ;  and  the  term 


616  THE  BRAIN. 

"  basal  ganglia  "  often  applied  to  them  is,  therefore,  unfortunate.  Neverthe- 
less, it  will  render  the  description  of  their  topographical  relations  easier,  if 
for  a  little  while  we  consider  them  together. 

When  the  lateral  ventricle  is  laid  open  from  above,  part  of  the  corpus 
striatum  is  seen  projecting  into  the  cavity  of  the  ventricle.  In  front  the 
projecting  part  is  broad,  forming  the  lateral  wall  and  part  of  the  floor  of 
the  ventricle,  and  to  its  median  side  lies  the  cavity  of  the  ventricle,  sepa- 
rated from  its  fellow  by  the  septum  lucidum.  Further  back  the  projecting 
part,  becoming  gradually  narrower,  assumes  a  more  lateral  position  and 
passes  into  the  descending  horn.  In  this  part  of  its  course  there  lies  on  its 
median  side,  separated  from  it  by  a  narrow  band  called  the  tsenia  semicir- 
cularis  or  stria  terminalis,  the  optic  thalamus,  a  narrow  strip  of  the  sur- 
face of  which  is  seen  projecting  outside  the  edge  of  the  choroid  plexus. 
If  now,  both  lateral  ventricles  be  laid  open  by  removal  of  the  corpus  cal- 
losum,  and  the  fornix  with  the  velum  interpositum  and  choroid  plexus  be 
taken  away,  so  as  fully  to  expose  the  third  ventricle,  and  also,  in  order  to 
obtain  a  better  view,  the  whole  of  the  hinder  part  of  the  cerebrum  contain- 
ing the  posterior  horns  of  the  lateral  ventricle,  be  completely  cut  away,  it 
is  seen  (Fig.  138)  that  the  two  optic  thalami  (0.  T.)  present  themselves  as 
two  large  oval  bodies,  placed  obliquely  athwart  the  diverging  crura  cerebri 
and  converging  in  front  to  form  the  immediate  walls  of  the  third  ventricle. 
In  front  and  to  the  sides  of  the  optic  thalami  are  seen  the  corpora  striata 
(C.  S.)  forming  anteriorly  the  lateral  walls  of  the  two  lateral  ventricles, 
and  diverging  behind  to  allow  of  the  interposition  of  the  optic  thalami. 
On  each  side  of  the  brain,  then,  these  two  bodies,  the  corpus  striatum  and 
optic  thalamus,  appear  as  two  masses  of  gray  matter  placed  on  the  crus 
cerebri  as  this,  diverging  from  its  fellow,  begin  to  spread  out  into  the  cere- 
bral hemisphere,  the  corpus  striatum  being  placed  somewhat  in  front  of  the 
optic  thalamus.  The  relations  of  the  two  bodies,  moreover,  are  such  that 
while  the  optic  thalamus  alone  forms  the  wall  of  the  third  ventricle  to 
which  it  properly  belongs,  and  the  corpus  striatum  forms  part  of  the  wall 
of  the  lateral  ventricle  to  which  it  in  turn  properly  belongs,  the  optic  thala- 
mus also  projects  into  and  seems  to  form  part  of  the  wall  of  the  lateral 
ventricle,  though  at  its  origin  it  had  nothing  to  do  with  the  cerebral  vesicle. 

We  spoke  just  now  of  these  bodies  as  being  placed  on  the  crura  cerebri, 
but  though  their  dorsal  surfaces  thus  project  from  the  dorsal  surface  of  the 
diverging  crura,  a  large  portion  of  each  body  is,  so  to  speak,  imbedded  in 
the  substance  of  the  diverging  crus,  and  what  is  seen  in  the  above  surface 
view  is  only  a  part  of  each  body,  and,  indeed,  in  the  case  of  the  corpus 
striatum,  only  a  small  part.  In  order  to  understand  the  nature  and  rela- 
tions of  these  two  important  bodies  we  must  study  sections  taken  through  a 
cerebral  hemisphere  in  various  planes  (Figs.  139-146). 

Each  crus  is  made  up,  as  we  have  seen,  of  a  dorsal  portion  or  tegmentum 
consisting  largely  of  gray  matter,  and  a  ventral  portion  or  pes  consisting 
exclusively  of  longitudinally  disposed  fibres.  The  tegmentum  ends  partly 
in  structures  lying  ventral  to  the  thalamns,  partly  in  the  thalamus  itself; 
and  we  may  for  the  present  leave  this  part  of  the  crus  out  of  consideration. 
The  fibres  of  the  pes,  while  continuing  their  oblique  course  forward  and 
outward,  soon  rise  dorsally  by  the  side  of  the  thalamus  and  hence,  in  a 
transverse  dorso-ventral  section  at  the  level  of  the  hind  part  of  the  thala- 
mus (Fig.  139),  are  seen  leaving  their  previous  position  ventral  to  the  sub- 
stantia  nigra  (8n~)  and  passing  (Op}  by  the  side  of  the  thalamus  on  their 
way  to  the  central  white  matter  of  the  hemisphere.  In  this  part  of  their 
course  they  form  a  thick  strand  separating  the  thalamus  (In.)  from  a  large 
mass  of  gray  matter  which,  roughly  triangular  in  section,  is  divided  by 


THE  GRAY  MATTER. 


617 


partitions  of  white  matter  into  three  parts  (Gp',  Gp'\  Pt),  and  of  which 
we  shall  speak  directly  as  the  nucleus  lenticularis. 


FIG.  139. 


Diagrammatic  Outline  of  a  Transverse  Dorso-ventral  Section  through  the  Right  Hemisphere 
(Man),  at  Level  Posterior  to  the  Knee  of  the  Internal  Capsule.  (Sherrington.)  Natural  size, 
Nc,  nucleus  caudatus;  in  the  upper  part  of  the  figure  the  section  of  the  nucleus  is  through  the 
narrower  portion  which  succeeds  the  wider  front  end  or  head  ;  in  the  lower  part  of  the  figure  the 
section  passes  through  the  tail  of  the  nucleus  near  its  end,  and  this  portion  of  it  has  for  the  sake 
of  clearness  been  sundered  from  the  gray  matter  at  Na,  nucleus  amygdalae,  more  distinctly  than 
in  reality  is  the  case.  Gp',  Gp",  globus  pallidus,  seen  here  in  two  segments,  and  Pt,  putarnen  of 
nucleus  lenticularis  ;  an,  the  anterior;  in,  the  inner  ;  and  In,  the  lateral  nucleus  of  the  optic  thala- 
mus  ;  at  II  is  seen  the  "  latticed  layer"  lying  next  to  dp,  the  posterior  limb  of  the  internal  capsule 
aud  containing  many  strands  of  fibres  which  mingle  with  it.  In  the  thalamus  between  the  ante- 
rior and  internal  nuclei  on  the  one  hand  and  the  lateral  nucleus  on  the  other  is  a  layer  shaded 
less  deeply  in  the  figure,  representing  the  internal  medullary  lamina  of  the  thalamus.  consisting 
largely  of  white  matter.  Other  collections  of  white  matter  within  the  thalamus  are  Vb,  the  bun- 
dle of  Vicq.  d'Azyr,  and  F',  the  lower  end  of  the  anterior  pillar  of  the  fornix  ;  F,  the  upper  end 
of  the  anterior  pillar  of  the  fornix,  below  cc  the  corpus  callosum  ;  C.sb,  corpus  subthalamicum; 
forming  a  fairly  continuous  mass  with  the  thalamus;  Sn,  substantia  nigra ;  cl,  claustrum ;  ce, 
external  capsule ;  Ca,  terminal  portion  of  anterior  commissure ;  In,  the  instila  or  island  of  Reil ; 
Iv,  lateral  ventricle ;  l.v.d.,  descending  horn  of  lateral  ventricle ;  V.  3,  in  the  position  of  the  third 
ventricle;  the  outlines  of  the  cavities  are  made  diagrammatically  distinct  by  thick  black  lines; 
Op  optic  tract ;  P  P,  parietal  lobe ;  T,  temporal  lobe. 

If  instead  of  taking  a  transverse  we  take  a  longitudinal  dorso-ventral 


618  THE  BRAIN. 

(or,  as  it  is  called,  sagittal)  section  (Fig.  145)  we  find  that  the  fibres  form- 
ing the  strand  in  question  do  not  continue  parallel  to  each  other  as  they  rise 
dorsally  but  diverge  in  a  radiating  manner,  forming  the  so-called  corona 
rndiata.  If  again  we  take  horizontal  sections  at  proper  levels  (Figs.  138, 
144),  \ve  find  that  this  strand  or  rather  thick  band  of  dorsally  directed  radi- 
ating fibres  not  only  stretches  (  Cip]  between  the  thalamus  and  the  gray  mass 
just  spoken  of,  but  reaching  further  forward  passes  (Cia)  between  the  same 
gray  mass  on  the  lateral  side  and  another  gray  mass  (Nc)  on  the  median  side, 
the  latter  from  its  position  being  evidently  the  part  of  the  corpus  striatum 
which  projects  into  the  lateral  ventricle.  The  same  horizontal  sections  further 
teach  us  that  the  front  part  of  the  band  (CVV)  is  bent  at  an  angle  upon  the 
hind  part  (Oip). 

It  appears,  then,  from  these  sections  that  the  fibres  of  the  pes  as  they  rise 
up  dorsally  into  the  hemisphere  spread  out  in  the  form  of  a  fan  bent  upon 
itself.  This  fan-like  expansion  of  the  pes  is  called  the  internal  capsule,  the 
angle  formed  by  the  bend  being  called  its  genu  or  knee  (C%),  the  part  in 
front  of  the  knee  the  front  limb,  and  the  part  behind  the  knee  the  hind  limb. 
And  horizontal  sections  at  levels  more  dorsal  than  those  given  in  Figs.  138- 
144  would  show  that  the  fibres  composing  this  fan-like  internal  capsule,  as 
they  rose  dorsally,  curved  away  in  various  directions  to  reach  nearly  all  parts 
of  the  surface  of  the  hemisphere.  We  may  add  that  though  the  internal 
capsule  is  mainly  composed  of  fibres  which  thus  stretch  all  the  way  from  the 
cerebral  cortex  to  the  pes  of  the  crus,  it  also  contains  other  fibres  of  which 
we  shall  speak  later  on. 

§  540.  The  gray  mass  separated  from  the  thalamus  by  the  hind  limb  of 
the  internal  capsule  is  called  as  a  whole  the  nucleus  lenticularis,  since  in  hori- 
zontal section  it  presents  a  certain  though  distant  resemblance  to  a  lens.  Of 
the  three  divisions  into  which  it  is  split  up  by  the  partitions  of  white  matter, 
the  two  median  ones  Gp ',  Gp",  are  spoken  of  together  as  the  globus  pallidus, 
the  name  being  given  to  them  on  account  of  their  paler  color.  The  third, 
lateral  division  Pt,  is  called  the  putamen.  The  use  of  these  two  names  for 
the  two  different  parts  of  the  one  body,  appears  to  be  justified  by  the  different 
connections  and  features  of  the  two  parts. 

The  gray  mass  which  in  a  horizontal  section  (Fig.  138,  JVb)  is  separated 
from  the  nucleus  lenticularis  by  the  front  limb  of  the  external  capsule,  and 
which  projects  into  the  lateral  ventricle,  is  called  the  nucleus  caudatus.  The 
nucleus  caudatus  and  the  nucleus  lenticularis  form  together  the  corpus  stria- 
tum ;  the  former,  since  it  projects  into  the  lateral  ventricle,  being  the  part  of 
the  corpus  striatum  seen  when  the  lateral  ventricle  is  laid  open, is  sometimes 
spoken  of  as  the  intra-ventricular  portion  of  the  whole  body,  while  the 
nucleus  lenticularis,  which  is  wholly  hidden  in  the  hemisphere  and  in  no 
part  projects  into  the  lateral  ventricle,  is  called  the  extra-ventricular  por- 
tion. 

But  only  a  part,  indeed  only  a  relatively  small  part,  of  the  nucleus  cauda- 
tus is  disclosed  in  such  a  horizontal  section  ;  to  learn  the  somewhat  peculiar 
form  and  relations  of  the  whole  nucleus  a  number  of  sections  of  a  hemi- 
sphere taken  in  different  planes  must  be  studied ;  and  these  will  at  the  same 
time  explain  why  the  nucleus  is  called  "  caudatus."  These  teach  us  that  the 
nucleus  has  somewhat  the  form  of  a  comma  (Fig.  142).  The  thick,  rounded 
head  forms  the  lateral  wall  of  the  front  part  of  the  lateral  ventricle  ;  thence 
the  body  passes  backward,  narrowing  rapidly  and  diverging  somewhat  later- 
ally ;  in  its  course  it  arches  over  the  nucleus  lenticularis,  curving  so  much 
that  the  end  of  the  tail,  sweeping  round  the  hinder  border  of  that  body  and 
changing  its  direction,  runs  eventually  ventral  to  it.  In  a  horizontal  section 
taken  at  a  certain  depth,  such  as  that  represented  in  Fig.  138,  only  a  portion 


THE  GRAY   MATTER. 


619 


of  the  head  or  body  (Ne)  in  the  front  part  of  the  figure,  and  a  transverse 
section  of  the  end  of  the  tail  (JVc)  in  the  hind  part  of  the  figure  are  seen  ;  all 
the  intervening  portion  of  the  nucleus  lies  above  the  plane  of  the  section. 
In  a  transverse,  dorso-ventral  section  taken  somewhat  anteriorly  through  the 
front  limb  of  the  capsule  (Fig.  140),  the  head  or  body  of  the  nucleus  cauda- 


FIG.  140. 


Diagrammatic  Outline  of  a  Transverse  Dorso-ventral  Section  through  the  Right  Hemisphere 
(Man)  at  a  Level  Anterior  to  Fig.  139.  (Sherrington.)  Natural  size.  Nc,  nucleus  caudatus ;  Gp', 
Gp",  globus  pallidus,  seen  here  in  two  segments,  and  Pt,  putamen  of  nucleus  lenticularis ;  OT, 
optic  thalamus,  with  ca,  anterior  commissure,  in  close  relation  to  cia,  anterior  limb  of  internal 
capsule ;  ce,  external  capsule ;  op,  optic  tract ;  cc,  corpus  callosum  ;  /,  fornix ;  Iv,  a  space  that  in  its 
upper  part  belongs  to  the  lateral  ventricle,  in  its  lower  was  filled  by  the  fold  of  subarachnoid  tis- 
sue and  pia  mater,  the  side  fringe  of  which,  covered  with  epithelium,  forms  the  choroid  plexus; 
this  fold  was  detached  in  the  making  of  the  section  and  was  removed  ;  In,  the  insula  ;  F,  frontal 
lobe;  P,  parietal  lobe ;  T,  temporal  lobe.  For  greater  clearness  the  cortical  gray  matter,  which 
is  shaded  in  Fig.  139,  is  in  this  figure  left  unshaded. 

tus  C/Vc),  which  has  not  yet  reached  its  greatest  dimensions,  is  seen  lying 
dorsal  to  the  nucleus  lenticularis,  separated  from  it  by  the  white  mass  of  the 
front  limb  (cia)  of  the  capsule,  though  this  is  somewhat  broken  up  by  strands 
of  gray  matter  passing  from  one  nucleus  to  the  other.  In  a  transverse  dorso- 


620 


THE  BRAIN. 


ventral  section,  taken  still  more  anteriorly  through  the  frontal  lobe  (Fig. 
141),  the  head  of  the  nucleus  caudatus  is  seen  at  about  it  greatest  size,  and 
the  diminishing  nucleus  lenticularis  (NE),  represented  by  the  putamen  alone, 
is  becoming  fused  with  it,  the  two  nuclei  being  separated  by  a  small  quantity 
of  white  matter  of  the  internal  capsule,  and  that  largely  broken  up  by 
bundles  of  gray  matter,  giving  rise  to  a  striated  appearance.  In  a  similar 
section  still  further  forward,  the  nucleus  lenticularis  would  be  absent,  the 
head  of  the  nucleus  caudatus  appearing  by  itself.  Returning  to  the  hinder 
part  of  the  hemisphere,  we  find  in  a  dorso-ventral  section  taken  through  the 
hind  limb  of  the  capsule  (Fig.  139)  that  while  the  nucleus  lenticularis  is 

FIG.  141. 


Diagrammatic  Outline  of  a  Transverse  Dorso-ventral  Section  of  Right  Hemisphere  (Man) 
through  the  Frontal  Lobe.  (Sherrington.)  Natural  size.  Nc,  head  of  nucleus  caudatus,  and  Nl, 
the  front  end  of  the  putamen  of  the  nucleus  lenticularis  becoming  fused  with  it ;  cc,  corpus  cal- 
losum,  cut  through  at  its  front  bend  or  rostrum,  so  that  both  dorsal  and  ventral  portions  are 
shown ;  between  these  is  seen  the  fifth  ventricle  or  cavity  in  the  septum  lucidum  SI;  Iv,  lateral 
ventricle  ;  Cl,  claustrum  ;  F,  frontal  lobe.  Cortical  gray  matter,  as  in  Fig.  140,  left  unshaded. 

here  at  its  greatest  size,  the  head  of  the  nucleus  caudatus  (Nc),  lying  dorsal 
to  the  nucleus  lenticularis  and  separated  from  it  by  a  considerable  thickness 
of  internal  capsule,  has  much  diminished ;  the  same  section,  moreover, 
shows  ventral  to  the  nucleus  lenticularis  and  clinging  to  the  descending 
•horn  of  the  lateral  ventricle  (Lv.d.~),  the  extreme  tip  of  the  tail  of  the  nucleus 
caudatus  (Nc)  soon  about  to  fuse  with  the  small  mass  of  gray  matter  called 
the  nucleus  amygdalce  (JVa).  A  sagittal  (longitudinal  dorso-ventral)  section 
taken  at  some  distance  from  the  median  line  (Fig.  142)  shows  the  curved 
course  of  the  larger  portion  of  the  nucleus  caudatus,  the  extreme  head  as 
well  as  the  latter  part  of  the  tail  lying  out  of  the  plane  of  the  section ;  and 


THE  GRAY   MATTER.  621 

a  similar  section  taken  nearer  the  middle  line  (Fig.  145)  shows  how  the 
nucleus  in  the  middle  portion  is  broken  up  by  bands  of  fibres  of  the  internal 
capsule  traversing  it,  and  thus  contributing  to  the  striated  appearance;  the 
same  section  also  shows  that  the  globus  pallidus,  as  well  as  the  putamen, 
becomes  continuous  with  the  nucleus  caudatus. 

FIG.  142. 


Diagrammatic  Outline  of  a  Sagittal  Section  taken  through  the  Right  Hemisphere  (Man),  Seen 
from  the  Mesial  Surface.  (Sherrington.)  Half  natural  size.  The  plane  of  the  section  is  not 
truly  sagittal,  but  slightly  inclined.  Nc,  the  caudate  nucleus  exposed  to  the  left  of  the  letters  Nc 
in  nearly  its  entire  anterior  extent,  to  right  of  the  letters  in  a  considerable  part  of  its  posterior 
extent.  It  forms  an  arch  of  gray  matter  over  the  gray  matter  of  Pt,  the  putamen,  and  Gp,  the 
globus  pallidus  of  the  lenticular  nucleus ;  Na,  the  amygdaloid  nucleus  ;  Oi,  Ci,  Ci,  the  internal 
capsule ;  Ca,  the  anterior  commissure ;  cc,  the  hinder  limit  of  fibres  of  the  splenium  corporis 
callosi,  P,  the  parietal  lobe ;  T,  the  temporal. 

Thus,  when  we  speak  of  the  corpus  striatum  as  a  whole  we  mean  a  large 
mass  of  gray  matter  lying  lateral  to  the  optic  thalamus,  reaching  nearly  as 
far  back  as  that  body  and  stretching  much  further  forward,  as  far  forward  in 
fact  as  does  the  lateral  ventricle ;  but  it  is  important  to  remember  that  it  is 
divided  into  two  masses  or  nuclei,  which  are  fused  together,  and  that  im- 
perfectly at  the  very  front  only.  These  two  nuclei  are,  the  one  the  comma- 
shaped  nucleus  caudatus,  the  bulk  of  which  is  placed  forward  projecting  into 
the  lateral  ventricle,  and  which  on  the  whole  is  the  more  dorsal  portion  of 
the  whole  body,  the  other  the  irregularly  shaped  nucleus  lenticularis,  the 
bulk  of  which  is  placed  further  back  than  the  lateral  ventricle,  by  the  side 
of  the  optic  thalamus,  and  which  on  the  whole  is  the  more  ventral  portion  of 
the  whole  body.  It  is  no  less  important  to  remember  that  the  radiating 
fibres,  which  we  call  the  internal  capsule,  pass  in  the  hinder  region  of  the 
whole  body  between  the  thalamus  and  the  nucleus  lenticularis,  forming  the 
hind  limb  of  the  capsule,  and  in  the  front  region  between  the  nucleus  cau- 
datus and  the  nucleus  lenticularis,  forming  the  front  limb  of  the  capsule,  the 
front  and  hind  limbs  being  bent  on  each  other  so  as  to  form  an  angle,  the 
so-called  knee. 

§  541.  The  optic  thalamus  as  a  whole  is  a  somewhat  oval  mass  of  gray 


622 


THE  BRAIN. 


matter  lying,  as  we  have  said,  athwart  the  diverging  crus,  in  which  it  is 
partly  imbedded.  Its  curved  median  side  covered  with  a  thin  layer  of  cen- 
tral gray  matter  forms  the  lateral  wall  of  the  third  ventricle  (Figs.  138, 139, 
144),  and  in  a  longitudinal  vertical  section  of  the  brain  taken  in  the  line  of 
the  middle  of  the  third  ventricle  (Fig.  143,  O.T.)  is  seen  occupying  the  space 
between  the  fornix  and  hind  end  (splenium)  of  the  corpus  callosum  above 
and  the  diverging  crus  below.  Its  more  or  less  straight  lateral  border  abuts 

FIG.  143. 


View  of  Right  Half  of  Brain  of  Man  as  Disclosed  by  a  Longitudinal  Section  in  the  Median 
Line  through  the  Longitudinal  Fissure.  (Sherrington.)  Half  natural  size.  The  bulb,  seen  in 
longitudinal  section  at  B,  passes  into  thepons,  P,  and  into  the  crus  cerebri,  which  last  is  cut  obliquely 
across  as  it  diverges  into  the  hemisphere  and  passes  out  of  the  section.  A  part  of  the  ventral 
surface  of  the  crus  is  shown  in  the  shaded  part  marked  CR.  At  GL  the  central  canal  of  the  spinal 
cord  is  seen  opening  out  into  the  fourth  ventricle  (4th),  overhung  by  the  cerebellum  (bisected  in 
the  middle  line),  and  passing  on  by  the  aqueduct  beneath  the  posterior,  QP,  and  anterior,  QA, 
corpora  quadrigemina  into  the  third  ventricle  (3).  The  posterior  corpus  quadrigeminum  is  continuous 
behind  with  the  valve  of  Vieussens,  attached  to  the  superior  peduncle  of  the  cerebellum,  and  seen 
in  a  longitudinal  section  overhanging  the  front  part  of  the  fourth  ventricle.  The  corpora  quad- 
rigemina appear  relatively  small  because  the  section  passes  in  the  median  line  in  the  depression 
between  the  right  and  left  bodies  of  the  two  pairs ;  and  immediately  in  front  of  them  is  the  sec- 
tion of  the  mesially  placed  pineal  gland  P,  which  overhangs  the  opening  of  the  aqueduct  into  the 
third  ventricle,  and  the  right  arm  of  which,  running  in  the  lateral  wall  of  the  third  ventricle,  is 
shown  by  an  unshaded  tract.  The  roof  of  the  third  ventricle  is  seen  to  be  furnished  by  the  arch 
of  the  fornix  F,  shown  unshaded  in  longitudinal  section.  Posteriorly  the  body  of  the  fornix  passes 
into  the  diverging  right  posterior  pillar,  where  F  is  shaded,  and  is  lost  to  view  under  the  over- 
hanging rounded  hind  end  or  splenium  (Sp)  of  the  corpus  callosum.  In  front  the  body  of  the  fornix 
is  seen  passing  just  behind  the  transverse  section  of  the  anterior  commissure  A  into  the  diverging 
right  antei-ior  pillar  /,  which  is  lost  to  view  as  it  stretches  in  the  lateral  wall  of  the  ventricle  toward 
the  corpus  mammillare  or  albicans  M.  The  small  white  cross  immediately  behind  /  indicates  the 
position  of  the  foramen  of  Monro.  The  bulging  median  surface  of  the  optic  thalamus  OT,  is  seen 
forming  the  lateral  wall  of  the  hinder  (and  owing  to  the  cranial  flexure,  the  more  dorsal)  part  of 
the  third  ventricle,  and  on  this,  below  the  area  of  the  pineal  gland,  is  seen,  unshaded,  the  section 
of  the  soft  or  middle  commissure  C.  Between  the  pineal  gland  (P)  and  the  splenium  (Sp)  is  seen 
the  hind  end  or  pulvinar  of  the  thalamus  projecting  into  the  so-called  transverse  fissure  of  the 
brain,  shown  shaded  in  the  figure,  by  which  the  pia  mater,  passing  on  beneath  the  posterior 
part  of  the  cerebrum  and  above  the  cerebellum,  gains  access  to  the  third  ventricle,  the  position  of 
the  velum  being  shown  by  the  thin  black  line  stretching  from  the  splenium  to  the  fornix.  The 
front  (and  more  ventral)  part  of  the  third  ventricle  is  seen  to  end  in  the  infundibulum,  attached 
to  which  is  the  pituitary  body  H,  seen  in  section  at  L.  In  front  of  the  infundibulum  is  seen  the 


THE  GRAY  MATTER.  623 

on  the  internal  capsule  (Figs.  138, 139, 144).  Its  dorsal  surface,  as  we  have 
already  seen,  also  forms  part  of  the  wall  of  the  third  ventricle  and  is  free ; 
but  there  lies  close  above  it  the  prolongation  of  the  pia  mater,  forming  the 
velum  interpositum  with  its  choroid  plexus  (§  515),  which  creeps  in  over  it 
beneath  the  projecting  hind  end  of  the  corpus  callosum  and  the  fornix 
(Fig.  143).  Its  ventral  surface  is  fused  with  the  crus ;  indeed  the  tegmental 
or  dorsal  portion  of  the  cms  may  be  said  to  end  in  it  and  in  certain  struc- 
tures lying  ventral  to  the  thalamus,  in  what  is  called  the  "  subthalamic 
region"  (Fig.  139),  while  the  fibres  of  the  pes  pass  first  ventral  and  then 
lateral  to  it  to  form  the  internal  capsule. 

The  gray  matter  of  the  whole  body  is  more  or  less  distinctly  divided  by 
sheets  of  white  matter,  as  seen  both  in  horizontal  and  in  vertical  sections 
(Figs.  136,  139,  144),  into  three  parts  which  have  received  the  name  of 
nuclei,  namely,  the  median  or  inner  nucleus  (Fig.  139  m),  which  with  the 
thin  layer  of  central  gray  matter  forms  the  side  wall  of  the  third  ventricle ; 
the  larger  lateral  nucleus  (ln\  which  abuts  upon  the  internal  capsule  ;  and 
the  smaller  anterior  nucleus  (an),  which  lies  on  the  dorsal  surface  of  the 
front  part  of  the  body,  and  which  thus  at  its  front  end  appears  to  project 
into  the  lateral  ventricle. 

These  three  nuclei  form,  however,  not  the  whole  of  the  optic  thalamus, 
but  only  the  larger  front  portion ;  behind  them  lies  the  important  portion 
called  the  pulvinar,  into  which  the  hind  part  of  the  median  nucleus  merges; 
this  is  partly  imbedded  in  the  crus  ventrally,  and  in  the  hemisphere  laterally, 
and  is  partly  free,  coming  to  the  surface  beneath  the  hind  end  of  the  corpus 
callosum.  In  a  median  longitudinal  section  of  the  brain  (Fig.  143)  it  is  the 
pulvinar  which  forms  the  cushion-like  (hence  the  name)  end  of  the  thalamus 
beneath  the  overhanging  splenium  of  the  corpus  callosum,  by  the  side  of  the 
pineal  gland ;  and  in  the  horizontal  view  (Fig.  138,  Pvr),  in  which  the 
hemispheres  are  supposed  to  have  been  removed,  the  same  pulvinar  is  seen 
projecting  over  the  crus  by  the  side  of  the  anterior  corpus  quadrigeminum. 
The  buried  portion  of  the  pulvinar  is  exposed  in  a  transverse  section  taken 
through  the  anterior  corpus  quadrigeminum  (Fig.  137) ;  the  extreme  end  of 
this  part  of  the  pulvinar  (Pvr)  is  here  seen  lying  dorsal  and  lateral  to  the 
pes  of  the  crus,  immediately  above  two  masses  of  gray  matter,  the  corpora 
geniculata  (Cgl.  Cgm.),  of  which  we  shall  speak  later  on.  One  of  these, 
the  lateral  corpus  geniculatum  ((7.  g.  /.),  is  especially  connected  with  the 
optic  tract  (op),  and,  as  we  shall  see  hereafter,  the  pulvinar  itself  is  also 
connected  with  the  optic  tract,  and  is  an  important  part  of  the  central  appa- 
ratus of  vision. 

§  542.  The  substantia  nigra,  the  red  nucleus,  and  other  gray  matter  of  the 
tegmentum.  Nerve-cells  and  groups  of  nerve-cells,  or  areas  of  gray  matter, 
too  small  to  deserve  special  names,  are  scattered  throughout  the  tegmentum 

optic  nerve  cut  across  at  the  optic  decussation  OP,  stretching  from  which  to  the  anterior  commissure 
is  the  lamina  terminalis.  Stretching  between  the  corpus  callosum  cc  (seen  in  longitudinal  section 
with  a  striated  appearance,  and  ending  in  front  at  the  rostrum  R  and  behind  at  the  splenium  Sp) 
dorsally  and  the  fornix  ventrally  is  seen  (unshaded)  the  septum  lucidum  SL,  but  the  greater  part 
of  this  has  been  cut  away  in  order  to  disclose  the  right  lateral  ventricle,  in  the  wall  of  which  is 
seen  the  bulging  nucleus  caudatus  NC.  Above  the  corpus  callosum  is  seen  the  mesial  surface  of 
the  right  hemisphere  forming  the  right  lateral  wall  of  the  longitudinal  fissure.  On  this  mesial 
surface  appears  immediately  above  the  corpus  callosum  the  arched  gyrus  fornicatus  GF,  defined 
above  by  the  calloso-marginal  fissure  f.  cm.  The  whole  of  the  surface  seen  in  the  frontal  region  in 
front  of  the  calloso-marginal  fissure,  though  divided  by  fissures,  is  called  the  marginal  convolution. 
In  the  middle  parietal  region  a  block  of  the  cerebral  substance  has  been  removed  in  order  to 
show  the  position  of  the  central  fissure  or  fissure  of  Rolando,  f.  c  ,  and  immediately  below  this  is 
seen  a  part  of  PA.  C.,  the  paracentral  lobule.  In  the  occipital  region,  PR.  C.,  is  the  precuneus  or 
quadrate  lobule,  and  C,  the  cuneus,  while  at  G.  L.  is  seen  a  part  of  the  lingual  lobule.  T.  i.  is  a  part 
of  the  inferior  temporo-occipital  convolution,  the  greater  part  of  which  is  hidden  from  view  by  the 
pons  and  crus. 


624  THE  BRAIN. 

along  its  course.  But,  besides  these  and  the  nuclei  of  the  third  and  fourth 
cranial  nerves,  of  which  we  have  already  spoken,  certain  larger  collections 
of  gray  matter  deserve  attention.  A  conspicuous  mass  of  gray  matter,  cir- 
cular in  transverse  section,  placed  in  the  midst  of  the  tegmentum  on  each 
side  but  somewhat  near  the  middle  line,  and  stretching  from  the  hinder 
margin  of  the  third  ventricle  beneath  the  anterior  corpus  quadrigeminum 
(Figs.  137,  138),  is,  from  the  red  tint  it  possesses,  called  the  red  nucleus, 
nucleus,  or  locus  ruber.  It  is  traversed  by  fibres  of  the  third  nerve  as  these 
make  their  way  ventrally  from  the  nucleus  to  the  surface. 

We  must  consider  also  as  belonging  to  the  tegmentum  a  large  area  of 
gray  matter,  somewhat  lens-shaped  in  section  (Fig.  137,  Sn),  which  lies 
between  the  pes  and  tegmentum,  sharply  marking  off  the  one  from  the  other. 
From  its  dark  appearance,  due  to  the  abundance  of  black  pigment,  it  is 
called  the  substantia  nigra  or  locus  niger.  It  acquires  its  largest  dimensions 
at  about  the  middle  of  the  length  of  the  crus,  coming  to  an  end  in  front 
(Fig.  139,  Sn)  and  fading  away  behind  (Fig.  136),  as  the  crus  passes  beneath 
the  posterior  corpora  quadrigemina.  These  two,  the  red  nucleus  and  the 
substantia  nigra,  are  perhaps  the  most  important  collections  of  gray  matter 
in  the  tegmentum,  but  we  may  add  that  at  the  front  of  the  crus  as  the  sub- 
stantia nigra  comes  to  an  end  there  is  seen  in  a  somewhat  similar  position 
ventral  to  the  hind  part  of  the  optic  thalamus  a  collection  of  gray  matter 
called  the  corpus  subthalamicum  (Fig.  139,  C.  sb). 

At  the  hinder  part  of  the  crus,  as  it  is  about  to  plunge  into  the  pons, 
while  the  pes,  now  decreasing  relatively  in  size,  still  continues  to  be  ordinary 
white  matter  composed  of  longitudinal  bundles  of  medullated  fibres,  the 
tegmentum  takes  on  more  and  more  the  structure  which  in  speaking  of  the 
bulb  we  called  reticular  formation,  and  which,  as  we  saw,  deserves  to  be  con- 
sidered as  a  kind  of  gray  matter. 

The  gray  matter  of  the  pons.  When  the  conjoined  crura  as  we  trace  them 
backward  plunge  beneath  the  pons,  the  longitudinal  fibres  of  the  pes  of  each 
crus  are,  as  we  have  said,  soon  split  up  into  bundles  and  scattered  among  the 
transverse  fibres  belonging  to  the  pons  itself.  Dorsal  to  this  system  of  trans- 
verse and  longitudinal  fibres  forming  the  pons  proper,  between  it  on  the  ven- 
tral surface  and  the  central  gray  matter  with  the  posterior  corpora  quadri- 
gemina on  the  dorsal  surface,  is  a  region  which  may  be  called  tegmental, 
since  it  is  a  continuation  of  the  tegmentum  of  the  crus.  In  the  front  part 
of  the  pons  (Fig.  136),  where  the  posterior  corpora  quadrigemina  still  form 
the  dorsal  roof  of  the  section,  this  tegmental  area,  which  is  much  broken  up 
by  certain  strands  of  longitudinal  fibres,  of  which  we  shall  speak  later  on, 
contains  scattered  nerve-cells,  and  is  largely  composed  of  reticular  formation. 
In  this  is  placed  on  each  side  a  group  of  nerve-cells,  the  locus  cceruleus  (Fig. 
136,  I.  c.),  to  which  we  have  already  referred  (§  534)  as  probably  serving  in 
part  as  the  origin  of  the  descending  root  of  the  fifth  nerve  (  V.  d.),  just  ven- 
tral to  which  it  lies.  This  acquires  larger  dimensions  further  back,  in  the 
front  part  of  the  fourth  ventricle  (Fig.  138,  /.  c.)  between  the  levels  repre- 
sented in  Figs.  135  and  136,  and  is  a  collection  of  large  spindle-shaped 
nerve-cells ;  it  has  a  bluish  tint  when  its  black  pigment  is  seen  shining 
through  the  surrounding  more  or  less  transparent  material,  hence  the  name. 

In  the  hinder  parts  of  the  pons  (Figs.  134, 135),  where  the  cerebellum  is 
seen  overhanging  the  open  fourth  ventricle,  the  reticular  formation  of  the 
tegmental  area  is  still  more  conspicuous.  The  only  special  collection  of 
gray  matter  in  this  region  to  which  we  need  call  attention  is  one  which,  con- 
sisting, like  the  olivary  body  of  the  bulb  (or  inferior  olive),  of  a  wall  of 
gray  matter  surrounding  and  surrounded  by  white  matter,  is  called  the 
upper  olive  (Figs.  134,  135,  s.  o.). 


THE  GRAY  MATTER.  625 

The  ventral  part  of  the  pons,  or  the  pons  proper,  unlike  the  pes  of  the 
crus,  contains,  mixed  with  the  fibres,  a  very  considerable  quantity  of  gray 
matter.  This  is  fairly  abundant  in  the  front  part  of  the  pons  (Fig.  136)  below 
the  corpora  quadrigemina,  but  increases  even  more  behind  this  (Figs.  125, 
135).  Hence,  though  the  pons  proper  is  largely  built  up  of  transverse  and 
longitudinal  fibres,  and  though  it  contains  no  compact  aggregations  of  gray 
matter  receiving  special  names,  it  does  contain  scattered  throughout  it  a  very 
large  quantity  of  gray  matter,  far  more,  indeed,  than  is  present  in  the  teg- 
mental  portion  ;  the  gray  matter  of  the  pons — that  is,  of  the  pons  proper — 
must  be  regarded  as  forming  a  very  important  part  of  the  gray  matter  of 
the  crural  system,  and  of  no  little  physiological  significance. 

Behind  the  pons  the  crural  system  is  continued  into  the  bulb,  with  whose 
structure  we  have  already  dealt. 

4.   Other  Collections  of  Gray  Matter. 

§  543.  Of  these,  three  deserve  chief  attention,  and  may  be  classed 
together,  though  they  differ  in  nature. 

T he  gray  matter  of  the  corpora  quadrigemina.  On  each  side  of  and  some- 
what dorsal  to  the  central  gray  matter  of  the  aqueduct,  which,  as  we  have 
seen,  is  well  developed,  especially  on  the  ventral  side,  collections  of  gray 
matter  form  the  chief  part  of  the  corpora  quadrigemina,  both  anterior  and 
posterior. 

The  gray  matter  of  the  anterior  corpora  quadrigemina  (Fig.  137,  A.  Q.  n.} 
is  more  distinctly  marked  off  from,  and  separated  by  a  wider  tract  of  white 
matter  from,  the  central  gray  matter  of  the  aqueduct  than  is  that  of  the  pos- 
terior corpora  quadrigemina  (Fig.  137,  nPQ}  ;  it  is,  moreover,  of  a  different 
nature.  Indeed  the  two  pairs  of  bodies  have  quiet  different  relations,  are  of 
different  nature,  and  perform  different  functions. 

Corpora  geniculata.  The  two  optic  nerves,  as  we  shall  see  in  detail  later 
on,  give  rise,  through  the  optic  decussation,  to  the  two  optic  tracts.  Each 
optic  tract  (Figs.  131,  137,  Op]  winds  round  the  crus  cerebri  on  its  ventral 
surface  to  reach  the  substance  of  the  hemisphere  in  the  region  below  the 
optic  thalamus,  and  as  it  does  so  is  described  as  dividing  into  a  lateral  and 
median  portion.  The  lateral  portion  just  as  it  sweeps  round  the  far  edge, 
that  is  the  outer  or  lateral  edge,  of  the  crus  bears  a  rounded  swelling  (Fig. 
131,  B  and  C,  Cgl.^>,  the  lateral  or  outer  corpus  geniculatum,  the  interior  of 
which  consists  largely  of  gray  matter  (Fig.  137,  Cgl).  The  median  portion 
similarly  bears  another  like  swelling  occupying  a  more  median  position,  the 
median  or  inner  corpus  geniculatum  (Fig.  131,  A  and  B,  Cgm),  the  interior 
of  which  (Fig.  137,  Cgm)  also  consists  of  gray  matter.  It  is  to  be  regretted 
that  these  two  bodies  should  bear  the  same  name,  for  they  are  different  in 
their  origin,  in  their  connections,  and  in  their  functions.  The  lateral  body 
is  said  to  be  derived  from  the  fore-brain,  that  is  from  the  vesicle  of  the  third 
ventricle,  has  definite  connections  with  the  retinal  optic  fibres,  and  is  dis- 
tinctly concerned  in  vision  ;  the  median  body  is  derived  from  the  mid-brain, 
is  not  definitely  connected  with  the  retinal  fibres,  and  appears  to  be  in  no 
way  concerned  in  vision.  We  shall,  however,  return  later  on  to  the  connec- 
tions and  probable  functions  of  these  bodies. 

Corpus  dentatum  of  the  cerebellum.  In  the  midst  of  the  mass  of  white 
matter  which  is  formed  in  the  interior  of  the  cerebellum  by  the  confluence 
of  the  three  peduncles  is  found  (Fig.  134,  C  D)  an  area  of  gray  matter 
arranged,  like  the  olivary  body  of  the  bulb,  as  a  sharply  folded  or  plaited 
band  in  the  shape  of  a  flask  or  bowl.  As  in  the  similar  olivary  body  the 
gray  matter  of  the  flask  is  covered  up  by  and  its  interior  filled  up  with  white 

40 


626 


THE  BRAIN. 


matter ;  the  mouth  of  the  flask  is,  on  each  side,  directed  toward  the  median, 
line  ;  the  fibres  pass  chiefly  to  the  superior  peduncle. 

FIG.  144. 


Outline  of  Horizontal  Section  of  Brain,  to  show  the  Internal  Capsule.  Natural  size.  The  sec- 
tion is  taken  at  a  level  more  ventral  than  shown  in  Fig.  138.  The  gray  matter  of  the  cortex  and 
claustrum  is  left  unshaded,  but  that  of  the  corpus  striatum  and  optic  thalamus  is  shaded  ;  OT, 
optic  thalamus,  showing  the  median,  lateral,  and  anterior  nuclei ;  NL,  nucleus  lenticularis,  show- 
ing the  putamen  large,  and  the  inner  division  of  the  globus  pallidus  very  small;  NC,  nucleus 
caudatus,  the  large  head  in  front  of,  and  the  diminishing  tail  behind,  the  thalamus  ;  G,  the  knee 
of  the  internal  capsule.  From  "Eye"  to  "  Dig,"  marks  the  position  of  the  pyramidal  tract  as  a 
whole,  and  the  several  letters  indicate  broadly  the  relative  positions  of  the  several  constituents 
of  the  tract,  named  according  to  the  movements  with  which  they  are  concerned :  thus  Eye,  move- 
ments of  the  eyes  ;  Hd,  of  the  head  ;  Tg,  of  the  tongue ;  mth,  of  the  mouth  ;  Shi,  of  the  shoulder; 
Elh,  of  the  elbow  ;  Dig,  of  the  hand  ;  Abd,  of  the  abdomen ;  Hip,  of  the  hip ;  Kn,  of  the  knee ; 
Dig,  of  the  foot ;  S,  the  temporo-occipital  tract ;  oc,  fibres  to  the  occipital  lobe ;  op,  optic  radiation. 
At  this  level  the  fibres  of  the  frontal  tract,  in  the  fore  limb  of  the  capsule  in  front  of  the  pyram- 
idal tract,  run  almost  horizontally,  parallel  with  the  plane  of  the  section.  (Cf.  Fig.  146,  Fron).  cc, 
the  rostrum  of  the  corpus  callosum,  Spl,  the  splenium  of  the  same,  both  cut  across  horizontally. 
The  thick  dark  line  indicates  the  boundary  of  the  cavities  of  the  anterior  and  descending  horns 
of  the  lateral  ventricle  and  of  the  third  ventricle,  the  two  ventricles  being  laid  open  into  one  by 
the  removal  of  the  velum  and  ehoroid  plexus,  etc.  The  oval  outline  in  the  fore  part  of  this  cavity 
indicates  the  fornix.  Lateral  to  the  nucleus  lenticularis  is  seen  in  outline  the  claustrum,  the  cor- 
tex of  the  island  of  Reil  and  the  operculum  or  convolution  overlapping  the  island  of  Reil.  P  is 
inserted  to  show  which  is  the  hind  part  of  the  section. 


THE  ARRANGEMENT  OF  THE  FIBRES  OF  THE  BRAIN.       627 

There  are  also  other  collections  of  gray  matter  in  the  central  white  mat- 
ter of  the  cerebellum,  one  of  which,  called  the  "nucleus  of  the  roof,"  is  con- 
nected with  the  two  inferior  peduncles. 

THE  ARRANGEMENT  OF  THE  FIBRES  OF  THE  BRAIN. 

§  544.  The  systems,  tracts,  and  bundles  of  fibres  in  which  the  white 
matter  of  the  brain  is  arranged,  may  be  distinguished  from  each  other,  partly 
through  mere  mechanical  separation  by  means  of  the  scalpel,  partly  by  being 
traced  out  with  the  help  of  the  microscope,  but,  as  in  the  spinal  cord,  much 
more  fully  and  completely  by  differences  of  development,  and  by  the  method 
of  degeneration. 

FIG.  145. 


Outline  of  a  Sagittal  Section  through  the  Hemisphere— Man.  (Sherrington.)  The  section  is 
taken  not  far  to  the  right  of  the  median  plane  and  is  one-half  linear  or  natural  size.  The  gray 
matter  of  the  corpus  striatnm  and  thalamus  is  shaded.  Nc,  Nc,  the  caudate  nucleus ;  Pt,  the 
putamen,  and  Gp,  the  globus  pallidus  of  the  lenticular  nucleus;  0  T,  the  optic  thalamus;  CI,  the 
internal  capsule  with  a  streaked  appearance  revealing  approximately  the  direction  taken  by  fibre- 
bundles  passing  into  it  from  the  portion  of  corona  radiata  over  it.  In  these  sets  of  bundles  may 
be  broadly  distinguished  a  frontal  system,  fron,  a  pyramidal  system,  P  Y (subdivisible  into  cranial 
(cran.),  brachial  (brack.),  dorso-lumbar  (dors,  lum.),  and  lumbo-sacral  (lum.  sac.)  parts,  and  a  tern- 
poro-occipital  system,  sens.;  the  situation  of  the  genu  of  the  internal  capsule  is  indicated  by  g. 
CR,  the  crus  cerebri ;  Oc,  the  so-called  optic  radiations  passing  into  the  occipital  lobe;  cc,  the 
splenial  end  of  the  corpus  callosum ;  v,  v,  v.  the  lateral  ventricle  cut  across  in  three  different 
places ;  F,  the  fornix  in  cross-section  ;  Op,  the  optic  tract  in  cross-section.  Part  of  the  cerebellum 
is  seen  in  outline  to  the  right. 

We  have  seen  that  a  marked  feature  of  the  brain  is  presented  by  the  two 
crura  cerebri  which,  running  forward  from  the  hind  parts  of  the  brain, 
spread  out  into  each  cerebral  hemisphere.  We  have  also  seen  that  the  crus 
in  the  wide  sense  of  the  word  consists  of  two  parts,  a  dorsal  part,  the  teg- 
mentum,  and  a  ventral  part,  the  pes  or  crusta,  and  that  these  two  parts  differ 
very  strikingly  from  each  other  in  structure  and  in  relations.  The  pes  con- 
sists exclusively  of  bundles  of  longitudinal  fibres,  and  we  may  trace  these 
from  the  cerebral  hemispheres  into  the  pons  and  some  of  them  beyond  the 
pons  into  the  bulb  and  spinal  cord.  The  tegmentum  is  more  complex  in 
structure ;  it  consists  of  gray  matter,  and  of  fibres  and  bundles  of  fibres 
having  various  relations  both  with  the  collections  of  gray  matter  lying 
within  itself  and  with  surrounding  structures.  It  too  has  connections  with 
the  parts  lying  in  front  of  it,  and  with  the  parts  lying  behind  it ;  we  may 
trace  it,  too,  backward  through  the  pons  into  the  bulb  and  forward  to  the 
optic  thalamus.  If  we  allow  ourselves  to  conceive  of  the  optic  thalamus  as 


628  THE  BKAIN. 

constituting  the  front  ending  of  the  tegmentum,  we  may  arrange  a  large  part 
of  the  brain  into  two  main  regions — into  a  tegmental  region  stretching  from 
the  optic  thalamus  through  the  dorsal  portion  of  the  pons  to  the  dorsal  por- 
tion of  the  bulb,  and  into  a  region,  which  we  may  call  the  pedal  region, 
stretching  from  the  internal  capsule  through  the  ventral  portion  of  the  pons 
to  the  ventral  portion  of  the  bulb. 

The  fibres  of  the  brain,  as  a  whole,  may  be  broadly  classified  into  longi- 
tudinal tracts  connecting  parts  of  the  brain  with  succeeding  parts  and  into 
transverse  or  commissural  tracts  between  one  lateral  half  and  the  other,  and 
into  tracts  connected  with  the  several  cranial  nerves.  Taking  the  longitu- 
dinal fibres  first,  we  may  in  accordance  with  the  division  just  explained  into 
a  pedal  and  a  tegmental  region,  consider  these  as  forming,  on  the  one  hand, 
a  pedal  and,  on  the  other  hand,  a  tegmental  system. 

Both  systems  begin,  as  we  shall  see,  in  the  cortex  of  the  cerebral  hemi- 
spheres. We  shall  have  to  deal  with  the  topography  of  the  cortex  later  on, 
but  may  here  say  that  the  first  broad  division  of  the  whole  surface  of  a 
hemisphere  is  into  four  main  regions  :  frontal,  parietal,  occipital,  and  tem- 
poral (Figs.  139,  140, 144). 

LONGITUDINAL    FIBRES    OF    THE   PEDAL   SYSTEM. 

§  545.  The  pyramidal  tract.  We  have  already  (§  488)  said  that  the 
pyramidal  tract  of  the  spinal  cord  may  be  traced  to  a  particular  region  of 
the  cerebral  cortex.  We  shall  study  the  details  of  this  region,  which  is 
often  spoken  of  as  the  "  motor  area  "  later  on,  but  may  here  say  that  broadly 
speaking  it  is  parietal  in  position  and  corresponds  to  the  parts  of  the  cortex 
gathered  round  the  fissure  of  Rolando.  Fibres  passing  from  the  gray  matter 
of  the  cortex  of  this  region  to  the  white  matter  below,  and  so  contributing 
their  share  to  the  central  white  matter  of  the  hemisphere,  converge  (Figs. 
145,  146)  to  form  part  of  the  internal  capsule,  namely,  that  part  which  in  a 
horizontal  section  (Fig.  144,  Eye  to  Dig)  occupies  the  knee  and  stretches  for 
more  than  half,  or  nearly  two-thirds,  along  the  hind  limb  of  the  capsule, 
between  the  optic  thalamus  on  the  inside  and  the  nucleus  lenticularis  on  the 
outside.  From  the  knee  and  hind  limb  of  the  capsule  they  pass  by  the  side 
of  and  ventral  to  the  optic  thalamus  (Fig.  139,  146),  and  so  contribute  to 
form  the  beginning  of  the  crus  cerebri.  In  thus  converging  to  take  up  their 
position  in  the  capsule  ancl  in  their  further  passage  to  the  crus  the  fibres 
follow  a  course  of  somewhat  complicated  curvature.  As  we  trace  the  cap- 
sule from  more  dorsal  to  more  ventral  levels,  we  find  it  continually  changing 
in  form  ;  the  exact  shape  of  the  capsule  shown  in  Fig.  144  only  holds  good 
for  the  level  at  which  the  section  was  taken  ;  it  differs  somewhat  from  that 
shown  in  Fig.  138  taken  at  a  slightly  different  level,  and  sections  still  more 
dorsal  or  still  more  ventral  would  present  still  greater  differences.  When 
we  examine  a  series  of  horizontal  sections,  taken  in  succession  from  the 
dorsal  to  the  ventral  regions,  we  find  that  the  knee  shifts  its  position  and 
changes  in  the  width  of  its  angle,  that  the  two  limbs  vary  in  direction,  in 
size,  and  in  shape,  and  that  at  last  the  bent,  flattened  capsule  passes  into  the 
more  or  less  rounded  crus  by  the  rapid  disappearance  of  the  fore  limb,  and 
the  consequent  extinction  of  the  angle ;  so  that  in  one  sense  it  is  the  hind 
limb  which  becomes  the  crus,  and  the  fibres  of  the  fore  limb  may  be  said  to 
pass  into  the  crus  through  the  ventral  portion  of  the  hind  limb.  Hence  it 
is  obvious  that  the  fibres  of  the  pyramidal  tract,  like  the  other  fibres  of 'the 
capsule,  are  continually  changing  their  direction  as  they  pass  through  the 
capsule.  Moreover,  while  the  fibres  from  the  different  parts  of  the  "  motor 
area  "  assume  definite  positions  in  relation  to  each  other  as  they  pass  into 


LONGITUDINAL  FIBRES  OF  THE  PEDAL  SYSTEM. 


629 


the  capsule,  their  relative  positions  are  not  constant,  but  vary  somewhat.  To 
this  point,  however,  we  shall  return  when  we  come  to  speak  of  the  function 
of  this  tract. 


FIG.  146. 


fc 


lud 


Outline  of  a  Transverse  Dorso-ventral  Section  of  the  Right  Half  of  the  Brain.  Natural  size. 
(Sherrington.)  The  section,  which  is  taken  at  the  level  of  the  knee  of  the  capsule,  and  is  therefore 
intermediate  between  those  shown  in  Figs.  139  and  140,  is  introduced  to  illustrate  the  course  of  the 
constituents  of  the  pyramidal  tract.  OT,  optic  thalamus;  Nc,  nucleus  caudatus— the  head  only 
appears  in  this  section ;  Pt,  putamen  ;  Gp",  Gp,'  the  two  parts  of  the  globus  pallidus  of  the  nucleus 
lenticularis;  C,  the  claustrum  ;  CE,  the  external  capsule  ;  In,  the  island  of  Reil ;  ca,  the  anterior 
commissure,  shaded  to  render  it  distinct  and  the  fibres  from  the  temporo-sphenoidal  lobe  which 
pass  into  it  being  indicated  by  broken  lines ;  Op,  the  optic  tract ;  I  v  d,  the  end  of  the  descending 
horn  of  the  lateral  ventricle ;  F,  the  fornix  ;  F',  the  end  of  the  anterior  pillar  of  the  fornix  in  the 
base  of  the  thalamus ;  cc,  corpus  callosum ;  OP,  anterior  part  of  the  occipital  lobe,  fc  is  the  central 
fissure,  or  fissure  of  Rolando.  The  course  of  the  fibres  of  the  pyramidal  tract  connected  respectively 
with  the  trunk,  leg,  and  arm  and  hence  with  spinal  nerves,  and  of  those  connected  with  the  face 
and  hence  with  cranial  nerves,  is  shown  by  broken  lines.  These  are  all  seen  converging  into  the 
internal  capsule,  CI.  This  figure  should  in  respect  to  the  course  of  these  fibres  be  compared  with 
the  horizontal  section  shown  in  Fig.  144,  and  the  sagittal  figure  shown  in  Fig.  145.  S  indicates  the 
course  of  the  most  anterior  and  dorsal  part  of  the  lemporo-occipital  tract.  The  fine  dotted  lines 
converging  to  the  corpus  callosum,  c  c,  indicate  the  course  of  the  callosal  fibres. 

In  the  cms  these  fibres  run  exclusively  in  the  pes  and  form  a  compact 
strand  (Fig.  137,  Py)  occupying  the  central  and  larger  portion  of  the  pes 
between  a  small  median  portion  on  the  inside  and  a  lateral  portion  on  the 


630  THE  BRAIN. 

outside.  Maintaining  this  position  along  the  cms  they  enter  the  pons,  but 
here  the  previously  compact  strand  is  split  up  by  the  interlacing  transverse 
fibres  of  the  pons  into  a  number  of  scattered  bundles,  which,  however,  as  a 
whole,  still  keep  their  central  position.  They  form  the  greater  part  of,  but 
not  all,  the  bundles  seen  cut  transversely  in  transverse  sections  of  the  pons 
(Figs.  135,  136).  Further  backward  they  become  the  pyramid  of  the  bulb, 
and  so  give  rise  in  the  spinal  cord  to  the  direct  and  crossed  pyramidal  tracts. 
These  fibres  from  the  motor  area  of  the  cortex  of  the  cerebrum  are  thus  the 
source  of  the  pyramidal  tracts  of  the  spinal  cord,  and  hence  the  whole 
strand  of  fibres  from  the  cortex  downward  has  been  called  the  pyramidal 
tract.  We  have  said  (§  488)  that  we  have  reasons  for  thinking  that  the 
pyramidal  tract  in  the  spinal  cord  makes  connections  through  the  gray 
matter  of  the  anterior  horn  with  the  anterior  roots  of  all  the  spinal  nerves 
in  succession ;  and  similarly  we  have  reason  to  think  that  along  its  course  in 
the  crus,  in  the  pons,  and  in  the  bulb,  before  it  reaches  the  cord,  the  tract 
also  makes  connections  with  the  nuclei  of  those  cranial  nerves  which  are 
motor  in  function.  During  the  passage  of  the  tract  through  the  internal 
capsule  the  fibres  destined  for  cranial  nuclei  occupy  the  knee,  while  those 
belonging  to  the  spinal  cord  run  in  the  hind  limb.  Some  authors  limit  the 
term  pyramidal  tract  to  the  spinal  moiety,  since  this  alone  forms  the  pyra- 
mid ;  but  this  is  undesirable. 

This  tract  is  well  marked  out  by  the  degeneration  method,  and  the  de- 
generation in  it  is  a  descending  one,  the  trophic  centres  of  the  fibres  being 
cells  in  the  gray  matter  of  the  cortex.  Removal  of  or  injury  to  the  cortex 
of  the  whole  motor  area  gives  rise  to  a  degeneration  along  the  whole  tract, 
and  removal  of  or  injury  to  part  of  the  area  gives  rise  to  degeneration  of 
some  of  the  strands.  The  tract  is  also  well  marked  out  by  the  embryologi- 
cal  method  ;  the  fibres  belonging  to  it  acquire  their  medulla  at  times  different 
from  those  of  other  fibres. 

Anterior  or  frontal  cortical.  Fibres  from  the  gray  matter  of  the  cortex 
in  front  of  the  motor  area  also  pass  to  the  internal  capsule,  but  occupy  the 
fore  limb  (Fig.  145,  /row).  Thence  they  pass  to  the  crus,  of  which  they 
form  the  small  inner,  median  portion  of  the  pes  (Fig.  137,  Fr.\  and  from 
the  crus  pass  into  the  pons ;  in  transverse  sections  of  the  pons  they  are  seen 
as  scattered  bundles  (Fig.  136,  F.  C.)  to  the  median  side  of  the  pyramidal 
fibres.  But  here  they  seem  to  end  ;  the  degeneration  of  the  tract  is  a  de- 
scending one,  and  ceases  here.  Most  probably  the  fibres  end  in  the  nerve- 
cells  of  the  gray  matter,  which,  as  we  have  seen,  is  abundant  in  the  pons. 
It  is  also  probable  that  through  these  nerve-cells  the  fibres  of  this  tract  are 
connected  with  transverse  fibres  passing  along  the  middle  cerebellar  pedun- 
cle into  the  cerebellum  of  the  opposite  side  ;  but  this  has  not  been  definitely 
proved. 

Posterior  or  temporo-occipital  cortical.  Fibres  from  the  gray  matter  of 
parts  of  the  cortex  behind  the  motor  area  also  converge  to  the  internal  cap- 
sule, forming  the  hinder  end  of  the  hind  limb  behind  the  pyramidal  tract 
(Fig.  144,  £).  These  fibres  also  contribute  to  form  the  crus  cerebri,  passing 
into  the  pes,  of  which  they  occupy  the  outer  lateral  portion  (Fig.  137,  Pr.  0.}. 
From  the  crus  they  pass  into  the  pons,  where,  like  the  fibres  of  the  preceding 
tract,  they  appear  to  end,  and  probably  in  a  like  manner.  This  tract  has 
been  described  as  one  of  ascending  degeneration,  but  in  all  probability  like 
the  preceding  is  one  of  descending  degeneration. 

The  above  three  tracts  of  fibres  may,  therefore,  all  be  regarded  as  start- 
ing from  or  having  their  trophic  centres  in  the  cortical  gray  matter  of  the 
hemispheres,  as  all  helping  to  form,  first,  the  internal  capsule  and  then  the 
pes  of  the  crus  cerebri.  But  while  the  pyramidal  tract  passes,  in  part,  to  the 


LONGITUDINAL  FIBRES  OF  THE  TEGMENTAL  SYSTEM.       631 

spinal  cord,  the  other  two  cease  at  the  pons,  and  probably  through  the  gray 
matter  of  the  pons  make  connections  with  the  cerebellum.  Further,  while 
the  pyramidal  tract  coming  from  the  middle  region  of  the  cortex  occupies  a 
middle  position  in  the  capsule  and  a  middle  position  in  the  crus,  the  system 
from  the  front  part  of  the  cortex  occupies  a  front  position  in  the  capsule 
and  an  inner  or  median  position  in  the  crus,  and  the  system  from  the  hind 
part  of  the  cortex,  a  hind  position  in  the  capsule  and  an  outer  or  lateral 
position  in  the  crus.  As  the  three  systems  pass  from  the  cortex  through  the 
capsule  to  form  the  pes  of  the  crus,  their  positions  in  relation  to  each  other 
are  shifted  from  one  plane  into  another.  As  the  fibres  spread  out  from  the 
pes  through  the  capsule  to  all  parts  of  the  cortex,  or,  put  in  another  way,  as 
they  converge  from  the  cortex  through  the  capsule  to  the  pes,  they  form  a 
f&n,  the  corona  radiata,  which  is  not  only  curved,  but  the  constituent  parts 
of  which  cross  each  other. 

Besides  these  three  systems  all  passing  from  various  regions  of  the  cortex 
to  the  crus,  there  is  yet  a  fourth  strand  contributed  to  the  pes  by  the  cere- 
bral hemisphere,  though  not  starting  in  the  cortex.  From  the  nucleus 
caudatus  fibres  pass  down  to  the  crus,  and  take  up  a  position  in  the  pes 
dorsal  to  the  tract  just  mentioned,  occupying  a  lens-shaped  area  immediately 
ventral  to  the  substantia  nigra,  and  probably  passing  into  the  substantia 
nigra  itself.  These  cannot  be  traced  further  down  than  the  pons,  where 
they  appear  to  end,  though  possibly  some  terminate  higher  up  in  the  sub- 
stantia nigra.  This  tract  has  a  descending  degeneration,  and  may  be  re- 
garded as  a  tract  analogous  to  the  front  and  hind  cortical  tracts,  though  it 
begins  not  in  the  cortex  but  in  the  nucleus  caudatus ;  it  is  not,  however, 
a  very  pure  tract,  many  fibres  of  the  pyramidal  tract  passing  into  it  in  the 
pes. 

These  are  the  main  tracts  of  the  pedal  system.  For,  though  the  nucleus 
lenticularis  gives  off  fibres  to  the  internal  capsule,  our  knowledge  of  the 
further  course  of  these  is  at  present  imperfect,  and  though  there  seem  to  be 
longitudinal  fibres  connecting  the  bulb,  the  pons,  and  the  pes  at  various 
levels,  these  are  not  numerous,  and  at  all  events  do  not  form  conspicuous 
strands. 


LONGITUDINAL    FIBRES    OF    THE    TEGMENTAL    SYSTEM. 

§  546.  Cortical  fibres.  Although  the  fibres  of  the  pedal  system  form,  as 
we  have  seen,  the  greater  part,  they  do  not  form  the  whole,  of  the  internal 
capsule.  Fibres  coming  from  all  or  nearly  all  parts  of  the  cortex,  though 
they  help  to  form  the  internal  capsule,  do  not  go  on  to  form  the  pes,  but 
pass  to  the  optic  thalamus  (Fig.  139,  /.  I.)  and  appear  to  end  in  the  gray 
matter  of  that  body.  In  their  passage  through  the  capsule  the  fibres  of  this 
nature  from  the  frontal  and  parietal  regions  of  the  cortex,  occupy  the  ex- 
treme end  of  the  anterior  limb  in  front  of  the  strand  of  the  fibres  of  the 
pedal  system  (Fig.  144,  2Vi.).  The  fibres  from  the  occipital  and  tem- 
poral regions — those  from  the  occipital  regions  being  the  most  numerous, 
and  indeed  being  very  conspicuous — occupy  the  extreme  hind  end  of  the 
hind  limb  of  the  capsule,  behind  the  temporo-occipital  division  of  the  pedal 
system  (Fig.  144,  Op.\  Since,  as  we  shall  see,  we  have  reason  to  associate 
the  occipital  region  of  the  cortex  with  vision,  the  fibres  thus  radiating  to  (or 
from)  the  thalamus  through  the  extreme  hind  limb  of  the  capsule  from  (or 
to)  the  occipital  cortex  have  been  called  the  optic  radiation. 

All  the  above  tracts  of  fibres,  though  joining  the  thalamus  and  not  pass- 
ing on  to  the  pes,  take  part  in  the  formation  of  the  internal  capsule.  But 
a  considerable  number  of  fibres  coming  from  the  temporo-occipital  region, 


632  THE  BRAIN. 

and  especially  from  the  temporal  region,  pass  to  the  thalamus  without  join- 
ing the  capsule ;  they  pass  ventral  to  and  behind  the  pes  as  this  plunges  into 
the  hemisphere  to  become  the  capsule,  and  so  reach  the  thalamus. 

We  may  here  perhaps  diverge  for  a  moment  to  point  out  the  contrast 
between  the  optic  thalamus  and  the  corpus  striatum,  or  at  least  the  nucleus 
caudatus.  The  former  does  not  contribute  to  the  pedal  system,  the  latter 
supplies  a  marked  contribution.  The  former  receives  fibres  from  all  parts 
of  the  cortex  ;  there  are  no  such  special  contributions  from  the  cortex  to  the 
latter.  And  this  difference  accords  with  the  experience  that  when  parts  of 
the  cortex  are  removed,  or  are  congenitally  absent,  no  degeneration  or  want 
of  development  is  observed  in  the  corpus  striatum,  while  degeneration  or 
want  of  development  is  observed  in  the  optic  thalamus  as  well  as  in  parts  of 
the  pedal  and  tegmental  systems.  Hence,  while  we  may  regard  the  optic 
thalamus  as  an  intermediate  mass  of  gray  matter  receiving  fibres  from  the 
cortex,  and  connecting  the  cortex  with  lower  parts  of  the  tegmental  region, 
the  corpus  striatum  appears  rather  to  be  analogous  to  the  cortex  itself,  to 
be  a  special  modification  of  the  cortex,  sending  fibres  down  into  the  pedal 
system,  but  itself  receiving  no  special  tracts  of  fibres  from  the  cortex.  Indeed 
we  may  probably  regard  the  corpus  striatum  as  the  oldest  part  of  the  super- 
ficial gray  matter  of  the  hemisphere,  the  more  ordinary  cortex  being  a  later 
development. 

The  tegmentum  proper,  lying  ventral  to  the  hind  end  of  and  behind  the 
thalamus,  in  which  region,  as  we  have  seen,  the  conspicuous  red  nucleus  is 
situated,  is  thus,  by  reason  of  its  connection  with  the  thalamus,  indirectly 
connected  with  the  cortex.  But  besides  this,  it  has  direct  connections  of  its 
own  with  the  cortex.  Some  of  the  fibres  of  the  optic  radiation,  as  well  as 
fibres  from  the  temporal  and  occipital  regions  described  above  as  sweeping 
round  the  base  of  the  internal  capsule,  are  said  to  pass  not  to  the  thalamus, 
but  to  the  tegmentum.  Other  fibres  from  the  frontal  and  parietal  regions 
traversing  the  lenticular  nucleus  in  the  sheets  of  white  matter  splitting  the 
nucleus  into  parts  are  also  said  to  reach  the  tegmentum  either  by  piercing 
through  or  by  sweeping  round  the  internal  capsule.  On  their  path  these 
fibres  are,  according  to  some  observers,  joined  by  fibres  coming  from  the  len- 
ticular nucleus  itself,  and  possibly  from  the  caudate  nucleus,  which  fibres, 
on  the  view  that  these  nuclei  are  modified  cortex,  may  also  be  considered  as 
cortical.  Thus  the  fore  part  of  the  tegmental  region  is  brought  into  ample 
connection  with  the  cerebral  hemisphere  partly  by  fibres  joining  the  thala- 
mus, partly  by  fibres  passing  directly  to  the  tegmentum  proper. 

The  mode  of  degeneration  of  these  cortical  fibres  of  the  tegmental  system 
is  at  present  a  matter  of  dispute.  Nor  is  the  general  nature  of  the  fibres 
conclusively  determined,  though  it  is  generally  supposed  that  they  carry 
impulses  from  the  thalamus  and  parts  of  the  tegmentum  to  the  cortex. 

§  547.  In  the  tegmentum  from  the  subthalamic  region  to  the  bulb  the 
reticular  formation  is,  as  we  have  seen,  more  or  less  abundant ;  this,  and  the 
occurrence  of  various  bundles  of  fibres,  gives  the  region  great  complexity ; 
and  we  must  confine  ourselves  here  to  touching  on  one  or  two  important 
longitudinal  strands  which  traverse  it. 

The  superior  peduncle  of  the  cerebellum  is  one  of  the  most  important  of 
these.  This,  on  each  side  consists  of  a  bundle  of  fibres  which,  taking  origin 
chiefly  from  the  gray  matter  of  the  nucleus  dentatus  and  the  smaller  neighbor- 
ing collection  of  gray  matter,  and  also  in  part  from  the  superficial  gray  mat- 
ter, leaves  the  cerebellum  in  front  of  and  to  the  median  side  of  the  restiform 
body  and  passes  forward  toward  the  corpora  quadrigemina  to  converge  with 
its  fellow.  At  first  the  two  peduncles  are  superficial  and  dorsal  in  position 
(Figs.  134,  135,  S.  P.)  and  the  space  between  them  is  bridged  over  by  the 


LONGITUDINAL   FIBRES  OF  THE  TEGMENTAL  SYSTEM.       633 

valve  of  Vieussens  (Fig.  135,  Via);  but,  still  converging,  they  soon  sink 
ventrally  beneath  the  posterior  corpora  quadrigeniina  and  at  the  level  of  the 
junction  between  the  anterior  and  posterior  corpora  quadrigeniina  meet  and 
decussate  ventral  to  those  bodies  in  the  ventral  region  of  the  tegmentum 
(Fig.  136,  S.  P.).  Beyond  the  decussation  they  are  continued  forward  in 
the  tegmentum  ventral  to  the  anterior  corpora  quadrigemina  as  two  strands, 
one  on  each  side,  which  appear  to  end  in  the  red  nuclei. 

In  this  way  the  peduncles  connect  certain  parts  of  the  gray  matter  of  the 
cerebellum  with  the  tegmental  region,  and  more  particularly  with  the  red 
nucleus,  and  thus  indirectly  with  the  structures  with  which  that  region  is 
itself  connected. 

The  fillet  This,  as  we  have  seen  (§  525),  takes  origin  in  the  bulb,  in  the 
inter-olivary  layer  between  the  inferior  olives,  from  fibres  which  are  derived 
through  the  supra-pyramidal  or  sensory  decussation  from  the  gracile  and 
cuneate  nuclei.  From  this  origin  it  passes  forward  on  each  side  as  a  flat 
band  into  the  tegmental  region  of  the  pons,  receiving  accessions  from  the 
superior  olive  and  other  collections  of  gray  matter,  and  dividing  there  into 
two  strands,  the  median  (Figs.  135, 136,  Fin)  and  lateral  (Figs.  135,  136,  Fl, 
and  Fig.  131,  B  F)  fillet.  The  lateral  division  ends  partly  in  the  gray 
matter  of  the  posterior  corpus  quadrigeminum,  and  partly  in  the  white 
matter  underlying  (Fig.  137,  dm)  the  anterior  corpus  quadrigeminum ;  the 
median  division,  passing  further  forward  appears  partly  to  end  in  the  gray 
matter  of  the  anterior  corpus  quadrigeminum,  but  partly  to  be  continued  on 
to  the  subthalarnic  region  of  the  tegrneutum  ventral  to  the  thalamus,  thence 
to  the  thalamus,  and  so  to  the  cortex. 

The  longitudinal  posterior  bundles.  In  a  transverse  section  through  the 
fore  part  of  the  pons  at  the  level  of  the  posterior  corpora  quadrigemina  a 
rather  conspicuous  bundle  of  longitudinal  fibres  (called  the  longitudinal 
posterior  bundle)  is  seen  on  each  side,  cut  transversely,  in  the  dorsal  region 
of  the  tegmentum  just  ventral  to  the  nucleus  of  the  fourth  nerve  (Fig.  136, 1). 
Traced  backward  from  the  aqueduct  beneath  the  fourth  ventricle,  it  becomes 
less  conspicuous  (Fig.  135,  /),  though  maintaining  its  position  dorsal  to  the 
reticular  formation,  and  at  the  hind  end  of  the  bulb  appears  to  be  a  con- 
tinuation forward  of  those  fibres,  "ground  fibres,"  of  the  anterior  column  of 
the  cord  which  probably  serve  as  successive  short  longitudinal  commissures 
between  the  segments  of  the  cord.  While  the  somewhat  analogous  fillet 
runs  ventral  to  the  reticular  formation,  this  posterior  longitudinal  bundle 
runs  always  dorsal  to  that  structure.  It  may  be  traced  forward  as  far  as 
the  nucleus  of  the  third  nerve,  as  is  seen  in  transverse  sections  lying  imme- 
diately ventral  to  that  group  of  cells  (Fig.  137,  /),  but  its  further  connections 
forward  have  not  as  yet  been  determined.  It  is  relatively  more  prominent 
in  the  lower  than  in  the  higher  animals,  and  its  fibres  acquire  their  medulla 
relatively  early.  It  is  supposed  to  be  connected  with  the  nuclei  of  the  nerves 
governing  the  muscles  of  the  eye,  and  so  to  be  concerned  in  the  movements 
of  that  organ. 

Tracts  from  the  corpora  quadrigemina.  From  each  corpus  quadrigeminum 
there  passes  obliquely  forward  and  downward  on  each  side  a  band  of  fibres, 
connected  with  the  gray  matter  of  the  corpus  and  known  as  the  brachium. 
The  anterior  brachium  (Fig.  137,  Ba),  as  we  shall  see  in  dealing  with  the 
optic  nerve,  joins  the  lateral  corpus  geniculatum  and  helps  to  form  the  optic 
tract,  but  some  of  its  deeper-lying  fibres  proceed  to  the  occipital  cortex  form- 
ing part  of  the  fibres  which  we  have  (§  545)  described  as  passing  from  the 
occipital  cortex  to  and  past  the  thalamus.  The  posterior  brachium  passes  to 
the  median  corpus  geniculatum  ;  having  received  fibres  from  and  probably 
given  fibres  up  to  that  body,  it  is  continued  on  to  the  tegmentum,  and,  ac- 


634  THE   BRAIN. 

cording  to  some  authors,  through  the  tegmentum  by  the  hind  part  of  the 
hind  limb  of  the  internal  capsule  to  the  temporal  region  of  the  cortex, 
mingling  in  its  course  with  fibres  from  the  thalamus. 

TRANSVERSE  OR  SO-CALLED  COMMISSURAL  FIBRES. 

§  548.  The  two  chief  masses  are  those,  on  the  one  hand,  belonging  to 
the  cerebrum,  and  those,  on  the  other  hand,  belonging  to  the  cerebellum. 

In  the  cerebrum  the  most  imposing  mass  of  transverse  fibres  form  the 
corpus  callosum.  Starting  from  the  cortex  in  nearly  all  parts  of  the  hem- 
isphere, the  fibres  converge  toward  the  thick  body  of  the  corpus  callosum 
placed  in  the  middle  line,  and  thence  diverge  to  nearly  all  parts  of  the 
cortex  of  the  hemisphere  on  the  other  side,  interlacing  in  their  course 
with  the  cortical  fibres  of  the  pedal  and  tegmental  systems.  It  is  sup- 
posed that  by  means  of  these  fibres,  each  part  of  the  cortex  of  one  hemi- 
sphere is  brought  into  connection  with  the  corresponding  part  of  the  other 
hemisphere. 

Besides  these  callosal  fibres  from  one  hemisphere  to  another,  the  white 
matter  of  each  hemisphere  contains  fibres  called  "  association  fibres,"  passing 
from  one  convolution  to  another  of  the  same  hemisphere. 

The  small  anterior  white  commissure  though  it  is  placed  in  the  front  part 
of  the  third  ventricle  (Fig.  143,  A)  and,  in  part  of  its  course,  lies  along  the 
thalamus  (Fig.  140,  Ca),  is  really  a  commissure  of  particular  parts  of  the 
cerebral  hemispheres.  A  portion,  very  small  in  man,  belongs  to  the  olfactory 
tract ;  the  rest  takes  origin  on  each  side  in  a  limited  portion  of  the  cortex 
(Fig.  139,  Ca\  which  we  shall  later  on  speak  of  as  the  temporo-sphenoidal 
convolution,  and  in  which  callosal  fibres  are  deficient,  whence  it  arches  for- 
ward through  the  globus  pallidus,  past  the  thalamus  (Figs.  146,  ca,  140,  Ca) 
to  the  front  part  of  the  third  ventricle.  It  may  be  remarked  that  this  com- 
missure is  still  found  in  those  lower  animals  which  do  not  possess  an  obvious 
corpus  callosum. 

The  small  posterior  commissure  may  be  regarded  as  simply  a  commissure 
between  the  two  thalami,  but  it  also  helps  to  unite  the  tegmentum  of  the  two 
sides,  and  some  fibres  are  said  to  pass  on  each  side  into  the  hemisphere.  The 
middle  or  soft  commissure  of  the  third  ventricle  (Fig.  138,  c),  though  it  con- 
tains transverse  fibres,  is,  in  the  main,  a  collection  of  gray  matter,  indeed,  a 
part  of  the  central  gray  matter. 

The  fornix,  together  with,  at  all  events,  part  of  the  septum  lucidum 
which  joins  it  with  the  corpus  callosum,  must  also  be  regarded  as  a  com- 
missural  structure.  But  its  relations  are  peculiar ;  for  while,  behind,  the 
diverging  posterior  pillars  begin  in  the  cerebral  hemispheres,  namely,  in  the 
walls  of  the  descending  horn  of  the  lateral  ventricle  on  each  side,  in  front 
the  anterior  pillars  or  columns,  leaving  the  cerebral  hemispheres,  pass  along 
the  lateral  walls  of  the  third  ventricle  (Fig.  143, /),  and  apparently  end 
in  the  gray  matter  of  the  corpora  albicantia.  Whether  the  band  of  fibres, 
known  as  Vicq  d'Azyr's  bundle  (Fig.  139,  Vb~),  which  running  in  the  lat- 
eral wall  of  the  third  ventricle  leads  dorsally  from  each  corpus  albicans  up 
to  the  anterior  nucleus  of  the  thalamus,  is  really  to  be  considered  as  a 
continuation  of  the  foruix  is  disputed  ;  it  may  more  probably  be  regarded 
as  a  part  of  the  system  spoken  of  above  as  connecting  the  cortex  with  the 
thalamus. 

In  the  cerebellum  true  commissural  fibres  are  supplied  by  the  middle 
peduncles;  but  by  no  means  all  the  fibres  of  these  peduncles  are  of  this 
nature.  The  fibres  of  the  middle  peduncle,  in  contrast  to  those  of  the 
superior  peduncle  which  starts  chiefly  from  the  nucleus  dentatus,  or  other 


SUMMARY.  635 

internal  gray  matter,  and  to  those  of  the  inferior  peduncle  which  start 
chiefly  from  'the  superficial  gray  matter  of  the  verrnis,  appear  to  start  from 
the  superficial  gray  matter  of  the  whole  surface,  from  that  of  the  median 
vermis  as  well  as  from  that  of  the  lateral  hemispheres ;  they  thus  form  the 
greater  part  of  the  central  white  matter.  Sweeping  down  into  the  pons, 
they  form  the  transverse  fibres  of  that  body,  interlacing  with  the  longitu- 
dinal fibres  of  the  crural  system  and  intermingling  with  the  abundant  gray 
matter. 

Of  these  transverse  fibres  of  the  pons,  a  certain  number  are  truly  com- 
missural ;  they  make  no  connections  with  cells  in  the  pons,  but  continue 
their  way  unbroken  across  it ;  they  start  in  the  superficial  gray  matter  of 
one  side  of  the  cerebellum  and  end  in  the  superficial  gray  matter  of  the  other 
side,  the  parts  of  the  gray  matter  thus  united  being  probably  corresponding 
parts.  The  most  ventrally  placed  transverse  fibres  of  the  pons  which  form 
a  superficial  layer  of  white  matter,  free  from  gray  matter  (Fig.  134,  tr.  P.) 
are  probably  of  this  nature,  as  are  also  the  transverse  fibres  placed  most 
dorsally,  just  ventral  to  the  tegmental  region. 

A  large  number  of  the  transverse  fibres  are  not  of  this  nature.  They 
cross  from  one  side  of  the  cerebellum  to  the  opposite  side  of  the  pons,  but 
end  in  the  pons  apparently  in  the  nerve-cells  of  the  gray  matter ;  and  it  is 
supposed  that  by  these  nerve-cells  they  are  brought  into  connection  with  the 
longitudinal  fibres  of  the  pedal  system  and  thus  with  the  cerebrum.  They 
are  transverse  appendages  of  the  pedal  system,  not  true  commissural  fibres, 
though  they  do  cross  the  median  line. 

It  is  further  supposed  that  other  fibres  of  the  middle  peduncle  reaching 
the  pons  do  not  cross  the  median  line,  but  keeping  to  the  same  side  and 
changing  their  direction,  take  a  longitudinal  upward  course  either  with  or 
without  the  intervention  of  nerve-cells,  and  so  make  their  way  to  the  teg- 
mentum.  But  this  is  not  certain. 

We  must  also  consider  as  commissural  structures  the  numerous  fibres 
crossing  or  serving  to  form  the  median  raphe  in  the  bulb.  This  raphe,  with 
similar  commissural  fibres,  is  present  in  the  tegmental  portion  of  the  pons,  and, 
indeed,  in  the  tegmentum  itself. 

Fibres  also  cross  from  one  side  to  the  other  in  connection  with  the 
cranial  nerves,  but  these,  as  well  as  all  the  tracts  specially  connected  with 
the  cranial  nerves,  including  the  olfactory  and  optic  nerves,  had  better  be 
considered  by  themselves. 

SUMMARY. 

§  549.  It  may  perhaps  appear  from  the  foregoing  that  the  brain  consists 
of  a  number  of  isolated  masses  of  gray  matter,  and  some  large,  some  small, 
connected  together  by  a  multitude  of  ties  of  white  matter  arranged  in  per- 
plexing intricacy ;  and  the  addition  of  numerous  collections  of  gray  matter 
and  strands  of  white  matter,  of  which  we  have  made  no  mention,  would  still 
further  increase  the  perplexity.  Nevertheless,  a  systematic  arrangement  may 
be  recognized,  at  least,  to  a  certain  extent. 

The  least  conspicuous,  but  perhaps  in  point  of  origin  the  oldest  part  of  the 
brain,  seems  to  be  what  we  have  called  the  central  gray  matter.  This  seems 
to  serve  chiefly  as  a  bed  for  the  development  of  the  nuclei  of  the  cranial 
nerves. 

Next  to  the  central  gray  matter  and  more  or  less  associated  with  it  comes 
what  is  called  the  tegmental  region,  of  which  the  reticular  formation,  com- 
ing into  prominence  in  the  bulb  and  continued  on  to  the  subthalamic  region, 
forms,  as  it  were,  the  core.  Belonging  to  the  tegmental  system  are  numerous 


636  THE  BRAIN. 

masses  of  gray  matter  from  the  conspicuous  optic  thalamus  and  the  red 
nucleus  in  front  to  the  several  nuclei  of  the  bulb  behind.  This  complex 
tegmental  system,  which  may,  perhaps,  be  regarded  as  a  more  or  less  con- 
tinuous column  of  gray  matter,  comparable  to  the  gray  matter  of  the  spinal 
cord,  serves  as  a  sort  of  backbone  to  the  rest  of  the  central  nervous  system. 
With  the  spinal  cord  it  is  connected  by  various  ties,  besides  being,  as  it 
were  a  continuation  of  the  spinal  gray  matter,  and  around  it  are  built  up 
the  great  mass  of  the  cerebrum,  and  the  smaller  but  still  larger  mass  of 
the  cerebellum ;  the  less  important  corpora  quadrigemina  we  may  for  sim- 
plicity's sake  neglect. 

At  the  hind  end  we  find  various  parts  of  the  spinal  cord  becoming  con- 
nected with  this  tegmental  system,  either  passing  into  it  and  becoming,  as 
far  as  our  present  knowledge  goes,  lost  in  it,  or  supplying  strands  or  fibres 
which  passing  into  it  become  through  it  connected  with  other  parts.  Thus 
the  anterior  column  of  the  cord  exclusive  of  the  direct  pyramidal  tract, 
the  lateral  column  exclusive  of  the  crossed  pyramidal  and  cerebellar 
tracts  (and  possibly  the  anter'o-lateral  ascending  tract),  together  with  part 
of  the  posterior  column,  appear  to  join  the  tegmental  system,  while  part 
of  the  posterior  column,  after  the  relay  of  the  gracile  and  cuneate  nuclei, 
passes  through  the  system  as  the  fillet  destined  for  various  structures. 

At  the  front  end  we  find  all  parts  of  the  cerebral  cortex  (though  some 
regions,  namely,  the  temporo-occipital,  to  a  greater  extent  than  others),  con- 
nected with  the  thalamus  and  other  parts  of  the  tegmental  system  ;  and  the 
corpus  striatum  may  possibly  possess  like  connections. 

The  relations  of  the  cerebellum  to  this  system  are  notable.  On  the  one 
hand  the  cerebellum  is  directly  connected  with  the  system,  partly  by  fibres 
which  pass  from  the  bulb  to  join  the  restiform  body  or  inferior  peduncle, 
partly  by  the  superior  peduncles  which  are  in  a  measure  lost  in  the  tegmen- 
tum,  and  partly  probably  by  fibres  of  the  middle  peduncles  also  making 
connections  with  the  tegmentum.  On  the  other  hand,  the  cerebellum  forms 
around  the  tegmental  system  a  great  junction  between  the  spinal  cord  and 
the  cerebrum.  To  the  spinal  cord  it  is  joined  in  a  direct  manner  by  the 
cerebellar  tract  and  possibly  by  the  antero-lateral  ascending  tract,  and  in 
an  indirect  manner  by  the  relay  of  the  gracile  and  cuneate  nuclei.  To  all 
parts  of  the  cerebral  cortex  it  appears  to  be  joined  by  those  conspicuous 
strands  of  the  pedal  system  which  end  in  the  pons,  and  there  make  connec- 
tions with  the  fibres  of  the  middle  peduncle.  And  we  may  here  perhaps 
remark  that  while  this  connection  between  the  cerebrum  and  cerebellum  is 
wholly  a  crossed  one,  each  cerebral  hemisphere  being  joined  with  the  oppo- 
site half  of  the  cerebellum,  the  connections  between  the  spinal  cord  and  the 
cerebellum  are  largely  uncrossed  ones,  that  by  the  cerebellar  tract  being 
wholly  uncrossed,  and  that  with  the  posterior  column  by  the  relay  of  the 
gracile  and  cuneate  nuclei  being  in  part  uncrossed. 

Thus  the  cerebral  cortex  has  a  double  hold,  so  to  speak,  on  the  rest  of 
the  central  nervous  system,  first  through  the  tegmental  system,  and  secondly 
through  the  cerebellar  junction.  But  in  addition  to  this  there  is  another 
tie  between  the  cerebral  cortex  and  the  whole  length  of  the  cerebro-spinal 
axis,  or  at  least  between  it  and  the  whole  series  of  motor  mechanisms  in 
succession  from  the  nucleus  of  the  third  nerve  to  the  nucleus,  if  we  may  so 
call  it,  of  the  anterior  root  of  the  coccygeal  nerve,  namely,  the  great  pyram- 
idal tract,  which  thus  appears  as  a  something  superadded  to  all  the  rest  of 
the  central  nervous  system. 

When  the  cerebral  hemispheres  are  removed  this  pyramidal  tract  falls 
away,  as  does  also  the  pedal  system  leading  from  the  cerebrum  to  the  pons, 
but  there  still  remains  the  tegmental  system  with  its  cerebellar  and  other 


WITHOUT  CEREBRAL  HEMISPHERES.  637 

adjuncts,  and  this,  as  we  shall  see,  constitutes  a  nervous  machinery  capable 
of  carrying  out  exceedingly  complicated  acts. 

ON  THE  PHENOMENA  EXHIBITED  BY  AN  ANIMAL  DEPRIVED  OF  ITS 
CEREBRAL  HEMISPHERES. 

§  550.  The  cerebral  hemispheres,  as  we  have  more  than  once  insisted, 
seem  to  stand  apart  from  the  rest  of  the  brain.  In  the  case  of  some  ani- 
mals it  is  possible  to  remove  the  cerebral  hemispheres  and  to  keep  the  ani- 
mal not  only  alive,  but  in  good  health  for  a  long  time — days,  weeks,  or 
even  months  after  the  operation.  In  such  case  we  are  able  to  study  the 
behavior  of  an  animal  possessing  no  cerebral  hemispheres,  and  to  compare 
it  with  that  of  an  intact  animal.  Such  an  experiment  is  best  carried  out 
on  a  frog.  In  this  animal  it  is  comparatively  easy  to  remove  the  cerebral 
hemispheres,  including  the  parts  corresponding  to  the  corpora  striata,  leav- 
ing behind  intact  and  uninjured  the  optic  thalami  with  the  optic  lobes  (or 
representatives  of  the  corpora  quadrigemina),  the  small  cerebellum,  and  the 
bulb.  If  the  animal  be  carefully  fed  and  attended  to,  it  may  be  kept  alive 
for  a  very  long  time — for  more  than  a  year,  for  instance. 

The  salient  fact  about  a  frog  lacking  the  cerebral  hemispheres  is  that,  as 
in  the  case  of  a  frog  deprived  of  its  whole  brain,  the  signs  of  the  working 
of  an  intelligent  volition  are  either  wholly  absent  or  extremely  rare.  The 
presence  of  the  bulb  and  the  middle  parts  of  the  brain  (for  so  we  may  con- 
veniently call  the  cerebral  structures  lying  between  the  cerebral  hemispheres 
and  the  bulb)  insures  the  healthy  action  of  the  vascular,  respiratory,  and 
other  nutritive  systems  ;  food  placed  in  the  mouth  is  readily  and  easily  swal- 
lowed ;  the  animal  when  stimulated  executes  various  movements  ;  but  if  it 
be  left  entirely  to  itself,  and  care  be  taken  to  shield  it  from  adventitious 
stimuli,  either  it  remains  perfectly  and  permanently  quiescent,  or  the  appar- 
ently spontaneous  movements  which  it  carries  out  are  so  few  and  so  limited 
as  to  make  it  very  doubtful  whether  they  can  fairly  be  called  volitional. 
Such  a  frog,  for  instance,  after  being  kept  alive  for  some  time  and  made  to 
exhibit  the  phenomena  of  which  we  are  about  to  speak,  has  been  placed  on 
a  table  with  a  line  drawn  in  chalk  around  the  area  covered  by  its  body,  and 
left  to  itself  has  subsequently  been  found  dead  without  having  stirred  out- 
side the  chalked  circle. 

We  must  here,  however,  repeat  the  caution  laid  down  in  §  495,  as  to  the 
ultimate  effects  of  an  operation  on  the  central  nervous  system.  The  longer 
the  frog  is  kept  alive  and  in  good  health  after  the  removal  of  the  cerebral 
hemispheres,  the  greater  is  the  tendency  for  apparently  spontaneous  move- 
ments to  show  themselves.  For  days,  or  even  weeks,  after  the  operation 
there  may  be  no  signs  whatever  of  the  working  of  any  volition  ;  but  after 
the  lapse  of  months,  movements,  previously  absent,  of  such  a  character  as 
to  suggest  that  they  ought  to  be  called  voluntary,  may  make  their  appear- 
ance. To  this  point  we  shall  return,  but  may,  in  the  meanwhile,  state  that 
even  in  their  most  complete  development  such  movements  do  not  negative 
the  view  that  the  frog  in  the  absence  of  the  cerebral  hemispheres  is  want- 
ing in  what  we  ordinarily  call  a  "  will." 

§  551.  We  have  seen  that  a  frog  from  which  the  whole  brain  has  been 
removed,  and  the  spinal  cord  only  left,  appears  similarly  devoid  of  a  "will;" 
but  the  phenomena  presented  by  a  frog  possessing  the  middle  portions  of  the 
brain  differ  widely  from  those  presented  by  a  frog  possessing  a  spinal  cord 
only.  We  may,  perhaps,  broadly  describe  the  behavior  of  a  frog  from  which 
the  cerebral  hemispheres  only  have  been  removed  by  saying  that  such  an 


638  THE  BRAIN. 

animal,  though  exhibiting  no  spontaneous  movements,  can  by  the  application 
of  appropriate  stimuli  be  induced  to  perform  all,  or  nearly  all,  the  move- 
ments which  an  entire  frog  is  capable  of  executing.  It  can  be  made  to  swim, 
to  leap,  and  to  crawl.  Left  to  itself,  it  assumes  what  may  be  called  the  natu- 
ral posture  of  a  frog,  with  the  fore  limbs  erect  and  the  hind  limbs  flexed,  so 
that  the  line  of  the  body  makes  an  angle  with  the  surface  on  which  it  is 
resting.  When  placed  on  its  back  it  immediately  regains  this  natural  pos- 
ture. When  placed  on  a  board  it  does  not  fall  from  the  board  when  the 
latter  is  tilted  up  so  as  to  displace  the  animal's  centre  of  gravity ;  it  crawls 
up  the  board  until  it  gains  a  new  position  in  which  its  centre  of  gravity  is 
restored  to  its  proper  place.  Its  movements  are  exactly  those  of  an  entire 
frog,  except  that  they  need  an  external  stimulus  to  call  them  forth.  They 
differ,  moreover,  fundamentally  from  those  of  an  entire  frog  in  the  following 
important  feature :  they  inevitably  follow  when  the  stimulus  is  applied  ;  they 
come  to  an  end  when  the  stimulus  ceases  to  act.  By  continually  varying  the 
inclination  of  a  board  on  which  it  is  placed,  the  frog  may  be  made  to  con- 
tinue crawling  almost  indefinitely ;  but  directly  the  board  is  made  to  assume 
such  a  position  that  the  body  of  the  frog  is  in  equilibrium,  the  crawling 
ceases  ;  and  if  the  position  be  not  disturbed  the  animal  will  remain  impassive 
and  quiet  for  an  almost  indefinite  time.  When  thrown  into  water  the 
creature  begins  at  once  to  swim  about  in  the  most  regular  manner,  and  will 
continue  to  swim  until  it  is  exhausted,  if  there  be  nothing  present  on  which 
it  can  come  to  rest.  If  a  small  piece  of  wood  be  placed  on  the  water  the 
frog  will,  when  it  comes  in  contact  with  the  wood,  crawl  upon  it  and  so 
come  to  rest.  If  disturbed  from  its  natural  posture,  as  by  being  placed  on 
its  back,  it  immediately  struggles  to  regain  that  posture ;  only  by  the  appli- 
cation of  continued  force  can  it  be  kept  lying  on  its  back.  Such  a  frog,  if 
its  flanks  be  gently  stroked,  will  croak ;  and  the  croaks  follow  so  regularly 
and  surely  upon  the  strokes  that  the  animal  may  almost  be  played  upon  like 
a  musical,  or,  at  least,  an  acoustic,  instrument.  Moreover,  provided  that  the 
optic  nerves  and  their  arrangements  have  not  been  injured  by  the  operation, 
the  movements  of  the  animal  appear  to  be  influenced  by  light ;  if  it  be  urged 
to  move  in  any  particular  direction,  it  seems  in  its  progress  to  avoid  obstacles, 
at  least  such  as  cast  a  strong  shadow ;  it  turns  its  course  to  the  right  or  left, 
or  sometimes  leaps  over  the  obstacle.  In  fact,  even  to  a  careful  observer,  the 
differences  between  such  a  frog  and  an  entire  frog  which  was  simply  very 
stupid  or  very  inert  would  appear  slight  and  unimportant,  except  in  this, 
that  the  animal  without  its  cerebral  hemispheres  is  obedient  to  every  stimulus, 
and  that  each  stimulus  evokes  an  appropriate  movement ;  whereas,  with  the 
entire  animal  it  is  impossible  to  predict  whether  any  result  at  all,  and,  if  so, 
what  result,  will  follow  the  application  of  this  or  that  stimulus.  Both  may 
be  regarded  as  machines  ;  but  the  one  is  a  machine  and  nothing  more;  the 
other  is  a  machine  governed  and  checked  by  a  dominant  volition. 

Now,  such  movements  as  crawling,  leaping,  swimming,  and,  indeed,  as  we 
have  already  urged,  to  a  greater  or  less  extent  all  bodily  movements  are 
carried  out  by  means  of  coordinate  nervous  motor  impulses,  influenced, 
arranged,  and  governed  by  coincident  sensory  or  afferent  impulses.  Muscu- 
lar movements  are  determined  by  afferent  influences  proceeding  from  the  mus- 
cles and  constituting  the  foundation  of  the  muscular  sense ;  they  are  also 
directed  by  means  of  afferent  impulses  passing  centripetally  along  the  sensory 
nerves  of  the  skin,  the  eye,  the  ear,  and  other  organs.  Independently  of  the 
particular  afferent  impulses,  which,  acting  as  a  stimulus,  call  forth  the  move- 
ment, very  many  other  afferent  impulses  are  concerned  in  the  generation 
and  coordination  of  the  resultant  motor  impulses.  Every  bodily  move- 
ment, such  as  those  of  which  we  are  speaking,  is  the  work  of  a  more  or  less 


WITHOUT  CEREBRAL  HEMISPHERES.  639 

complicated  nervous  mechanism,  in  which  there  are  not  only  central  and 
efferent  but  also  afferent  factors.  And,  putting  aside  the  question  of  con- 
sciousness, with  which  we  have  here  no  occasion  to  deal,  it  is  evident  that  in 
the  frog  deprived  of  its  cerebral  hemispheres  all  these  factors  are  present, 
the  afferent  no  less  than  the  central  and  the  efferent.  The  machinery  for  all 
the  necessary  and  usual  bodily  movements  is  present  in  all  its  completeness. 
We  may  regard  the  share,  therefore,  which  the  cerebral  hemispheres  take  in 
executing  the  movements  of  which  the  entire  animal  is  capable,  as  that  of 
putting  this  machinery  into  action  or  of  limiting  its  previous  activity.  The 
relation  which  the  higher  nervous  changes  concerned  in  volition  bear  to  this 
machinery  may  be  compared  to  that  of  a  stimulus,  always  bearing  in  mind 
that  the  effect  of  a  stimulus  on  a  nervous  centre  may  be  either  to  start 
activity  or  to  increase  or  to  curb  or  to  stop  activity  already  present.  We 
might  almost  speak  of  the  will  as  an  intrinsic  stimulus.  Its  operations  are 
limited  by  the  machinery  at  its  command.  We  may  infer  that  in  the  frog 
the  action  of  the  cerebral  hemispheres  in  giving  shape  to  a  bodily  movement 
is  that  of  throwing  into  activity  particular  parts  of  the  nervous  machinery 
situated  in  the  lower  parts  of  the  brain  and  in  the  spinal  cord ;  precisely  the 
same  movement  may  be  initiated  in  the  absence  of  the  cerebral  hemispheres 
by  applying  such  stimuli  as  shall  throw  precisely  the  same  parts  of  that 
machinery  into  the  same  activity. 

Very  marked  is  the  contrast  between  the  behavior  of  such  a  frog  which, 
though  deprived  of  its  cerebral  hemispheres,  still  retains  the  other  parts  of 
the  brain,  and  that  of  a  frog  which  possesses  a  spinal  cord  only.  The  latter 
when  placed  on  its  back  makes  no  attempt  to  regain  its  normal  posture ;  in 
fact,  it  may  be  said  to  have  completely  lost  its  normal  posture,  for  even  when 
placed  on  its  belly  it  does  not  stand  with  its  fore  feet  erect,  as  does  the  other 
animal,  but  lies  flat  on  the  ground.  When  thrown  into  water,  instead  of 
swimming  it  sinks  like  a  lump  of  lead.  When  pinched  or  otherwise  stimu- 
lated it  does  not  crawl  or  leap  forward ;  it  simply  throws  out  its  limbs  in 
various  ways.  When  its  flanks  are  stroked  it  does  not  croak ;  and  when  a 
board  on  which  it  is  placed  is  inclined  sufficiently  to  displace  its  centre  of 
gravity  it  makes  no  effort  to  regain  its  balance,  but  falls  off  the  board  like  a 
lifeless  mass.  Though,  as  we  have  seen,  the  various  parts  of  the  spinal  cord 
of  the  frog  contain  a  large  amount  of  coordinating  machinery,  so  that  the 
brainless  frog  may,  by  appropriate  stimuli,  be  made  to  execute  various  pur- 
poseful coordinate  movements,  yet  these  are  very  limited  compared  with 
those  which  can  be  similarly  carried  out  by  a  frog  possessing  the  middle  and 
lower  parts  of  the  brain  in  addition  to  the  spinal  cord.  It  is  evident  that  a 
great  deal  of  the  more  complex  machinery  of  this  kind,  especially  all  that 
which  has  to  deal  with  the  body  as  a  whole,  and  all  that  which  is  concerned 
with  equilibrium  and  is  specially  governed  by  the  higher  senses,  is  seated 
not  in  the  spinal  cord  but  in  the  brain.  We  do  not  wish  now  to  discuss  the 
details  of  this  machinery ;  all  we  desire  to  insist  upon  at  present  is  that  in 
the  frog  the  nervous  machinery  required  for  the  execution,  as  distinguished 
from  the  origination,  of  bodily  movements  even  of  the  most  complicated  kind, 
is  present  after  complete  removal  of  the  cerebral  hemispheres,  though  these 
movements  are  such  as  to  require  the  cooperation  of  highly  differentiated 
afferent  impulses. 

§  552.  In  warm-blooded  animals  the  removal  of  the  cerebral  hemi- 
spheres is  attended  with  much  greater  difficulties  than  in  the  case  of  the 
frog.  Nevertheless,  in  the  bird  the  operation  may  be  carried  out  with 
approximate  success.  Pigeons,  for  instance,  have  been  kept  alive  for  five 
or  six  weeks  after  complete  removal  of  the  cerebral  hemispheres,  with  the 
exception  of  portions  of  the  crura  and  corpora  striata  immediately  sur- 


640  THE  BEAIN. 

rounding  the  optic  thalami  ;  these  parts  were  left  in  order  to  ensure  the 
intact  condition  of  the  latter  bodies. 

When  the  immediate  effects  of  the  operation  have  passed  off,  and  for 
some  time  afterward,  the  appearance  and  behavior  of  the.  bird  are  strik- 
ingly similar  to  those  of  a  bird  exceedingly  sleepy  and  stupid.  It  is  able 
to  maintain  what  appears  to  be  a  completely  normal  posture,  and  can  bal- 
ance itself  on  one  leg,  after  the  fashion  of  a  bird  which  has  in  a  natural 
way  gone  to  sleep.  Left  alone  in  perfect  quiet,  it  will  remain  impassive  and 
motionless  for  a  long  time.  When  stirred  it  moves,  shifts  its  position  ;  and 
then,  on  being  left  alone,  returns  to  a  natural,  easy  posture.  Placed  on  its 
side  or  its  back  it  will  regain  its  feet ;  thrown  into  the  air,  it  Hies  with  con- 
siderable precision  for  some  distance  before  it  returns  to  rest.  It  frequently 
tucks  its  head  under  its  wings,  and  at  times  may  be  seen  to  clean  its  feath- 
ers ;  when  its  beak  is  plunged  into  corn,  it  eats.  It  may  be  induced  to 
move  not  only  by  ordinary  stimuli  applied  to  the  skin,  but  also  by  sudden 
loud  sounds,  or  by  flashes  of  light ;  in  its  flight  it  will,  though  imperfectly, 
avoid  obstacles,  and  its  various  movements  appear  to  be  to  a  certain  extent 
guided  not  only  by  touch  but  also  by  visual  impressions. 

In  a  certain  number  of  cases  this  sleepy,  drowsy  condition  passes  off  and 
is  succeeded  by  a  phase  in  which  the  bird,  apparently  spontaneously,  without 
the  intervention  of  any  obvious  stimulus,  moves  rapidly  about.  It  does 
not  fly,  that  is  to  say,  it  does  not  raise  itself  from  the  ground  in  flight,  but 
walks  about  incessantly  for  a  long  while  at  a  time,  the  periods  of  activity 
alternating  with  periods  of  repose.  It  seems,  from  time  to  time,  to  wake  up 
and  move  about,  and  then  to  go  to  sleep  again ;  and  it  has  been  observed 
that  during  the  night  it  appears  to  be  always  asleep.  It  is  obvious,  there- 
fore, that  the  sleepy,  quiescent  condition  is  not  due  simply  to  the  absence 
of  the  cerebral  hemispheres,  but  is  a  temporary  effect  of  the  operation,  and 
that  spontaneous  movements,  that  is  to  say,  movements  not  started  by  any 
obvious  stimulus,  may  occur  after  removal  of  the  cerebral  hemispheres. 
But  the  movements  so  witnessed  differ  from  those  of  an  intact  bird.  They 
are,  it  is  true,  varied  ;  and  the  variations  are  in  part  dependent  on  external 
circumstances,  the  bird  being  guided  by  tactile,  and  as  we  have  said,  visual 
sensations,  or,  to  be  more  exact,  by  impressions  made  upon  the  sensory 
nerves  of  the  skin  and  on  the  retina ;  but  they  do  not  show  the  wide  varia- 
tions of  voluntary  movements.  The  bird  never  flies  up  from  the  ground, 
never  spontaneously  picks  up  corn,  and  its  aimless,  monotonous,  restless 
walks,  resembling  the  continuous  swimming  of  the  frog  thrown  into  the 
water  after  being  deprived  of  its  cerebral  hemispheres,  forcibly  suggest  that 
the  activity  is  the  outcome  of  some  intrinsic  impulse  generated  in  the 
nervous  machinery  in  some  way  or  other,  but  not  by  the  working  of  a  con- 
scious intelligence  as  in  the  impulse  which  we  call  the  will. 

Still  we  must  not  shut  our  eyes  to  the  fact  that  spontaneous  movements, 
whatever  their  exact  nature,  are  manifested  by  a  bird  in  the  absence  of  the 
cerebral  hemispheres,  and  become  the  more  striking  the  more  complete  the 
recovery  from  the  passing  effects  of  the  mere  operation.  Could  such  birds 
be  kept  alive  for  any  considerable  time,  possibly  further  developments 
might  be  witnessed,  and  indeed  cases  are  on  record  where  birds  have  been 
kept  alive  for  months  after  the  operation,  and  have  shown  spontaneous 
movements  of  a  still  more  varied  character  than  those  just  described  ;  but 
in  such  cases  the  removal  of  the  hemispheres  has  not  been  complete,  por- 
tions of  the  ventral  regions  being  left  behind ;  and,  though  a  mere  remnant 
left  around  the  optic  thalami  can  hardly  be  regarded  as  a  sufficient  cause 
for  the  spontaneity  of  which  we  are  speaking,  a  larger  mass,  still  more  or 
less  retaining  its  normal  structure,  might  have  a  marked  effect.  And  we 


WITHOUT  CEREBRAL  HEMISPHERES.  641 

may  here  perhaps  remark  that  all  these  facts  seem  to  point  to  the  conclu- 
sion that  what  may  be  called  mechanical  spontaneity,  sometimes  spoken  of 
as  "  automatism,''  differs  from  the  spontaneity  of  the  "  will "  in  degree 
rather  than  in  kind.  Looking  at  the  matter  from  a  purely  physiological 
point  of  view  (the  only  one  which  has  a  right  to  be  employed  in  these 
pages),  the  real  difference  between  an  automatic  act  and  a  voluntary  act  is 
that  the  chain  of  physiological  events  between  the  act  and  its  physiological 
cause  is  in  the  one  case  short  and  simple,  in  the  other  long  and  complex. 
We*have  seen  that  a  frog  lacking  its  cerebral  hemispheres,  viewed  from  one 
standpoint,  appears  in  the  light  of  a  mechanical  apparatus,  on  which  each 
change  of  circumstances  produces  a  direct,  unvarying,  inevitable  effect. 
And  yet  it  is  on  record  that  such  a  frog,  if  kept  alive  long  enough  for  the 
most  complete  disappearance  of  the  direct  effects  of  the  operation,  will 
bury  itself  in  the  earth  at  the  approach  of  winter,  and  is  able  to  catch  and 
swallow  flies  and  other  food  coming  in  its  neighborhood,  although  in  other 
respects  it  shows  no  signs  of  an  intelligent  volition,  and  answers  with  un- 
erring mechanical  certainty  to  the  play  of  stimuli.  We  may  add  that  in 
some  fishes  the  removal  of  their  cerebral  hemispheres,  which  in  these  ani- 
mals form  a  relatively  small  part  of  the  whole  brain,  produces  exceedingly 
little  change  in  their  general  behavior. 

These,  however,  are  not  the  considerations  on  which  we  wish  here  to 
dwell ;  we  have  quoted  the  behavior  of  the  bird  deprived  of  its  cerebral 
hemispheres  mainly  to  show  that  in  this  warm-blooded  animal,  as  in  the 
more  lowly  cold-blooded  frog,  the  parts  of  the  brain  below  or  behind  the 
cerebral  hemispheres  constitute  a  nervous  machinery  by  which  all  the  ordi- 
nary bodily  movements  may  be  carried  out.  The  bird,  like  the  frog,  suffers 
no  paralysis  when  the  cerebral  hemispheres  are  removed ;  on  the  contrary, 
though  its  movements  have  not  been  studied  so  closely  as  those  of  the  frog, 
the  bird  without  its  cerebral  hemispheres  seems  capable  of  executing  at  all 
events  all  the  ordinary  bodily  movements  of  a  bird.  And  in  the  bird  as  in 
the  frog,  the  afferent  impulses  passing  into  the  central  nervous  system, 
whether  they  give  rise  to  consciousness  or  not,  play  an  important  part  not 
only  in  originating  but  in  guiding  and  coordinating  the  efferent  impulses 
which  stir  the  muscles  to  contract,  the  coordination  being  effected  partly  in 
the  spinal  cord,  but  largely  and  indeed  chiefly  in  the  parts  of  the  brain 
lying  behind  the  cerebral  hemispheres.  It  is  further  worthy  of  notice  that 
spontaneity  of  movement  of  the  kind  which  we  have  described  is  much 
more  prominent  in  the  more  highly  developed  bird  than  in  the  more  lowly 
frog.  The  cerebral  hemispheres  are  not  the  only  part  of  the  central  nerv- 
ous system  which  has  undergone  a  greater  development  in  the  bird ;  the 
other  parts  of  the  brain  have  also  acquired  a  far  greater  complexity  than 
in  the  frog. 

§  553.  In  the  mammal  the  removal  of  the  cerebral  hemispheres  is  still 
more  difficult  than  in  the  bird  ;  the  animal  cannot  be  kept  alive  for  more 
than  a  few  hours;  but  in  some  mammals  it  is  possible  to  observe  during 
those  few  hours  phenomena  kindred  to  those  witnessed  in  the  bird 
and  in  the  frog.  The  rabbit  or  rat,  from  which  the  whole  of  both  hemi- 
spheres has  been  removed  with  the  exception  of  the  parts  immediately 
surrounding  the  optic  thalami,  can  stand,  run,  and  leap.  Placed  on  its 
side  or  back  it  at  once  regains  its  feet.  Left  alone  it  generally  remains  as 
motionless  and  impassive  as  a  statue,  save  now  and  then  when  a  passing 
impulse  seems  to  stir  it  to  a  sudden  but  brief  movement ;  but  sometimes  it 
seems  subject  to  a  more  continued  impulse  to  move,  in  which  case  death 
usually  follows  very  speedily.  Such  a  rabbit  will  remain  for  minutes  together 
utterly  heedless  of  a  carrot  or  cabbage-leaf  placed  just  before  its  nose,  though 

41 


642  THE  BRAIN. 

if  a  morsel  be  placed  within  its  mouth  it  at  once  begins  to  eat.  When  stirred 
it  will  with  ease  and  steadiness  run  or  leap  forward  ;  and  obstacles  in  its 
course  are  very  frequently,  with  more  or  less  success,  avoided.  In  some  cases 
the  animal  (rat)  has  been  described  as  following  by  movements  of  the  head  a 
bright  light  held  in  front  of  it  (provided  that  the  optic  nerves  and  tracts 
have  not  been  injured  during  the  operation),  as  starting  when  a  shrill  and 
loud  noise  is  made  near  it,  and  as  crying  when  pinched,  often  with  a  long 
and  seemingly  plaintive  scream.  So  plaintive  is  the  cry  which  it  thus  gives 
forth  as  to  suggest  to  the  observer  the  existence  of  passion  ;  this,  however,  is 
probably  a  wrong  interpretation  of  a  vocal  action  ;  the  cry  appears  plaintive 
simply  because,  in  consequence  of  the  completeness  of  the  reflex  nervous 
machinery  and  the  absence  of  the  usual  restraints,  it  is  prolonged. 

Without  insisting  too  much  on  such  results  as  these,  and  allowing  full 
weight  to  the  objection  which  may  be  urged,  that  in  some  of  these  cases 
parts  of  the  cerebral  hemispheres  surrounding  the  optic  thalami  were  left, 
there  still  remains  adequate  evidence  to  show  that  a  mammal  such  as  a 
rabbit,  in  the  same  way  as  a  frog  and  a  bird,  may  in  the  complete  or  all  but 
complete  absence  of  the  cerebral  hemispheres  maintain  a  natural  posture, 
free  from  all  signs  of  disturbance  of  equilibrium,  and  is  able  to  carry  out 
with  success,  at  all  events  all  the  usual  and  common  bodily  movements. 
And  as  in  the  bird  and  frog,  the  evidence  also  shows  that  these  movements 
not  only  may  be  started  by,  but  in  their  carrying  out  are  guided  by  and 
coordinated  by  afferent  impulses  along  afferent  nerves,  including  those  of  the 
special  senses.  But  in  the  case  of  the  rabbit  it  is  even  still  clearer  than  in 
the  case  of  the  bird  that  the  effects  of  these  afferent  impulses  are  different 
from  those  which  result  when  the  impulses  gain  access  to  an  intact  brain. 
The  movements  of  the  animal  seem  guided  by  impressions  made  on  its  retina, 
as  well  as  on  other  sensory  nerves ;  we  may  perhaps  speak  of  the  animal  as 
the  subject  of  sensations ;  but  there  is  no  satisfactory  evidence  that  it  pos- 
sesses either  visual  or  other  perceptions,  or  that  the  sensations  which  it  ex- 
periences give  rise  to  ideas.  Its  avoidance  of  objects  depends  not  so  much 
on  the  form  of  these  as  on  their  interference  with  light.  No  image,  whether 
pleasant  or  terrible,  whether  of  food  or  of  an  enemy,  produces  an  effect  on 
it,  other  than  that  of  an  object  reflecting  more  or  less  light.  And  we  may 
infer  that  it  lacks  the  possession  of  an  intelligent  will.  But  it  must  always 
be  remembered  that  some  of  the  phenomena  are  due  to  the  operation  pro- 
ducing other  results  than  the  mere  absence  of  the  part  removed.  We  roust 
bear  in  mind  that  in  all  the  above  experiments,  while  the  positive  phenomena, 
the  things  which  the  animal  continues  able  to  do,  are  of  great  value,  the 
negative  phenomena,  the  things  which  the  animal  can  no  longer  do,  are  of 
much  less,  indeed  of  doubtful  value.  The  more  carefully  and  successfully 
the  experiments  are  carried  out,  the  narrower  become  what  we  may  call  the 
"  deficiency  phenomena,"  the  phenomena  which  are  alone  and  directly  due 
to  something  having  been  taken  away.  Were  it  possible  to  keep  the  rabbit 
alive  long  enough  for  the  mere  effects  of  the  operation  to  pass  completely 
away,  we  should  not  only  probably  witness,  as  in  the  case  of  the  bird,  a 
greater  scope  of  movement  and  more  frequent  spontaneity,  but  possibly  find 
a  difficulty  in  describing  the  exact  condition  of  the  animal. 

§  554.  Hitherto  attempts  to  witness  similar  phenomena  in  more  highly 
organized-  mammals,  such  as  the  dog,  have  failed  ;  these  animals  do  not 
recover  from  the  operation  of  removing  the  whole  of  both  their  hemispheres 
sufficiently  to  enable  us  to  judge  whether  they,  like  the  frog,  the  bird,  and 
the  rabbit,  can  carry  out  coordinate  bodily  movements  in  the  absence  of  the 
hemispheres,  or  whether  in  them  this  part  of  the  brain,  so  largely  developed, 
has  usurped  functions  which  in  the  lower  animals  belong  to  other  parts. 


THE  MACHINERY  OF  COORDINATED  MOVEMENTS.  643 

Our  knowledge  is  largely  confined  to  the  experience  that  when  in  a  dog  the 
cerebral  convolutions  are  removed  piecemeal  at  several  operations,  the 
animal  may  be  kept  alive  and  in  good  health  for  a  long  time,  many  months 
at  least,  even  after  these  parts  of  the  brain  have  been  reduced  to  very  small 
dimensions,  and  that  under  these  circumstances  the  animal  is  not  only  able 
to  carry  out  with  some  limitations  his  ordinary  bodily  movements,  but  also 
exhibits  a  spontaneity  obviously  betokening  the  possession  not  merely  of  a 
conscious  volition  but  of  a  certain  amount  of  intelligence.  Unless  we  are 
willing  to  believe  that  a  mere  fragment,  so  to  speak,  of  the  hemispheres  can 
take  on  most  extended  powers,  such  an  experience  seems  to  show  that  in  the 
dog  as  in  the  rabbit  and  in  the  bird,  the  development  of  so-called  higher 
functions  is  not  limited  to  the  cerebral  hemispheres,  that  the  middle  and 
lower  portions  of  the  brain  in  the  higher  animals  as  compared  with  the 
lower  do  not  increase  in  bulk  merely  as  the  instruments  of  the  hemispheres, 
but  like  the  hemispheres  acquire  more  and  more  complex  functions.  We 
may  perhaps  go  so  far  as  to  ask  the  question  whether  the  volition  and  intel- 
ligence which  such  a  dog  exhibits  is  not  as  much  the  product  of  the  parts 
lying  behind  the  hemispheres  as  of  the  stump  left  in  the  front. 

If  we  can  thus  say  little  about  the  condition  of  a  dog  without  the  cerebral 
hemispheres,  we  can  say  still  less  about  the  monkey,  which  in  all  matters 
touching  the  cerebral  nervous  system  serves  as  our  best,  indeed  our  only 
guide  for  drawing  inferences  concerning  man  ;  but  in  all  probability  the 
monkey  in  this  respect  bears  somewhat  the  same  relation  to  the  dog  that  the 
dog  bears  to  the  bird. 

In  short,  the  more  we  study  the  phenomena  exhibited  by  animals  pos- 
sessing a  part  only  of  their  brain,  the  closer  we  are  pushed  to  the  conclusion 
that  no  sharp  line  can  be  drawn  between  volition  and  the  lack  of  volition, 
or  between  the  possession  and  absence  of  intelligence.  Between  the  muscle- 
nerve  preparation  at  the  one  limit  and  our  conscious  willing  selves  at  the 
other  there  is  a  continuous  gradation  without  a  break ;  we  cannot  fix  on 
any  linear  barrier  in  the  brain  or  in  the  general  nervous  system,  and  say, 
"  Beyond  this  there  is  volition  and  intelligence,  but  up  to  this  there  is  none." 

This,  however,  is  not  the  question  with  which  we  are  now  dealing.  What 
we  want  to  point  out  is  that  in  the  higher  animals,  including  at  least  some 
mammals,  as  in  the  frog,  after  the  removal  of  the  cerebral  hemispheres,  even 
though  conscious  volition  and  intelligence  appear  to  be  largely,  if  not  en- 
tirely lost,  the  body  is  still  capable  of  executing  all  the  ordinary  movements 
which  the  animal  in  its  natural  life  is  wont  to  perform,  in  spite  of  these 
movements  necessitating  the  cooperation  of  various  afferent  impulses ;  and 
that  therefore  the  nervous  machinery  for  the  execution  of  these  movements 
lies  in  some  part  of  the  brain  other  than  the  cerebral  hemispheres.  We 
have  reason  for  thinking  that  it  is  situated  in  the  structures  forming  the 
middle-  and  hind-brain  ;  as  we  shall  see,  interference  with  these  parts  pro- 
duces at  once  remarkable  disorders  of  movement. 

THE  MACHINERY  OF  COORDINATED  MOVEMENTS. 

§  555.  We  may  now  direct  our  attention  for  a  while  to  some  considerations 
concerning  the  nature  of  this  complex  nervous  machinery  for  the  coordina- 
tion of  bodily  movements,  and  especially  concerning  the  part  played  by 
afferent  impulses.  Most  of  our  knowledge  on  this  point  has  been  gained 
by  a  study  of  animals  not  deprived  of,  but  still  possessing,  their  cerebral 
hemispheres,  or  by  deductions  from  the  data  of  our  own  experience ;  but  it 
is  possible  in  most  cases  to  eliminate  from  the  total  results  the  phenomena 
which  are  due  to  the  working  of  a  conscious  intelligence.  Some  of  the  most 


644  THE  BRAIN. 

striking  facts  bearing  on  this  matter  have  been  gained  by  studying  the  effects 
of  operative  interference  with  certain  parts  of  the  internal  ear,  known  as  the 
semicircular  canals. 

When  in  a  pigeon  the  horizontal  membranous  semicircular  canal  is  cut 
through,  the  bird  is  observed  to  be  continually  moving  its  head  from  side  to 
side.  If  one  of  the  vertical  canals  be  cut  through,  the  movements  are  up 
and  down.  The  peculiar  movements  may  not  be  witnessed  when  the  bird  is 
perfectly  quiet,  but  they  make  their  appearance  whenever  it  is  disturbed  or 
attempts  in  any  way  to  stir.  When  the  injury  is  confined  to  one  canal  only, 
or  even  to  the  canals  of  one  side  of  the  head  only,  the  condition  after  a 
while  passes  away;  when  the  canals  of  both  sides  have  been  divided,  it 
becomes  much  exaggerated,  lasts  much  longer,  and  in  some  cases  is  said  to 
remain  permanently.  After  such  injuries  it  is  found  that  these  peculiar 
movements  of  the  head  are  associated  with  what  appears  to  be  a  great  want 
of  coordination  of  bodily  movements.  If  the  bird  be  thrown  into  the  air, 
it  flutters  and  falls  down  in  a  helpless  and  confused  manner ;  it  appears  to 
have  lost  the  power  of  orderly  flight.  If  placed  in  a  balanced  position,  it 
may  remain  for  some  time  quiet,  generally  with  its  head  in  a  peculiar  pos- 
ture :  but  directly  it  is  disturbed,  the  movements  which  it  attempts  to  execute 
are  irregular  and  fall  short  of  their  purpose.  It  has  great  difficulty  in  pick- 
ing up  food  and  in  drinking;  and  in  general  its  behavior  very  much  resem- 
bles that  of  a  person  who  is  exceedingly  dizzy. 

It  can  hear  perfectly  well,  and  therefore  the  symptoms  cannot  be  regarded 
as  the  result  of  any  abnormal  auditory  sensations,  such  as  a  "  roaring  "  in  the 
ears.  Besides  any  such  stimulation  of  the  auditory  nerve  as  the  result  of  the 
section  would  speedily  die  away,  whereas  these  phenomena  may  last  for  a 
very  considerable  time. 

The  movements  are  not  occasioned  by  any  partial  paralysis,  by  any  want 
of  power  in  particular  muscles  or  group  of  muscles  ;  though  removal  of 
the  canals  of  one  side  has  been  described  as  leading  to  diminished  muscular 
force  on  the  same  side  of  the  body,  the  mere  diminution  of  force  is  insuffi- 
cient to  explain  the  phenomena.  Nor,  on  the  other  hand,  are  the  movements 
due  to  any  uncontrollable  impulse ;  a  very  gentle  pressure  of  the  hand  suf- 
fices to  stop  the  movements  of  the  head,  and  the  hand  in  doing  so  experi- 
ences no  strain.  The  assistance  of  a  very  slight  support  enables  movements 
otherwise  impossible  or  most  difficult  to  be  easily  executed.  Thus,  though 
when  left  alone  the  bird  has  great  difficulty  in  drinking  or  picking  up  corn, 
it  will  continue  to  eat  with  ease  if  its  beak  be  plunged  into  water  or  into  a 
heap  of  barley ;  the  slight  support  of  the  water  or  the  grain  seems  sufficient 
to  steady  its  movements.  In  the  same  way  it  can,  even  without  assistance, 
clean  its  feathers  and  scratch  its  head,  its  beak  and  foot  being  in  these  ope- 
ations  guided  by  contact  with  its  own  body. 

The  amount  of  disorder  thus  induced  differs  in  different  birds,  and  some 
movements  are  more  affected  than  others.  As  a  general  rule,  it  may  be  said 
that  the  more  complex  and  intricate  a  movement,  the  fuller  and  more  del- 
icate the  coordination  needed  to  carry  it  out  successfully,  the  more  markedly 
is  it  disordered  by  the  operation  ;  thus,  after  injury  of  the  canals,  while  a 
pigeon  cannot  fly,  a  goose  is  still  able  to  swim. 

In  mammals  (rabbits)  section  of  the  canals  also  produces  a  certain 
amount  of  loss  of  coordination,  but  much  less  than  that  witnessed  in  birds ; 
and  the  movements  of  the  head  are  not  so  marked,  peculiar  oscillating 
movements  of  the  eyeballs,  differing  in  direction  and  character  according 
to  the  canal  or  canals  operated  upon,  becoming,  however,  prominent.  In 
the  frog  no  deviations  of  the  head  are  seen,  but  there  is  some  loss  of  coordi- 
nation in  the  movements  of  the  body.  In  fishes  no  effect  at  all  is  produced. 


THE  MACHINERY  OF  COORDINATED  MOVEMENTS.  645 

Injury  to  the  bony  canals  alone  is  insufficient  to  produce  the  symptoms ; 
the  membranous  canals  themselves  must  be  divided  or  injured.  The  cha- 
racteristic movements  of  the  head  may,  however,  be  brought  about  in  a  bird 
without  opening  the  bony  canal,  by  suddenly  heating  or  cooling  a  canal, 
especially  its  ampullar  terminations,  or  by  the  making  or  breaking  of  a  con- 
stant current  directed  through  the  canal. 

There  can  be  no  doubt  that  these  characteristic  movements  of  the  head 
are  the  result  of  afferent  impulses  started  in  the  nervous  endings  of  the 
auditory  nerve  over  the  ampulla  of  the  canal  and  conveyed  to  the  brain 
along  that  nerve.  And  that  injury  to  or  other  stimulation  of  each  of  the 
three  canals  should  produce  in  each  case  a  different  movement  of  the  head, 
the  direction  of  the  movement  being  different  according  to  the  plane  in 
which  the  canal  lies,  shows  that  these  impulses  are  of  a  peculiar  nature. 
This  is  further  illustrated  by  the  following  experiment :  If  the  horizontal 
canal  be  carefully  laid  bare,  and  the  membranous  canal  opened  so  as  to  ex- 
pose the  endolymph,  blowing  gently  over  the  opened  canal  with  a  fine  glass 
canula  will  produce  a  definite  movement  of  the  head,  which  is  turned  to  the 
one  side  or  to  the  other,  according  as  the  current  of  air  drives  the  endolymph 
toward  or  away  from  the  ampulla.  From  this  it  is  inferred  that  a  movement 
of  the  endolymph  over,  or  an  increased  pressure  of  the  endolymph  on,  the 
nervous  endings  in  the  ampulla  gives  rise  to  afferent  impulses  which  in 
some  way  determine  the  issue  of  efferent  impulses  leading  to  the  movement 
of  the  head.  It  is  further  suggested  that  since  the  planes  of  the  three  canals 
lie  in  the  three  axes  of  space,  any  change  in  the  position  of  the  head  must 
lead  to  changes  in  the  pressure  of  the  endolymph  on  the  walls  of  the  ampul- 
lae or  to  movements  of  endolymph  over  those  walls,  and  so  must  give  rise 
to  impulses  passing  up  the  auditory  nerve ;  and  that  since  every  change  of 
position  will  affect  the  three  canals  differently  (whereas,  the  changes  of  pres- 
sure of  the  endolymph  involved  in  a  "  wave  of  sound  "  will  affect  all  three 
ampullae  equally),  those  impulses  will  differ  according  to  the  direction  of 
the  change.  A  still  further  extension  of  this  view  supposes  that  since  in 
any  one  position  of  the  head  the  pressure  of  the  endolymph  will  differ  in 
the  three  ampullae,  mere  position  of  the  head,  as  distinguished  from  change 
of  position,  is  adequate  to  generate  afferent  impulses  differing  in  the  differ- 
ent positions. 

Let  us  now  for  a  while  turn  aside  to  ourselves  and  examine  the  coordina- 
tion of  the  movements  of  our  own  bodies.  When  we  appeal  to  our  own  con- 
sciousness we  find  that  our  movements  are  governed  and  guided  by  what 
we  may  call  a  sense  of  equilibrium,  by  an  appreciation  of  the  position  of  our 
body  and  its  relations  to  space.  When  this  sense  of  equilibrium  is  dis- 
turbed we  say  we  are  dizzy,  and  we  then  stagger  and  reel,  being  no  longer 
able  to  coordinate  the  movements  of  our  bodies  or  to  adapt  them  to  the 
position  of  things  around  us.  What  is  the  origin  of  this  sense  of  equilib- 
rium ?  By  what  means  are  we  able  to  appreciate  the  position  of  our 
body  ?  There  can  be  no  doubt  that  this  appreciation  is  in  large  measure 
the  product  of  visual  and  tactile  sensations  ;  we  recognize  the  relations  of 
our  body  to  the  things  around  us  in  great  measure  by  sight  and  touch  ;  we 
also  learn  much  by  our  muscular  sense.  But  there  is  something  besides 
these.  Neither  sight  nor  touch  nor  muscular  sense  can  help  us  when,  placed 
perfectly  flat  and  at  rest  on  a  horizontal  rotating  table,  with  the  eyes  shut 
and  not  a  muscle  stirring,  we  attempt  to  determine  whether  or  not  the  table 
and  we  with  it  are  being  moved,  or  to  ascertain  how  much  it  and  we  are 
turned  to  the  right  or  to  the  left.  Yet  under  such  circumstances  we  are 
conscious  of  a  change  in  our  position,  and  some  observers  have  been  even 
able  to  pass  a  tolerably  successful  judgment  as  to  the  angle  through  which 


646  THE  BRAIN. 

they  have  been  moved.  There  can  be  no  doubt  that  such  a  judgment  is 
based  upon  the  interpretation  by  consciousness  of  afferent  impulses  which 
are  dependent  on  the  position  of  the  body,  but  which  are  not  afferent  im- 
pulses belonging  to  sensations  of  touch  or  sight,  or  taking  part  in  the  mus- 
cular sense.  And  it  is  urged  with  great  plausibility  that  the  afferent  im- 
pulses in  question  are  those  which  we  have  just  referred  to  as  started  in  the 
semicircular  canals. 

If  we  admit  the  existence  of  such  ampullar  impulses,  if  we  may  venture 
so  to  call  them,  and  recognize  them  as  contributing  largely  not  only  to  our 
direct  perception  of  the  position  of  the  head  and  thus  of  the  body,  but  also 
in  a  more  indirect  way  to  what  we  have  called  the  sense  of  equilibrium,  we 
should  expect  to  find  that  when  they  are  abnormal  the  sense  of  equilibrium 
is  disturbed,  and  that  in  consequence  a  failure  of  coordination  in  our  move- 
ments results.  And  the  loss  of  coordination  which  we  described  above  as 
resulting  from  injury  to  the  semicircular  canals  has  accordingly  been  attrib- 
uted to  a  deficiency  or  disorder  of  normal  ampullar  impulses. 

But  we  must  here  distinguish  between  two  things.  It  seems  clear  that 
when  the  membranous  canals  are  injured  or  otherwise  stimulated,  afferent 
impulses  are  generated  which,  on  the  one  hand,  may  produce  peculiar  move- 
ments of  the  head,  and,  on  the  other  hand,  seem  able  when  the  injury  is  large 
to  cause  a  loss  of  coordination  of  bodily  movements.  But  it  does  not  neces- 
sarily follow  from  this  that  in  a  normal  condition  of  things  afferent  impulses 
are  continually  passing  up  to  the  brain  from  the  semicircular  canals,  and 
that  the  loss  of  coordination  which  follows  upon  injury  to  the  canals  is  due 
to  these  normal  impulses  being  deficient  or  altered.  It  may  be  that  such 
normal  impulses  do  not  exist,  and  that  the  loss  of  coordination  is  the  result 
of  the  central  machinery  for  coordination  being  interfered  with  by  quite  new 
impulses  generated  by  the  injury  to  the  canal  with  the  consequent  loss  of 
eudolymph  acting  as  a  stimulus  to  the  endings  of  the  nerve.  For  the  expe- 
rience quoted  above,  though  it  proves  that  afferent  impulses  other  than  those 
of  sight,  touch,  and  the  muscular  sense  do  reach  the  brain  and  afford  a  basis 
for  a  judgment  as  to  the  position  of  the  body,  does  not  by  itself  prove  that 
those  impulses  come  from  the  semicircular  canals ;  the  arrangement  of  the 
canals  is  undoubtedly  suggestive  ;  but  it  is  quite  possible  that  the  afferent 
impulses  in  question  may  be  generated  by  one  or  other  of  various  changes, 
vasomotor  and  others,  of  the  tissues  of  the  body  which  are  involved  in  a 
change  of  position.  And  if  it  be  true,  as  affirmed  by  some  observers,  that 
both  auditory  nerves  may  be  completely  and  permanently  severed  without 
any  effect  on  the  coordination  of  movements,  it  is  obvious  that  the  incoordi- 
nation  which  follows  upon  section  of  the  semicircular  canals  is  due  to  some 
special  irritation  setup  by  the  operation,  and  not  to  the  mere  absence  of  any 
normal  ampullar  impulses.  On  the  other  hand,  if  the  effects  are  those  of 
irritation,  it  is  difficult  to  understand  how  they  can,  as  according  to  certain 
observers  they  certainly  do,  become  permanent.  It  has,  however,  been 
strongly  urged  that  in  such  cases  of  permanent  incoordination,  the  operation 
has  set  up  secondary  mischief  in  the  brain,  in  the  cerebellum  for  instance, 
with  which,  as  we  have  seen  (§  531),  the  vestibular  auditory  nerve  makes 
special  connections,  and.  that  the  permanent  effects  are  really  due  to  the  dis- 
ease going  on  here;  and  we  have  reason,  as  we  shall  see,  to  think  that  the 
cerebellum  is  concerned  in  the  coordination  of  movements.  It  cannot, 
therefore,  be  regarded  as  settled  that  the  canals  are  the  source  of  normal 
impulses,  or  that  our  conscious  appreciation  of  the  position  of  the  head  and 
so  of  the  body  in  space  is  based  on  such  impulses.  But  such  a  view  is  not 
disproved  ;  and  in  any  case  it  remains  true  that  injury  to  the  canals  does  in 
some  way  or  other,  either  by  generating  new  impulses  or  by  altering  pre- 


THE  MACHINERY  OF  COORDINATED  MOVEMENTS.  647 

existing  ones,  so  modify  the  flow  of  afferent  impulses  into  the  machinery  of 
coordination  as  to  throw  that  machinery  out  of  gear. 

§  556.  We  have  dwelt  on  these  phenomena  of  the  semicircular  canals 
because  they  illustrate  in  a  striking  manner  the  important  part  played  by 
afferent  impulses  in  the  coordination  of  movements.  We  saw  reason  to 
think  (§  502)  that  even  in  an  ordinary  reflex  movement  carried  out  by  the 
spinal  cord  or  by  a  portion  of  the  cord,  afferent  impulses,  other  than  those 
which  excite  the  movement  are  at  work,  determining  such  coordination  as 
is  present.  In  such  a  case  the  coordinating  afferent  impulses  are  relatively 
simple  in  character  and  start  chiefly  at  all  events  in  the  muscles  concerned. 
In  an  animal  possessing  the  lower  parts  of  the  brain,  though  deprived  of  the 
cerebral  hemispheres,  the  coordinating  afferent  impulses,  in  accordance  with 
the  greater  diversity  and  complexity  of  the  movements  which  the  animal  is 
able  to  execute,  are  far  more  potent  and  varied.  Besides  afferent  impulses 
from  the  muscles,  forming  the  basis  of  what  we  have  called  the  muscular 
sense,  afferent  impulses  from  the  skin,  forming  the  basis  of  the  sense  of  touch 
in  the  wide  meaning  of  that  word,  other  afferent  impulses  of  obscure  cha- 
racter from  the  viscera  and  various  tissues,  and  the  peculiar  afferent  ampullar 
impulses  of  which  we  have  just  spoken,  important  special  afferent  impulses 
borne  along  the  nerves  of  sight  and  hearing  come  into  play.  The  frog,  the 
bird,  and  even  the  mammal,  deprived  of  the  cerebral  hemispheres,  though  it 
may  show  little  signs  or  none  at  all  of  having  a  distinct  volition,  is,  as  we 
have  urged,  indubitably  affected  by  visual  and  auditory  impressions,  and 
whether  we  admit  or  not  that  such  an  animal  can  rightly  be  spoken  of  as 
being  conscious,  we  cannot  resist  the  conclusion  that  afferent  impulses  started 
in  its  retina  or  internal  ear  produce  in  its  central  nervous  system  changes 
similar  to  those  which  in  a  conscious  animal  form  the  basis  of  visual  and 
auditory  sensations,  and  we  must  either  call  these  changes  sensations  or  find 
for  them  some  new  word.  Whatever  we  call  them,  and  whether  conscious- 
ness is  distinctly  involved  in  them  or  not,  they  obviously  play  an  important 
part  as  factors  of  the  coordination  of  movements.  Indeed,  when  we  appeal 
to  the  experience  of  ourselves  in  possession  of  consciousness,  we  find  that 
though  various  sensations  clearly  enter  into  the  coordination  of  our  move- 
ments, we  carry  out  movements  thus  coordinated  without  being  distinctly 
aware  of  these  coordinating  factors.  In  every  movement  which  we  make 
the  coordination  of  the  movement  is  dependent  on  the  impulses  or  influences 
which  form  the  basis  of  the  muscular  sense,  yet  we  are  not  distinctly  con- 
scious of  these  impulses  ;  it  is  only,  as  we  shall  see,  by  special  analysis  that 
we  come  to  the  conclusion  that  we  do  possess  what  we  shall  call  a  muscular 
sense.  So  again,  taking  the  matter  from  a  somewhat  different  point  of  view, 
many  of  our  movements,  markedly,  as  we  shall  see,  those  of  the  eyeballs,  are 
coordinated  by  visual  sensations,  and  when  we  sing  or  when  we  dance  to 
music  our  movements  are  coordinated  by  the  help  of  sensations  of  sound.  In 
these  cases  distinct  sensations  in  the  ordinary  sense  of  the  word  intervene; 
if  we  cannot  see  or  cannot  hear,  the  movement  fails  or  is  imperfect ;  yet 
even  in  these  cases  we  are  not  directly  conscious  of  the  sensations  as  coordi- 
nating factors;  it  needs  careful  analysis  to  prove  that  the  success  of  the 
movement  is  really  dependent  on  the  sound  or  on  the  sight.  These  and 
other  facts  suggest  the  view  that  the  point  at  which  the  various  afferent  im- 
pulses which  form  the  basis  of  the  sensations  of  a  conscious  individual  enter 
into  the  coordination  mechanism  is  or  may  be  some  way  short  of  the  stage 
at  which  the  complete  conversion  of  the  impulse  into  a  perfect  sensation 
takes  place.  The  events  which  constitute  what  we  may  call  visual  impulses, 
as  these  leave  the  retina  to  sweep  along  the  optic  nerve  are,  we  must  admit, 
very  different  from  those  which  in  the  appropriate  parts  of  the  brain  con- 


G48  THE  BRAIN. 

stitute  what  we  may  call  conscious  vision  ;  and  probably  between  the  begin- 
ning and  the  end  there  are  progressive  changes.  It  is  probable,  we  say,  that 
these  visual  events  may  affect  the  coordinating  mechanism  at  some  stage  of 
their  progress  before  they  reach  their  final  and  perfect  form.  If  this  be  so 
we  may  further  conclude  that  though,  when  the  whole  nervous  machinery  is 
present  in  its  entirety,  the  afferent  impulses  which  take  part  in  coordination 
must  inevitably  at  the  same  time  give  rise  to  conscious  sensations,  they 
might  still  effect  their  coordinating  work  when,  owing  to  their  imperfec- 
tion or  lack  of  the  terminal  part  of  the  nervous  machinery,  the  impulses 
failed  to  receive  their  final  transformation,  and  conscious  sensations  were 
absent.  In  other  \vords,  the  coordinating  influences  of  sensory  or  afferent 
impulses  are  not  essentially  dependent  on  the  existence  of  a  distinct  con- 
sciousness. 

§  557.  We  have  raised  this  point  partly  for  the  sake  of  illustrating  the 
working  of  the  coordination  machinery  in  the  absence  of  the  cerebral  hemi- 
spheres, but  also  in  order  to  aid  in  the  interpretation  of  the  subjective  condi- 
tion which  we  speak  of  as  giddiness  or  dizziness  or  vertigo.  The  condition 
of  the  pigeon  after  an  injury  to  the  semicircular  canals  is  comparable  to  that 
of  a  person  who  is  giddy  or  dizzy,  and,  indeed,  vertigo  is  the  subjective 
expression  of  a  disarrangement  of  the  coordination  machinery,  especially 
of  that  concerned  in  the  maintenance  of  bodily  equilibrium.  It  may  be 
brought  about  in  many  ways.  When  a  constant  current  of  adequate  strength 
is  sent  through  the  head  from  ear  to  ear,  we  experienced  a  sense  of  vertigo  ; 
our  movements  then  appear  to  a  bystander  to  fail  in  coordination,  in  fact  to 
resemble  those  of  a  pigeon  whose  semicircular  canals  have  been  injured ; 
and,  indeed,  the  effects  are  probably  produced  in  the  same  way  in  the  two 
cases.  In  what  is  called  Meniere's  disease  attacks  of  vertigo  seem  to  be  asso- 
ciated with  disease  in  the  ear,  being  attributed  by  many  to  disorder  of  the 
semicircular  canals,  and  cases  have  been  recorded  of  giddiness  as  well  as 
deafness  resulting  from  disease  of  the  auditory  nerve.  Visual  sensations  are 
very  potent  in  producing  vertigo.  Many  persons  feel  giddy  when  they  look 
at  a  waterfall ;  and  this  is  a  case  in  which  both  the  sense  of  giddiness  and 
the  disarrangement  of  coordination  is  the  result  of  the  action  of  a  pure  sen- 
sation and  nothing  else.  In  the  well-known  intense  vertigo  which  is  caused 
by  rapid  rotation  of  the  body  visual  sensation  plays  a  part  when  the  rotation 
is  carried  on  with  the  eyes  open,  but  only  a  part ;  for  vertigo  may  be  induced, 
though  not  so  readily,  by  rotation  with  the  eyes  completely  shut.  In  the 
latter  case  it  has  been  suggested  that  the  vertigo  is  caused  by  abnormal  am- 
pullar  impulses,  but  these  can  only  contribute  to  the  result,  which  is  in  the 
main  caused  by  direct  disturbance  of  the  brain.  When  the  rotation  is  car- 
ried out  with  the  eyes  open,  the  vertigo  which  is  felt  when  the  rotation  ceases 
is  partly  caused  by  the  visual  sensations,  on  account  of  the  behavior  of  the 
eyeballs,  ceasing  to  be  in  harmony  with  the  rest  of  the  sensations  and  affer- 
ent impulses  which  help  to  make  up  the  coordination.  The  rotation  sets  up 
peculiar  oscillating  movements  of  the  eyeballs,  which  continue  for  some  time 
after  the  rotation  has  ceased  ;  owing  to  these  movements  of  the  eyeballs  the 
visual  sensations  excited  are  such  as  would  be  excited  if  external  objects 
were  rapidly  moving,  whereas  all  the  other  sensations  and  impulses  which 
are  affecting  the  central  nervous  system  are  such  as  are  excited  by  objects  at 
rest.  In  a  normal  state  of  things  the  visual  and  the  other  sensations  and 
impulses  which  go  to  make  up  the  coordinating  machinery  are  in  accord 
with  each  other  in  reference  to  the  events  in  the  external  world  which  are 
giving  rise  to  them  ;  after  rotation  they  are  for  a  time  in  disaccord,  and  the 
coordinating  machinery  is  in  consequence  disarranged. 

When  we  interrogate  our  own  consciousness,  we  find  that  we  are  not  dis- 


THE  MACHINERY  OF  COORDINATED  MOVEMENTS.  649 

tinctly  conscious  of  this  disaccord  ;  the  visual  sensations  are  so  prepotent  in 
consciousness  that  we  really  think  the  external  world  is  rapidly  whirling 
round  ;  all  that  we  are  further  conscious  of  is  the  feeling  of  giddiness  and  our 
inability  to  make  our  bodily  movements  harmonize  with  our  visual  sensations. 
So  that  even  in  the  cases  where  the  loss  of  coordination  is  brought  about  by 
distinct  sensations,  what  we  really  appreciate  by  means  of  our  consciousness 
is  the  disarrangement  of  the  coordinating  machinery.  It  is  the  apprecia- 
tion of  this  disorder  which  constitutes  the  feeling  of  vertigo ;  both  the  feel- 
ing of  giddiness  and  the  disordered  movements  are  the  outcome,  one  sub- 
jective and  the  other  objective,  of  the  same  thing.  It  is  not  because  we 
feel  giddy  that  we  stagger  and  reel ;  our  movements  are  wrong  because  the 
machinery  is  at  fault,  and  it  is  the  faulty  action  of  the  machinery  which  also 
makes  us  feel  giddy. 

We  may  here  perhaps  remark  that  it  is  an  actually  disordered  condition 
of  the  coordinating  mechanism  which  gives  rise  to  the  affection  of  conscious- 
ness which  we  call  giddiness,  not  a  mere  curtailing  of  the  mechanism  or  any 
failure  on  its  part  to  make  itself  effective.  Complete  blindness  limits  the 
range  of  activity  of  the  machinery  but  leaves  the  remainder  intact,  and  no 
giddiness  is  felt.  So  again  in  certain  diseases  of  the  nervous  system  the 
muscular  sense  is  interfered  with  over  considerable  regions  of  the  body,  and 
in  these  regions  coordination  fails  or  is  imperfect,  but  the  central  machinery 
is  not  thereby  affected,  though  its  area  of  usefulness  is  limited,  and  no  giddi- 
ness is  experienced  ;  and  so  in  other  instances. 

§  558.  Forced  movements.  So  far  we  have  dwelt  on  disorders  of  the  co- 
ordinating machinery  brought  about  by  the  action  of  various  afferent  im- 
pulses. We  have  now  to  call  attention  to  some  peculiar  phenomena  which 
result  from  operative  interference  with  parts  of  the  brain,  and  which  in  some 
instances  at  least  may  be  taken  to  illustrate  how  this  complex  machinery 
works  when  some  of  its  inner  wheels  are  broken. 

All  investigators  who  have  performed  experiments  On  the  brain  have  ob- 
served, as  the  result  of  injury  to  various  parts  of  it,  remarkable  movements 
which  have  the  appearance  of  being  irresistible,  compulsory,  forced.  They 
vary  much  in  the  extent  to  which  they  are  developed  ;  some  are  so  slight  as 
hardly  to  deserve  the  name,  while  others  are  strikingly  intense.  One  of  the 
most  common  forms  is  that  in  which  the  animal  rolls  incessantly  round  the 
longitudinal  axis  of  its  own  body.  This  is  especially  common  after  section 
of  one  of  the  crura  cerebri,  or  of  the  middle  and  inferior  peduncles  of  the 
cerebellum,  or  after  unilateral  section  of  the  pons,  but  has  also  been  wit- 
nessed after  injury  to  the  bulb  and  corpora  quadrigemina.  Sometimes  the 
animal  rotates  toward  and  sometimes  away  from  the  side  operated  on.  An- 
other form  is  that  in  which  the  animal  executes  "  circus  movements,"  i.  e., 
continually  moves  round  and  round  in  a  circle  of  longer  or  shorter  radius, 
sometimes  toward  and  sometimes  away  from  the  injured  side.  This  may  be 
seen  after  several  of  the  above-mentioned  operations,  and  in  one  form  or  an- 
other is  not  uncommon  after  various  unilateral  injuries  to  the  brain.  There 
is  a  variety  of  the  circus  movement,  the  "  clock-hand  movement,"  said  to 
occur  frequently  after  lesions  of  the  posterior  corpora  quadrigemina,  in 
which  the  animal  moves  in  a  circle,  with  the  longitudinal  axis  of  its  body 
as  a  radius  and  the  end  of  its  tail  for  a  centre.  And  this  form  again  may 
easily  pass  into  a  simple  rolling  movement.  In  yet  another  form  the  animal 
rotates  over  the  transverse  axis  of  its  body,  tumbles  head  over  heels  in  a 
series  of  somersaults ;  or  it  may  run  incessantly  in  a  straight  line  backward 
or  forward  until  it  is  stopped  by  some  obstacle.  These  latter  forms  of  forced 
movements  are  sometimes  seen  after  injury  to  the  corpus  striatum  even  when 
a  very  limited  portion  of  the  gray  matter  is  affected.  And  many  of  these 


650  THE   BRAIN. 

forced  movements  may  result  from  injuries  which  appear  to  be  confined  to 
the  cerebral  cortex. 

When  the  phenomena  are  well  developed,  every  effort  of  the  animal  brings 
on  a  movement  of  this  forced  character.  Left  to  itself  and  at  rest  the  ani- 
mal may  present  nothing  abnormal,  its  posture  and  attitude  may  be  quite 
natural ;  but  when  it  is  excited  to  move  or  when  it  attempts  of  itself  to  move, 
it  executes  not  a  natural  movement,  but  a  forced  one,  turning  round  or  roll- 
ing over  as  the  case  may  be.  In  severe  cases  the  movement  is  continued 
until  the  animal  is  exhausted ;  when  the  exhaustion  passes  off  the  animal 
may  remain  for  some  little  time  quiet,  but  some  stimulus,  intrinsic  or  ex- 
trinsic, soon  inaugurates  a  fresh  outbreak,  to  be  again  followed  by  exhaustion. 

In  some  of  the  milder  forms,  that  for  instance  of  the  circus  movement 
with  a  long  radius,  the  curved  character  of  the  progression  appears  simply 
due  to  the  fact  that  in  the  effort  of  locomotion  volitional  impulses  do  not 
gain  such  ready  access  to  one  side  of  the  body  as  to  the  other,  the  injury 
having  caused  some  obstacle  or  other.  Hence  the  contractions  of  the  mus- 
cles of  one  side  (the  left,  for  instance)  of  the  body  are  more  powerful  than 
the  other,  and  in  consequence  the  body  is  continually  thrust  toward  the  other 
(the  right)  side.  As  is  well  known,  we  ourselves,  when  our  walk  is  not 
guided  by  visual  sensations,  tend  to  describe  a  circle  of  somewhat  wide 
radius,  the  deviation  being  due  to  a  want  of  bilateral  symmetry  in  our  limbs  ; 
and  the  above  circus  movement  is  only  an  exaggeration  of  this. 

But  the  other  more  intense  forms  of  forced  movements  are  more  compli- 
cated in  their  nature.  No  mere  blocking  of  volitional  impulses  will  explain 
why  an  animal  whenever  it  attempts  to  move  rolls  rapidly  over  or  rushes 
irresistibly  forward  or  backward.  It  is  not  possible  with  our  present  know- 
ledge to  explain  how  each  particular  kind  of  movement  is  brought  about ; 
and,  indeed,  the  several  kinds  are  probably  brought  about  in  different  ways, 
for  they  differ  so  greatly  from  each  other  that  we  only  class  them  together 
because  it  is  difficult  to  know  where  to  draw  the  line  between  them.  But 
we  may  regard  the  more  intense  forms  as  illustrating  the  complex  nature  of 
what  we  have  called  the  coordinating  machinery,  the  capabilities  of  which 
are,  so  to  speak,  disclosed  by  its  being  damaged.  Such  gross  injuries  as  are 
involved  in  dividing  cerebral  structures  or  in  injecting  corrosive  substances 
into  this  or  that  part  of  the  brain,  must,  of  necessity,  partly  by  blocking  the 
way  to  the  impulses  which  in  a  normal  state  of  things  are  continually  pass- 
ing from  one  part  of  the  brain  to  another,  partly  by  generating  new  unusual 
impulses,  seriously  affect  the  due  working  of  the  general  coordinating  machin- 
ery. The  fact  that  an  animal  can,  at  any  moment,  by  an  effort  of  its  own 
will,  rotate  on  an  axis  or  run  straight  forward,  shows  that  the  nervous 
mechanism  for  the  execution  of  those  movements  is  ready  at  hand  in  the 
brain,  waiting  only  to  be  discharged ;  and  it  is  easy  to  conceive  how  such  a 
discharge  might  be  affected  either  by  the  substitution  for  the  will  of  some 
potent  intrinsic  afferent  impulse  or  by  some  misdirection  of  volitional  im- 
pulses. Persons  who  have  experienced  similar  forced  movements  as  the 
result  of  disease  report  that  they  are  frequently  accompanied,  and  seem  to 
be  caused,  by  disturbed  visual  or  other  sensations;  thus  they  attribute  their 
suddenly  falling  forward  to  the  occurrence  of  the  sensation  that  the  ground 
in  front  of  them  is  suddenly  sinking  away  beneath  their  feet.  Without 
trusting  too  closely  to  the  interpretations  the  subjects  of  these  disorders  give 
of  their  own  feelings,  and  remembering  what  was  said  above  concerning  ver- 
tigo, we  may  at  least  conclude  that  the  unusual  movements  are  in  many 
cases  due  to  a  disorder  of  the  coordinating  mechanism,  brought  about  by 
strange  or  disordered  sensory  impulses.  And  this  view  is  supported  by  the 
fact  that  many  of  these  forced  movements  are  accompanied  by  a  peculiar 


THE  MACHINERY  OF  COORDINATED  MOVEMENTS.  651 

and  wholly  abnormal  position  of  the  eyes,  which  alone  might  perhaps  explain 
many  of  the  phenomena. 

§  559.  The  phenomena  presented  by  animals  deprived  of  their  cerebral 
hemispheres  show  that  this  machinery  of  coordination  is  supplied  by  cerebral 
structures  lying  between  the  cerebral  hemisphere  above  and  the  top  of  the 
spinal  cord  below.  But  when  we  ask  the  further  question,  How  is  this 
machinery  related  to  the  various  elements  which  go  to  make  up  this  part 
of  the  brain  ?  the  only  answers  which  we  receive  are  of  the  most  imperfect 
kind. 

In  the  case  of  the  frog  we  can,  after  removal  of  the  cerebral  hemispheres, 
make  an  experimental  distinction  in  the  parts  left  between  the  optic  thalami 
with  the  optic  nerves  and  tracts,  the  optic  lobes,  and  the  bulb  with  the  rudi- 
mentary cerebellum.  When  the  optic  thalami  are  removed,  as  might  be 
expected,  the  evidence  of  visual  impressions  modifying  the  movements  of 
the  animal  disappears ;  and  it  is  stated  that  apparently  spontaneous  move- 
ments are  much  more  rare  than  when  the  thalami  are  intact.  When  the 
optic  lobes  as  well  as  the  cerebral  hemispheres  are  removed,  the  power  of 
balancing  is  lost ;  when  such  a  frog  is  thrown  off  its  balance  by  inclining 
the  plane  on  which  it  is  placed,  it  slips  back  or  falls  down ;  the  special  co- 
ordinating mechanism  for  balancing  must,  therefore,  in  this  animal  have  a 
special  connection  with  the  optic  lobes.  But  after  removal  of  these  organs 
the  animal  is  still  capable  of  a  great  variety  of  coordinate  movements ; 
unlike  a  frog  retaining  its  spinal  cord  only,  it  can  swim  and  leap,  it  main- 
tains a  normal  posture,  and  when  placed  on  its  back  immediately  regains 
the  normal  posture.  The  cerebellum  of  the  frog  is  so  small,  and  in  re- 
moving it  injury  is  so  likely  to  be  done  to  the  underlying  parts,  that  it 
becomes  difficult  to  say  how  much  of  the  coordination  apparent  in  a  frog 
possessing  cerebellum  and  bulb  is  to  be  attributed  to  the  former  or  to  the 
latter ;  probably,  however,  the  part  played  by  the  former  is  small. 

In  the  case  neither  of  the  bird  nor  of  the  mammal  have  we  any  exact 
information  as  to  the  behavior  of  the  animal  after  removal  of  the  parts 
behind  the  hemispheres,  in  addition  to  the  hemispheres  themselves.  Our 
knowledge  is  confined  to  the  results  of  the  ablation  or  of  the  stimulation  of 
parts,  the  cerebellum  for  instance,  in  animals  in  which  the  rest  of  the  brain 
has  been  left  intact.  Observations  of  this  kind  have  disclosed  many  inter- 
esting facts,  besides  the  forced  movements  just  referred  to,  but  they  have  not 
led  to,  and  indeed  could  hardly  be  expected  to  lead  to,  any  clear  views  as  to 
the  point  which  we  are  now  discussing.  It  does  not  follow  that  every  part, 
injury  or  stimulation  of  which  interferes  with  coordinated  movements,  or 
gives  rise  to  definite,  forced,  or  other  movements,  is  to  be  considered  as  part 
of  the  machinery  under  consideration.  The  corpora  striata  and  cerebral 
hemispheres  form,  as  we  have  seen,  no  part  of  the  machinery,  yet  injury  to 
them  may  disorder  the  machinery ;  and  the  fact  that  removal  of  or  injury 
to  the  cerebellum  disorders  the  machinery  is  no  proof  by  itself  that  the  cere- 
bellum is  an  essential  part  of  the  machinery. 

If  we  may  trust  to  deductions  from  structural  arrangements,  we  might 
be  inclined  to  infer  that  the  anatomical  relations  of  the  tegmental  region 
from  the  bulb  upward  point  to  its  serving 'as  the  foundation  of  the  machin- 
ery in  question.  Behind  it  has  full  connections  with  various  parts  of  the 
cord,  while  in  front  by  means  of  the  optic  thalami  and  anterior  corpora 
quadrigemina,  if  not  by  other  ways  as  well,  it  is  so  far  associated  with  the 
optic  nerves  that  the  path  seems  open  for  visual  impulses  to  gain  access  to 
it.  To  this  foundation,  however,  we  must  add  the  cerebellum,  on  account  of 
its  relations  to  it,  to  the  cord,  and  to  the  bulb  through  the  restiform  bodies, 
including  its  ties  with  the  auditory  nerve.  And  if  we  add  the  cerebellum 


652  THE  BRAIN. 

we  must  also  probably  add  the  pons.  We  may  exclude  the  pes  of  the  crus, 
since  this  is  composed  exclusively  of  fibres,bringing  the  cerebral  hemispheres, 
including  the  corpora  striata,  into  connection  with  the  pons,  bulb,  and  cord, 
and  so  with  the  coordinating  machinery  itself,  as  well  as  with  other  parts  of 
the  nervous  system.  And  observation,  as  far  as  its  goes,  supports  this  deduc- 
tion from  anatomical  relationships.  We  will,  however,  defer  what  else  we 
have  to  say  on  this  point  until  after  we  have  discussed  the  carrying  out  of 
voluntary  movements. 

ON  SOME  HISTOLOGICAL  FEATURES  OF  THE  BRAIN. 

§  560.  The  white  matter  of  the  brain,  as  we  have  already  said,  like  that 
of  the  spinal  cord,  consists  of  medullated  fibres  of  various  sizes  imbedded 
in  neuroglia  and  supported  by  septa  of  connective  tissue  derived  from  the  pia 
mater.  Save  that  cells,  or  even  groups  or  rows  of  cells,  for  the  most  part 
small  cells,  about  many  of  which  it  may  be  debated  whether  they  are  nerve- 
cells  or  neuroglia  cells,  arejfrequently  seen  between  the  fibres  and  bundles  of 
fibres,  the  white  matter  of  the  brain  seems  essentially  identical  with  that  of 
the  spinal  cord. 

The  gray  matter  of  the  brain  in  general  also  corresponds  to  the  gray 
matter  of  the  cord  in  consisting  of  branching  nerve-cells,  fine  medullated 
fibres  of  peculiar  nature,  non-medullated  fibres  and  fibrils,  with  a  few  ordi- 
nary medullated  fibres,  all  supported  in  neuroglia. 

The  "  central "  gray  matter  is  extremely  like  that  of  the  cord  except 
that  the  nervous  elements  are  imbedded  in  a  relatively  larger  quantity  of 
neuroglia.  Immediately  underneath  the  epithelium  lining  the  several  ven- 
tricles and  the  aqueduct,  the  neuroglia  is  especially  developed,  forming  a 
distinct  layer  which  may  be  regarded  as  a  continuation  of  the  central  gelat- 
inous substance  of  the  spinal  cord,  and  which,  with  the  epithelium  overlying 
it,  forms  what  is  known  as  the  ependyma.  The  "  nuclei "  of  the  cranial 
nerves  are,  as  we  have  seen,  comparable  to  the  groups  of  nerve-cells  in  the 
spinal  cord. 

A  great  deal  of  gray  matter  of  the  brain  may  be  spoken  of  as  more 
"  diffuse  "  or  "  scattered,"  more  broken  up  by  bundles  of  fibres  than  is  the 
case  in  the  spinal  cord.  The  "  reticular  formation  "  of  the  bulb  and  of  the 
tegmental  region  is  an  extreme  form  of  this  diffuse  gray  matter.  And  even 
in  such  collections  of  indubitable  gray  matter  as  the  corpus  striatum,  optic 
thalamus,  and  the  like,  the  pure  gray  matter,  if  we  may  use  the  term,  is 
much  more  interrupted  and  broken  up  by  conspicuous  bundles  of  white 
fibres  than  is  the  case  in  any  region  of  the  spinal  cord.  In  the  corpora 
quadrigemina  the  gray  matter  is  broken  up  by  sheets  or  bundles  of  white 
matter. 

The  nerve-cells  of  the  several  collections  of  gray  matter  are  not  all  alike  ; 
they  present  in  different  regions  differences  in  size,  form,  and  in  other  cha- 
racters. The  cells  of  the  nucleus  caudatus,  for  instance,  are  rather  small  and 
often  round  or  spindle-shaped,  while  those  of  the  optic  thalamus  are  large, 
branched,  and  rich  in  pigment.  The  cells  of  the  substantia  nigra  are  spindle- 
shaped,  of  moderate  size,  and  so  loaded  with  black  pigment  (in  man)  as  to 
justify  the  name ;  those  of  the  locus  cseruleus  are  very  large  and  spherical, 
with  just  so  much  pigment  as  to  give  a  bluish  tint.  But  our  knowledge  of 
the  finer  histological  details  of  the  various  masses  of  gray  matter  is  at  present 
too  imperfect  to  afford  any  basis  whatever  for  physiological  deductions;  and 
it  will  be  hardly  profitable  to  dwell  upon  them.  Two  regions  of  gray  matter 
alone  call  for  special  description,  the  cortex  cerebri  and  the  superficial  gray 
matter  of  the  cerebellum. 


ON   SOME  HISTOLOGICAL  FEATUKES  OF  THE  BRAIN.         653 

The  Superficial  Gray  Matter  of  the  Cerebellum. 

§  561.  The  surface  of  the  cerebellum  is  increased  by  being  folded  or 
plaited  into  leaf-like  folds,  and  each  of  these  primary  folds  is  similarly- 
folded  into  a  number  of  secondary,  also  leaf-like,  folds  or  lamellae.  Each 
of  these  lamellae  consists  of  a  central  core  of  white  matter,  the  fibres  of 
which  pass  inward  to  and  contribute  to  form  the  central  white  matter  of  the 
cerebellum,  and  of  a  superficial  layer  of  gray  matter.  A  section  through 
a  lamella  perpendicular  to  the  surface  shows  that  the  gray  matter  consists 
essentially  of  two  layers :  a  layer  lying  next  to  the  white  matter  formed  by 
densely  crowded  small  cells,  called  the  nuclear  layer,  and  between  this  and 
the  superficial  pia  mater  a  much  thicker  layer  of  peculiar  nature,  called  the 
molecular  layer.  Between  these  two  layers,  and  connected,  as  we  shall  see, 
with  both  of  them,  lies  a  row  of  very  large  and  remarkable  cells,  called  the 
cells  of  Purkinje,  the  bodies  of  which  abut  on  the  nuclear  layer,  and  the 
long  branches  of  which  traverse  the  molecular  layer ;  these  cells  so  placed 
may  be  said  to  constitute  a  third  layer.  Before  proceeding  further,  we  may 
here  remark  that  a  section  of  the  lamellae,  that  is,  one  of  the  secondary,  not 
one  of  the  primary,  folds,  while  still  remaining  a  vertical  section  (that  is, 
perpendicular  to  the  surface),  may  be  carried  through  the  lamella  in  different 
planes,  and  that,  of  these  several  planes,  the  sections  taken  in  two  of  them 
are  especially  instructive,  namely,  the  one  taken  in  what  we  may  call  the 
longitudinal  plane,  passing  from  the  top  of  the  lamella  to  its  base,  and  the 
one  taken  at  right  angles  to  the  former,  in  what  we  may  call  the  transverse 
plane.  The  nuclear  layer  and  the  molecular  layer  present  the  same  broad 
features  in  both  longitudinal  and  transverse  sections,  but  the  long-branched 
processes  of  the  cells  of  Purkinje  since  they  run  in  the  transverse  plane  are 
adequately  seen  in  transverse  sections  only  ;  longitudinal  sections  show  only 
their  profiles. 

The  molecular  layer  is  of  a  peculiar  nature.  In  many  modes  of  prepara- 
tion and  in  many  sections  it  appears  chiefly  composed  of  a  granular  or  dotted 
ground  substance;  hence  the  name  molecular,  as  if  it  were  an  aggregation 
of  molecules.  The  dots,  however,  are  sections  of  fine  fibrils,  some  of  which 
are  neuroglia  fibrils  but  others  are  undoubtedly  nervous.  The  layer  consists 
in  fact  partly  of  nervous  elements,  and  here  perhaps  even  more  than  else- 
where it  is  extremely  difficult  to  say  with  regard  to  many  of  the  elements 
whether  they  are  neuroglial  or  nervous  in  nature.  A  considerable  portion 
of  the  whole  area  of  the  molecular  layer  is  taken  up  by  the  conspicuous 
branched  processes  of  the  cells  of  Purkiuje;  and  scattered  about  lie  numer- 
ous small  cells,  some  of  which  are  neuroglia  cells,  but  some  of  which  are 
undoubtedly  nerve-cells.  The  most  conspicious  feature  of  the  layer,  how- 
ever, is  the  presence  in  large  numbers  of  the  fine  fibrils  ;  but  before  we  speak 
of  these  it  will  be  desirable  to  turn  to  the  cells  of  Purkinje  and  the  nuclear 
layer. 

The  cell  of  Purkinje  possesses  a  large  (40  /j.  by  30  ,«)  flask-shaped  body, 
surrounding  a  large,  conspicuous,  clear,  rounded  nucleus;  it  has  much  the 
appearance  of  a  large  ganglion  cell.  The  base  of  the  flask  rests  on  the  nuclear 
layer,  and  from  it  there  proceeds  a  single  axis-cylinder  process  which,  passing 
through  the  nuclear  layer  somewhat  obliquely,  and  in  its  passage  acquiring 
a  medulla,  joins  the  central  white  substance  as  a  medullated  fibre.  The 
cells,  as  we  have  said,  form  a  single  layer  only,  but  since  this  covers  the  nu- 
clear layer  over  the  whole  of  the  lamella,  a  considerable  number  of  the  fibres 
of  the  white  central  matter,  though  only  a  very  small  fraction  of  the  whole, 
are  thus  derived  from  these  cells  of  Purkinje.  The  narrowed  neck  of  the 
flask  running  outward  in  the  molecular  layer  divides  in  an  arborescent 


654  THE  BRAIN. 

fashion  into  a  large  number  of  branches  which,  spreading  out  laterally  in 
the  transverse  plane  and  stretching  as  far  as  the  surface,  ramify  through  the 
molecular  layer,  and  are  eventually  lost  to  view  as  exceedingly  fine  fibrils. 
Some  observers  maintain  that  some  of  the  fine  processes  are  continuous 
with  processes  of  the  small  nerve-cells  of  the  molecular  layer,  but  this  is  not 
admitted  by  all.  In  any  case  the  fibrillar  terminations  of  these  cells  of 
Purkinje  contribute  to  the  fine  fibrils  of  the  molecular  layer. 

The  nuclear  layer  in  ordinary  stained  specimens  has 'the  appearance  of 
a  mass  of  nuclei  closely  crowded  together  in  a  bed  of  reticular  nature ;  and 
since  the  nuclei  usually  stain  deeply,  the  layer  stands  out  in  strong  contrast 
to  the  much  less  deeply  stained  molecular  layer.  Careful  examination  with 
special  modes  of  preparation  shows,  however,  that  while  some  of  the  nuclei 
are  nuclei  belonging  to  neuroglia  and  bloodvessels,  the  majority  belong  to 
small  nerve-cells  of  a  peculiar  nature.  In  these  cells  the  nucleus  is  sur- 
rounded by  cell  substance,  which,  forming  a  thin  layer  immediately  around 
the  nucleus,  is  chiefly  disposed  as  thin  spreading  branches,  some  of  which  end 
in  a  peculiar  arborescence  not  unlike  a  muscle  end-plate ;  these  processes 
contribute  writh  the  neuroglia  to  form  the  reticular-looking  bed  spoken  of 
above.  No  process  can  be  traced  inward  to  the  central  white  matter ;  but 
one  of  the  processes  gives  off  a  branch,  which  passing  vertically  outward 
takes  on  the  appearance  of  a  delicate  axis-cylinder  process  and  runs,  with- 
out dividing,  into  the  molecular  layer  for  a  variable  distance,  sometimes 
reaching  close  to  the  surface,  but  at  last  divides  at  right  angles  into  two 
fibrils,  which  run  in  the  longitudinal  plane  in  opposite  directions  for  a  con- 
siderable distance,  and  are  ultimately  lost  to  view.  Since  these  cells  in  the 
nuclear  layer  are  very  numerous  and  each  gives  rise  in  the  above  manner  to 
longitudinal  fibrils,  the  molecular  layer  is  traversed  by  a  multitude  of  fibrils, 
visible  as  such  in  longitudinal  sections,  but  appearing  as  dots  in  transverse 
sections,  in  which  the  cells  of  Purkinje  are  best  displayed. 

Besides  these  longitudinal  fibrils  proceeding  from  the  cells  of  the  nuclear 
layer,  special  modes  of  preparation  similarly  disclose  numerous  transverse 
as  well  as  more  or  less  oblique  fibrils.  Many  of  these  appear  to  result  from 
the  branching  of  the  small  nerve-cells  of  the  molecular  layer,  and  some  of 
those  so  arising  descend  to  the  layer  of  the  cells  of  Purkinje  and  end  around 
the  bodies  of  those  cells  in  remarkable  nests  of  fibrils,  without,  however, 
actually  making  connections  with  them. 

The  medullated  fibres  of  the  central  white  matter  of  a  lamella  pass  on  all 
sides  into  the  nuclear  layer;  or,  put  in  another  way,  medullated  fibres  pass- 
ing out  of  the  nuclear  layer  at  all  points  converge  to  form  the  central  white 
matter.  Some  of  these  fibres,  as  we  have  seen,  begin  or  end  in  the  cells  of 
Purkinje.  None  of  them  appear  to  join  the  cells  of  the  nuclear  layer,  and 
we  have  no  evidence  that  any  of  them  end  or  begin  in  any  way  in  the  nu- 
clear layer.  A  certain  number,  however,  may  be  seen  to  pass  through  the 
nuclear  layer  and  between  the  cells  of  Purkinje  into  the  molecular  layer, 
where  losing  their  medulla  they  divide  and  apparently  contribute  to  the 
numerous  fibrils  of  the  molecular  layer.  The  presumption,  therefore,  is  that 
all  the  fibres  of  the  white  matter  begin  or  end  either  in  the  cells  of  Pur- 
kinje or  the  fibrils  of  the  molecular  layer. 

The  superficial  gray  matter  of  the  cerebellum  then  resembles  the  gray 
matter  of  the  spinal  cord  in  so  far  as  it  consists  of  branching  nerve-cells, 
nerve-fibres,  and  nerve-fibrils  imbedded  in  neuroglia ;  but  the  disposition 
and  features  of  the  several  factors  are  peculiar.  We  may  take,  perhaps,  as 
the  key  of  the  structure  the  fibrils  of  the  molecular  layer  ;  this  layer  is  rela- 
tively very  thick,  about  400 /x,  much  thicker  than  the  nuclear  which,  how- 
ever, varies  in  thickness,  being  generally  thickest  at  the  top  of  the  fold ; 


ON  SOME  HISTOLOGICAL  FEATURES  OF  THE  BRAIN.         655 

hence  the  number  of  fibrils  in  it  may  be  spoken  of  as  enormous.  These 
fibrils  seem  certainly  to  be  connected  on  the  one  hand  with  the  cells  of  the 
nuclear  layer,  and  on  the  other  hand  with  the  scattered  small  cells  of  their 
own  layer ;  but  we  have  no  evidence  that  these  two  sets  of  fibrils  are  con- 
tinuous with  each  other ;  on  the  contrary,  it  seems  more  probable  that  the 
two  sets  of  cells  represent  two  independent  systems.  We  can  hardly  doubt 
that  these  fibrils  are  in  functional  connection  with  the  medullated  fibres  of 
the  central  white  matter ;  but  we  have  no  clear  evidence  that  the  system 
of  scattered  cells  is  continuous  either  with  the  cells  of  Purkinje,  and  so  with 
the  medullated  fibres  belonging  to  those  cells,  or  with  the  medullated  fibres 
which  end  independently  in  the  molecular  layer ;  and  we  have  no  evidence 
at  all  that  the  system  of  the  cells  of  the  nuclear  layer  is  connected  with 
either.  We  can  hardly  think  otherwise  than  that  the  molecular  changes 
which  sweep  to  and  fro  along  the  tangle  of  these  fibrils  (whose  nutrition  is 
probably  governed  and  hence  whose  functional  activity  is  probably  regulated 
by  the  nuclear  and  scattered  cells  respectively)  are  influenced  by  or  originate 
the  nervous  impulses  passing  along  the  medullated  fibres  of  the  white  matter ; 
and  hence  we  must  conclude  that  either  a  continuity  exists  which  has  as  yet 
escaped  detection,  or,  what  is  quite  possible  if  not  probable,  that  one  fibril 
can  act  upon  another  by  simple  contact  or  even  at  a  distance.  Further, 
while  the  cell  of  Purkinje,  with  its  large  cell-body  and  nucleus,  its  conspicu- 
ous axis-cylinder  process  and  its  other  branched  processes  presents  many 
analogies  with  motor  cells,  such  as  those  of  the  anterior  horn  of  the  spinal 
cord,  and  raises  the  presumption  that  the  impulses  which  move  along  its 
axis-cylinder  process  proceed  outward  from  the  cell  as  motor  or  at  least  as 
efferent  impulses,  we  have  no  direct  proof  that  this  is  so.  And  though  it  is 
tempting  to  suppose  that  the  other  medullated  fibres,  which  like  the  fibres 
of  a  posterior  root  are  lost  in  the  gray  matter,  without  the  intervention  of  a 
conspicuous  cell,  carry  efferent  impulses,  we  have  as  yet  no  proof  of  this. 
All  we  can  say  is  that  the  gray  matter  is  connected  in  two  different  ways 
with  at  least  two  sets  of  fibres,  which  probably  therefore,  have  different 
functions. 

We  may  here  add  the  remark  that  the  large  body  of  the  cell  of  Purkinje 
lies,  as  indeed  do  the  other  nervous  elements,  in  an  appropriate  space  in 
the  bed  of  neuroglia.  Between  the  surface  of  the  cell  and  the  wall  of  neur- 
oglia  is  a  space,  generally  so  narrow  as  to  be  potential  rather  than  actual, 
but  which  may  sometimes  be  considerable.  Whether  small  or  large  it  con- 
tains lymph,  and  the  cavity  in  which  the  cell  lies  is  in  connection  with  the 
lymphatics  of  the  brain.  Each  cell  then  lies  in  a  lymph-space ;  but  we 
merely  mention  the  fact  now ;  we  shall  have  to  return  to  the  matter  when 
we  come  to  deal  with  the  lymphatic  and  vascular  arrangements  of  the  brain 
and  spinal  cord. 

The  Cerebral  Cortex. 

§  562.  While  the  superficial  gray  matter  of  the  cerebellum  does  not 
differ  strikingly  as  to  its  histological  features  in  different  regions,  very  con- 
siderable differences  are  observed  in  different  regions  of  the  cerebral  cortex. 
A  general  plan  of  structure  may  perhaps  be  recognized,  but  as  we  pass  from 
one  part  of  the  cerebral  surface  to  another  we  find  modifications  continually 
taking  place.  We  must  content  ourselves  here  with  attempting  a  descrip- 
tion of  the  general  plan,  followed  by  an  indication  of  the  more  striking  cha- 
racteristics of  certain  regions. 

The  cortical  gray  matter,  having  an  average  thickness  of  about  3  mm., 
but  varying  considerably  in  different  regions  from  1.8  mm.  in  some  parts  of 


656  THE  BRAIN. 

the  occipital  lobe  to  4.2  mm.  at  the  dorsal  summit  of  the  precentral  convolu- 
tion, is,  like  other  gray  matter,  composed  of  nerve-cells,  and  of  nerve-fibres 
and  fibrils  supported  by  neuroglia.  The  nerve-cells,  at  least  the  conspicuous 
and  easily  recognized  nerve-cells,  are  scattered,  and  appear,  in  sections,  to  be 
imbedded  in  and  separated  from  each  other  by  a  not  inconsiderable  but 
variable  quantity  of  somewhat  peculiar  ground  substance,  not  unlike  that 
which  forms  so  large  a  part  of  the  molecular  layer  of  the  cerebellum.  Part 
of  this  ground  substance,  which  apparently  is  not  confined  to  any  particular 
layer,  but  stretches  throughout  the  thickness  of  the  cortex,  is  undoubtedly 
neuroglial  in  nature,  but  part,  and  probably  the  greater  part,  is  nervous  in 
nature  ;  it  is  largely  composed  of  fine  fibrils  traversing  it  in  various  direc- 
tions, the  transverse  sections  of  the  fibrils  giving  it  a  characteristic  dotted 
or  "  molecular  "  appearance  ;  and  the  majority  of  these  fine  fibrils  are  prob- 
ably the  continuations  of  branching  nerve-cells  or  dividing  nerve-fibres,  the 
remainder  being  neuroglial  fibrils.  In  this  respect  it  resembles  the  molec- 
ular layer  of  the  cerebellum,  but  it  is,  to  a  much  greater  extent  than  is  that 
layer,  traversed  by  medullated  nerve-fibres,  especially  by  fine  medullated 
fibres  like  those  seen  in  the  gray  matter  of  the  spinal  cord  (§  476). 

The  nerve-cells  imbedded  in  this  ground  substance  in  more  or  less  dis- 
tinct layers  are  of  various  kinds.  The  most  conspicuous,  abundant  and  cha- 
racteristic nerve-cells  found  in  the  cortex  of  all  regions  of  the  cerebellum 
are  those  which  from  their  shape  are  called  pyramidal  cells.  These  vary 
very  much  in  size  and  have  been  distinguished  as  "  small  pyramidal "  cells 
averaging  12  p  in  length  by  8  /j.  in  breadth,  and  "  large  pyramidal"  cells, 
sometimes  called  "  ganglionic  cells,"  of  which  the  medium  size  is  about  40  t>. 
in  length  by  20  A*  in.  breadth.  Some  of  the  latter,  occurring  in  special 
regions,  are  of  very  large  size,  120  /*  by  50  fj.,  and  have  been  called  "  giant 
cells." 

The  features  of  a  "  large  pyramidal "  cell  are  very  characteristic.  Such  a 
cell  appears  in  a  well-prepared  vertical  section  of  the  cortex  as  an  elongated 
isosceles  triangle  placed  vertically,  with  the  base  looking  toward  the  under- 
lying white  substance  and  the  tapering  apex  pointing  to  the  surface.  The 
cell  substance  is  finely  granulated  or  fibrillated,  the  fibrilla3  sweeping  round 
in  various  directions  ;  it  not  unfrequently  contains  pigment.  In  the  midst  of 
this  cell  substance  rather  near  the  base  lies  a  large,  clear,  conspicuous  round 
or  oval  nucleolated  nucleus.  At  the  base  the  cell  substance  is  prolonged 
into  a  number  of  processes.  One  of  these,  generally  starting  from  about  the 
middle  of  the  base,  runs  for  some  distance  without  dividing,  and  soon  acquir- 
ing a  medulla  may  be  recognized  as  an  axis-cylinder  process ;  the  fibre  to 
which  it  gives  origin  sweeps  with  a  more  or  less  curved  course  into  the  sub- 
jacent white  matter.  In  some  instances  the  axis-cylinder  process,  by  a 
T-division  like  that  seen  in  a  ganglion  of  a  posterior  root  (§  93),  gives  rise 
to  two  fibres,  one  of  which  may  take  a  horizontal  direction ;  in  some  regions 
of  the  cortex,  the  occipital  for  instance,  the  axis-cylinder  process  is  said  to 
give  rise  by  division  to  several  fibres.  The  other  processes  from  the  base, 
especially  those  from  the  angles  of  the  triangle,  rapidly  branch  into  fine 
fibrils  which  are  soon  lost  to  view  in  the  ground  substance.  The  apex  of 
the  triangle  is  also  prolonged  into  a  process,  which,  giving  off  fine  lateral 
branches,  makes,  as  it  were,  straight  for  the  surface,  but  ultimately  branch- 
ing into  fine  fibrils  is  lost  to  view  at  some  distance  from  the  body  of  the  cell. 
The  cell  lies  in  a  cavity  of  the  ground  substance,  which  it  appears  normally 
to  fill,  but  from  the  walls  of  which  it  sometimes  shrinks,  developing  between 
itself  and  the  wall  of  the  cavity  a  space  which  may  contain  not  only  lymph, 
but  occasionally  leucocytes.  In  prepared  specimens  the  retraction  within  its 
cavity  of  the  artificially  shrunken  cell  may  be  often  observed. 


ON  SOME  HISTOLOGICAL  FEATURES  OF  THE  BRAIN.         657 

The  "  small  pyramidal "  cells  have  much  the  same  features ;  that  is  to 
say,  the  cells  are  characterized  by  their  pyramidal  form,  though  this  is  nat- 
urally not  so  distinct,  by  their  vertical  position,  and  by  the  possession  of 
branching  processes  which  are  lost  in  the  molecular  ground  substance ;  the 
presence,  however,  of  a  midbasal  axis-cylinder  process  has  not  been  clearly 
demonstrated. 

Other  nerve-cells  are  more  like  the  ordinary  nerve-cells  of  the  spinal 
cord  and  of  the  internal  cerebral  gray  matter ;  they  are  branched  cells,  of 
irregular,  not  of  pyramidal,  form,  and  for  the  most  part  small,  18  //.  by  10  n. 
They  may  be  characterized  by  the  relatively  large  size,  7  /-/,  of  the  nucleus, 
and  do  not  possess  an  axis-cylinder  process ;  at  least,  such  a  process  has  not 
yet  been  demonstrated.  They  are  frequently  spoken  of  as  "  angular  "  cells. 

Another  kind  of  cell,  the  "  fusiform  cell,"  which  is  found  in  all  regions 
of  the  cortex,  has  a  characteristic  spindle-shape,  the  cell  substance  being 
prolonged  at  the  opposite  poles  into  tapering,  ultimately  branched  processes. 
The  long  axis  of  the  cell  is  generally  placed  horizontally,  following  the  cur- 
vature of  the  cortex,  and  being  thus  at  the  sides  of  the  sulci  vertical  to  the 
surface  of  the  brain  ;  it  is,  however,  at  times  inclined  at  various  angles. 

Still  another  kind  of  cell,  the  "  granule  cell,"  or  "  nuclear  cell,"  is  one  in 
which  the  nucleus  is  surrounded  by  a  relatively  small  quantity  of  cell  sub- 
stance, 9  p  by  7  />-,  more  or  less  spherical  in  form  in  ordinary  preparations, 
but  probably  breaking  up  into  delicate  branched  processes.  Cells  of  this 
kind  are  sparsely  scattered  throughout  the  cortex  generally,  but  in  particu- 
lar regions — e.  g.,  the  occipital — are  crowded  together  into  a  layer,  which  in 
many  respects  resembles  the  nuclear  layer  of  the  cerebellum,  and  has  been 
called  the  "  granular"  or  "nuclear"  layer. 

Lastly,  throughout  the  cortex  are  found,  besides  indubitable  nerve-cells 
and  indubitable  neuroglial  cells,  numerous  small,  somewhat  irregular  cells, 
concerning  which  it  may  be  debated  whether  they  are  really  nervous  or 
simply  neuroglial  in  nature.  Moreover,  in  using  the  names  given  above  for 
the  various  kinds  of  nerve-cells,  it  must  be  remembered  that  many  transi- 
tional forms  are  observed.;  cells,  for  instance,  may  be  seen  intermediate  in 
form  between  pyramidal  cells  and  "  fusiform  "  or  "  angular  "  cells. 

The  medullated  nerve-fibres  which  take  part  in  the  cortex  may  be  con- 
sidered provisionally  as  forming  two  categories.  In  the  first  place,  fibres 
sweep  up  vertically  into  the  cortex  from  the  subjacent  "  central  white 
matter,"  taking  at  first  a  curved  course  as  they  enter  into  the  gray  matter, 
and  then  appearing  to  run  straight  toward  the  surface.  These  are  arranged 
in  the  deeper  levels  in  bundles,  leaving  vertical  columns  of  the  gray  matter 
between  them ;  but  at  more  superficial  levels  the  bundles  spread  out  and  are 
gradually  lost  to  view.  Besides  these  distinct  vertical  fibres  and  bundles  of 
fibres  of  the  ordinary  medullated  kind,  which  we  have  reason  to  think  are 
the  ends  (or  beginnings)  on  the  one  hand  of  fibres  of  the  pedal  and  tegmen- 
tal  systems,  and  on  the  other  hand  of  fibres  of  the  corpus  callosum,  or  the 
other  commissural  fibres  spoken  of  as  "  association  "  fibres  (§  548),  an  ex- 
ceedingly large  number  of  fibres  of  the  peculiar  fine  medullated  kind  run 
in  various  directions,  forming  a  dense  network  in  the  ground  substance  of 
the  gray  matter  between  the  cells.  We  may  add  that  this  system  of  fine 
medullated  fibres  is  of  late  growth,  and  is  not  fully  developed  in  man  until 
two  or  three  years  after  birth.  Many  of  the  medullated  fibres,  coarse  as  well 
as  fine,  take  a  horizontal  direction  parallel  to  the  surface,  and  in  certain  re- 
gions are  specially  developed  into  a  layer  or  into  two  layers  so. as  to  form  a 
horizontal  streak  or  streaks. 

The  vascular  pia  mater  invests  closely,  as  we  have  said,  the  whole  surface 
of  the  cortex,  dipping  down  into  the  sulci ;  and  from  it,  as  in  the  case  of  the 

42 


658  THE  BRAIN. 

spinal  cord,  processes  carrying  bloodvessels  and  bearing  lymph-spaces  pass 
inward  to  supply  the  gray  matter  with  blood.  But  while,  as  we  shall  see 
later  on,  the  supply  of  bloodvessels  to  the  gray  matter  is  considerable,  the 
truly  connective-tissue  elements  of  the  pia  mater  processes  are  soon  merged 
into  neuroglia.  Immediately  beneath  the  pia  mater  forming  the  immediate 
surface  of  the  cortex  is  a  thin  layer  consisting  of  neuroglia  only. 

§  563.  The  nerve-cells  of  the  above  several  kinds  are  arranged  more  or 
less  distinctly  in  layers  parallel  to  the  surface,  so  that  the  whole  thickness  of 
the  cortex  may  by  means  of  them  be,  more  or  less  successfully,  divided  into 
a  series  of  zones,  one  above  the  other ;  and  we  may,  as  we  have  said,  recog- 
nize on  the  one  hand  a  general  arrangement  common  to  the  whole  surface, 
and  on  the  other  hand  modifications  existing  in  the  several  regions.  The 
general  arrangement  may  be  said  to  be  one  of  five  layers  or  zones,  usually 
counted  from  the  surface  inward. 

The  fifth  layer,  lying  next  to  the  central  white  matter,  fairly  uniform  in 
characters  and  thickness  (about  1  mm.)  over  the  greater  part  of  the  brain, 
is  characterized  by  the  presence  of  somewhat  sparsely  scattered  "  fusiform  " 
cells,  though  other  branched  cells  are  present.  It  is  broken  up  into  vertical 
columns  by  the  bundles  of  vertical  fibres,  and  its  demarcation  from  the  white 
matter  below  is  somewhat  indistinct,  owing  to  the  fact  that  in  the  brain  the 
white  matter,  especially  that  lying  beneath  the  cortex,  contains  cells  and 
small  groups  of  cells  lying  between  the  bundles  of  fibres  to  a  much  greater 
extent  than  does  the  white  matter  of  the  spinal  cord. 

The  fourth  layer,  lying  above  the  preceding,  varies  much  more  both  in 
thickness  (0.35  mm.  to  0.15  mm.)  and  in  its  characters.  The  constituent 
cells  are  on  the  one  hand  large  pyramidal  cells,  and  on  the  other  hand 
"  granule  "  or  "nuclear  "  cells.  In  some  regions  it  may  be  subdivided  into 
two  layers,  the  small  "  nuclear  "  cells  being  so  abundant  as  to  form  in  the 
upper  part  of  the  layer  a  separate  layer  called  the  "granule"  or  "nuclear" 
layer.  This  fourth  layer,  like  the  preceding  fifth  layer  beneath  it,  is  split 
up  into  vertical  columns  by  the  bundles  of  vertical  fibres,  but  to  a  less  degree. 
It  is  marked  in  its  lower  part  by  a  horizontal  streak  due  to  numerous,  mostly 
fine,  medullated  fibres  running  horizontally.  In  the  cortex  of  the  island  of 
Reil  this  horizontal  layer  is  developed  into  a  conspicuous  sheet  of  medullated 
fibres,  separating  the  fourth  and  fifth  layers  by  a  distinct  interval  of  obvious 
white  matter.  This  fifth  layer  of  fusiform  cells,  thus  detached  from  the  rest 
of  the  cortex,  is  what  is  called  the  claustrum  (Figs.  138,  139,  c/.). 

In  the  third  layer,  the  constituent  cells  are  the  characteristic  pyramidal 
cells.  These  are  for  the  most  part  large,  though  diminishing  in  size  from 
below  upward,  and  the  layer  has  been  called  the  "  layer  of  large  pyramidal 
cells,"  though  in  certain  regions  the  largest  pyramidal  cells,  and  notably  the 
giant-cells,  are  found  in  the  preceding,  fourth,  layer.  The  cells  are,  on  the 
whole,  scattered  somewhat  sparsely,  though  frequently  gathered  into  small 
groups,  and  among  them  occur  small  "  nuclear "  and  other  cells.  The 
bundles  of  vertical  fibres  spread  out  rapidly  in  this  layer,  so  that  the 
columnar  arrangement  becomes  lost,  and  many  of  the  fibres  undoubtedly 
become  axis-cylinder  processes  of  the  pyramidal  cells.  Though  the  layer 
varies  in  thickness  (1  mm.  to  0.4  mm.),  and  in  some  of  its  features  in  different 
regions,  the  characteristic  pyramidal  cells  are  present  over  the  whole  surface 
of  the  hemisphere.  In  the  lower  part  of  the  layer  a  second  horizontal  streak 
of  closely  interwoven  horizontal  fibres  frequently  makes  its  appearance. 

The  second  layer,  generally  a  thin  one,  though  varying  from  0.25  mm.  to 
0.75  mm.  in  thickness,  is  also  formed  by  pyramidal  cells,  but  is  distinguished 
from  the  layer  below  by  the  absence  of  large  and  medium-sized  cells  and  by 
the  presence  of  numerous  small  cells  closely  packed  together ;  it  has  been 


ON  SOME  HISTOLOGICAL  FEATURES  OF  THE  BRAIN.         659 

called  "  the  layer  of  small  pyramidal  cells."  As  we  have  said,  these  smaller 
pyramidal  cells  differ  somewhat  from  the  larger  cells ;  and  the  cells  in  this 
layer  are  sometimes  described  as  "  angular." 

The  first  and  most  superficial  layer  is  characterized  by  the  predominance 
of  the  molecular  ground  substance,  the  cells  being  few.  far  between,  small, 
and  irregular.  The  ground  substance  itself  seems  to  be  more  largely  neur- 
oglial  iii  nature  than  in  the  other  layers,  and,  as  we  said  above,  its  extreme 
surface  appears  to  be  furnished  by  neuroglia  alone.  The  layer  is  generally 
spoken  of  as  the  "  peripheral "  or  "  superficial  layer,"  or  sometimes  as  the 
"  molecular  "  layer.  The  tapering  vertical  processes  of  the  pyramidal  cells 
may  be  traced  into  this  layer,  which  indeed  varies  in  thickness  according  to 
the  abundance  of  pyramidal  cells  in  the  subjacent  layers ;  numerous  some- 
what fine  medullated  fibres  also  traverse  it  in  a  horizontal  direction. 

§  564.  The  general  arrangement  just  described  varies,  as  we  have 
said,  in  different  regions  of  the  cerebral  surface.  We  must  content  our- 
selves here  with  pointing  out  the  characteristics  of  two  or  three  important 
regions, 

The  region  which  we  have  (§  545)  called  the  "  motor  area  "  or  "  region  " 
is  characterized  on  the  one  hand  by  the  great  thickness  (1  mm.)  of  the  third 
layer,  that  of  large  pyramidal  cells,  as  well  as  by  the  number  and  size  of 
the  cells  contained  in  it,  and  on  the  other  hand,  and  especially,  by  the 
prominence  in  the  fourth  layer  of  remarkable  clusters  of  very  large  pyram- 
idal cells,  of  the  kind  which  are  referred  to  above  (§  562)  as  being  fre- 
quently called  "  ganglionic  ;  "  it  is  in  this  region  that  "  giant  cells  "  are 
found  in  the  fourth  layer,  namely,  in  the  upper  part  of  the  precentral  and 
at  the  summit  of  the  postcentral  convolution,  and  in  the  paracentral 
lobule,  acquiring  their  greatest  size  at  the  top  of  the  precentral  con- 
volution. 

The  occipital  region  is  characterized  by  the  prominence  of  the  "  granule  " 
or  "  nuclear  "  cells.  These  not  only  form  a  distinct  division  of  the  fourth 
layer,  but  are  also  conspicuous  in  other  layers,  their  arrangements  being  such 
that  some  authors  have  been  led  to  divide  the  cortex  of  this  region  into  seven 
or  even  eight  layers.  In  the  present  state  of  our  knowledge  we  may  be  con- 
tent with  insisting  that  the  great  mark  of  this  occipital  region  is  the  abun- 
dance of  these  small  "nuclear"  cells,  together  with  other  small  "  angular" 
cells,  whereby  the  pyramidal  cells  seem  to  be  made  less  conspicuous.  It  is 
worthy  of  notice,  however,  that  in  the  third,  but  more  especially  in  the 
fourth  layer,  a  few  cells  of  very  large  size  are  met  with,  which  by  their 
large  branched  cell  substance  and  conspicuous  axis-cylinder  process  resemble 
the  large  cells  in  the  motor  region ;  but  it  should  be  noted  that  while  these 
large  cells  occur  (at  least  in  man  and  in  the  monkey,  though  not  in  some 
of  the  lower  animals,  as  the  rabbit)  in  very  definite  clusters  in  the  motor 
region,  they  occur  singly  in  the  occipital  region.  In  this  occipital  region 
the  layer  of  horizontal  fibres  in  the  fourth  layer  is  very  conspicuous,  and 
owing  to  the  number  of  ordinary  medullated  fibres  present,  forms  a  white 
streak  visible  even  to  the  naked  eye. 

In  the  frontal  region,  in  front  of  the  motor  region,  the  arrangement  is 
more  in  accordance  with  what  we  have  described  as  the  general  plan.  The 
two  pyramidal  layers  are  well  marked,  as  is  also  the  fourth  layer ;  but  the 
layer  of  large  pyramidal  cells  is  much  thinner  than  in  the  motor  region,  as 
is  also,  though  to  a  less  extent,  the  fourth  layer,  while  the  fifth  layer,  that  of 
fusiform  cells,  is  thicker  than  elsewhere.  Small  "  nuclear  "  cells  are  perhaps 
more  abundant  in  this  region  throughout  all  layers  than  in  the  motor  region, 
but  are  far  less  conspicuous  than  in  the  occipital  region. 

We  may  here  remark  that  the  transition  in  structure  from  one  region  to 


660  THE  BKAIN. 

another  is  very  gradual,  not  sharp  and  distinct,  and  is  perhaps  especially 
gradual  in  passing  from  the  motor  region  backward  to  the  occipital  region. 
It  is  not  possible  to  recognize  histologically  the  limit,  for  instance,  of  the 
motor  region  as  determined  experimentally. 

In  special  regions  of  the  brain,  for  instance  in  the  olfactory  bulb,  of  which 
we  shall  speak  later  on,  very  great  modifications  of  the  general  plan  may  be 
observed  in  the  cortex.  We  cannot  enter  upon  these,  but  may  just  refer  to 
the  cornu  ammonis  or  hippocampus.  At  the  ventral  end  of  the  temporal 
lobe  the  gyrus  hippocampi,  the  structure  of  whose  cortex  follows  the  general 
plan,  is  thrust  inward  so  as  to  project  into  the  cavity  of  the  descending  horn 
of  the  lateral  ventricle,  forming  the  ridge-like  prominence  known  by  the 
above  name.  The  substance  of  the  cornu  ammonis  is  therefore  cortical  sub- 
stance covered  on  the  side  of  the  ventricle  by  a  thin  prolongation  of  the 
central  white  matter,  which  is  in  turn  covered  by  the  ependyma  lining  the 
ventricle.  A  vertical  section  of  this  substance  shows  that  while  the  fifth 
and  fourth  layers  are  reduced  to  small  dimensions,  the  third  layer,  that  of 
large  pyramidal  cells,  is  well  developed,  though  narrow.  The  cells  are  large 
and  remarkably  long,  and  the  tapering  processes  are  arranged  so  regularly 
as  to  give  rise,  especially  in  stained  preparations,  to  a  marked  radiate  ap- 
pearance. At  the  level  of  the  second  layer  there  occurs  a  large  develop- 
ment of  capillary  bloodvessels  and  a  scarceness  of  cells,  giving  rise  to  a 
"  lacunar  "  appearance ;  and  the  first  or  molecular  layer  is  of  some  consid- 
erable thickness.  From  the  prominence  of  the  pyramidal  cells  in  this 
region,  the  third  layer  in  the  general  plan  of  the  cortex  has  sometimes  been 
spoken  of  as  the  "  formation  of  the  coruu  ammonis." 

§  565.  In  the  present  state  of  knowledge  it  is  impossible  to  come  to  any 
satisfactory  conclusion  concerning  the  meaning  of  the  variety  and  arrange- 
ment of  the  cells  and  other  constituents  of  the  cortex.  The  cells  with  their 
branches,  the  nerve-fibres  and  the  nerve-fibrils  form  a  network  of  gray 
matter  which  we  may  compare  with  the  gray  matter  of  the  spinal  cord 
(§  492),  but  which  is  obviously,  as  we  might  expect,  far  more  complex  than 
that  is.  We  may  conclude,  and  experimental  observation  confirms  the  con- 
clusion, that  the  large  pyramidal  cells  with  recognizable  axis-cylinder  pro- 
cesses serve  as  trophic  centres  for  the  fibres  which  appear  to  start  from  them. 
And  we  may,  though  with  less  confidence,  explain  the  large  size  of  these 
cells  in  the  motor  region,  by  the  fact  that  they  give  rise  to  fibres  of  the 
pyramidal  tract  stretching  a  long  way  from  their  origin  in  the  cell,  and 
therefore  demanding  great  nutritive  activity  on  the  part  of  the  cell.  We 
may  perhaps  also  conclude  that  these  fibres  are  efferent,  motor  fibres,  des- 
tined to  carry  impulses  from  the  cortex  to  the  peripheral,  or  at  least  distant 
parts.  And  we  may  further,  with  distinctly  less  confidence,  however,  assume 
that  the  size  of  the  cell  is  correlated  to  the  energy  which  has  to  be  expended 
in  the  discharge  of  efferent,  motor  impulses.  If  we  accept  these  conclusions 
we  must  also  bear  in  mind  that  such  cells,  with  axis-cylinder  processes  con- 
tinued on  as  fibres,  are  not  limited  to,  though  most  abundant  in  the  motor 
region,  but  are  found  in  all  regions  of  the  cortex ;  and  we  must  hence  con- 
clude that  impulses,  which  we  must  call  efferent,  proceed  from  all  parts  of 
the  cortex. 

It  is  obvious,  however,  that  the  connection  of  the  cortical  network  of 
gray  matter  with  the  fibres  of  the  white  matter  is  effected  in  part  only,  and 
that  a  small  part,  by  the  method  of  axis-cylinder  processes  definitely  pro- 
longed from  the  cell  substance  of  cells.  A  part,  and  probably  a  greater  part, 
of  the  fibres  sweeping  up  from  the  subjacent  white  matter,  whether  they  be 
fibres  of  the  pedal  and  tegrnental  systems  or  callosal  or  "  association  "  fibres, 
end  in  the  gray  matter  in  some  other  way  than  by  bodily  being  continued 


ON  VOLUNTARY  MOVEMENTS.  661 

in  the  cell  substance  of  cells;  they  plunge  into  and  break  up  within  the 
network,  of  which  fibrils  no  less  than  cells  form  a  conspicuous  part ;  and  we 
may  here  repeat  the  remark  which  we  made  in  speaking  of  the  cerebellum 
concerning  the  actual  continuity  of  the  elements  of  the  network.  Moreover, 
besides  the  vertical  fibres  obviously  coming  from  the  subjacent  white  matter, 
we  have  in  this  gray  matter  to  deal  with  the  fibres  of  horizontal  and  other 
directions,  which  may  come  from  white  matter  not  far  off,  but  which  may 
come  from  some  neighboring  gray  matter ;  our  present  knowledge  will  not 
enable  us  to  settle  this  point. 

In  the  spinal  cord  we  were  able  to  divide  all  the  fibres  into  afferent  and 
efferent  respectively  ;  though  even  here  we  met  with  some  difficulty.  Deal- 
ing with  the  cerebral  cortex,  which,  as  we  have  already  seen,  is  certainly 
especially  concerned  in  voluntary  movements  and  in  the  development  of  full 
sensations,  we  may  be  tempted  to  consider  the  fibres  connected  with  the  gray 
matter  as  similarly  divisible  into  motor  and  sensory  ;  and  we  may  go  on  to 
suppose  that  the  fibres  joining  the  cortex  as  axis-cylinder  processes  of  recog- 
nizable cells  are  motor  fibres,  and  that  all  the  other  fibres  joining  the  gray 
matter  in  some  other  way  are  sensory  fibres.  But  in  doing  so  we  are  going 
beyond  our  tether;  in  all  probability  the  nervous  processes  going  on  in  the 
cortex  are  far  too  complex  to  permit  such  a  simple  classification  of  the 
functions  of  fibres  as  that  into  motor  and  sensory ;  and  any  attempt  to 
arrange  either  fibres  or  regions  of  the  cortex  as  simply  motor  or  sensory  is 
probably  misleading.  But  we  shall  have  to  return  to  these  matters  when  we 
deal  with  the  functions  of  the  cortex. 

ON  VOLUNTARY  MOVEMENTS. 

§  566.  When  we  examine  ourselves  we  recognize  certain  of  our  move- 
ments as  "  voluntary  ;"  we  say  that  we  carry  them  out  by  an  effort  of  the 
"  will."  And  when  we  witness  the  movements  of  other  people  or  of  animals 
we  regard  as  also  voluntary  such  of  those  movements  as  by  their  characters 
and  by  the  circumstances  of  their  occurrence  seem  to  be  carried  out  in  the 
same  way  as  our  own  voluntary  movements.  Even  in  the  case  of  some  of 
our  own  movements  we  are  not  always  clear  whether  they  are  really  volun- 
tary or  no ;  and  in  the  case  of  other  people  and  of  animals  it  is  still  more 
difficult  to  decide  the  question.  It  would  be  out  of  place  to  attempt  to  dis- 
cuss here  how  voluntary  movements  really  differ  from  involuntary  move- 
ments, or,  in  other  words,  what  is  the  nature  of  the  will ;  we  must  be  content 
to  take  a  somewhat  rough  use  of  the  words  "  voluntary,"  "  volitional,"  and 
"  will "  as  a  basis  for  physiological  discussion.  We  may,  however,  remark  that 
as  far  as  the  muscular  side  of  the  act,  if  we  may  use  such  an  expression,  is 
concerned,  a  voluntary  movement  does  not  differ  in  kind  from  an  involun- 
tary movement.  It  is  perfectly  true  that  a  skilled  man  may  by  practice 
learn  to  execute  muscular  manoeuvres  which  he  would  not  have  learned  to 
execute  had  not  intelligent  volition  been  operative  within  him  ;  but  our  own 
experience  teaches  us  that  many  more  or  less  intricate  movements  which  have 
undoubtedly  been  learned  by  help  of  the  will  may  be  carried  out  under  cir- 
cumstances of  such  a  kind  that  we  feel  compelled  to  regard  them  as,  at  the 
time,  involuntary ;  and  it  may  at  least  be  debated  whether  every  movement 
which  we  can  carry  out  by  an  effort  of  the  will  may  not  appear,  under  ap- 
propriate circumstances,  as  part  of  an  involuntary  act.  In  the  case  of  the 
lower  animals,  in  the  frog  deprived  of  its  cerebral  hemispheres,  for  instance, 
we  have  seen  that  voluntary  differ  from  involuntary  movements,  not  by 
their  essential  nature,  but  by  the  relation  which  their  occurrence  bears  to 
circumstances.  We  have,  therefore,  to  seek  for  the  distinction  between 


662  THE  BEAIN. 

voluntary  and  involuntary,  not  in  the  coordination  of  the  muscular  and 
nervous  components  of  a  movement,  but  in  the  nature  of  the  process  which 
starts  the  whole  act. 

The  histories  related  in  a  preceding  section,  of  various  animals  deprived 
of  their  cerebral  hemispheres,  while  they  have  further  shown  the  difficulty 
of  drawing  a  sharp  line  between  the  presence  and  absence  of  volition,  such  as 
when  we  appeal  to  our  own  consciousness  we  seem  able  to  draw,  have  taught 
us  that  in  a  broad  sense  the  presence  of  volition  is,  in  the  higher  vertebrate, 
dependent  on  the  possession  of  the  cerebral  hemispheres ;  and  we  have  now 
to  inquire  what  we  know  concerning  the  way  in  which  the  cerebral  cortex— 
for  this,  as  we  have  seen,  is  the  important  part  of  the  cerebral  hemisphere — 
by  the  help  of  other  parts  of  the  nervous  system  carries  out  a  voluntary 
movement. 

§  567.  With  this  view  we  may  at  once  turn  to  the  results  of  experi- 
mental interference  with  the  cortex.  When  the  surface  of  the  brain  is  laid 
bare  by  removal  of  the  skull  and  dura  mater,  mechanical  stimulation  of  the 
cortex  produces  little  or  no  effect,  thus  affording  a  contrast  with  the  results 
of  mechanically  stimulating  other  portions  of  the  brain,  or  other  nervous 
structures.  And  for  a  long  time  the  cortex  was  spoken  of  as  insensible  to 
stimulation.  When,  however,  the  electric  current  is  employed,  either  the 
make  and  break  of  the  constant  current  or  the  more  manageable  inter- 
rupted current,  very  marked  results  follow.  It  is  found  that  certain  move- 
ments follow  upon  electric  stimulation  of  certain  regions  or  areas.  The  re- 
sults, moreover,  differ  in  different  animals.  It  will  be  convenient  to  begin 
with  the  dog,  on  which  animal  the  observations  of  this  kind  were  first  con- 
ducted. 

When  the  surface  of  the  dog's  brain  is  viewed  from  the  dorsal  surface  a 
short  but  deep  sulcus  it  seen  toward  the  front,  running  outward  almost  at 
right  angles  from  the  great  longitudinal  fissure ;  this  is  called  the  crucial 
sulcus  (Fig.  147),  the  gyrus  or  convolution  in  front  and  behind  it,  and 
sweeping  around  its  end,  being  called  the  sigmoid  gyrus.  It  will  hardly 
be  profitable  to  discuss  here  either  the  homology  of  this  sulcus  or  the 
names  of  the  other  sulci  and  convolutions  of  the  dog's  brain.  We  men- 
tion this  sulcus  because  it  is  found  that  stimulation  of  the  cortex  in  a  region 
which  may  be  broadly  described  as  that  of  the  neighborhood  of  this  crucial 
sulcus  gives  rise  to  movements  of  various  parts  of  the  body,  whereas  no  such 
movements  result  from  stimulation  of  the  extreme  frontal  region  in  front 
of  the  area  around  the  crucial  sulcus,  or  from  stimulation  of  the  occipital 
region  behind  this  area.  Certain  exceptions  may  be  made  to  this  broad 
statement,  but  these  it  will  be  best  to  discuss  in  reference  to  the  more 
highly  developed  monkey. 

The  region  of  the  cortex  in  the  neighborhood  of  the  crucial  sulcus  may 
then  be  termed  an  "  excitable  "  or  "  motor  "  region,  inasmuch  as  stimulation 
of  this  region  leads  to  movements  carried  out  by  skeletal  muscles,  while 
stimulation  of  other  regions  does  not.  Further,  stimulation  of  particular 
districts  or  areas  of  the  region  leads  to  particular  movements  carried  out 
by  particular  muscles.  For  instance,  stimulation  of  the  more  median 
parts  of  the  gyrus  behind  the  crucial  sulcus  (Fig.  147,  C)  leads  to  move- 
ments of  the  hind  limb,  whereas  stimulation  of  the  lateral  part  or  outer  end 
of  the  same  gyrus  leads  to  movements  of  the  fore  limb,  and  we  may  here 
distinguish  between  an  area,  stimulation  of  which  (Fig.  147,  E)  leads  to 
flexion  of  the  fore  limb,  and  an  area  (Fig.  147,  A),  stimulation  of  which 
leads  to  extension  of  the  same  limb.  In  a  similar  way  stimulation  of  other 
areas  within  the  "  motor  "  region  leads  to  movements  of  this  kind  or  of  that 
kind  of  the  tail,  of  the  eyes,  of  the  mouth,  of  other  parts  of  the  face,  of  the 


ON   VOLUNTARY   MOVEMENTS. 


663 


tongue,  and  so  on.  Obviously  in  the  dog  this  region  of  the  cortex  has  con- 
nections with  the  skeletal  muscles  which  do  not  obtain  between  other  regions 
of  the  cortex  and  those  muscles ;  and  further,  the  region  in  question  is  topo- 
graphically differentiated,  so  that  certain  areas  or  districts  of  this  region  are 
specially  connected  with  certain  skeletal  muscles  or  groups  of  muscles.  We 
may  speak  of  a  "  localization  of  function  "  in  this  region  as  compared  with 

FIG.  147. 


The  Areas  of  the  Cerebral  Convolutions  of  the  Dog,  according  to  Hitzig  and  Fritsch.  (1)  B, 
The  area  for  the  muscles  of  the  neck.  (2)  A,  The  area  for  the  extension  and  adduction  of  the 
fore  limb.  (4)  E,  The  area  for  the  flexion  and  rotation  of  the  fore  limb.  (4)  C,  The  area  for  the 
hind  limb.  Running  transversely  toward  and  separating  1  and  2  from  3  and  4  is  seen  the  crucial 
sulcus,  (5)  D,  The  facial  area. 

other  regions  of  the  cortex,  and  in  the  several  areas  within  the  region  as 
compared  with  each  other. 

The  muscles  which  are  thus  thrown  into  contraction  are  the  muscles  of 
the  opposite  side  of  the  body.  When  "  the  fore-limb  area,"  as  we  may  call 
it,  of  the  right  hemisphere  is  stimulated,  it  is  the  left  fore  limb  which  is 
moved;  and  so  with  the  other  areas;  it  is  only  in  exceptional  cases,  as  in 
certain  movements  of  the  eyes,  that  the  effect  is  bilateral ;  a  movement 
confined  to  the  same  side  as  that  stimulated  is  never  witnessed. 

The  results  are  most  clear  when  the  current  employed  as  a  stimulus  is  not 
stronger  than  is  just  sufficient  to  produce  the  appropriate  movement  (roughly 
speaking,  a  current  just  perceptible  to  the  tongue  of  the  operator  is  in  ordi- 
nary cases  a  useful  one),  and  when  the  cortex  is  in  good  nutritive  condition. 
In  any  experiment  the  results  obtained  by  the  earlier  stimulations,  soon  after 
the  cortex  has  been  exposed,  are  the  best ;  after  repeated  stimulations  the 
surface  is  apt  to  become  hyperamic,  and  it  is  then  frequently  observed  that 
the  movements  resulting  from  the  stimulation  of  a  particular  area  are  not 
confined  to  the  appropriate  muscles,  but  spread  to  the  corresponding  muscles 
of  the  opposite  side,  then  to  muscles  connected  with  other  cortical  areas,  and 
at  last  to  the  muscles  of  the  body  generally ;  at  the  same  time  the  move- 
ments lose  their  distinctive  purposeful  character  and  the  animal  is  thrown 
into  convulsions  of  an  epileptiform  kind.  It  not  unfrequently  happens  that 
an  experiment  has  to  be  stopped  in  consequence  of  the  onset  of  these  epilep- 
tiform convulsions.  The  response  of  movement  to  stimulation  may  be  ob- 
served while  the  animal  is  under  the  moderate  influence  of  an  anaesthetic, 
but  a  too  profound  anaesthesia  lessens  or  annuls  the  effects. 


664  THE  BRAIN. 

In  order  to  carry  out  a  closer  analysis  of  the  phenomena  it  is  desirable 
to  watch  or  record  the  contraction  of  a  particular  group  of  muscles,  or  per- 
haps better  still  a  particular  muscle,  e.  g.,  the  area  for  extension  of  the  hind 
limb  may  be  studied  by  help  of  the  extensor  digitorum  communis  of  the 
limb.  When  this  is  done,  the  following  important  facts  may  be  observed  : 
The  area  of  cortex  having  been  found  which  gives  the  best  movements,  and 
the  stimulus  being  no  stronger  than  is  necessary,  isolation  of  the  area  from 
its  lateral  surroundings  by  a  circular  incision  carried  to  some  little  depth 
will  not  prevent  the  development  of  contractions  in  the  muscle ;  but  these 
do  cease,  even  without  the  circular  incision,  if  by  a  horizontal  section  the 
gray  cortex  is  separated  from  the  subjacent  white  matter.  After  removal 
of  the  cortex,  stimulation  of  the  white  matter  underlying  the  area  produces 
the  appropriate  contraction ;  not  only,  however,  is  a  stronger  stimulus 
necessary,  but  also  the  latent  period,  that  is  the  time  intervening  between 
the  beginning  of  the  application  of  the  stimulating  current  and  the  begin- 
ning of  the  muscular  contraction,  is  appreciably  shortened.  The  appropriate 
contractions  not  only  appear  when  the  white  matter  immediately  below  the 
cortex  is  stimulated,  but  by  making  successive  horizontal  sections  and  stim- 
ulating each  in  turn,  the  effect  may,  so  to  speak,  be  traced  through  the  cen- 
tral white  matter  of  the  hemisphere  down  to  the  internal  capsule.  We  may 
conclude  from  these  results,  that  when  the  current  is  applied  to  the  surface 
of  the  cortex,  certain  parts  of  certain  structures  in  the  gray  matter  are 
stimulated,  the  process  having  a  marked  latent  period,  and  that  as  the  out- 
come of  the  changes  induced  in  the  gray  matter,  impulses  pass  along  the 
fibres  leading  down  from  the  gray  matter  to  the  internal  capsule  and  so  by 
the  pedal  system  of  fibres  to  the  spinal  cord  and  motor  spinal  roots.  The 
anatomical  considerations  advanced  in  a  previous  section  lead  us  to  suppose 
that  the  fibres  in  question  belong  to  the  great  pyramidal  tract,  on  which  we 
have  so  much  insisted  ;  and,  as  we  shall  see,  all  our  knowledge  confirms  this 
view. 

It  must  not,  however,  be  supposed  that  the  several  areas  stimulation  of 
which  produces  each  its  distinctive  movement,  are  in  the  dog  sharply  de- 
fined from  each  other ;  when  the  term  area  for  extension  of  the  hind  limb 
is  used,  it  must  not  be  supposed  that  the  area  can  be  defined  by  an  outline 
within  which  stimulation  produces  nothing  but  extension  of  the  hind  limb, 
and  outside  which  stimulation  never  produces  extension  of  the  hind  limb. 
All  that  is  meant  is  that  extension  of  the  hind  limb  is  the  salient  and  strik- 
ing result  of  stimulating  the  area.  When  we  study  the  various  movements, 
and  especially  perhaps  when  we  study,  by  help  of  a  graphic  record,  the  con- 
tractions of  various  individual  muscles  resulting  from  the  stimulation  of 
various  parts  of  the  motor  region,  we  find  not  only  that  the  areas  for 
particular  movements  or  particular  muscles  are  very  diffuse,  but  that  the 
several  areas  largely  overlap  each  other.  If,  for  instance,  we  were  to  map 
out  on  the  same  diagram  the  several  areas  belonging  to  four  or  five  muscles 
of  different  parts  of  the  body,  such  as  the  extensors  of  the  digits  of  the  fore 
and  of  the  hind  limb,  the  flexors  of  the  same,  and  the  orbicular  muscle  of 
the  eyelid,  that  is  to  say,  the  several  areas  within  which  in  turn  stimulation 
of  the  cortex  produced  contraction  of  the  particular  muscle,  the  overlap- 

fing  would  be  so  great  that  the  whole  figure  would  appear  highly  confused. 
n  a  similar  way  the  excitable  motor  region  as  a  whole  would  gradually 
merge  into,  be  broken  up  into,  the  unexcitable  frontal,  occipital,  and  tem- 
poral regions,  in  front,  behind,  and  below.     In  other  words,  the  localization 
in  the  cortex  of  the  dog  is  to  a  marked  degree  imperfect. 

In  this  respect  the  dog,  corresponding  to  its  position  in  the  animal  hier- 
archy, is  intermediate  between  such  animals  as  the  rabbit,  the  bird,  and  the 


ON  VOLUNTAKY  MOVEMENTS.  665 

frog,  on  the  one  hand,  and  the  more  highly  developed  monkey  on  the  other  ; 
and  that  is  one  reason  why  we  have  taken  the  dog  first  and  dwelt  so  long 
upon  it.  In  the  rabbit,  a  similar  localization  may  be  observed,  but  far  less 
definite,  far  more  diffuse  ;  it  becomes  still  less  in  the  bird,  and  is  hardly 
recognizable  in  the  frog.  It  will  not  be  profitable  to  dwell  on  the  details  of 
the  phenomena  of  these  lower  animals;  but  the  phenomena  of  the  monkey, 
leading  up  as  they  do  to  those  of  man,  call  for  special  notice. 

§  568.  When  in  a  monkey,  in  an  individual,  for  instance,  belonging  to 
the  genus  Macacus,  the  surface  of  the  cerebrum  is  explored  with  reference 
to  the  effects  of  electric  stimulation,  it  is  found  that  when  the  current  is 
applied  to  the  precentral  or  ascending  frontal  and  the  post-central  or 
ascending  parietal  convolutions  which  lie  respectively  in  front  of  and  behind 
the  important  central  fissure  or  fissure  of  Rolando  (ef.  Fig.  148),  movements 
of  the  fore  limb  follow.  The  "motor  area  for  the  fore  limb"  thus  discov- 
ered is  more  circumscribed  and  definite  than  is  the  corresponding  area  in 
the  dog.  Its  outline  (Fig.  149)  is  roughly  that  of  a  truncated  triangle  bi- 
sected by  the  central  fissure,  with  the  broad  base  at  some  distance  from  the 
mesial  line,  and  the  truncated  apex  reaching  on  the  lateral  surface  of  the 
hemisphere  to  a  well-marked  bend  in  the  lower  part  of  the  central  fissure. 
Behind,  it  reaches  as  far  as  the  intra-parietal  fissure  which  somewhat  sharply 
defines  its  hind  border,  and  in  front  it  ceases  no  less  definitely  at  some  little 
distance  behind  the  precentral  fissure.  Further  examination  shows  that  the 
whole  area  is  divided  into  areas  corresponding  to  movements  of  particular 
parts  of  the  forearm,  and  that  these  are  arranged  in  a  definite  relation  to 
each  other.  In  the  more  dorsal  part  of  the  area,  at  the  base  of  the  triangle, 
stimulation  produces  movements  of  the  shoulder  (Fig.  149)  ;  if  the  elec- 
trodes be  shifted  ventrally,  movements  of  the  elbow  make  their  appear- 
ance ;  if  still  more  ventrally,  movements  of  the  wrist  come  in,  and  these  are 
in  turn  succeeded  ventrally  by  movements  of  the  digits  generally,  of  the 
forefinger,  and  lastly  of  the  thumb.  A  very  striking  experiment  may  be 
made  by  applying  a  current  of  suitable  strength,  first  at  the  lower  ventral 
border  of  the  area,  and  then  gradually  advancing  upward  toward  the  mesial 
line;  the  thumb  is  moved  first,  then  the  forefinger,  then  the  rest  of  the 
digits,  then  the  wrist,  next  the  elbow,  and  lastly  the  shoulder.  Further,  in 
certain  parts  of  the  area  the  resulting  movement  is  flexion  of  the  appropri- 
ate segment  of  the  limb,  in  other  parts  extension,  in  certain  parts  abduc- 
tion, in  other  parts  adduction,  and  so  on. 

Similar  exploration  shows  that  the  "  area  for  the  hind  limb  "  lies  on  the 
median  side  of  the  area  for  the  fore  limb,  stretching  besides  on  to  the  mesial 
surface  along  the  marginal  convolution  which  forms  the  dorsal  portion  of 
the  wall  of  the  great  longitudinal  fissure  ;  it  reaches  as  far  back  as  the  intra- 
parietal  sulcus,  and  is  succeeded  in  front  by  the  "  area  for  the  trunk  "  (Fig. 
150).  Within  this  general  area  for  the  hind  limb  we  may  similarly  distin- 
guish special  areas  for  the  hip  (Figs.  149,  150)  in  the  front  portion,  for  the 
knee  and  ankle  behind  this,  and  for  the  digits  still  further  backward,  the 
area  for  the  great  toe  being,  however,  in  front  of  the  area  for  the  other 
dig-its. 

In  front  of  the  areas  for  the  limbs  and  trunk,  on  the  median  dorsal  sur- 
face, dipping  down  into  the  mesial  surface  along  the  marginal  convolution 
(Fig.  150)  and  reaching  laterally  on  the  lateral  dorsal  surface  to  the  dorsal 
extremity  of  the  precentral  sulcus  (Fig.  149),  is  the  "area  for  the  head," 
that  is  to  say,  for  the  movements  of  the  head  brought  about  by  contractions 
of  the  muscles  of  the  neck. 

Ventral  to  this  again,  in  front  of  the  precentral  sulcus,  is  the  "  area  for 
the  eyes,"  that  is  to  say,  for  contractions  of  the  ocular  muscles  ;  and  behind 


666 


THE  BKAIN. 


the  precentral  suleus,  ventral  to  the  arm  area  lies  a  small  area  for  move- 
ments of  the  eyelids,  brought  about  by  contractions  of  the  orbicularis  mus- 
cle. Ventral  to  this  again  is  the  "  area  for  the  face,"  in  which  we  may  dis- 
tinguish an  area  for  the  mouth,  that  is  an  area  stimulation  of  which  pro- 
duces changes  in  the  buccal  orifice,  opening,  shutting,  drawing  to  one  side, 
etc.,  and  an  area  for  movements  of  the  tongue.  These  two  areas  reach  down- 
ward to  the  fissure  of  Sylvius  and  backward  to  the  line  of  the  intra-parietal 
sulcus.  In  front  of  them,  occupying  all  the  ventral  part  of  the  precentral 
convolution  and  reaching  forward  as  far  as  the  precentral  sulcus,  where  it 

FIG.  148. 


Outline  of  Brain  of  Monkey  (Macacvs)  to  show  Principal  Sulci  (Fissures)  and  Gyri  (Convolu- 
tions). (Sherrington,  after  Horsley  and  Schafer.)  Natural  size.  The  brain  figured  is  the  same  as 
that  in  Fig.  14'J,  and  the  two  figures  should  be  consulted  together.  Over  each  sulcus.  purposely 
printed  very  thick,  the  name  is  written  in  SMALL  CAPITALS,  over  each  gyrus  in  italics,  x  indicates 
the  small  depression,  hardly  to  be  called  a  sulcus,  which  is  supposed  to  be  homologous  with  the 
superior  frontal  sulcus  of  man  ;  and  w,  y,  z,  similarly  indicate  sulci  whose  homologies  are  not 
certain.  For  some  synonyms  see  Figs.  152,  154. 

meets  the  area  for  the  eyes,  lies  an  area  stimulation  of  which  produces 
movements  of  the  pharynx  or  larynx,  as  well  as  of  the  mouth  or  face,  and 
which  may  be  divided  into  areas  for  mastication,  for  swallowing,  and  for  the 
production  of  the  voice. 

We  might  speak  of  these  several  areas  in  another  way  by  referring  to  the 
nerves  concerned  in  carrying  out  the  several  movements,  though  in  doing 


ON  VOLUNTAKY   MOVEMENTS. 


667 


so  we  must  remember  that  there  is  not  an  exact  correspondence  between  the 
relative  position  of  a  muscle  along  the  axis  of  the  body  or  along  the  axis 
of  a  limb  and  the  relative  position  along  the  cerebro-spinal  axis  of  the 
nerve  or  nerves  governing  the  muscle.  We  may,  however,  adopting  this 
method,  note  that  the  sacral  and  lumbar  nerves  are  represented  by  the 
most  mesial  portion  of  the  whole  motor  area  and  by  the  hind  division  of 
this  mesial  portion  ;  that  the  lumbar  and  thoracic  nerves  are  represented  by 

FIG.  149. 


Left  Hemisphere  of  the  Cerebrum  of  Monkey  (Macacus),  Viewed  from  its  Left  Side  and  from 
Above.  (Sherrington,  after  Horsley  and  Beevor.)  Natural  size.  The  figure  shows  the  positions 
of  the  portions  of  the  cortex  concerned  with  movement  of  various  parts,  and  with  the  senses  of 
sight,  smell,  and  hearing,  The  cortical  area  connected  with  the  movements  of  the  leg  is  shaded 
vertically  across,  that  with  the  movements  of  the  arm  horizontally,  and  that  with  the  movements 
of  the  trunk  in  a  slanting  direction  ;  the  area  connected  with  the  movements  of  the  head  (neck), 
face,  and  eyes  is  dotted.  The  course  of  the  chief  fissures  is  indicated  by  single  lines. 

the  front  division  of  the  same  mesial  portion  ;  that  the  upper  thoracic  with 
the  lower  cervical  nerves  belong  to  a  region  lying  lateral  to,  mid  the  upper 
cervical  nerves  to  one  lying  in  front  of,  the  preceding  area ;  and,  lastly, 
that  the  remaining  lateral  and  ventral  portions  of  the  whole  motor  region 
appertain  to  the  cranial  nerves.  But  the  topographical  differentiation  does 
not  come  out  so  clearly  by  this  method  as  by  that  of  taking  for  our  guide 
distinctive  movements  of  the  several  parts  of  the  body. 

It  will  be  observed  that  all  these  areas  taken  together,  represented  by 


668 


THE  BRAIN. 


the  portion  of  Figs.  149,  150  shaded  in  one  way  or  another,  occupy  chiefly 
the  parietal  region  of  the  cerebral  surface,  though  they  also  reach  into  the 
frontal  region.  Stimulation  of  the  frontal  region  in  front  of  this  motor 
area  or  of  the  occipital  region  behind,  whether  on  the  lateral  or  on  the 
mesial  surface,  or  of  the  temporal  region,  whether  also  on  the  latter  or  on  the 
mesial  surface,  or  of  the  gyrus  fornicatus  (Fig.  150)  connecting  the  frontal 
and  occipital  regions  on  the  mesial  surface,  and  running  ventral  to  the 
marginal  gyrus,  does  not  give  rise  to  movements ;  or,  to  be  more  exact,  does 
not  give  rise  to  movements  comparable  to  those  just  described  as  resulting 
from  stimulation  of  various  parts  of  the  motor  region.  Movements  do  take 
place  when  certain  parts  of  the  occipital  or  of  the  temporal  region  are  stim- 
ulated, but  these  are  not  only  feeble  and  experimentally  uncertain,  but 
appear  to  be  of  a  different  nature  from  those  resulting  from  stimulation  of 


FIG.  150. 


Po.F 


Mesial  Aspect  of  the  Left  Half  of  the  Brain  of  Macacus,  displayed  by  Section  in  the  Median 
Sagittal  Plane  and  Removal  of  the  Cerebellum.  (Sherrington,  after  Horsley  and  Beevor.)  Natural 
size.  The  hatched  and  stippled  parts  of  the  surface  show  the  regions  of  the  cortex  connected 
with  movements  of  the/oo<,  knee,  hip,  tail,  trunk,  and  neck  respectively.  The  several  positions  of 
the  areas  of  cortex  connected  with  vision  and  smell  and  with  cutaneous  sensation  are  indicated  by 
the  appropriate  words.  The  plane  of  section  has  passed  through  the  corpus  callosum,  cc.,  cc.,  cc., 
and  through  the  anterior  commissure,  c.,  sparing  the  left  pillar  of  the  lornix,  F.;  behind  it  has 
bisected  the  anterior  part  of  the  pons,  laying  open  the  aqueduct,  Aq.  (iter  a  tertio  ad  quartum 
ventrieulum) ;  Pons,  the  left  half  of  the  pons  in  frontal  section ;  Op.,  the  optic  commissure  cut 
across  ;  in,  the  root  of  the  third  cranial  nerve  ;  FR.,  the  frontal  pole  ;  Oc.,  the  occipital  pole ; 
On.,  the  cuneus  ;  Pen  ,  the  precuneus  ;  Gfn,  Gfn,  Gfn,  the  gyrus  fornicatus ;  the  unlettered  fissure 
seen  to  form  the  upper  boundary  of  this  gyrus  in  its  supra-callosal  part  is  the  calloso-marginal ; 
Pof,  the  parieto-occipital  fissure. 

the  motor  region  ;  it  will  be  convenient  to  speak  of  the  nature  and  meaning  of 
this  kind  of  movement  when  we  come  to  discuss  the  development  of  sensations. 
§  569.  It  is  obvious  from  the  foregoing  that  the  mechanism  for  the 
development  of  these  movements  of  cerebral  origin  are  far  more  highly 
differentiated  in  the  monkey  than  in  the  dog.  But  even  in  the  monkey 
(Macac-us  and  allied  forms)  the  differentiation  is  still  very  incomplete.  If 
we  explore,  for  instance,  the  area  for  the  wrist,  we  find  that  its  limits  are 
ill-defined.  In  some  parts  of  the  area  we  obtain  movements  of  the  wrist 
only,  but  in  other  parts  of  the  area  stimulation  produces  not  only  move- 
ments of  the  wrist,  but  also  of  the  shoulder  or  of  the  digits,  or  of  the  neck ; 
and  so  with  the  other  areas. 


ON  VOLUNTARY  MOVEMENTS.  669 

If,  however,  not  a  Maeacus  or  other  ordinary  monkey,  but  the  more 
highly  developed  orang-outang  be  taken  as  the  subject  of  experiments,  the 
differentiation  is  found  to  be  distinctly  advanced ;  the  several  areas  are 
more  sharply  defined,  and  what  is  important  to  note,  the  respective  areas 
tend  to  be  separated  from  each  other  by  portions  of  cortex  stimulation  of 
which  gives  rise  to  no  movement  at  all. 

The  opportunities  of  stimulating  the  cortex  of  man  himself  have  been 
few  and  far  between,  and  have,  for  the  most  part,  been  conducted  under 
unfavorable  circumstances ;  but  as  far  as  the  results  so  obtained  go,  they 
show  that  the  topographical  distribution  of  areas  for  the  several  movements 
is  carried  out  on  the  same  plan  as  in  the  monkey  (we  are  purposely  confin- 
ing ourselves  now  to  the  results  of  artificial  stimulation)  ;  and,  moreover, 
justify  the  conclusion,  which  a  priori  reasons  would  lead  us  to  adopt,  that 
in  man  the  differentiation  is  advanced  still  further  than  in  the  monkey. 

Thus,  when  we  survey  a  series  of  brains  in  succession,  from  the  more 
lowly  frog,  through  the  bird,  the  rabbit,  the  dog,  and  other  lower  mammals 
up  to  the  monkey,  the  anthropoid  ape,  and  so  to  man  himself,  we  find  an 
increasing  differentiation  of  the  cerebral  cortex,  by  which  certain  areas  of 
the  cortex  are  brought  into  special  connection  with  certain  skeletal  or  other 
muscles  in  such  a  way  that  stimulation  of  a  particular  portion  of  the  gray 
matter  gives  rise  to  a  particular  movement,  and  to  that  alone. 

§  570.  In  treating  of  the  structure  of  the  brain  we  spoke  (§  545)  of 
the  pyramidal  tract  as  starting  from  the  motor  region  of  the  cortex ;  and  it 
is  obvious  that  the  fibres  of  this  tract  must  be  concerned  in  the  develop- 
ment of  the  movements  which  we  have  just  described.  When  the  move- 
ments are  brought  about  by  stimulation  of  the  fibres  in  some  part  of  their 
course,  in  the  internal  capsule,  for  instance,  there  can  be  no  doubt  that  the 
stimulation  starts  impulses  which  travelling  down  the  tract  to  the  origin  of 
certain  cranial  or  spinal  nerves,  in  some  way  give  rise  to  coordinate  motor 
impulses  along  the  motor  fibres  of  the  nerves ;  and  we  may  with  reason 
speak  of  the  impulses  then  passing  along  the  tract  as  motor  or  efferent  in 
nature.  When  the  stimulus  is  applied  direct  to  the  cortex,  we  may  assume 
that  processes,  started  in  the  gray  matter,  eventuate  in  similarly  efferent 
impulses  along  the  fibres  of  the  tract.  All  the  evidence  leads  us  to  regard 
this  tract  as  an  efferent  tract. 

When  the  spinal  cord  is  divided  in  the  lower  dorsal  region  and  the  elec- 
trodes of  an  electrometer  are  brought  into  connection  with  the  transverse 
cut  surface  and  with  some  point  of  the  longitudinal  surface  above,  the 
electrometer  gives  evidence  of  currents  of  action  (manifested  as  negative 
variations  of  a  demarcation  current  or  current  of  rest  (§  67)  whenever  the 
motor  area  of  the  hind  limb  is  stimulated,  but  not  when  other  parts  of  the 
cortex  are  stimulated.  We  have  already  said  that  stimulation  of  any  part 
of  the  motor  region  may,  under  abnormal  conditions,  give  rise  to  general 
epileptiform  convulsions ;  when  these  occur  during  such  an  experiment  as 
the  above,  currents  of  action  manifest  themselves  in  the  lower  dorsal  cord, 
whether  the  stimulation  giving  rise  to  the  convulsions  be  applied  to  the  area 
for  the  hind  limb  or  to  any  part  of  the  motor  region.  It  has  been  further 
observed  that  the  currents  of  action  developed  within  the  spinal  cord  tally 
in  a  very  exact  manner  with  the  muscular  movements.  The  convulsions 
begin  with  a  sustained  "  tonic "  contraction  of  the  muscles,  and  the  elec- 
trometer shows  a  similar  sustained  current  of  action ;  this  is  followed  by 
rhythmic  movements  of  the  muscles,  accompanied  by  corresponding  rhyth- 
mic movements  of  the  mercury  of  the  electrometer.  Without  insisting  too 
much  on  the  exact  interpretation  of  these  results,  we  may  take  them  as  at 
least  showing  that,  when  the  motor  region  of  the  cortex  is  excited,  nervous 


070  THE   BRAIN. 

impulses  accompanied  by  "  currents  of  action  "  pass  downward  along  the 
fibres  of  the  pyramidal  tract. 

The  results  of  stimulating  the  fibres  of  the  tract  in  their  course  through 
the  corona  radiata  and  the  internal  capsule,  and  the  results  obtained  by 
studying  the  degenerations  following  upon  injury  to  or  removal  of  the  several 
parts  of  the  cortical  motor  region,  agree  in  marking  out  the  paths  taken  by 
the  several  constituents  of  the  tract  through  the  central  white  riiatter  of  the 
hemisphere,  the  corona  radiata,  and  the  capsule.  Comparing  Figs.  149, 150, 
with  Figs.  144,  145,  and  146,  it  will  be  seen  that  the  portions  of  the  tract 
destined  for  the  cranial  nerves,  and  so  for  the  movements  of  the  eyes,  the 
mouth,  face,  tongue,  pharynx,  and  larynx,  starting  from  the  ventral  parts  of 
the  more  frontal  district  of  the  motor  region,  take  up  their  position  at  the 
knee  of  the  internal  capsule ;  and  the  portion  destined  for  those  upper 
cervical  nerves  which  carry  out  movements  of  the  head  through  the  muscles 
of  the  neck,  starting  from  the  extreme  frontal  and  dorsal  parts  of  the  area, 
is  also  apparently  directed  to  the  knee  of  the  capsule.  The  rest  of  the  tract, 
starting  from  the  part  of  the  area  lying  at  once  behind  and  mesial  to  the 
above,  occupies  in  the  capsule  a  position  posterior  to  them  in  the  hind  limb 
of  the  capsule  ;  and  it  will  be  observed  that  the  tract  for  the  fore  limb  which 
begins  on  the  lateral  surface  of  the  tracts  for  the  trunk  and  hind  limb,  shifts 
its  course  in  relation  to  theirs,  so  that  in  the  capsule  it  is  front  of  them, 
not  lateral  to  them.  It  may  further  be  observed  that  while  in  the  tracts  for 
the  trunk  and  hind,  limb  the  same  fore-and-aft  order  which  obtains  on  the 
surface  is  reproduced  in  the  capsule,  even  apparently  to  the  strange  prece- 
dence of  the  ankle  over  the  knee,  the  order  of  the  several  elements  in  the 
fore-limb  tract  which  is  lateral  on  the  surface  becomes  regularly  fore  and  aft 
in  the  capsule.  In  the  capsule  the  several  elements  are  arranged  in  a  linear 
order,  corresponding  broadly  to  that  of  the  distribution  of  the  muscles  along 
the  longitudinal  axis  of  the  body;  on  the  cortex  they  are  disposed  in  an 
order  the  cause  of  which  is  at  present  not  very  clear,  but  which  is  probably 
determined  by  the  respective  relations  of  the  several  parts  of  the  motor 
region  to  the  functional  activity  of  the  other  parts  of  the  cortex.  In  the 
shifting  from  the  one  order  to  the  other,  the  several  constituent  fibres,  as  we 
have  said,  describe  a  somewhat  peculiar  course ;  and  when  we  remember,  as 
stated  in  §  545,  that  the  order  shown  in  Fig.  144  is  only  the  order  obtaining 
at  one  particular  level  of  the  capsule,  and  that  from  the  dorsal  beginnings 
of  the  capsule  in  the  corona  radiata  to  its  ventral  end  in  the  pes  the  capsule 
is  continually  changing  in  form,  and  its  fibres  therefore  continually  shifting 
their  relations  to  each  other,  the  whole  course  of  the  several  fibres  of  the 
tract  from  their  origin  in  the  cortex  until  they  are  gathered  up  into  the 
central  portion  of  the  pes  (Fig.  137,  Py.)  must  be  a  very  complicated  one. 

When  the  .area  of  one  hemisphere  is  stimulated,  the  movement  which 
results  is  in  most  cases  seen  on  the  other  side  of  the  body,  and  on  that  other 
side  alone.  Thus  when  the  area  for  the  fore  limb  on  the  left  hemisphere 
is  stimulated  it  is  the  right  fore  limb  which  is  moved.  This  is  in  accordance 
with  what  we  have  learned  of  the  pyramidal  tract  and  its  ultimate  entire 
decussation  before  it  reaches  the  motor  nerves,  the  decussation  either  occur- 
ring massively  as  in  the  case  of  the  crossed  pyramidal  tract,  or  in  a  more 
scattered  manner  along  the  upper  part  of  the  spinal  cord  in  the  case  of  the 
direct  pyramidal  tract ;  and,  as  we  have  seen,  there  is  a  similar  decussation 
for  such  part  of  the  pyramidal  tract  as  is  connected  with  the  cranial  nerves 
above  the  decussation  of  the  pyramids.  Except  in  the  case  of  certain  areas 
for  movements  naturally  bilateral,  of  which  we  shall  speak  presently,  the 
movement  is  normally  on  the  crossed  side,  and  on  the  crossed  side  only. 
Under  abnormal  conditions,  however,  the  limb  on  the  other  side — that  is,  of 


ON  VOLUNTARY  MOVEMENTS.  671 

the  same  side  as  the  hemisphere  stimulated — may  move  also.  But  such  an 
abnormal  movement  of  the  same  side  lias  not  the  same  characters  as  the 
proper  movement  of  the  crossed  limb.  Instead  of  being  an  orderly  coordi- 
nate movement,  it  is  a  more  simple,  either  tetanic  or  perhaps  tonic,  or 
rhythmic,  clouic,  contraction  of  the  muscles.  Obviously  its  mechanism  is 
of  a  different  nature  from  that  by  which  the  proper  movement  of  the  crossed 
limb  is  effected  ;  but  it  is  important  to  bear  in  mind  that  a  movement  of 
the  uncrossed  limb  may  take  place ;  and  further,  that  the  abnormal  condi- 
tions continuing,  similar  movements  of  an  uncoordinated  character  may 
spread  to  the  hind  limb  and  other  parts  of  the  crossed  side,  though  the  stimu- 
lation be  still  confined  to  the  arm  area,  then  to  other  parts  of  the  uncrossed 
side,  until,  as  we  have  said,  the  whole  body  is  thrown  into  epileptiform  con- 
vulsions. This  feature  must  not  be  forgotten.  In  fact,  it  may  be  fairly 
insisted  upon  that  while  we  may  speak  of  a  particular  coordinate  movement 
as  being  the  normal  outcome  of  an  ordinary  careful  stimulation  of  a  par- 
ticular area  in  a  normal  condition,  it  is  no  less  true  that  diffuse  uncoordinated 
movements,  culminating  in  general  epileptiform  convulsions,  are  the  natural 
outcome  of  the  stimulation  of  any  area  in  an  abnormal  condition.  And  in 
attempting  to  form  any  opinion  of  the  nature  of  the  first  act  we  must  bear 
the  second  in  mind. 

As  we  said  above,  the  movements  resulting  from  cortical  stimulation 
are  most  conveniently  described  in  terms  of  parts  of  the  body — of  the  arm, 
of  the  thumb,  of  the  tongue,  etc.  The  movements  of  the  same  part  may 
be  further  distinguished  by  means  of  the  nomenclature  usually  adopted  in 
speaking  of  muscular  movements,  such  as  flexion,  extension,  abduction, 
adduction,  etc. ;  so  that,  within  the  area  bearing  the  name  of  some  particular 
part — such  as  the  wrist,  for  instance — we  have  to  distinguish  an  area  for  the 
flexion  and  another  for  the  extension  of  that  joint;  and  in  like  manner  in 
reference  to  other  parts.  But  it  will  be  readily  understood  that  it  is  easier 
to  map  out  the  area  for  a  particular  part  than  to  distinguish  the  areas  cor- 
responding to  the  several  movements  of  that  part.  Hence  the  nomenclature 
usually  adopted  in  speaking  of  the  motor  region  is  one  based  on  the  parts  of 
the  body  moved,  rather  than  on  the  character  of  the  movements.  The  more 
closely,  however,  the  movements  in  question  are  studied,  the  more  probable 
it  appears  that  the  localization  which  obtains  in  the  cortex  is  essentially  a 
localization  corresponding  not  to  parts  of  the  body  or  to  nerves  or  to  mus- 
cles, but  to  movements.  In  considering  this  point  it  must  be  remembered 
how  rude  and  barbarous  a  method  of  stimulation  is  that  of  applying  electrodes 
to  the  surface  of  the  gray  matter  compared  with  the  natural  stimulation  which 
takes  place  during  cerebral  action ;  the  one  probably  is  about  as  much  like 
the  other  as  is  striking  the  keys  of  a  piano  at  a  distance  with  a  broomstick 
to  the  execution  of  a  skilled  musician.  Were  it  in  our  power  to  stimulate 
the  cortex  in  any  way  at  all  approaching  the  natural  method,  we  should,  in 
all  probability,  arrive  at  two  results  :  on  the  one  hand,  we  should  be  able  to 
produce  at  will  a  variety  of  movements  of  different  degrees  of  complexity, 
some  very  simple,  others  very  complex,  and  for  these  we  should  have  to  use 
names  suggested  by  the  characters  and  purpose  of  each  movement,  and  by 
these  alone;  on  the  other  hand,  we  should  find  very  decided  limits  to  the 
number  and  kind  of  movements  which  we  could  evoke,  limits  fixed  in  the 
case  of  each  subject  partly  by  inherited  organization,  partly  by  the  training 
of  the  individual. 

Some  such  results  of  refined  experimentation  are,  indeed,  already  fore- 
shadowed by  the  rude  results  of  our  present  rough  methods.  The  move- 
ments which  usually  follow  stimulation  of  the  motor  region,  and  which  we 
have  described  as  flexion,  etc.,  are,  so  to  speak,  the  elementary  factors  of 


672  THE  BKAIN. 

ordinary  bodily  movements,  the  detached  and  imperfect  chords  of  a  musical 
piece ;  and  in  the  following  facts  relating  to  their  production  we  can  recog- 
nize the  influences  of  organization  and  habit.  As  we  have  said,  stimulation 
of  the  motor  area  of  one  hemisphere  produces  movements,  as  a  rule,  which 
are  limited  to  one  side  of  the  body,  and  that  the  opposite  side.  Now  both 
in  ourselves  and  in  the  higher  animals  a  large  number  of  bodily  movements, 
especially  of  the  limbs,  are  habitually  unilateral  ;  and,  putting  aside  the 
question  why  there  should  be  two  halves  of  the  brain,  and  why  the  one  half 
of  the  brain  should  be  associated  with  the  cross  half  of  the  body,  we  may 
recognize  in  the  unilateral  crossed  movement  resulting  from  stimulation  of 
the  cortex  an  accordance  with  natural  habits.  But  some  movements  of  the 
body  are  ordinarily  bilateral ;  the  two  eyes,  for  instance,  are  ordinarily 
moved  together,  and  the  two  sides  of  the  trunk  move  together  very  much 
more  frequently  than  do  the  two  fore  limbs  or  the  two  hind  limbs.  And 
in  accordance  with  this  we  find  that  stimulation  of  the  motor  area  for  the 
eyes  on  either  hemisphere  produces  movements  of  both  eyes,  and  stimula- 
tion of  the  trunk  area  of  one  hemisphere  is  also  very  apt  to  produce  bilat- 
eral action  of  the  trunk  muscles;  in  such  instances  the  movements  on  both 
sides  are  quite  normal  movements.  We  may  incidentally  remark  that  re- 
moval of  the  trunk  area  leads  to  a  good  deal  of  bilateral  degeneration,  that 
is,  to  degeneration  of  strands  in  the  pyramidal  tracts  of  both  sides,  whereas 
such  a  bilateral  degeneration  is  comparatively  scanty  after  removal  of  the 
leg  or  arm  area. 

That  it  is  the  movement  and  not  the  part  moved  which  is,  so  to  speak, 
represented  on  the  cortex  is  further  shown  by  the  relative  magnitude  of 
the  several  cortical  areas  when  they  are  mapped  out  according  to  parts 
of  the  body.  The  area  for  the  arm,  for  instance  (c/.  Figs.  149,  150),  is, 
so  to  speak,  enormous  compared  to  that  of  the  trunk  when  the  relative 
bulks  of  these  two  parts  of  the  body  are  considered ;  and  within  the  arm 
area  itself  the  space  occupied  by  the  thumb  and  forefinger  and  digits  is, 
bulk  for  bulk,  out  of  proportion  to  the  space  allotted  to  the  shoulder ;  so 
also  the  area  for  the  eyes,  or  for  the  mouth  is  out  of  proportion  to  the 
size  of  those  organs.  But  these  relative  sizes  of  the  respective  areas  be- 
come intelligible  when  we  bear  in  mind  relative  mobility,  nimbleness, 
and  delicacy  of  execution ;  in  these  respects  the  shoulder  is  far  behind 
the  thumb,  while  the  eyes  and  mouth  surpass  most  other  parts  of  the 
body. 

We  are  brought  yet  a  step  further  when  we  compare,  in  respect  of  the 
cortical  motor  region,  animals  of  different  grades  of  organization  ;  and  the 
results  thus  obtained  lead  us  to  the  conclusion  that  the  motor  region  is  cor- 
related not  to  movements  in  general,  but  to  movements  of  a  particular  kind. 
Taking  in  series  the  rabbit,  the  dog,  the  monkey,  and  man,  we  find  in  pass- 
ing from  one  to  the  other,  an  increase  in  prominence  and  in  differentiation 
of  the  motor  region  accompanied  by  an  increase  in  the  bulk  of  the  pyram- 
idal tract ;  among  the  many  striking  differences  between  the  brains  of  these 
several  animals,  these  two  features,  the  increasing  complexity  of  the  motor 
region  and  the  increasing  size  of  the  pyramidal  tract,  are  among  the  most 
striking.  The  size  of  the  pyramidal  tract  is  itself  correlated  to  the  com- 
plexity of  the  motor  region,  and,  being  the  more  easily  determined,  may  be 
used  as  indicating  both ;  the  difference  in  the  size  of  the  pyramidal  tract  in 
these  animals  is  seen  all  along  the  whole  length  of  the  cord  (Fig.  151).  Now 
as  regards  mere  quantity  of  movement,  if  we  may  use  such  an  expression, 
the  differences  between  these  animals  are  of  no  great  moment.  If  we 
were  to  take  the  amount  of  energy  expended  as  movement  in  twenty-four 
hours  per  gramme  of  muscle  present  in  the  body  in  each  of  the  four  cases. 


ON   VOLUNTARY   MOVEMENTS. 


673 


we  should  certainly  not  find  any  correspondence  between  that  and  the  size 
of  the  pyramidal  tract.  If,  however,  we  take  a  particular  kind  of  move- 
ment, what  we  may  perhaps  call  skilled  movement,  that  is,  movement  car- 
ried out  by  means  of  intricate  changes  in  the  central  nervous  system,  we  do 
find  a  remarkable  parallelism  in  the  above  cases  between  the  amount  of  such 
skilled  movement  entering  into  the  daily  life  of  the  individual  and  the  size 
of  the  pyramidal  tract.  In  these  two  respects  man  is  much  above  the  monkey, 
and  the  monkey  far  above  the  dog.  We  may  conclude  then  that  the  cortical 


FIG.  151. 


MAN 


MONKEY 


DOG 


Diagram  to  illustrate  the  Relative  Size  of  the  Pyramidal  Tract  in  the  Dog,  Monkey,  and  Man. 
(Sherrington.)  The  figure  shows  in  outline  the  lateral  half  of  the  cord,  at  the  level  of  the  fifth 
thoracic  nerve,  in  A,  man  ;  B,  monkey  ;  C,  dog.  A  is  a  reproduction  of  IP  in  Fig.  127  ;  B  and  C 
are  drawn  of  the  same  size  as  A.  Py.,  shaded  obliquely,  the  pyramidal  tract ;  the  depth  of  shad- 
ing indicates  that  the  tract  is  more  crowded  with  true  pyramidal  fibres  as  well  as  larger  in  A 
than  in  B,  and  in  B  than  in  C.  In  B,  Py'  is  an  outlying  portion  of  the  pyramidal  tract  separated 
from  the  rest  by  the  cerebellar  tract;  Py.  d.,  the  direct  pyramidal  tract,  present  in  man  only. 
The  gray  matter  seems  relatively  large  in  C  because  the  section  was  taken  from  a  very  young 
puppy. 

motor  region  is  in  some  way  especially  concerned  with  the  kind  of  movement 
which  we  have  called  "  skilled." 

§  571.  These  skilled  movements  are,  to  a  large  extent,  though  not 
exclusively,  voluntary  movements.  We  have,  in  a  previous  section,  seen 
reason  to  believe  that  the  cerebral  cortex  is  in  some  way  especially  asso- 
ciated with  the  development  of  voluntary  movements.  Putting  together 
this  conclusion  and  the  conclusions  just  arrived  at,  we  are  naturally  led  to 
the  further  conclusion  that  the  cortical  motor  region,  with  the  pyramidal 
tract  belonging  to  it,  plays  an  important  part  in  carrying  out  voluntary 
movements.  Do  other  facts  support  this  view ;  and  if  so,  what  light  do 
they  throw  on  the  question  as  to  what  part  and  what  kind  of  part  the 
motor  region  thus  plays  ? 

In  this  connection  we  naturally  desire  to  know  what  are  the  results  of 
removing  from  an  otherwise  intact  animal  the  whole  motor  region  and  more 
especially  this  or  that  particular  portion  of  it.  Before  proceeding  further, 
however,  we  may  once  more  call  attention  to  the  caution  given  in  §  495  and 
repeated  in  §  553  ;  indeed,  when  we  consider  the  high  organization  and  com- 
plex functions  which  obviously  belong  to  the  cortex,  when  we  bear  in  mind 
that  it  appears  to  govern,  and  must  therefore  be  bound  by  close  ties  to  almost 
all  the  rest  of  the  central  nervous  system,  we  must  be  prepared  to  find  after 
removing  a  portion  of  cortex  that  the  pure  "  deficiency"  phenomena,  those 
which  result  from  the  mere  absence  of  a  piece  of  the  cortex,  are  largely 
obscured  by  the  other  effects  of  the  operation. 

In  the  rabbit  the  results  have  been  almost  purely  negative.  When  in  this 
animal  the  part  of  the  cortex  which  may  be  considered  as  the  motor  region 

43 


674  THE  BRAIN. 

is  removed,  nothing  remarkable  is  observed  in  the  movements  of  the  animal 
We  can  hardly  suppose  that  the  operations  of  the  central  nervous  system  are 
the  same  in  an  injured  as  in  an  intact  animal,  and  the  differences  induced 
ought  to  be  betrayed  by  the  movements  of  the  body  ;  but  at  present  they 
have  escaped  observation. 

In  the  dog  the  removal  of  an  area  is  followed  by  a  loss  or  diminution  of 
voluntary  movement  in  the  corresponding  part  of  the  body.  When,  for 
instance,  the  area  for  the  fore  limb  is  removed  from  the  left  hemisphere,  the 
right  fore  limb  is  completely  or  partially  "  paralyzed."  In  carrying  out  its 
ordinary  movements  the  operated  animal  makes  little  or  no  use  of  its  right 
fore  limb.  But  this  state  of  things  is  temporary  only.  After  a  while  the 
animal  regains  power  over  the  limb,  and  in  successful  cases  recovery  is  so 
complete  that  it  is  impossible  to  point  out  in  the  limb  any  appreciable  devia- 
tion from  the  normal  use.  And  careful  examination  after  death  has  shown 
not  only  that  the  area  had  been  wholly  removed,  but  also  that  there  was  no 
regeneration  of  the  lost  parts  ;  the  removal  of  the  cortex  leads  in  such  cases, 
as  usual,  to  degeneration  of  the  corresponding  strand  in  the  pyramidal  tract 
right  away  from  the  cerebral  surface  to  the  endings  of  the  strand  in  the 
cervical  and  dorsal  spinal  cord.  Nor  can  it  be  urged  in  such  cases  that  dif- 
fused remnants  of  the  arm  area  had  been  left  in  the  remaining  parts  of  the 
motor  region ;  for  the  whole  motor  region  has  been  removed,  and  yet  the 
animal  has  recovered  to  such  an  extent  that  a  casual  observer  could  detect 
no  differences  between  the  movements  of  the  two  sides  of  the  body.  Closer 
examination  did  disclose  certain  imperfections  of  movement ;  but  the  opera- 
tion had  involved  injury  to  or  produced  changes  in  structures  other  than 
the  motor  region,  and  the  imperfections  might  have  been  due  to  the  addi- 
tional damage.  Nor  can  it  be  urged  that,  in  such  a  case,  where  one  side  is 
removed,  the  remaining  hemisphere  takes  on  double  functions ;  for  the 
greater  part  of  the  motor  areas  have  been  removed  on  both  sides,  and  yet 
the  animal's  movements  have  been  so  far  apparently  complete  that  a  casual 
observer  would  see  nothing  strange  in  them.  Again,  the  whole  motor  region 
has  been  removed  from  one  hemisphere  in  a  young  puppy,  and  some  time 
later  when  the  movements  seemed  to  have  recovered  their  normal  condition, 
the  removal  of  the  motor  region  of  the  other  hemisphere  has  produced  merely 
a  paralysis  of  the  crossed  side  of  the  body,  and  that  as  before  only  of  a  tem- 
porary character. 

Two  things  have  to  be  noted  here.  In  the  first  place,  the  removal  of  an 
area  does  affect  the  movements  which  are  brought  about  by  stimulating  that 
area,  it  leads  to  their  disappearance  or  at  least  to  great  diminution  of  them  ; 
and  this  affords  an  additional  argument  that  the  connection  between  the  area 
and  the  movement  is  a  real  and  important  one.  In  the  second  place,  the 
physiological  effect  is  temporary  only,  though  the  anatomical  results  of  the 
operation  are  permanent,  for  the  cortex  is  never  renewed,  and  the  pyramidal 
tract  degenerates  along  its  whole  length,  never  to  be  restored ;  this  shows 
that  we  have  to  deal  here  with  events  of  a  very  complex  character.  When 
a  particular  movement  results  from  stimulation  of  the  appropriate  cortical 
area,  we  may  be  sure  that  whatever  takes  place  in  the  cortex  and  along  the 
pyramidal  tract,  motor  impulses,  duly  coordinated,  pass  along  certain  ante- 
rior roots  to  certain  muscles ;  and  we  know  that  if  we  removed  a  sufficient 
length  of  each  of  those  anterior  roots  that  particular  movement  would  be 
lost  for  the  rest  of  the  life  of  the  individual.  We  may,  therefore,  infer  that 
the  events  which,  whatever  be  their  exact  nature,  taking  place  in  the  cortex 
and  along  the  pyramidal  tract  lead  ultimately  to  the  issue  of  motor  impulses 
along  the  anterior  roots,  differ  essentially  from  _the  events  attending  the 
transmission  of  ordinary  motor  impulses. 


ON  VOLUNTARY  MOVEMENTS.  675 

In  the  case  of  the  monkey,  the  results  of  removing  parts  of  the  cortical 
motor  region  have  not  been  so  accordant  as  in  the  case  of  the  dog.  The 
two  animals  agree  perfectly  in  so  far  that  the  removal  of  a  particular  area 
leads,  as  an  immediate  result,  to  the  loss  of  the  corresponding  movement ; 
but  while  in  some  instances  recovery  of  the  movement  has  in  the  monkey  as 
in  the  dog  after  a  while  taken  place,  in  other  instances  the  "  paralysis  "  has 
appeared  to  be  permanent.  As  a  rule  the  paralysis  caused  by  a  large  lesion 
is  not  only  more  extensive,  but  also  of  longer  duration  than  that  caused  by 
a  small  one ;  and  natural  bilateral  movements,  as  of  the  eyes,  reappear 
earlier  than  unilateral  movements.  The  facts,  however,  within  our  know- 
ledge relating  to  the  permanence  of  the  effect  are  neither  numerous  nor  exact 
enough  to  justify  at  present  a  definite  conclusion.  On  the  one  hand,  the 
positive  cases  where  recovery  has  taken  place  are  of  more  value  than  the 
negative  ones,  since  in  the  latter  the  recovery  may  have  been  hindered  by 
concomitant  events  of  a  nature  which  we  may  call  accidental ;  and  it  is  at 
least  a  priori  most  unlikely  that  the  pyramidal  tract  mechanism,  if  we  may 
use  the  expression,  though  it  may  differ  in  the  monkey  and  the  dog  in  degree 
of  development,  differs  so  essentially  in  kind  that  damage  of  it  leads  in  the 
one  case  to  permanent,  and  in  the  other  to  mere  temporary,  loss  of  function. 
We  may  add  that  we  should  further  expect  to  meet  in  the  monkey  with 
more  prominent  and  more  lasting  complications  due  to  the  subsidiary  effects 
of  the  operation,  and  it  may  be  doubted  whether  in  any  of  the  recorded 
experiments  the  animal  has  been  allowed  to  live  a  sufficient  time  for  these 
subsidiary  events  to  have  cleared  away,  leaving  only  what  we  have  called 
the  "  deficiency"  phenomena,  due  to  the  loss  of  the  cortical  area  alone.  On 
the  other  hand,  it  must  be  remembered  that  the  movements  of  the  monkey 
are  more  intricate  in  origin,  more  "  skilled,"  than  those  of  the  dog ;  and  it 
may  be  that  differences  in  the  characters  of  movements  determine  the  possi- 
bility of  their  recovery.  In  illustration  of  this  we  may  quote  the  expe- 
rience that,  after  the  removal  of  the  arm  area  in  the  monkey,  a  certain 
awkwardness  in  the  movements  of  the  thumb  is  one  of  the  last  effects  of  the 
operation. 

§  572.  Before  we  proceed,  however,  any  further  in  the  discussion,  it  will 
be  of  advantage  to  turn  aside  to  what  is  known  concerning  the  cortical 
motor  region  in  man.  As  we  have  already  said,  theoretical  considerations 
lead  us  to  believe  that  the  cortical  motor  region  in  man  is  disposed  in  accord- 
ance with  the  plan  of  the  anthropoid  ape  as  ascertained  experimentally,  but 
with  the  differentiation  carried  still  further ;  and  the  few  cases  of  experi- 
mental stimulation  of  the  human  cortex  support  this  view.  Our  chief 
knowledge  in  this  matter  is  derived  from  the  study  of  disease ;  and  in  this 
the  advantages  of  dealing  with  one  of  ourselves  are  largely  counterbalanced 
by  the  disadvantages  due  to  disease  being  so  often  anatomically  diffuse  and 
physiologically  changeful  and  progressive. 

We  said  above  that  during  experiments  on  animals  stimulation  of  any 
part  of  the  motor  region  may  under  abnormal  conditions  lead  to  general 
epileptiform  convulsions.  Now  clinical  study  has  shown  that  in  man  certain 
kinds  of  epileptic  attacks  are  of  similar  cortical  origin.  In  these  cases  it 
has  been  observed  that  the  attack  begins  in  a  particular  movement,  by  con- 
tractions of  particular  muscles  or  of  the  muscles  of  a  particular  region  of 
the  body,  of  the  hand,  foot,  toe,  thumb,  etc.,  and  then  spreads  in  a  definite 
order  or  "  march  "  over  the  muscles  of  other  regions  until  the  whole  body  is 
involved.  When  in  an  experiment  on  an  animal  epileptiform  convulsions 
supervene,  they  similarly  start  from  the  region  of  the  body,  the  motor  area 
of  which  is  beneath  the  electrodes  at  the  time,  and  similarly  spread  by  a 
definite  "  march  "  over  the  whole  body.  Hence,  in  the  human  epileptiform 


676  THE  BRAIN. 

attacks  of  which  we  are  speaking,  it  has  been  inferred  that  the  immediate 
exciting  cause  of  the  attack  is  to  be  sought  in  events  taking  place  in  that 
part  of  the  cortex  which  serves  as  the  area  for  the  movement  which  ushers 
in  the  attack.  Further  inquiry  has  not  only  confirmed  this  view,  but  has 
also  shown  that  the  topography  of  the  cortical  areas  in  man,  as  thus  deter- 
mined, very  closely  follows  that  of  the  monkey. 

Other  diseases  of  the  cortex  have  been  marked,  among  other  symptoms, 
by  loss  or  impairment  of  particular  movements.  In  most  of  such  cases  the 
cortical  lesion  has  been  of  such  an  extent  as  to  involve  a  number  of  special 
areas  at  the  same  time,  and  so  to  lead  to  loss  or  impairment  of  movement 
over  relatively  considerable  regions  of  the  body,  such  as  the  whole  of  one 
arm ;  and  in  general  the  teaching  of  these  cases  of  disease,  while  confirming 
the  deductions  from  the  monkey,  and  giving  us  some  general  idea  of  the 
topography  of  the  human  motor  cortical  region,  has  at  present  given  us 
approximate  results  only.  Figs.  154  and  155  show  in  broad  diagrammatic 
manner  the  position  and  relative  extent  of  the  motor  areas  for  the  leg,  arm, 
and  face  in  man  as  far  as  has  yet  been  ascertained.  To  assist  the  reader  we 
give  at  the  same  time  diagrams  (Figs.  152  and  153)  illustrating  the  nomen- 
clature of  the  surface  of  the  human  brain. 

One  area  is  of  special  and  instructive  interest.  Speech  is  an  eminently 
"  skilled  "  movement.  We  have  seen  that  in  the  monkey  the  area  for  the 
mouth  and  tongue  lies  at  the  ventral  end  of  the  central  fissure  or  fissure  of 
Rolando,  ventral  to  the  arm  area,  and  that  the  extreme  ventral  and  front 
part  of  the  motor  region  just  above  the  fissure  of  Sylvius  supplies  an  area 
which  we  marked  as  that  of  phonation  (Fig.  149).  In  the  monkey  the  area 
of  phonation  is  determined  by  experimental  stimulation ;  in  man,  in  a 
similar  position,  on  the  third  or  lowest  frontal  convolution,  sometimes  called 
Broca's  convolution,  ventral  to  and  in  front  of,  and  probably  overlapping 
backward,  the  area  which  in  Fig.  154  is  marked  "face,"  and  which  includes 
the  mouth  and  tongue,  clinical  study  has  disclosed  the  existence  of  an  area 
which  may  be  spoken  of  as  the  area  of  "  speech."  Lesions  of  the  cortex  in 
this  area  cause  a  loss  of  or  interference  with  speech,  the  condition  being 
known  as  aphasia;  to  this  we  shall  presently  return.  In  Fig.  154  this  area 
is  shown  in  an  approximate  manner. 

The  movements  of  speech  are  essentially  bilateral  movements.  In  the 
dog  and  monkey  various  bilateral  movements  may  be  excited  by  stimulation 
of  the  appropriate  area  in  either  hemisphere ;  and  analogy  would  lead  us  to 
suppose  that  in  man  the  movements  of  speech  would  be  connected  with  the 
speech  area  in  both  one  and  the  other  hemisphere.  The  results  of  lesions, 
however,  show  that  it  is  in  most  cases  especially  the  left  hemisphere  which 
is  connected  with  speech ;  it  is  a  lesion  in  the  third  frontal  convolution  of 
the  left  hemisphere,  often  associated  with  other  lesions  of  the  same  hemi- 
sphere leading  to  paralysis  of  the  right  side  of  the  body  and  face,  which 
causes  aphasia,  it  being  only  in  exceptional  cases  that  the  condition  results 
from  a  lesion  of  the  corresponding  area  of  cortex  on  the  right  hemisphere. 

In  man,  then,  clinical  study  corroborates  the  conclusions  deduced  from 
the  experimental  investigation  of  the  dog  and  of  the  monkey,  but  still  leaves 
us  in  uncertainty  as  to  the  question  what  and  what  alone  are  the  absolutely 
permanent  effects  of  the  loss  of  a  cortical  area  and  nothing  else.  On  the 
one  hand,  in  the  cases  in  which  recovery  of  a  movement  follows  upon  its 
loss  or  impairment,  it  is  open  for  us  to  suppose  that  the  lesion  itself  was  tem- 
porary, and  that  with  the  cure  of  the  malady  the  cortical  area  regained  its 
normal  condition.  On  the  other  hand,  where  the  disease  continues,  the  per- 
manency of  the  loss  of  any  movement  may  be  attributed  to  the  disease 
doing  more  than  merely  suspend'  the  function  of  the  cortical  area.  Aphasia, 


ON  VOLUNTARY   MOVEMENTS. 


677 


especially  in  young  persons,  has  been  followed  by  recovery,  but  in  such 
cases  it  has  been  supposed  that  the  dormant  area  on  the  right  side  has  been 
awakened  to  activity  by  the  loss  of  the  left  area  ;  and  in  support  of  this 


FIG.  152. 


JPar.Oc.F. 


LOBE 


TEMPORAL 


Diagram  of  the  Gyri  (convolutions)  and  Sulci  (fissures)  on  the  Lateral  Surface  of  the  Right 
Hemisphere  of  Man.    (Gowers.) 


FIG.  153. 


F.Rolando 


The  same  on  the  Mesial  Surface.  (Gowers.)  In  both  figures  the  sulci  are  indicated  by  italic 
and  the  convolutions  by  roman  type.  The  following  list  of  some  synonyms  may  perhaps  be  of 
use  in  connection  with  these  figures  and  those  of  the  brain  of  the  monkey  (Figs.  149  and  150). 

Gyri  or  convolutions.  Precentral  or  anterior  central  =  ascending  frontal.  Postcentral  or  pos- 
terior central  =  ascending  parietal.  Superior  temporal  =  infra-marginal  =  first  temporal.  Tri- 
angular lobule  =  cuneus.  Central  lobe  =  island  of  Reil.  Paracentral  lobule  =  the  mesial  face 
of  the  superior  frontal  within  the  marginal  gyrus.  Cingulum  =  the  part  of  the  gyrus  fornicatus 
which  adjoins  the  corpus  callosum.  Gyrus  hippocampi  =  uncinate  gyrus,  though  the  latter  name 
is  sometimes  restricted  to  the  front  part  of  the  hippocampal  gyrus ;  the  two  may  be  considered 
as  a  continuation  of  the  gyrus  fornicatus,  and  the  three  together,  forming  a  series,  have  been 
called  "  the  great  limbic  lobe." 

Sulci  or  fissuers.  Central  =  Rolandic,  or  of  Rolando.  Perpendicular  ptari  eto-occipital. 
Parietal  =  mtra-parietal  or  sometimes  inter-parietal.  Tempor=o-sphenoidal  lobe  =mporal  lobe. 

view  cases  have  been  recorded  in  which  a  first  aphasia,  due  to  a  lesion  on 
the  left  side  has  been  followed  by  a  second  aphasia  due  to  a  subsequent  lesion 


678 


THE   BRAIN. 


occurring  on  the  right  side.     On  the  whole,  perhaps,  the  evidence  of  clinical 
study  tends  to  show  that  in  man  the  loss  of  movement  due  to  the  destruction 


FIG.  154. 


Oc.L. 


Fr-.L. 


Te.L. 


The  Lateral  Surface  of  the  Right  Cerebral  Hemisphere  of  Man  in  Outline,  to  illustrate  the 
Cortical  Areas.  (Reduced  from  nature.)  The  position  of  the  areas  of  the  cortex  concerned  with 
movements  of  the  face,  arm,  and  leg,  and  with  the  senses  of  sight  and  hearing  are  approximately 
shown.  The  position  of  the  area  connected  with  speech  (Broca's  centre)  is  also  shown  for  the  sake 
of  comparison  of  it  with  the  position  of  the  other  areas;  the  representation  of  speech  in  the  cor- 
tex cerebri  lies,  however,  in  the  left  hemisphere  chiefly.  Oc.  L,  occipital  lobe  ;  Fr.  L,  frontallobe  ; 
Te.  L,  temporal  lobe  ;  Sy.f,  the  fissure  of  Sylvius;  C.f,  the  central  fissure  (Rolandic) ;  Om.f,  indi- 
cates the  position  of  the  posterior  end  of  the  calloso-marginal  fissure. 

FIG.  155. 


Fr.L. 


Oc.L. 


Te.L. 

The  Mesial  Surface  of  the  Right  Cerebral  Hemisphere  of  Man  in  Outline,  to  illustrate  the 
Cortical  Areas.  The  areas  shown  are  those  connected  with  the  movements  of  the  leg  and  with 
the  senses  of  sight  and  smell.  Fr.  L,  the  frontal  lobe  of  the  hemisphere  :  Oc.  L,  the  occipital 
lobe;  Te.  L,  the  temporal  lobe  ;  Cm./,  the  calloso-marginal  fissure  separating  the  marginal  gyrus 
above  from  the  gyrus  fornicatus  below;  Cf,  marks  the  situation  of  the  central  fissure,  the  fissure 
itself  not  being  apparent  on  the  mesial  aspect  of  the  hemisphere.  The  corpus  callosum  and  the 
anterior  commissure  are  seen  in  cross  section. 

by  disease  of  an  area  is  a  permanent  one,  though  actual  demonstration  of 
this  is  wanting. 


ON  VOLUNTARY  MOVEMENTS.  679 

§  573.  We  may  now  return  to  the  discussion  of  the  question,  What  is 
the  part  played  by  a  motor  area,  and  by  the  contribution  from  that  area  to 
the  pyramidal  tract  in  carrying  out  the  movements  with  which  the  area  is 
associated  ? 

We  may  premise  that  the  evidence  points  very  distinctly  to  the  con- 
clusion that  whatever  be  the  nature  of  the  whole  chain  of  events  of  which 
the  cortical  area  seems  to  be  a  sort  of  centre,  the  fibres  of  the  pyramidal 
tract  serve  as  the  channel  of  processes  which  we  must  regard  as  efferent  in 
nature.  It  is  perfectly  true  that  in  many  cases  at  least  the  removal  of  a 
cortical  area  has  led  to  diminished  sensibility  of  the  part  in  which  move- 
ments are  excited  by  stimulation  of  the  area ;  and  there  are  many  facts,  of 
which  we  shall  presently  quote  a  very  striking  one,  which  go  to  show  that 
the  cortex  of  the  motor  region  is  largely  influenced  by  sensory  impulses  from 
various  parts  of  the  body  ;  but  we  cannot  suppose  that  the  pyramidal  tract 
is  the  channel  by  which  such  sensory  impulses  reach  the  cortex.  As  we 
have  previously  (§  481)  urged,  the  fact  that  the  degeneration  of  the  fibres 
in  the  tract  is  a  descending  one,  and  cannot  be  trusted  by  itself  to  prove 
that  the  direction  in  which  the  fibres  carry  impulses  is  only  that  from  the 
cortex  downward  ;  but  this  added  to  the  fact  that  when  the  fibres  of  the 
tract  are  stimulated  at  any  part  of  their  course,  movements,  the  signs  of  the 
occurrence  of  efferent  centrifugal  impulses,  are  produced,  leaves  no  doubt 
that  the  tract  is  one  of  efferent  fibres.  Hence  we  may  infer  that  whatever 
be  the  nature  of  the  events  taking  place  in  a  motor  area  during  the  carry- 
ing out  of  a  movement,  the  part  played  by  the  fibres  of  the  pyramidal 
tract  is  that  of  carrying  efferent  impulses  from  the  area  to  the  muscles  con- 
cerned. 

Let  us  consider  first  the  movements  of  speech  in  man,  the  evidence  touch- 
ing the  connection  of  which  with  an  area  on  the  third  frontal  convolution 
appears  so  very  clear.  Speech  is  eminently  a  "  skilled "  movement ;  it 
involves  the  most  delicate  coordination  of  several  muscular  contractions, 
and  we  may  certainly  say  of  it  that  it  has  to  be"  learned."  The  whole  chain 
of  coordinated  events  by  which  the  utterance  of  a  sentence,  a  word,  or  any 
vocal  sign  is  accomplished  consists  of  many  links,  the  breaking  of  any  of 
which  will  lead  to  failure  of  one  kind  or  another  in  the  act:  Something  may 
go  wrong  in  the  glossal  or  other  muscles,  in  the  nerve-endings  in  those 
muscles,  or  in  the  fibres  of  the  nerves,  hypoglossal  and  others,  between  the 
central  nervous  system  and  the  muscles,  or  something  may  go  wrong  in  that 
part  of  the  central  nervous  system,  the  bulb  to  wit,  in  which  a  certain  amount 
of  coordination  is  carried  out  just  previous  to  the  issue  of  the  motor  impulses. 
Damage  done  to  any  of  these  parts  of  the  mechanism  may  lead  to  dumbness 
or  to  imperfect  speech.  In  the  latter  case  the  imperfections  have  a  certain 
character  ;  if  we  are  at  all  able  to  gather  the  wish  of  the  speaker,  we  recog- 
nize that  he  is  attempting  to  utter  the  right  words  in  the  right  sequence,  but 
that  his  efforts  are  frustrated  by  imperfect  coordination  or  imperfect  mus- 
cular action  ;  his  speech  is  "  thick,"  the  syllables  are  blurred  and  the  like. 
Disease  of  the  bulb  at  times  leads  to  imperfect  speech  of  this  kind  in  which 
the  imperfection  may  be  recognized  as  due  to  the  lack  of  proper  coordina- 
tion of  motor  impulses.  The  affection  of  speech  known  as  "aphasia," 
which  is  caused  by  lesions  of  the  cortex,  is  of  a  different  character,  and  the 
forms  of  imperfect  speech  caused  by  bulbar  disease  have  justly  been  distin- 
guished from  true  aphasia  by  the  use  of  other  terms.  Cases  of  complete 
aphasia  in  which  all  power  of  speech  is  lost,  do  little  more  than  help  us  to 
ascertain  the  topographical  position  in  the  cortex  of  the  "  speech  "  area,  but 
cases  of  partial  aphasia  are  especially  instructive.  Without  attempting  to 
go  into  the  details  of  the  subject  and  into  the  many  considerations  which 


680  THE  BKAIN. 

have  to  be  had  in  mind  in  dealing  with  it,  for  there  are  different  kinds  of 
aphasia,  we  may  venture  to  say  that  the  striking  feature  of  partial  aphasia 
is  the  failure  to  say  certain  words  or  syllables,  and  the  tendency  to  substi- 
tute some  wrong  word  or  syllable  for  the  right  one.  The  words  or  syllables 
which  are  uttered  are  rightly  pronounced  without  defect  of  articulation  ; 
and  in  many  cases,  though  the  right  word  cannot  be  produced  as  a  direct 
effort  of  the  will,  it  may  be  uttered  under  the  influence  of  an  emotion,  or, 
indeed,  sometimes  as  the  result  of  some  physical  processes  more  complex 
than  those  involved  in  the  mere  volitional  effort  to  say  the  word.  An  in- 
structive case  is  recorded  of  a  man  suffering  from  slight  aphasia,  who,  after 
several  failures  to  say  the  word  "no"  by  itself,  at  last  said,  "I  can't  say 
no,  sir." 

From  the  phenomena  of  partial  aphasia  we  may  draw  the  deduction 
that  the  cortical  speech  area  does  not  carry  out  the  whole  of  the  coordina- 
tion of  the  impulses  involved  in  articulation.  That  coordination  is  exceed- 
ingly complex,  and  we  ought,  perhaps,  to  recognize  in  it  more  than  one  de- 
gree or  kind  of  coordination.  The  failure  of  articulation  in  disease  of  the 
bulb  shows  that  a  certain  amount  of  coordination  takes  place  there  ;  for  the 
affections  of  speech  due  to  bulbar  disease  are  not  the  same  as  those  resulting 
from  the  mere  loss  of  this  or  that  muscle  or  nerve.  We  must,  of  course, 
admit  that  some,  possibly  a  great  deal,  of  coordination  of  a  certain  kind 
takes  place  in  the  cortex,  for  the  bulb  cannot  by  itself  be  made  to  speak ; 
exactly  how  much,  the  knowledge  at  present  at  our  disposal  leaves  a  matter 
of  great  uncertainty;  but  it  is  sufficient  for  our  present  purpose  to  recog- 
nize that  whatever  may  be  the  nature  of  the  events  taking  place  in  the 
cortical  area  during  the  act  of  speech,  those  events  make  use  of  the  machin- 
ery already  provided  in  the  bulb.  The  word  spoken  does  not  start,  so  to 
speak,  ready  made  in  the  cortex  ;  it  is  not  that  a  group  of  impulses  start 
from  the  cortex  with  their  coordination  fully  achieved,  and  pass  along  cer- 
tain nerve-fibres  to  certain  muscles,  making  their  way  without  change 
through  the  tangle  of  the  bulb,  as  if  this  were  merely  a  bundle  of  lines 
offering  paths  for,  but  exercising  no  influence  over  the  impulses.  We  must 
rather  suppose  that  something  takes  place  in  the  cortex  of  the  third  frontal 
convolution,  as  the  result  of  which  efferent  impulses  pass  along  the  appro- 
priate fibres  of  the  pyramidal  tract  to  the  bulb,  and  there  start  a  series  of 
events  leading  to  the  issue  of  the  coordinated  impulses  by  which  the  word  is 
spoken. 

§  574.  We  have  no  reason  whatever  to  think  that  the  cortical  area  for 
speech  differs  in  its  fundamental  characters  from  other  divisions  of  the  motor 
region,  and  are  justified  in  carrying  on  to  other  areas  the  deduction  we  have 
just  drawn  in  connection  with  the  speech  area.  With  that  end  in  view  we 
may  now  turn  back  to  the  experimental  results  obtained  on  the  dog,  and  it 
will  make  our  discussion  simpler  if  we  take  as  an  illustration  some  large  area 
such  us  the  fore-limb  area. 

We  have  seen  that  stimulation  of  this  area  produces  what  we  may,  to 
start  with,  speak  of  simply  as  movements  of  the  fore  limb ;  and  guided  by 
the  analogy  of  speech  in  man  we  may  confidently  conclude  that  when  the 
dog  voluntarily  moves  the  fore  limb,  the  act  is  carried  out  by  means  of 
events  taking  place  in  the  fore-limb  cortical  area.  The  simplicity  of  the 
electrical  phenomena  resulting  from  cortical  stimulation,  which  we  described 
in  §  658,  might  at  first  sight  lead  us  to  conclude  that  the  whole  matter  was 
fairly  simple ;  and,  indeed,  some  writers  appear  to  entertain  the  conception 
that  in  a  voluntary  movement  such  as  that  of  the  fore  limb,  all  that  takes 
place  is  that  the  "  will "  stimulates  certain  cells  in  the  cortical  area,  causing 
the  discharge  of  motor  impulses  along  the  pyramidal  fibres  connected  with 


ON  VOLUNTARY  MOVEMENTS.  681 

those  cells,  and  that  these  motor  impulses  travel  straight  down  the  pyram- 
idal tract  to  the  motor  fibres  of  the  appropriate  nerves,  undergoing  possibly 
some  change  at  the  place  in  the  cord  where  the  pyramidal  fibre  makes  junc- 
tion with  the  fibre  of  the  anterior  root,  but  deriving  their  chief  if  not  their 
whole  coordination  from  the  cortex  itself,  that  is  to  say,  being  coordinated 
at  their  very  starting-point.  That  such  a  view  is  untenable  and  that  the 
simplicity  of  the  electrical  phenomena  is  misleading  is  shown  by  the  follow- 
ing two  considerations  among  others  :  On  the  one  hand,  as  was  shown  in  a 
previous  section,  the  coordination  of  movements  may  be  carried  out  apart 
from  the  cortex,  namely,  in  the  absence  of  the  hemispheres ;  and  we  can 
hardly  suppose  that  there  should  be  two  quite  distinct  systems  of  coordina- 
tion to  carry  out  the  same  movement,  one  employed  when  volition  was  the 
moving  cause,  and  the  other  when  something  else  led  to  the  movement.  On 
the  other  hand,  the  analogy  of  speech  justifies  us  in  concluding  that  the  cor- 
tical processes  do  take  advantage  of  coordination  effected  by  the  action  of 
other  parts  of  the  nervous  system. 

Bearing  this  in  mind,  we  may  recall  attention  to  the  remarkable  effects 
which  result  from  the  removal  of  the  area.  These  are  twofold.  In  the  first 
place,  there  is  more  or  less  complete  paralysis  of  the  limb  ;  all  the  movements 
of  the  limb  are  for  a  time  ineffective.  It  is  not  that  purely  voluntary  move- 
ments are  alone,  so  to  speak,  cut  out ;  the  reflex  and  other  movements  are 
also  impaired  or  temporarily  abolished,  and,  as  we  have  already  said,  in 
many  cases  at  least  the  sensations  of  the  limb  are  interfered  with.  These 
troubles  are,  of  course,  in  part  the  effects  of  the  mere  operative  interference 
belonging  to  what  we  spoke  of  in  §  495,  as  being  of  the  nature  of  shock. 
But,  even  giving  full  weight  to  this  consideration,  there  remains  the  fact  that 
the  cortical  area  is  associated  with  the  various  coordinating  and  other  ner- 
vous mechanisms  belonging  to  the  limb  by  such  close  ties  that  these  are 
thrown  into  disorder  when  it  is  injured.  And  side  by  side  with  this  we  may 
put  the  remarkable  fact  previously  stated,  that  during  an  abnormal  condi- 
tion of  the  cortical  area  stimulation  of  the  area,  instead  of  producing  the 
appropriate  movements  confined  to  the  limb,  may  give  rise  to  movements 
of  other  parts  culminating  in  epileptiform  convulsions. 

In  the  second  place,  this  paralysis  is  temporary  only  ;  the  voluntary  move- 
ments are  after  a  while  regained,  and  that  in  spite  of  the  fore-limb  moiety  of 
the  pyramidal  tract  permanently  degenerating  along  its  whole  length,  neither 
it  nor  the  cortical  area  ever  being  regenerated.  This  shows  that  whatever 
be  the  chain  of  events  in  the  intact  animal,  it  is  possible  for  the  "  will  "  of 
the  animal  to  get  at  the  muscles  and  motor  mechanisms  of  the  fore  limb  by 
some  other  path  than  that  provided  by  the  appropriate  cortical  area  and 
corresponding  path  of  the  pyramidal  tract ;  and  the  facts  previously  recorded 
(§  571)  show  that  that  other  part  is  not  the  corresponding  part  of  the  pyram- 
idal system  belonging  to  the  other  half  of  the  hemisphere,  and,  indeed,  is 
not  any  part  at  all  of  the  whole  pyramidal  system.  The  "  will,"  whatever 
be  the  processes  by  which  it  takes  origin,  and  wherever  be  the  place  where 
they  are  carried  on,  is  able  in  the  absence  of  the  pyramidal  system  to  produce 
its  effect  on  the  motor  fibres  of  the  brachial  nerves  by  working  on  other 
parts  of  the  central  nervous  system. 

Hence  while  admitting,  as  we  must  do,  that  in  the  intact  animal  the  cor- 
tical area  and  pyramidal  tract  play  their  part  in  carrying  out  voluntary 
movements,  their  action  is  not  of  that  simple  character  supposed  by  the  view 
referred  to  above.  On  the  contrary,  we  are  driven  to  regard  them  rather  as 
links,  important  links,  it  is  true,  but  still  links  in  a  complex  chain.  As  we 
have  already  urged,  we  may  probably  speak  of  the  changes  taking  place  in 
the  pyramidal  fibres  as  being  on  the  whole  of  the  nature  of  efferent  impulses; 


682  THE  BRAIN. 

but  we  should  be  going  beyond  the  evidence  if  \ve  concluded  that  they  were 
identical  with  the  ordinary  efferent  impulses  of  motor  nerves.  And  above 
all  it  must  not  be  left  unnoticed  that  the  cortical  area  has  close,  if  not  direct, 
connections  of  a  sensory  nature  with  the  part  in  whose  movements  it  is  con- 
cerned. This  is  shown  by  the  following  remarkable  results  which  may  make 
their  appearance  when  stimulation  of  the  cortex  is  carried  on  while  the  animal 
(dog)  is  in  a  particular  stage  of  the  influence  of  morphine.  If  a  submini- 
mal  stimulus  be  found,  that  is,  a  current  of  such  intensity  that  applied  to  a 
motor  area  it  will  produce  no  movement,  but  if  increased  ever  so  slightly 
will  give  a  feeble  constriction  of  the  appropriate  muscle,  it  may  be  observed 
that  a  slight  stimulus,  such  as  gently  stroking  the  skin  over  the  muscles  in 
question,  will  render  the  previous  subminimal  stimulus  effective,  and  so  call 
forth  a  movement.  Thus,  if  the  area  experimented  on  be  that  connected 
with  the  lifting  of  the  forepaw,  and  the  subminimal  stimulus  be  applied  to 
the  area  at  intervals,  after  several  applications  followed  by  no  movements,  a 
gentle  stroke  or  two  over  the  skin  of  the  paw  wrill  lead  to  the  paw  being* 
lifted  the  next  time  the  stimulus  is  applied  to  the  area.  A  similar  result, 
but  less  sure  and  striking,  may  follow  upon  the  stimulation  of  parts  of  the 
body  other  than  the  part  corresponding  to  the  area  stimulated.  Then,  again, 
it  has  been  observed  that  in  certain  other  stages  of  the  influence  of  morphine, 
the  cortex  and  the  rest  of  the  nervous  system  are  in  such  a  condition  that 
the  application  of  even  a  momentary  stimulus  to  an  area  leads  not  to  a  sim- 
ple movement,  but  to  a  long-continued  tonic  contraction  of  the  appropriate 
muscles.  Under  these  circumstances  a  gentle  stimulus,  such  as  stroking  the 
skin  or  blowing  on  the  face,  applied  immediately  after  the  application  of  the 
electric  stimulus  to  the  area,  suddenly  cuts  short  the  contraction,  and  brings 
the  muscles  at  once  to  rest  and  normal  flaccidity. 

These  experiments  show  that  the  development  of  the  processes  in  the 
cortex  leading  to  the  issue  of  what  we  have  agreed  to  call  efferent  impulses 
along  the  pyramidal  fibres  is  markedly  affected  by  sensory  impulses,  and 
especially  by  sensory  impulses  started  in  the  skin  overlying  and  corresponding 
to  the  muscles  put  into  movement.  How  these  sensory  impulses  reach  the 
cortex  we  do  not  exactly  know ;  but  we  have  no  evidence  to  show  that 
afferent,  centripetal  impulses  can  travel  backward,  so  to  speak,  along  the 
pyramidal  fibres ;  and  it  is  more  reasonable  to  suppose  that  the  sensory  im- 
pulses in  question  reach  the  cortex  by  the  ordinary  paths  of  sensory  impulses, 
which  we  shall  presently  discuss.  We  may  therefore  take  the  results  of  the 
experiments  as  showing  how  close  is  the  connection  of  the  motor  area  with 
the  sensory  mechanisms  of  the  spinal  cord  and  lower  parts  of  the  brain,  and 
as  illustrating  the  complexity  of  the  chain  of  events  by  which  the  motor 
area  brings  about  voluntary  movements. 

§  575.  We  have  above  used  the  general  phrase  "  movements  of  the  limb," 
since  in  the  dog  it  is  not  easy  to  pick  out  certain  movements  as  being  particu- 
larly skilled  movements.  In  the  monkey  such  a  distinction  is  easier.  In 
this  animal,  as  we  have  said,  recovery  of  voluntary  movement  also  takes 
place  after  removal  of  a  cortical  area,  or  at  least  has  done  so  in  many  cases ; 
and  while  the  phenomena  immediately  following  removal  on  the  whole  re- 
semble those  witnessed  in  the  dog,  a  certain  order  of  recovery  may  be  ob- 
served ;  the  more  skilled  movements  are  the  last  to  return.  When,  for 
instance,  the  arm  area  is  removed,  the  delicate  movements  of  the  hand,  of 
the  thumb  and  finger,  are  the  last  to  be  reestablished  ;  and  a  condition  of 
things  may  be  met  with  in  which  the  animal  after  removal,  say  of  the  arm 
area  in  the  left  hemisphere,  uses  by  preference  the  left  hand  at  a  time  when, 
if  prevented  from  using  that  hand,  he  is  able  to  use  the  right ;  that  is  to  say, 
the  recovery  in  the  right  limb  after  the  removal  of  the  area  on  the  left  side 


ON  VOLUNTARY  MOVEMENTS.  683 

is  nearly  but  not  quite  complete ;  the  "  will "  can  gain  access  to  the  right 
hand,  but  not  so  easily  as  to  the  left  hand,  and  this  latter  is  used,  though 
under  ordinary  circumstances  it  would  not  be  used. 

When  we  turn  to  man,  in  whom  the  great  development  of  the  pyramidal 
system  and  differentiation  of  the  cortical  area  is  paralleled  by  the  promi- 
nence of  skilled  and  trained  movements,  the  analogy  of  the  phenomena  of 
speech,  if  it  be  true,  as  clinical  histories  seem  to  show,  that  destruction  by 
disease  of  the  speech  area  of  both  sides  causes  permanent  aphasia,  would 
lead  us  to  conclude  that  at  least  highly  skilled  voluntary  movements  are 
carried  out  by  the  pyramidal  system  and  by  that  alone.  But  in  reference  to 
this  it  must  be  remembered  that  such  a  permanent  aphasia  may  be  due,  not 
to  mere  loss  of  the  pyramidal  channel,  not  to  the  will  being  merely  unable 
to  gain  access  to  lower  coordinating  mechanisms,  but  to  the  absence  of  the 
differentiated  cortical  gray  matter,  by  reason  of  which  absence  the  will  can- 
not initiate  the  first  processes  of  the  act  of  speech  ;  it  may  be  that  were  it 
able  to  do  so,  the  processes  so  started  might,  in  the  absence  of  the  pyramidal 
tract,  find  some  other  way  to  the  bulbar  mechanism,  as  in  the  case  of  the  un- 
skilled movements  of  the  dog.  This  point,  however,  clinical  histories  have 
not  definitely  settled.  Moreover,  in  dealing  with  the  phenomena  of  the 
nervous  system  of  man,  as  revealed  by  disease,  we  meet  in  reference  to  the 
cerebral  cortex  the  same  difficulty  that  we  dwelt  upon  in  dealing  with  the 
spinal  cord  (§  504).  Lesions  of  the  pyramidal  system,  of  the  internal  capsule 
for  instance,  lead  to  the  loss  not  only  of  skilled,  but  of  all  voluntary  move- 
ments ;  according  to  the  character  and  position  of  the  lesion  this  or  that  part 
of  the  body  is  wholly  withdrawn  from  the  influence  of  the  will.  And  it  is 
possible  to  maintain  the  thesis  that  man  has  become  so  developed  as  to  his 
nervous  system  and  the  motor  cortex,  so  accustomed  to  make  use  exclusively 
of  the  pyramidal  system,  that  the  will  has  lost  the  power,  still  possessed  by 
lower  animals,  to  gain  access  by  some  path  other  than  the  pyramidal  one  to 
the  immediate  nervous  mechanisms  of  movement.  The  data  for  forming  a 
satisfactory  conclusion  as  to  this  point  are  so  few  and  uncertain  that  it  would 
be  unprofitable  to  discuss  the  question  here  ;  but  we  may  venture  to  point  out 
that,  great  as  is  the  development  of  the  cerebral  cortex  and  the  pyramidal 
system  in  man,  that  development  is  accompanied  by  a  hardly  less  striking 
expansion  of  other  parts  of  the  brain  not  directly  connected  with  the  pyram- 
idal system  which  we  have  previously  seen  reason  to  associate  with  the 
coordination  of  movements,  for  example  the  cerebellum.  And,  indeed,  it  is 
clear  that,  admitting  the  pyramidal  tract  to  be  the  ordinary  channel  by 
which  volitional  impulses  pass  to,  or  by  which  the  will  gains  access  to,  the 
motor  mechanisms  immediately  associated  with  the  anterior  roots  of  this  or 
that  spinal  nerve,  we  must  also  admit  that  those  volitional  impulses  passing 
along  the  pyramidal  tract,  or  at  least  some  of  the  processes  constituting  the 
will,  are  in  connection  with,  and  thus  are  influenced  by  the  condition  of, 
other  parts  of  the  brain.  When,  for  instance,  a  gymnast  executes  a  skilled 
voluntary  movement  in  which  all  his  four  limbs  and  other  parts  as  well  per- 
haps of  his  body  are  involved,  it  is  probably  the  case  that  changes  of  the 
nature  of  efferent  impulses  sweep  down  his  pyramidal  tract,  and  that  these 
impulses,  starting  in  a  definite  order  from  his  cortex,  that  is  to  say,  having 
undergone  a  certain  amount  of  initial  coordination  at  their  very  origin,  meet 
with  further  coordination  in  the  spinal  gray  matter,  which  serves  as  a  set  of 
nuclei  of  origin  for  the  motor  nerves  concerned  in  the  movement  before  they 
issue  as  ordinary  motor  impulses  along  the  anterior  roots.  But  this  is  not 
all.  Should  the  gymnast's  semicircular  canals  happen  to  be  injured  and  his 
cerebellum  thereby  be  troubled,  or  mischief  fall  on  some  other  part  of  the 
brain  which  like  this  has  no  direct  connection  with  either  the  pyramidal 


684  THE  BRAIN. 

tract  or  the  motor  cortex,  the  movement  fails  through  lack  of  coordination, 
though  both  the  cortex,  the  pyramidal  tract,  and  the  spinal  motor  mechan- 
isms remain  as  they  were  before.  Obviously  the  carrying  out  of  a  voluntary 
movement  is  a  very  complex  proceeding,  and  the  motor  cortex  with  the 
pyramidal  tract  is  only  one  part  of  the  whole  mechanism  ;  so  far  from  the 
whole  business  being  confined  to  these,  it  is  perhaps  no  exaggeration  to  say 
that  in  each  movement  of  the  kind  most  parts  of  the  whole  brain  have  a 
greater  or  less  share. 

The  exact  nature  of  the  part  played  by  the  cortex  and  the  pyramidal 
tract  in  voluntary  movements  our  present  knowledge  is  inadequate  to  define. 
When  we  pass  in  review  a  series  of  brains  from  the  lower  to  the  higher  and 
see  how  the  pyramidal  system  is,  so  to  speak,  grafted  on  to  the  rest  of  the 
brain,  when  we  observe  how  the  increasing  differentiation  of  the  motor  cor- 
tex runs  parallel  to  the  increasing  possession  of  skilled  educated  movements, 
we  may  perhaps  suppose  that  a  "  short  cut "  from  the  cortex  to  the  origins 
of  the  several  motor  nerves,  such  as  is  afforded  by  the  pyramidal  fibres, 
from  the  advantages  it  offers  to  the  more  primitive  path  from  segment  to 
segment  along  the  cerebro-spinal  axis  has  by  natural  selection  been  developed 
into  being  in  man  the  chief  and  most  important  instrument  for  carrying  out 
voluntary  movements  ;  but,  we  repeat,  it  remains  even  in  its  highest  develop- 
ment a  link  in  a  chain,  and  a  knowledge  of  how  the  whole  chain  works  is 
at  present  hidden  from  us. 

We  must  not  here  wander  into  psychological  problems,  but  may  repeat 
that  in  the  above  discussion  we  have  used  the  word  "will"  in  a  general 
sense  only.  A  man  maybe  brought  into  a  condition,  for  instance  in  certain 
hypnotic  phases,  in  which  he  can  carry  out  all  the  various  skilled  movements 
which  he  has  inherited  or  which  he  has  learned  ;  and  yet,  according  to  some 
definitions  of  the  word  "  will,"  those  movements  could  not  be  said  to  be  initi- 
ated by  his  will.  It  can  hardly  be  doubted  that  in  such  cases  the  motor 
cortex  and  pyramidal  tract  play  their  usual  part.  But  we  may  pass  from 
such  cases  as  these  through  others,  until  we  come  to  cases  where  a  skilled 
movement  which  has  been  learned  and  practised  by  the  working  of  an  in- 
telligent will,  may  continue  to  be  carried  out  under  circumstances  which 
seem  to  preclude  the  intervention  of  any  conscious  will  at  all ;  and  the 
transition  from  one  case  to  another  is  so  gradual,  that  it  is  impossible  to  sup- 
pose that  there  has  been  any  shifting  of  the  machinery  employed  for  carry- 
ing out  the  movement.  So  that  a  volitional  origin  is  not  an  essential  feature 
of  these  so-called  voluntary  movements,  and  the  machinery  of  the  motor 
cortex  and  pyramidal  tract  is  available  for  other  things  than  pure  voli- 
tional impulses. 

§  576.  The  preceding  discussion  will  enable  us  to  be  very  brief  concern- 
ing a  question  which  has  from  time  to  time  been  much  discussed,  and  which 
has  acquired  perhaps  fictitious  importance,  viz.,  the  question  as  to  how 
volitional  impulses  leading  to  voluntary  movements  travel  along  the  spinal 
cord.  The  conclusions  at  which  we  have  arrived,  namely,  that  in  the  normal 
carrying  out  of  voluntary  movements  the  chief  part  is  played  by  efferent 
impulses  passing  along  the  pyramidal  tract,  carries  with  it  the  answer  that 
volitional  impulses  travel  in  the  spinal  cord  along  the  pyramidal  tract. 

In  the  dog,  in  which  the  whole  pyramidal  tract  crosses  at  the  decussation 
of  the  pyramids,  we  should  expect  to  find  that  a  break  in  the  pyramidal 
tract  of  one  side  of  the  cord  at  any  point  along  its  length  caused  loss  of 
voluntary  movement  on  the  same  side  below  the  level  of  the  break.  And 
experiments  as  far  as  they  go  support  this  view.  No  one  it  is  true  has 
attempted  to  divide  or  otherwise  cause  a  break  in  the  pyramidal  tract  alone, 
leaving  the  rest  of  the  cord  intact ;  and,  indeed,  even  if  an  injury  were 


ON  VOLUNTARY  MOVEMENTS.  685 

limited  to  the  area  marked  out  as  the  pyramidal  tract,  fibres  other  than 
pyramidal  fibres  would  be  injured  at  the  same  time,  since  the  tract  is  never 
a  u  pure  "  one.  But  it  has  been  found  that  a  section  of  a  lateral  half  of  the 
cord,  a  lateral  hemisectiou,  or  a  section  limited  to  the  lateral  column  of  one 
side,  has  for  one  of  its  principal  effects  loss  of  voluntary  movement  on  the 
same  side  in  the  parts  supplied  by  motor  nerves  leaving  the  cord  below  the 
level  of  the  section.  We  say  "  one  of  its  principal  effects  "  because,  besides 
the  concomitant  interference  with  sensations  concerning  which  we  shall  speak 
presently,  the  loss  of  voluntary  movement  is  not  absolutely  confined  to  the 
same  side  ;  there  is  some  loss  of  power  on  the  crossed  side,  at  least  in  a  large 
number  of  cases.  We  must  not  lay  stress  on  this  crossed  paralysis  because 
it  is  probably  one  of  the  effects  of  the  mere  operation,  not  a  pure  "  defi- 
ciency "  phenomenon,  and,  indeed,  appears  soon  to  pass  away.  But  taking 
into  consideration  what  was  said  above  concerning  the  effects  of  removing 
cortical  areas,  it  is  important  to  note  that  in  the  experience  of  many  experi- 
menters the  loss  of  voluntary  power  on  the  operated  side  diminishes  after  a 
while,  and  that  the  animal  if  kept  alive  and  in  good  health  long  enough 
appears  to  regain  almost  full  voluntary  power  over  the  affected  parts.  In 
such  cases,  as  in  other  operations  on  the  central  nervous  system,  there  is  no 
regeneration  of  nervous  tissue  ;  the  two  surfaces  of  the  section  unite  by  con- 
nective not  nervous  tissue,  and  the  tracts  which  as  the  result  of  the  section 
degenerate  downward  or  upward  are  permanently  lost.  Hence,  even  if  we 
admit  that  in  the  intact  animal  a  voluntary  movement  is  chiefly  carried  out 
by  means  of  efferent  impulses  passing  along  the  pyramidal  tract  right  down 
to  the  motor  mechanisms  of  the  cord  immediately  connected  with  the  motor 
nerves,  we  must  also  admit  that  the  "  will "  under  changed  circumstances 
can  find  other  channels  for  gaining  access  to  the  same  mechanisms. 

It  has  been  further  observed  that  if  in  the  dog  a  hemisection  be  made  at 
one  level,  for  instance  in  the  lower  thoracic  region  of  the  cord,  and  then, 
after  waiting  until  the  voluntary  power  over  the  hind  limb  of  that  side  has 
returned,  a  second  hemisection,  this  time  on  the  other  side,  be  made  at  a 
higher  level,  this  second  operation  is  followed  by  results  similar  to  those  of 
the  first ;  there  is  loss  of  voluntary  power  on  the  side  operated  on,  with  some 
loss  of  power  on  the  crossed  side,  and  as  in  the  first  case  this  loss  of  power 
not  only  on  the  same  but  also  on  the  crossed  side  may  eventually  disappear. 
This  shows  among  other  things  that  the  recovery  after  the  first  operation  was 
not  due  to  the  remaining  pyramidal  tract  doing  the  work  of  both.  Further, 
the  hemisection  may  be  repeated  a  third  time,  the  third  hemisection  being 
on  the  same  side  as  the  first,  with  at  least  a  very  considerable  return  of  power 
over  both  limbs.  That  is  to  say,  under  such  abnormal  circumstances  volun- 
tary impulses  may,  so  to  speak,  thread  their  way  in  a  zigzag  manner  from 
side  to  side  along  the  mutilated  cord  until  they  reach  the  appropriate  spinal 
motor  mechanisms.  Such  an  abnormal  state  of  things  does  not,  however, 
really  militate  against  the  view  that  under  normal  circumstances  volitional 
impulses  normally  travel  along  the  pyramidal  tract ;  but  it  does  show  what, 
indeed,  has  already  been  shown  by  the  phenomena  of  strychnine  poisoning 
(§  499)  that  in  the  central  nervous  system  the  passage  of  nervous  impulses 
(using  these  words  in  the  general  sense  of  changes  propagated  along  ner- 
vous material)  is  not  rigidly  and  unalterably  fixed  by  the  anatomical  distri- 
bution of  tracts  of  fibres ;  in  all  such  discussions  as  those  in  which  we  are 
engaged  we  must  bear  in  mind  that  physiological  conditions  as  well  as  ana- 
tomical continuity  are  potent  in  determining  the  passage  of  these  impulses. 

§  577.  When  we  reflect  on  the  great  prominence  of  the  pyramidal  tract 
in  the  spinal  cord  of  man  as  compared  with  that  of  the  dog,  we  may  justly 
infer  not  only  that  the  pyramidal  tract  is  under  normal  circumstances  more 


686  THE  BRAIN. 

exclusively  the  channel  of  volitional  impulses  in  man  than  in  such  lower 
animals,  but  also,  bearing  in  mind  the  discussion  in  a  previous  chapter 
(§  504)  concerning  the  activities  of  the  spinal  cord  of  man,  that  the  potential 
alternatives  presented  by  the  spinal  cord  of  the  dog  are  greatly  reduced  in 
that  of  man.  And  such  clinical  histories  of  disease  or  accidental  injury  in 
man  as  we  possess  support  this  conclusion.  Lesions  confined  to  one-half  of 
the  cord,  or  even  lesions  confined  to  the  lateral  column  of  one-half,  appear 
to  lead  to  loss  of  voluntary  power  on  the  same  side,  and  the  same  side  only, 
in  the  parts  below  the  level  of  the  lesion  ;  and  the  same  symptoms  have  been 
observed  to  accompany  disease  limited  apparently  to  the  pyramidal  tract  of 
one  side.  Moreover,  though  cases  of  recovery  of  power  have  been  recorded, 
we  have  not  such  satisfactory  evidence  as  in  animals  of  the  volitional  im- 
pulses ultimately  making  their  way  along  an  alternative  route ;  but  here  the 
same  doubts  may  be  entertained  as  were  expressed  in  discussing  the  reflex 
acts  of  the  cord  in  man. 

When  we  say  that  the  loss  of  voluntary  power  is  seen  on  the  side  of  the 
lesion  only,  we  should  add  that  this  statement  appears  to  apply  chiefly  to  the 
thoracic  and  lower  parts  of  the  cord.  We  have  seen  that  in  man,  in  the 
upper  regions  of  the  cord,  the  pyramidal  tract  is  only  partly  crossed  ;  a  vari- 
able but  not  inconsiderable  number  of  the  pyramidal  fibres  do  not  cross  at 
the  decussation  of  pyramids,  but  running  straight  down  as  the  direct  pyram- 
idal tract  effect  their  crossing  lower  down  in  the  cervical  and  upper  thoracic 
regions.  Hence,  we  should  infer  that  a  hemisection  of,  or  a  lesion  confined 
to  one  side  of,  the  cervical  cord  would  affect  the  voluntary  movements  of  the 
crossed  side  as  well  as  of  the  same  side,  though  not  to  the  same  extent.  But 
we  have  no  exact  information  as  to  this  point.  And,  indeed,  the  purpose  of 
the  direct  tract  is  not  clear ;  there  is  no  adequate  evidence  for  the  view  which 
has  been  held  that  these  direct  fibres  are  destined  for  the  upper  limbs  and 
upper  part  of  the  body ;  since  they  are  the  last  to  cross  we  should,  a  priori, 
be  inclined  to  suppose  that  they  were  distributed  to  lower  rather  than  higher 
parts. 

§  578.  We  may  now  briefly  summarize  what  we  know  concerning  volun- 
tary movements.  And  it  will  be  convenient  to  trace  the  events  in  order 
backward. 

Certain  muscles  are  thrown  into  a  contraction  which  even  in  the  briefest 
movements  is  probably  of  the  nature  of  a  tetanus.  In  almost  every  move- 
ment more  than  one  muscle,  as  defined  by  the  anatomists,  is  engaged,  and  in 
many  movements  a  part  of  several  muscles  is  employed,  and  not  the  whole 
of  each.  It  is  perhaps  partly  owing  to  the  latter  fact  that  a  muscle  which 
has  become  tired  in  one  kind  of  movement,  may  show  little  or  no  fatigue 
when  employed  for  another  movement,  though  we  must  bear  in  mind  that  in 
a  voluntary  movement  fatigue  is  much  more  of  nervous  than  of  muscular 
origin. 

Besides  the  active  muscles,  if  we  may  so  call  them,  which  directly  carry 
out  the  movement,  the  metabolism  of  which  supplies  the  energy  given  out  as 
work  done,  other  muscles,  some  of  which  are  antagonistic  to  the  active  mus- 
cles and  some  of  which  may  be  spoken  of  as  adjuvant,  enter  into  the  whole 
act.  In  flexion,  for  instance,  of  the  forearm  on  the  arm  it  is  not  the  flexor 
muscles  only  but  the  extensors  also  which  are  engaged.  According  to  the 
immediately  preceding  position  and  use  of  the  arm,  and  according  to  the 
kind  and  amount  of  flexion  which  is  to  be  carried  out,  the  extensors  will  be 
either  relaxed,  that  is  to  say,  inhibited,  or  thrown  into  a  certain  amount  of 
contraction.  And  in  some  of  the  more  complicated  voluntary  movements 
the  part  played  by  adjuvant  muscles  is  considerable.  Hence,  in  a  voluntary 
movement  the  will  has  to  gain  access  not  only  to  the  active  muscles,  but  also 


ON  VOLUNTARY  MOVEMENTS.  687 

to  the  antagonistic  and  adjuvant  muscles;  and  every  voluntary  movement, 
even  one  of  the  simplest  kind,  is  a  more  or  less  complex  act. 

The  impulses  which  lead  to  the  contraction  of  the  active  muscles  reach 
the  muscles  along  the  fibres  of  the  anterior  roots  (we  may  for  the  sake  of 
simplicity  take  spinal  nerves  alone,  neglecting  the  peculiar  cranial  nerves), 
and  such  evidence  as  we  possess  goes  to  show  that  the  impulses  governing 
the  antagonistic  and  adjuvant  muscles  travel  by  the  anterior  roots  also ;  the 
question  whether  the  inhibition  of  the  antagonistic  muscles  when  it  takes 
place  is  carried  out  by  inhibitory  impulses  passing  as  such  along  the  fibres, 
or  simply  by  central  inhibition  of  previously  existing  motor  impulses,  need 
not  be  considered  now.  These  anterior  roots  are  connected,  as  we  have 
seen,  with  the  gray  matter  of  the  cord,  and  in  each  hypothetical  segment 
of  the  cord  we  may  recognize  the  existence  of  an  area  of  gray  matter  which, 
though  we  cannot  define  its  limits,  we  may,  led  by  the  analogy  of  the  cranial 
nerves,  call  the  nucleus  of  the  nerve  belonging  to  the  segment ;  and  we  may 
further  recognize  in  such  a  nucleus  what  we  may  call  its  efferent  and  its 
afferent  side. 

Every  voluntary  movement,  even  the  simplest,  is,  as  we  have  repeatedly 
insisted,  a  coordinated  movement,  and  in  its  coordination  afferent  impulses 
play  an  important  part.  The  study  of  reflex  actions  (§  502)  has  led  us  to 
suppose  that  each  spinal  segment  presents  a  nervous  mechanism  in  which  a 
certain  amount  of  coordination  is  already  present,  in  which  efferent  impulses 
are  adjusted  to  afferent  impulses.  But  the  results  obtained  by  stimulating 
separate  anterior  nerve-roots  show  that,  in  the  case  of  most  muscles  at  all 
events,  the  especially  active  muscles  of  the  limbs  for  instance,  each  muscle  is 
supplied  by  fibres  coming  from  more  than  one  nerve-root,  that  is  to  say,  the 
spinal  nucleus,  or  at  least  the  spinal  motor  mechanism  for  any  one  muscle, 
extends  over  two  or  three  segments.  Hence  a  fortiori  in  a  voluntary  move- 
ment, involving  as  this  does  in  most  cases  more  than  one  muscle,  the  spinal 
mechanism  engaged  in  the  act  spreads  over  at  least  two  or  three  segments, 
thus  allowing  of  increased  coordination.  In  that  coordination  the  impulses 
serving  as  the  foundation  of  muscular  sense  play  an  important  part,  but 
other  afferent  impulses,  such  as  those  from  the  adjoining  skin,  also  have  their 
share  in  the  matter  ;  and  it  is  worthy  of  notice  that  not  only  is  the  skin  over- 
lying a  muscle  served,  broadly  speaking,  by  nerve-roots  of  the  same  segment 
as  th*e  muscle  itself,  afferent  in  one  case,  efferent  in  the  other,  but  in  the 
parts  of  the  body  where  coordination  is  especially  complex,  in  the  fingers 
for  instance,  not  only  is  each  muscle  supplied  from  more  than  one  segment, 
but  also  each  piece  of  skin  is  supplied  in  the  same  way  by  the  posterior  roots 
of  more  than  one  nerve. 

In  the  case  of  the  frog  it  is  clear  that  in  reflex  movements  a  large  amount 
of  coordination  is  carried  out  by  these  various  spinal  mechanisms ;  and  as  we 
have  urged,  we  may  safely  infer  that  in  the  voluntary  movements  of  the  frog, 
the  will  makes  use  of  this  already  existing  coordination,  whatever  be  the 
exact  path  by  which  in  this  animal  the  will  gains  access  to  the  spinal  mechan- 
isms. In  the  dog  we  may  conclude  that  in  voluntary  movements  the  spinal 
mechanisms,  with  coordinating  functions,  are  also  set  in  action,  in  this  case 
by  impulses  passing  straight  from  the  cortex  to  the  mechanisms  by  the  pyram- 
idal tract,  though  apparently,  in  the  absence  of  the  pyramidal  tract,  the 
will  can  work  upon  the  mechanisms  by  changes  travelling  through  other 
parts  of  the  cerebro-spinal  axis.  And  in  the  monkey  and  man,  subject  to 
the  doubts  already  expressed  as  to  the  potentialities  of  the  human  spinal 
cord,  we  may  probably  also  infer  that  in  each  voluntary  movement  some, 
perhaps  we  may  say  much,  of  the  coordination  is  carried  out  by  the  spinal 
mechanism  set  into  action  through  impulses  along  the  pyramidal  tract.  We 


688  THE  BRAIN. 

may  probably  further  infer  that  a  careful  adjustment  obtains  between  the 
beginnings  of  the  pyramidal  tract  in  the  cortex  and  its  endings  in  the  cord, 
so  that  the  topography  of  "  areas  "  or  "  foci  "  in  the  cortex  above  is  an  image 
or  projection  of  the  spinal  mechanisms  below. 

The  complex  character,  on  which  we  insisted  just  now,  of  almost  every 
voluntary  movement  necessitates  that  in  every  such  movement  a  large  area 
of  spinal  mechanism  is  involved.  But  this  is  not  all.  The  movements  of 
any  part  of  the  legs,  for  instance,  are  not  determined,  nor  is  the  coordination 
of  the  movements  affected,  simply  by  what  is  going  on  in  the  legs  and  the 
part  of  the  spinal  cord  belonging  to  them.  The  discussion  in  a  previous 
section  has  shown  that  much  of  the  coordination  of  the  body  is  carried  out 
by  the  middle  portions  of  the  brain,  and  on  these  the  motor  area  must  have 
its  hold  as  on  the  spinal  mechanisms. 

The  details  of  the  nature  of  that  hold  are  at  present  unknown  to  us ;  but 
it  must  be  remembered  that  not  all  the  fibres  passing  down  from  the  motor 
region,  not  all  those  even  proceeding  from  the  densest  and  most  clearly 
defined  motor  areas,  are  pyramidal  fibres.  With  the  pyramidal  fibres  are 
mingled  fibres  having  other  destinations,  and  some  of  these  probably  pass 
to  the  thalamus  and  so  join  the  great  tegmental  region.  Moreover,  the 
motor  region  must  have  close  ties  with  other  regions  of  the  cortex  whence,  as 
we  have  seen  (§  545)  fibres  pass  to  the  pons  to  make  connections  with  the 
cerebellum.  On  the  other  hand,  as  we  have  seen  (§  525)  the  cerebellum  is 
especially  connected  with  what  we  may  fairly  consider  the  afferent  side  of 
the  spinal  cord  and  bulb.  These  facts  must  merely  be  taken  as  indicating  the 
possibilities  by  which  the  motor  region  is  kept  in  touch  with  the  great  coordi- 
nating mechanism  ;  it  would  be  venturesome  at  present  to  say  much  more. 

In  an  ordinary  voluntary  movement  an  intelligent  consciousness  is  an 
essential  element.  But  many  skilled  movements  initiated  and  repeated  by 
help  of  an  intelligent  conscious  volition  may,  when  the  nervous  machinery 
for  carrying  them  out  has  acquired  a  certain  facility  (and  in  all  the  higher 
processes  of  the  brain  we  must  recognize  that,  in  nervous  material  at  all 
events,  action  determines  structure,  meaning  by  structure  molecular  arrange- 
ment and  disposition),  be  carried  out  under  appropriate  circumstances  with 
so  little  intervention  of  distinct  consciousness  that  the  movements  are  then 
often  spoken  of  as  involuntary.  All  the  arguments  which  go  to  show  that 
the  distinctly  conscious  voluntary  skilled  movement  is  carried  out  by  help 
of  the  appropriate  motor  area  go  to  show  that  the  motor  area  must  play  its 
part  in  these  involuntary  skilled  movements  also.  So  that  distinct  con- 
sciousness is  not  a  necessary  adjunct  to  the  activity  of  a  motor  area.  And 
it  is  worthy  of  notice  that  some  of  these,  in  their  origin,  purely  voluntary 
skilled  movements,  which  by  long-continued  training  have  become  almost 
as  purely  involuntary,  are  hampered  rather  than  assisted  by  being  "  thought 
about."  ' 

The  word  "  training  "  suggests  the  reflection  that  the  physiological  inter- 
pretation of  becoming  easy  by  practice  is  that  new  paths  are  made,  or  the 
material  of  old  paths  made  more  mobile  by  effort  and  use.  We  have  already 
urged  (§  494)  that  the  gray  matter  of  the  spinal  cord  is  a  network,  in  which 
the  passage  of  impulses  is  determined  by  physiological  conditions  rather  than 
anatomical  continuity,  and  the  same  considerations  may  with  still  greater 
force  be  applied  to  the  brain.  We  must  suppose  that  training  promotes  the 
growth  and  molecular  mobility  of  the  motor  area  and  of  all  its  connections. 
There  are  doubtless  limits  to  the  changes  which  can  be  effected,  but  within 
these  limits  the  will,  blundering  at  first  in  the  maze  of  the  nervous  network, 
gradually  establishes  easy  paths  ;  though  even  to  the  end  it  blunders — in 
trying  to  carry  out  one  movement  it  often  accomplishes  another. 


VISUAL  AND  OTHER  SENSATIONS.  689 

Lastly,  without  attempting  to  enter  into  psychological  questions,  we  may 
at  least  say  that  the  birthplace  of  what  we  call  the  "  will,"  is  not  conter- 
minous with  the  motor  area  ;  the  will  arises  from  a  complex  series  of  events, 
some  of  which  take  place  in  other  regions  of  the  cortex,  and  probably  in 
other  parts  of  the  brain  as  well.  With  these  parts  the  motor  area  has  ties 
concerned  not  in  the  carrying  out  of  volition,  but  in  the  generation  of  the 
will.  So  that,  looking  round  on  all  sides,  it  is  obvious,  as  we  have  said,  that 
the  motor  area  is  a  mere  link  in  a  complex  chain.  It  is,  moreover,  a  link  of 
such  a  kind,  that  while  the  changes  which  the  breaking  of  it  makes  in  the 
daily  life  of  a  lowly  animal,  such  as  the  dog,  in  whom  the  experience  of  the 
individual  adds  relatively  little  to  the  nervous  and  psychical  storehouse 
transmitted  from  his  ancestors,  can  hardly  be  appreciated  by  a  bystander, 
those  which  the  breaking  of  it  makes  in  the  daily  life  of  a  man,  whose  brain 
at  any  moment  is  not  only  a  machine  fitted  for  present  and  future  work  but 
a  closely  packed  record  of  his  past  life,  are  obvious  not  only  to  the  individual 
himself,  but  to  his  fellows. 


ON  THE  DEVELOPMENT  WITHIN  THE  CENTRAL  NERVOUS  SYSTEM  OF 

VISUAL    AND   OF   SOME   OTHER   SENSATIONS. 

Visual  Sensations. 

§  579.  In  the  chain  of  events  through  which  some  influence  brought  to 
bear  on  the  periphery  of  a  sensory  nerve  gives  rise  a  sensation,  we  are  able, 
with  more  or  less  success,  to  distinguish  between  those  events  which  are 
determined  by  the  changes  at  the  periphery  and  those  which  are  the  expres- 
sion of  changes  induced  in  the  central  nervous  system.  Thus  when  certain 
rays  of  light  proceeding  from  an  object  and  falling  upon  the  eye  give  rise  to 
visual  perception  of  the  object,  two  sets  of  events  happen  :  the  rays  of  light, 
by  help  of  the  mechanisms  of  the  eye,  partly  dioptric,  partly  nervous,  give 
rise  to  certain  changes  in  the  fibres  of  the  optic  nerve,  which  we  may  call 
visual  impulses  ;  and  these  visual  impulses  reaching  the  brain  along  the" optic 
nerve  give  rise  to  visual  sensations  and  so  to  visual  perception  of  the  object. 
We  shall  later  on,  under  the  heading  of  "  the  senses,"  deal  chiefly  with  the 
peripheral  events,  and  have  now  to  consider  some  points  connected  with  the 
central  events,  to  learn  what  we  know  concerning  how  the  various  sensory 
impulses  travelling  along  the  several  kinds  of  sensory  nerves  behave  within 
the  central  nervous  system.  In  doing  so  we  shall  have  from  time  to  time  to 
refer  to  peripheral  events,  but  only  occasionally,  and  never  in  any  great 
detail.  It  will  be  convenient  to  begin  with  the  special  sense  of  sight,  and 
we  must  first  briefly  call  attention  to  a  few  points  which  we  shall  have  to 
study  in  fuller  detail  hereafter. 

The  eye  is  so  constructed  that  images  of  external  objects  are  brought  to 
a  focus  on  the  retina,  the  stimulation  of  which  by  light  starts  the  visual  im- 
pulses along  the  fibres  of  the  optic  nerve ;  and  the  distinctness  with  which, 
by  means  of  the  visual  sensations  arising  out  of  these  visual  impulses,  we 
perceive  external  objects  is  dependent  on  the  sharpness  of  the  retinal  images. 
The  eye  is  further  so  constructed  that,  in  any  position  of  the  eye,  the  rays 
of  light  proceeding  from  a  portion  only  of  the  external  world  fall  upon  the 
retina ;  or  in  other  words,  in  any  one  position  of  the  eye  only  a  portion  of 
the  external  world  is  visible  at  the  same  time.  The  portion  so  seen  is  spoken 
of  as  the  visual  field  for  that  position. 

The  image  thrown  on  the  retina  is  an  inverted  one,  so  that  the  top  of  an 
actual  object  is  represented  by  the  lower,  and  the  bottom  by  the  upper,  part 

44 


690  THE  BRAIN. 

of  the  retinal  image ;  similarly  the  actual  left-hand  side  of  the  retinal  image 
corresponds  to  the  right-hand  side  of  the  actual  object,  and  the  right-hand 
side  to  the  left-hand  side.  Hence  the  right-hand  half  of  the  visual  field 
corresponds  to  the  left-hand  side  of  the  retina,  and  the  left-hand  half  to  the 
right-hand  side. 

The  eye  can  be  moved  in  various  directions,  and  since  in  the  visual  field 
the  portion  of  external  nature  which  can  be  seen  at  the  same  time  differs 
with  each  different  position,  a  large  range  of  vision  is  thus  secured  ;  and  this 
can  be  further  increased  by  movements  of  the  head.  Moreover,  we  nor- 
mally make  use  of  two  eyes ;  our  normal  vision  is  binocular,  and  the  visual 
field  of  the  right  eye  differs  from  that  of  the  left  eye.  There  is  one  striking 
difference  which  must  always  be  borne  in  mind.  A  section  carried  through 
the  eye  in  a  vertical  and  front-to-back  plane,  through  what  we  shall  learn 
to  call  the  optic  axis  (Fig.  156,  o.  x.~)  (the  exact  details  of  the  plane  may  be 
left  for  the  present),  will  divide  the  retina  into  two  lateral  halves,  and  in 
each  retina  one-half  will  be  on  the  nasal  side  next  to  the  nose,  and  the  other 
half  will  be  on  the  malar  or  temporal  side,  next  to  the  cheek  or  temple.  It 
must  be  remembered  that  the  nasal  halves  and  temporal  halves  of  the  two 
retinas  do  not  occupy  corresponding  positions  in  space.  The  temporal  half 
of  the  left  retina  is  on  the  left  side  of  its  own  eye,  whereas  the  temporal  half 
of  the  right  retina  is  not  on  the  left,  but  on  the  right,  side  of  its  eye ;  and 
so  with  the  nasal  halves.  Now,  in  the  right  eye,  the  right-hand  side  of  the 
visual  field  corresponds  to  the  nasal  half  of  the  retina,  and  the  left-hand  side 
of  the  visual  field  to  the  temporal  half  of  the  retina,  whereas  in  the  left  eye 
the  right-hand  side  of  the  visual  field  corresponds  to  the  temporal  half  of 
the  retina,  and  the  left-hand  side  to  the  nasal  half.  This  is  shown  in  Fig. 
156,  where  the  left-hand  visual  field  and  the  retinal  area  concerned  are 
shown  shaded  in  each  eye. 

When  we  look  at  an  object  with  the  two  eyes,  though  two  retinal  images 
are  produced,  one  in  one  eye  and  one  in  the  other,  we  perceive  one  object 
only,  not  two.  This  is  the  essential  fact  of  binocular  vision  ;  when  certain 
parts  of  each  retina  are  stimulated  at  the  same  time  we  are  conscious  of  one 
sensation  only,  not  two ;  and  the  parts  of  the  two  retinas  which,  stimulated 
at  the  same  time,  give  rise  to  one  sensation  are  spoken  of  as  "  corresponding 
parts."  From  the  structure  and  relations  of  the  two  eyes  it  follows  that  the 
temporal  side  of  the  right,  and  the  nasal  side  of  the  left,  eye  are  such  cor- 
responding parts,  while  the  nasal  side  of  the  right  eye  corresponds  to  the 
temporal  side  of  the  left  eye.  But  the  whole  of  each  retina  is  not  employed 
in  binocular  vision.  Owing  to  the  position  of  the  two  eyes  in  relation  to  the 
nose,  it  comes  about  that  an  object  held  very  much  on  one  side,  to  the  left- 
hand  side  for  instance,  while  it  is  capable  of  producing  an  image  on  the 
extreme  nasal  side  of  the  left  eye,  and  can  be  seen,  therefore,  by  that  eye, 
cannot  produce  an  image  on  the  temporal  side  of  the  right  eye ;  the  nose 
blocks  the  way.  It  is,  therefore,  not  seen  by  the  right  eye,  and  the  vision 
of  it  is  monocular  by  the  left  eye  only.  In  Fig.  156  it  may  be  seen  that  the 
left  visual  field  of  the  left  eye  (L.  F.  _L.)  extends  more  to  the  left,  and  is 
larger  than  the  left  visual  field  of  the  right  eye  (L.  F.  #.),  and  that  the  right 
retinal  area,  corresponding  to  the  left  visuaffield,  extends  further  along  the 
nasal  side  of  the  left  side  (a')  than  it  does  along  the  temporal  side  of  the 
right  eye  (a'),  the  difference  being  due  to  the  presence  of  the  nose  (f\). 
And  similar  conditions  obtain  with  regard  to  the  extreme  right-hand  side 
of  the  visual  field. 

§  580.  After  these  preliminary  statements,  we  may  now  turn  to  con- 
sider some  anatomical  facts  concerning  the  ending  of  the  optic  nerve  in  the 
brain. 


VISUAL  AND  OTHER  SENSATIONS. 
FIG.  156. 


691 


Diagram  to  illustrate  the  Nervous  Apparatus  of  Vision  in  Man.    (Sherrington.)    L,  the  left  eye ; 
E,  the  right  eye  ;  o,x.,  the  optic  axis ;  F,  the  outline  of  the  face  between  the  eyes ;  Op  T,  the  right 


692  THE  BKAIN. 

The  optic  nerve  of  each  eye  consists  of  nerve-fibres  coming  from  all 
parts  of  the  retina  of  that  eye ;  but  the  two  optic  nerves  meet,  ventral  to 
the  floor  of  the  third  ventricle,  cross  each  other  at  the  optic  chiasma  (Fig. 
156,  Op.  De.~),  and  are  thence  continued  on  under  the  name  not  of  optic 
nerves,  but  of  optic  tracts  (Op.  T.).  The  decussation  of  fibres  which  takes 
place  in  the  chiasma  has  peculiar  characters.  At  their  decussation  (we  are 
speaking  now  of  man)  the  fibres  in  the  optic  nerve  belonging  to  the  tem- 
poral half  of  the  eye  in  which  the  nerve  ends  pass  into  one  optic  tract, 
namely,  the  optic  tract  of  the  same  side,  while  the  fibres  belonging  to  the 
nasal  half  pass  into  another  optic  tract,  namely  the  optic  tract  of  the  oppo- 
site side.  Thus  the  fibres  of  the  temporal  half  of  the  right  eye  and  of  the 
nasal  half  of  the  left  eye  pass  into  the  right  optic  tract,  and  the  fibres  of 
the  nasal  half  of  the  right  eye  and  of  the  temporal  half  of  the  left  eye  pass 
into  the  left  optic  tract.  Compare  Fig.  156,  in  which  the  fibres  forming  the 
right  optic  tract  are  shaded,  while  those  forming  the  left  optic  tract  are  left 
unshaded.  Now  the  nasal  half  of  one  retina  and  the  temporal  half  of  the 
other  retina  are  "  corresponding "  parts.  Hence,  while  each  optic  tract 
contains  fibres  belonging  to  half  of  each  eye,  the  two  halves  thus  repre- 
sented in  each  tract  are  corresponding  halves. 

The  amount  and  character  of  the  decussation  taking  place  in  the  optic 
chiasma  differs  in  different  animal  types,  the  difference  having  relation  to 
the  amount  of  binocular  vision,  which  in  turn  depends  on  the  position  of 
the  eyes  in  the  head,  that  is,  on  the  prominence  of  the  face  between  the 
eyes.  In  the  fish,  for  instance,  with  laterally  placed  eyes,  no  binocular 
vision  at  all  is  possible,  and  the  decussation  is  complete ;  the  whole  optic 
nerve  of  each  eye  crosses  over  to  the  other  optic  tract.  Between  this  and 
the  arrangement  in  man,  just  described,  various  stages  obtain  in  various 
animals. 

The  chiasma  also  contains  at  its  hinder  part  fibres  which  have  no  con- 
nection with  the  optic  nerves  or  the  eyes,  but  are  simply  commissural  tracts 
passing  from  one  side  of  the  brain,  namely,  from  the  median  corpus  genicu- 
latum (§  543)  along  one  optic  tract,  through  the  chiasma  to  the  other  optic 
tract,  and  so  to  the  median  corpus  geniculatum  of  the  other  side  of  the 
brain.  These  fibres  are  spoken  of  as  the  inferior  or  posterior  (optic)  com- 
missure or  arcuate  commissure,  or  Gudden's  commissure.  It  was  once 
thought  that  in  a  similar  way  fibres  passed  from  one  retina  along  one  optic 
nerve  through  the  front  part  of  the  chiasma  to  the  other  optic  nerve,  and 
to  the  other  retina,  forming  an  anterior  (optic)  commissure ;  but  this  seems 
to  be  an  error. 

§  581.  The  optic  vesicle  is,  as  we  have  seen,  budded  off  from  the  fore- 
brain  or  forerunner  of  the  third  ventricle,  and  the  optic  chiasma  is 

optic  tract  (shaded)  supplying,  through  Op  De,  the  optic  decussation,  the  temporal  side  of  the 
retina  of  the  right  eye  and  the  nasal  side  of  the  retina  of  the  left  eye;  L.F.L  and  L.F.R,  the 
left  visual  fields  of  the  left  and  right  eye  respectively ;  the  two  fields  and  the  parts  of  the  two 
retinas  whose  excitation  produces  vision  over  the  fields  are  shaded,  the  object  a  in  the  field  of  the 
right  side  giving  rise  to  an  image  at  a',  and  a  011  the  left  side  an  image  at  a'.  The  right  optic  tract 
is  represented  as  ending  in  GL,  the  lateral  corpus  geniculatum  ;  in  Pv,  the  pulvinar ;  and  in  AQ, 
the  anterior  corpus  quadrigeminum,  all  three  stippled;  op  rod,  the  optic  radiation  from  these 
bodies  to  R.Oc,  the  right  occipital  lobes  whose  stippled  cortex  indicates  the  "  visual  area ;"  d, 
the  "direct "  tract  to  the  cortex ;  c c,  corpus  callosum, cut  across  at  the  splenium ;  I.  v.  d,  descend- 
ing horn  of  the  lateral  ventricle;  the  left  side  has  been  utilized  to  indicate  at  F,  shaded  with 
lines,  the  cortical  motor  area  for  the  eyes ;  fm.  c.  indicates  the  path  from  it  to  iii,  iv,  vi,  the  nuclei 
of  the  third,  fourth,  and  sixth  nerves ;  p.  b,  the  posterior  longitudinal  bundle,  shown  as  a  broken 
line;  NC,  the  nucleus  caudatus;  LN,  the  nucleus  lenticularis;  and  TH,  optic  thalamus  shown  in 
outline;  cia,  the  front  limb;  cig,  the  knee,  and  tip,  the  hind  limb  of  the  internal  capsule.  The 
outlines  of  the  fourth  ventricle,  4tb  Vn,  and  of  the  posterior  corpora  quadrigemina  are  shown  by 
dotted  lines,  that  of  the  bulb  is  shown  by  a  fine  line;  p,  the  pineal  gland. 


VISUAL  AND  OTHER  SENSATIONS.  693 

attached  to  and  forms  part  of  the  floor  or  ventral  wall  of  that  ventricle. 
In  a  view  of  the  basal  or  ventral  surface  of  the  brain  the  diverging  optic 
tracts  are  seen  to  separate  the  anterior  perforated  space  and  lamina  cinerea 
in  front  from  the  posterior  perforated  space,  tuber  cinereum  with  the  in- 
fundibulum,  and  corpora  albicantia  behind,  all  these  being  parts  of  the 
floor  of  the  third  ventricle.  From  the  gray  matter  in  this  floor,  fibres 
forming  what  is  sometimes  spoken  of  as  Meynert's  commissure,  belonging 
neither  to  the  optic  nerves  nor  to  the  inferior  commissure,  join  the  optic 
tracts,  eventually  leaving  them  to  pass  to  the  pes.  Hence  the  whole  of  the 
optic  tract  is  by  no  means  derived  from  the  optic  nerve ;  the  fibres  just 
mentioned  and  the  inferior  commissure  form  parts  of  the  optic  tract  not 
connected  with  the  retina. 

Each  optic  tract  crosses  obliquely,  being  in  crossing  firmly  attached  to 
the  ventral  surface  of  the  crus  cerebri  of  the  same  side  (Fig.  131,  (?),  and 
is  soon  lost  to  view,  being  covered  up  by  the  temporo-sphenoidal  lobe  of 
the  hemisphere.  When  this  is  removed  the  tract  is  seen  to  sweep  dorsally 
round  the  crus  toward  the  dorsal  aspect,  and  as  we  have  already  (§  543) 
said,  to  become  connected  on  the  further  side  of  the  crus  with  the  two  cor- 
pora geniculata,  lateral  and  median.  We  may  say  at  once  that  the  median 
corpus  geuiculatum  has  no  connection  with  that  part  of  the  tract  which  is 
derived  from  the  optic  nerve,  and  is  not  concerned  in  vision,  but  is  con- 
nected with  that  part  of  the  tract,  sometimes  called  the  median  part,  which 
goes  to  form  the  inferior  commissure.  We  may  confine  our  attention  to 
that  part  of  the  tract  which  consists  exclusively  of  fibres  coming  from  the 
retinas  of  the  two  eyes,  for  it  is  this  part,  and  this  part  only,  which  is  con- 
cerned in  vision. 

§  582.  This  ends  in  three  main  ways,  as  shown  diagrammatically  in 
Fig.  156  In  the  first  place,  part  of  the  tract  ends  in  the  lateral  corpus 
geniculatum  (GL.\  formed  of  alternating  layers  of  white  and  gray  matter, 
the  gray  matter  containing  in  some  parts  large  nerve-cells,  and  in  others 
small  nerve-cells.  In  these  cells  of  one  kind  or  another,  many  of  the  fibres 
appear  to  end.  In  the  second  place,  a  very  large  number  of  fibres  passing 
the  corpus  geniculatum  on  its  ventral  and  lateral  surfaces  spread  out  into 
the  pulvinar  (PV.).  In  the  third  place,  others  in  considerable  number, 
taking  a  more  median  direction,  reach  the  anterior  corpus  quadrigeminum 
(AQ.).  These  two  sets  also,  like  the  first,  end  apparently  in  the  nerve-cells 
of  the  respective  bodies.  Thus,  the  really  optic  fibres  of  the  optic  tract 
end  in  one  of  three  collections  of  gray  matter,  the  lateral  corpus  genicula- 
tum, the  pulvinar,  or  the  anterior  corpus  quadrigeminum.  Further,  we 
have  reasons  for  thinking  that  a  considerable  part  at  all  events  of  the  gray 
matter  of  these  three  bodies  is  associated  with  and,  in  a  certain  sense, 
dependent  on  the  fibres  of  the  optic  nerves ;  the  reasons  are  as  follows :  We 
know  that  when  a  nerve-fibre  is  cut  away  from  its  trophic  centre  it  degen- 
erates ;  but  the  division  and  the  loss  of  the  peripheral  degenerating  portion 
has  no  obvious  effect  on  the  trophic  centre ;  when  a  spinal  nerve,  for  in- 
stance, is  divided  below  the  spinal  ganglion,  though  the  nerve  below  the 
section  degenerates,  the  ganglion  and  the  piece  of  nerve  in  connection  with 
it  remain  very  much  as  before ;  we  have  it,  however,  in  our  power  to  bring 
about  changes  of  a  deeper  and  wider  character,  a  cessation  of  growth 
amounting  to  atrophy,  by  operative  interference  with  nervous  structures 
before  they  are  fully  developed.  Thus,  in  an  adult  animal,  a  section  of  an 
optic  nerve  or  removal  of  the  eye  leads  to  degeneration  in  the  optic  nerve 
and  optic  tract ;  the  optic  fibres  have  their  trophic  centre  in  certain  cells  of 
the  retina,  of  which  we  shall  speak  in  treating  of  vision,  and  cut  away  from 
that  centre  they  degenerate ;  by  this  means  the  nature  of  the  optic  decussa- 


694  THE   BRAIN. 

tion  in  animals,  and,  indeed,  in  man,  has  been  ascertained.  But  if  the  eyes 
be  removed  (removal  of  both  eyes  being  desirable  on  account  of  the  cha- 
racters of  the  optic  decussation)  in  a  newborn  animal,  not  only  do  both  the 
optic  nerves  and  the  greater  part  of  both  optic  tracts  cease  only  to  be  fur- 
ther developed  and  degenerate,  but  the  bodies  mentioned  above,  the  two 
lateral  corpora  geniculata,  the  pulvinar  on  each  side,  and  the  two  anterior 
corpora  quadrigemina,  do  not  fully  develop ;  certain  parts  of  them  undergo 
atrophy.  The  development  of  these  nervous  structures  seems  therefore  to 
be  largely  dependent  on  their  functional  connection  with  the  eyes  by  means 
of  the  optic  tracts  and  nerves. 

The  same  method  confirms  the  view  expressed  above,  that  the  median 
corpus  geniculatum  has  no  connection  with  vision.  When  the  eyes  of  new- 
born animals  are  extirpated,  neither  the  median  corpora  geniculata  nor  the 
posterior  corpora  quadrigemiua  show  any  signs  of  atrophy,  and  the  part  of 
the  optic  tract  which  does  not  degenerate  is  the  inferior  commissure  con- 
necting the  two  median  corpora  geuiculata.  Obviously  these  parts  are  asso- 
ciated with  functions  of  the  brain  other  than  those  of  sight.  The  lateral 
corpora  geniculata,  the  pulviuar,  and  the  anterior  corpora  quadrigemina,  are, 
we  may  repeat,  alone  to  be  regarded  as  the  chief  central  parts  in  which  the 
optic  nerve  ends.  We  may  also  repeat  that  owing  to  the  peculiarity  of  the 
optic  decussation  each  optic  nerve  thus  finds  its  endings  in  both  sides  of  the 
brain. 

While  the  optic  chiasma  is,  as  we  have  seen,  helping  to  form  the  floor  of 
the  third  ventricle,  it  gives  off  fibres  to  the  posterior  perforated  spot.  Some 
of  these  have  been  supposed  to  pass  directly  in  the  wall  of  the  ventricle  to 
the  nucleus  of  the  third  (oculo-motor)  nerve,  and  to  serve  as  a  channel  for 
afferent  impulses,  causing  constriction  of  the  pupil ;  but  to  this  we  shall 
return  in  dealing  hereafter  with  the  movements  of  the  pupils. 

§  583.  Though  the  above  three  bodies  are  undoubtedly  the  chief  endings 
of  the  optic  nerve,  three  primary  visual  centres,  if  we  may  so  call  them,  it 
is  also  believed  that  some  fibres  of  the  optic  tract,  making  connections  with 
neither  of  these  three  bodies,  pass  by  the  crus  cerebri  straight  to  certain  parts 
of  the  cerebral  hemisphere  (Fig.  156,  d) ;  but  this  fourth  ending  is  by  no' 
means  so  clearly  established  as  are  the  other  three. 

And  undoubtedly  the  main  connection  of  the  cerebral  hemisphere  with 
the  optic  tract  is  not  a  direct  one,  but  an  indirect  one,  through  the  three 
bodies  in  question.  A  number  of  fibres  proceeding  from  the  occipital  cortex 
and  reaching  the  thalamus  through  the  hind  limb  of  the  internal  capsule 
formed  what  was  called  the  "  optic  radiation."  These  fibres  beginning  (or 
ending)  in  the  cortex  of  the  occipital  region,  end  (or  begin)  (Fig.  156,  op.  rad.} 
to  a  large  extent  in  the  pulvinar  and  in  the  lateral  corpus  geniculatum,  but 
also  in  the  anterior  corpus  quadrigeminum,  reaching  it  by  the  anterior 
brachium  (§  547).  When  even  in  a  grown  animal  the  occipital  cortex  is 
destroyed,  not  only  these  fibres,  but  also  parts  of  the  pulvinar  and  external 
corpus  geniculatum,  undergo  degeneration,  and  there  is  some  change  in  the 
anterior  corpus  quadrigeminum.  When  the  same  cortex  is  destroyed  in  a 
newborn  animal  the  same  parts  atrophy  ;  and  in  such  cases  the  optic  tract 
and  nerve,  which  are  but  little  affected  by  the  operation  in  the  adult  animal, 
are  also  involved  in  the  atrophy.  We  may  add  that  removal  of  both  eyes 
in  the  newborn  animal  is  said  to  lead,  besides  the  atrophy  of  the  three  bodies 
in  question,  to  diminished  occipital  lobe  due  to  lack  of  white  matter.  We 
may  therefore  conclude  that  in  the  complex  act  of  vision  two  orders  of  cen- 
tral apparatus  are  involved  ;  we  may  speak  of  two  kinds  of  centres  for 
vision — the  primary  or  lower  visual  centres  supplied  by  the  three  bodies  of 
which  we  are  speaking,  and  a  secondary  or  higher  visual  centre  supplied  by 


VISUAL  AND  OTHER  SENSATIONS.  695 

the  cortex  in  the  occipital  region  of  the  cerebrum.  And  experimental  results 
accord  with  this  view. 

Before  we  proceed  to  discuss  those  results,  one  or  two  preliminary  obser- 
vations may  prove  of  use. 

In  the  first  place,  as  we  have  previously  urged,  the  interpretation  of  the 
results  of  an  experiment,  in  which  we  have  to  judge  of  sensory  effects,  are 
far  more  uncertain  than  when  we  have  to  judge  of  motor  effects,  that  is,  of 
course,  when  the  experiment  is  conducted  on  an  animal.  We  can  estimate 
the  motor  effect  quantitatively,  we  can  measure  and  record  the  contraction 
of  the  muscle,  but  in  estimating  a  sensory  effect  we  have  to  depend  on  signs, 
our  interpretation  of  which  is  based  on  analogies  which  may  or  may  not  be 
misleading.  We  are  on  safer  ground  when  we  can  appeal  to  man  himself 
in  the  experiments  instituted  by  disease ;  but  the  many  advantages  thus 
secured  are  often  more  than  counterbalanced  by  the  diffuse  characters  or 
the  complex  concomitants  of  the  lesion.  In  dealing  with  sensory  effects  we 
must  expect  and  be  content  for  the  present  with  conclusions  less  definite 
and  more  uncertain  even  than  those  gained  by  the  study  of  motor  effects. 

In  the  second  place,  in  dealing  with  vision,  it  will  be  desirable  to  know 
the  meaning  which  we  are  attaching  to  the  words  which  we  employ.  By 
blindness,  that  is,  "  complete  "  or  "total "  blindness,  we  mean  that  the  move- 
ments and  other  actions  of  the  body  are  in  no  way  at  all  influenced  by  the 
amount  of  light  falling  on  the  retina.  Of  partial  or  incomplete  or  imperfect 
vision,  using  the  word  vision  in  its  widest  sense,  there  are  many  varieties  ;  and 
we  may  illustrate  some  of  the  defects  of  the  visual  machinery,  regarded  as  a 
whole,  with  its  central  as  well  as  its  peripheral  parts,  by  referring  to  certain 
defects  of  vision  due  to  changes  in  the  eye  itself.  The  eye  may  fall  into  such 
a  condition  that  the  mind  can  only  appreciate,  and  that  to  a  varying  degree, 
the  difference  between  light  and  darkness  ;  the  mind  is  aware  that  the  retina 
(or  it  may  be  part  of  the  retina)  is  being  stimulated  to  a  less  or  greater 
degree,  but  cannot  perceive  that  one  part  of  the  retina  is  being  stimulated 
in  a  different  way  from  another  part ;  a  sensation  of  light  is  excited,  but  not 
a  set  of  visual  sensations  corresponding  to  the  sets  of  pencils  of  luminous 
rays,  which,  reflected  or  emanating  from  external  objects  in  a  definite  order, 
are  falling  upon  the  eye.  The  eye  again  may  fall  into  another  condition,  in 
which  such  sets  of  visual  sensations  are  excited,  but  on  account  of  dioptric 
imperfections  or  for  other  reasons  the  several  sensations  are  not  adequately 
distinct ;  the  mind  is  aware  through  the  eye  of  the  existence  of  "  things," 
but  cannot  adequately  recognize  the  character  of  those  things ;  the  visual 
images  are  blurred  and  indistinct.  And  a  large  number  of  gradations  are 
possible  between  the  extreme  condition  in  which  only  those  objects  which 
present  the  strongest  contrast  with  their  surroundings  are  visible  to  a  condi- 
tion which  only  just  falls  short  of  normal  vision.  Imperfections  of  this  kind, 
of  varying  degree,  may  result  from  failure,  not  in  the  peripheral  apparatus, 
not  in  the  retina  or  optic  nerve  or  other  parts  of  the  eye,  but  in  the  central 
apparatus ;  the  retinal  image  may  be  sharp,  the  retina  and  the  optic  fibres 
may  be  duly  responsive,  but  from  something  wrong  in  some  part  or  other  of 
the  brain  the  visual  sensations  excited  by  the  visual  impulses  may  fail  in 
distinctness,  and  that  in  varying  degree  ;  imperfections  of  vision,  whether  of 
central  or  peripheral  origin,  in  wrhich  visual  sensations  fail  in  distinctness, 
are  generally  spoken  of  under  the  not  wholly  unexceptionable  name  of 
amblyopia. 

If  one  optic  nerve  be  divided,  total  blindness  of  one  eye  will  result ;  but 
if  one  optic  tract  be  divided,  it  follows  from  what  has  been  said  above  that 
half-blindness  in  the  corresponding  halves  of  both  eyes  will  result.  If,  for 
instance,  the  right  optic  tract  (Fig.  154,  Op.  T.)  be  divided,  the  left  visual 


6%  THE   BRAIN. 

fields  of  both  eyes  will  be  blotted  out.  The  same  condition  will  be  brought 
about  by  failure  in  the  optic  tract  at  its  central  ending,  provided  of  coursi? 
the  mischief  be  confined  to  the  ending  of  the  one  tract.  Such  a  half-blind- 
ness or  half-vision  is  spoken  of  as  hemianopsia  or  hemianopia  or  kemiopia ; 
the  words  left  and  right  are  generally  used  in  reference  to  the  visual  field ; 
thus,  left  hemiauopsia  is  the  blotting  out  of  both  left  visual  fields  through 
failure  of  the  right  optic  tract. 

If,  instead  of  the  whole  optic  nerve  being  divided,  certain  bundles  only 
were  cut  across,  partial  blindness  would  be  the  result,  a  portion  of  the  visual 
field  would  be  blotted  out,  and  mischief  limited  to  a  few  bundles  of  one  optic 
tract  would  lead  to  corresponding  blots  in  the  corresponding  halves  of  the 
visual  fields  of  both  eyes. 

Further,  an  affection  of  half  the  retina  or  of  a  limited  area  in  the  retina 
might  occur  of  such  a  character  as  to  lead,  not  to  complete,  but  to  partial 
blindness,  to  a  hemi-amblyopia  or  to  a  partial  amblyopia.  The  part  of  the 
retina  so  affected  might  be  central,  or  peripheral,  or  a  quadrant,  or  any  patch 
of  any  size,  form,  and  relative  position.  And  we  may  further  imagine  it  at 
least  possible  that  mischief  in  the  brain  might  be  so  limited  as  to  produce 
any  of  the  above  partial  effects,  though  the  retina,  optic  nerve,  and  optic 
tracts  all  remained  intact. 

The  above  visual  imperfections  we  have  illustrated  by  changes  in  the 
peripheral  apparatus,  but  there  is  a  kind  of  imperfection  which  we  may  still 
call  a  visual  imperfection,  though  it  is  of  purely  central  origin.  In  a  normal 
state  of  things  a  visual  sensation  excited  in  the  brain  is  or  maybe  linked  on 
to  a  chain  of  physical  events ;  we  often,  then,  speak  of  it  as  a  visual  idea. 
When  we  see  a  dog  the  visual  sensation,  or  rather  the  group  of  sensations 
making  up  the  visual  perception  of  the  dog,  does  not  exist  by  itself,  apart 
from  all  the  other  events  of  the  brain  ;  it  joins  and  affects  them,  and  among 
the  events  which  it  so  affects  may  be  and  often  are  psychical  events;  the 
visual  perception  "enters  into  our  thoughts"  and  modifies  them.  Between 
the  visual  impulse,  as  it  travels  along  the  optic  nerve  or  tract,  and  its  ulti- 
mate psychical  effect,  a  whole  series  of  events  intervene ;  and  we  may  take 
it  for  granted  that  the  chain  may  be  broken  or  spoiled  at  any  of  its  links,  alt 
the  later  as  well  as  at  the  earlier  ones.  We  may  therefore  consider  it  possi- 
ble that  the  break  or  damage  may  occur  at  the  links  by  which  the  fully 
developed  visual  sensation  joins  on  to  psychical  operations.  We  may  sup- 
pose that  an  object  is  seen  and  yet  does  not  affect  the  mind  at  all,  or  affects 
it  in  an  abnormal  way. 

These  foregoing  considerations  emphasize  the  difficulty  and  uncertainty  of 
interpreting  the  visual  condition  of  an  animal  which  has  been  experimented 
upon.  When,  for  instance,  after  an  operation,  an  animal  ceases  to  be  influ- 
enced in  its  previous  normal  manner  by  the  visual  effects  of  external  objects, 
a  most  careful  psychical  analysis  is  often  necessary  to  enable  us  to  judge 
whether  the  newly  introduced  disregard  of  this  or  that  object  is  due  to  the 
mere  visual  sensation  being  blurred  or  blunted,  or  to  some  failure  in  the 
psychical  appreciation  of  the  sensations ;  and  in  most  cases  such  an  analy.sis 
is  beyond  our  reach.  The  greatest  caution  is  needful  in  drawing  conclusions 
from  experiments  of  this  kind,  especially  from  such  as  appear  to  have  been 
hastily  carried  out  or  hastily  observed  ;  and  we  must  be  content  here  to 
dwell  on  some  of  the  broader  features  only  of  the  subject. 

§  584.  Since  we  have  in  this  matter  to  trust  so  much  to  analogies  with  our 
own  experience,  we  may  turn  at  once  to  the  monkey  as  being  more  instruc- 
tive than  any  of  the  lower  animals.  We  have  already  said  that  electrical 
excitation  of  the  occipital  cortex  behind  the  motor  region  may  produce 
movements,  but  that  these  movements  are  in  character  different  from  those 


VISUAL   AND  OTHER   SENSATIONS.  697 

caused  by  stimulation  of  the  motor  region  itself.  In  the  monkey  stimulation 
of  parts  of  the  occipital  region,  the  occipital  lobe,  and  the  angular  gyrus,  for 
instance,  may  give  rise  to  movements  of  the  eyes,  of  the  eyelids,  and  of  the 
head,  that  is  of  the  neck,  all  the  movements  so  produced  being  such  as  are 
ordinarily  connected  with  vision.  It  will  not  be  profitable  to  enter  here  into 
the  details  concerning  the  exact  topography  of  the  excitable  parts  or  of  the 
special  characters  of  the  movements  so  called  forth.  But  it  is  important  to 
note  that  these  movements  are  unlike  the  movements  excited  by  stimulation 
of  the  appropriate  motor  area,  inasmuch  as  their  occurrence  is  far  less  cer- 
tain, they  need  a  stronger  stimulus  to  bring  them  out ;  when  evoked  they  are 
feeble,  being  easily  antagonized  by  appropriate  stimulation  of  the  motor 
area,  and  they  have  a  much  longer  latent  period.  They  are  not  due  to  any 
indirect  stimulation  of  the  motor  area,  through  "  association  "  fibres  con- 
necting the  spot  stimulated  with  the  motor  area,  or  otherwise,  since  they 
persist  after  removal  of  the  motor  area.  Movements  of  this  kind  may  also 
be  witnessed  in  the  dog.  They  are  obviously  the  result  of  impulses  trans- 
mitted in  some  direct  manner  from  the  cortex  to  some  parts  below,  and  may 
be  taken  as  an  indication  that  the  parts  of  the  cortex  in  question  are  in  some 
way  connected  with  vision.  The  exact  manner,  however,  in  which  they  are 
brought  about  is  at  present  obscure.  The  explanation  of  their  genesis  which 
is  frequently  offered,  namely,  that  the  stimulation  so  affects  the  cortical  gray 
matter  as  to  give  rise  to  visual  sensations,  and  that  the  movements  express 
these  sensations,  does  not  seem  satisfactory.  For,  if  it  be  possible  that  the 
gross  changes  which  the  electric  current  sets  going  in  the  cortical  gray  matter 
can  reproduce  the  psychical  events  which  take  place  in  that  gray  matter  in 
the  normal  action  of  the  brain,  we  should  expect  stimulation  of  any  and 
every  part  of  the  cortex  to  call  forth  some  movement  or  other,  since  it  cannot 
be  doubted  that  every  part  of  the  cortex  is  in  some  way  or  other  engaged  in 
psychical  operations,  and  that  every  psychical  phase  tends  to  express  itself 
in  movement :  whereas,  outside  the  motor  region,  with  the  exceptions  we 
are  now  discussing,  the  cortex  is,  as  we  have  seen,  "  inexcitable,"  and  even 
within  the  motor  region  itself  the  excitable  substance  is  scattered,  with  in- 
creasing segregation  as  we  advance  along  the  animal  scale,  among  inexcitable 
substance.  When  we  speak  of  the  region  or  substance  as  inexcitable,  we  do 
not  mean  that  the  electric  current  produces  no  effect;  we  only  mean  that 
the  effect  is  not  manifested  by  movement ;  the  real  difference  between  the 
excitable  motor  region  and  the  inexcitable  rest  of  the  cortex  is  probably 
that  in  the  several  motor  areas  the  current,  playing  upon  the  beginnings  of 
the  pyramidal  fibres,  is  able  to  inaugurate  simple  motor  impulses  or  some- 
thing like  them,  whereas  elsewhere  the  molecular  changes  induced  by  the 
current  are  too  confused  to  reach  their  normal  expression.  There  can  be  no 
doubt,  of  course,  that  molecular  changes  in  this  or  that  part  of  the  brain, 
set  going  by  processes  other  than  actual  visual  impulses  along  the  optic 
nerves,  may  give  rise  to  visual  sensations ;  and,  as  we  shall  see  in  dealing 
with  the  senses,  the  subject  of  such  "subjective"  sensations  is  unable  to 
distinguish  them  from  sensations  of  "objective"  origin  ;  but  it  is  at  least 
unlikely  that  the  coarse  disturbances  started  by  a  tetanizing  current  should 
take  such  a  definite  form.  Moreover,  the  view  in  question  is  disproved  by 
the  experimental  result  that  the  same  movements  are  brought  about  when 
the  cortex  is  pared  away  and  the  electrodes  are  applied  to  the  subjacent 
white  matter.  This  result  suggests  the  existence  of  efferent  tracts  or  bundles 
of  a  special  kind,  differing  from  those  of  the  pyramidal  kind,  though  like 
them  making  connections  with  the  ocular  and  other  muscles;  we  have, 
however,  as  yet  no  other  evidence  of  such  tracts  existing. 

§  585.  The  results  of  removal  of  the  cortex  support  the  same  general 


698  THE   BRAIN. 

conclusion,  though  there  is  much  discordance  among  the  various  observers, 
both  as  to  the  particular  results  and  especially  as  to  their  interpretation. 
One  broad  fact  comes  out  in  all  the  observations,  namely,  that  the  removal 
of  or  injury  to  the  hind  region  of  the  cortex  always  produces  some  disturb- 
ance of  vision,  and  produces  disturbance  of  vision  more  surely  and  to  a 
greater  extent  than  does  injury  to  or  removal  of  any  other  region  of  the 
cortex ;  but  beyond  this  broad  fact  there  is  much  dispute,  and  we  must  be 
content  here  with  a  very  brief  statement. 

In  the  monkey,  some  observers  have  found  that  removal  of  the  occipital 
lobe  on  one  side  (the  region  marked  "  vision  "  in  Figs.  149  and  150)  caused 
hemiopia,  the  effect  on  the  visual  fields  being  a  crossed  one ;  when  the  right 
lobe  was  removed  there  was  blindness  in  the  left  visual  fields,  that  is,  in  the 
right  halves  of  the  retinas  of  both  eyes  ;  in  other  words,  the  visual  impulses 
passing  along  the  right  optic  tract  failed  to  produce  their  usual  effect,  so 
that  the  animal  disregarded  objects  on  its  left-hand  side.  We  may  remark 
that  the  decussation  of  the  optic  nerves  in  the  monkey  is  very  similar  to  that 
in  man.  When  both  occipital  lobes  were  removed,  total  blindness  resulted. 
But,  and  this  is  most  important,  not  only  was  the  hemiopia  caused  by  the 
removal  of  one  lobe  transient,  but  also  according  to  some  observers,  the  lost 
vision  returned  after  the  total  removal  of  both  lobes,  though  some  impair- 
ment might  be  noticed  long  afterward,  so  long  in  fact  as  the  animal  was  kept 
alive. 

In  the  hands  of  other  observers  destruction  of  the  angular  gyrus  of  one 
side  (Fig.  148)  has  led  to  hemiopia,  failure  in  the  left  (or  right)  visual  field, 
indicating  failure  in  the  central  endings  of  the  right  (or  left)  optic  tract, 
being  caused  by  removal  of  the  right  (or  left)  gyrus,  and  destruction  of  both 
angular  gyri  has  led  to  total  blindness,  not  only  the  hemiopia,  but  the  total 
blindness  being,  however,  apparently  transitory.  And  cases  have  been  ob- 
served in  which  the  transient  blindness  due  to  removal  of  the  occipital  lobes 
has  been  succeeded  by  permanent  hemiopia  upon  the  subsequent  removal  of 
the  angular  gyrus.  Indeed,  the  general,  but  not  uniform,  tendency  of  the 
many  experiments  which  have  been  made  is  to  connect,  in  the  monkey,  both 
the  occipital  lobe  and  the  angular  gyrus  with  vision. 

In  the  dog,  removal  of  portions  of  the  occipital  cortex  have  also  led  to 
partial  and  transient  blindness,  or,  according  to  some,  to  permanent  blind- 
ness ;  but  the  difficulties  of  judging  the  visual  condition  of  a  dog  are  very 
considerable,  and  his  vision  is  so  different  from  that  of  a  man,  so  much  less 
binocular,  for  instance,  than  his,  that  it  would  not  be  profitable  to  relate  at 
length  the  results  obtained  in  the  dog  or  to  discuss  the  conclusions  which 
have  been  derived  from  them.  We  will  only  say  that  some  observers  have 
been  led  to  think  that  the  lateral  part  of  the  retina  is  connected  with  the 
lateral  part  of  the  visual  occipital  area,  the  front  part  with  the  front  part, 
and  so  on,  the  retina  being,  as  it  were,  projected  on  to  the  occipital  cortex ; 
but  the  facts  are  not  clear  enough  to  make  it  worth  while  to  dwell  upon 
them  here. 

In  man,  clinical  histories  so  far  conform  to  the  results  of  experiments  on 
the  monkey  as  to  associate  the  occipital  cortex,  and  more  particularly  the 
cuneus  (see  Figs.  152  and  153),  with  vision.  They  have,  however,  raised  a 
point  on  which  we  have  not  yet  touched.  In  the  experiments  on  the  mon- 
key quoted  above,  the  result  (putting  aside  transient  effects  due  probably  to 
"  shock  ")  of  interference  with  one  side  of  the  brain  was  hemiopia ;  and  this 
is  what  we  might  expect  from  the  anatomical  relations  ;  the  optic  tract  goes 
straight  to  the  tegmental  masses  of  its  own  side,  and  the  optic  radiation 
passes  from  those  masses  to  the  occipital  cortex  of  the  same  side ;  there  is 
no  decussation,  save  of  the  fibres  of  the  optic  nerve,  as  they  pass  into  the 


VISUAL  AND  OTHER  SENSATIONS.  699 

optic  tract  at  the  chiasma.  Clinical  histories  teach  the  same  lessons  as  these 
experiments  on  animals ;  lesions  limited  to  the  occipital  lobe  have  for  a 
symptom  hemiopia ;  and  this  is  said  to  be  especially  the  result  of  mischief 
limited  to  the  apex  of  the  occipital  lobe,  that  is,  to  the  cuneus.  But  experi- 
ments on  monkeys  have  been  made  in  which  destruction  of  one  angular 
gyrus  has  produced,  not  hemiopia,  but  crossed  blindness  or  crossed  ambly- 
opia,  that  is  to  say,  has  affected  the  whole  of  the  retina  of  one  eye,  and  that 
the  crossed  eye,  the  eye  of  the  same  side  not  being,  or  being  supposed  not  to 
be,  at  all  affected  ;  similar  results  have  also  been  stated  to  follow  upon 
removal  of  one  occipital  lobe.  And  a  few  clinical  cases  have  been  recorded 
in  which  disease,  especially  of  the  angular  gyrus,  seemed  to  affect  the  vision 
of  the  whole  of  the  crossed  eye.  (It  must  be  remembered  that  the  angular 
gyrus  of  man  corresponds  to  a  part  only  of  the  whole  angular  gyrus  of  the 
monkey.  (Cf.  Fig.  148  with  Fig.  152.)  Some  authors  have,  in  accordance 
with  this,  put  forward  the  theory  that  the  occipital  lobe  serves  as  a  cortical 
centre  for  the  optic  tract  of  its  own  side  only,  and  so  for  one-half  of  each 
retina,  while  in  front  of  this  on  the  angular  gyrus  is  a  centre  in  which  both 
optic  tracts  are  represented.  But  the  clinical  histories  bearing  on  this  point 
cannot  be  regarded  as  wholly  satisfactory  ;  and  with  reference  to  the  experi- 
mental results  we  may  once  more  insist,  and  the  warning  applies  perhaps 
with  particular  force  to  these  experiments  on  vision,  on  the  danger  of  con- 
founding those  immediate  effects  of  operative  interference,  which  are  of  the 
nature  of  "shock"  in  the  wide  sense  of  that  word,  with  those  pure  "de- 
ficiency "  phenomena  which  are  alone  the  outcome  of  the  loss  of  the  part 
removed.  It  is  difficult  to  resist  the  conclusion  that  much  of  the  transitory 
blindness  which  is  observed  in  these  experiments  belongs  to  the  former 
category,  that  the  effect  is  transient  because  it  is  of  the  nature  of  shock,  and 
not  because  the  loss  of  faculty  is  supplied  by  some  other  cortical  area  being 
subsequently  substituted  for  the  one  removed.  In  the  dog,  injury  to  the 
frontal  region  of  the  cortex  unaccompanied  by  any  secondary  mischief  in 
the  occipital  region  has  led  to  impaired  vision ;  and  this  was  probably  an 
instance  of  "shock,"  for  we  have  no  other  reason  to  connect  the  frontal 
region  of  the  cortex  with  vision.  We  must  be  very  careful  in  drawing  the 
conclusion  that,  because  an  operation  produces  transient  blindness,  the  part 
operated  on  has  a  direct  share  in  vision ;  and  we  may  well  hesitate  to 
accept  the  view  that  the  whole  retina  is  represented  in  the  crossed  hemi- 
sphere. 

In  conclusion  we  may  say  that,  when  all  the  many  results  which  have 
been  arrived  at  by  experiment  or  by  clinical  observation  are  duly  weighed, 
it  will  be  felt  that  while  the  evidence  for  the  occipital  lobe,  especially  the 
cuneus,  being  concerned  in  the  matter  is  convincing,  we  cannot  in  the  pres- 
ent state  of  our  knowledge  dogmatically  exclude  the  angular  gyrus,  and  that 
hence  the  only  clear  and  consistent  statement  which  can  be  made  with  any 
confidence  is  the  broad  and  simple  one  that  the  hind  region  of  the  cortex  is 
in  some  way  intimately  concerned  in  vision. 

§  586.  Such  an  attitude  becomes  all  the  more  necessary  when  we  ask 
ourselves  the  question,  What  is  it  which  actually  takes  place  in  the  cortex 
during  vision  ?  Are  we  to  conceive  of  it  as  if  a  visual  impulse  set  going 
along  the  fibres  of  the  optic  tract  underwent  no  essential  change  until  it 
reached  the  cortex,  as  if  it  there  suddenly  developed  into  a  "  visual  sensa- 
tion?" We  can  hardly  suppose  this.  Between  the  cortex  and  the  optic 
tract,  the  lower  visual  centres,  the  tegmental  masses,  intervene ;  and  we  can 
hardly  suppose  that  interference  with  these  bodies  produces  the  same  effect 
on  vision  as  simple  section  of  the  optic  tract.  We  have  seen  in  a  previous 
section  that  the  frog  and  the  bird  certainly,  and  according  to  some  observers 


700  THE  BRAIN. 

also  the  rabbit,  are  in  the  absence  of  the  cerebral  hemispheres  not  totally 
blind,  their  movements  being  guided  by  retinal  impressions ;  and  cases  are 
recorded  of  the  dog  being  obviously  still  guided  in  some  measure  by  retinal 
impressions  after  the  occipital  lobes  had  been  wholly  or  almost  wholly 
removed.  And,  though  this  is  a  matter  at  present  outside  exact  knowledge, 
and  though  it  is  perhaps  possible  for  simple  afferent  impulses  to  determine 
even  complex  movements  without  the  intervention  of  "  consciousness,"  we 
are  probably  justified  in  assuming  that  the  simple  visual  impulses,  travelling 
along  the  fibres  of  the  optic  tract,  undergo  important  transformations  in  the 
tegmental  masses,  and  that  the  changes  which  are  propagated  along  the 
fibres  of  the  optic  radiation  constitute  something  quite  different  from  the 
impulses  along  the  optic  tract  or  nerve. 

Judging  from  the  analogy  of  the  motor  region  we  may  probably  assume 
that  in  vision  the  cortical  events  are  psychical  in  nature,  and  that  the  func- 
tion of  the  optic  radiation  is  to  furnish  what  we  may  call  crude  visual  sensa- 
tions for  further  psychical  elaboration. 

Nor  need  this  view  compel  us  to  suppose  that  injury  to  or  removal  of 
the  cortex  must  produce  only  psychical  blindness  or  psychical  impairment 
of  vision,  though  this  point  has  probably  not  been  sufficiently  held  in  view 
during  the  various  experiments,  sufficient  care  not  having  been  taken  to 
determine  how  far  the  blindness  was  purely  psychical.  Bearing  in  mind  the 
degeneration  following  upon  lesions  of  the  occipital  cortex,  and  the  far- 
reaching  effects  of  any  operation  on  the  brain,  we  may  suppose  that  injury 
to  the  cortex  affects  the  lower  centres  as  well;  and  some  of  the  transient 
impairment  of  vision,  on  which  we  have  just  dwelt,  may,  perhaps,  be  ex- 
plained as  the  effect  of  the  cortical  injury  on  the  lower  centres. 

Although  the  matter  is  thus  in  many  of  its  details  at  present  outside  our 
exact  knowledge,  we  may  probably  conclude  that  in  the  complex  act  of 
complete  vision,  while  part,  especially  the  more  psychical  part,  is  carried 
out  in  the  cortex,  more  particularly  of  the  occipital  region,  part  is  accom- 
plished in  the  lower  centres,  the  tegmental  masses.  As  to  the  several  func- 
tions of  the  three  masses,  we  know  almost  absolutely  nothing.  Electric 
stimulation,  and,  it  is  said,  mechanical  stimulation  also,  of  the  anterior  cor- 
pora quadrigemina  in  mammals,  or  the  optic  lobes  in  lower  animals,  calls 
forth  movements  of  the  eyes,  and  of  various  parts  of  the  body  ;  and  removal 
of  them  causes  blindness  and  in  some  cases  loss  of  coordination  of  move- 
ments. Our  knowledge  on  these  points  is  not  very  exact;  but  from  the 
above  facts  as  well  as  from  the  connections  of  the  anterior  corpora  quad- 
rigemina with  the  parts  of  the  brain  behind  we  may  suppose  that  these 
bodies  are  more  especially  concerned  with  the  part  visual  impulses  play  in 
determining  the  coordination  of  movements.  We  must  remember,  however, 
that  all  three  masses  are  connected  with  the  cortex,  and  probably  all  three 
play  a  part  in  vision  even  of  the  highest  psychical  kind. 

Sensations  of  Smell. 

§  587.  In  many  animals  in  whom  the  sense  of  smell  is  acute,  a  portion 
of  the  cortex,  known  as  the  "  pyriforrn  lobe  "  or  "  hippocampal  lobule,"  and 
which  is  anatomically  continuous  with  the  front  end  of  the  hippocampal 
gyrus  (the  part  to  which  the  name  uncinate  gyrus  is  often  restricted),  ac- 
quires relatively  large  dimensions.  This  and  the  anatomical  relations  just 
mentioned  would  lead  us  to  suppose  that  a  part  of  the  cortex  which  is  con- 
tinuous with  the  front  end  of  the  hippocampal  gyrus  is  in  some  way  con- 
nected with  smell.  The  argument  from  comparative  anatomy,  however,  is 
one  which  must  be  used  with  caution  ;  since,  besides  the  great  difficulty  of 


, 


VISUAL  AND  OTHER  SENSATIONS.  701 

determining  the  homologies  of  parts  of  the  brain  in  different  animals,  rela- 
tive increase  in  the  part  in  question  might  be  correlated  to  other  things  than 
the  power  of  smell,  and  might  be  determined  by  circumstances  having  no 
relation  to  smell. 

The  experimental  evidence,  though,  on  the  whole,  it  gives  support  to  the 
view,  is  conflicting  ;  and  when  the  difficulty  of  determining  whether  a 
"  dumb  animal  "  can  or  cannot  smell  is  borne  in  mind,  this  will  not  be  won- 
dered at.  The  observation  that  electrical  stimulation  of  the  region  in  ques- 
tion gives  rise  to  movements  of  the  nostrils,  which  have  been  interpreted  as 
sniffing  in  response  to  subjective  olfactory  sensations,  cannot  have  much 
weight ;  and  while  some  observers  have  found  that  the  removal  of  this 
part  of  the  brain  destroys  the  sense  of  smell,  others  have  obtained  negative 
results. 

The  few  clinical  histories  which  bear  upon  the  matter  are,  perhaps,  more 
trustworthy.  These  seem  to  show  that  a  lesion  involving  the  cortex  of  this 
region,  but  leaving  the  olfactory  bulb  and  tract,  as  well  as  other  parts  of 
the  brain,  intact,  may  destroy  or  greatly  impair  smell.  And  we  may,  per- 
haps, give  particular  weight  to  the  cases  in  which  epileptiform  attacks,  pre- 
ceded by  an  "  aura  "  in  the  form  of  a  peculiar  smell,  have  been  associated 
with  disease  limited  to  this  region  ;  for  the  phenomena  of  "  aura  "  seem  to 
be  connected  with  cortical  processes. 

Though  the  evidence  on  the  whole  goes  to  show  that  the  cortex  at  the 
front  end  of  the  hippocampal  gyrus  is  especially  connected  with  smell,  and 
we  have  so  marked  it  (in  Fig.  155),  yet  the  whole  matter  stands  on  a  some- 
what different  footing  from  the  sense  of  sight.  In  man  the  relations  of 
smell  to  the  other  operations  of  the  brain  (though,  as  we  shall  see  in  deal- 
ing with  the  senses,  somewhat  peculiar)  are  far  more  limited  than  are  those 
of  vision,  and  the  psychical  development  of  simple  olfactory  sensations  is 
extremely  scanty. 

Sensations  of  Taste. 

§  588.  This  special  sense,  though  so  closely  associated  with  smell,  stands, 
together  with  the  special  sense  of  hearing,  on  a  different  footing  from  the 
two  preceding  special  senses,  since  the  nerves  concerned  belong  to  the  cate- 
gory of  ordinary  cranial  nerves,  and  we  lack,  in  reference  to  them,  the  ana- 
tomical leading  which  is  offered  to  us  in  the  case  of  the  optic  and  olfactory 
nerves. 

We  shall  see  in  dealing  with  the  senses  that  the  fifth  nerve  and  the 
glosso-pharyngeal  nerve  have  been  considered  as  nerves  of  taste,  but  that 
the  matter  is  one  subject  to  controversy ;  the  gustatory  function  of  the  fifth 
is  attributed  to  the  peculiar  chorda  tympani  nerve,  and  other  questions  have 
been  raised.  Whatever  view  we  take,  however,  the  nerves  of  taste  are  ordi- 
nary cranial  nerves,  and  we  have  no  anatomical  guidance  as  to  the  fibres  of 
either  of  the  above  two  nerves  making  special  connections  with  any  part  of 
the  cortex.  Though  sensations  of  taste  enter  largely  into  the  life  of  ani- 
mals, and,  indeed,  of  man  himself,  we  have  no  satisfactory  indications  which 
will  enable  us  to  connect  this  special  sense  with  any  part  of  the  cortex ;  the 
view,  indeed,  has  been  put  forward  that  some  part  of  the  cortex  in  the 
lower  portion  of  the  temporal  lobe,  not  far  from  the  centre  for  smell,  serves 
as  a  centre  for  taste ;  but  the  arguments  in  favor  of  this  view  are  not,  as 
yet  at  least,  convincing. 

Sensations  of  Hearing. 

§  589.  The  cochlear  division  of  the  eighth  or  auditory  nerve  may  be 
assumed  to  be  a  nerve  of  the  special  sense  of  hearing,  and  of  that  alone ; 


702  THE   BRAIN. 

the  vestibular  division  serves,  as  we  have  seen,  for  other  functions  than  those 
of  hearing  (§  555),  but,  as  we  shall  urge  in  dealing  with  the  senses,  is  not  to 
be  regarded  as  wholly  useless  for  the  purposes  of  that  sense.  The  cochlear 
division  we  have  traced  (§  531)  into  the  bulb,  and  the  vestibular  division 
into  the  lateral  auditory  nucleus  (which,  perhaps,  maybe  regarded  as  a 
continuation  or  segmental  repetition  forward  of  the  cuneate  nucleus  or  of 
part  of  that  nucleus),  and  into  the  cerebellum,  the  cerebellar  continuation 
being  probably  the  part  of  the  nerve  which  serves  for  coordinating  func- 
tions. The  connections  of  the  auditory  nerve  with  the  cerebral  hemisphere 
belong  to  the  same  category  as  those  of  other  afferent  cranial,  and  we  may 
add  spinal,  nerves;  we  have  no  very  clear  anatomical  guide  toward  any 
particular  part  of  the  cortex. 

When  we  turn  to  the  empirical  results  furnished  by  experiment  and 
clinical  observations,  we  find  that  these,  though  even  less  definite  and  less 
accordant  than  in  the  case  of  the  senses  of  sight  and  smell,  point  to  part  of 
the  first  or  superior  temporal  (temporo-sphenoidal)  convolution  (Figs.  149, 
152,  and  154)  lying  in  the  temporal  lobe  just  ventral  to  the  Sylvian  fissure, 
as  being  especially  concerned  in  hearing  in  some  such  way  as  the  occipital 
lobe  is  concerned  in  vision. 

Electrical  stimulation  of  this  region  of  the  cortex  gives  rise  to  "  pricking 
of  the  ears,"  and  other  movements  such  as  are  frequently  connected  with 
auditory  sensations ;  but  such  phenomena  are  in  this  instance,  perhaps,  to 
be  depended  upon  even  less  than  in  other  similar  instances.  While  some 
observers  maintain  that  this  convolution,  the  operation  including  other  por- 
tions of  the  temporal  lobe  as  well,  may  be  removed  from  a  monkey  without 
producing  any  certain  signs  of  deafness,  other  observers  have  found  that 
removal  of  it  on  one  side  affected  the  hearing  of  the  ear  on  the  opposite  side, 
and  removal  on  both  sides  brought  the  animal  into  a  condition  in  which, 
without  being,  perhaps,  absolutely  deaf,  it  reacted  toward  sound  in  a  very 
imperfect  manner  indeed,  very  different  from  its  normal  behavior.  The 
scanty  clinical  histories  bearing  on  this  matter  are  not  very  decisive ;  for 
though  deafness  has  been  observed  in  connection  with  disease  affecting  the 
superior  temporal  convolution,  the  lesion  has  usually  invaded  other  parts  as 
well,  and  the  deafness  has  been  associated  with  other  symptoms,  notably 
aphasia.  An  auditory  "  aura  "  has,  however,  at  times  been  observed  in  con- 
nection with  disease  of  this  region,  as  also  a  peculiar  psychical  failure, 
known  as  "  word-deafness,"  in  which,  though  sounds  are  heard — that  is  to 
say,  auditory  sensations  are  felt,  it  may  be  even  as  usual — the  perception  or 
psychical  appreciation  of  the  sounds  is  lacking,  and  a  spoken  word  is  not 
recognized. 

Lastly,  we  may  add  that  though,  as  we  said,  the  anatomical  leading  is 
not  definite,  observers  have  found  that  in  newborn  animals,  on  the  one 
hand,  destruction  of  the  part  of  the  cortex  probably  corresponding  to  the 
region  mentioned  above  leads  to  atrophy  of  the  median  corpus  geniculatum, 
and  to  some  extent  of  the  posterior  corpus  quadrigeminum ;  and,  on  the 
other  hand,  destruction  of  the  internal  ear  leads  to  an  atrophy  of  part  of 
the  lateral  fillet  of  the  opposite  crossed  side,  which  may  be  traced  to  the 
posterior  corpus  quadrigeminum  ;  and  thence  to  the  median  corpus  geuicu- 
latum  ;  and  section  of  the  lateral  fillet  on  one  side  leads,  among  other  re- 
sults, to  atrophy  of  the  striae  acusticaB  and  tuberculum  acusticum  (§  531)  of 
the  crossed  side.  This  suggests  that  the  path  of  auditory  impulses  is  along 
the  cochlear  nerve  to  the  lateral  fillet  of  the  crossed  side,  and  so  by  the  pos- 
terior corpus  quadrigeminum  and  median  corpus  geniculatum  to  the  cortex 
of  the  temporal  lobe  of  that  crossed  side,  the  two  latter  bodies  bearing  toward 
hearing  a  relation  somewhat  like  that  borne  toward  sight  by  the  anterior 


CUTANEOUS  AND  SOME  OTHER  SENSATIONS.  703 

corpus  quadrigeminum  and  lateral  corpus  geniculatum.     But  the  matter 
needs  further  investigation. 

There  remains  the  special  sense  of  touch,  but  this  we  had  better  consider 
in  connection  with  sensations  in  general. 

ON  THE  DEVELOPMENT  OF  CUTANEOUS  AND  SOME  OTHER  SENSATIONS. 

§  590.  The  sensations  with  which  we  have  just  dealt  arise  through  im- 
pulses passing  along  special  nerves  or  parts  of  special  nerves,  the  optic  nerve, 
the  olfactory  nerve,  etc.  We  have  now  to  deal  with  sensations  arising 
through  impulses  along  the  nerves  of  the  body  generally.  These  are  of 
several  kinds.  In  the  first  place,  there  are  sensations  which  we  may  speak 
of  as  "  cutaneous  sensations,"  the  impulses  giving  rise  to  which  are  started 
in  the  skin  covering  the  body,  or  in  the  so-called  mucous  membrane  lining 
certain  passages.  These  sensations,  which,  as  we  shall  see  in  dealing  with 
the  senses,  are  dependent  on  the  existence  of  special  terminal  organs  in  or 
near  the  skin,  are  sensations  of  "touch,"  in  the  narrow  meaning  of  that 
word,  by  which  we  appreciate  contact  with  and  pressure  on  the  skin,  and 
the  sensations  of  "  temperature,"  which  again  we  may,  as  we  shall  see, 
divide  into  sensations  of  "  heat "  and  sensations  of  "  cold."  These  sensations 
may  be  excited  in  varying  degree  by  impulses  passing  along  any  nerve 
branches  of  which  are  supplied  to  the  skin.  Then  there  are  the  sensations 
constituting  the  "  muscular  sense,"  to  which  we  have  already  referred,  and 
these  again  may  be  excited  in  any  nerve  having  connections  with  the  skeletal 
muscles. 

As  we  shall  see  in  dealing  with  the  senses,  when  a  nerve  is  laid  bare  and 
its  fibres  are  stimulated  directly  either  by  pressure,  such  as  pinching,  or  by 
heat,  or  by  cold,  or  in  other  ways,  the  sensations  which  are  caused  do  not 
enable  us  to  appreciate  whether  the  stimulation  is  one  of  contact  or  pressure, 
or  of  temperature,  or  of  some  other  kind  ;  we  only  experience  a  "  feeling," 
which  at  all  events  when  it  reaches  a  certain  intensity  we  speak  of  as  "  pain." 
And  we  have  reason  to  think  that  at  least  from  time  to  time  impulses  along 
various  nerves  gives  rise  to  sensations  which  have  been  spoken  of  as  those  of 
"  general  sensibility,"  by  which  in  addition  to  other  sensations,  such  as  those 
of  touch  and  of  the  muscular  sense,  we  become  aware  of  changes  in  the  con- 
dition and  circumstances  of  our  body.  When  the  stimulation  of  the  skin 
exceeds  a  certain  limit  of  intensity,  the  sense  of  touch  or  temperature  is  lost 
in,  that  is  to  say,  is  not  appreciated  as  separate  from,  the  sense  of  pain  ;  and 
under  abnormal  circumstances  acute  sensations  of  pain  are  started  by 
changes  in  parts,  for  example  tendons,  the  condition  of  which  under  normal 
circumstances  we  are  not  conscious  of  appreciating  through  any  distinct 
sensation,  though  it  may  be  that  these  parts  do  normally  give  rise  to  feeble 
impulses  contributing  to  "  general  sensibility."  It  may  therefore  be  debated 
whether  "  pain  "  is  a  phase  of  all  sensations,  or  of  general  sensibility  alone, 
or  a  sensation  sui  generis.  We  shall  have  something  further  to  say  on  this 
matter  when  we  treat  of  the  senses ;  meanwhile  it  will  be  convenient  for 
present  purposes  if  we  consider  that  the  sensations  we  have  to  deal  with  just 
now  are  the  sensations  of  touch  and  of  temperature,  those  of  the  muscular 
sense,  and  those  of  general  sensibility,  including  those  of  pain. 

§  591.  The  fairly  convincing  evidence  that  the  occipital  cortex  has  special 
relations  with  vision,  and  the  less  clear  evidence  that  other  regions  have 
special  relations  with  smell  and  hearing,  suggest  that  special  parts  of  the 
cortex  have  special  relations  with  the  sensations  now  under  consideration. 
But  in  the  cases  of  the  senses  of  sight  and  smell  we  had  a  distinct  anatomical 
leading ;  and  we  have  seen  how  uncertain  is  the  evidence  where  such  an 


704  THE   BRAIN. 

anatomical  leading  fails,  as  in  hearing  and  taste.  In  the  case  of  sensations 
of  the  body  at  large,  the  anatomical  leading  similarly  fails.  Moreover,  any 
attempt  to  push  the  analogy  of  sight  raises  the  following  question  :  If  there 
were  two  optic  nerves  on  each  side  of  the  head,  would  there  be  two  cortical 
areas,  one  for  each  nerve,  in  each  hemisphere,  or  one  visual  area  only  ?  And 
again,  if  the  optic  nerve  were  the  instrument  for  some  sense  in  addition  to 
that  of  sight,  would  there  be  two  cortical  areas,  one  for  each  sensation,  or 
one  area  only  serving  as  the  cortical  station,  so  to  speak,  of  the  whole  nerve  ? 
If  we  push  the  analogy  of  sight  it  is  open  for  us,  since  we  cannot  give  a 
definite  answer  to  the  above  question,  to  suppose  either  that  there  is  one  area 
for  touch,  another  area  for  temperature,  and  so  on,  each  for  the  whole  body, 
or  that  there  is  an  area  for  sensations  of  all  kinds  for  each  afferent  nerve,  or, 
that  there  is  an  intricate  arrangement  which  supplies  all  the  combinations  of 
the  two  which  are  required  for  the  life  of  the  individual.  Of  the  three 
hypotheses  the  latter  is  the  more  probable;  but  if  so,  it  is  by  its  very  nature 
almost  insusceptible  of  experimental  proof,  especially  when  we  bear  in  mind 
what  we  have  already  said  touching  the  difficulty  of  judging  the  sensations 
of  animals.  If  the  judgment  of  visual  sensations  is  difficult,  how  much  more 
difficult  must  be  the  judgment  of  sensations  of  touch  and  temperature? 
Indeed,  sensations  of  pain  are  the  only  sensations  of  which  we  can  form  a 
quantitative  judgment  in  animals ;  and  our  method  of  judging  even  these, 
namely,  by  studying  the  movements  or  other  effects  indirectly  produced,  is  a 
most  imperfect  one. 

We  can  learn,  therefore,  almost  absolutely  nothing  in  this  matter  from 
experimental  stimulation  of  the  cortex  in  animals.  As  we  have  previously 
(§  584)  urged,  the  absence  of  movements  when  parts  of  the  cortex  other  than 
the  motor  regions  are  stimulated  is  no  evidence  that  the  stimulation  does  not 
give  rise  to  psychical  events  into  which  sensations  enter ;  and  movements 
follow  stimulation  of  the  motor  area,  not  because  that  area  is  wholly  given 
up  to  motor  events,  but  because  from  the  histological  arrangement  the 
stimulus  gets  ready  access  to  relatively  simple  motor  mechanisms.  That  the 
motor  region  has  close  connections  with  sensory  factors  is  not  only  almost 
certain  on  theoretical  grounds,  but  is  shown  in  many  ways,  for  example  by 
the  experiment,  described  in  §  574,  of  exalting  the  sensitiveness  of  a  motor 
area  by  generating  peripheral  sensory  impulses. 

Nor  can  the  effects  on  sensation  of  removal  of  parts  of  the  cortex  be  in- 
terpreted with  clearness  and  certainty.  In  the  monkey  removal  or  destruc- 
tion of  the  gyrus  fornicatus  (Figs.  148  and  150)  on  the  mesial  surface  of 
the  brain,  ventral  to  the  calloso-marginal  sulcus  which  forms  on  the  mesial 
surface  the  ventral  limit  of  the  motor  region  (an  operation  of  very  great  dif- 
ficulty), has  brought  the  whole  of  the  opposite  side  of  the  body  to  a  condition 
which  has  been  described  as  an  anaesthesia,  that  is,  a  loss  of  all  cutaneous 
tactile  sensations,  and  an  analgesia,  that  is,  a  loss  of  sensations  of  pain,  the 
condition  being  accompanied  by  little  or  no  impairment  of  voluntary  move- 
ments and,  though  apparently  diminishing  as  time  went  on,  lasting  until  the 
death  of  the  animal  some  weeks  afterward.  Again,  removal  of  the  contin- 
uation of  the  gyrus  fornicatus  into  the  gryus  hippocampi  has,  in  other  in- 
stances, led  to  a  more  transient  anaesthesia  also  of  the  whole  or  greater  part 
of  one  side  of  the  body.  And  it  is  asserted  that  removal  of  no  other  region 
of  the  cortex  interferes  with  cutaneous  and  painful  sensations  in  so  striking 
and  lasting  a  manner  as  does  the  removal  of  parts  or  of  the  whole  of  this 
mesial  region. 

These  results,  however,  do  not  accord  with  clinical  experience  which, 
though  scanty,  seems,  as  far  as  it  goes,  to  show  that  in  man,  when  mischief 
apparently  limited  to  the  cortex  produces  loss  of  sensation,  it  is  the  parietal 


CUTANEOUS  AND  SOME  OTHER  SENSATIONS.  705 

lobe  corresponding  to  the  motor  region  which  is  affected ;  but  there  appears 
to  be  no  record  of  any  case  of  a  cortical  lesion  affecting  sensation  without 
affecting  movement.  We  have  previously  called  attention  to  the  fact  that 
the  temporary  loss  or  impairment  of  movement  which  follows  removal  of  an 
area  is  frequently,  if  not  always,  accompanied  by  an  impairment  of  cutane- 
ous sensations  in  the  limb  or  part  "  paralyzed  ;"  and  side  by  side  with  this 
we  may  put  the  experience  that  in  the  human  epileptiform  attacks  of  cortical 
origin,  the  seizure  is  at  times  ushered  in  by  peculiar  sensations,  called  the 
"  aura,"  in  the  part  movements  of  which  inaugurate  the  march  of  convul- 
sive movements.  But  these  things  do  not  show  that  the  cortical  area  is  the 
"  seat  of  sensations ;"  they  rather  illustrate  what  we  said  concerning  the 
complexity  of  the  chain  of  which  the  events  in  the  cortical  area  are  links, 
and  the  close  tie  between  sensory  factors  and  the  characteristic  elements  of 
the  motor  region. 

In  the  dog,  while  removal  of  almost  any  considerable  portion  of  the 
cortex  affects  sensation,  removal  of  parts  of  the  frontal  region  producing, 
perhaps,  less  effect  than  removal  of  parts  in  other  regions,  the  loss  or  im- 
pairment of  sensation  appears  to  be  transient,  though  having  a  duration 
broadly  proportionate  to  the  extent  of  cortex  removed ;  and  when  a  very 
large  portion  of  the  cortex  is  removed,  some  imperfection  appears  to  remain 
to  the  end.  We  have  already  referred  to  the  case  of  a  dog  from  which  the 
greater  part  of  both  cerebral  hemispheres  had  been  removed,  but  which 
remained  capable  of  carrying  out  most  of  the  ordinary  bodily  movements, 
and  that  apparently  in  a  voluntary  manner ;  in  this  case  the  "  blunting  "  of 
cutaneous  sensations  was  perhaps  more  striking  than  the  imperfection  of 
movement.  It  will  be  worth  while  to  consider  the  condition  of  this  dog  a 
little  closely,  on  account  of  the  light  which  it  throws  on  the  problem  which 
we  are  now  discussing. 

Clinical  experience  shows  that  in  man  the  integrity  of  the  cerebral  hemi- 
spheres, and  of  the  connection  of  the  hemispheres  with  the  rest  of  the  central 
nervous  system,  is  essential  to  the  full  development  of  sensations ;  and  that 
in  this  respect,  each  hemisphere  is  related  to  the  crossed  side  of  the  body.  A 
very  common  form  of  paralysis  or  "  stroke  "  is  that  due  to  a  lesion  of  some 
part  of  one  hemisphere  (the  exact  position  of  the  lesion  need  not  concern  us 
now),  frequently  caused  by  rupture  of  a  bloodvessel,  in  which  the  patient 
loses  all  power  of  voluntary  movement  and  all  sensations  on  the  crossed  side 
of  his  body  (including  the  face) ;  he  is  said  to  be  suffering  from  hemiplegia, 
"  one-sided  stroke."  Not  only  do  voluntary  impulses  fail  to  reach  the  mus- 
cles of  the  affected  side,  but  sensory  impulses,  such  as  those  which  started, 
for  instance,  in  the  skin,  would,  under  normal  conditions,  lead  to  sensations 
of  touch,  of  heat  or  cold,  or  of  pain,  fail  to  affect  consciousness,  when  they 
originate  on  the  affected  side ;  the  patient  cannot  on  that  side  feel  a  rough 
surface,  or  a  hot  body,  or  the  prick  of  a  pin.  For  the  sake  of  clearness  we 
suppose  the  loss  of  movement  and  sensation  to  be  complete,  but  it  might,  of 
course,  be  partial.  Such  a  case  shows,  we  repeat,  that  the  integrity  of  the 
cerebral  hemisphere,  and  of  the  connection  of  that  hemisphere,  we  may  say 
of  the  cortex  of  that  hemisphere,  with  the  other  parts  of  the  nervous  system, 
is  essential  to  the  development  of  the  sensations ;  but  it  does  not  prove  that 
the  cortex  of  the  hemisphere  is  the  "  seat "  of  the  sensations,  it  does  not  prove 
that  the  afferent  and  sensory  impulses  started  in  the  skin  undergo  no  mate- 
rial change  until  they  reach  the  cortex  and  are  then  suddenly  converted 
into  sensations  ;  it  only  proves  that  in  the  complex  chain  of  events  by  which 
sensory  impulses  give  rise  to  full  conscious  sensations  the  events  in  the  cortex 
furnish  an  indispensable  link.  And  the  phenomena  of  the  dog  in  question, 
on  the  one  hand,  illustrate  how  complex  the  chain  is,  and,  on  the  other  hand, 
45 


706  THE  BRAIN. 

suggest  that  the  completeness  of  the  loss  of  sensation  in  the  hemiplegic  man 
is  not  a  pure  "  deficiency  "  phenomenon,  but  is  due  to  the  lesion  affecting  the 
chain  of  events  in  some  way  or  other  besides  merely  removing  the  link  fur- 
nished by  means  of  the  cortex.  For,  as  we  previously  urged,  the  dog  in 
question,  however  curtailed  its  psychical  life  may  have  been,  seemed  to  a 
casual  observer  to  feel  and  move  much  as  usual.  Neglecting  visual  and 
auditory  sensations  with  which  we  are  not  now  dealing,  it  needed  careful 
observation  to  ascertain  that  some  of  the  animal's  movements  fell  short,  the 
failure  being  apparently  due  to  the  lack  of  adequately  energetic  coordinating 
sensory  impulses ;  a  stronger  stimulus  than  usual  had  to  be  applied  to  the 
skin  in  order  to  call  forth  the  usual  movements  and  other  tokens  that  the 
stimulus  was  "  felt."  As  we  have  before  urged,  it  is  impossible  to  suppose 
that  the  mere  stump  of  cerebrum  left  in  this  case  could  have  taken  on  all  the 
functions  of  the  lost  hemispheres ;  and  making,  as  we  have  previously  done, 
full  allowance  for  the  differentiation  between  man  and  dog,  we  must  conclude 
that  in  the  more  general  sensations  with  which  we  are  now  dealing,  as  with 
the  more  special  visual  sensations,  the  full  development  of  a  complete  sensa- 
tion is  a  complex  act  of  more  stages  than  one  between  the  afferent  impulse 
along  the  afferent  nerve  and  the  affection  of  consciousness  which  we  subject- 
ively recognize  as  "  the  sensation  ;"  the  cortical  events  are  only  some  among 
several.  It  follows  that  any  analogy  between  the  cortical  events  which  play 
their  part  in  a  sensation  and  the  cortical  events  which  immediately  precede 
the  issue  of  impulses  from  the  motor  region  along  the  fibres  of  the  pyram- 
idal tract  is  misleading;  the  highly  differentiated  motor  localization  does 
not  justify  us  in  concluding  that  there  exists  a  similar  topographical  distri- 
bution of  sensation. 

§  592.  We  may  now  attack  the  problem  in  a  different  way,  and  instead 
of  beginning  with  the  cortex  begin  with  afferent  impulses  started  along  affer- 
ent nerves  from  their  peripheral  endings,  and  attempt  to  trace  them  central- 
ward.  And  first  we  may  call  to  mind  what  anatomical  guidance  we  possess 
(§482). 

The  fibres  of  posterior  roots,  the  channels  of  afferent  impulses,  end  in 
the  spinal  cord  in  at  least  two  main  ways.  One  set  is  continued  on,  not 
broken  by  any  relays,  as  the  median  posterior  tract,  and  by  this  tract  repre- 
sentatives of  all  the  spinal  nerves  are  connected  with  the  gracile  nucleus  in 
which  (§  523)  the  median  posterior  column  ends.  The  other  fibres  of  a  pos- 
terior root  appear  to  end  in  the  gray  matter  not  far  from  their  entrance ; 
but  from  the  gray  matter  there  starts  the  cerebellar  tract,  which,  though  not 
conclusively  proved  to  be,  may  be  assumed  to  be,  an  efferent  tract.  We  may 
therefore  probably  suppose  that  afferent  impulses  along  certain  of  the  fibres 
of  the  posterior  root  make  their  way  upward  along  the  cerebellar  tract,  and 
there  are  some  reasons  for  regarding  the  vesicular  cylinder  and  the  cells 
which  represent  this  where  it  is  not  conspicuous  in  the  regions  of  the  cord, 
as  a  relay  between  the  two  systems  of  fibres.  There  are  also  the  more  scat- 
tered fibres  of  the  ascending  antero-lateral  tract  (§  480),  which  probably  is 
also  an  afferent  tract,  and  therefore  probably  also  connected  with  the  poste- 
rior roots;  but,  as  we  have  seen,  our  knowledge  of  this  tract  is  imperfect, 
though,  if,  as  some  urge,  it  ends  in  the  restiform  body,  we  may  perhaps  con- 
sider it  as  similar  at  least  to  the  cerebellar  tract,  and  treat  the  two  as  one. 

Thus  there  seem  to  be  at  least  two  main  recognized  paths,  in  the  form  of 
tracts  of  fibres,  for  afferent  impulses  along  the  cord ;  one  along  the  median 
posterior  column,  the  other  along  the  lateral  column  in  the  cerebellar  tract. 
The  latter  passes  straight  up  to  the  cerebellum  by  the  restiform  body,  trav- 
elling along  the  same  side  of  the  cord,  and  any  crossing  of  impulses  passing 
along  this  tract  must  take  place  before  they  enter  the  tract ;  we  have,  how- 


CUTANEOUS  AND  SOME  OTHER  SENSATIONS.  707 

ever,  no  anatomical  guidance  for  such  a  crossing.  The  other  path,  along  the 
median  posterior  tract,  comes  to  end  in  the  gracile  nucleus ;  it  has,  indeed, 
heen  urged  that  the  gracile  nucleus  is  thus  connected  chiefly  with  the  lower 
limbs  and  lower  part  of  the  body,  and  that  the  analogous  posterior  root 
fibres  from  the  upper  limbs  and  neck  pass  similarly  into  the  cuneate  nucleus, 
or  at  least  into  the  median  division  of  that  nucleus,  but  this  cannot  be  con- 
sidered as  proved.  Moreover,  both  the  posterior  columns,  median  and  ex- 
ternal, bring  to  these  nuclei  fibres  which  have  started  from  some  relay  in  the 
gray  matter  lower  down,  and  which  are  not  fibres  coming  straight  without 
any  relay  from  the  posterior  roots ;  these,  however,  we  cannot  distinguish 
from  each  other  in  their  course  beyond  the  nuclei.  From  the  gracile  and 
cuneate  nuclei  the  path  onward  is  a  double  one,  one  broad,  one  narrow.  The 
broad  path,  the  one  having  most  fibres  and  presumably  carrying  most  im- 
pulses, leads  to  the  cerebellum  by  the  restiform  body;  and  here  the  path, 
previously  continued  exclusively  along  the  same  side  of  the  cord,  becomes 
partly  crossed  though  remaining  partly  uncrossed,  the  sensory  decussation  in 
the  bulb  being  the  crossed  and  the  other  fibres  passing  from  the  nuclei 
straight  to  the  restiform  body  being  the  uncrossed  one  (§  525)  ;  the  uncrossed 
one  we  may,  perhaps,  look  upon  as  really  an  upper  part  of  the  cerebellar 
tract.  The  narrow  path  is  the  fillet  (§  547),  by  which  some  of  the  fibres 
from  the  nuclei  are  continued  on  toward  the  cerebrum.  This  path  is  a 
crossed  one,  the  crossing  taking  place  in  the  sensory  decussation,  and  it  car- 
ries relatively  few  impulses,  the  chief  increase  in  the  size  of  the  fillet  as  it 
passes  onward  being  due  to  fibres  coming  from  structures  other  than  the 
gracile  and  cuneate  nuclei. 

Hence  of  the  sensory  impulses  travelling  along  continuous  tracts  in  the 
spinal  cord,  these  tracts  apparently  keeping  always  to  the  same  side,  the 
great  majority  pass  to  the  cerebellum ;  and  of  these  again  the  greater  num- 
ber, all  those  along  the  cerebellar  tract,  and  some  of  those  passing  through 
the  gracile  and  cuneate  nuclei,  remain  uncrossed  to  the  end.  The  only  path 
by  which  all  these  impulses  thus  passing  to  the  cerebellum  can  gain  access 
to  the  cortex  of  the  cerebrum,  is  by  some  or  other  of  the  ties  between  the 
cerebellum  and  the  cerebral  cortex.  The  relatively  few  impulses  which  pass 
along  the  fillet  are  for  the  most  part  carried  to  the  middle  parts  of  the  brain, 
for  only  a  small  portion  of  the  fillet  passes  to  the  cortex  (§  547),  and  it  is 
not  clear  that  this  part  of  the  fillet  comes  from  the  gracile  and  cuneate 
nuclei,  so  that  most  of  these  impulses  can  gain  access  to  the  cortex  only  by 
the  relays  of  these  middle  parts  of  the  brain. 

Very  striking,  indeed,  are  these  constant  relays  along  the  path  of  sensory 
impulses ;  in  this  respect  the  sensory  impulses  offer  a  strong  contrast  to  the 
motor  impulses.  But  a  still  more  complex  system  of  relays  has  to  be  men- 
tioned ;  for  yet  a  third  path  is  open  for  sensory  afferent  impulses  along  the 
•cord.  We  must  admit  the  possibility  of  afferent  impulses  travelling  along 
the  network  of  the  gray  matter,  their  path  being  either  absolutely  confined 
to  the  gray  matter,  or  leaving  the  gray  matter  at  intervals,  and  joining  it 
again  by  means  of  those  longer  or  shorter  commissural  or  internuncial 
fibres  which  unite  the  longitudinal  segments  of  gray  matter,  and  form  no 
inconsiderable  portion  of  the  whole  white  matter  of  the  cord.  We  have 
seen  (§  499)  that  under  abnormal  circumstances  impulses  pass  freely  in  all 
directions  along  the  gray  matter,  and  we  may  conclude  that  under  normal 
circumstances  they  can  pass  along  it  under  restrictions  and  along  lines  de- 
termined by  physiological  conditions.  The  fibres  in  the  white  matter  which 
do  not  show  either  descending  or  ascending  degeneration  are  probably,  as  we 
have  said  (§  494),  internuncial  fibres,  connecting  segments  of  gray  matter 
in  a  longitudinal  direction  ;  and,  though  we  have  no  exact  knowledge  touch- 


708  THE   BRAIN. 

ing  this  matter,  we  may  suppose  that  some  of  these  convey  impulses  upward, 
and  others  downward. 

If,  as  some  maintain,  the  fibres  of  the  ascending  antero-lateral  tract  end 
not  in  the  cerebellum,  but  in  the  gray  matter  of  the  bulb,  or  higher  up,  we 
have  a  fourth  path  for  sensory  impulses,  which,  after  the  primary  relay  in 
the  segmental  gray  matter,  pass  straight  up  to  the  bulb. 

§  593.  How  do  experimental  results  and  clinical  histories  accord  with 
such  an  anatomical  programme? 

We  may  first  call  attention  to  an  experiment,  which,  though  somewhat 
old,  carried  out  on  rabbits,  and  confined  to  one  region  only  of  the  cord,  the 
lower  thoracic,  has  nevertheless  a  certain  value  on  account  of  its  affording 
more  or  less  distinctly  quantitative  and  measurable  results.  We  have  seen 
(§  161)  that  afferent  impulses  started  in  afferent  fibres,  in  those,  for  instance, 
of  the  sciatic  nerve,  so  affect  the  vasomotor  centre  in  the  bulb  as  to  cause  a 
rise  of  blood-pressure,  at  least  in  an  animal  under  urari.  Those  afferent 
impulses  must  pass  by  some  path  or  other  from  the  roots  which  supply  the 
sciatic  nerves  with  afferent  fibres  along  the  thoracic  and  cervical  cord  to  the 
bulb.  If  the  path  be  blocked,  the  stimulation  of  the  sciatic  nerve  will  fail 
to  produce  the  usual  rise  of  blood-pressure.  Now  in  a  rabbit  the  amount  of 
rise  of  blood-pressure  following  upon  the  stimulation  of  one  sciatic  nerve 
with  a  certain  strength  of  current  having  been  ascertained,  it  is  found  that 
a  much  less  rise  of  blood-pressure  or  none  at  all  follows  the  same  stimulation 
after  division  of  certain  parts  of  the  cord  in  the  mid  or  upper  thoracic 
region  ;  that  is  to  say,  the  section  of  the  cord  has  partially  or  completely 
blocked  the  path  of  the  afferent  impulses.  Further,  the  block  is  conspicuous 
when  the  lateral  column  is  divided,  and  is  not  increased  by  other  parts  of 
the  cord  being  divided  at  the  same  time ;  when  both  lateral  columns  are 
divided,  the  block  is  almost  complete.  And  further,  supposing  one  sciatic, 
say  the  right,  is  the  one  which  is  stimulated,  a  block  occurs  both  when  the 
lateral  column  of  the  same,  right,  side  and  when  that  of  the  crossed,  left, 
side  is  divided,  but  is  greater  when  the  division  is  on  the  crossed  than  when 
it  is  on  the  same  side.  We  may  infer  that  the  impulses,  which  reach  the 
lumbar  cord  by  the  roots  of  the  sciatic  nerve,  travel  up  the  cord,  or  give 
rise  within  the  lumbar  cord  to  events  which  we  may  compare  to  nervous 
impulses,  and  which  travel  up  the  cord  in  such  a  manner  that  in  the  lower 
thoracic  region  they  pass  almost  exclusively  along  the  fibres  of  the  lateral 
column,  some  having  kept  to  the  same  side  of  the  cord,  but  more  having 
crossed  over  to  the  opposite  side  before  reaching  the  thoracic  region. 

This  result  was  obtained  in  rabbits,  and  the  experiment  was  carried  out 
in  the  lower  thoracic  region  only ;  the  conclusions  to  be  drawn  from  it  hold 
good  for  that  animal  only,  and  for  that  part  only  of  its  cord.  Moreover, 
the  experiment  only  tests  the  path  of  such  impulses  as  reach  and  affect  the 
vasomotor  centre  in  the  bulb.  It  is,  however,  exceedingly  probable  that  the 
impulses  which,  generated  in  sensory  nerves,  affect  the  vasomotor  centre  are 
impulses  which,  in  the  conscious  animal,  give  rise  to  sensations  of  pain  ;  in 
an  intact  animal  changes  in  the  vasomotor  centre  occasioned  by  the  stimula- 
tion of  sensory  nerves  are  accompanied  by  signs  of  more  or  less  pain.  And 
indeed  this  is  confirmed  by  the  fact  that  similar  results  were  obtained  when, 
the  experiment  being  conducted  in  a  similar  way,  signs  of  pain  instead  of 
variations  in  blood-pressure  were  taken  as  the  tokens  of  the  blocking  of 
impulses.  Hence,  assuming  this,  we  may  regard  the  experiment  as  indicat- 
ing that  the  impulses  which  form  the  basis  of  painful  sensations  pass  by  the 
lateral  columns  in  the  lower  thoracic  region  of  the  cord  of  the  rabbit,  and 
therefore,  though  this  is  a  further  assumption,  by  the  same  columns  along  the 
whole  length  of  the  cord.  We  further  may  infer  that  while  some  of  the 


CUTANEOUS  AND  SOME  OTHER  SENSATIONS.  709 

impulses  keep  to  the  same  side  of  the  cord,  others,  and  indeed  the  greater 
number,  cross  to  the  opposite  side. 

These  conclusions  entail  assumptions,  but  the  main  interpretation  of  the 
whole  experiment  entails  a  still  greater  assumption.  The  testing  of  the 
influence  of  the  sciatic  stimulation  was  carried  out  soon  after  the  section  of 
the  cord,  and  yet  we  have  assumed  that  the  block  of  the  impulses  was  due 
to  a  pure  deficiency  phenomenon,  the  absence  of  a  usual  path.  But  we 
have  no  right  to  do  this.  It  is  possible  that  the  section  produced,  in  some 
way  or  other,  a  depressing  or  inhibitory  effect  lower  down  in  the  cord, 
affecting  structures  other  than  the  lateral  columns ;  all  our  experience,  in- 
deed, of  the  effects  of  operations  on  the  cord  would  lead  us  to  expect  this. 
It  is  further  possible  that  a  section  of  the  lateral  column  might  produce  this 
depressing  effect,  while  sections  of  other  parts  did  not,  or  might  produce 
more  effect  than  the  former.  It  is  possible,  for  instance,  that  the  section  of 
the  thoracic  lateral  column  inhibited,  for  the  period  during  which  the  ex- 
periment was  carried  out,  the  gray  matter  of  the  lumbar  cord,  and  that  the 
block  really  took  place  in  this  gray  matter.  Until  the  uncertainties  thus 
attending  the  interpretation  are  removed  the  experiment  is  not  valid  as  a 
proof  that  the  lateral  columns  are  the  paths  of  afferent  impulses ;  it  would, 
however,  still  serve  to  indicate  that  the  afferent  impulses  reaching  the  cord 
along  the  sciatic  nerve  crossed  over  to  a  large  extent  before  they  came  under 
the  influence  of  the  inhibition,  since  we  have  no  evidence  to  show  that  such 
an  inhibitory  action  of  the  section  would  be  exerted  chiefly  on  the  crossed 
side. 

Again,  we  have  seen  that  the  afferent  impulses  affecting  the  vasomotor 
centre  gain  access  to  that  centre  without  the  help  of  the  parts  of  the  brain 
above  the  bulb  ;  the  existence  of  the  vasomotor  centre  was  made  out  (§  162), 
by  combining  stimulation  of  a  sciatic  nerve  with  a  series  of  operations  con- 
sisting in  making  successive  transverse  sections  of  the  bulb  from  above 
downward ;  and  it  was  not  until  the  sections  reached  the  vasomotor  centre 
that  the  blood-pressure  effects  of  the  sciatic  stimulation  were  modified. 
Hence,  if  the  experiment  be  taken  as  showing  that  not  only  afferent  im- 
pulses affecting  the  vasomotor  centre,  but  other  afferent  impulses  also  travel 
by  the  lateral  columns,  it  would  also  seem  to  show  that  these  other  impulses 
pass  in  like  manner  to  the  bulb,  and  gain  access  to  the  cortex  through  the 
bulb.  This  increases  a  difficulty  which  presents  itself  even  when  the  afferent 
impulses  affecting  the  vasomotor  centre  are  alone  considered.  If  the  experi- 
ment means  anything,  it  means  that  the  impulses  having  in  some  way  or 
other  reached  the  lateral  column,  travel  up  that  column  by  some  continuous 
path,  and  indeed  is  generally  taken  as  having  that  meaning.  But  if  we  put 
aside  the  very  doubtful  view  that  the  ascending  antero-lateral  tract  ends  in 
the  bulb,  there  is  no  continuous  afferent  tract  in  the  lateral  column  ending 
in  the  bulb  ;  the  only  definite  continuous  afferent  tract  in  the  lateral  column 
of  which  we  have  any  clear  knowledge,  namely,  the  cerebellar  tract,  ends 
not  in  the  bulb  but  in  the  cerebellum.  And  if  we  attempt  to  get  out  of 
the  difficulty  by  supposing  that  those  impulses  at  least  which  affect  the 
vasomotor  centre,  after  travelling  for  some  distance  in  the  cerebellar  tract, 
leave  that  tract  for  some  path  leading  to  the  bulb  (and  the  cerebellar  tract 
does  probably  give  off  as  well  as  receive  fibres  along  its  course),  we  practi- 
cally admit  that  the  experiment  does  not  prove  the  existence  of  a  continuous 
path. 

A  further  difficulty  is  raised  by  the  fact  that,  according  to  the  interpreta- 
tion which  we  are  discussing,  the  "section  of  the  lateral  column  breaks  the 
paths  of  what  we  may  consider  two  kinds  of  impulses :  those,  the  larger 
number,  which  have  already  crossed  from  one  side  of  the  cord  to  the  other, 


710  THE   BRAIN. 

and  those  which  have  remained  on  the  same  side.  For,  as  we  have  already 
said,  we  have  evidence,  in  man  at  least  and  some  other  animals,  that  afferent 
impulses  cross  completely  over  somewhere  or  other  on  their  path  before 
they  are  developed  into  full  sensations ;  and  we  have  also  evidence,  though 
less  strong,  that  they  cross  not  long  after  their  entrance  into  the  cord.  But, 
if  we  suppose  this  to  be  the  case  in  the  rabbit  also,  it  follows  that  in  the 
experiment  in  question  the  impulses  which  were  blocked  on  their  passage 
along  the  lateral  column  of  the  same  side,  whatever  the  way  by  which  they 
reached  that  lateral  column,  were  pursuing  a  path  which  would  eventually 
have  led  them  to  the  other  side  of  the  cord.  Hence,  the  section  of  the 
lateral  column,  in  breaking  their  path,  broke  not  a  continuous  path  keeping 
to  the  lateral  column  up  the  length  of  the  cord,  but  a  path  which  soon  left 
the  lateral  column  to  pass  elsewhere.  The  experiment,  therefore,  as  far  as 
the  impulses  passing  up  the  same  side  are  concerned,  does  not  prove  that 
they  pursue  a  continuous  path  along  the  lateral  column  ;  and  if  so,  what  be- 
comes of  the  validity  of  the  experiment  as  regards  the  impulses  crossing  over 
from  the  other  side?  for  the  experiment  in  itself  makes  no  distinction  between 
the  two. 

We  may  add,  however,  that  though  the  point  has  not  been  specially 
investigated,  it  is  possible  that  in  the  rabbit,  in  whose  hind  limbs  bilateral 
movements  are  so  predominant,  there  is  associated  with  the  movements  a 
bilateral  arrangement  for  sensations,  and  that  those  impulses  which  remain 
along  the  same  side  of  the  cord  as  the  nerve  in  which  they  originate,  are 
carried  up  to  the  brain  without  any  crossing  at  all. 

§  594.  The  results  of  this  vasomotor  experiment  then,  though  they  are 
frequently  quoted,  do  not  when  closely  considered  afford  adequate  proof  that 
afferent  impulses  pursue  a  continuous  path  along  the  lateral  columns  of  the 
cord,  and,  moreover,  the  facts  brought  to  light  by  the  experiment  show  but 
little  accord  with  the  anatomical  programme.  We  have  dwelt  on  it  so  long 
because  it  is  more  or  less  illustrative  of  the  many  difficulties  attending  the 
interpretation  of  experiments  of  this  kind ;  and  it  is  in  this  respect  all  the 
more  valuable  because  the  actual  experimental  results  are  sharp  and  clear. 
We  may  pass  over  more  rapidly  the  numerous  experiments  on  the  lower 
mammals,  such  as  rabbits  and  dogs,  in  which  other  indications  of  sensation 
have  been  made  use  of,  chiefly  those  which  are  the  signs  of  painful  sensa- 
tions; these  have  been  carried  out  in  various  regions  of  the  cord,  but  chiefly 
in  the  thoracic  region,  and  in  them  a  like  uncertainty  of  interpretation  is 
further  increased  by  the  want  of  exactness  and  agreement  in  the  results. 

If  we  content  ourselves  with  making  no  distinction  between  the  different 
kinds  of  afferent  impulses,  and  in  the  case  of  these  animals  it  would  hardly 
be  profitable  to  attempt  to  make  a  distinction,  we  may  say  that  the  several 
experiments  so  far  agree  that  they  point  to  the  lateral  columns  as  being  the 
chief  paths  of  afferent,  sensory  impulses,  or  to  speak  more  exactly,  to  the 
passage  of  these  impulses  being  especially  blocked  by  section  of  the  lateral 
columns.  Some  observers  find  that  in  the  dog  and  other  lower  mammals  a 
section  of  the  lateral  column  on  one  side,  or  at  least  a  hemisection  of  the 
cord,  produces  "  loss  of  sensation  "  on  the  opposite  side  greater  than  on  the 
same  side,  or  confined  to  the  opposite  side  and  even  accompanied  by  an 
exaltation  of  sensation,  a  hypersesthesia,  on  the  same  side.  Other  observers 
again,  and  these  certainly  competent  observers,  find  that,  in  the  dog,  section 
of  one  side  affects  sensations  on  both  sides,  and,  indeed,  chiefly  on  the  same 
side.  We  may  perhaps  once  more  repeat  the  warning  of  the  difficulty  attend- 
ing the  quantitative  and  qualitative  determination  of  sensations  in  such 
an  animal  as  the  dog ;  and  may  remark  that  in  all  these  cases  of  unilateral 
section  the  increased  blood-supply  due  to  failure  of  the  normal  vaso-cou- 


CUTANEOUS  AND  SOME  OTHER  SENSATIONS.  711 

stricter  tone  must  influence    the  peripheral   development   of  sensory  im- 
pulses. 

In  these  experiments,  as  in  those  on  voluntary  movements,  it  is  most 
important  to  distinguish  between  immediate  or  temporary  and  more  lasting 
effects ;  and  observers  have  found  that  the  loss  of  sensation  following  a 
hemisection  of  the  cord,  like  the  loss  of  voluntary  movement,  is  temporary 
only,  and  eventually  disappears,  though  the  recovery  is  slower  and  less 
complete  than  is  the  case  with  movements.  As  with  voluntary  movement 
(§  576)  so  with  sensation,  recovery,  though  less  complete  than  that  of 
movement,  is  possible  when  a  hemisection  on  one  side  has  been  at  a  later 
date  followed  by  a  hemisection  on  the  other  side.  We  may,  therefore, 
repeat  in  reference  to  sensations  the  remarks  which  we  made  in  reference 
to  movement.  There  is,  however,  an  important  difference  between  the  two 
cases :  in  respect  to  movement  we  have  evidence  that  under  normal  condi- 
tions the  pyramidal  tract  plays  an  important  part  and  that  any  other  path 
for  volitional  impulses  is  more  or  less  an  alternative  one,  whereas  in  respect 
to  sensation  we  have  no  anatomical  or  other  distinct  proof  of  any  such 
normal  path. 

The  experiments  on  monkeys  are  in  like  manner  neither  accordant  nor 
decisive ;  and  even  in  these  animals  with  their  more  varied  signs  of  sensa- 
tions, the  interpretation  of  these  signs  is  beset  with  fallacies.  Some  observ- 
ers have  found  that  a  hemisection  (in  the  thoracic  region)  produced  loss 
of  sensation  on  the  crossed  side,  accompanied  by  little  or  no  loss  on  the 
same  side ;  other  observers  again  have  failed  to  obtain  after  a  hemisection 
satisfactory  proof  of  any  such  marked  loss  on  the  crossed  side.  Further,  large 
portions  of  the  lateral  column,  the  more  internal  parts  adjacent  to  the  gray 
matter  being  left,  have  been  removed  without  any  very  obvious  and  certainly 
without  any  lasting  defects  of  sensation  on  the  one  side  or  on  the  other. 

§  595.  The  clinical  histories  of  diseases  of  the  spinal  cord  in  man  bring 
to  light  in  a  fairly  clear  manner  a  fact  of  some  importance,  namely,  that 
the  several  impulses  which  form  the  bases  of  the  several  kinds  of  sensa- 
tions, of  touch,  heat,  cold,  and  pain,  and  of  the  muscular  sense,  are  trans- 
mitted along  the  cord  in  different  ways  and  presumably  by  different  struc- 
tures, for  disease  may  impair  one  of  these  sensations  and  leave  the  others 
intact.  Thus  cases  of  spinal  disease  are  recorded,  in  which  on  one  side  of 
the  body  or  in  one  limb  ordinary  tactile  sensations  seemed  to  be  little  im- 
paired, and  yet  sensations  of  pain  were  absent ;  when  a  needle  was  thrust 
into  the  skin  no  pain  was  felt,  though  the  patient  was  aware  that  the  needle 
had  been  pressed  upon  the  skin  at  a  particular  spot ;  and  conversely,  in 
other  cases  pain  has  been  felt  upon  the  insertion  of  a  needle,  though  mere 
contact  with  or  pressure  on  the  skin  could  not  be  appreciated.  Again, 
cases  are  recorded  in  which  the  skin  was  sensitive  to  touch  or  pain,  but  not 
to  variations  of  temperature ;  it  is  further  stated  that  cases  have  been  met 
with  in  which  cold  could  be  appreciated,  but  not  heat,  and  vice  versa;  and 
there  are  some  facts  which  point  to  sensations  of  pain  being  more  closely 
associated  with  those  of  heat,  and  tactile  sensations  with  those  of  cold,  than 
those  of  pain  with  those  of  touch  or  those  of  heat  with  those  of  cold.  Cases 
of  spinal  disease  are  also  recorded  in  which  the  muscular  sense  appeared  to 
be  affected  apart  from  other  sensations.  We  shall  return  to  these  matters 
later  on  in  dealing  with  the  senses ;  we  refer  to  them  now  simply  as  show^ 
ing  that  disease,  limited  as  far  as  can  be  ascertained  to  the  spinal  cord,  may 
affect  the  several  sensations  separately,  and  therefore  as  suggesting  that  the 
several  kinds  of  impulses,  forming  the  bases  of  the  several  kinds  of  sensa- 
tions, are  transmitted  in  different  ways  and  follow  different  "paths"  along 
the  spinal  cord. 


712  THE  BRAIN. 

Clinical  histories,  moreover,  agree,  at  least  to  a  large  extent,  in  showing 
that  when  the  lesion  is  confined  to  one-half  of  the  cord,  the  sensations 
affected  in  the  parts  below  the  level  of  the  lesion  are  chiefly  or  even  exclu- 
sively those  of  the  crossed  side.  But  there  is  not  entire  accordance,  espe- 
cially as  to  the  crossing  being  complete.  And  with  regard  to  the  muscular 
sense  there  is  a  distinct  conflict  of  opinion ;  the  majority  of  cases  seem  to 
show  that  in  unilateral  disease  or  injury  to  the  cord,  the  muscular  sense  in 
company  with  the  voluntary  movements  fails  on  the  same  side ;  but  cases 
have  been  recorded  in  which  the  muscular  sense  in  company  with  other 
sensations  seemed  to  be  affected  on  the  crossed  side ;  it  must  be  remem- 
bered, however,  that  it  is  very  difficult  to  appreciate  a  deficiency  of  muscu- 
lar sense  mingled  with  deficiencies  in  other  sensations,  and  we  should  a 
priori  expect  the  muscular  sense  to  run  parallel  with  motor  impulses. 

When,  however,  we  appeal  to  clinical  histories  or  indications  as  to  the 
several  paths  within  the  spinal  cord  taken  by  these  several  impulses,  the 
answer  is  a  most  uncertain  one,  as  indeed  might  be  expected  from  the  too 
often  diffuse  character  of  the  lesions  of  disease ;  and  it  is  perhaps  not  too 
much  to  say  that  no  satisfactory  deductions  at  all  can  be  made. 

§  596.  Whether,  then,  we  turn  to  experiments  on  animals  or  to  the  study 
of  disease,  the  teachings  with  regard  to  sensation,  in  contrast  to  those  with 
regard  to  voluntary  movement,  are  in  the  highest  degree  uncertain  and 
obscure.  A  few  general  reflections  will  perhaps  help  us  to  appreciate  the 
value  of  such  facts  as  we  possess. 

We  have  seen  reason  to  think  that  in  every  movement,  whether  voluntary 
and  of  cortical  origin,  or  involuntary  and  started  either  as  a  simple  spinal 
reflex  or  through  the  working  of  some  part  or  other  of  the  brain,  the  motor 
impulses,  which  sweep  down  the  motor  fibres  to  the  muscles,  issue  mar- 
shalled and  coordinated  from  the  gray  matter  of  the  cord  (for  the  sake  of 
clearness  we  may  omit  the  cranial  nerves),  from  what  we  have  called  the 
motor  mechanisms  of  the  cord.  Analogy  would  lead  us  to  suppose  that 
the  afferent  impulses,  forming  the  bases  of  the  several  kinds  of  sensations, 
similarly  left  the  afferent  fibres  to  join  the  gray  matter  of  the  cord  in  what 
we  may  call  the  sensory  mechanism.  And  such  anatomical  leading  as  we 
possess  seems  to  support  this  view;  with  the  exception  of  the  median  pos- 
terior tract,  to  which  we  will  return  immediately,  all  the  fibres  of  a  poste- 
rior root  seem  to  end  in  the  gray  matter  not  very  far  from  the  entrance  of 
the  root.  We  have  seen  that  a  coordinate  reflex  movement  may  be  carried 
out  by  at  least  a  few  segments  of  the  cord ;  that  a  reflex  movement  may  be 
started  by  stimuli  of  various  kinds  and  therefore  presumably  by  afferent 
impulses  of  various  kinds ;  and  that  impulses  forming  the  basis  of  the 
muscular  sense  are  essential  to  the  coordination  of  the  movement.  All 
our  knowledge  goes  to  show  that  in  reflex  movement  carried  out  by  a  few 
segments  of  the  cord,  the  whole  chain  of  events  between  the  arrival  of  the 
afferent  impulses  along  the  posterior  root  and  the  issue  of  efferent  impulses 
along  the  anterior  root  may  be  carried  out  by  gray  matter,  and  gray  matter 
alone.  We  may  further  infer  that,  while  on  the  one  hand  the  same  pro- 
cedure might  obtain  not  through  a  few  segments  only  but  along  the  whole 
length  of  the  cord,  there  would  be  an  advantage,  especially  in  respect  to 
the  rapidity  of  transmission,  in  employing  internuncial  tracts  of  fibres  be- 
tween the  several  segments,  the  advantage  being  greater  the  more  distant 
the  segments  which  have  to  work  together. 

We  might  further  suppose  that  it  would  be  of  advantage  to  possess  some 
direct  path  between  the  cerebral  cortex  and  the  spinal  sensory  mechanism 
immediately  connected  with  the  posterior  root,  such  as  is  afforded  by  the 
pyramidal  tract  between  the  cortex  and  the  spinal  motor  mechanism 


CUTANEOUS   AND  SOME  OTHER  SENSATIONS.  713 

immediately  connected  with  the  anterior  root.  But  no  anatomical  evi- 
dence of  such  a  tract  is  forthcoming ;  and,  as  we  have  before  remarked, 
along  all  the  tracts  which  seem  to  be  sensory  in  nature,  in  contrast  to  what 
takes  place  in  the  motor  tracts,  relays  of  gray  matter  are  continually  being 
interpolated. 

The  median  posterior  tract,  since  it  gathers  up  representatives  of  succes- 
sive nerves,  presents  itself  as  the  nearest  approach  to  such  a  sensory  homo- 
logue  of  the  pyramidal  tract,  though  it  ends  in  the  bulb  and  is  not  con- 
tinued directly  on  to  the  cortex.  And  possibly  it  does  play  a  somewhat 
analogous  part,  in  so  far  as  it  serves  as  a  special  connection  between  the 
brain  and  the  whole  series  of  spinal  nerves.  But  we  are  wholly  ignorant  as 
to  what  it  really  does ;  and  whatever  be  the  exact  nature  of  the  part  which 
it  plays,  it  probably  has  relations  not  to  one  kind  of  sensation  only,  but  to 
all  the  different  kinds  of  sensation.  It  has  indeed  been  supposed  by  some 
to  be  especially  a  tract  for  the  impulses  of  the  muscular  sense ;  but  neither 
experiment  nor  clinical  study  affords  adequate  proof  of  this  view.  The  con- 
dition known  as  locomotor  ataxia,  the  salient  feature  of  which  is  loss  or 
impairment  of  muscular  sense,  is  associated  with  disease  of  the  posterior 
root  and  of  its  entrance  into  the  cord,  not  with  disease  confined  exclusively 
to  the  median  posterior  column.  Moreover,  the  tract  cannot  carry  all  the 
impulses  of  muscular  sense,  since  some  of  them  must  pass  at  once  into  the 
gray  matter  to  take  part  in  the  coordination  of  reflex  movements,  and  must 
therefore  travel  by  fibres  which  do  not  form  this  tract.  Similarly  there  is 
no  adequate  proof  of  the  tract  being  an  exclusive  channel  for  tactile  or  for 
painful  sensations. 

We  may  also,  perhaps,  urge  similar  considerations  with  regard  to  the 
cerebellar  tract,  which,  though  starting  from  a  relay  of  gray  matter,  is  thence 
onward  to  the  cerebellum  a  continuous  tract.  This  tract  also  has  been  sup- 
posed to  carry  impulses  of  a  particular  kind,  and  more  particularly  those  of 
muscular  sense.  There  is  \essa  priori  objection  to  this  view,  since  the  tract 
starts  from  the  gray  matter,  where  the  impulses  of  muscular  sense  may  have 
already  done  their,  so  to  speak,  local  work,  and  ends  in  the  cerebellum, 
which,  as  we  have  seen,  seems  especially  connected  with  the  coordination  of 
movements.  But  with  respect  to  this  tract  also  neither  experiment  nor 
clinical  study  affords  any  clear  and  decisive  proof  that  it  is  solely  or  even 
especially  concerned  with  the  muscular  sense. 

With  regard  to  the  antero-lateral  ascending  tract  our  knowledge  is  too 
imperfect  to  justify  us  in  supposing  that  it  is  the  special  or  exclusive  channel 
for  any  one  kind  of  sensation,  or  indeed  in  drawing  any  conclusions  at  all 
concerning  it. 

But  when  we  subtract  from  the  white  matter  of  the  cord  these  con- 
tinuous tracts  of  ascending  degeneration  of  presumably  sensory  or  afferent 
function,  and  the  continuous  tracts  of  descending  degeneration,  which  we 
may  confidently  speak  of  as  motor  or  at  least  efferent,  there  are  left  only  the 
fibres  which  we  have  (§  494)  supposed  to  be  longitudinal  commissural  or 
internuncial  fibres  between  successive  segments.  We  are  thus  driven  back 
to  our  former  conclusion,  that  sensory  impulses  pass  either  by  the  gray  matter 
alone,  or  by  a  series  of  steps  as  it  were,  by  relays  of  gray  matter  connected 
by  iuternuncial  tracts  of  fibres,  whose  length  we  cannot  ascertain,  but  which 
may  be  short.  That  such  internuncial  tracts  intervene  is  rendered  probable 
on  the  one  hand  by  the  fact  that  section  of  the  white  matter,  leaving  the 
gray  matter  untouched,  does  affect  sensations,  and  on  the  other  hand  by  the 
fact  that  the  several  kinds  of  sensation  appear  to  travel  along  the  cord  by 
separate  paths,  or  at  least  may  be  separately  blocked.  It  is  of  course,  as  we 
have  already  urged,  possible  that  the  effect  of  a  section  of  a  tract  of  fibres 


714  THE  BRAIN. 

may  not  be  the  mere  block  due  to  loss  of  continuity,  but  some  action  on  the 
gray  matter  with  which  the  fibres  are  connected,  whereby  that  gray  matter 
fails  of  its  usual  functions  and  ceases  to  carry  onward  the  sensory  impulses 
reaching  it  from  below ;  it  is  also  possible  that  this  or  that  lesion  of  disease 
may,  directly  or  indirectly,  affect  particular  parts  of  the  gray  matter  or 
affect  the  gray  matter  in  a  particular  way,  so  that  a  certain  kind  of  sensory 
impulse  and  none  other  is  blocked.  On  the  other  hand,  we  have  reason  to 
think  that  the  rate  at  which  impulses  travel  along  the  gray  matter  is  very 
slow  compared  with  that  along  nerve-fibres  ;  and  in  the  struggle  for  life 
rapidity  of  transmission  of  nervous  impulses  is  of  great  importance.  Hence 
the  view  that  the  internuncial  fibres  intervene  has  more  to  commend  it ;  it 
is,  moreover,  to  a  certain  extent  supported  by  clinical  histories.  But,  if  we 
accept  this  view,  we  must  at  the  same  time  admit  that,  in  animals  at  least, 
the  lines  provided  by  the  internuncial  tracts  are  not  rigid — that  within  limits 
and  under  circumstances  alternative  routes  are  possible. 

§  597.  We  may  here,  perhaps,  raise  once  more,  and  this  time  more  point- 
edly than  before,  the  doubt  whether  we  are  justified  in  assuming,  as  we 
generally  do  assume,  that  the  events  which  take  place  in  the  fibres  connect- 
ing relays  of  gray  matter  within  the  central  nervous  system  are  exactly  the 
same  as  those  which  take  place  in  the  fibres  of  nerves  outside  the  central 
system  during  the  passage  of  what  \ve  call  a  nervous  impulse.  Most  of  our 
knowledge  of  a  nervous  impulse  has  been  gained  by  the  study  of  the  motor 
nerve  of  a  muscle-nerve  preparation.  Our  knowledge  of  the  processes  in  af- 
ferent nerves  is  much  more  imperfect ;  but  there  are  many  facts  which  at  least 
suggest  that  the  molecular  events  constituting  an  afferent  impulse  along  an 
afferent  nerve  are  different  from,  and  probably  more  complicated  than,  those 
constituting  an  efferent  impulse  along  an  efferent  nerve.  And,  with  regard 
to  the  processes  taking  place  in  fibres  \vithin  the  central  nervous  system,  we 
have  hardly  any  exact  experimental  knowlege  at  all.  It  has  been  main- 
tained by  many  observers  that  not  only  the  gray  matter,  but  also  the  tracts 
of  white  matter  in  the  spinal  cord,  while  they  are  capable  of  conveying  im- 
pulses in  one  direction  or  the  other,  are  incapable  of  being  so  excited  by 
artificial  stimuli  as  to  generate  new  impulses.  These  observers  maintain  that 
when  movements  or  signs  of  sensation  follow  the  direct  stimulation  of  various 
parts  of  the  cord,  the  effects  are  due  to  issuing  motor  fibres  or  entering  sen- 
sory fibres  having  been  stimulated,  and  not  to  a  stimulation  of  the  intrinsic 
substance  of  the  parts  themselves ;  they  propose  accordingly  to  call  these 
parts  "  kinesodic  "  and  "  sesthesodic  "  respectively,  that  is  to  say,  serving  as 
paths  for  motor  or  sensory  impulses  without  being  themselves  either  motor 
or  sensory.  The  evidence  on  the  whole  goes  to  show  that  this  view  is  a  mis- 
taken one ;  that  the  various  tracts  of  the  spinal  cord,  like  the  pyramidal 
tract,  and  indeed  other  parts  of  the  brain,  are  excitable  toward  artificial 
stimuli.  The  question  cannot,  however,  be  considered  as  definitely  closed  ; 
and  the  very  fact  that  it  has  been  raised  illustrates  the  point  on  which  we 
are  now  dwelling.  We  may  further  quote,  in  similar  illustration  of  the 
same  point,  the  following  remarkable  fact  which  was  observed  in  the  series 
of  experiments  referred  to  in  §  576  on  the  effects  of  repeated  hemisection  of 
the  spinal  cord  in  dogs :  The  animal  had  partially  recovered  voluntary 
movements  in  his  hind  limbs  after  a  third  hemisection  of  the  thoracic  cord, 
and  yet  when,  at  his  death,  a  strong  tetanizing  current  was  directed  through 
the  bulb  and  cervical  cord,  no  movements  of  the  hind  limbs  followed;  the 
impulses  started  by  artificial  stimulation  could  not  pass  the  bridge  which 
sufficed  for  volitional  impulses  of  natural  origin.  It  is  not  too  much  to  say 
that  our  experimental  knowledge  as  to  the  events  which  accompany  the 
activity  of  the  structures  within  the  central  nervous  system  is  almost  entirely 


CUTANEOUS  AND  SOME  OTHER  SENSATIONS.  715 

limited  to  the  recognition  of  the  "  currents  of  action  "  referred  to  in  §  570. 
We  are  already  going  beyond  our  tether  when  we  assume  on  the  strength  of 
this  that  the  processes  started  in  the  fibres  of  the  pyramidal  tract  by  artifi- 
cial stimulation  are  in  all  respects  identical  with  those  started  in  the  fibres 
of  a  motor  nerve.  We  are  going  still  more  beyond  our  tether  when  we 
assume  that  the  processes  started  in  the  same  pyramidal  fibres  as  the  out- 
come of  natural  events  in  the  motor  cortex  are  of  the  same  kind.  But  these 
assumptions  are  trifles  compared  with  the  assumption  that  the  events  taking 
place  in  the  fibres  of  the  optic  radiation  passing  from  the  pulvinar  to  the 
occipital  cortex  are  identical  with  the  events  taking  place  in  the  fibres  of 
the  optic  tract  on  the  way  to  the  pulvinar,  or  that  the  events  travelling  along 
the  spinal  cord  to  the  brain  as  the  result  of  a  prick  of  the  little  finger  are 
identical  with  those  which  the  prick  has  started  in  the  fibres  of  the  ulnar 
nerve.  Of  the  latter  events  we  know  a  little ;  of  the  former  events  we  know 
next  to  nothing.  And  we  may  here  ask  the  question,  What  is  the  meaning 
of  these  continual  relays  of  gray  matter  along  the  sensory  tract,  unless  it  be 
that  at  each  relay  some  transformation,  some  further  elaboration  of  the  im- 
pulses take  place,  until  what  were  the  relatively,  but  only  relatively,  simple 
impulses  along  the  fibres  of  the  peripheral  nerve  are  by  successive  steps 
changed  in  the  complex  events  which  we  call  a  conscious  sensation  ?  This 
is  what  we  had  in  mind  when  we  gave  (§  565)  a  note  of  warning  concerning 
the  danger  of  considering  all  the  events  in  the  central  nervous  system  as  either 
motor  or  sensory  in  nature.  It  is  perhaps  not  an  exaggeration  to  represent 
the  views  of  some  observers  as  if  they  supposed  that  afferent  impulses,  say 
tactile  impulses,  that  is,  impulses  eventually  giving  rise  to  tactile  sensations, 
travelled  unchanged  from  the  skin  to  the  cortex,  and  there  suddenly  blos- 
somed into  sensations.  If  such  a  view  were  true,  undoubtedly  the  chief  task 
of  physiology,  almost  the  only  one,  would  be  to  ascertain  the  tract  along 
which  these  impulses  passed.  But  if,  on  the  other  hand,  the  views  just  now 
urged  have  any  real  foundation,  the  question  of  tracts  or  paths  sinks  into 
insignificance  compared  with  the  almost  untouched  problems  as  to  what  are 
the  several  changes  by  which  simple  impulses  are  developed  into  full  sensa- 
tions, and  when  and  how  the  changes  are  effected. 

§  598.  Seeing  how  unsatisfactory  is  our  present  knowledge  with  regard 
to  the  tracts  or  paths  of  sensations  in  the  relatively  simple  spinal  cord,  it 
would  be  useless  to  attempt  any  discussion  as  to  their  paths  in  the  much 
more  complex  brain.  If  it  be  probable  that  the  passage  is  effected  by  relays 
of  gray  matter  in  the  former,  the  same  method  is  much  more  probable  in  the 
latter  ;  and  if  neither  experiment  nor  clinical  study  throws  much  light  on 
the  path  up  to  the  bulb,  these  cannot  be  expected  to  give  much  help  in 
the  maze  of  gray  matter  and  fibres  by  which  the  bulb  is  joined  to  the  cortex. 
The  several  defined  areas  or  collections  of  gray  matter,  and  the  several 
strands  and  tracts  of  fibres  which  we  briefly  described  in  a  previous  section, 
must  have  of  course  a  meaning ;  but  it  may  be  doubted  whether  we  have 
even  so  much  as  a  correct  glimpse  of  that  meaning  in  any  one  case,  if  we 
except  those  which  are  in  immediate  connection  with  the  cranial  nerves  and 
their  nuclei.  Seeing  that  the  thalamus  appears,  on  the  one  hand,  to  be  con- 
nected with  all  or  nearly  all  parts  of  the  cortex,  and,  on  the  other  hand,  to 
serve  as  the  front  of  the  tegrnental  system,  it  is  tempting  to  suppose  that  it 
plays  an  important  part  in  sensations  pertaining  to  the  body  generally,  as 
part  of  it,  the  pulvinar,  certainly  does  with  reference  to  the  special  sense  of 
sight ;  but  we  have  no  decisive  indications  as  to  what  part  it  plays.  And 
the  part  which  it  plays,  whatever  that  may  be,  is  not  an  exclusively  sensory 
one,  since  both  experimental  and  morbid  lesions  of  the  thalamus  are  apt  to 
produce  disorders  of  movement  as  well  as  other  efferent  effects.  We  ought 


716  THE  BRAIN. 

perhaps  to  say  the  parts  which  it  plays,  for  it  is  a  complex  body,  having 
many  ties  and  probably  performing  many  duties. 

The  conspicuous  fillet  again,  seeming  as  it  does  to  be  a  special  internun- 
cial  tract  connecting  what  appear  to  be  more  particularly  afferent  or  sensory 
parts  of  the  bulb,  such  as  the  gracile  and  cuneate  nuclei,  with  various  parts 
of  the  middle  brain  and  probably  with  the  cortex,  presents  itself  as  a 
probable  path  of  sensations  of  one  kind  or  another  from  the  body  at  large, 
the  "  narrow  path  "  of  the  anatomical  programme  (§  592)  ;  but  in  reference 
to  this  too,  beyond  its  probable  connection  with  the  auditory  sensations 
(§  589),  we  lack  evidence. 

A  conspicuous  part  of  the  brain,  namely,  the  cerebellum,  naturally  ar- 
rests our  attention  on  account  of  its  large  connections  with  what  appear  to 
be  afferent  structures  ;  in  the  anatomical  programme  we  called  it "  the  broad 
path."  By  the  cerebellar  tract  it  has  an  uncrossed  grip  upon  what  is  prac- 
tically the  whole  length  of  the  spinal  cord  ;  by  the  other  constituents  of  the 
inferior  peduncle  it  has  a  like  uncrossed  grip  upon  what  appear  to  be  affer- 
ent structures  in  the  bulb,  the  gracile  and  cuneate  nuclei,  as  well  as  on  the 
eighth  (vestibular)  nerve  and  probably  representatives  of  other  afferent 
cranial  nerves  ;  it  has  further  a  crossed  grip  through  the  gracile  and  cuneate 
nuclei  on  the  afferent  posterior  columns  of  the  whole  cord.  It  is,  of  course, 
possible  that  the  cerebellar  tract,  though  in  itself  uncrossed,  lays  its  hand, 
by  means  of  the  vesicular  cylinder  for  instance,  on  impulses  which  have 
already  crossed  from  the  posterior  roots  of  the  other  side ;  for  as  we  have 
seen  the  evidence  as  a  whole  shows  that  sensory  impulses  do  cross  over ;  but 
neither  has  the  crossing  of  the  impulses  been  definitely  proved,  nor  has  the 
path  of  the  crossing  been  clearly  demonstrated  ;  while,  on  the  contrary,  the 
fibres  of  the  auditory  nerve  which  pass  to  the  cerebellum,  and  which,  as  we 
have  suggested  (§  531),  may  be  compared  to  an  outlying  part  of  the  cere- 
bellar tract,  certainly  continue  uncrossed  into  the  peduncle  of  the  same 
side.  We  may  conclude,  therefore,  that  the  ties  of  the  cerebellum  with 
the  posterior  roots  are  both  crossed  and  uncrossed.  And  we  may  regard 
this  double  grip  of  the  cerebellum  on  the  cord,  this  grip  oil  both  sides  of 
it,  as  an  additional  evidence  that  the  ties  of  the  cerebellum  with  the 
spinal  cord  are  not  merely  for  the  purpose  of  serving  as  the  channel  for 
the  impulses  of  muscular  sense,  but  are  the  means  by  which  the  cere- 
bellum transforms  or  elaborates  sensory  impulses,  not  of  muscular  sense 
alone  or  chiefly,  but  probably  of  all  kinds,  in  order  that  they  may  take 
part  in  cerebral  operations,  of  which  the  coordination  of  bodily  move- 
ments may  be  one,  but  probably  is  only  one,  of  several  or  even  of  many. 

SOME  OTHER  ASPECTS  OF  THE  FUNCTIONS  OF  THE  BRAIN. 

§  599.  It  is  difficult  to  say  anything  definite  concerning  the  transmis- 
sion of  sensory  impulses  and  the  development  of  sensations ;  it  is  still  more 
difficult  to  say  anything  definite,  beyond  what  has  been  already  incidentally 
said,  concerning  the  parts  played  in  the  work  of  the  brain  by  the  various 
aggregations  of  gray  matter  and  tracts  of  fibres  forming  the  middle  part  of 
the  brain.  Neither  experiment  nor  clinical  study  has  as  yet  afforded  any 
clear  or  sure  leading. 

To  what  has  already  been  said  about  the  cerebellum,  we  may  add  the 
following ; 

Electrical  stimulation  of  the  surface  of  the  cerebellum,  in  the  monkey 
and  in  other  animals,  has  led  to  movements  of  the  eyes,  and  of  other 
parts  of  the  head ;  but  we  cannot  from  such  results  draw  any  satisfactory 
inferences. 


OTHER   ASPECTS  OF  THE  FUNCTIONS  OF  THE  BRAIN.        717 

The  removal  of  various  parts  of  the  cerebellum,  especially  of  the 
medium  parts,  has  led  to  a  want  of  coordination  in  bodily  movements  ; 
and  an  unsteady  gait,  due  to  a  like  want  of  adequate  coordination,  is  a 
frequent  symptom  of  cerebellar  disease.  But  the  incoordination  which 
makes  its  appearance  immediately  after  removal  of,  or  injury  to,  the  cere- 
bellum may  eventually  disappear,  even  though  large  portions  have  been 
removed ;  and  many  cases  of  cerebellar  disease  have  been  recorded  in 
which  the  most  perfect  coordination  of  movements  was  retained.  Hence, 
the  results  of  experimental  and  clinical  study,  while  on  the  whole  support- 
ing the  conclusion  that  the  cerebellum  has  in  some  way  'to  do  with  coor- 
dination, throw  little  or  no  light  on  the  exact  nature  of  the  part  which  the 
organ  plays  in  the  complex  process,  but  perhaps  rather  show  that  we  are 
at  present  wholly  ignorant  of  how  coordination  is  brought  about. 

Many  hypotheses  have  been  put  forward  as  to  the  work  carried  out  by 
the  cerebellum,  but  none  of  these  can  be  said  to  have  an  adequate  basis. 
And,  indeed,  if  there  be  any  value  in  the  reflections  we  have  repeatedly 
made  in  previous  pages,  the  physiologist  ought  not  to  use  the  words  "  func- 
tions of  the  cerebellum."  From  a  physiological  point  of  view  it  is,  so  to 
speak,  a  matter  of  accident  that  various  structures,  the  seats  of  various 
physiological  processes,  have,  from  morphological  causes,  been  gathered 
together  into  the  body  which  anatomists  call  the  cerebellum.  The  task 
of  the  physiologist  is  to  unravel  the  ties  binding  these  various  cerebellar 
structures  with  other  parts  of  the  central  nervous  system,  and  so  with 
various  parts  of  the  body  at  large. 

We  must  content  ourselves  here  with  calling  attention  to  two  or  three 
broad  and  suggestive  facts  concerning  its  structure  and  connections. 

In  the  first  place,  one  striking  fact  about  the  cerebellum  is  the  very  large 
development  of  commissural  fibres  connecting  together  the  superficial  gray 
matter  of  the  two  hemispheres  for  the  greater  part  of  their  extent,  and 
passing,  not  only  tjirough  the  pons  (§  548)  as  part  of  the  middle  peduncle, 
but  also  through  the  median  vermis.  This  great  commissure  is  second  only 
to  the  great  callosal  commissure  of  the  cerebrum  ;  and  from  the  fact  that 
median  lesions  of  the  cerebellum,  those  which  do  most  damage  to  this  com- 
missure, are  the  most  effective  in  causing  incoordination  and  forced  move- 
ments, we  may  infer  that  it  in  some  way  plays  an  important  part  in  co- 
ordination. 

A  second  striking  fact  is  one  on  which  we  have  already  just  dwelt,  the 
connection,  chiefly  an  uncrossed  one,  through  the  inferior  peduncle,  with  the 
afferent  structures  of  the  bulb  and  spinal  cord.  We  may  now  add  that  the 
fibres  of  this  peduncle  passing  into  the  centre  of  the  white  matter  of  the 
cerebellar  hemisphere  of  the  same  side  enclose  the  gray  matter  of  the  nu- 
cleus deutatus  and  appear  largely  to  end  in  that  body,  though  some  pass 
on  to  the  vermis. 

A  third  striking  fact  is  the  connection,  this  being,  as  far  as  we  know, 
wholly  a  crossed  one,  through  the  pons  and  pes,  with  the  cerebral  cortex 
both  of  the  extreme  frontal  region  and  of  the  temporo-occipital  region 
and  possibly  or  even  probably  with  more  scattered  cortical  elements  of  the 
parietal  (motor)  region.  This  connection  is  one  between  cortex  and  cortex, 
or,  at  least,  between  cerebral  cortex  and  cerebellar  superficial  gray  matter, 
for  the  fibres  of  the  middle  peduncle  passing  from  the  gray  matter  of  the 
pons  which  serves  as  a  relay  end  in  the  surface  of  the  lateral  hemisphere 
of  the  cerebellum.  The  frontal  cortical  fibres  passing  to  the  pes  have  a 
descending  degeneration,  that  is,  from  the  cortex  to  the  pons,  and  we  may 
probably  assume  that  the  similar  temporo-occipital  fibres  similarly  degener- 
ate downward  to  the  pons  (§  545).  From  this  it  has  been  inferred  that 


718  THE  BRAIN. 

this  cerebro-cerebellar  connection  carries  impulses  from  the  cebral  cortex 
to  the  cerebellum  ;  and  it  has  been  further  inferred  that  these  impulses 
are  of  the  nature  of  motor  impulses.  As  we  have  more  than  once  urged, 
the  character  of  degeneration,  that  is,  whether  "  ascending  "  or  "  descend- 
ing," is  not  a  satisfactory  proof  of  the  direction  taken  by  impulses ;  but  it 
is,  perhaps,  of  more  importance  to  remember  that,  as  we  have  also  urged, 
we  have  no  right  to  assume  that  the  impulses  passing  along  such  a  tract  as 
the  one  in  question  must  be  either  sensory  or  motor,  or  indeed  that 
such  a  tract  serves  as  an  instrument  for  producing  effects  in  one  direction 
only. 

That  during  life  the  fibres  of  which  we  are  speaking  serve  as  an  important 
chain  by  which  cerebral  cortex  and  cerebellum  affect  each  other,  there 
can  be  but  little  doubt ;  but  we  are  wholly  in  the  dark  as  to  what  really 
takes  place  along  the  fibres.  We  have  seen  (§  506)  reason  to  think  that  the 
brain  may  and  does  exert  an  inhibitory  influence  over  the  spinal  cord ;  and 
the  mechanical  certainty  with  which  an  animal  deprived  of  its  cerebral 
hemispheres  responds  to  stimuli,  in  contrast  to  the  uncertainty  attending  the 
result  of  stimuli  applied  to  an  intact  animal,  as  well  as  all  the  experience  of 
our  own  daily  life,  shows  that  the  cerebral  cortex  can  work  in  an  inhibitory 
manner  on  other  parts  of  the  brain  ;  the  remarkable  "  forced  movements  " 
on  which  we  dwelt  in  a  previous  section  seem,  in  some  instances,  to  be  the 
result  of  the  abrupt  snap  of  some  inhibitory  bond.  Conversely  all  the  ex- 
perience of  our  daily  life,  many  of  the  phenomena  of  the  condition  known 
as  hypnotism  and  of  allied  conditions,  as  well  as  various  experimental  re- 
sults such  as  that  quoted  in  §  574,  where  a  sensory  impulse  seems  to  inhibit 
the  activity  of  a  motor  area,  show  that  the  cortex  may  itself  in  turn  be  in- 
hibited by  other  parts  of  the  central  nervous  system.  But  we  have  at  present 
no  satisfactory  indications  as  to  the  paths  of  inhibitory  impulses  or  as  to  how 
inhibition  is  brought  about ;  nor  have  we  any  proof  that  the  cerebro-cerebellar 
tract  is  an  inhibitory  one  in  either  direction. 

We  may  add  that  some  of  the  fibres  of  the  middle  peduncle  appear  to 
be  neither  commissural  nor  connected  with  the  cortical  fibres  in  the  pes,  but 
to  end  in  other  ways ;  and  tracts  have  been  described  as  continuing  on- 
ward, some  of  the  cerebellar  fibres  of  the  middle  peduncle  on  the  one  hand 
upward  toward  the  cerebrum,  and,  on  the  other  hand,  downward  toward  the 
spinal  cord.  It  has  been  further  urged  that  these  tracts  are  efferent  in 
function. 

Lastly,  we  may  call  attention  to  the  superior  peduncles.  These,  which, 
as  we  have  seen,  appear  to  come  largely  from  the  gray  matter  of  the  nucleus 
dentatus  and  to  end  in  the  tegmentum,  largely  in  the  red  nucleus,  may  be 
regarded  as  constituting  through  the  relay  of  the  front  part  of  the  tegmen- 
tum another  tie,  presumbly  of  a  different  nature  from  the  foregoing,  between 
the  cerebellum  and  the  cortex ;  indeed,  it  used  to  be  called  processes  a 
cerebello  ad  cerebrum.  It  is  an  obviously  crossed  tract  (Fig.  136,  $P);  it 
connects  one  nucleus  dentatus,  and  so  presumably  by  that  relay  the  fibres 
of  the  inferior  peduncle  ending  in  that  body,  and  perhaps  other  fibres  pro- 
ceeding from  the  superficial  gray  matter  of  one  side  of  the  cerebellum,  with 
the  red  nucleus  and  other  parts  of  the  tegmentum  of  the  crossed  side,  and 
thus  with  the  cortex  of  the  crossed  side.  It  has  been  supposed  that  the 
direction  of  impulses  passing  along  it  is  from  the  cerebrum  to  the  cere- 
bellum, but  we  have  no  clear  proof  of  this  ;  indeed,  as  to  what  it  does,  we 
have  no  satisfactory  evidence  either  experimental  or  clinical. 

We  may  here  incidentally  remark  that,  in  consequence  of  afferent  tracts 
being  traced  to  or  toward  the  tegmentum,  and  of  the  sharp  contrast  pre- 
sented between  the  tegmentum  and  the  conspicuously  motor  pyramidal  tract 


OTHER   ASPECTS  OF  THE  FUNCTIONS  OF  THE  BRAIN.        719 

in  the  pes,  the  view  has  gained  ground  that  the  tegmeutum  is  essentially  a 
sensory  structure.  But  there  does  not  appear  to  be  adequate  evidence  either 
clinical  or  experimental  for  such  a  conclusion.  The  thalamus,  which  we  have 
regarded  as  the  front  so  to  speak  of  the  tegmentum,  cannot,  as  we  have 
already  urged  (§  598),  be  considered  exclusively  or  especially  sensory.  And 
many  of  the  ties  of  the  tegmentum,  such  as  the  fibres  from  the  corpora  striata 
ending  in  the  substantia  nigra,  for  this  may  be  considered  as  properly  be- 
longing to  the  tegmentum,  are  of  the  kind  which  we  may  suppose  to  be 
efferent  or  motor.  Indeed,  we  may  probably  regard  the  whole  tegmentum 
as  being  broadly  the  analogue  in  the  forward  segments  of  the  cerebro-spinal 
axis  of  both  the  anterior  and  posterior  gray  matter  of  the  spinal  segments 
behind. 

Though  we  are  thus  in  the  dark  concerning  what  goes  on  in  the  cere- 
bellum, it  may  be  worth  while  to  call  attention  once  more  to  the  remarkable 
characters  of  the  superficial  gray  matter  (§  561).  The  many  points  of  re- 
semblance between  it  and  the  cerebral  cortex  cannot  but  suggest  that  the 
processes  taking  place  in  it  have  some  analogies  with  cortical  events.  And 
it  is,  at  least,  a  fact  of  some  significance  that  congenital  deficiency,  or 
atrophy  of  the  cerebral  hemisphere  of  one  side,  is  frequently  accompanied 
by  a  corresponding  deficiency  of  the  crossed  cerebellar  hemisphere. 

§  600.  Both  the  anterior  and  posterior  corpora  quadrigemina  are  com- 
plex in  structure  ;  not  only  do  they  differ  from  each  other,  but  also  in  each 
the  gray  matter  differs  in  different  parts,  both  as  to  its  nature  and  appear- 
ance and  as  to  its  connections  with  tracts  of  fibres.  If  we  have  little  right 
to  speak  of  the  "  functions  of  the  cerebellum,"  we  have  even  less  right  to 
speak  of  the  "  functions  of  the  corpora  quadrigemina  "  or  of  either  pair  of 
them.  The  anterior  pair,  as  we  have  seen,  has  to  do  in  some  way  with 
vision ;  but  we  have  reason  to  think  that  a  part  only  of  the  whole  body  is 
thus  concerned  ;  and  there  is  some  foundation  for  the  view  that  of  this  part 
one  portion  belongs,  so  to  speak,  to  the  optic  tract,  and  another  portion  to 
the  cortical  fibres  of  the  optic  radiation.  Possibly  still  another  part  is  con- 
cerned in  bringing,  as  we  have  (§  586)  suggested,  visual  impulses  to  bear  on 
the  coordination  of  movements. 

Stimulation  of  the  surface  of  the  posterior  pair,  besides  giving  rise  to 
movements  of  various  parts  of  the  body,  has  in  monkeys  and  some  other 
animals  the  singular  effect  of  producing  a  vocal  utterance  in  the  form  of  a 
cry  or  bark.  But  we  cannot  make  much  use  of  these  results  for  the  pur- 
pose of  drawing  conclusions  as  to  what  share  these  bodies  take  in  the  whole 
work  of  the  brain.  In  the  frog,  the  optic  lobes  correspond  to  the  two  pair 
of  corpora  quadrigemina  together  ;  and  the  cry  just  mentioned  may,  perhaps, 
be  put  side  by  side  with  the  fact  that  in  the  frog  the  optic  lobes  seem  to 
furnish  a  mechanism  for  croaking ;  when  the  optic  lobes  are  destroyed,  the 
reflex  croaking  mentioned  in  §  551  is  done  away  with.  The  probable  con- 
nection of  the  posterior  corpora  quadrigemina  with  hearing  is  also  interest- 
ing in  this  connection  ;  but  we  have  no  satisfactory  evidence  of  any  special 
ties  between  the  bodies  in  question  and  either  the  cortical  area  for  phonation 
or  the  vocal  mechanism  in  general ;  the  occurrence  of  the  cry  remains  so  far 
an  isolated  fact. 

In  frogs,  in  which  the  cerebellum  is  very  small,  the  optic  lobes  seem  to 
be  particularly  concerned  in  the  coordination  of  movements.  When  the 
brain  is  removed  by  means  of  a  section  behind  the  optic  lobes  the  animal 
loses  the  power  of  balancing  itself  (§  551),  which  it  possesses  when  the  sec- 
tion passes  in  front  of  the  optic  lobes  ;  and  injury  to  the  optic  lobes  pro- 
duces incoordination  of  movement  and  often  "forced  movements."  It  has 
been  maintained  that  the  loss  of  coordination  is  in  these  cases  due  to  re- 


720  THE  BRAIN, 

inoval  of  or  injury  to  the  central  gray  matter  in  the  walls  of  the  third  ven- 
tricle, and  not  to  mere  removal  of  or  injury  to  the  optic  lobes  ;  but  the 
whole  evidence  goes  to  show  that  in  the  frog  and  in  the  bird  the  optic  lobes 
do  play  a  part  in  the  coordination  of  movement,  though  lesions  of  the  cen- 
tral gray  matter  around  the  third  ventricle,  or,  indeed,  of  the  thalamus  or 
other  parts  of  the  tegmentum,  may  give  rise  to  loss  of  coordination  or  to 
"  forced  movements." 

In  the  mammal  removal  of  or  injury  to  the  posterior  corpora  quadri- 
gemina  does  not  cause  blindness,  but  may,  like  a  lesion  of  the  anterior  pair, 
give  rise  to  loss  of  coordination  or  to  forced  movements  ;  the  effect,  how- 
ever, is,  in  most  instances,  very  temporary.  The  connection  of  the  anterier 
pair  with  vision  suggests  a  clew  as  to  how  this  pair  takes  part  in  coordina- 
tion ;  but  as  to  how  the  posterior  pair  could  intervene  in  the  matter  we  have 
hardly  so  much  as  a  hint ;  for,  even  if  we  admit  a  connection  between  them 
and  the  sense  of  hearing,  and,  remembering  that  a  loud  sound  would  often 
cause  a  person  to  reel,  further  admit  that  purely  auditory  impulses,  as  dis- 
tinct from  what  we  have  called  ampullar  impulses,  may  take  part  in  the 
general  coordination  of  bodily  movements  and  in  the  maintenance  of  equi- 
librium, as  they  certainly  do  in  the  special  coordination  of  laryngeal  move- 
ments, still  we  are  not  much  nearer  an  understanding  of  the  matter.  We 
may  add  that  section  of  the  lateral  fillet,  which  appears  as  a  conspicuous  tie 
between  the  posterior  corpora  quadrigemina  and  the  parts  of  the  nervous 
system  behind  them,  does  not  appear  to  have  any  marked  effect  in  produ- 
cing incoordination. 

In  fine,  beyond  the  broad  facts  on  which  we  dwelt  in  a  previous  section, 
namely,  that  we  maintain  our  equilibrium  and  carry  out  complex  movements 
involving  often  several  parts  of  our  body,  through  what  we  call  coordination, 
that  afferent  impulses  supply  important  factors  of  this  coordination,  and 
that  the  cerebellum,  through  the  vestibular  nerves  in  part  at  all  events, 
together  with  other  portions  of  the  middle  brain,  are  in  some  way  its  chief 
instruments,  we  as  yet  know  very  little.  We  have  certainly  no  adequate 
knowledge  as  to  how  either  pair  of  corpora  quadrigemina  exactly  intervene 
in  the  matter,  or,  indeed,  as  to  what  other  parts  they  play  in  the  general 
work  of  the  brain. 

With  regard  to  other  tracts  of  fibres  or  areas  of  gray  matter  we  have 
nothing  to  say,  except  as  regards  those  which  are  more  or  less  immediately 
connected  with  certain  of  the  cranial  nerves,  such  for  instance  as  the  nerves 
for  movements  of  the  eyes,  and  these  it  will  be  best  to  consider  when  we  have 
to  deal  with  the  nerves  themselves. 

§  601.  Besides  the  somatic  functions  which  in  previous  discussions  we 
have  chiefly  had  in  view,  the  brain  as  a  whole  undoubtedly  carries  out 
splanchnic  functions  ;  concerning  these,  however,  we  must  be  very  brief. 

Of  the  respiratory  and  vasomotor  functions  of  the  bulb  we  have  already 
treated  in  their  appropriate  places,  and  we  have  referred  (§  449)  to  the 
experimental  evidence  that  a  lesion  of  the  corpus  striatum  or  of  the  front 
part  of  the  optic  thalamus  has  a  remarkable  influence  on  the  development 
of  heat  in  the  body.  We  have  further  seen  that  the  higher  parts  of  the 
brain,  acting  through  the  bulb,  exercise  powerful  influences  on  respiration, 
on  the  vasomotor  system,  and  on  the  beat  of  the  heart.  Daily  experience 
affords  abundant  instances  of  actions  such  as  these,  as  well  as  of  the  influence 
of  the  brain  on  other  organic  functions.  We  can  bring  our  will  to  bear  on 
the  mechanism  of  micturition  (§  364)  which  is  almost  wholly,  and  on  the 
mechanism  of  defecation  (§  236)  which  is  largely,  splanchnic  in  nature. 
These  movements,  however,  are  not  skilled  movements ;  and  as  we  explained 
in  dealing  with  them,  the  action  of  the  brain  as  regards  them  seemed  limited 


OTHER  ASPECTS  OF  THE  FUNCTIONS  OF  THE  BRAIN.        721 

to  augmenting  or  inhibiting  the  activity  of  spinal  centres.  We  should,  there- 
fore, hardly  expect  them  to  be  specially  represented  in  the  cortical  motor 
region.  But  emotions  have  a  much  wider  and  more  powerful  influence  over 
the  splanchnic  functions  than  has  the  will,  and  have  the  power  of  affecting 
the  work  of  certain  organs,  for  instance  the  heart  and  secreting  glands,  which 
the  will  is  unable  to  touch.  And  since  we  have  every  reason  to  believe  that 
the  cortex  is  closely  associated  with  the  emotions,  we  may  naturally  infer 
that  elements  of  the  cortex  supply  a  link  in  the  chain  through  which  an 
emotion  influences  this  or  that  splanchnic  activity  ;  we  may,  accordingly, 
expect  to  find  that  stimulation  of  some  part  or  other  of  the  cortex  produces 
splanchnic  effects.  The  results  of  experimental  investigation,  however,  are 
both  scanty  and  discordant ;  but  the  greater  weight  should  perhaps  be 
attached  to  the  positive  results.  Thus,  some  observers  find  that  stimulation 
of  the  cortex,  the  locality  being  in  the  dog  some  part  of  the  sigmoid  gyrus, 
produces  movements  of  the  bladder ;  and  they  trace  the  path  of  this  influence 
through  the  front  part  of  the  thalamus  and  the  tegmentum  to  the  bulb  and 
so  to  the  cord,  excluding  the  cerebellum,  which  other  observers  believed  to 
be  concerned  in  the  matter.  Some  observers  again  find  that  stimulation  of 
the  cortex  produces  a  flow  of  "  chorda  saliva,"  while  others  maintain  that 
the  secretion,  when  it  does  occur,  is  an  indirect  and  not  a  direct  effect  of  the 
cortical  stimulation ;  and  it  may  be  remarked  that  the  cortical  area,  which 
is  claimed  to  be  a  "  salivation  area,"  lying  in  the  dog  on  the  convolutions 
dorsal  to  and  in  front  of  the  Sylvian  fissure,  is  not  either  the  area  connected 
with  the  facial  nerve,  or  that  allotted  to  taste  or  smell. 

Similarly,  stimulation  of  parts  of  the  cortex  has  in  the  hands  of  various 
observers  led  to  movements  or  to  arrest  of  movements  of  the  intestines,  to 
changes  in  the  beat  of  the  heart,  and  to  various  vasomotor  and  other  effects  ; 
but  it  will  not  be  profitable  to  enter  into  any  further  details.  We  may, 
however,  add  the  remark  that  when  the  cortical  motor  area  for  a  limb  is 
removed,  or  suffers  a  lesion,  the  temporary  paralysis  which  is  thereby  caused 
is  accompanied  by  a  rise  of-  temperature  in  the  limb ;  this  may  be  at  times 
very  great  indeed — in  the  monkey,  for  instance,  the  hand  or  foot  on  the 
paralyzed  side  may  be  as  much  as  10°  C.  higher  than  that  of  the  other  side. 
The  effect  is  partly  due  to  vasomotor  paralysis,  but,  especially  considering 
that  the  muscles  of  the  limb  are  relatively  quiescent  and  so  producing  less 
heat  than  usual,  cannot  be  due  to  that  alone.  The  remarkable  result  may 
be  taken  as  still  further  illustrating  the  complexity  of  the  processes  connected 
with  the  cortical  motor  area  ;  the  area  is  in  some  way  associated  with  the 
vascular  arrangements  and  nutrition  of  the  muscles  with  whose  movements 
it  is  concerned. 

§  602.  There  remain  yet  a  few  words  to  be  said  about  the  cortex.  We 
regard,  and  justly  so,  the  spontaneous  intrinsic  activity  of  the  brain  as  the 
most  striking  feature  of  its  life.  The  nearest  approach  to  it  which  we  find 
elsewhere  in  the  body  is  perhaps  the  rhythmic  beat  of  the  heart.  The 
analogy  between  the  "  regular  automatism  "  of  the  one,  and  the  "  irregular 
automatism  "  of  the  other  is  a  striking  one ;  and  indeed  our  knowledge  of 
the  relatively  simple  spontaneity  of  the  heart  has  probably  influenced  to  a 
large  extent  our  conceptions  of  the  complex  spontaneity  of  the  brain.  In 
the  heart  the  rhythmic  discharge  of  energy  is  chiefly  determined  by  in- 
trinsic chemical  changes,  by  the  metabolism  of  the  cardiac  substance ;  the 
influence  of  external  circumstances,  apart  from  those  which  provide  an 
adequate  supply  of  proper  blood,  is  wholly  subsidiary  and  serves  only  to 
raise  or  to  lower  the  intrinsic  changes  from  time  to  time,  as  occasion  may 
demand.  And  the  analogy  of  the  heart  has  perhaps  led  us  to  exaggerate 
the  part  played  in  the  brain  by  the  like  intrinsic  chemical  metabolism. 

46 


722  THE   BRAIN. 

(We  are  here,  of  course,  viewing  the  action  of  the  brain  from  the  only 
standpoint  admissible  in  these  pages,  the  purely  physiological  one  ;  but  such 
a  mode  of  treatment  does  not  prejudge  other  points  of  view.)  Some  writers 
use  expressions  which  seem  to  imply  the  conception  that  the  nervous 
changes  forming  the  basis  of  the  psychical  and  other  processes  of  the  brain 
are  chiefly  the  direct  outcome  of  the  chemical  metabolism  of  the  gray 
matter  and  especially  of  the  nerve-cells.  They  speak  of  "  the  discharge  of 
energy  "  from  these  cells  in  the  same  way  that  we  can  speak  of  the  dis- 
charge of  energy  from  a  cardiac  fibre.  But,  to  say  nothing  of  the  low  rate 
of  nervous  metabolism  as  measured  in  terms  of  chemical  energy,  we  have 
no  experimental  or  other  evidence  of  nervous  substance  in  any  part  of  the 
body  being,  like  the  cardiac  substance,  the  seat  of  an  important  metabolism 
carried  on  irrespective  of  influences  other  than  purely  nutritive  ones.  In 
the  case  of  nerve- eel  Is  interpolated  along  nerves  composed  of  fibres  of  the 
same  kind,  as  in  the  sporadic  ganglia,  all  the  instances  where  the  nerve-cells 
were  supposed  to  initiate  active  processes  have,  on  examination,  broken 
down  ;  as  we  have  seen,  the  ganglia  of  the  heart  do  not  supply  the  moving 
cause  of  the  heart-beat.  It  is  only  in  the  central  nervous  system,  where 
nerve-cells,  as  part  of  gray  matter,  are  found  at  the  meeting  of  nerve-fibres 
of  different  kinds,  that  we  have  any  evidence  of  "  discharge  of  energy  "  from 
the  cells. 

As  we  pointed  out  (§  510)  in  speaking  of  the  spinal  cord,  the  discharge 
of  efferent  impulses  from  the  central  nervous  system,  though  it  undoubt- 
edly must  have  a  certain  chemical  basis,  namely,  the  metabolism  of  the 
nervous  substance,  is,  in  the  first  place,  dependent  on  the  advent  of  afferent 
impulses.  But  this,  if  true  of  the  spinal  cord,  is  still  more  true  of  the 
brain,  which  receives  or  may  receive  not  only  all  the  impulses  which  reach 
it  through  the  cord,  but  especially  potent* and  varied  impulses  directly 
through  the  cranial  nerves.  All  life  long,  the  never-ceasing  changes  of  the 
external  world  continually  break  as  waves  on  the  peripheral  endings  of  the 
afferent  nerves;  all  life  long,  nervous  impulses, now  more  now  fewer, are  con- 
tinually sweeping  inward  toward  the  centre ;  and  the  nervous  metabolism, 
which  is  the  basis  of  nervous  action,  must  be  at  least  as  largely  dependent 
on  these  influences  from  without,  as  on  the  mere  chemical  supply  furnished 
by  the  blood. 

We  have  developed  this  point  because  of  the  influence  it  must  have  on 
our  conceptions  of  the  physiological  processes  taking  place  in  the  cortex. 
If  we  accept  the  view  just  laid  down,  we  must  regard  the  superemiuent 
activity  of  the  cortex  and  the  characters  of  the  processes  taking  place  in  it 
as  due  not  so  much  to  the  intrinsic  chemical  nature  of  the  nervous  substance 
which  is  built  up  into  the  cortical  gray  matter  as  to  the  fact  that  impulses 
are  continually  streaming  into  it  from  all  parts  of  the  body,  that  almost  all 
influences  brought  to  bear  on  the  body  make  themselves  felt  by  it.  To  put 
the  matter  in  a  bald  way  we  may  ask  the  question,  What  would  happen  in 
the  cortex  if,  its  ordinary  nutritive  supply  remaining  as  before,  it  were  cut 
adrift  from  afferent  impulses  of  all  kinds?  We  can  hardly  doubt  but  that 
volitional  and  other  physical  processes  would  soon  come  to  a  standstill  and 
consciousness  vanish.  This  is,  indeed,  roughly  indicated  by  the  remarkable 
case  of  a  patient  whose  almost  only  communication  with  the  external  world 
was  by  means  of  one  eye,  he  being  blind  of  the  other  eye,  deaf  of  both  ears, 
and  suffering  from  general  anaesthesia.  Whenever  the  sound  eye  was  closed, 
he  went  to  sleep.  It  is  further  indirectly  illustrated  by  the  following  ex- 
perimental result.  We  have  seen  (§  567)  that  a  vertical  incision  carried 
through  the  depth  of  the  gray  matter  around  an  area  does  not  prevent 
stimulation  of  the  surface  of  the  area  producing  the  usual  movements. 


OTHER  ASPECTS  OF  THE   FUNCTIONS  OF  THE  BRAIN.        723 

But  after  such  an  incision  the  animal  suffers  a  paralysis  of  the  movements 
connected  with  the  area,  like  that  resulting  from  the  removal  of  the  gray 
matter  of  the  area ;  and  the  operation  is  said  to  be  followed  by  degenera- 
tive changes  in  the  area,  and  degeneration  of  the  pyramidal  fibres  starting 
from  it.  Some  of  this  effect  may  be  due  to  nutritive  changes  brought  about 
by  injury  to  the  pia  mater  and  division  of  bloodvessels ;  but  it  cannot  be 
wholly  accounted  for  in  this  way ;  it  appears  as  if  the  life  of  the  area  is 
curtailed  when  its  nervous  ties  are  broken. 

We  may  conclude  then  that  we  are  not  justified  in  speaking  of  con- 
sciousness of  volition,  or  other  psychical  processes,  even  admitting  that  these 
fail  when  the  cortex  is  removed,  as  being  functions  of  the  cortex  in  the  same 
way  that  we  speak  of  the  functions  of  other  organs ;  they  are  rather  func- 
tions of  the  connections  of  the  cortex  with  the  other  parts  of  the  central 
nervous  system. 

We  should  add  that  they  are  also  functions  of  the  connections  of  the 
several  parts  of  the  cortex  with  each  other.  All  our  knowledge  goes  to 
show  that  psychical  processes  are  dependent  on,  or  are  in  some  way  asso- 
ciated with  the  cortex ;  but  whatever  classification  of  psychical  functions 
we  adopt,  we  are  wholly  unable  to  make  out  any  localization  of  functions, 
such  as  we  can  make  out  for  movements,  visual  sensations  and  the  like. 
Even  taking  the  broad  and  elementary  division  into  "  the  emotions"  and 
"the  intellect,"  we  cannot  satisfactorily  allot  either  division  to  any  particu- 
lar part  of  the  hemisphere.  In  dogs,  removal  of  particular  parts  of  the 
hemispheres  has  indeed  been  observed  to  change  the  character  of  the  ani- 
mal, converting  for  instance  a  vicious,  morose  dog  into  a  mild  and  inoffen- 
sive one ;  and  removal  of  the  front  part  of  the  hemisphere  seems  to  have 
frequently  a  marked  effect  in  rendering  the  animal  more  impressionable  and 
excitable ;  he  becomes  much  more  demonstrative  and  "  gushing "  in  his 
behavior  than  before.  But  these  are  mere  hints,  and  the  clinical  histories 
of  disease  in  man  do  not  enable  us  to  say  much  more.  Such  knowledge  as 
we  do  possess  rather  tends  to  show  that  the  psychical  processes  in  proportion 
as  they  become  more  complex  involve  a  greater  number  of  nervous  factors, 
and,  therefore,  have  for  their  material  basis  a  greater  width  of  nervous  area, 
or  in  other  words  their  localization  becomes  less  definite.  Thus,  while  we 
may  localize  the  beginning  of  a  psychical  process,  a  visual  sensation  for 
instance,  and  one  of  its  terminal  acts,  such  as  the  issue  of  impulses  along 
the  pyramidal  tract,  we  cannot  put  our  finger  on  the  seat  of  the  interme- 
diate transactions.  These  even  in  the  simplest  process  must  be  complex, 
and  must  involve  many  factors.  Our  simplest  conceptions  of  the  external 
world  are  based  on  a  combination  of  visual  sensations  and  tactile  sensations. 
It  being  granted  that  the  visual  sensation,  in  one  phase  of  its  development, 
is  connected  with  certain  changes  in  some  spot  of  the  occipital  cortex,  there 
must  be  some  tie  between  this  and  the  corresponding  nervous  seat  of  the 
tactile  sensation  wherever  that  may  be,  and  further  ties  between  these  and 
other  parts  of  the  cortex.  Hence,  as  we  said,  the  psychical  process  is  a 
function  of  connections. 

Many  of  these  ties  are  most  probably  furnished  by  the  association  fibres 
passing  from  one  part  of  the  cortex  to  a  neighboring  part.  We  must  also 
probably  admit  that  impulses,  or,  to  use  a  more  general  word,  processes,  may 
travel  laterally  along  the  tangle  of  the  cortical  gray  matter,  for  this,  like 
the  gray  matter  of  the  spinal  cord,  seems  to  form  a  physiological  continuity, 
no  more  broken  by  the  fissures  than  is  the  cord  by  its  segmental  arrange- 
ment ;  and  we  know  nothing  as  to  the  limits  which  must  be  placed  on  the 
distance  to  which  such  processes  may  travel  from  their  focus  of  origin. 
Further,  seeing  how  completely  in  the  dark  we  are  as  to  the  reason  why  we 


724  THE   BRAIN. 

possess  two  hemispheres,  and  especially  seeing  that,  as  shown  by  speech,  the 
whole  of  each  hemisphere  is  not  identical  in  action  with  the  whole  of  the 
other,  we  may  perhaps  suppose  that  the  fibres  of  the  corpus  callosum,  which 
form  so  large  a  part  of  the  central  white  matter  of  the  hemisphere,  have 
other  duties  than  that  of  merely  keeping  the  points  of  one  hemisphere  in 
touch  with  the  corresponding  points  of  the  other  hemisphere.  But,  when 
we  have  made  every  allowance  for  all  these  direct  intercortical  connections, 
we  are  driven  to  the  conclusion  that  the  indirect  ties  between  one  part  of 
the  cortex  and  another  through  the  lower  parts  of  the  brain  are  of  no  less, 
perhaps  of  greater  importance.  This,  indeed,  is  shown  by  the  relations  of 
the  motor  region.  We  have  already  urged,  that  even  as  regards  the  mere 
carrying  out  of  a  skilled  movement  (and  we  may  add,  whether  that  be  vol- 
untary or  involuntary  in  the  ordinary  common  use  of  the  words)  the  motor 
region  must  have  other  ties  with  the  part  moved  than  merely  the  efferent 
tie  of  the  pyramidal  fibres ;  it  must  have  sensory  afferent  ties,  and  the 
course  of  these,  including  perhaps  even  those  which  belong  to  the  muscular 
sense,  we  may  regard  as  an  indirect  one  along  the  spinal  cord  and  middle 
parts  of  the  brain,  though  the  details  are  as  yet  unknown  to  us.  It  must, 
moreover,  as  we  have  also  seen,  have  ties,  at  least  in  many  cases,  with  parts 
other  than  the  part  moved,  for  instance  with  the  general  coordinating 
machinery.  And  the  ease  with  which  some  not  very  obvious  change  will 
permit  the  stimulation  of  a  limited  motor  area  to  start  epileptiform  convul- 
sions shows  how  many  and  close  are  the  ties  in  another  direction.  Further, 
when  we  go  beyond  the  final  phases  of  the  process  in  the  motor  cortex  to 
those  which  precede  the  issue  of  the  efferent  impulses,  we  find  the  ties  mul- 
tiplying. For  instance,  since  our  movements  are  so  largely  guided  by  visual 
sensations,  there  must  be  ties  between  the  motor  cortex  and  the  central 
visual  apparatus,  it  may  be  of  the  occipital  cortex,  but  it  may  also  be  of 
the  lower  visual  centres.  As  we  insisted,  the  motor  area  is  only  a  link  in  a 
complex  chain ;  and  what  we  can  see,  dimly  though  it  be,  in  reference  to 
the  cortical  motor  processes,  probably  holds  good  for  those  other  cortical 
processes  as  well,  of  whose  nervous  genesis  we  know  at  present  nothing. 
Hence  even  the  higher  psychical  events  cannot  truly  be  spoken  of  as  func- 
tions of  the  cortex,  meaning  that  they  are  simply  the  outcome  of  molecular 
changes  in  the  cortical  gray  matter  ;  they  are  rather  to  be  regarded  as  the 
outcome  of  complex  processes  in  which  the  parts  of  the  brain  below  the 
cortex  play  a  part  no  less  important  than  that  of  the  cortex  itself.  If  so, 
the  fibres  passing  down  from  the  cortex  to  the  middle  brain  have  functions 
by  which  they  take  part  even  in  our  psychical  life,  functions  for  which 
neither  the  words  motor  nor  sensory  are  fitting. 

ON  THE  TIME  TAKEN  UP  BY  CEREBRAL  OPERATIONS. 

§  603.  We  have  already  seen  (§  507)  that  a  considerable  time  is  taken 
up  in  a  purely  reflex  act,  such  as  that  of  winking,  though  this  is  perhaps 
the  most  rapid  form  of  reflex  movement.  When  the  movement  which  is 
executed  in  response  to  a  stimulus  involves  cerebral  operations  a  still  longer 
time  is  needed  ;  and  the  interval  between  the  application  of  the  stimulus 
and  the  commencement  of  the  muscular  contraction  varies  according  to  the 
nature  of  the  mental  labor  involved. 

The  simplest  case  is  that  in  which  a  person  makes  a  signal  immediately 
that  he  perceives  a  stimulus — ex.  gr.,  closes  or  opens  a  galvanic  circuit  the 
moment  that  he  feels  an  induction-shock  applied  to  the  skin,  or  sees  a  flash 
of  light,  or  hears  a  sound.  By  arrangements  similar  to  those  employed  in 
measuring  the  velocity  of  nervous  impulses,  the  moment  of  the  application 


ON   THE  TIME  TAKEN   UP  BY   CEREBRAL  OPERATIONS.       725 

of  the  stimulus  and  the  moment  of  the  making  of  the  signal  are  both  re- 
corded on  the  same  travelling  surface,  and  the  interval  between  them  is 
carefully  measured.  This  interval,  which  has  been  called  the  "  reaction 
period  "  or  "  reaction  time,"  may  be  divided  into  three  stages  :  1.  The  time 
during  which  afferent  impulses  are  generated  in  the  peripheral  sense  organs 
and  transmitted  along  the  afferent  nerves  to  the  central  nervous  system  ;  this 
may  be  called  the  "  afferent  stage."  2.  The  time  during  which,  through 
the  operations  of  the  central  nervous  system,  the  afferent  impulses  are  trans- 
formed into  efferent  impulses ;  this  may  be  called  the  "  central  stage." 
3.  The  time  taken  up  by  the  passage  of  the  efferent  impulses  along  the 
efferent  nerves  and  the  transformation  of  the  nervous  impulses  into  mus- 
cular contractions  ;  this  may  be  called  the  "  efferent  stage."  In  the  efferent 
stage  the  events  are  comparatively  simple,  and  though  not  absolutely  con- 
stant, do  not  vary  largely  ;  we  are  able  to  form  a  fairly  satisfactorily  esti- 
mate of  its  duration,  and  so  of  the  share  in  the  whole  reaction  period  which 
may  be  allotted  to  it.  The  events  of  the  afferent  stage  are  much  more  com- 
plex, and  the  estimates  of  its  duration,  being  arrived  at  in  an  indirect 
manner  and  chiefly  based  upon  calculations  of  the  whole  reaction  time,  are 
very  uncertain.  Hence  all  attempts  to  estimate  the  length  of  the  "  central  " 
stage,  the  "  reduced  reaction  period,"  as  it  is  sometimes  called,  by  subtract- 
ing the  efferent  and  afferent  stages,  must  be  subject  to  much  error.  But  a 
good  deal  may  be  learned  by  studying  the  variations  under  different  cir- 
cumstances of  the  reaction  period  as  a  whole. 

Taking  first  of  all  the  cases  in  which  the  events  of  the  central  stage  are 
simple,  such  as  those  where  the  subject  has  merely  to  make  a  signal  upon 
feeling  a  sensation,  we  find  that  the  length  of  the  reaction  period  is  dependent 
on  the  intensity  of  the  stimulus,  being  shorter  with  the  stronger  stimulus. 
But  variations  in  the  strength  of  the  stimulus,  especially  in  the  case  of 
minimal  stimuli,  have  a  much  more  striking  effect  in  determining  the  cer- 
tainty of  the  reaction  than  in  affecting  the  length  of  the  period.  Thus,  when 
the  signal  is  made  in  response  to  some  visual  sensation,  upon  seeing  an  elec- 
tric spark,  for  instance,  if  the  spark  be  a  very  weak  one  the  subject  of  the 
experiment  often  fails  to  make  a  signal  at  all,  though  he  may  rarely  fail  if 
the  spark  be  a  strong  one. 

Some  of  the  most  marked  variations  in  the  length  of  the  reaction  period 
are  determined  by  the  individuality  of  the  subject.  Thus,  with  the  same 
stimulus  applied  under  the  same  circumstances,  the  reaction  period  of  one 
person  will  be  found  very  different  from  that  of  another. 

The  length  of  the  reaction  period  varies  also  according  to  the  nature  and 
disposition  of  the  peripheral  organs  stimulated.  In  general,  it  may  be  said 
that  cutaneous  sensations  produced  by  the  stimulus  of  an  electric  shock 
applied  to  the  skin  (the  signal,  for  instance,  being  made  by  the  right  hand 
when  the  shock  is  felt  by  the  left  hand)  are  followed  by  a  shorter  reaction 
period  than  are  auditory  sensations,  while  the  period  of  these  is  in  turn 
shorter  than  that  of  visual  sensations  produced  by  luminous  objects ;  on  the 
other  hand,  the  shortest  period  of  all  is  said  to  be  that  of  visual  sensations 
produced  by  direct  electrical  stimulation  of  the  retina.  Roughly  speaking, 
we  may  say  that  the  reaction  period  is  for  cutaneous  sensations  one-seventh, 
for  hearing  one-sixth,  and  for  sight  one-fifth  of  a  second. 

Practice  materially  shortens  the  reaction  period  ;  indeed,  after  long  prac- 
tice, making  the  signal,  at  first  a  distinct  effort  of  the  will,  takes  on  the  cha- 
racters of  a  reflex  act,  with  a  correspondingly  shortened  interval.  Lastly, 
we  may  add  that  in  the  same  individual  and  with  the  same  stimulus,  the 
length  of  the  period  will  vary  according  to  circumstances,  such  as  the  time  of 
year,  the  weather,  and  the  like,  as  well  as  according  to  the  condition  of  the 


726  THE  BRAIN. 

individual,  whether  fresh  or  fatigued,  fasting  or  replete,  having  taken  more 
or  less  alcohol,  and  the  like. 

The  reaction  period  of  vision  has  long  been  known  to  astronomers.  It 
was  early  found  that  when  two  observers  were  watching  the  appearance  of 
the  same  star  a  considerable  discrepancy  existed  between  their  respective 
reaction  periods,  and  that  the  difference,  forming  the  basis  of  the  so-called 
"  personal  equation,"  varied  from  time  to  time,  according  to  the  personal 
condition  of  the  observers. 

§  604.  The  events  taking  place  in  the  central  stage  are,  of  course,  com- 
plex, and  this  stage  may  be  subdivided  into  several  stages.  Without  at- 
tempting to  enter  into  psychological  questions,  we  may  at  least  recognize 
certain  elementary  distinctions.  The  afferent  impulses  started  by  the  stimu- 
lus, whatever  be  their  nature,  when  they  reach  the  central  nervous  system 
undergo  changes,  and  as  we  have  seen,  probably  complex  changes,  before 
they  become  sensations  ;  and  further  changes,  now  of  a  more  distinctly 
psychical  character,  are  necessary  before  the  mind  can  duly  appreciate  the 
characters  of  these  sensations  and  act  accordingly.  Then  come  the  psychical 
processes  through  which  these  appreciated  sensations,  or  perceptions,  or 
apperceptions,  as  they  are  sometimes  called,  determine  an  act  of  volition. 
Lastly,  there  are  the  executive  processes  of  volition,  the  processes  which r 
psychical  to  begin  with,  end  in  the  issue  of  coordinate  motor  impulses,  or, 
in  other  words,  start  the  distinctly  physiological  processes  of  the  efferent 
stage.  We  may  thus  speak  of  the  time  required  for  the  perception  of  the 
stimulation,  of  the  time  required  for  the  action  of  the  will,  and  of  the  time 
required  for  the  complex  psychical  processes  which  link  these  two  together. 
Accepting  this  elementary  analysis,  it  is  obvious  that  the  total  length  of  the 
central  stage  may  be  varied  by  differences  in  the  length  of  each  of  these 
parts;  and  a  more  complete  analysis  would,  of  course,  open  the  way  for 
further  distinctions.  Hence,  by  studying  the  variations  of  the  whole  reac- 
tion time  under  varying  forms  of  psychical  activity,  we  may  form  an  esti- 
mate of  time  taken  up  by  various  psychical  processes. 

We  may  take  as  an  instance  the  case  in  which  the  subject  of  the  experi- 
ment has  to  exercise  discrimination.  The  mode  of  making  the  signal  being 
the  same,  and  the  stimulus  being  of  the  same  order  in  each  trial — that  is  to 
say,  visual,  or  cutaneous,  or  auditory,  etc. — and  general  circumstances 
remaining  the  same,  two  different  stimuli  are  employed,  and  the  subject  is 
required  to  make  a  signal  in  response  to  the  one  stimulus,  but  not  to  the 
other ;  the  subject  has  to  discriminate  between  the  psychical  effects  of  the 
two  stimuli.  Suppose,  for  example,  the  stimulus  is  the  sound  of  a  spoken  or 
sung  vowel,  and  the  subject  is  required  to  make  a  signal  when  a  is  spoken 
or  sung,  but  not  when  o  is  spoken  or  sung.  If  the  subject's  whole  reaction 
period  be  determined  (1)  in  the  usual  way,  with  either  a  or  o  spoken  (and 
the  result  will  be  found  not  to  differ  materially  whether  a  or  o  be  used),  the 
subject  knowing  that  only  a  or  only  o  will  be  spoken,  and  then  be  determined 
again  (2)  when  he  has  to  discriminate  in  order  that  he  may  make  the  signal 
when  a  is  spoken,  but  not  when  o  is  spoken,  he  not  knowing  which  is  about 
to  be  spoken,  the  whole  reaction  period  will  be  found  to  be  distinctly  longer 
in  the  second  case.  The  experiment  may  be  varied  by  making  use  of  all  the 
vowel  sounds  taken  irregularly  as  the  stimulus,  the  subject  responding  by  a 
signal  to  one  only,  as  arranged  beforehand.  And,  of  course,  other  orders 
of  stimulus  may  be  used,  either  visual,  the  signal  being  made  when  a  red 
light  is  shown  but  not  when  other  colors  are  shown,  or  tactile,  the  signal 
being  made  when  one  part  of  the  body  is  touched  but  not  when  other  parts 
are  touched,  and  the  like. 

In  such  experiments,  where  the  subject  has  to  distinguish,  to  discrimi- 


ON  THE  TIME  TAKEN  UP  BY  CEREBRAL  OPERATIONS.   7'27 

nate,  between  two  or  more  events,  the  prolongation  of  the  reaction  period  is 
obviously  due  to  the  longer  time  required  for  the  psychical  processes  taking 
place  during  what  we  have  called  the  central  stage.  In  the  two  cases,  one 
without  and  the  other  with  discrimination,  not  only  are  the  afferent  and 
efferent  stages  the  same  in  both,  but  we  have  no  reason  to  suppose  that  in 
the  central  stage  there  is  any  difference  between  the  two  cases  as  to  the  time 
taken  up  by  the  transformation  of  simple  sensory  impulses  into  perceptions, 
or  as  to  that  taken  up  by  the  will  in  gaining  access  to  the  motor  apparatus 
and  so  starting  the  processes  of  the  efferent  stage ;  the  delay  takes  place  in 
the  psychical  processes  intervening  between  these  two  parts,  and  the  amount 
of  delay  is  the  measure  of  the  time  needed  for  the  processes  involved  in  the 
discrimination.  This  "discrimination  period"  has  been  found  to  differ  in 
the  same  individual  according  to  the  sensation  employed,  visual,  auditory, 
etc.,  and  according  to  the  kind  of  difference  in  the  sensation  which  has  to 
be  discriminated,  for  instance  in  visual  sensations  between  colors  or  between 
objects  in  different  parts  of  the  field  of  vision.  In  a  series  of  observations 
made  in  this  way,  the  discrimination  period,  i.  e.,  the  prolongation  of  the 
simple  reaction  period  due  to  having  to  discriminate,  was  found  to  range 
from  0.011  second  to  0.062  second. 

Another  series  of  observations  may  be  made  in  the  following  way  :  The 
signal  being  one  made  with  the  hand,  the  simple  reaction  period  for  a 
stimulus  is  determined  with  the  signal  given  by  the  right  hand.  Two  kinds 
of  stimuli  are  then  employed,  both  of  the  same  order,  two  vowel  sounds  for 
instance,  and  the  subject  is  directed  to  respond  to  one  vowel  with  the  right 
hand  and  to  the  other  with  the  left  hand.  It  is  found,  the  subject  being 
right-handed,  that  the  reaction  period  is  greater  when  the  signal  is  made 
with  the  left  hand.  In  this  case  the  delay  takes  place  not  in  the  recognition 
of  the  effects  of  the  stimulus,  nor  in  the  processes  through  which  the  will  is 
formed  upon  that  recognition  ;  these  are  the  same  in  the  two  cases ;  it  takes 
place  in  the  processes  by  which  the  will  is  brought  to  bear  on  the  nervous 
motor  apparatus  for  making  the  signal,  on  the  cortical  origin,  for  example, 
of  the  pyramidal  tract ;  these  processes  take  a  longer  time  in  the  case  of  the 
unaccustomed  left  hand  than  in  the  case  of  the  usual  right  hand.  In  this 
way  we  obtain  a  measure,  so  to  speak,  of  the  volitional  side  of  psychical 
processes. 

In  a  somewhat  similar  way  we  may  obtain  a  measure  of  the  time  re- 
quired for  perception.  A  strong  sensation  following  too  closely  upon  a  weak 
one  will  prevent  the  psychical  recognition  of  the  weaker  one.  If,  for 
instance,  two  or  three  letters  in  white  on  a  black  background  be  presented 
to  the  eye,  and  a  large  white  surface  be  presented  afterward  at  an  interval 
which  is  made  successively  shorter  and  shorter,  it  is  found  that  when  the 
interval  is  made  very  brief  indeed  the  letters  cannot  be  perceived  at  all. 
In  proportion  as  the  interval  is  prolonged,  the  recognition  of  the  letters  in- 
creases, until  at  an  interval  of  about  0.05  second  they  are  fully  and  clearly 
recognized.  That  is  to  say,  the  time  required  for  perception  is  in  such  a 
case  of  about  that  length.  * 

The  duration  of  all  these  psychical  processes,  as  of  the  simple  reaction 
period  itself,  varies  of  course  under  different  circumstances,  and  the  dis- 
crimination period  may  be  conveniently  used  for  measurements  of  the  vary- 
ing effects  of  circumstances.  Practice  shortens  the  discrimination  period  as 
it  does  the  simple  reaction  period.  One  of  the  most  powerful  influences  is 
that  of  attention.  And  it  is  stated  that  the  shortening  of  the  period  is 
greater  when  the  attention  is  concentrated  on  the  making  of  the  signal  than 
when  it  is  more  especially  directed  to  recognition  of  the  stimulus;  in  other 
words,  the  volitional  processes  are  more  amenable  than  are  the  perceptive 


728  THE   BRAIN. 

processes  to  the  psychical  action  which  we  call  attention.  On  the  other 
hand,  the  period  is  distinctly  prolonged  if  the  observer  be  distracted  by  con- 
comitant sensations.  For  example,  the  period  for  discriminating  between 
two  visual  sensations  is  prolonged  if  powerful  auditory  sensations  be  excited 
at  the  same  time. 

The  same  method  of  measurement  may  be  used  in  other  ways  and  under 
other  circumstances  with  reference  to  psychical  processes.  It  must  be  re- 
membered, however,  that  all  such  observations  are  open  to  many  fallacies 
and  need  particular  caution.  It  not  unfrequently  happens  that  false  results 
are  obtained  ;  for  instance,  the  subject,  expecting  the  stimulus  to  be  brought 
to  bear  upon  him  and  straining  his  attention,  makes  the  signal  before  the 
stimulus  actually  comes  off.  And  the  interpretation  of  the  results  obtained 
are  in  many  cases  very  difficult ;  but  it  would  be  out  of  place  to  dwell  upon 
these  matters  any  further  here. 

THE  LYMPHATIC  ARRANGEMENTS  OF  THE  BRAIN  AND  SPINAL  CORD. 

§  605.  The  membranes  of  the  brain  and  spinal  cord.  The  cerebro-spinal 
canal  is  lined  by  a  tough  lamellated  membrane,  composed  of  connective 
tissue  with  a  small  amount  of  elastic  network,  called  the  dura  mater,  which, 
somewhat  closely  adherent  to  the  walls  of  the  cranial  cavity,  is  separated 
from  those  of  the  vertebral  canal  by  a  considerable  space,  containing  blood- 
vessels, especially  large  venous  sinuses,  and  some  fat.  It  may  be  considered 
as  a  development  of  the  periosteum  lining  the  cerebro-spinal  cavity.  It 
sends  tubular  sheaths  lor  some  distance  along  the  several  cranial  and  spinal 
nerves ;  and  forms  between  the  cerebral  hemispheres,  in  the  longitudinal 
fissure,  a  conspicuous  sickle-shaped  vertical  fold,  the  falx  cerebri,  as  well  as 
a  smaller  horizontal  or  oblique  fold  between  the  cerebellum  and  cerebrum 
known  as  the  tentorium. 

The  vascular  pia  mater  is  closely  attached  to  the  surface  of  the  brain  and 
spinal  cord,  dipping  down,  as  we  have  seen,  into  the  ventral  or  anterior  fissure 
of  the  cord  as  well  as  into  the  fissures  of  the  brain.  Sheath-like  investments 
of  pia  mater  are  continued  along  the  several  nerves  as  they  leave  the  cerebro- 
spinal  cavity ;  and  in  the  vertebral  canal  an  imperfect  partition  half-way 
between  the  dorsal  and  ventral  surfaces  of  the  cord  is  furnished  by  a  mem- 
brane of  connective  tissue  which,  continuous  along  its  whole  length  with  the 
pia  mater,  is  attached  to  and  fused  with  the  dura  mater  at  intervals  only, 
namely,  between  the  successive  nerve-roots.  Since  its  outer  edge  has  thus  a 
toothed  appearance,  this  membrane  is  called  the  ligamentum  denticulalum. 
Between  the  pia  mater  next  to  the  brain  and  cord  and  the  dura  mater  next 
to  the  bony  walls  is  a  cavity,  which  is  divided  into  two  by  a  thin  membrane, 
the  arachnoid,  composed  of  interwoven  bundles  of  connective  tissue.  The 
space  between  the  arachnoid  and  the  dura  rnater  is  called  the  subdural  space, 
and  the  space  between  the  arachnoid  and  the  pia  mater  is  called  the  sub- 
arachnoid  space.  When  the  brain  is  exposed  by  removing  the  roof  of  the 
skull  and  slitting  open  the  dura  mater,  the  subdural  space  is  laid  bare,  and 
the  arachnoid  is  seen  stretching  over  the  pia  mater ;  in  the  vertebral  canal 
the  arachnoid  lies  close  to  the  dura  mater,  so  that  usually,  when  the  dura 
mater  is  slit  open  and  turned  back,  the  arachnoid  is  carried  with  it  and  the 
cavity  exposed  is  that  of  the  subarachnoid  space.  The  arachnoid,  like  the 
dura  mater  and  the  pia  mater,  is  continued  for  some  distance  over  the  nerves 
as  they  leave  the  cerebro-spinal  cavity;  so  that  each  nerve  at  its  exit  is  sur- 
rounded by  a  tubular  prolongation  of  the  subdural  space,  and  within  this  a 
similar  tubular  prolongation  of  the  subarachnoid  space. 

The  subdural  space  is  broken  up  to  a  slight  extent  only  by  bridles  carry- 


THE  LYMPHATIC  ARRANGEMENTS  OF  THE  BRAIN.  729 

ing  nerves  and  bloodvessels,  especially  venous  sinuses,  between  the  pia  mater 
a  lid  dura  mater,  and  over  the  surface  of  the  brain  by  villus-like  projections 
of  the  arachnoid  called  Pacchionian  glands,  some  of  which  pierce  the  venous 
sinuses  of  the  dura  mater.  It  is  lined  throughout,  both  on  its  dural  and  on 
its  arachnoid  wall,  by  an  epithelium  of  flat  epithelioid  cells,  and  may  be 
compared  to  a  serous  cavity,  such  as  that  of  the  peritoneum.  Like  the  serous 
cavities  it  contains  normally  a  small  quantity  only  of  fluid,  and  its  size  is 
potential  rather  than  actual. 

The  subarachnoid  space  on  the  other  hand  is,  especially  in  certain  regions, 
such  as  the  dorsal  portions  of  the  vertebral  canal  and  the  base  of  the  brain, 
much  broken  up  by  bridles  of  connective  tissue  passing  from  it  to  the  pia 
mater,  as  well  as  by  a  network  of  sponge-like  arrangement  of  bundles  of 
connective  tissue  lying  immediately  beneath  itself,  and  giving  it,  when  viewed 
from  below,  a  honeycomb  or  fenestrated  appearance.  The  under  surface  of 
the  membrane  itself,  as  well  as  all  the  trabeculse  of  the  sponge-work  and  the 
bridles,  are  covered  with  an  epithelium  of  flat  epithelioid  cells,  which  is  con- 
tinued also  over  the  pia  mater  and  the  ligamentum  denticulatum,  and  lines 
the  tubular  sheath-like  prolongations  of  the  space  along  the  issuing  nerve- 
roots.  The  subarachnoid  space,  therefore,  like  the  subdural  space,  may  be 
regarded  as  a  serous  or  large  lymphatic  space,  but  it  is  an  actual  not  a  mere 
potential  space ;  it  always  contains  an  appreciable  quantity  of  fluid,  which, 
however,  is  not  ordinary  lymph,  but  is  furnished  in  a  particular  way  and 
deserves  special  study.  To  understand  the  nature  and  origin  of  this  cerebro- 
spinal  fluid,  as  it  is  called,  we  must  turn  to  some  special  arrangements  of  the 
pia  mater. 

§  606.  The  pia  mater  proper,  consisting  of  interwoven  bundles  of  con- 
nective tissue  with  some  elastic  fibres  and  a  considerable  number  of 
connective-tissue  corpuscles,  serves,  as  we  have  said,  as  the  bearer  of  blood- 
vessels to  the  nervous  structures  which  it  invests.  The  small  arteries,  as 
they  pass  into  the  nervous  substance  by  way  of  the  septa,  are  surrounded  by 
perivascular  lymphatic  canals,  with  which  spaces  in  the  neuroglial  ground- 
work, both  of  the  brain  and  spinal  cord,  especially  spaces  surrounding  the 
larger  nerve-cells,  are  continuous.  As  is  the  case  with  other  tissues,  so  with 
the  central  nervous  system,  the  several  elements  of  the  tissue  are  bathed 
with  lymph  derived  from  the  blood  ;  and  this,  oozing  through  the  spaces  into 
the  perivascular  canals  and  the  other  lymphatic  vessels  of  the  pia  mater, 
makes  its  way  into  the  subarachnoid  space  ;  but  the  fluid  in  the  subarachnoid 
space  has  other  sources  besides. 

The  roof  of  the  fourth  ventricle  is,  as  we  have  said  (§  514),  reduced  to  a 
single  layer  of  non-nervous  columnar  epithelium,  which  appears  as  a  mere 
lining  to  the  pia  rnater  overlying  it.  In  the  hinder  part  of  the  ventricle  this 
roof  is  perforated  by  a  distinct  narrow  oval  orifice,  the  foramen  of  Majendie. 
By  this  orifice,  which  passes  right  through  both  the  pia  mater  and  the 
underlying  layer  of  epithelium,  the  cavity  of  the  fourth  ventricle,  and  so  the 
whole  series  of  cavities  derived  from  the  original  medullary  canal,  the  lateral 
and  third  ventricles,  the  aqueduct,  and  the  central  canal  of  the  spinal  cord, 
are  made  continuous  with  the  subarachnoid  space.  There  are  also  other  less 
conspicuous  communications  between  the  subarachnoid  space  and  the  fourth 
ventricle.  Hence  the  cerebro-spinal  fluid  is  made  common  to  all  these  cavi- 
ties, and  is  furnished  not  only  by  the  pia  mater  investing  the  outside  of  the 
brain  and  spinal  cord,  but  also,  and  indeed  probably  to  a  larger  extent,  by 
the  epithelium  lining  the  several  cavities  of  the  cerebro-spinal  axis,  especially 
perhaps  by  those  portions  of  that  epithelium  which  coat  the  processes  of  pia 
mater  projecting  into  those  cavities  at  certain  places. 

We  saw  previously  (§  515)  that  a  large  fold  of  the  pia  mater,  carrying  in 


730  THE    BRAIN. 

with  it  the  thin  non-nervous  epithelium  which  alone  represents  at  the  place 
the  original  wall  of  the  medullary  canal,  is  thrust  inward  at  the  transverse 
fissure  of  the  brain,  beneath  the  fornix,  to  form  the  velum  interpositum,  thus 
supplying  a  roof  to  the  third  ventricle,  and  that  it  thence  projects  into  each 
lateral  ventricle  as  the  choroid  plexus  of  each  side,  reaching  from  the  fora- 
men of  Monro  in  front  along  the  edge  of  the  fornix  to  the  tip  of  the  descend- 
ing horn.  The  velum  being  a  fold  of  the  pia  mater  consists  theoretically  of 
two  layers,  and  between  the  upper  dorsal  layer  and  the  lower  ventral  layer 
lies  a  thin  bed  of  connective  tissue  carrying  arteries  forward  from  the  hind 
edge  of  the  corpus  callosum,  and  similarly  carrying  veins  backward  ;  these 
vessels  supply  the  choroid  plexus  with  an  abundant  supply  of  blood.  In  the 
choroid  plexus  the  folded  pia  mater  is  developed  into  a  number  of  villus- 
like  processes,  the  primary  processes  bearing  secondary  ones.  Each  process 
consists,  like  a  villus,  of  a  basis  of  connective  tissue,  in  which  the  bloodvessels 
end  in  close-set  capillary  loops,  covered  with  an  epithelium.  The  epithelium, 
though  continuous  with  the  rest  of  the  epithelium  lining  the  lateral  ventricle, 
and  thus,  as  we  have  said,  shutting  off  the  lateral  from  the  third  ventricle 
(except  at  the  foramen  of  Monro),  and  though  like  it  derived  from  the  wall 
of  the  original  medullary  canal,  is  different  in  structure.  Over  the  ventricle 
generally  the  epithelium  consists  of  ordinary  short  columnar,  apparently 
ciliated  cells,  with  more  or  less  transparent  cell  substance  ;  the  cells  over  the 
choroid  plexus  are  cubical,  often  irregular  in  form,  and  their  cell  substance 
is  loaded  with  granules,  some  of  which  are  pigmentary.  They  have  very 
much  the  appearance  of  "  active  "  secreting  cells ;  and  indeed  a  branched 
process  of  the  plexus  may  be  compared  to  an  everted  alveolus  of  a  secreting 
gland,  with  the  epithelium  outside  and  the  bloodvessels  within.  It  cannot 
be  doubted  that  these  cells  play  an  important  part  in  secreting  into  the  cavity 
of  the  ventricle  fluid  which,  passing  thence  by  the  foramen  of  Monro  into 
the  third  and  so  into  the  fourth  ventricle,  finds  its  way  by  the  foramen  of 
Majendie  into  the  subarachnoid  space. 

As  the  velum  overhangs  the  third  ventricle  it  sends  down  vertically  two 
longitudinal  linear  fringes,  which,  resembling  in  structure  the  choroid  plex- 
uses of  the  lateral  ventricle,  are  called  the  choroid  plexuses  of  the  third  ven- 
tricle. From  the  roof  of  the  fourth  ventricle  there  hangs  down  on  each  side 
a  similar  linear  fringe,  the  choroid  plexus  of  the  fourth  ventricle,  which  is 
especially  developed  at  its  front  end  beneath  the  overhanging  cerebellum. 
These  subsidiary  choroid  processes  doubtless  assist  in  furnishing  cerebro- 
spinal  fluid,  but  their  share  is  small  compared  with  that  of  the  main  choroid 
plexuses  of  the  lateral  ventricle. 

§  607.  The  cerebro-spinal  fluid.  The  specimens  of  cerebro-spinal  fluid 
which  have  been  examined  as  to  their  composition  are  not  quite  comparable 
with  each  other,  since  while  some  (such  as  those  obtained  from  cases  where 
a  fracture  of  the  base  of  the  skull  has  placed  the  subarachnoid  space  at  the 
base  of  the  brain,  where  it  is  largely  developed,  in  communication  with  the 
external  meatus,  and  the  fluid  escapes  by  the  ear)  may  be  regarded  as 
normal,  others  (such  as  those  obtained  from  case*  of  hydrocephalus  where 
the  ventricles  contain  an  unusual  quantity  of  fluid,  or  from  cases  of  spinal 
malformations)  must  be  considered  as  abnormal.  In  most  of  the  more  com- 
plete analyses,  the  fluid  examined  has  belonged  to  the  latter  class;  and  the 
following  statements  apply,  strictly  speaking,  to  them  alone. 

With  this  caution,  we  may  say  that  cerebro-spinal  fluid  is  a  transparent 
colorless  or  very  slightly  yellowish  fluid,  of  faint  alkaline  reaction,  free  from 
histological  elements.  The  specific  gravity  is  about  1010  or  less,  the  amount 
of  solids  being  on  an  average  1  per  cent.  Of  these  by  far  the  greater  part, 
0.8  or  0.9  per  cent.,  is  supplied  by  salts,  the  total  quantity  of  which  a?  well 


THE  LYMPHATIC  ARRANGEME^7TS  OF  THE  BRAIN.          731 

as  the  relative  amount  of  the  several  constituents  being  about  the  same  as 
obtain  in  blood  and  lymph.  The  comparative  deficiency  of  solids  is  due  to 
the  scantiness  of  the  proteids,  which  rarely  exceed  0.1  per  cent.  These  are 
chiefly  globulin  and  a  form  of  albumose,  or  even  peptone ;  albumin  is  said 
to  be  generally  absent.  The  fluid,  save  apparently  in  exceptional  cases,  does 
not  clot,  and  contains  neither  fibrogenous  factors  nor  fibrin  ferment.  It 
very  frequently  contains  a  substance  which  like  dextrose  reduces  Fehling's 
solution  but  which  is  not  sugar ;  it  appears  to  be  pyrocatechin  or  a  closely 
allied  body. 

Seeing  that  a  fluid  of  such  a  composition  is  of  a  different  nature  from 
ordinary  lymph,  furnished  entirely  in  the  ordinary  way,  we  might  be  inclined 
to  infer  that  probably  a  very  large  part  of  the  whole  mass  of  the  fluid  is 
furnished  by  the  secreting  epithelium  of  the  choroid  plexus.  But  it  must 
be  borne  in  mind,  that  the  foregoing  analyses  refer  chiefly  to  fluid  appearing 
under  abnormal  circumstances,  and  it  would  be  hazardous  to  draw  any  wide 
inference  from  them.  We  have  little  or  no  exact  experimental  evidence  as 
to  how  much  fluid  is  actually  secreted  by  the  choroid  plexuses ;  and  if  the 
fluids  which  have  been  analyzed  do  represent  a  mixture  of  ordinary  lymph 
supplied  through  the  pia  mater  with  the  peculiar  secretion  of  the  choroid 
plexus  and  cerebro-spinal  canal,  some  further  change  beyond  the  mere  min- 
gling of  the  two  fluids  is  needed  to  explain  the  remarkable  absence  of  albumin 
which  has  been  so  strongly  insisted  upon  by  various  authors. 

§  608.  We  may  fairly  suppose  that  during  life  the  fluid  is  continually 
being  supplied,  from  the  one  source  or  the  other ;  but  we  have  no  very  exact 
knowledge  as  to  the  rate  at  which  it  is  furnished.  In  the  dog,  the  fluid  has 
been  observed  to  escape  at  a  rate  varying  very  largely  under  different  cir- 
cumstances, and  ranging  from  1  c.c.  in  forty  minutes  to  as  much  as  1  c.c.  in 
six  minutes,  the  total  quantity  discharged  in  twenty-four  hours  varying  from 
36  c.c.  to  240  c.c.  In  the  cases  of  fracture  of  the  base  of  the  skull  men- 
tioned above,  a  very  considerable  flow  has  been  frequently  observed  ;  but  it 
may  be  doubted  whether  the  abnormal  circumstances  of  such  cases  have  not 
raised  the  secretion  above  normal.  The  rate  of  flow  was  found  in  the  dog 
to  be  much  increased  by  the  injection  of  substances  (normal  saline  solution) 
into  the  blood,  but  to  be  relatively  little  influenced  by  artificial  heightening 
of  arterial  pressure.  This  has  been  put  forward  as  indicating  that  the  fluid 
is  chiefly  furnished  as  a  secretion  and  not  as  an  ordinary  transudation  of 
lymph  ;  but  it  cannot  be  regarded  as  affording  a  valid  argument.  The  pres- 
sure under  which  the  fluid  exists  is  also  very  variable ;  it  is  closely  dependent 
on  the  vascular  arrangements  of  which  we  shall  have  to  speak  presently. 
In  the  dog  the  average  pressure  has  been  estimated  at  about  10  mm.  of 
mercury. 

If  the  fluid  is  thus  continually  formed  it  mnst  always  find  a  means  of 
escape.  This  is  probably  supplied  by  the  tubular  prolongations  of  the  sub- 
arachnoid  space  along  the  nerve-roots ;  these  are  continuous  with  the  lym- 
phatic vessels  of  the  nerves,  and  so  with  the  lymphatics  of  the  body  generally  ; 
and  in  the  skull,  the  passages  of  this  kind  along  the  cranial  nerves,  especially 
along  the  two  optic  nerves  into  the  orbits,  afford  a  ready  means  of  escape. 
It  is  also  urged  that  some  of  the  fluid  escapes  through  the  Pacchionian 
glands  directly  into  the  blood  of  the  venous  sinuses.  In  a  dead  body  fluid 
introduced  into  the  subarachnoid  space  through  an  opening  over  the  bulb, 
disappears  at  even  a  very  low  pressure  with  great  rapidity.  The  circum- 
stances then  are,  however,  not  the  same  as  in  life ;  and  the  few  experiments 
which  have  been  made  seem  to  show  that,  during  life,  a  somewhat  high 
pressure  is  required  to  secure  the  escape  of  fluid  introduced  in  addition  to 
that  naturally  secreted.  Thus  it  is  stated  that  when  in  a  dog  normal  saline 


732  THE   BRAIN. 

solution  is  introduced  into  the  subarachnoid  cavity  at  the  lower  end  of  the 
spinal  cord  very  little  resorption  takes  place  so  long  as  the  pressure  remains 
as  low  as  about  10  c.c.  of  mercury ;  as  the  pressure  is  increased  beyond  this 
resorption  quickly  increases.  But  it  may  be  doubted  whether  the  resorptiou 
of  added  fluid  is  a  fair  test  of  the  escape  of  fluid  naturally  present ;  and  tli£ 
experiment  is  of  value  rather  as  showing  simply  that  there  are  means  of 
escape  than  as  affording  a  measure  of  the  rate  of  escape.  Besides,  the 
immediate  effects  of  applying  pressure  at  the  caudal  end  of  the  spinal  cord 
are  not  the  same  as  those  of  applying  the  pressure  within  the  skull. 

The  rate  of  possible  escape  is  not  without  importance  as  regards  the 
mechanical  importance  of  the  cerebro-spinal  fluid.  Thus  it  has  been  urged 
that  when  an  extra  quantity  of  blood  is  driven  into  the  skull,  any  injurious 
intercranial  compression  is  prevented,  not  only  by  the  transference  of  a  cor- 
responding quantity  of  cerebro-spiual  fluid  through  the  foramen  of  Majendie 
from  the  cranium  into  the  spinal  canal,  the  walls  of  which  are  less  rigidly 
complete,  but  also  by  the  direct  escape  of  the  fluid  from  the  cavity  of  the 
skull  along  the  cranial  nerves  in  the  manner  described.  It  has  also  been 
urged  that  the  fluid  at  the  base  of  the  skull,  in  the  large  subarachuoid  spaces 
of  which  it  gathers  in  larger  quantity  than  elsewhere,  acts  as  a  sort  of  pro- 
tective water  cushion  to  the  delicate  cerebral  substance,  and  that,  in  general, 
the  presence  of  the  fluid  is  mechanically  useful  to  the  welfare  of  the  brain, 
removal  of  the  fluid  by  aspiration  being  said  to  lead  to  hemorrhage  from  the 
pia  mater  and  to  various  nervous  disorders.  But  our  knowledge  as  to  the 
part  which  the  fluid  plays  is  at  present  very  imperfect ;  and  its  very  peculiar 
chemical  characters  suggest  that  it  has  some  chemical  functions. 


THE  VASCULAR  ARRANGEMENTS  OF  THE  BRAIN  AND  SPINAL  CORD. 

§  609.  The  bloodvessels  reach  the  nervous  structures  by  means  of  the 
pia  mater.  In  the  spinal  cord  arteries  coming  from  the  vertebral,  inter- 
costal, and  other  arteries,  and  travelling  along  the  nerve-roots  join  the  pia 
mater,  and  then  through  the  fissures  and  septa  reach  all  parts  of  the  cord  ; 
but,  as  we  have  previously  remarked,  the  capillary  network  is  much  denser, 
and,  therefore,  the  blood-supply  much  greater  in  the  gray  than  in  the  white 
matter.  The  veins,  also  gathered  up  along  the  septa  and  fissures  into  the 
pia  mater,  those  coming  from  the  gray  matter  forming,  before  they  reach  the 
external  pia  mater,  a  conspicuous  longitudinal  vein  on  each  side  of  the  poste- 
rior gray  commissure,  pass  from  the  pia  mater  to  the  large  venous  sinuses 
of  the  dura  mater  and  so  to  adjoining  veins. 

In  the  brain  two  important  features  of  the  distribution  of  the  arteries 
deserve  special  attention.  In  the  first  place,  the  quadruple  supply  by  the 
right  and  left  vertebral  and  internal  carotid  arteries  is  made  one  by  remark- 
able anastomoses  forming  the  circle  of  Willis.  The  right  and  left  vertebral 
arteries  entering  the  vertebral  canal  at  the  level  of  the  sixth  cervical  ver- 
tebra and  running  forward  toward  the  brain,  join  beneath  the  ventral  surface 
of  the  bulb  to  form  the  single  median  basilar  artery.  This,  after  giving  off 
branches  to  the  bulb,  cerebellum,  and  pons,  divides  into  the  right  and  left 
posterior  cerebral  arteries.  Each  internal  carotid  entering  the  skull  reaches 
the  base  of  the  brain  in  the  region  of  the  floor  of  the  third  ventricle,  and, 
passing  ventral  to  and  athwart  the  optic  tract,  gives  off  the  large  and  impor- 
tant middle  cerebral  artery  along  the  fissure  of  Sylvius,  and,  then,  turning 
forward  and  toward  the  median  line,  passes  dorsal  to  the  optic  nerve  to  end 
in  the  anterior  cerebral  artery.  Just,  however,  as  it  gives  off  the  middle 
artery,  it  sends  backward,  inclining  to  the  middle  line,  a  relatively  large 


THE  VASCULAR  ARRANGEMENTS  OF  THE  BRAIN.  733 

branch,  the  posterior  communicating  artery,  which  joins  the  posterior  cere- 
bral near  the  origin  of  this  from  the  basilar  artery.  Moreover,  the  two 
anterior  cerebral  arteries,  soon  after  they  have  crossed  the  optic  nerves,  just 
as  they  are  about  to  run  straight  forward  along  the  frontal  lobes,  are  joined 
together  by  a  short,  wide  branch,  the  anterior  communicating  artery.  In 
this  way  the  vertebral  arteries  through  the  basilar  artery  join  with  the  carotid 
arteries  to  form  around  the  optic  chiasma  beneath  the  floor  of  the  third  ven- 
tricle an  arterial  circle,  the  circle  of  Willis. 

Blood  can  pass  along  this  circle  in  various  ways — from  the  basilar  artery 
along  the  right  posterior  communicating  artery  to  the  right  internal  carotid, 
and  so  by  the  right  anterior  cerebral  artery  and  anterior  communicating 
artery  to  the  left  side  of  the  circle,  and  similarly  from  the  basilar  artery 
along  the  left  side  to  the  right,  or  from  the  right  or  from  the  left  carotid 
through  the  circle,  to  the  right  hand  or  to  the  left  hand  in  each  case.  Since 
the  channel  of  the  circle  is  a  fairly  wide  one,  the  passage  in  various  direc- 
tions is  an  easy  one  ;  all  the  vessels  radiating  from  the  circle,  including  the 
basilar  artery  and  its  branches,  can  be  supplied  by  the  carotids  alone,  or  by 
the  vertebrals  alone,  or  even  by  one  carotid  or  one  vertebral  alone.  In  this 
way  an  ample  supply  of  blood  to  the  brain  is  secured  in  the  face  of  any 
hindrance  to  the  flow  of  blood  along  any  one  of  the  four  channels. 

In  what  may,  perhaps,  be  considered  the  usual  arrangement,  the  calibre 
of  the  posterior  communicating  arteries  is  rather  smaller  than  the  other  parts 
of  the  circle,  so  that,  other  things  being  equal,  most  of  the  vertebral  blood 
will  pass  by  the  posterior  cerebral  arteries,  while  the  carotid  blood  passes  to 
the  middle  and  anterior  cerebral  arteries  ;  but  many  variations  are  met  with. 
We  may  also  here,  perhaps,  call  to  mind  the  fact  that  the  left  carotid  com- 
ing off  from  the  top  of  the  aorta  offers  a  shorter  path  for  the  blood  than 
does  the  right  carotid  which  comes  off  from  the  innominate  artery. 

Another  special  feature  of  the  arterial  supply  to  the  brain  is  that  the 
three  large  cerebral  arteries — posterior,  middle,  and  anterior — are  distrib- 
uted almost  exclusively  to  the  cortex  and  to  the  subjacent  white  matter, 
while  the  deeper  parts  of  the  hemisphere,  the  nucleus  caudatus,  thalamus, 
and  the  like,  with  the  capsule  and  other  adjoining  white  matter,  are  supplied 
by  smaller  arteries  coming  direct  from  the  circle  of  Willis,  or  from  the  very 
beginnings  of  the  three  cerebral  arteries.  It  is  stated  that  these  two  sys- 
tems make  no  anastomoses  with  each  other ;  but  this  appears  to  vary  much 
in  different  individuals.  We  may  add  that  the  anterior  cerebral  artery 
supplies  the  cortex  of  the  dorsal  aspect  of  the  frontal  lobe  as  well  as  the 
front  and  middle  portions  of  the  whole  mesial  surface  of  the  hemisphere ; 
while  the  middle  cerebral,  always  large,  is  distributed  to  the  side  of  the 
brain,  that  is,  the  parietal  lobe,  with  the  ventral  part  of  the  frontal  lobe 
and  the  dorsal  part  of  the  temporal  lobe ;  the  posterior  cerebral  sup- 
plying the  rest  of  the  cortex,  that  is  to  say,  the  occipital  lobe,  including  the 
hind  part  of  the  mesial  surface  of  the  hemisphere,  together  with  the  ventral 
part  of  the  temporal  lobe.  The  distribution  of  these  arteries,  therefore,  does 
not  correspond  to  functional  divisions,  for  while  the  middle  cerebral  supplies 
a  large  part  of  the  motor  region,  it  does  not  supply  the  whole  of  it,  and  does 
supply  parts  outside  of  it.  Though  the  small  arteries  as  they  run  in  the  pia 
mater  on  the  surface  of  the  cortex  anastomose  freely,  there  is  very  little 
anastomosis  between  the  small  arteries  which,  leaving  the  pia  mater,  dip 
down  into  the  substance  of  the  brain  ;  hence,  when  these  latter  arteries  are 
blocked,  the  nutrition  of  the  part  of  the  cortex  supplied  by  them  is  apt  to 
be  impaired. 

§  610.  The  venous  arrangements  of  the  brain  have  very  special  cha- 
racters. 


734  THE  BRAIN. 

Along  the  upper  convex  border  of  the  sickle-shaped  fold  of  dura  mater, 
the  falx  cerebri,  is  developed  a  large  venous  sinus,  the  superior  longitudinal 
sinus.  This,  triangular  in  section,  increasing  in  calibre  from  before  back- 
ward, is  a  sinus,  not  a  vein  ;  its  walls  are  formed  of  nothing  but  connective 
tissue  lined  with  epithelium,  muscular  elements  being  entirely  absent. 
Though  its  channel  is  broken  by  bridles  of  connective  tissue  passing  across 
it,  it  possesses  no  valves,  and,  indeed,  these  are  absent  from  all  the  sinuses 
and  veins  of  the  brain.  Most  of  the  blood  returning  from  the  cortex  and 
subjacent  white  matter  is  carried  into  this  sinus  by  veins,  the  mouths  of 
which  are  for  the  most  part  directed  forward,  that  is  to  say,  against  the 
direction  of  the  blood  stream.  Along  the  lower  concave  border  of  the  falx 
is  a  similar  sinus,  the  inferior  longitudinal  sinus,  which,  however,  is  small, 
and  into  which  relatively  few  veins  open. 

From  the  deeper  parts  of  the  brain,  and  especially  from  the  choroid 
plexus,  blood  is  conveyed  by  the  veins  of  Galen  along  the  velum  interposi- 
tum  to  the  transverse  fissure,  where  the  veins  of  Galen  join  the  inferior  longi- 
tudinal sinus  to  form  the  straight  sinus.  This,  running  along  the  line  formed 
by  the  intersection  of  the  vertical  falx  with  the  (more  or  less)  horizontal 
tentorium,  joins  the  end  of  the  superior  longitudinal  sinus  to  form  the  reser- 
voir or  cellar,  called  the  torcular  Herophili,  from  which  the  lateral  sinus, 
passing  on  each  side  along  the  convex  border  of  the  tentorium  and  gather- 
ing veins  from  the  cerebellum  and  hind  regions,  as  well  as  from  the  base  of 
the  brain,  delivers  the  blood  into  the  internal  jugular  vein. 

It  should  be  added  that  veins  from  the  nose  and,  through  the  ophthalmic 
veins,  from  the  face  join  the  veins  and  sinuses  of  the  brain,  and  that  the  so- 
called  emissary  veins  pass  through  the  cranium  from  the  scalp  to  the  superior 
longitudinal  and  lateral  sinuses. 

The  channels  for  the  venous  blood  of  the  brain  are  therefore  not  veins, 
but  sinuses ;  not  so  much  tubes  for  maintaining  a  uniform  current,  as  longi- 
tudinal reservoirs,  which,  while  affording  an  easy  onward  path,  can  also  be 
easily  filled  and  easily  emptied,  and  in  which  the  blood  can  move  to  and  fro 
without  the  restrictions  of  valves.  This  arrangement  is  correlated  to  the 
peculiar  surroundings  of  the  brain,  which  is  not,  like  other  organs,  protected 
merely  by  skin  or  other  extensible  or  elastic  tissue,  but  is  encased  by  a  fairly 
complete  inextensible  envelope,  the  skull.  As  a  consequence  of  this,  when 
at  any  time  an  extra  quantity  of  blood  is  sent  from  the  heart  to  the  brain 
room  must  be  made  for  it  by  the  increased  exit  of  the  fluids  already  present. 
For  any  pressure  on  the  brain-substance  beyond  a  certain  limit  is  injurious 
to  its  welfare  and  activity,  as  is  seen  in  certain  maladies,  where  blood  pass- 
ing by  rupture  of  the  bloodvessels  out  of  its  normal  channels  remains  effused 
on  the  surface  of  the  brain  or  elsewhere,  and  thus  taking  up  the  room  of  the 
proper  brain-substance  leads,  by  "  compression,"  as  it  is  called,  to  paralysis, 
loss  of  consciousness,  or  death.  Some  room  may,  as  we  have  seen  (§  608),  be 
provided  by  the  escape  of  cerebro-spinal  fluid  from  the  skull.  But,  within 
the  limits  of  the  normal  cerebral  circulation,  the  characteristic  venous  sinuses 
especially  serve  to  regulate  the  internal  pressure  ;  they  form  temporary  reser- 
voirs from  which  a  comparatively  large  quantity  of  blood  can  be  rapidly  dis- 
charged from  the  cranium,  the  flow  from  the  sinuses  being  greatly  assisted 
by  the  low  or  negative  pressure  obtaining  in  the  veins  of  the  neck  at  each 
inspiratory  movement  of  the  chest. 

§  611.  The  supply  of  blood  to  the  brain  seems  at  first  sight  not  to  cor- 
respond to  the  importance  of  this  the  chief  organ  of  the  body.  In  the  rabbit 
it  would  appear  that  hardly  more  than  one  per  cent,  of  the  total  quantity  of 
the  blood  of  the  body  is  present  at  any  one  time  in  the  brain,  a  quantity  but 
little  more  than  half  that  which  is  found  in  the  kidneys ;  and  while  the 


THE  VASCULAK  ARRANGEMENTS  OF  THE  BRAIN.  735 

weight  of  bloocUin  the  brain  at  anyone  time  amounts  to  about  five  per  cent, 
of  the  total  weight  of  the  organ,  being  about  the  same  as  in  the  muscles,  in 
the  kidney  it  amounts  to  nearly  twelve  per  cent.,  and  in  the  liver  to  as  much 
as  nearly  thirty  per  cent.  Making  every  allowance  for  the  relative  small 
size  and  functional  importance  of  the  rabbit's  brain,  the  blood-supply  of 
even  the  human  brain  must  still  be  small ;  and  making  every  allowance  for 
rapidity  of  current,  the  interchange  between  the  blood  and  the  nervous  ele- 
ments must  also  be  small.  In  other  words,  the  metabolism  of  the  brain-sub- 
stance is  of  importance  not  so  much  on  account  of  its  quantity  as  of  its  special 
qualities. 

The  circulation  in  the  brain  may  be  studied  by  help  of  various  methods. 
A  manometer  may  be  connected  with  the  peripheral  end  of  the  divided  inter- 
nal carotid  artery,  a  second  manometer  being  attached  in  the  usual  way  to 
the  central  portion.  Since  the  peripheral  manometer  records  the  blood-pres- 
sure in  the  circle  of  Willis  transmitted  along  the  peripheral  portion  of  the 
carotid  artery,  variations  of  pressure  in  the  circle  of  Willis  may  thus  be 
studied ;  and  a  comparison  of  the  peripheral  with  the  central  manometer 
will  indicate  what  general  changes  are  taking  place  in  the  circulation 
through  the  brain.  Thus  a  fall  of  pressure  in  the  peripheral  manometer 
unaccompanied  by  any  corresponding  fall  in  the  central  manometer  would 
show  that  the  "  peripheral  resistance  "  in  the  brain  was  being  lowered,  in 
other  words,  that  the  vessels  were  being  dilated. 

In  another  method,  in  the  dog,  the  outflow  of  venous  blood  from  the 
lateral  sinus  through  the  posterior  facial  vein  has  been  measured.  The 
freedom  with  which  blood  passes  along  the  sinuses  justifies  the  assumption 
that  the  outflow  through  the  open  vein  gives  an  approximate  measure  of  the 
rate  of  flow  under  natural  conditions  ;  still  the  results  are  only  approximate, 
and  besides,  the  continued  loss  of  blood  introduces  error. 

A  third  method  is  a  plethysmographic  one.  The  skull  is  made. to  serve 
as  the  box  of  the  plethysmograph  or  oncometer  (§  346) ;  a  small  piece  of 
the  roof  having  been  removed  by  the  trephine,  a  membrane  is  fitted  to  the 
hole,  and  the  movements  of  the  membrane  are  recorded  by  help  of  a  piston 
and  lever  or  directly  by  a  lever.  In  young  subjects,  the  fontanelle,  or  por- 
tion of  the  cranium  not  yet  ossified,  may  be  utilized  as  a  natural  membrane, 
and  its  movements  recorded  in  a  similar  manner.  When  the  instrument  is 
fitted  to  the  hole  in  a  water-tight  manner,  this  method  records  variations  in 
internal  pressure;  and  we  may  take  it  for  granted,  unless  otherwise  indi- 
cated, that  greater  or  less  pressure  is  due  to  more  or  less  blood  passing  to 
the  brain.  But  the  amount  of  pressure  brought  to  bear  on  the  recording 
instrument  will  also  depend  on  the  readiness  with  which  the  cerebro-spinal 
fluid  escapes  from  the  cavity  of  the  skull ;  if  there  be  a  hindrance  to  the 
escape,  or,  on  the  other  hand,  an  increased  facility  of  escape,  the  same  in- 
crease of  supply  of  blood  will  produce  in  one  case  a  less,  in  the  other  a 
greater  movement  of  the  lever.  If  the  membrane  be  attached  loosely  to 
the  hole  so  as  to  allow  free  escape  of  the  cerebro-spinal  fluid,  the  lever 
practically  resting  on  the  surface  of  the  cerebral  hemisphere,  the  method 
records  variations  in  the  dorso-ventral  diameter  of  the  hemisphere,  and 
these  may  be  taken  as  measuring  variations  in  the  volume  of  the  brain  and 
so  in  the  blood-supply.  In  neither  form,  however,  does  the  method  by  itself 
give  us  all  the  information  which  we  want.  An  increase  of  blood  in  the 
brain,  and  therefore  an  expansion  of  the  brain,  and  so  a  movement  of  the 
recording  instrument,  may  result  either  from  a  fuller  arterial  supply  or 
from  hindrance  to  the  venous  outflow ;  the  former  condition  is,  at  least  in 
most  cases,  favorable  to,  the  latter  always  and  distinctly  injurious  to,  the 
activity  of  the  nervous  structures ;  hence  the  teachings  of  the  lever  must 


736  THE  BRAIN. 

be  corrected  by  a  simultaneous  observation  of  the  general  arterial  pressure 
and  of  the  blood-pressure  in  the  veins  of  the  neck.  Moreover,  the  argument 
which  we  used  (§  353)  in  reference  to  the  kidney  may  be  applied  here  and 
probably  with  equal  force,  namely,  that  the  value  of  the  blood  stream  for 
the  nutrition  of  the  tissue  is  dependent  not  alone  on  the  amount  of  blood- 
pressure  but  also  and  especially  on  the  rapidity  of  the  flow;  indeed,  this 
second  factor  is  of  particular  importance  in  view  of  the  need  of  supplying 
the  nervous  elements  with  an  adequate  interchange  of  gases.  Now  of  the 
rapidity  of  flow  the  plethysmographic  method  can  give  us  indirect  informa- 
tion only. 

§  612.  By  one  or  other  or  all  of  these  methods  certain  important  facts 
have  been  made  out.  The  volume  of  the  brain  as  determined  by  the  amount 
of  blood  present  in  it,  is  continuously  undergoing  changes  brought  about  by 
various  causes.  Each  heart-beat  makes  itself  visible  on  the  cerebral  as  on 
the  renal  plethysmographic  tracing,  and  as  we  have  seen  in  speaking  of  res- 
piration, the  diminution  of  pressure  in  the  great  veins  of  the  neck  during  in- 
spiration leads  to  a  shrinking,  and  the  reverse  change  during  expiration  to 
a  swelling  of  the  brain.  The  plethysrnograph  also  shows  variations,  larger 
and  slower  than  the  respiratory  undulations,  and  brought  about  by  various 
causes,  such  as  the  position  of  the  head  in  relation  to  the  trunk,  movements 
of  the  limbs,  modifications  of  the  respiratory  movements,  and  apparently 
phases  of  activity  of  the  brain  itself,  as  in  waking  and  sleeping  ;  undulations 
corresponding  to  the  Traube-Hering  variations  (§  330)  of  blood-pressure  may 
not  unfrequently  be  observed. 

All  the  various  methods  show  that  the  flow  through  the  brain  is  largely 
determined  by  a  vasomotor  action  of  some  kind  or  another.  And  this  we 
might  indeed  infer  from  ordinary  experience.  When  the  head  is  suddenly 
shifted  from  the  erect  to  a  hanging  position,  there  must  be  a  tendency  for 
the  blood  to  accumulate  in  the  cranial  cavity,  and  conversely  when  the  head 
is  suddenly  shifted  from  a  hanging  to  an  erect  position,  there  must  be  a 
tendency  for  the  supply  of  blood  within  the  cranium  to  be  for  a  while  less 
than  normal.  Either  change  of  position,  and  especially  perhaps  the  latter, 
would  lead  to  cerebral  disturbances,  which  in  turn  would  in  ourselves  be 
revealed  by  affections  of  our  consciousness.  That  a  perfectly  healthy  and 
especially  young  organism,  whose  vasomotor  mechanisms  are  at  once  effective 
and  delicately  responsive,  can  pass  swiftly  from  one  position  of  the  head  to 
the  other  without  inconvenience,  whereas  those  in  whom  the  vasomotor 
mechanisms  have  by  age  or  otherwise  become  imperfect  are  giddy  when 
they  attempt  such  rapid  changes,  is  in  itself  adequate  evidence  of  the  im- 
portance of  the  vasomotor  arrangements  affecting  the  circulation  through 
the  brain.  The  several  methods  agree  in  showing  that  increased  general 
arterial  pressure,  such  as  that,  for  instance,  induced  by  stimulation  of  a 
sensory  nerve,  leads  to  a  greater  flow  of  blood  to  the  brain ;  the  volume  of 
the  brain  is  increased  arid  the  venous  outflow  by  the  lateral  sinus  is  quick- 
ened. Conversely,  a  lowering  of  arterial  pressure  leads  to  a  lessened  flow 
of  blood  to  the  brain. 

Seeing  that  the  cerebral  arteries  have  well-developed  muscular  coats,  the 
basilar  artery  in  fact  being  conspicuous  in  this  respect,  one  would  be  led  to 
suppose  that  the  brain  possessed  special  vasomotor  nerves  of  its  own ;  and 
recognizing  the  importance  of  blood-supply  to  rapid  functional  activity  one 
would  perhaps  anticipate  that  by  special  vasomotor  action,  the  supply  of 
blood  to  this  or  that  particular  part  of  the  brain  might  be  regulated  apart 
from  changes  in  the  general  supply.  The  various  observations,  however, 
which  have  hitherto  been  made  have  failed  to  demonstrate  with  certainty 
any  such  special  vasomotor  nerves  or  fibres  directly  governing  cerebral 


THE  VASCULAR  ARRANGEMENTS  OF  THE  BRAIN.  737 

vessels.  It  would  be  hazardous  to  insist  too  much  on  this  negative  result, 
especially  since  the  observations  have  been  chiefly  directed  to  the  nerves  of 
the  neck,  the  experimental  difficulties  of  investigating  the  presence  of  vaso- 
motor  fibres  in  the  cranial  nerves  being  very  great.  Still  it  may  be  urged 
and  indeed  has  been  urged  that  the  flow  of  blood  through  the  brain  is  so 
delicately  responsive  to  the  working  of  the  general  vasomotor  mechanism 
just  because  it  has  no  vasomotor  nerves  of  its  own.  In  such  an  organ  as 
the  kidney,  an  increase  of  general  blood-pressure,  as  we  have  more  than 
once  insisted,  may  or  may  not  lead  to  a  greater  flow  through  the  kidney 
according  as  the  vessels  of  the  kidney  itself,  through  the  action  of  the  renal 
vasomotor  nerves,  are  dilated  or  constricted ;  and  as  we  have  seen,  a  con- 
striction of  the  renal  vessels  may  be  one  of  the  contributors  to  the  increased 
general  pressure.  In  the  brain,  on  the  other  hand,  an  increase  of  general 
arterial  pressure  seems  always  to  lead  to  increase  of  flow.  Thus  in  the 
Traube-Hering  undulations  just  mentioned,  the  expansions  of  the  brain 
are  coincident  with  the  rises  of  the  general  pressure,  whereas  in  the  normal 
kidney  and  in  other  organs  the  local  Traube-Hering  undulation  reverses  the 
general  one,  the  shrinkings  are  synchronous  with  the  rises  of  pressure,  the 
local  constriction  being  one  of  the  factors  of  the  general  rise.  It  is  argued 
that  in  the  absence  of  vasomotor  nerves  of  their  own,  the  cerebral  vessels 
are  wholly,  so  to  speak,  in  the  hands  of  the  general  vasomotor  system,  so 
that  when  the  blood-pressure  is  high  owing  to  a  large  vaso-constriction  in 
the  abdominal  viscera,  more  blood  must  necessarily  pass  to  the  brain,  and 
when  again  the  blood-pressure  falls  through  the  opening  of  the  splanchnic 
flood-gates  (§  159)  less  blood  necessarily  flows  along  the  cerebral  vessels. 
And  indeed  one  may  recognize  here  a  sort  of  self-regulating  action  ;  for 
diminishing  the  supply  of  blood  to  the  vasomotor  centre  in  the  bulb  acts,  as 
we  know,  as  a  powerful  stimulus  in  producing  vaso-constriction,  and  so  leads 
to  a  rise  of  blood-pressure ;  but  this  very  rise  of  blood-pressure  drives  more 
blood  to  the  brain,  including  the  bulb,  and  thus  the  injurious  effects  to  the 
brain  threatened  by  an  anaemic  condition  are  warded  off  by  the  very  be- 
ginning of  the  anaemia  itself.  All  these  advantages  are,  however,  quite 
compatible  with  the  coexistence  of  special  vasomotor  mechanisms. 

§  613.  Moreover,  the  flow  of  blood  to,  and  consequent  change  in  the  bulk 
of,  the  brain,  and  indeed  the  flow  of  blood  through  the  brain,  as  measured 
by  the  venous  outflow,  may  be  modified  independently  of  changes  in  the 
general  blood-pressure.  For  instance,  stimulation  of  the  motor  region  of  the 
cortex  quickens  the  venous  outflow,  without  producing  any  marked  change 
in  the  general  blood-pressure  ;  this  feature  becomes  very  striking  at  the  onset 
of  epileptiform  convulsions  when  these  make  their  appearance.  It  is  diffi- 
cult not  to  connect  such  a  result  of  functional  activity  with  some  special 
vasomotor  nervous  arrangement  comparable  to  that  so  obvious  in  the  case 
of  a  secreting  gland.  Again,  it  has  been  observed  that  certain  drugs  have 
an  effect  on  the  volume  of  the  brain,  quite  incommensurate  with  their  effect 
on  the  vasomotor  system ;  thus  in  particular  the  injection  into  the  general 
blood-stream  of  a  weak  acid  produces  a  large  and  immediate  expansion  of 
the  brain,  while  the  introduction  of  a  weak  alkali  similarly  gives  rise  to 
similar  considerable  shrinking.  It  is  suggested  that  these  effects  are  pro- 
duced by  the  acid  or  alkali  acting  directly  on  the  muscular  coats  of  the 
minute  arteries  and  so  leading  to  relaxation  or  contraction  respectively.  In 
treating  of  the  chemistry  of  nervous  substance  (§  70)  we  stated  that  "the 
gray  matter  of  the  central  nervous  system  is  said  to  be  slightly  acid  during 
life  and  to  become  more  acid  after  death."  Recent  observations  go  to  show 
that  the  gray  matter  of  the  cortex  is  faintly  alkaline  during  life  and  under 
normal  conditions,  but  becomes  acid  after  death  or  when  its  blood-supply  is 

47 


738  SIGHT. 

interfered  with ;  and  it  has  been  urged  that  nervous  gray  matter  like  mus- 
cular substance  develops  acidity  during  activity  as  well  as  upon  death,  the 
acidity  being  probably  due  in  each  case  to  some  form  of  lactic  acid.  And 
just  as  it  has  been  suggested  that  the  dilatation  of  the  minute  arteries  of  a 
skeletal  muscle,  accompanying  or  following  the  contraction  of  the  muscle, 
is  brought  about  by  the  acid  generated  during  the  contraction  causing  a 
relaxation  of  the  muscular  coats  of  the  minute  arteries,  so  it  has  been  sug- 
gested that  a  similar  acidity,  the  product  of  nervous  activity,  similarly  leads 
in  nervous  tissue  to  a  dilatation  of  the  vessels  of  the  part.  The  existence  of 
special  vasomotor  mechanisms  would,  however,  afford  a  more  satisfactory 
explanation  of  these  and  other  phenomena ;  in  spite  of  the  negative  results 
so  far  obtained,  the  matter  is  obviously  one  needing  further  investigation. 
Meanwhile  we  have  abundant  evidence  that,  however  brought  about,  the 
flow  of  blood  through  the  brain,  and  probably  through  particular  parts  of 
the  brain,  is  varied  in  accordance  with  the  needs  of  the  brain  itself  and  the 
events  taking  place  elsewhere  in  the  body. 


CHAPTER    III. 

SIGHT. 

§  614.  A  RAY  of  light  falling  on  the  retina  gives  rise  to  what  we  call  a 
sensation  of  light ;  but  in  order  that  distinct  vision  of  any  object  may  be 
gained,  an  image  of  the  object  must  be  formed  on  the  retina,  and  the  better 
defined  the  image  the  more  distinct  will  be  the  vision.  Hence,  in  studying 
the  physiology  of  vision,  our  first  duty  is  to  examine  into  the  arrangements 
by  which  the  formation  of  a  satisfactory  image  on  the  retina  is  eifected ; 
these  we  may  call  briefly  the  dioptric  mechanisms.  We  shall  then  have  to 
inquire  into  the  laws  according  to  which  rays  of  light  impinging  on  the 
retina  give  rise  to  sensory  impulses,  and  those  according  to  which  the  im- 
pulses thus  generated  giVe  rise  in  turn  to  sensations.  Sere  we  shall  come 
upon  the  difficulty  of  distinguishing  between  the  unconscious  or  physical 
and  the  conscious  or  psychical  factors.  And  we  shall  find  our  difficulties 
increased  by  the  fact,  that  in  appealing  to  our  own  consciousness  we  are  apt 
to  fall  into  error  by  confounding  primary  and  direct  sensations  with  states 
of  consciousness  which  are  produced  by  the  weaving  of  these  primary  sen- 
sations with  other  operations  of  the  central  nervous  system,  or,  in  familiar 
language,  by  confounding  what  we  see  with  what  we  think  we  see.  These 
two  things  we  will  briefly  distinguish  as  visual  sensations  and  visual  judg- 
ments ;  and  we  shall  find  that  both  in  vision  with  one  eye,  but  more 
especially  in  binocular  vision,  visual  judgments  form  a  very  large  part  of 
what  we  frequently  speak  of  as  our  sight. 

§  615.  [The  eyeball  is  of  as  pheroidal  shape.  It  consists  of  two  seg- 
ments of  different-sized  spheres.  The  larger  segment  is  situated  posteriorly, 
and  constitutes  about  five-sixths  of  the  walls  of  the  eyeball.  From  its  free 
margin  projects  the  smaller  segment,  which  is  that  of  a  smaller  sphere.  The 
posterior  segment  is  composed  of  a  whitish,  opaque,  firm  wall,  consisting  of 
three  coats  or  tunics — the  sclerotic,  the  choroid,  and  retina.  The  anterior 
segment  is  continuous  with  the  sclerotic  coat.  (Fig.  157.)  It  is  a  trans- 
parent, elastic,  convex  organ,  called  the  cornea.  The  cornea  consists  of  three 
layers — an  anterior  and  posterior  elastic  lamina,  having  between  them  a 


SIGHT. 


739 


layer  which  is  the  proper  tissue  of  the  organ.  This  middle  layer  is  com- 
posed of  about  sixty  superimposed  laminae  of  fusiform  fibrous  cells.  In  the 
interstices  between  the  laminae  are  found  tubular  spaces,  which  contain  a 
transparent  fluid.  The  anterior  and  posterior  elastic  laminae  are  structure- 
less and  highly  elastic.  When  separated  from  the  proper  corneal  tissue  they 
have  a  great  tendency  to  curl  up,  suggesting  that  they  are  active  agents  in 
the  retention  of  a  proper  curvature  of  the  cornea.  The  cornea  is  covered 
on  its  anterior  surface  by  the  conjunctival  mucous  membrane,  which  consists 
of  three  or  four  layers  of  pavement  epithelial  cells ;  the  deeper  layers  of 
cells  are  oblong,  and  placed  perpendicularly.  The  conjunctiva  at  this  point 
has  no  perceptible  basement  membrane.  The  posterior  surface  of  the  cornea 
is  covered  by  a  transparent  serous  membrane,  which  consists  of  a  simple 
layer  of  polygonal  pavement  epithelial  cells  resting  on  an  elastic  mem- 

FIG.  157. 


Diagram  of  a  Horizontal  Section  of  the  Eyeball  a,  outer  or  sclerotic  coat ;  d,  the  cornea  ;  6, 
middle  or  choroidal  coat ;  m,  ciliary  ligament ;  s,  ciliary  process  ;  e,  ciliary  muscle,  and/,  iris  ;  c, 
inner  coat  of  retina,  continuous  with  the  optic  nerve  behind,  with  a  dark  layer  outside  it ;  g,  lens ; 
t,  suspensory  ligament  of  the  lens  ;  h,  vitreous  body  •  n,  hyaloid  membrane  ;  i,  posterior  chamber ; 
o,  canal  of  Petit ;  r,  sinus  circularis  iridis ;  I,  optic  nerve.  The  dotted  line  through  the  centre  is 
the  longitudinal  axis  of  the  ball. 


cornea 


has 


no 


brane.     This  is  called  the  membrane  of  Demours.     The 
bloodvessels,  and  therefore  derives  its  nutriment  by  diffusion. 

The  sclerotic  coat  is  so  named  on  account  of  the  firmness  of  its  texture  and 
of  its  hardness.  It  forms  the  outer  tunic  of  the  posterior  segment.  It  is 
whitish,  opaque,  smooth,  excepting  at  the  points  of  attachment  of  the  muscles 
of  the  eyeball.  It  is  composed  of  white  fibrous  tissue,  arranged  more  or  less 
in  bundles,  which  interlace  each  other  in  various  directions.  Anteriorly  the 
interlacements  are  in  a  generally  transverse  direction  ;  posteriorly  the  direc- 
tion is  longitudinal.  This  coat  also  contains  yellow  elastic  fibres  and 
fusiform  nucleated  cells.  It  is  continuous  anteriorly  with  the  cornea,  and 
posteriorly  with  the  perineurium  of  the  optic  nerve.  At  the  internal  border 
of  the  junction  with  the  cornea  is  a  venous  sinus  called  the  sinus  circularis 
iridis  or  canal  of  Schlemm.  The  optic  nerve  pierces  it  about  2.6  mm.  internal 
to  the  antero-posterior  axis  of  the  eyeball.  At  this  point  the  coat  is  perfo- 


740 


SIGHT. 


FIG.  158. 


rated  by  minute  openings  for  the  passage  of  the  nerve-filaments.  One  of 
these  openings,  which  is  relatively  large,  gives  passage  to  the  arteria  centralis 
retinae.  Surrounding  this  point  of  entrance  of  the  optic  nerve  are  many 
small  openings  for  the  passage  of  the  ciliary  nerves  and  vessels.  The  in- 
ternal surface  of  the  sclera  contains  some 
pigment  granules.  It  is  separated  from 
the  choroid  coat  by  a  delicate  flocculent 
cellular  tissue,  called  the  lamina  fusca. 

The  choroid  coat  is  a  vascular  membrane 
containing  some  pigment  granules.  The 
external  portion  is  composed  principally 
of  bloodvessels  and  nerves.  Between  the 
vessels  are  found  numerous  stellate  pig- 
ment-cells, which  form  a  fibrous  network. 
The  internal  surface,  where  it  adjoins  the 
pigment  layer  of  the  retina,  also  contains 
pigment-cells.  Posteriorly  it  is  pierced 
by  the  optic  nerve ;  anteriorily  it  is  con- 
tinuous with  the  ciliary  processes,  and  is 
separated  from  the  sclerotic  coat  by  the 
ciliary  muscle. 

The  ciliary  processes  are  arranged  in  the 
form  of  a  ring.  They  consist  of  about  sixty 
to  eighty  somewhat  conical-shaped  bodies, 
situated  with  their  bases  internally.  (Fig. 
158.)  They  are  placed  posterior  to  the 
iris,  and  are  attached  by  their  thickened 
or  internal  extremities  to  the  suspensory  ligament  of  the  lens. 

The  ciliary  muscle  arises  from  the  point  of  junction  of  the  sclerotic  coat 
and  the  cornea.  It  consists  of  two  portions — a  radiating  or  meridional  and 
a  circular  layer.  The  radiating  fasciculi  are  situated  externally  and  have  a 
meridional  direction.  (Fig.  159.)  From  this  layer  numerous  fasciculi  inter- 


inner  View  of  the  Front  of  the 
Choroid  Coat  with  its  Ciliary  Processes, 
and  the  Back  of  the  Iris,  a,  anterior 
piece  of  the  choroid  coat ;  b,  ciliary  pro- 
cesses ;  c,  iris  ;  d,  sphincter  of  the  pupil ; 
e,  bundles  of  fibres  of  the  dilator  of  the 
pupil. 


FIG.  159. 


Section  of  the  Ciliary  Region  of  the  Eye  in  Man.  a,  meridional  muscular  fasciculi  of  the 
musculus  ciliaris  ;  b,  deeper-seated  radiating  fasciculi ;  c,  c.  c,  annular  plexus  ;  d,  annular  muscle 
of  Muller :  /,  muscular  lamina  on  the  posterior  surface  of  the  iris ;  g,  muscular  plexus  at  the 
ciliary  border  of  the  iris ;  e,  annular  tendon  of  the  musculus  ciliaris;  h,  ligamentum  pectinatum. 

lace  between  the  fasciculi  of  the  circular  layer,  which  occupies  an  internal 
position  to  the  radiating  layer.  The  ciliary  muscle  is  inserted  into  the 
external  surface  of  the  anterior  portion  of  the  choroid  coat,  the  fibres  extend- 
ing somewhat  posterior  to  the  anterior  margin  of  the  retina.  This  muscle  is 
a  very  important  factor  in  the  mechanism  of  accommodation. 


THE  EYEBALL. 


741 


The  iris  is  a  fibre-muscular  curtain  which  is  suspended  between  the 
cornea  and  the  crystalline  lens.  It  is  attached  by  its  circumference  to  the 
internal  wall  of  the  sinus  c.  iridis.  In  its  centre  is  a  round  perforation  called 
the  pupil,  which  is  susceptible  of  considerable  variations  in  size.  This  mem- 
brane is  composed  of  a  fibre-connective  tissue  having  a  general  radiating 
direction  from  the  pupillary  border.  Within  this  tissue  are  found  pigment- 
cells  and  unstriated  muscular  tissue.  The  muscular  tissue  element  consists 
of  radiating  and  circular  fasciculi.  (Fig.  160.)  The  circular  fasciculi  form 

FIG.  160. 


Muscular  Structure  of  the  Iris  of  a  White  Rabbit,    a,  sphincter  of  the  pupil;  b,  b,  radiating 
fasciculi  of  dilator  muscle;  c,  c,  connecting  tissue  with  its  corpuscles. 

a  sphincter  at  the  pupillary  margin  ;  the  radiating  fasciculi  radiate  from 
the  sphincter  to  the  circumference.  At  the  circumference  of  the  iris  the 
membrane  lining  the  anterior  chamber  forms  fibrous  processes,  which  are 
termed  the  ligamentum  iridis  pectinatwn.  The  posterior  surface  is  covered 
with  a  pigmentary  layer,  which  is  a  continuation  of  the  pigment  layer  of 
the  retina. 

The  retina  or  third  coat  consists  of  two  portions :  the  pigmentary  mem- 
brane and  terminal  elements  of  the  optic  nerve.  The  pigmentary  membrane 
or  external  layer,  which  has  been  called  the  system  of  the  uvea,  covers  the 
whole  of  the  internal  surface  of  the  ciliary  processes,  the  iris,  and  the 
choroid.  It  consists  of  a  single  layer  of  hexagonal  nucleated  pigment-cells 
of  a  dark-brown  color.  From  the  internal  surface  of  this  membrane  delicate 
fibres  are  continued  between  the  cellular  elements  of  the  nervous  layer.  It 
is  frequently  dissected  with  the  choroid  coat,  and  spoken  of  as  one  of  its 
laminae.  The  color  of  the  iris  in  different  individuals  is  dependent  upon  the 
density  of  the  fibro-connective  tissue  anterior  to  the  uvea,  and  to  the  amount 
of  pigment-granules  in  it.  In  persons  with  dark  eyes  the  pigment  in  this 
tissue  is  relatively  more  abundant. 

The  internal  or  nervous  layer  of  the  retina  is  composed  essentially  of  the 
terminal  nerve  elements  of  the  optic  nervre.  Externally  it  is  covered  with 
the  pigmentary  layer ;  internally  it  is  lined  by  a  homogeneous  transparent 
structure  called  the  hyaloid  membrane.  The  structure  of  the  retina  is  one 
of  great  complexity.  It  consists  of  nine  distinct  layers,  seven  of  which  are 
layers  of  nerve  elements.  All  of  these  layers  are  bound  together  and  sup- 
ported by  a  connective  tissue  which  contains  bloodvessels.  This  layer  ex- 


742 


SIGHT. 


tends  from  the  entrance  of  the  optic  nerve  to  a  point  where  the  annular 
fasciculi  of  the  ciliary  muscle  are  found  ;  at  this  point  the  nervous  elements 
cease  to  exist,  and  the  layer  has  an  irregular  dentated  margin  called  the 
ora  serrata.  Beyond  this  the  nervous  layer  is  continued  as  a  mere  fibrous 
extenuation. 

The  optic  nerve  pierces  the  sclerotic  and  the  choroid  coats,  and  the  pig- 
mentary membrane  of  the  retina,  when  it  rapidly  divides  into  vast  numbers 
of  fibres,  which  consist  alone  of  the  axis-cylinders  or  their  ultimate  fibrillse. 


FIG.  161. 


FIG.  162. 


FIG.  163. 


FIG.  161.— Diagrammatic  Representation  of  the  Connections  of  the  Nerve-fibres  in  the  Retina. 
1,  membrana  limitans  interna ;  2,  optic  nerve-fibre  layer  ;  3,  layer  of  ganglion  cells ;  4,  internal 
granulated  or  molecular  layer ;  5,  internal  granule  layer ;  6,  external  granulated  or  molecular 
layer;  7,  external  granule  layer;  8,  membrana  limitans  exterior;  9,  bacillary  layer,  or  layer  of 
rods  and  cones. 

FIG.  162.— Rod  and  Cone  from  the  Retina  of  Man,  preserved  in  a  two  per  cent,  solution  of 
perosmic  acid  to  show  the  fine  fibres  of  the  surface,  and  the  different  lengths  of  the  internal  seg- 
ment. The  outer  segment  of  the  cone  is  broken  up  into  disks,  which,  however,  are  still  adherent 
to  one  another  ;  at  the  base  of  the  cone  are  seen  a  few  fine  hairs.  (  +  1000  diameters.) 

FIG.  163.— Diagrammatic  Representation  of  the  Connective  Tissue  and  Radial  Fibres  of  Miiller 
of  the  Retina  as  seen  near  the  Ora  Serrata.  The  numbers  correspond  to  those  of  the  several 
layers  of  the  retina  shown  in  Fig.  161. 

This  layer  of  fibres  is  continuous  over  nearly  the  whole  of  the  internal  sur- 
face, and  is  called  the  second  or  optic  nerve-fibre  layer.  On  its  internal  sur- 
face, between  it  and  the  hyaloid  membrane,  is  a  delicate  structure  called  the 
first  layer,  or  membrana  limitans  interna.  The  third  or  ganglion  layer  is  com- 
posed of  multipolar  ganglion  cells,  similar  to  those  found  in  the  cerebral 
substance.  In  the  posterior  portion  of  the  retina  these  ganglion  cells  are  in 
several  layers ;  at  the  macula  lutea  there  are  as  many  as  eight,  and  at  the 
anterior  portion  of  the  retina  there  is  but  a  single  layer.  From  each  of  these 


THE  EYEBALL. 


743 


FIG.  164. 


cells  fibres  are  continued  to  the  fifth  or  internal  granule  layer,  which  consists 
of  granular  cells  with  nuclei.  Between  the  third  and  fifth  layers  is  a  layer 
of  vesicular  matter  containing  nerve-fibrils  of  extreme  minuteness.  This 
layer  is  the  fourth  or  internal  granulated  or  molecular  layer.  The  sixth  or 
external  granulated  or  molecular  layer  consists  of  parallel  interlaced  fibres, 
containing  nuclei  and  smooth  cells.  The  seventh  or  external  granule  layer  is 
very  similar  to  the  fifth.  The  eighth  layer  consists  of  a  delicate  membrane 
of  connective  tissue,  called  the  membrana  limitans  externa.  The  ninth  or 
bacillary  layer,  or  layer  of  rods  and  cones,  or  Jacob's  membrane,  is  composed 
of  two  elements,  the  rods  and  cones.  These  layers  are  supported  by  con- 
nective tissue  and  a  peculiar  neuroglial  structure  known  as  the  radial  fibres 
of  Muller.  (Fig.  163.)  The  rods  are  cylindrical  bodies,  each  ending  exter- 
nally in  a  truncated,  flattened  extremity,  and  internally  as  an  attenuated 
fibre,  which  probably  communicates  with  the  deeper  layer  of  ganglion  cells. 
The  cones,  as  their  name  indicates,  are  conical-shaped  bodies.  Each  consists 
of  two  portions,  a  conical  body  having  projecting  from  its  apex  a  rod-like 
segment,  which  appears  in  all  respects  like  the  rods.  This  segment  is  called 
the  cone  rod.  The  terminal  extremities  of  the  cone  rods  do  not  extend  as 
far  externally  as  the  extremities  of  the  rods.  The  rods  and  cones  have 
been  demonstrated  to  consist  of  two  segments  or  limbs,  which  are  com- 
posed of  filaments,  granular  matter,  and  nuclei.  The  outer  limb  of  the 
rods  contains  a  pinkish  pigment  known  as  "  visual  purple/'  which  is 
extremely  sensitive  to  light. 

The  optic  nerve,  where  it  pierces  the  coats  of  the  eye,  projects  somewhat 
beyond  the  surface  of  the  retina,  as  a  papilla;  here  the  essential  nerve  ele- 
ments of  the  retina  are  absent,  and  luminous 
rays  are  unperceived  ;  hence,  it  is  called  the 
blind  spot.  About  2.6  mm.  external  to  the 
point  of  entrance  of  the  optic  nerve,  and  in 
the  exact  centre  of  the  retinal  surface  corre- 
sponding to  the  antero-posterior  axis  of  the  eye, 
is  the  "  yellow  spot  of  Sommerring,"  or  macula 
lutea.  (Fig.  164.)  It  is  an  elliptical-shaped  spot, 
having  its  long  diameter  transverse.  In  the 
centre  of  the  macula  lutea  is  a  depression  called 
the  fovea  centralis.  At  this  point  the  nervous 
layer  of  the  retina  is  very  much  modified  in 
the  composition  of  the  different  layers.  The 
nervous  layer  is  much  thicker  than  at  any  other 
part  of  the  membrane.  The  ganglion  (third) 
and  external  granulated  (sixth)  layers  are  even 
more  thickened.  The  ganglion  layer  consists  of 
six  or  eight  laminae  of  cells.  The  rods  of  the 
ninth  layer  are  absent,  and  are  replaced  by 
cones.  In  the  fovea  centralis  the  internal  gran- 
ulated (fourth),  the  internal  granule  (fifth),  and  the  optic  nerve-fibre 
(second)  layers  are  wanting.  The  ganglion  cell  (third),  the  external  gran- 
ulated (sixth),  and  the  external  granule  (seventh)  layers  are  increased  in 
thickness.  The  ganglion  layer  of  cells  in  the  fovea  consists  of  three 
laminae.  In  all  portions  of  the  nervous  layer  the  rods  greatly  predomi- 
nate in  number  over  the  cones,  excepting  in  the  macula  lutea,  where  they 
are  entirely  absent.  The  retina  is  much  thicker  posteriorly,  becoming  thin- 
ner as  it  extends  forward,  the  nervous  layer  gradually  disappearing  in  the 
anterior  portion  of  the  membrane. 

The  interior  of  the  eyeball  is  divided  into  two  portions  by  the  crystalline 


Objects  on  the  Inner  Surface 
of  the  Retina.  In  the  centre  of 
the  ball  is  the  yellow  limbus 
luteus,  here  represented  by  shad- 
ing, and  in  its  middle  the  dark 
spot.  To  the  inner  side  is  the 
nerve,  with  its  accompanying 
artery.  (After  Sommerring.) 


744  SIGHT. 

lens  and  its  suspensory  ligament.  The  anterior  portion  contains  the  aqueous 
humor,  the  posterior  contains  the  vitreous  body. 

The  crystalline  lens  measures  about  7  mm.  in  transverse  diameter,  and 
about  4  mm.  antero-posterior  diameter.  It  is  a  transparent  biconvex  body, 
somewhat  flattened  anteriorly.  It  consists  of  a  number  of  segments  which 
radiate  from  the  centre,  similar  to  the  segments  of  an  orange.  These  seg- 
ments are  composed  of  superimposed  laminae  of  varying  density.  The  most 
superficial  are  soft  and  gelatinous ;  the  deeper  are  relatively  hard,  so  that 
they  form  a  kernel  or  nucleus.  The  laminae  are  made  up  of  parallel  fibres, 
with  an  undulating  course,  the  convexities  and  concavities  of  the  adjoining 
fibres  fitting  accurately  into  each  other.  The  lens  is  covered  with  a  capsule 
consisting  of  a  transparent,  elastic,  fragile  membrane,  which  has  a  tendency 
to  curl  up,  with  its  external  surface  innermost. 

The  suspensory  ligament  of  the  lens  is  formed  by  a  continuation  of  the 
hyaloid  membrane  which  lines  the  vitreous  body.  The  hyaloid  membrane  is 
a  delicate  transparent  structure  situated  between  the  vitreous  body  and  mem- 
brana  limitans  interna  of  the  retina.  It  is  continued  in  front  of  the  ora 
serrata,  where  it  divides  into  two  layers.  The  posterior  is  attached  to  the 
posterior  portion  of  the  capsule  of  the  lens ;  the  anterior  portion  gradually 
becomes  thicker  as  it  extends  forward  behind  the  ciliary  processes,  and  is 
attached  to  the  anterior  surface  of  the  capsule.  This  thickened  portion  of 
the  membrane,  which  is  corrugated  where  it  has  attached  the  ciliary  pro- 
cesses, is  called  the  zone  of  Zinn.  These  two  layers  constitute  the  suspensory 
ligament.  Between  them  is  a  triangular  canal,  with  its  base  corresponding 
to  the  crystalline  lens.  This  is  called  the  canal  of  Petit. 

The  vitreous  body  is  contained  within  the  cavity  formed  by  the  hyaloid 
membrane  and  the  posterior  surface  of  the  lens.  It  consists  of  a  clear,  color- 
less, albuminous  fluid,  having  an  extremely  delicate  interlacement  of  fibres 
extending  in  all  directions  through  it.  These  fibres  are  not  discernible  in 
the  adult,  but  can  readily  be  seen  in  the  foetus. 

The  aqueous  humor  is  contained  within  the  space  formed  by  the  posterior 
surface  of  the  cornea  and  the  anterior  surface  of  the  lens.  The  space,  which 
is  divided  into  two  chambers  by  the  iris,  is  filled  with  a  clear,  colorless, 
limpid  fluid  containing  saline  and  proteid  substances  in  solution.  This  fluid 
constitutes  the  aqueous  humor. 

The  anterior  external  portion  of  the  eyeball,  comprising  the  surface  of  the 
cornea  and  about  6  or  8  mm.  of  the  sclerotic  coat,  is  covered  by  the  conjunc- 
tival  mucous  membrane.] 

DIOPTRIC  MECHANISMS. 

The  Formation  of  the  Image. 

§  616.  The  eye  is  a  camera,  consisting  of  a  series  of  surfaces  and  media 
arranged  in  a  dark  chamber,  the  iris  serving  as  a  diaphragm  ;  and  the  object 
of  the  apparatus  is  to  form  on  the  retina  a  distinct  image  of  external  objects. 
That  a  distinct  image  is  formed  on  the  retina  may  be  ascertained  by  removing 
the  sclerotic  from  the  back  of  an  eye,  and  looking  at  the  hinder  surface  of 
the  transparent  retina  while  rays  of  light  proceeding  from  any  external  object 
are  allowed  to  fall  on  the  cornea. 

§  617.  A  dioptric  apparatus  in  its  simplest  form  consists  of  two  media 
separated  by  a  (spherical)  surface  ;  and  the  optical  properties  of  such  an  ap- 
paratus depend  upon  (1)  the  curvature  of  the  surface,  (2)  the  relative  refrac- 
tive power  of  the  media.  The  eye  consists  of  several  media,  bounded  by 
surfaces  which  are  approximately'spherical,  but  of  different  curvature.  The 


DIOPTRIC  MECHANISMS,  745 

surfaces  are  all  centred  on  a  line  called  the  optic  axis,  which  meets  the  retina 
at  a  point  somewhat  above  and  to  the  inner  (nasal)  side  of  the  fovea  cen- 
tral is.  In  passing  from  the  outer  surface  of  the  cornea  to  the  retina  the 
rays  of  light  traverse  in  succession  the  cornea,  the  aqueous  humor,  the  lens, 
and  the  vitreous  humor.  Refraction  takes  place  at  all  the  surfaces  bound- 
ing these  several  media,  but  particularly  at  the  anterior  surface  of  the 
cornea,  and  at  both  the  anterior  and  posterior  surfaces  of  the  lens.  Since 
the  anterior  and  posterior  surfaces  of  the  cornea  are  parallel,  or  very  nearly 
so,  the  rays  of  light  would  suffer  little  or  no  change  of  direction  in  passing 
through  the  cornea,  if  it  were  bounded  on  both  sides  by  the  same  medium. 
The  direction  of  the  rays  of  light  in  the  aqueous  humor  would,  therefore, 
remain  the  same  if  the' cornea  were  made  exceedingly  thin  ;  if,  in  fact,  its 
two  surfaces  were  made  into  one,  forming  a  single  anterior  surface  to  the 
aqueous  humor;  or,  which  comes  to  the  same  thing  in  the  end,  since  the 
refractive  power  of  the  substance  of  the  cornea  is  almost  exactly  the  same 
as  that  of  the  aqueous  humor,  the  refraction  at  the  posterior  surface  of  the 
cornea  may  be  neglected  altogether.  Thus  the  two  surfaces  of  the  cornea 
are  practically  reduced  to  one.  The  lens  varies  in  density  in  different  parts, 
the  refractive  power  of  the  central  portions  being  greater  than  that  of  the 
external  layers ;  but  the  refractive  power  of  the  whole  may,  without  any 
serious  error,  be  assumed  to  be  uniform.  The  refractive  power  of  the  vitreous 
humor  is  almost  exactly  the  same  as  that  of  the  aqueous  humor. 

§  618.  Thus  the  apparently  complicated  natural  eye  may  be  simplified 
into  a  "  diagrammatic  eye,"  in  which  the  refracting  surfaces  are  reduced  to 
three,  viz. :  (1)  the  anterior  surface  of  the  cornea,  (2)  the  anterior  surface  of 
the  lens  separating  the  lens  from  the  aqueous  humor,  and  (3)  the  posterior 
surface  of  the  lens  separating  the  lens  from  the  vitreous  humor.  The  media 
will  similarly  be  reduced  to  two :  the  substance  of  the  lens  and  the  aqueous 
or  vitreous  humor.  This  "  diagrammatic  eye  "  is  of  great  use  in  the  various 
calculations  which  become  necessary  in  studying  physiological  optics ;  for 
the  magnitudes  which  are  derived  by  calculation  from  it  represent  the  cor- 
responding magnitudes  in  an  average  natural  eye  with  sufficient  accuracy  to 
serve  for  all  practical  purposes.  The  values  adopted  by  Listing  for  'the 
constants  of  this  "  diagrammatic  eye,"  and  to  him  we  are  indebted  for  the 
introduction  of  it,  are  as  follows : 

Radius  of  curvature  of  cornea 8  mm. 

of  anterior  surface  of  lens ]0    ' 

of  posterior  6     " 

Refractive  index  of  aqueous  or  vitreous  humor \fif 

Mean  refractive  index  of  lens |£ 

Distance  from  anterior  surface  of  cornea  to  anterior  surface  of  lens  4  mm. 

Thickness  of  lens 4    " 

The  calculated  position  of  the  principal  posterior  focus,  i.  e.,  the  point  at 
which  all  rays  falling  on  the  cornea  parallel  to  the  optic  axis  are  brought  to 
a  focus,  is  in  the  diagrammatic  eye  14.6470  mm.  behind  the  posterior  surface 
of  the  lens,  or  22.6470  mm.  behind  the  anterior  surface  of  the  cornea. 
That  is  to  say,  the  fovea  centralis  must  occupy  this  position  in  order  that  a 
distinct  image  of  a  distant  object  may  be  formed  upon  it.  It  must  be  under- 
stood that  these  values  refer  to  the  eye  when  at  rest,  i.  e.,  when  it  is  not 
undergoing  any  strain  of  accommodation. 

Accommodation. 

§  619.  When  an  object,  a  lens,  and  a  screen  to  receive  the  image  are  so 
arranged  in  reference  to  each  other  that  the  image  falls  upon  the  screen  in 


746  SIGHT. 

exact  focus,  the  rays  of  light  proceeding  from  each  luminous  point  of  the 
object  are  brought  into  focus  on  the  screen  in  a  point  of  the  image  corre- 
sponding to  the  point  of  the  object.  If  the  object  be  then  removed  further 
away  from  the  lens,  the  rays  proceeding  in  a  pencil  from  each  luminous  point 
will  be  brought  to  a  focus  at  a  point  in  front  of  the  screen,  and,  subsequently 
diverging,  will  fall  upon  the  screen  as  a  circular  patch  composed  of  a  series 
of  circles,  the  so-called  diffusion  circles,  arranged  concentrically  round  the 
principal  ray  of  the  pencil.  If  the  object  be  removed,  not  further  from,  but 
nearer  to  the  lens,  the  pencil  of  rays  will  meet  the  screen  before  they  have 
been  brought  to  focus  in  a  point,  and  consequently  will  in  this  case  also 
give  rise  to  diffusion  circles.  When  an  object  is  placed  before  the  eye,  so 
that  the  image  falls  into  exact  focus  on  the  retina,  and  the  pencils  of  rays 
proceeding  from  each  luminous  point  of  the  object  are  brought  into  focus 
in  points  on  the  retina,  the  sensation  called  forth  is  that  of  a  distinct  image. 
When,  on  the  contrary,  the  object  is  too  far  away,  so  that  the  focus  lies  in 
front  of  the  retina,  or  too  near,  so  that  the  focus  lies  behind  the  retina,  and 
the  pencils  fall  on  the  retina  not  as  points,  but  as  systems  of  diffusion  circles, 
the  sensation  produced  is  that  of  an  indistinct  and  blurred  image.  In  order 
that  objects  both  near  and  distant  may  be  seen  with  equal  distinctness  by 
the  same  dioptric  apparatus,  the  focal  arrangements  of  the  apparatus  must 
be  accommodated  to  the  distance  of  the  object,  either  by  changing  the  refrac- 
tive power  of  the  lens  or  by  altering  the  distance  between  the  lens  and  the 
screen. 

§  620.  That  the  eye  does  possess  such  a  power  of  accommodation  is  shown 
by  every-day  experience.  If  two  needles  be  fixed  upright  some  two  feet  or 
so  apart  into  a  long  piece  of  wood,  and  the  wood  be  held  before  the  eye  so 
that  the  needles  are  nearly  in  a  line,  it  will  be  found  that  if  attention  be 
directed  to  the  far  needle,  the  near  one  appears  blurred  and  indistinct,  and 
that  conversely,  when  the  near  one  is  distinct,  the  far  one  appears  blurred. 
By  an  effort  of  the  will  we  can  at  pleasure  make  either  the  far  one  or  the 
near  one  distinct ;  but  not  both  at  the  same  time.  When  the  eye  is  arranged 
so  that  the  far  needle  appears  distinct,  the  image  of  that  needle  falls  exactly 
on  the  retina,  and  each  pencil  from  each  luminous  point  of  the  needle  unites 
in  a  point  upon  the  retina ;  but  when  this  is  the  case  the  focus  of  the  near 
needle  lies  behind  the  retina,  and  each  pencil  from  each  luminous  point  of 
this  needle  falls  upon  the  retina  in  a  series  of  diffusion  circles.  Similarly, 
when  the  eye  is  arranged  so  that  the  near  needle  is  distinct,  the  image  of 
that  needle  falls  upon  the  retina  in  such  a  way,  that  each  pencil  of  rays 
from  each  luminous  point  of  the  needle  unites  in  a  point  on  the  retina,  while 
each  pencil  from  each  luminous  point  of  the  far  needle  unites  at  a  point  in 
front  of  the  retina,  and  then  diverging  again  falls  on  the  retina  in  a  series 
of  diffusion  circles.  If  the  near  needle  be  gradually  brought  nearer  and 
nearer  to  the  eye,  it  will  be  found  that  greater  and  greater  effort  is  required 
to  see  it  distinctly,  and  at  last  a  point  is  reached  at  which  no  effort  can  make 
the  image  of  the  needle  appear  anything  but  blurred.  The  distance  of  this 
point  from  the  eye  marks  the  limit  of  accommodation  for  near  objects. 
Similarly,  if  the  person  be  short-sighted,  the  far  needle  may  be  moved 
away  from  the  eye,  until  a  point  is  reached  at  which  it  ceases  to  be  seen 
distinctly,  and  appears  blurred.  In  the  one  case  the  eye,  with  all  its  power, 
is  unable  to  bring  the  image  of  the  needle  sufficiently  forward  to  fall  on  the 
retina ;  the  focus  lies  permanently  behind  the  retina.  In  the  other  the  eye 
cannot  bring  the  image  sufficiently  backward  to  fall  on  the  retina ;  the  focus 
lies  permanently  in  front  of  the  retina.  In  both  cases  the  pencils  of  rays 
from  the  needles  strike  the  retina  in  diffusion  circles. 

§621.  The   same  phenomena  may  be   shown   with  greater  nicety  by 


DIOPTRIC  MECHANISMS. 


747 


what  is  called  Schemer's  Experiment.  If  two  smooth  holes  be  pricked  in  a 
card,  at  a  distance  from  each  other  less  than  the  diameter  of  the  pupil, 
and  the  card  be  held  up  before  one  eye,  with  the  holes 
horizontal,  and  a  needle  placed  vertically  be  looked  at 
through  the  holes,  the  following  facts  may  be  observed  : 
When  attention  is  directed  to  the  needle  itself,  the  image 
of  the  needle  appears  single.  Whenever  the  gaze  is 
directed  to  a  more  distant  object,  so  that  the  eye  is  no 
longer  accommodated  for  the  needle,  the  image  appears 
double  and  at  the  same  time  blurred.  It  also  appears 
double  and  blurred  when  the  eye  is  accommodated  for 
a  distance  nearer  than  that  of  the  needle.  When  only 
one  needle  is  seen,  and  the  eye  therefore  is  properly  ac- 
commodated for  the  distance  of  the  needle,  no  effect  is 
produced  by  blocking  up  one  hole  of  the  card,  except 
that  the  whole  field  of  vision  seems  dimmer.  When, 
however,  the  image  is  double  on  account  of  the  eye  be- 
ing accommodated  for  a  distance  greater  than  that  of 
the  needle,  blocking  the  left-hand  hole  causes  a  disap- 
pearance of  the  right-hand  or  opposite  image,  and 
blocking  the  right-hand  hole  causes  the  left-hand  image 
to  disappear.  When  the  eye  is  accommodated  for  a 
distance  nearer  than  that  of  the  needle,  blocking  either 
hole  causes  the  image  on  the  same  side  to  vanish.  The 
diagram  will  explain  how  these  results  are  brought 
about. 

Let  a  (Fig.  165)  be  a  luminous  point  in  the  needle, 
and  ae,  a /the  extreme  right-hand  and  left-hand  rays 
of  the  pencil  of  rays  proceeding  from  it,  and  passing 
respectively  through  the  right-hand  e,  and  left-hand/, 
holes  in  the  card.  (The  figure  is  supposed  to  be  a 
horizontal  section  of  the  eye.)  When  the  eye  is  accom- 
modated for  a,  the  rays  e  and /meet  together  in  the 
point  c,  the  retina  occupying  the  position  of  the  plane 
n  n ;  the  luminous  point  appears  as  one  point,  and 
the  needle  will  appear  as  one  needle.  When  the 
eye  is  accommodated  for  a  distance  beyond  a,  the  retina  may  be  consid- 
ered to  lie l  no  longer  at  n  n,  but  nearer  the  lens,  at  m  m  for  example  ;  the 
rays  a  e  will  cut  this  plane  at  p,  and  the  rays  af  at  q;  hence  the  luminous 
point  will  no  longer  appear  single,  but  will  be  seen  at  two  points,  or  rather 
as  two  systems  of  diffusion  circles,  and  the  single  needle  will  appear  as  two 
blurred  needles.  The  rays  passing  through  the  right-hand  hole  e,  will  cut 
the  retina  at  p — i.  e.,  on  the  right-hand  side  of  the  optic  axis  ;  but,  as  we 
shall  see  in  speaking  of  the  judgments  pertaining  to  vision,  the  image  on  the 
right-hand  side  of  the  retina  is  referred  by  the  mind  to  an  object  on  the  left- 
hand  side  of  the  person  ;  hence  the  affection  of  the  retina  at  p,  produced  by 
the  rays  a  e  falling  on  it  there,  gives  rise  to  the  image  of  the  spot  a  at  P,  and 
similarly  the  left-hand  spot  q  corresponds  to  the  right-hand  Q.  Blocking 
the  left-hand  hole,  therefore,  causes  a  disappearance  of  the  right-hand  image, 
and  vice  versa.  Similarly,  when  the  eye  is  accommodated  for  a  distance 
nearer  than  the  needle,  the  retina  may  be  supposed  to  be  removed  to  /  I,  and 
the  right-hand  a  e  and  left-hand  a  f  rays,  after  uniting  at  c,  will  diverge 

1  Of  course  in  the  actual  eye,  as  we  shall  see,  accommodation  is  effected  by  a  change  in 
the  lens,  and  not  by  an  alteration  in  the  position  of  the  retina ;  but  for  convenience  sake, 
we  may  here  suppose  the  retina  to  be  moved. 


Diagram  of  Schemer's 
Experiment. 


748  SIGHT. 

again,  and  strike  the  retina  at  p'  and  q'.  The  blocking  of  the  hole  e  will 
now  cause  the  disappearance  of  the  image  q'  on  the  left-hand  side  of  the 
retina,  and  this  will  be  referred  by  the  mind  to  the  right-hand  side,  so  that 
§  will  seem  to  vanish. 

If  the  needle  be  brought  gradually  nearer  and  nearer  to  the  eye,  a  point 
will  be  reached  within  which  the  image  is  always  double.  This  point  marks 
with  considerable  exactitude  the  near  limit  of  accommodation.  With  short- 
sighted persons,  if  the  needle  be  removed  further  and  further  away,  a  point 
is  reached  beyond  which  the  image  is  always  double ;  this  marks  the  far 
limit  of  accommodation. 

The  experiment  may  also  be  performed  with  the  needle  placed  horizon- 
tally, in  which  case  the  holes  in  the  card  should  be  vertical. 

The  adjustment  of  the  eye  for  near  or  far  distances  may  be  assisted  by 
using  two  needles,  one  near  and  one  far.  In  this  case,  one  needle  should  be 
vertical  and  the  other  horizontal,  and  the  card  turned  round  so  that  the  holes 
lie  horizontally  or  vertically  according  to  whether  the  vertical  or  horizontal 
needle  is  being  made  to  appear  double. 

§  622.  In  what  may  be  regarded  as  the  normal  eye,  the  so-called  emme- 
tropic  eye  [Fig.  166],  near  the  limit  of  accommodation  is  about  10  or  12  cm. 

[FIG.  166. 


Emmetropic  Eye.    Parallel  Rays  focussed  on  the  Retina.] 


and  the  far  limit  may  be  put  for  practical  purposes  at  an  infinite  distance. 
The  "  range  of  distinct  vision,"  therefore,  for  the  emmetropic  eye  is  very 
great.  In  the  myopic,  or  short-sighted  eye  [Fig.  167]  the  near  limit  is 
brought  much  closer  (5  or  6  cm.)  to  the  cornea ;  and  the  far  limit  is  at  a 
variable,  but  not  very  great  distance,  so  that  the  rays  of  light  proceeding 
from  an  object  not  many  feet  away  are  brought  to  a  focus,  not  on  the  retina, 


Myopic  Eye.    Rays  coming  from  a  distance  focussed  too  soon.] 

but  in  the  vitreous  humor.  The  range  of  distinct  vision  is  therefore,  in  the 
myopic  eye  very  limited.  In  the  hypermetropic  [Fig.  168],  or  long-sighted 
eye,  the  rays  of  light  coming  from  even  an  infinite  distance  are,  in  the  pas- 
sive state  of  the  eye,  brought  to  a  focus  beyond  the  retina.  The  near  limit 
of  accommodation  is  at  some  distance  off,  and  a  far  limit  of  accommodation 
does  not  exist.  The  presbyopie  eye,  or  the  long  sight  of  old^people,  resembles 
the  hypermetropic  eye  in  the  distance  of  the  near  point  of  accommodation, 
but  differs  from  it  inasmuch  as  the  former  is  an  essentially  defective  condi- 
tion of  the  accommodation  mechanism,  whereas  in  the  latter  the  power  of 


DIOPTRIC  MECHANISMS.  749 

accommodation  may  be  good,  and  yet,  from  the  internal  arrangements  of  the 
eye,  be  unable  to  bring  the  image  of  a  near  object  on  to  the  retina.  When 
abnormal  eye  becomes  presbyopic,  the  far  limit  may  remain  the  same,  but 
since  the  power  of  accommodating  for  near  objects  is  weakened  or  lost,  the 
change  is  distinctly  a  reduction  of  the  range  of  distinct  vision.  In  the  nor- 
mal emmetropic  eye,  when  no  effort  of  accommodation  is  made,  the  principal 
focus  of  the  eye  lies  on  the  retina,  in  the  myopic  eye  in  front  of  it,  and  in 
the  hypermetropic  eye  behind  it. 


Hypermetropic  Eye.    Rays  coming  from  a  distance  focussed  too  late.] 

§  623.  Mechanism  of  accommodation.  In  directing  our  attention  from  a 
far  to  a  very  near  object,  we  are  conscious  of  a  distinct  effort,  and  feel  that 
some  change  has  taken  place  in  the  eye ;  when  we  turn  from  a  very  near  to 
a  far  object,  if  we  are  conscious  of  any  change  in  the  eye,  it  is  one  of  a  dif- 
ferent kind.  The  former  is  the  sense  of  an  active  accommodation  for  near 
objects  ;  the  latter,  when  it  is  felt,  is  the  sense  of  relaxation  after  exertion. 

Since  the  far  limit  of  an  emmetropic  eye  is  at  an  infinite  distance,*  no 
such  thing  as  active  accommodation  for  far  distances  need  exist.  The  only 
change  that  will  take  place  in  the  eye  in  turning  from  near  to  far  objects 
will  be  a  mere  passive  undoing  of  the  accommodation  previously  made  for 
the  near  object.  And  that  no  such  active  accommodation  for  far  distance 
takes  place  is  shown  by  the  facts — that  the  eye,  when  opened  after  being 
closed  for  some  time,  is  found  not  in  medium  state,  but  adjusted  for  distance  ; 
that  when  the  accommodation  mechanism  of  the  eye  is  paralyzed  by  atropine 
or  nervous  disease,  the  accommodation  for  distant  objects  is  unaffected ;  and 
that  we  are  conscious  of  no  effort  in  turning  from  moderately  distant  to  far 
distant  objects.  The  sense  of  effort  often  spoken  of  by  myopic  persons  as 
being  felt  when  they  attempt  to  see  things  at  or  beyond  the  far  limit  of  their 
range  seems  to  arise  from  a  movement  of  the  eyelids,  and  not  from  any  in- 
ternal changes  taking  place  in  the  eye. 

§  624.  What,  then,  are  the  changes  which  take  place  in  the  eye  when 
we  accommodate  for  near  objects  ?  It  might  be  thought,  and,  indeed,  once 
was  thought,  that  the  curvature  of  the  cornea  was  changed,  becoming  more 
convex,  with  a  shorter  radius  of  curvature,  for  near  objects.  Young,  how- 
ever, showed  that  accommodation  took  place  as  usual  when  the  eye  (and 
head)  is  immersed  in  water.  Since  the  refractive  powers  of  aqueous  humor 
and  water  are  very  nearly  alike,  the  cornea  with  its  parallel  surfaces,  placed 
between  these  two  fluids,  can  have  little  or  no  effect  on  the  direction  of  the 
rays  passing  through  it  when  the  eye  is  immersed  in  water.  And  accurate 
measurements  of  the  dimensions  of  an  image  on  the  cornea  have  shown  that 
these  undergo  no  change  during  accommodation,  and  that,  therefore,  the 
curvature  of  the  cornea  is  not  altered.  Nor  is  there  any  change  in  the  form 
of  the  bulb  ;  for  any  variation  in  this  would  necessarily  produce  an  altera- 
tion in  the  curvature  of  the  cornea,  and  pressure  on  the  bulb  would  act  in- 
juriously by  rendering  the  retina  anaemic  and  so  less  sensitive.  In  fact, 
there  are  only  two  changes  of  importance  which  can  be  ascertained  to  take 
place  in  the  eye  during  accommodation  for  near  objects. 


750  SIGHT. 

One  is  that  the  pupil  contracts.  When  we  look  at  near  objects,  the 
pupil  becomes  small  ;  when  we  turn  to  distant  objects,  it  dilates.  This, 
however,  cannot  have  more  than  an  indirect  influence  on  the  formation  of 
the  image  ;  the  chief  use  of  the  contraction  of  the  pupil  in  accommodation  for 
near  objects  is  to  cut  off  the  more  divergent  circumferential  rays  of  light. 

§  625.  The  other  and  really  efficient  change  is  that  the  anterior  surface  of 
the  lens  becomes  more  convex.  If  a  light  be  held  before  the  eye,  three  reflected 
images  may,  with  care  and  under  proper  precautions,  be  seen  by  a  bystander : 


a      b     c  a       b       c  a       b       c 

Diagram  of  Images  reflected  from  the  Eye.] 

one  a  very  bright  one  caused  by  the  anterior  surface  of  the  cornea  (a),  a 
second  less  bright,  by  the  anterior  surface  of  the  lens  (b),  and  a  third  very 
dim,  by  the  posterior  surface  of  the  lens  (c)  ;  when  the  images  are  those  of 
an*  object,  such  as  a  candle,  in  which  a  top  and  bottom  can  be  recognized, 
the  two  former  images  are  seen  to  be  erect,  but  the  third  inverted.  When 
the  eye  is  accommodated  for  near  objects,  no  change  is  observed  in  either 
the  first  or  the  third  of  these  images  ;  but  the  second,  that  from  the  ante- 
rior surface  of  the  lens,  is  seen  to  become  distinctly  smaller,  showing  that 
the  surface  has  become  more  convex.  When,  on  the  contrary,  vision  is 
directed  from  near  to  far  objects,  the  image  from  the  anterior  surface  of  the 
lens  grows  larger,  indicating  that  the  convexity  of  the  surface  has  dimin- 
ished, while  no  change  takes  place  in  the  curvature  either  of  the  cornea  or 
of  the  posterior  surface  of  the  lens.  And  accurate  measurements  of  the 
size  of  the  image  from  the  anterior  surface  of  the  lens  have  shown  that  the 
variations  in  curvature  which  do  take  place  are  sufficient  to  account  for  the 
power  of  accommodation  which  the  eye  possesses. 

The  observation  of  these  reflected  images  is  facilitated  by  the  simple  instrument 
introduced  by  Helmholtz  and  called  a  phakoscope.  It  consists  of  a  small,  dark 
chamber,  with  apertures  for  the  observed  and  observing  eyes ;  a  needle  is  fixed  at 
a  short  distance  in  front  of  the  former,  to  serve  as  a  near  object,  for  which  accom- 
modation has  to  be  made  ;  and  a  lamp  or  candle  is  so  disposed  as  to  throw  an  image 
on  each  of  the  three  surfaces  of  the  observed  eye.  Since  the  distance  between  two 
images  is  more  readily  appreciated  than  is  a  simple  change  of  size  of  a  single  image, 
two  prisms  are  employed  so  as  to  throw  a  double  image  of  the  lamp  on  each  of  the 
three  surfaces  [Fig.  169,  B.  C  ].  When  the  anterior  surface  of  the  lens  becomes 
more  convex  the  two  images  reflected  from  that  surface  approach  each  other  (C  ), 
when  it  becomes  less  convex  they  retire  from  each  other  (B). 

These  observations  leave  no  doubt  that  the  essential  change  by  which 
accommodation  is  effected  is  an  alteration  of  the  convexity  of  the  anterior 
surface  of  the  lens.  And  that  the  lens  is  the  agent  of  accommodation  is 
further  shown  by  the  fact  that  after  removal  of  the  lens,  as  in  the  operation 
for  cataract,  the  power  of  accommodation  is  lost.  In  the  cases  which  have 
been  recorded,  where  eyes  from  which  the  lens  had  been  removed  seemed 


DIOPTRIC  MECHANISMS.  751 

still  to  possess  some  accommodation,  we  must  suppose  that  no  real  accommo- 
dation took  place,  but  that  the  pupil  contracted  when  a  near  object  was 
looked  at,  and  so  assisted  in  making  vision  more  distinct. 

§  626.  This  increase  of  the  convexity  of  the  lens  has  been  supposed  to 
be  due  to  a  compression  of  the  circumference  of  the  lens  by  a  contraction  of 
the  iris  ;  but  this  is  disproved  by  the  fact  that  accommodation  may  take 
place  in  eyes  from  which  the  iris  is  congenitally  absent.  It  has  also  been 
attributed  to  vasomotor  changes,  to  increased  fulness  of  the  vessels  of  the  iris 
or  ciliary  processes  surrounding  the  lens  ;  but  this  also  is  disproved  by  the 
fact  that  accommodation  may  be  effected  after  death,  in  an  eye  which  is 
practically  bloodless,  by  stimulating  the  ciliary  ganglion  or  ciliary  nerves 
with  an  interrupted  current  or  by  other  means.  The  real  nature  of  the 
mechanism  seems  to  be  as  follows. 

§  627.  The  lens  when  examined  after  removed  from  the  eye  is  found  to 
be  a  body  of  considerable  elasticity.  When  the  curvature  of  the  anterior 
surface  of  the  lens  is  determined,  as  may  be  done  by  appropriate  means,  in 
its  natural  position  in  the  eye  at  rest,  and  then  again  determined  after  the 
lens  has  been  removed  from  the  eye,  the  anterior  surface  is  found  to  be  more 
convex  in  the  latter  than  in  the  former  case.  There  seems  to  be  in  the  eye 
in  its  natural  condition,  some  agency  at  work  keeping  the  anterior  surface 
of  the  lens  somewhat  flattened.  The  suspensory  ligament,  attached  to  the 
choroid  and  ciliary  processes  behind,  and  passing  over  the  front  of  the  lens,  is 
just  such  a  structure  as  would  produce  this  effect.  In  the  natural  position  of 
the  choroid  this  ligament  is  tense,  and  tends  to  flatten  the  front  of  the  lens. 
When  the  choroid  is  pulled  forward,  the  ligament  becomes  slack  and  the 
lens  bulges  out  forward.  Further,  the  ciliary  muscle  attached  on  the  one 
hand  to  a  fairly  fixed  region,  the  junction  of  the  sclerotic  and  cornea,  and 
on  the  other  to  the  looser  and  more  movable  choroid,  would  naturally,  when 
thrown  into  contraction,  pull  forward  the  choroid  and  so  slacken  the  suspen- 
sory ligament,  and  hence  permit  the  elastic  lens  to  bulge  out  forward.  And 
we  have  experimental  evidence,  carried  out  on  lower  animals,  that  stimula- 
tion of  the  ciliary  ganglion  or  of  its  so-called  radix  brevis  does  lead,  on  the 
one  hand,  to  a  contraction  of  the  ciliary  muscle  and  pulling  forward  of  the 
choroid,  and,  on  the  other  hand,  to  an  increased  curvature  of  the  anterior 
surface  of  the  lens.  Hence  we  may  conclude  that  accommodation  for  near 
objects  consists  essentially  in  a  contraction  of  the  ciliary  muscle,  which,  by 
pulling  forward  the  choroid  coat  and  the  ciliary  process,  slackens  the  sus- 
pensory ligament,  and  allows  the  lens  to  bulge  forward  by  virtue  of  its  elas- 
ticity, and  so  to  increase  the  convexity  of  its  anterior  surface  [Figs.  170  and 
171]. 

Accommodation  is  in  most  cases  a  voluntary  act ;  since,  however,  the 
change  in  the  lens  is  always  accompanied  by  movements  in  the  iris,  it  will 
be  convenient  to  consider  the  latter  before  we  discuss  the  nervous  mechan- 
ism of  the  whole  act. 

§  628.  Movements  of  the  pupil.  Though  by  making  the  efforts  required 
for  accommodation  we  can  at  pleasure  contract  or  dilate  the  pupil,  it  is  not 
in  our  power  to  bring  the  will  to  act  directly  on  the  iris  by  itself.  This 
fact  alone  indicates  that  the  nervous  mechanism  of  the  pupil  is  of  a  peculiar 
character,  and  such  indeed  we  find  it  to  be.  The  pupil  is  contracted  (1) 
when  the  retina  (or  optic  nerve)  is  stimulated,  and  when  light  falls  on  the 
retina,  the  brighter  the  light  the  greater  being  the  contraction,  (2)  when  we 
accommodate  for  near  objects.  The  pupil  is  also  contracted  when  the  eye- 
ball is  turned  inward,  when  the  aqueous  humor  is  deficient,  in  the  early 
stages  of  poisoning  by  chloroform,  alcohol,  etc. ;  in  nearly  all  stages  of 
poisoning  by  morphine,  physostigmine,  and  some  other  drugs ;  and  in  deep 


752 


SIGHT. 


slumber.  The  pupil  is  dilated  (1)  when  stimulation  of  the  retina  (or  optic 
nerve)  is  diminished  or  arrested  as  in  passing  from  a  bright  into  a  dim  light 
or  into  darkness,  (2)  when  the  eye  is  adjusted  for  far  objects.  Dilatation 
also  occurs  when  there  is  an  excess  of  aqueous  humor,  during  dyspnoea, 
during  violent  muscular  efforts,  as  the  result  of  a  stimulation  of  sensory 
nerves,  as  an  effect  of  emotions,  in  the  later  stages  of  poisoning  by  chloro- 
form, etc.,  and  in  all  stages  of  poisoning  by  atropine  and  some  other  drugs, 
and  in  pathological  conditions  of  the  nervous  system. 


N 


Diagram  to  illustrate  Accommodation.  C,  cornea  ;  S,  sclerotic  ;  F,  C,  A;  vertical  plane  of  the 
cornea  ;  B,  C,  D,  axis  of  the  eye  ;  s,  s,  canal  of  Schlemm  ;  p,  angle  formed  by  the  iris  and  cornea, 
or  margin  of  anterior  chamber  ;  m,  position  of  iris  and  curvature  of  lens  in  an  eye  converged  for 
parallel  rays,  distant  vision,  or  negative  accommodation ;  n,  position  of  iris  and  curvature  of  lens 
required  for  near  objects  or  for  positive  accommodation.] 

[Fio.  171. 


Emmetropic  Eye.  The  dotted  lines  show  how  accommodation  for  the  diverging  rays  of  near 
objects  is  effected  by  the  bulging  of  the  lens.  The  dotted  lines  indicate  the  change  in  the 
lens  and  the  effect  on  the  light  rays.] 

§  629.  Contraction  of  the  pupil  is  caused  by  contraction  of  the  circular 
fibres  or  sphincter  of  the  iris.  Dilatation  is  caused  by  contraction  of  the 
radial  fibres  of  the  iris ;  for  though  the  existence  of  radial  fibres  has  been 
denied  by  many  observers,  the  preponderance  of  evidence  is  clearly  in  favor 
of  their  being  really  present. 

Considering  how  vascular  the  iris  is,  it  does  not  seem  unreasonable  to 
interpret  some  of  the  variations  in  the  condition  of  the  pupil  as  the  results 
of  simple  vascular  turgescence  or  of  depletion  brought  about  by  vasomotor 
action  or  otherwise,  the  small  or  contracted  pupil  corresponding  to  the  di- 
lated and  filled  and  the  large  or  dilated  pupil  to  the  constricted  and  emptied 
condition  of  the  bloodvessels.  Thus  slight  oscillations  of  the  pupil  may  be 
observed  synchronous  with  the  heart-beat  and  others  synchronous  with  the 
respiratory  movements.  But  the  variations  in  the  pupil  seem  too  marked  to 
be  merely  the  effects  of  vascular  changes,  and  indeed  that  constriction  of  the 
pupil  cannot  be  wholly  the  result  of  turgescence,  nor  dilatation  wholly  the 
result  of  depletion  of  the  vessels  of  the  iris,  is  shown  by  the  facts  that  both 
these  events  may  be  witnessed  in  a  perfectly  bloodless  eye,  and  that  the 
movements  of  the  pupil  when  brought  about  by  agents  which  also  affect  the 


DIOPTRIC  MECHANISMS.  753 

bloodvessels  begin  some  time  before  the  changes  in  the  calibre  of  the  blood- 
vessels, and  indeed  may  be  over  before  these  have  arrived  at  their  maximum. 
Moreover,  the  fibres  of  the  sympathetic,  which,  as  we  shall  see,  are  con- 
cerned in  causing  dilatation  of  the  pupil,  run  a  somewhat  different  course 
from  those  which  govern  the  bloodvessels  of  the  eye.  We  may,  therefore, 
adhere  to  the  view  that  the  main  changes  of  the  pupil  in  the  direction  of 
narrowing  and  widening  are  brought  about  by  contractions  of  the  plain 
muscular  fibres  in  the  iris. 

§  630.  Muscular  contractions  leading  to  changes  of  the  pupil  may  be 
observed  in  the  eye  removed  from  the  body,  and  indeed  in  the  extirpated 
iris.  The  plain  muscular  fibres  of  the  iris,  like  other  plain  muscular  fibres, 
are  remarkably  sensitive  to  variations  in  temperature.  Besides  this  there 
seems  to  be,  in  certain  animals  at  least,  a  connection  within  the  eye  between 
the  iris  and  retina  of  such  a  kind  that  light  falling  into  an  extirpated  eye 
will  lead  to  a  narrowing  of  the  pupil.  Putting  aside,  however,  such  excep- 
tional events  we  may  lay  down  the  broad  principle  that  contraction  of  the 
pupil,  brought  about  by  light  falling  on  the  retina,  is  a  reflex  act,  of  which 
the  optic  is  the  afferent  nerve,  the  third  or  oculo-motor  the  efferent  nerve, 
and  the  centre  some  portion  of  the  brain  lying  below  the  corpora  quadri- 
gemina  in  the  front  part  of  the  floor  of  the  aqueduct  of  Sylvius.  This  is 
proved  by  the  following  facts :  When  the  optic  nerve  is  divided,  the  falling 
of  light  on  the  retina  no  longer  causes  a  contraction  of  the  pupil ;  when 
the  third  nerve  is  divided,  stimulation  of  the  retina  or  of  the  optic  nerve  no 
longer  causes  contraction  ;  but  direct  stimulation  of  the  peripheral  portion 
of  the  divided  third  nerve  causes  extreme  contraction  of  the  pupil.  If  the 
region  of  the  brain  spoken  of  above  as  a  centre  be  carefully  stimulated  con- 
traction of  the  pupil  will  take  place  even  in  the  absence  of  light  and  after 
division  of  the  optic  nerve.  After  removal  of  the  same  centre  stimulation 
of  the  retina  is  ineffectual  in  narrowing  the  pupil.  But  if  the  centre  and  its 
connections  with  the  optic  nerve  and  third  nerve  be  left  intact  and  in  thor- 
oughly sound  condition,  contraction  of  the  pupil  will  occur  as  a  result  of 
light  falling  on  the  retina,  though  all  other  nervous  parts  be  removed. 

§  631.  The  nervous  centre  is  not  a  double  centre  with  two  completely 
independent  halves,  one  for  each  eye  ;  there  is  a  certain  amount  of  functional 
communion  between  the  two  sides,  so  that  when  one  retina  is  stimulated  both 
pupils  contract.  It  might  be  imagined  that  this  cerebral  centre  acted  as  a 
tonic  centre,  whose  action  was  simply  increased,  not  originated,  by  the  stimu- 
lation of  the  retina ;  but  this  is  disproved  by  the  fact  that  if  the  optic  nerve 
be  divided  subsequent  section  of  the  third  nerve  produces  no  further  dilatation. 

In  considering  the  movements  of  the  pupil,  however,  we  have  to  deal  not 
only  with  a  narrowing  of  the  pupil  thus  brought  about  in  a  reflex  way  by 
contraction  of  the  circular  sphincter  fibres,  and  with  the  absence  of  such  a 
narrowing,  but  also  with  active  dilatation  due  to  a  contraction  of  the  radial 
dilator  fibres,  and  this  renders  the  whole  matter  much  more  complex  than 
might  be  supposed  to  be  the  case  from  the  simple  statement  just  made. 

§  632.  The  iris  is  supplied,  in  common  with  the  ciliary  muscle  and 
choroid,  by  the  short  ciliary  nerves  (Fig.  172,  s.  c.)  coming  from  the  oph- 
thalmic or  lenticular  (ciliary)  ganglion  (/.  c.)  which  is  connected  by  its  roots 
with  the  third  nerve  (r.  &.),  the  cervical  sympathetic  nerve  (sym.),  and 
with  the  nasal  branch  of  the  ophthalmic  division  of  the  fifth  nerve  (r.  I.). 
The  short  ciliary  nerves  are,  moreover,  accompanied  by  the  long  ciliary 
nerves  (I.  c.)  coming  from  the  same  nasal  branch  of  the  ophthalmic  division 
of  the  fifth  nerve.  What  are  the  uses  of  these  several  nerves  in  relation  to 
the  pupil? 

§  633.  If  the  cervical  sympathetic  in  the  neck  be  divided,  all  other  por- 

48 


754 


SIGHT. 


FIG.  172. 


tions  of  the  nervous  mechanism  being  intact,  a  contraction  of  the  pupil  (not 
always  very  well  marked)  takes  place,  and  if  the  peripheral  portion  (?'.  e.,  the 

upper  portion  still  connected  with  the 
head)  be  stimulated,  a  well-developed 
dilatation  is  the  result.  The  sympa- 
thetic has,  it  will  be  observed,  an  effect 
on  the  iris  the  opposite  of  that  which  it 
exercises  on  the  bloodvessels ;  when  it 
is  stimulated  the  pupils  are  dilated 
while  the  bloodvessels  are  constricted. 
This  dilating  influence  of  the  sympa- 
thetic may,  as  in  the  case  of  the  vaso- 
motor  action  of  the  same  nerve,  be 
traced  back  down  the  neck  to  the  upper 
thoracic  ganglion  and  thence  along  the 
raiui  communicantes  and  roots  of  the 
lower  cervical  and  first  dorsal  or  two 
first  dorsal  spinal  nerves,  to  a  region 
in  the  lower  cervical  and  upper  dorsal 
cord  (called  by  some  authors  the  cen- 
trum cilio-spinale  inferius),  and  from 
thence  up  through  the  medulla  oblon- 
gata  to  a  centre,  which  appears  to  be 
placed  in  the  floor  of  the  front  part 
of  the  aqueduct  of  Sylvius  not  far 
from  and  apparently  on  either  side 
of  the  centre  for  contraction  of  the 
pupil. 

§  634.  The  dilatation  of  the  pupil 
which  is  witnessed  in  dyspnoea,  and 
that  which  results  from  stimulation  of 
sensory  nerves  and  from  emotions,  ap- 
pears to  be  brought  about  by  the 
action  of  the  sympathetic,  the  venous 
blood  or  the  sensory  impulses  or  the 
emotional  impulses  so  affecting  the 
dilating  centre  as  to  augment  the  dilat- 
ing impulses  proceeding  from  it  along 
the  sympathetic.  The  existence  of  the 
subordinate  centre  in  the  cervical  or 
dorsal  cord,  spoken  of  just  now,  is  sup- 
posed to  be  indicated  by  the  fact  that 
after  division  of  the  medulla  oblongata,  and  consequent  severance  of  the 
efferent  paths  from  the  centre  in  the  aqueduct  of  Sylvius,  dilatation  of  the 
pupil  may  still  be  brought  about,  in  some  animals  at  least,  by  dyspnoea  or 
by  adequate  stimulation  of  sensory  nerves.  A  question  is  raised  here  in 
fact  somewhat  similar  to  that  raised  in  connection  with  the  medullary  respi- 
ratory centre  (p.  370)  ;  and  here  as  there  we  may  probably  conclude  that 
the  independent  action  of  such  a  spinal  centre  is  of  subordinate  importance. 
§  635.  The  pupil  then  seems  to  be  under  the  dominion  of  two  antagon- 
istic mechanisms :  one  a  contracting  mechanism,  reflex  in  nature,  the  third 
nerve  serving  as  the  efferent  and  the  optic  as  the  afferent  tract ;  the  other  a 
dilating  mechanism,  apparently  tonic  in  nature,  but  subject  to  augmentation 
from  various  causes,  and  of  this  the  cervical  sympathetic  is  the  efferent 
channel.  Hence,  when  the  third  or  optic  nerve  is  divided,  not  only  does 


sym; 


Diagrammatic  Representation  of  the 
Nerves  Governing  the  Pupil.  II.  optic 
nerve ;  I.  y.  lenticular  ganglion  ;  r.  b.  its 
short  root  from  ///. ;  oc.  m..  third  or  oculo- 
motor nerve  ;  sym.  its  sympathetic  root ;  r.  I. 
its  long  root  from  V.  ophthm.  the  nasal  branch 
of  the  ophthalmic  division  of  the  fifth  nerve  , 
s.  c.  the  short  ciliary  nerves  from  the  lenticu- 
lar ganglion  ;  I.  c.  the  long  ciliary  nerve  from 
the  nasal  branch  of  the  ophthalmic  division 
of  the  fifth  nerve. 


DIOPTRIC  MECHANISMS.  755 

contraction  of  the  pupil  cease  to  be  manifest,  but  active  dilatation  occurs,  on 
account  of  the  tonic  dilating  influence  of  the  sympathetic  being  left  free  to 
work.  When,  on  the  other  hand,  the  sympathetic  is  divided,  this  tonic 
dilating  influence  falls  away,  arid  contraction  results.  When  the  optic  or 
third  nerve  is  stimulated,  the  dilating,  effect  of  the  sympathetic  is  overcome, 
and  contraction  results ;  and  when  the  sympathetic  is  stimulated,  any  con- 
tracting influence  of  the  third  nerve  which  may  be  present  is  overcome,  and 
dilatation  ensues. 

§  636.  But  there  are  considerations  which  show  that  the  matter  is  still 
more  complex  than  this.  A  small  quantity  of  atropine  introduced  into  the 
eye  or  into  the  system  causes  a  dilatation  of  the  pupil.  This  might  be 
attributed  to  a  paralysis  of  the  third  nerve,  and,  indeed,  it  is  found  that 
after  atropine  has  produced  its  effects  the  falling  of  light  on  the  retina  no 
longer  causes  contraction  of  the  pupil.  A  difficulty,  however,  is  introduced 
by  the  fact  that  when  the  third  nerve  is  divided,  and  when,  therefore,  the 
contracting  effects  of  stimulation  of  the  retina  are  placed  entirely  on  one 
side,  and  there  is  nothing  to  prevent  the  sympathetic  producing  its  dilating 
effects  to  the  utmost,  dilatation  is  still  further  increased  by  atropine.  When 
physostigmine  is  introduced  into  the  eye  or  system,  contraction  of  the  pupil 
is  caused,  whether  the  third  nerve  be  divided  or  not ;  and  when  the  dose  is 
sufficiently  strong  the  contraction  is  so  great  that  it  cannot  be  overcome  by 
stimulation  of  the  sympathetic.  The  dilatation  which  is  caused  by  a  suffi- 
cient dose  of  atropine  may  be  greater  than  that  which  can  ordinarily  be 
produced  by  stimulation  of  the  sympathetic,  and  the  contraction  caused  by 
a  sufficient  dose  of  physostigmine  may  be  greater  than  that  which  is  ordi- 
narily produced  in  a  reflex  manner  by  stimulation  of  the  optic  nerve,  or 
even  than  that  produced  by  direct  stimulation  of  the  third  nerve.  Evi- 
dently these  drugs  act  either  directly  on  the  plain  muscular  fibres  of  the  iris 
or  on  some  local  mechanism,  the  one  in  such  a  way  as  to  cause  dilatation, 
the  other  in  such  a  way  as  to  cause  contraction.  Such  a  local  mechanism 
cannot,  however,  lie  in  the  ophthalmic  ganglion,  for  both  drugs  continue  to 
produce  these  effects  in  a  most  marked  degree  after  the  ganglion  has  been 
excised.  We  must  suppose,  therefore,  that  the  mechanism  if  it  exists  is 
situated  in  the  iris  itself  or  in  the  choroid,  where,  indeed,  ganglionic  nerve- 
cells  are  abundant.  The  movements  of  the  iris  in  the  extirpated  eye,  spoken 
of  just  now,  may  perhaps  be  attributed  to  the  same  local  mechanism.  Fur- 
ther it  is  stated  that  with  stimulation  of  the  sympathetic,  the  latent  period, 
i.  e.,  the  period  intervening  between  the  beginning  of  stimulation  and  the 
beginning  of  the  movement  of  the  iris,  is  much  greater  than  with  stimu- 
lation of  the  third  nerve,  indicating  that  the  former  acts  through  a  local 
mechanism  but  the  latter  more  directly  on  the  muscular  fibres.  The  whole 
question,  however,  of  this  local  mechanism,  and  of  the  exact  mode  of  action 
of  the  various  drugs  and  of  the  changes  in  the  body  which  lead  to  contrac- 
tion or  dilatation  respectively  of  the  pupil,  needs  fuller  discussion  than  we 
can  afford  to  give  to  it  here.  We  may  add  that  the  local  action  of  atropine 
in  contrast  to  any  -action  on  the  cerebral  centre  is  well  illustrated  by  apply- 
ing atropine  to  one  eye  locally.  The  pupil  of  that  eye  dilates  widely  ;  in 
consequence  more  light  falls  on  the  retina,  and  this  so  affects  the  cerebral 
centre,  which  as  we  have  seen  is  not  strictly  unilateral  but  in  communion 
with  its  fellow,  that  increased  constricting  impulses  pass  from  both  centres, 
and  these,  though  ineffectual  in  the  atropinized  eye,  lead  in  the  untouched 
eye  to  an  increased  narrowing  of  the  pupil. 

§  637.  The  share  of  the  fifth  nerve  in  the  work  of  the  iris  seems  to  be  in 
part  a  sensory  one ;  the  iris  is  sensitive,  and  the  sensory  impulses  which  are 
generated  in  it  pass  from  it  along  the  fibres  of  the  fifth  nerve. 


756  SIGHT. 

We  may  sum  up  the  nervous  mechanism  of  the  pupil  then  somewhat  as 
follows :  The  salient  and  most  frequently  repeated  event,  the  contraction  of 
the  pupil  upon  exposure  to  light,  is  a  reflex  act,  the  centre  of  which  is 
placed  in  the  brain  ;  and  the  correlative  widening  of  the  pupil  upon  dimi- 
nution of  light  is  due  to  the  tonic  action  of  the  sympathetic  making  itself 
felt  upon  the  waning  of  its  antagonist.  The  contraction  of  the  pupil  in  the 
earlier  stages  of  the  action  of  alcohol  and  chloroform  and  in  slumber  is 
probably  due  to  an  increased  action  of  the  contracting  centre,  but  the 
narrow  pupil  caused  by  such  drugs  as  morphine  and  physostigmine  is  due, 
chiefly  at  least,  to  a  local  action.  The  dilating  effects  of  such  drugs  as 
atropine  are  also  largely  due  to  a  local  action,  but  in  the  widened  pupil  of 
the  later  stages  of  alcohol  poisoning  and  of  dyspnoea  we  can  probably  trace 
the  effects  of  an  exhaustion  of  the  cerebral  contracting  centre,  assisted  pos- 
sibly by  an  increased  activity  of  the  dilating  centre. 

§  638.  There  remains  a  word  to  be  said  concerning  the  contraction  of 
the  pupil  which  takes  place  when  the  eye  is  accommodated  for  near  objects, 
and  when  the  pupil  is  turned  inward  (the  two  being  closely  allied,  since  the 
eyes  converge  to  see  near  objects),  and  the  return  to  the  more  dilated  condi- 
tion when  the  eye  returns  to  rest  and  regains  the  accommodation  for  far 
objects.  These  are  instances  of  what  are  called  "associated  movements." 
Two  movements  are  thus  spoken  of  as  "  associated  "  when  the  special  central 
nervous  mechanism  employed  in  carrying  out  the  one  act  is  so  connected  by 
nervous  ties  of  some  kind  or  other  with  that  employed  in  carrying  out  the 
other,  that  when  we  set  the  one  mechanism  in  action  we  unintentionally  set 
the  other  in  action  also.  The  ciliary  muscles  which  bring  about  accommo- 
dation are  governed  in  this  action  by  fibres  that  may  be  traced,  through  the 
ciliary  nerves  and  lenticular  ganglion,  along  the  third  or  oculo-motor  nerve, 
to  a  centre  which  lies  (in  dogs)  in  the  hind  part  of  the  floor  of  the  third 
ventricle,  and  which  is  especially  connected  with  the  most  anterior  bundles 
of  the  roots  of  the  third  nerve.  This  centre  is  under  the  command  of  our 
will ;  when  we  wish  to  accommodate  for  near  objects  we  throw  it  into  action, 
and,  when  in  action,  it  calls  also  into  action  by  "  association  "  the  centre  for 
the  contraction  of  the  pupil ;  when  the  action  of  the  accommodation  centre 
ceases  and  the  eye  falls  back  to  the  condition  of  rest,  in  which  it  is  accom- 
modated for  far  objects,  the  action  of  the  pupil-contracting  centre  ceases 
also,  and  the  pupil  therefore  widens. 

§  639.  The  mechanism  of  accommodation  may  also  be  affected  in  a  local 
manner.  And  the  drugs  which  have  a  special  action  on  the  pupil,  such  as 
atropine  and  Calabar  bean,  also  affect  the  mechanism  of  accommodation. 
Atropine  paralyzes  it,  so  that  the  eye  remains  adjusted  for  far  objects;  and 
physostigmine  throws  the  eye  into  a  condition  of  forced  accommodation  for 
near  objects.  This  double  action  has  been  explained  by  the  supposition  that 
while  atropine  paralyzes,  physostigmine  throws  into  tonic  or  tetanic  contrac- 
tion, on  the  one  hand,  the  circular  muscles  of  the  iris  and,  on  the  other,  the 
ciliary  muscles ;  but  the  phenomena,  on  inquiry,  appear  too  complicated  to 
be  explained  in  so  simple  a  manner. 

We  can  accomodate  at  will ;  but  few  persons  can  effect  the  necessary 
change  in  the  eye  unless  they  direct  their  attention  to  some  near  or  far 
object,  as  the  case  may  be,  and  thus  assist  their  will  by  visual  sensations.  By 
practice,  however,  the  aid  of  external  objects  may  be  dispensed  -with  ;  and 
it  is  when  this  is  achieved  that  the  pupil  may  be  made  to  dilate  or  contract 
at  pleasure,  accommodation  being  effected  without  the  eye  being  turned  to 
any  particular  object. 


DIOPTRIC  MECHANISMS.  757 

Imperfections  in  the  Dioptric  Apparatus. 

§  640.  The  emmetropic  eye  may  be  taken  as  the  normal  eye.  The  myopic 
and  hypermetropic  eyes  may  be  considered  as  imperfect  eyes,  though  the 
former  possesses  certain  advantages  over  the  normal  eye.  An  eye  might  be 
myopic  from  too  great  a  convexity  of  the  cornea  or  of  the  anterior  surface 
of  the  lens,  or  from  permanent  spasm  of  the  accommodation-mechanism,  or 
from  too  great  a  length  of  the  long  axis  of  the  eyeball.  The  last  appears  to 
be  the  usual  cause.  Similarly,  most  hypermetropic  eyes  possess  too  short  a 
bulb.  Moreover,  in  the  strongly-marked  myopic  eye  there  is  frequently  hy- 
pertrophy of  the  longitudinal  (meridional)  fibres  of  the  ciliary  muscle,  often 
spoken  of  exclusively  as  the  ciliary  muscle,  and  atrophy  or  absence  of  the 
circular  fibres ;  in  the  hypermetropic  eye,  on  the  other  hand,  the  circular 
fibres  are  well  developed  and  the  meridional  fibres  scanty.  The  presbyopic 
eye  is,  as  we  have  seen,  an  eye  normally  constituted  in  which  the  power  of 
accommodation  has  been  lost  or  is  failing  through  increasing  weakness  of  the 
ciliary  muscle  or  a  loss  of  elasticity  in  the  lens,  or  through  the  parts  be- 
coming rigid. 

§  641.  Spherical  aberration.  In  a  spherical  lens  the  rays  which  impinge 
on  the  circumference  are  brought  to  a  focus  sooner  than  those  which  pass 
nearer  the  centre,  and  the  rays  proceeding  from  a  luminous  point  are  no 
longer  brought  to  a  single  focus  at  one  point,  but  form  a  number  of  foci  at 
different  distances.  Hence  when  rays  are  allowed  to  fall  on  the  whole  of 
the  lens,  the  image  formed  on  a  screen  placed  in  the  focus  of  the  more  central 
rays  is  blurred  by  the  diffusion-circles  caused  by  the  circumferential  rays 
which  have  been  brought  to  a  premature  focus.  In  an  ordinary  optical  in- 
strument spherical  aberration  is  obviated  by  a  diaphragm  which  shuts  off 
the  more  circumferential  rays.  In  the  eye  the  iris  is  an  adjustable  diaphragm ; 
and  when  the  pupil  contracts  in  near  vision  the  more  divergent  rays  proceed- 
ing from  a  near  object  which  tend  to  fall  on  the  circumferential  parts  of 
the  lens  are  cut  off.  As,  however,  the  refractive  power  of  the  lens  does  not 
increase  regularly  and  progressively  from  the  centre  to  the  circumference, 
but  varies  most  irregularly,  the  purpose  of  the  narrowing  of  the  pupil  cannot 
be  simply  to  obviate  spherical  aberration ;  and  indeed  the  other  optical  im- 
perfections of  the  eye  are  so  great  that  such  spherical  aberrations  as  are 
caused  by  the  lens  produce  no  obvious  effect  on  vision. 

§  642.  Astigmatism.  We  have  hitherto  treated  the  eye  as  if  its  dioptric 
surfaces  were  all  parts  of  perfect  spherical  surfaces.  In  reality  this  is  rarely 
the  case,  either  with  the  lens  or  with  the  cornea.  Slight  deviations  do  not 
produce  any  marked  effect,  but  there  is  one  deviation,  known  as  regular 
astigmatism,  which,  present  to  a  certain  extent  in  most  eyes,  very  largely 
developed  in  some,  frequently  leads  to  very  imperfect  vision.  This  defect  is 
due  to  the  dioptric  surface  being  not  spherical  but  more  convex  along  one 
meridian  than  another,  more  convex,  for  instance,  along  the  vertical  than 
along  the  horizontal  meridian.  When  this  is  the  case  the  rays  proceeding 
from  a  luminous  point  are  not  brought  to  a  single  focus  at  a  point,  but 
possess  two  linear  foci,  one  nearer  than  the  normal  and  corresponding  to  the 
more  convex  surface,  the  other  further  than  the  normal  focus  and  corre- 
sponding to  the  less  convex  surface.  If  the  vertical  meridians  of  the  sur- 
face be  more  convex  than  the  horizontal,  then  the  nearer  linear  focus  will 
be  horizontal  and  the  further  linear  focus  will  be  vertical,  and  vice  versa. 
(This  can  be  shown  much  more  effectually  on  a  model  than  in  a  diagram  in 
which  we  are  limited  to  two  dimensions.)  Now,  in  order  to  see  a  vertical 
line  distinctly,  it  is  much  more  important  that  the  rays  which  diverge  from 
the  line  in  a  series  of  horizontal  planes  should  be  brought  to  a  focus  prop- 


758 


SIGHT. 


erly  than  those  which  diverge  in  the  vertical  plane  of  the  line  itself;  and 
similarly,  in  order  to  see  a  horizontal  line  distinctly,  it  is  much  more  im- 
portant that  the  rays  which  diverge  from  the  line  in  a  series  of  vertical 
planes  should  be  brought  to  a  focus  properly  than  those  which  diverge  in 
the  horizontal  plane  of  the  line  itself.  Hence  a  horizontal  line  held  before 
an  astigmatic  dioptric  surface,  most  convex  in  the  vertical  meridians,  will 
give  rise  to  the  image  of  a  horizontal  line  at  the  nearer  focus,  the  vertical 
rays  diverging  from  the  line  being  here  brought  to  a  linear  horizontal  focus. 
Similarly,  a  vertical  line  held  before  the  same  surface  will  give  rise  to  an 
image  of  a  vertical  line  at  the  further  focus,  the  horizontal  rays  diverging 
from  the  vertical  line  being  ,here  brought  to  a  linear  vertical  focus.  In 
other  words,  with  a  dioptric  surface  most  convex  in  the  vertical  meridians 
horizontal  lines  are  brought  to  a  focus  sooner  than  are  vertical  lines. 

Most  eyes  are  thus  more  or  less  astigmatic,  and  generally  with  a  greater 
convexity  along  the  vertical  meridians.  If  a  set  of  horizontal  or  vertical 
lines  be  looked  at,  or  if  the  near  point  of  accommodation  be  determined  by 
Schemer's  experiment  (p.  747),  for  the  needle  placed  first  horizontally  and 
then  vertically,  the  horizontal  lines  or  needle  will  be  distinctly  visible  at  a 
shorter  distance  from  the  eye  than  the  vertical  lines  or  needle.  Similarly, 
the  vertical  line  must  be  further  from  the  eye  than  a  horizontal  one  if  both 
are  to  be  seen  distinctly  at  the  same  time.  The  cause  of  astigmatism  is,  in 
the  great  majority  of  cases,  the  unequal  curvature  of  the  cornea ;  but  some- 
times the  fault  lies  in  the  lens,  as  was  the  case  with  Young. 

When  the  curvature  of  the  cornea  or  lens  differs  not  in  two  meridians 
only  but  in  several,  irregular  astigmatism  is  the  result.  A  certain  amount 
of  irregular  astigmatism  exists  in  most  lenses,  thus  causing  the  image  of  a 
bright  point,  such  as  a  star,  to  be  not  a  circle  but  a  radiate  figure. 

§  643.  Chromatic  aberration.  The  different  rays  of  the  spectrum  are 
of  different  refrangibility,  those  toward  the  violet  end  of  the  spectrum 
being  brought  to  a  focus  sooner  than  those  near  the  red  end.  This  in  optical 
instruments  is  obviated  by  using  compound  lenses  made  up  of  various  kinds 
of  glass.  In  the  eye  we  have  no  evidence  that  the  lens  is  so  constituted  as 
to  correct  this  fault ;  still  the  total  dispersive  power  of  the  instrument  is  so 
small  that  such  amount  of  chromatic  aberration  as  does  exist  attracts  little 


Diagram  illustrating  Chromatic  Aberration,    h  h  is  the  dioptric  surface ;  h  v  represents  the  blue 
and  h  r  the  red  rays ;  V  is  the  focal  plane  of  the  blue,  R  of  the  red  rays. 

notice.  Nevertheless,  some  slight  aberration  may  be  detected  by  careful 
observation.  When  the  spectrum  is  observed  at  some  distance  the  violet  end 
will  not  be  seen  in  focus  at  the  same  time  as  the  red.  If  a  luminous  point 
be  looked  at  through  a  narrow  orifice  covered  by  a  piece  of  violet  glass, 
which  while  shutting  out  the  yellow  and  green  allows  the  red  and  blue  rays 
to  pass  through,  there  will  be  seen  alternately  an  image  having  a  blue 
centre  with  a  red  fringe,  or  a  red  centre  with  a  blue  fringe,  according  as  the 
image  of  the  point  looked  at  is  thrown  on  one  side  or  other  of  the  true 
focus.  Thus  supposing /(Fig.  173)  to  be  the  plane  of  the  mean  focus  of 
A,  the  violet  rays  will  be  brought  to  a  focus  in  the  plane  V,  and  the  red 


VISUAL  SENSATIONS.  759 

rays  in  the  plane  R.  If  the  rays  be  supposed  to  fall  on  the  retina  between 
V  and/,  the  diverging  or  blue  rays  will  form  a  centre  surrounded  by  the 
still  converging  red  rays;  whereas  if  the  rays  fall  on  the  retina  between/ 
and  R,  the  converging  red  rays  will  form  a  centre  with  the  still  diverging 
blue  rays  forming  a  fringe  around  them.  If  the  rays  fall  on  the  retina  at 
/,  the  two  kinds  of  rays  will  be  mixed  together;  as  will  be  seen  from  the 
figure,  the  circumferential  converging  red  ray  h  r  as  it  cuts  the  plane  of 
the  retina  is,  in  ordinary  vision,  accompanied  by  the  diverging  violet  ray 
h  v,  and  thus  by  a  sort  of  compensation,  we  see  together  even  the  rays  which 
differ  most  in  refraction. 

§  644.  Entoptic  phenomena.  The  various  media  of  the  eye  are  not  uni- 
formly transparent ;  the  rays  of  light  in  passing  through  them  undergo  local 
absorption  and  refraction,  and  thus  various  shadows  are  thrown  on  the  retina, 
of  which  we  become  conscious  as  imperfections  in  the  field  of  vision,  espe- 
cially when  the  eye  is  directed  to  a  uniformly  illuminated  surface.  These 
are  spoken  of  as  entoptic  phenomena,  and  are  very  varied,  many  forms 
having  been  described. 

The  most  common  are  those  caused  by  the  presence  of  floating  bodies  in 
the  vitreous  humor,  the  so-called  muscce  volitantes.  These  are  readily  seen 
when  the  eye  is  turned  toward  a  uniform  surface,  and  are  frequently  very 
troublesome  in  looking  through  a  microscope.  They  are  especially  obvious 
when  divergent  rays  fall  upon  the  eye.  They  assume  the  form  of  rows  and 
groups  of  beads,  of  single  beads,  of  streaks,  patches,  and  granules,  and  may 
be  recognized  by  their  almost  continual  movement,  especially  when  the  head 
or  eye  is  moved  up  and  down.  When  an  attempt  is  made  to  fix  the  vision 
upon  them,  they  immediately  float  away.  Tears  on  the  cornea,  temporary 
unevenness  on  the  anterior  surface  of  the  cornea  after  the  eyelid  has  been 
pressed  on  it,  and  imperfections  in  the  lens  or  its  capsule,  also  give  rise  to 
visual  images.  Not  unfrequeutiy  a  radiate  figure  corresponding  to  the  ar- 
rangement of  the  fibres  of  the  lens  makes  its  appearance. 

Imperfections  in  the  margin  of  the  pupil  appear  in  the  shadow  of  the  iris 
which  bounds  the  field  of  vision ;  and  the  movements  of  the  iris  in  one  eye 
may  be  rendered  visible  by  looking  at  a  bright  point  or  luminous  surface 
through  a  pin-hole  in  a  card  placed  close  in  front  of  the  eye,  in  the 
anterior  focus  in  fact,  and  then  alternately  closing  and  opening  the  other 
eye;  the  field  of  the  first  may  be  observed  to  contract  when  light  enters  and 
to  expand  when  the  light  is  shut  off  from  the  second.  The  media  of  the  eye 
are  fluorescent :  a  condition  which  favors  the  perception  of  the  ultra-violet 
rays.  If  a  white  sheet  or  white  cloud  be  looked  at  in  daylight  through  a 
Nicol's  prism,  a  somewhat  bright  double  cone  or  double  tuft,  with  the  apices 
touching,  of  a  faint  blue  color,  is  seen  in  the  centre  of  the  field  of  vision, 
crossed  by  a  similar  double  cone  of  a  somewhat  darker  yellow  color.  These 
are  spoken  of  as  Haidinger's  brushes ;  they  rotate  as  the  prism  is  rotated, 
and  are  supposed  to  be  due  to  the  unequal'absorption  of  the  polarized  light 
in  the  yellow  spot.  The  prism  must  be  frequently  rotated,  as  when  the 
prism  remains  at  rest  the  phenomena  fade.  Lastly,  the  optical  arrange- 
ments have  a  further  imperfection  in  that  the  dioptric  surfaces  are  not  truly 
centred  on  the  optic  axis. 

VISUAL  SENSATIONS. 

§  645.  Light  falling  on  the  retina  excites  sensory  impulses,  and  these 
passing  up  the  optic  nerve  to  certain  parts  of  the  brain,  produce  changes  in 
certain  cerebral  structures,  and  thus  give  rise  to  what  we  call  a  sensation.  In 
a  sensation  we  ought  to  be  able  to  distinguish  between  the  events  through 


760  SIGHT. 

which  the  impact  of  the  rays  of  light  on  the  retina  is  enabled  to  generate 
sensory  impulses,  and  the  events,  or  rather  series  of  events,  through  which 
these  sensory  impulses  (for,  judging  by  the  analogy  of  motor  nerves,  we  have 
no  reason  to  think  that  they  undergo  any  fundamental  changes  in  passing 
along  the  optic  nerve),  by  the  agency  of  the  cerebral  arrangements,  develop 
into  a  sensation.  Such  an  analysis,  however,  is,  at  present  at  least,  in  most 
particulars,  quite  beyond  our  power ;  and  we  must  therefore  treat  of  the  sen- 
sations as  a  whole,  distinguishing  between  the  peripheral  and  central  phe- 
nomena, on  the  rare  occasions  when  we  are  able  to  do  so. 

The  Origin  of  Visual  Impulses. 

§  646.  Of  primary  importance  to  the  understanding  of  the  way  in  which 
luminous  undulations  give  rise  to  those  nervous  changes  which  pass  along 
the  optic  nerve  as  visual  impulses,  is  the  fact  that  the  rays  of  light  produce 
their  effect  by  acting  not  on  the  optic  nerve  itself  but  on  its  terminal  organs. 
They  pass  through  the  anterior  layers  of  the  retina  apparently  without  in- 
ducing any  effect ;  it  is  not  till  they  have  reached  the  region  of  the  rods  and 
cones  that  they  set  up  the  changes  concerned  in  the  generation  of  visual  im- 
pulses ;  and  the  impulses  here  generated  travel  back  to  the  layer  of  fibres 
in  the  anterior  surface  of  the  retina  and  thence  pass  along  the  optic  nerve. 
That  the  optic  fibres  are  themselves  insensible  to  light  and  that  visual  im- 
pulses begin  in  the  region  of  rods  and  cones  is  shown  by  the  phenomena  of 
the  blind  spot  and  of  Purkinje's  figures  respectively. 

§  647.  Blind  spot.  There  is  one  part  of  the  retina  on  which  rays  of  light 
falling  give  rise  to  no  sensations  ;  this  is  the  entrance  of  the  optic  nerve,  and 
the  corresponding  area  in  the  field  of  vision  is  called  the  blind  spot.  If  the 
visual  axis  of  one  eye,  the  right  for  instance,  the  other  being  closed,  be  fixed 
on  a  black  spot  in  a  white  sheet  of  paper,  and  a  small  black  object,  such  as 
the  point  of  a  quill  pen  dipped  in  ink,  be  moved  gradually  sideways  over 
the  paper  away  to  the  outside  of  the  field  of  vision,  at  a  certain  distance  the 
black  point  of  the  quill  will  disappear  from  view.  On  continuing  the  move- 
ment still  further  outward  the  point  will  again  come  into  view  and  continue 
in  sight  until  it  is  lost  in  the  periphery  of  the  field  of  vision.  If  the  pen  be 
used  to  make  a  mark  on  the  paper  at  the  moment  when  it  is  lost  to  view, 
and  at  the  moment  when  it  comes  into  sight  again,  and  if  similar  marks  be 
made  along  the  other  meridians  as  well  as  the  horizontal,  an  irregular  out- 
line will  be  drawn  circumscribing  an  area  of  the  field  of  vision  within  which 
rays  of  light  produce  no  visual  sensation.  This  is  the  blind  spot.  The 
dimensions  of  the  figure  drawn  vary,  of  course,  with  the  distance  of  the  paper 
from  the  eye.  If  this  distance  be  known,  the  size  as  well  as  the  position  of 
the  area  of  the  retina  corresponding  to  the  blind  spot  may  be  calculated 
from  the  diagrammatic  eye  (p.  745).  The  position  exactly  coincides  with 
the  entrance  of  the  optic  nerve,  and  the  dimensions  (about  1.5  mm.  diameter) 
also  correspond.  While  drawing  the  outline  as  above  directed  the  indica- 
tions of  the  large  branches  of  the  retinal  vessels  as  they  diverge  from  the 
entrance  of  the  nerve  can  frequently  be  recognized.  The  existence  of  the 
blind  spot  is  also  shown  by  the  fact  that  an  image  of  light,  sufficiently  small, 
thrown  upon  the  optic  nerve  by  means  of  the  ophthalmoscope,  gives  rise  to 
no  sensations. 

The  existence  of  the  blind  spot  proves  that  the  optic  fibres  themselves  are 
insensible  to  light;  it  is  only  through  the  agency  of  the  retinal  expansion 
that  these  can  be  stimulated  by  luminous  vibrations. 

§  648.  Purkinje's  figures.  If  one  enters  a  dark  room  with  a  candle,  and 
while  looking  at  a  plain  (not  parti-colored)  wall*  moves  the  candle  up  and 


VISUAL  SENSATIONS. 


761 


down,  holding  it  on  a  level  with  the  eyes  by  the  side  of  the  head,  there  will 
appear  in  the  h'eld  of  vision  of  the  eye  of  the  same  side,  projected  on  the 
wall,  an  image  of  the  retinal  vessels,  quite  similar  to  that  seen  on  looking 
into  an  eye  with  the  ophthalmoscope.  The  field  of  vision  is  illuminated  with 
a  glare,  and  on  this  the  branched  retinal  vessels  appear  as  shadows.  In  this 
mode  of  experimenting  the  light  enters  the  eye  through  the  cornea,  and  an 
image  of  the  candle  is  formed  on  the  nasal  side  of  the  retina ;  and  it  is  the 
light  emanating  from  this  image  which  throws  shadows  of  the  retinal  vessels 
on  to  the  rest  of  the  retina.  A  far  better  method  is  for  a  second  person  to 
concentrate  the  rays  of  light,  with  a  lens  of  low  power,  on  to  the  outside  of 
the  sclerotic  just  behind  the  cornea ;  the  light  in  this  case  emanates  from  the 
illuminated  spot  on  the  sclerotic  and  passing  straight  through  the  vitreous 
humor  throws  a  direct  shadow  of  the  vessels  on  to  the  retina.  Thus  the  rays 
passing  through  the  sclerotic  at  b,  Fig.  174,  in  the  direction  b  v,  will  throw  a 
shadow  of  the  vessel  »  on  to  the  retina  at  /? ;  this  will  appear  as  a  dark  line 
at  B  in  the  glare  of  the  field  of  vision.  This  proves  that  the  structures  in 
which  visual  impulses  originate  must  lie  behind  the  retinal  vessels,  otherwise 
the  shadows  of  these  could  not  be  perceived. 

If  the  light  be  moved  from  b  to  a,  the  shadow  on  the  retina  will  move 
from  ft  to  «,  and  the  dark  line  in  the  field  of  vision  will  move  from  B  to  A. 
If  the  distance  B  A  be  measured  when  the  whole  image  is  projected  at  a 

FIG.  175. 


Diagram  illustrating  the  Formation  of 
Purkinje's  Figures  when  the  Illumination 
is  Directed  through  the  Sclerotic. 


Diagram  illustrating  the  Formation  of 
Purkinje's  Figures  when  the  Illumination 
is  Directed  through  the  Cornea. 


known  distance,  k  B  from  the  eye,  k  being  the  optical  centre,1  then,  knowing 
the  distance  k  ft  in  the  diagrammatic  eye,"the  distance  /3  a  can  be  calculated. 
But  if  the  distance  ft  a  be  thus  estimated,  and  the  distance  b  a  be  directly 
measured,  the  distances  /?  v,  a  v,bv,a  v  can  be  calculated,  and  if  the  appearance 
in  the  field  of  vision  is  really  caused  by  the  shadow  of  v  falling  on  /?,  these 
distances  ought  to  correspond  to  the  distances  of  the  retinal  vessels  v  from  the 

1  For  the  properties  of  the  optical  centre  we  must  refer  the  reader  to  the  various 
treatises  on  optics.  The  optical  centre  of  a  lens  is  the  point  through  which  all  the  princi- 
pal rays,  of  the  various  pencils  of  rays  falling  on  the  lens,  pass.  The  diagrammatic  eye 
of  Listing  (p.  745)  has  two  optical  centres,  but  these  may,  without  serious  error,  be  further 
reduced  for  practical  purposes  to  one  lying  in  the  lens  near  its  posterior  surface,  at  about 
15  mm.  distance  from  the  retina. 


762  SIGHT. 

sclerotic  b  on  the  one  hand,  and  from  that  part  of  the  retina  /?  where  visual 
impressions  begin,  on  the  other.  H.  Miiller  found  that  the  distance  /?  v  thus 
calculated  corresponded  to  the  distance  of  the  retinal  vessels  from  the  layer 
of  rods  and  cones.  Thus  Purkinje's  figures  prove  in  the  first  place  that  the 
sensory  impulses  which  form  the  commencement  of  visual  sensations  origi- 
nate in  some  part  of  the  retina  behind  the  retinal  vessels,  i.  e.,  somewhere 
between  them  and  the  choroid  coat ;  and  H.  Miiller's  calculations  go  far  to 
show  that  they  originate  at  the  most  posterior  or  external  part  of  the  retina, 
viz.,  the  layer  of  rods  and  cones.  It  must  be  admitted,  however,  that  H. 
Miiller's  results  were  not  sufficiently  exact  to  allow  any  great  stress  to  be 
placed  on  this  argument. 

In  the  second  method  of  experimenting  the  image  always  moves  in  the 
same  direction  as  the  light,  as  it  obviously  must  do.  In  the  first  method, 
where  the  light  enters  through  the  cornea,  the  image  moves  in  the  same 
direction  as  the  light  when  the  light  is  moved  from  right  to  left,  provided 
the  movement  does  not  extend  beyond  the  middle  of  the  cornea,  but  in  the 
opposite  direction  to  the  light  when  the  latter  is  moved  up  and  down.  In 
Fig.  175,  which  represents  a  horizontal  section  of  an  eye,  if  a  be  moved  to  «, 
b  will  move  to  /?,  the  shadow  on  the  retina  c  to  y,  and  the  image  d  to  <5.  If, 
on  the  other  hand,  a  be  supposed  to  move  above  the  plane  of  the  paper,  b 
will  move  below,  in  consequence  c  will  move  above,  and  d  will  appear  to 
move  below,  i.  e.,  d  will  sink  as  a  rises. 

It  is  desirable  in  these  cases  to  move  the  light  to  and  fro,  especially 
in  the  first  method,  as  the  retina  soon  becomes  tired,  and  the  image 
fades  away.  Some  observers  can  recognize  in  the  axis  of  vision  a  faint 
shadow  corresponding  to  the  edge  of  the  depression  of  the  fovea  centralis. 

§  649.  The  retinal  vessels  may  also  be  rendered  visible  by  looking 
through  a  small  orifice,  such  as  a  pin-hole  in  a  card  placed  close  to  the  eye, 
at  a  bright  field  such  as  the  sky,  and  moving  the  orifice  very  rapidly  from 
side  to  side  or  up  and  down.  If  the  movement  be  from  side  to  side  the  ves- 
sels which  run  vertically  will  be  seen  ;  if  up  and  down,  the  horizontal  vessels. 
The  fine  capillary  vessels  are  seen  more  easily  in  this  way  than  by  Purkinje's 
method.  The  same  appearances  may  also  be  produced  by  looking  through 
a  microscope  from  which  the  objective  has  been  removed  and  the  eye-piece 
only  left  (or  in  which,  at  least,  there  is  no  object  distinctly  in  focus  in  the 
field),  and  moving  the  head  rapidly  from  side  to  side  or  backward  and  for- 
ward. Or  the  microscope  itself  may  be  moved  ;  a  circular  movement  of  the 
field  will  then  bring  both  the  vertically  and  horizontally  directed  vessels  into 
view  at  the  same  time. 

§  650.  The  photo-chemistry  of  the  retina.  In  seeking  to  understand  how7 
it  is  that  rays  of  light  falling  upon  the  region  of  the  rods  and  cones  can  give 
rise  to  sensory,  visual  impulses  in  the  optic  nerve,  we  may  adopt  one  or  other 
of  two  views.  On  the  one  hand,  we  may  suppose  that  the  vibrations  of  the 
ether  are  able,  through  the  means  of  the  retinal  apparatus  of  the  rods  and 
cones  for  example,  to  give  rise  in  some  way  or  other  to  molecular  vibrations 
which  are  the  beginning  of  the  nervous  impulses  in  the  optic  nerve.  No 
satisfactory  explanation  of  how  such  a  change  can  be  brought  about  has 
been  offered,  and  indeed  the  difficulties  of  such  a  conception  are  very  great. 
On  the  other  hand,  we  may  more  naturally  turn  to  a  chemical  explanation. 
We  are  familiar  with  the  fact  that  rays  of  light  are  able  to  bring  about  the 
decomposition  of  very  many  chemical  substances,  and  we  accordingly  speak 
of  these  substances  as  being  sensitive  to  light.  All  the  facts  dwelt  on  in  this 
book  illustrate  the  great  complexity  and  corresponding  instability  of  the 
composition  of  protoplasm.  And  we  might  reasonably  suppose  that  proto- 
plasm itself  would  be  sensitive  to  light ;  that  is  to  say,  that  rays  of  light 


VISUAL  SENSATIONS.  763 

falling  on  even  undifferentiated  protoplasm  might  set  up  a  decomposition  of 
that  protoplasm,  and  so  inaugurate  a  molecular  disturbance ;  in  other  words, 
that  light  might  act  as  a  direct  stimulus  to  protoplasm.  As  a  matter  of  fact, 
however,  such  evidence  as  we  at  present  possess  goes  to  show  that  native 
undifferentiated  protoplasm  is,  as  a  rule,  not  sensitive  to  light  (that  is,  to 
those  particular  waves  which,  when  they  fall  on  our  retina  give  rise  in  us  to 
the  sensation  of  light),  though  in  the  case  of  some  lowly  organisms,  whose 
protoplasm  exhibits  very  little  differentiation  and  in  particular  contains  no 
pigment,  a  sensitiveness  to  light  has  been  observed.  Nor  can  we  be  surprised 
at  this  indifference  of  protoplasm  when  we  reflect  that  what  we  may  call  pure 
protoplasm  is  remarkable  for  its  transparency,  that  is  to  say,  the  rays  of  light 
pass  through  it  with  the  slightest  possible  absorption.  But  in  order  that 
light  may  produce  chemical  effects,  it  must  be  absorbed ;  it  must  be  spent 
in  doing  the  chemical  work.  Accordingly,  the  first  step  toward  the  forma- 
tion of  an  organ  of  vision  is  the  differentiation  of  a  portion  of  protoplasm 
into  a  pigment  at  once  capable  of  absorbing  light  and  sensitive  to  light, 
i.  e.j  undergoing  decomposition  upon  exposure  to  light.  An  organism,  a  por- 
tion of  whose  protoplasm  had  thus  become  differentiated  into  such  a  pig- 
ment, would  be  able  to  react  toward  light.  The  light  falling  on  the  organ- 
ism would  be  in  part  absorbed  by  the  pigment,  and  the  rays  thus  absorbed 
would  produce  a  chemical  action  and  set  free  chemical  substances  which 
before  were  not  present.  We  have  only  to  suppose  that  the  chemical  sub- 
stances are  of  such  a  nature  as  to  act  as  a  stimulus  to  the  protoplasm  of 
other  parts  of  the  organism  (and  we  have  manifold  evidence  of  the  exqui- 
site sensitiveness  of  protoplasm  in  general  to  chemical  stimuli),  in  order  to 
see  how  rays  of  light  falling  on  the  organism  might  excite  movements  in  it, 
or  modify  movements  which  were  being  carried  on,  or  might  otherwise  affect 
the  organism  in  whole  or  in  part. 

Such  considerations  as  the  foregoing  may  be  applied  to  even  the  complex 
organ  of  vision  of  the  higher  animals.  If  we  suppose  that  the  actual  ter- 
minations of  the  optic  nerve  are  surrounded  by  substances  sensitive  to  light, 
then  it  becomes  easy  to  imagine  how  light  falling  on  these  sensitive  sub- 
stances should  set  free  chemical  bodies  possessed  of  the  property  of  acting 
as  stimuli  to  the  actual  nerve-endings,  and  thus  give  rise  to  visual  impulses 
in  the  optic  fibres.  We  say  "  easy  to  imagine,"  but  we  are  at  present  far 
from  being  able  to  give  definite  proofs  that  such  an  explanation  of  the 
origin  of  visual  impulses  is  the  true  one,  probable  and  enticing  as  it  may 
appear. 

§  651.  One  of  the  most  striking  features  in  the  structure  of  the  retina  is 
the  abundance  of  black  pigment  in  the  retinal,  or  as  it  is  sometimes  called 
choroidal,  epithelium.  It  is  difficult  to  suppose  that  the  sole  function  of 
this  pigment  is  to  absorb  the  superfluous  rays  of  light,  and  that  the  rays 
thus  absorbed  are  put  to  no  use,  but  simply  wasted.  And  indeed  it  has  been 
shown  that  the  pigment  is  sensitive  to  light ;  but  the  changes  in  it  induced 
by  light  are  excessively  slow.  Moreover,  its  presence  cannot  be  of  funda- 
mental importance,  since  vision  is  not  only  possible  but  fairly  distinct  with 
albinos,  in  which  this  pigment  is  absent. 

Then  again,  in  the  vast  majority  of  vertebrate  animals,  the  outer  limbs 
of  the  rods  are  suffused  with  a  purplish-red  pigment,  the  so-called  visual 
purple,  which  is  so  eminently  sensitive  to  light  that  images  of  external  objects 
may  by  appropriate  means  be  photographed  in  it  on  the  retina.  When  the 
eye  of  a  frog  or  of  a  rabbit  is  examined  in  an  ordinary  way,  with  full  expos- 
ure to  light,  the  retina  appears  colorless.  But  if  the  eye  be  kept  in  the  dark 
for  some  time  before  it  is  examined,  the  retina,  if  removed  rapidly,  will  be 
found  to  be  of  a  beautiful  purplish-red  color.  Upon  exposure  to  light  the 


764  SIGHT. 

color  changes  to  yellow  and  then  fades  away,  leaving,  however,  the  retina 
not  only  white,  but  more  opaque  than  it  was  before.  Upon  examination 
with  the  microscope  it  is  found  that  the  purple  color  is  confined  exclusively 
to  the  rods  and  to  the  outer  limbs  of  the  rods,  the  inner  limbs  being  wholly 
devoid  of  it. 

§  652.  The  color  of  the  rods  is  due  to  the  presence  of  a  distinct  pigment, 
the  "  visual  purple,"  diffused  through  the  substance  of  the  outer  limbs  ;  and 
this  may  be  extracted  from  the  rods  by  dissolving  these  in  an  aqueous  solution 
of  bile-salts.  A  clear  purple  solution  is  thus  obtained,  which  is  capable  of 
being  bleached  by  the  action  of  light,  and  in  its  general  features  and  be- 
havior is  similar  to  the  pigment  as  it  naturally  exists  in  the  retina. 

Visual  purple  is  found,  as  we  have  said,  exclusively  in  the  outer  limbs  of 
the  rods ;  it  has  never  yet  been  found  in  the  cones,  and  it  is  accordingly 
absent  from  the  retinas  (such  as  those  of  snakes),  which  are  composed  of 
cones  only,  and  from  the  macula  lutea  and  fovea  centralis  of  the  retinas  of 
man  and  the  ape.  The  intensity  of  the  coloration  varies  in  different  animals, 
and  the  retinas  even  of  some  animals  possessing  rods  (bat,  dove,  hen)  seem 
to  be  wholly  devoid  of  the  visual  purple ;  it  is  generally  well  marked  in 
retinas  in  which  the  outer  limbs  of  the  rods  are  well  developed.  Its  absence 
or  presence  is  not  dependent  on  nocturnal  habits,  since  the  intense  color  of 
the  retina  of  the  owl  is  in  strong  contrast  to  the  absence  of  color  in  the  bat 
It  has  been  found  in  the  retina  of  the  embryo. 

§  653.  The  visual  purple  is  bleached  not  only  by  white,  but  also  b] 
monochromatic,  light.  Of  the  various  prismatic  rays  the  most  active  are  tin 
greenish-yellow  rays,  those  to  the  blue  side  of  these  coming  next,  the  leas* 
active  being  the  red.  Now  it  is  precisely  the  greenish-yellow  rays  which  ar< 
most  readily  absorbed  by  the  color  itself.  A  natural  colored  retina  or  a  solu 
tion  of  visual  purple  gives  a  diffuse  spectrum  without  any  defined  absorption 
bands,  and  according  to  the  amount  of  coloring  material  through  which  tin 
light  passes,  absorption  is  seen  either  to  be  limited  to  the  greenish-yellow 
part  of  the  spectrum  or  to  spread  thence  toward  the  blue,  and,  to  a  mud. 
less  extent,  toward  the  red.  Thus  the  various  prismatic  rays  produce  *. 
photo-chemical  effect  on  the  visual  purple  in  proportion  as  they  are  absorber 
by  it.  Under  the  action  of  light  the  visual  purple,  whether  in  solution  o* 
in  its  natural  condition  in  the  rods,  passes  through  a  purplish-orange  to  a 
yellow,  and  finally  becomes  colorless ;  and  we  appear  to  be  justified  ih 
speaking  of  a  "  visual  yellow "  and  "  visual  white "  as  products  of  tin 
photo-chemical  changes  undergone  by  the  visual  purple. 

For  the  restoration  of  the  visual  purple,  after  it  has  been  destroyed  by 
light,  the  maintenance  of  the  circulation  of  the  blood  through  the  tissues  of 
the  eye  is  not  essential.  The  choroidal  epithelium  has  by  itself,  provided 
that  it  still  retains  its  tissue  life,  the  power  of  regenerating  the  purple.  If 
a  portion  of  the  retina  of  an  excised  eye  be  raised  from  its  epithelial  bed, 
bleached,  and  then  carefully  restored  to  its  natural  position,  the  purple  will 
return  if  the  eye  be  kept  in  the  dark.  The  choroidal  epithelium  may,  in 
fact,  be  spoken  of  as  a  "  purpurogenous  "  membrane. 

§  654.  If  the  image  of  some  bright  object,  such  as  a  lamp  or  a  window, 
be  thrown  on  to  the  retina,  either  of  an  eye  in  its  natural  position  or  of  one 
recently  excised,  care  having  been  taken  to  keep  the  retina  for  some  time 
previous  away  from  any  rays  of  light,  the  portion  of  the  retina  on  which 
the  rays  have  fallen  will  be  found  to  be  bleached,  the  rest  of  the  retina  re- 
maining purple.  In  fact,  an  "  optogram  "  of  external  objects  may  thus  be 
obtained ;  and  if  the  retina  be  removed  and  treated  with  a  four  per  cent, 
solution  of  potash  alum  before  the  choroidal  epithelium  has  had  time  to 
obliterate  the  bleaching  effects,  the  retina  may  remain  permanently  in  that 


VISUAL  SENSATIONS.  765 

condition ;    the  photo-chemical  effect  may,  as  the  photographers  say,  be 
"  fixed." 

It  seemed  very  tempting,  especially  upon  the  first  discovery  of  it,  to  sup- 
pose that  this  visual  purple  is  directly  concerned  in  vision.  If  we  suppose 
that  visual  purple  itself  is  inert  toward  the  endings  of  the  optic  nerve,  but 
that  either  visual  yellow  or  visual  white,  i.  e.,  some  product  of  the  action  of 
light  on  visual  purple,  may  act  as  a  stimulus  to  those  endings,  the  way  seems 
opened  to  understanding  how  rays  of  light  can  give  rise  to  sensory  impulses 
in  the  optic  nerve.  Unfortunately  visual  purple  is  absent  from  the  cones, 
and  from  the  fovea  centralis,  which,  as  we  shall  see,  is  the  region  of  distinct 
vision ;  it  is  further  entirely  wanting  in  some  animals  which  undoubtedly 
see  very  well ;  and,  lastly,  animals,  such  as  frogs,  naturally  possessing  the 
pigment,  continue  to  see  very  well,  and  even  apparently  to  see  colors,  when 
their  visual  purple  has  been  absolutely  bleached,  as  it  may  be  by  prolonged 
exposure  of  the  eyes  to  strong  light.  We  cannot  therefore,  at  present  at 
least,  explain  the  origin  of  visual  impulses  by  the  help  of  visual  purple.  At 
the  same  time  its  history  suggests  that  some  substances,  sensitive  like  it  to 
light,  but  unlike  it,  colorless  and  therefore  escaping  observation,  may  exist, 
and  by  photo-chemical  changes  be  the  means  of  exciting  the  optic  nerves. 
And,  as  we  shall  see  later  on,  one  theory  of  color  vision  is  based  on  the 
assumption  that  vision  is  carried  on  in  some  way  or  other  by  changes  in 
what  may  be  called  the  visual  substances  present  in  the  retina,  these  sub- 
stances being  used  up  and  regenerated  as  vision  is  going  on. 

But  even  admitting  as  probable  the  existence  of  these  sensitive  visual  sub- 
stances, the  changes  in  which  lead  to  stimulation  of  the  real  endings  of  the 
retinal  nervous  mechanism,  we  cannot  at  present  state  anything  definite  con- 
cerning those  nerve-endings  or  the  manner  of  their  stimulation.  It  may  be 
that  even  the  outer  limbs  of  the  rods  and  cones,  in  spite  of  the  apparent 
break  of  continuity  between  the  outer  and  inner  limbs,  are  really  nervous 
in  nature.  It  may  be,  on  the  other  hand,  that  the  outer  limbs  are  either 
purely  dioptric  in  function,  or  are  associated  with  the  sensitive  visual  sub- 
stances in  such  a  way  that  the  purely  nervous  structures  must  be  considered 
as  extending  no  further  at  least  than  the  inner  limbs.  We  cannot  as  yet 
make  any  definite  statement  in  the  one  direction  or  the  other. 

§  655.  In  connection  with  the  origin  of  visual  impulses,  we  may  per- 
haps call  attention  to  the  remarkable  changes  which  the  cells  of  the  retinal 
pigment  epithelium  undergo  under  the  influence  of  light.  When  an  eye  has 
been  shut  off  from  all  light  for  some  little  time  the  pigment  is  concentrated 
in  the  bodies  of  the  cells,  and  the  remarkable  filamentous  processes  of  the 
cells,  with  the  pigment  granules  or  crystals  which  they  carry,  extend  a  slight 
distance  only  between  the  limbs  of  the  rods  and  cones  (about  one-third  down 
the  length  of  the  outer  limbs  of  the  rods).  Under  the  influence  of  light 
these  processes,  loaded  with  pigment,  thrust  themselves  a  much  longer  way 
down  toward  the  external  limiting  membrane  ;  in  consequence  a  considerable 
quantity  of  pigment  is  found  massed  between  the  outer  and  even  the  inner 
limbs  of  the  rods  and  cones ;  indeed,  the  outer  limbs  of  the  rods  swelling  at 
the  same  time  become  jammed,  as  it  were,  between  the  masses  of  pigment, 
causing  the  epithelial  layer  to  adhere  very  closely  to  the  layer  of  rods  and 
cones. 

§  656.  The  retina  and  optic  nerve,  like  other  nervous  structures,  develop 
electric  currents,  which  may  be  spoken  of  as  currents  of  rest  and  currents 
of  action.  They  may  be  shown  by  placing  one  electrode  on  the  retina  of  a 
bisected  eye,  or  on  the  cornea  of  a  whole  one,  and  the  other  on  the  optic 
nerve,  or  hind  part  of  the  eyeball,  or  even  on  some  distant  part  of  the  body. 
They  are  also  manifested  by  the  isolated  retina  itself.  The  phenomena 


766  SIGHT 

appear  somewhat  complicated  by  the  appearance  now  of  positive,  now  of 
negative,  variations ;  but  this  fact  comes  out  clearly,  that  the  incidence  of 
light  on  the  irritable  retina  develops  an  electric  change,  the  magnitude  of 
which  is  to  a  certain  extent  proportionate  to  the  intensity  of  the  light  acting 
as  a  stimulus.  The  changes  accordingly  diminish  and  cease  to  appear  as 
the  retina  gradually  loses  its  irritability  after  death.  We  may  add  that 
these  electric  phenomena  appear  to  be  quite  independent  of  the  condition 
of  the  visual  purple. 

Simple  Sensations. 

§  657.  Relations  of  the  sensation  to  the  stimulus.  If  we  put  aside  for  the 
present  all  questions  of  color,  we  may  say  that  light,  viewed  as  a  stimulus 
affecting  the  retina,  varies  in  intensity,  that  is,  in  the  energy  of  the  luminous 
vibrations  as  manifested  by  their  amplitude,  and  in  duration,  that  is,  in  the 
length  of  time  a  succession  of  waves  continues  to  fall  upon  the  retina.  The 
effect  of  the  light  will  also  depend  on  the  extent  of  retinal  surface  exposed 
to  the  luminous  vibrations  at  the  same  time.  Taking  a  luminous  point,  in 
order  to  eliminate  the  latter  circumstance,  we  may  make  the  following  state- 
ments : 

The  sensation  has  a  duration  much  greater  than  that  of  the  stimulus,  and 
in  this  respect  is  comparable  to  a  muscular  contraction  caused  by  such  a 
stimulus  as  a  single  induction-shock.  The  sensation  of  a  flash  of  light,  for 
instance,  lasts  for  a  much  longer  time  than  that  during  which  luminous 
vibrations  are  falling  on  the  retina.  Hence,  when  two  stimuli,  such  as  two 
flashes  of  light,  follow  each  other  at  a  sufficiently  short  interval,  the  two 
sensations  are  fused  into  one ;  and  a  luminous  point  moving  rapidly  round 
in  a  circle  gives  rise  to  the  sensation  of  a  continuous  circle  of  light.  This 
again  is  quite  comparable  to  muscular  tetanus.  The  interval  at  which  fusion 
takes  place,  that  is,  the  interval  between  successive  stimuli  which  must  be 
exceeded  in  order  that  successive  distinct  sensations  may  be  produced,  varies 
according  to  the  intensity  of  the  light,  being  shorter  with  the  stronger  light ; 
with  a  faint  light  it  is  about  TV  second,  with  a  strong  light  ^  or  -fa  second. 
This  may  be  shown  by  rotating  rapidly  before  the  eye  a  disc  arranged  with 
alternate  black  and  white  sectors  of  equal  width.  With  a  faint  illumination 
the  flickering,  indicative  of  the  successive  sensations  from  the  white  sectors 
not  being  completely  fused,  ceases  when  the  rotation  becomes  so  rapid  that 
each  pair  of  black  and  white  sectors  takes  only  ^Q  second  in  passing  before 
the  eye.  When  a  brighter  illumination  is  used  the  rapidity  must  be  in- 
creased before  the  flickering  disappears.  That  part  of  the  sensation  which 
is  recognized  as  lasting  after  the  cessation  of  the  stimulus  is  frequently 
spoken  of  as  the  "  after-image." 

Though  the  duration  of  the  after-image  is  longer  with  the  stronger  light 
(that  caused  by  looking  even  momentarily  at  the  sun  lasting  for  some  time), 
the  commencement  of  the  decline  of  the  sensation  begins  relatively  earlier, 
hence  the  greater  difficulty  in  the  complete  fusion  of  successive  sensations 
with  the  stronger  light.  The  interval  at  which  fusion  takes  place  differs 
with  different  colors,  being  shortest  with  yellow7,  intermediate  with  red,  and 
longest  with  blue. 

The  duration  of  a  stimulus  necessary  to  call  forth  a  sensation  is  exceed- 
ingly short ;  thus  the  shortest  possible  flash,  such  as  that  of  an  electric 
spark,  gives  rise  to  a  sensation  of  light. 

Objects  in  motion  when  illuminated  by  a  single  electric  spark  appear 
motionless,  the  stimulus  of  the  light  reflected  from  them  ceasing  before  they 
can  make  an  appreciable  change  in  their  position.  When  a  moving  body  is 


VISUAL  SENSATIONS.  767 

illuminated  by  several  rapid  flashes  in  succession,  several  distinct  images 
corresponding  to  the  positions  of  the  body  during  the  several  flashes  are 
generated  ;  the  images  of  the  body  corresponding  to  the  several  flashes  fall 
on  different  parts  of  the  retina. 

§  658.  The  intensity  of  the  sensation  varies  with  the  luminous  intensity 
of  the  object ;  a  wax  candle  appears  brighter  than  a  rushlight.  The  ratio, 
however,  of  the  sensation  to  the  stimulus  is  not  a  simple  one.  If  the  lumi- 
nosity of  an  object  be  gradually  increased  from  a  very  feeble  stage  to  a  very 
bright  one,  it  will  be  found  that  though  the  corresponding  sensations  like- 
wise gradually  increase,  the  increments  of  the  sensations  due  to  increments 
of  the  luminosity  gradually  diminish ;  and  at  last  an  increase  of  the  lumi- 
nosity produces  no  appreciable  increase  of  sensation ;  a  light,  when  it  reaches 
a  certain  brightness,  appears  so  bright  that  we  cannot  tell  when  it  becomes 
any  brighter.  Hence  it  is  much  easier  to  distinguish  a  slight  difference  of 
brightness  between  two  feeble  lights  than  the  same  difference  between  two 
bright  lights ;  we  can  easily  tell  the  difference  between  a  rushlight  and 
a  wax  candle  ;  but  two  suns,  or  even  two  bright  lamps,  one  of  which  differed 
from  the  other  merely  by  just  the  number  of  luminous  rays  which  a  wax 
candle  emits  in  addition  to  those  sent  forth  by  a  rushlight,  would  appear  to 
us  to  have  exactly  the  same  brightness.  In  a  darkened  room  an  object 
placed  before  a  candle  will  throw  what  we  consider  a  deep  shadow  on  a 
sheet  of  paper  or  any  white  surface.  If,  however,  sunlight  be  allowed  to 
fall  on  the  paper  at  the  same  time  from  the  opposite  side  the  shadow  is  no 
longer  visible.  The  difference  between  the  total  light  reflected  from  that 
part  of  the  paper  where  the  shadow  was,  and  which  is  illuminated  by  the 
sun  alone,  and  that  reflected  from  the  rest  of  the  paper  which  is  illuminated 
by  the  candle  as  well  as  by  the  sun,  remains  the  same  ;  yet  we  can  no  longer 
appreciate  that  difference. 

On  the  other  hand,  if  using  two  rushlights  we  throw  two  shadows  on  a 
white  surface  and  move  one  rushlight  away  until  the  shadow  caused  by  it 
ceases  to  be  visible,  and,  having  noted  the  distance  to  which  it  had  to  be 
moved,  repeat  the  same  experiment  with  two  wax  candles,  we  shall  find 
that  the  wax  candle  has  to  be  moved  just  as  far  as  the  rushlight.  In  fact, 
it  is  found  by  careful  observation  that,  within  tolerably  wide  limits,  the 
smallest  difference  of  light  which  we  can  appreciate  by  visual  sensations  is 
a  constant  fraction  (about  T-^otn)  °f  tne  total  luminosity  employed.  The 
same  law  holds  good  with  regard  to  the  other  senses  as  well.  The  smallest 
difference  in  length  we  can  detect  between  two  lines,  one  an  inch  long  and 
the  other  a  little  less  than  an  inch,  is  the  same  fraction  of  an  inch  that  the 
smallest  difference  in  length  we  can  detect  between  a  line  a  foot  long  and 
one  a  little  less  than  a  foot,  is  of  a  foot.  Put  in  a  more  general  form,  then, 
the  law,  which  is  often  called  Weber's  law,1  is  as  follows :  When  a  stimulus 
is  continually  increased,  the  increase  of  stimulus  necessary  to  call  forth  the 
smallest  appreciable  increase  of  sensation  always  bears  the  same  proportion 
to  the  whole  stimulus. 

§  659.  Distinction  and  fusion  of  sensations.  When  light  falls  on  a  large 
portion  of  the  retina  the  total  sensation  produced  is  greater  in  amount  than 
when  a  small  portion  only  of  the  retina  is  affected ;  a  large  piece  of  white 
paper  produces  a  greater  total  effect  on  our  consciousness  than  a  small  one, 
though,  if  the  surfaces  be  uniformly  and  equally  illuminated,  the  intensity  of 
the  sensation  is  in  each  case  the  same  ;  the  small  piece  of  paper  appears  as 
bright  or  as  "  white"  as  the  large  one.  If  the  images  of  two  luminous  ob- 
jects fall  on  the  retina  at  sufficient  distances  apart,  the  consequent  sensations 

1  From  which  Fechner,  by  an  assumption,  obtained  a  mathematical  expression  or  for- 
mula, which  is  sometimes  incorrectly  spoken  of  as  Fechner's  law. 


768  SIGHT. 

are  distinct,  and  the  intensity  of  each  sensation  will  depend  solely  upon  the 
luminosity  of  the  corresponding  object.  If,  however,  the  two  objects  are 
made  to  approach  each  other,  a  point  will  be  reached  at  which  the  two  sen- 
sations are  fused  into  one.  When  this  occurs  the  intensity  of  the  total 
sensation  produced  will  be  greater  than  that  of  either  of  the  sensations 
caused  by  the  single  objects.  A  number  of  luminous  points  scattered  over 
a  wide  surface  would  appear  each  to  have  a  certain  brightness ;  each  would 
give  rise  to  a  sensation  of  a  certain  intensity.  If  they  were  all  gathered 
into  one  spot,  that  spot  would  appear  far  brighter  than  any  of  the  previous 
points  ;  the  intensity  of  the  sensation  would  be  greater.  We  may  therefore 
suppose  the  retina  to  be  divided  into  areas  corresponding  to  sensational 
units.  If  the  images  from  two  luminous  objects  fall  on  separate  visual  areas, 
if  we  may  so  call  them,  two  distinct  sensations  will  be  produced ;  if,  on  the 
contrary,  they  both  fall  on  the  same  visual  area,  one  sensation  only  will  be 
produced.  Where  the  sensations  are  separate,  the  intensity  of  the  one 
(with  exceptions  hereafter  to  be  mentioned)  is  not  affected  by  the  presence  of 
the  other ;  but  where  they  become  fused  the  intensity  of  the  united  sensa- 
tions is  greater  than  either  of,  though  not  equal  to  the  sum  of,  the  single 
sensations.  The  existence  of  these  sensational  units  is  the  basis  of  distinct 
vision.  When  we  speak  of  the  smallest  size  visible  or  distinguishable,  we 
are  referring  to  the  dimensions  of  the  retinal  areas  corresponding  to  these 
sensational  units.  The  retinal  area  must  be  carefully  distinguished  from  the 
sensational  unit,  for  the  sensation  is,  as  we  have  seen,  a  process  whose  arena 
stretches  from  the  retina  to  certain  parts  of  the  brain,  and  the  circumscrip- 
tion of  the  sensational  unit,  though  it  must  begin  as  a  retinal  area,  must  also 
be  continued  as  a  cerebral  area  in  the  brain,  the  latter  corresponding  to, 
and  being  as  it  were  the  projection  of,  the  former.  With  most  people  two 
stars  appear  as  a  single  star  when  the  distance  between  them  subtends  an 
angle  of  less  than  60  seconds ;  and  the  best  eyes  generally  fail  to  distinguish 
two  parallel  white  streaks  when  the  distance  between  the  two,  measured 
from  the  middle  of  each,  subtends  an  angle  of  less  than  73  seconds.  Some, 
however,  can  distinguish  objects  50  seconds  distant  from  each  other.  An 
angle  of  73  seconds  in  an  object  corresponds  in  the  diagrammatic  eye  (see 
p.  745)  to  the  length  of  5.36  //  in  the  retinal  image,1  and  one  of  50  seconds 
to  3.65  <j.. 

§  660.  In  the  human  eye  50  cones  may  be  counted  along  a  line  of  200  />. 
in  length  drawn  through  the  centre  of  the  yellow  spot ;  this  would  give  4  PL 
for  the  distance  between  the  centres  of  two  adjoining  cones  in  the  yellow 
spot,  the  average  diameter  of  a  cone  at  its  widest  part  being  3  //,  and  there 
being  slight  intervals  between  neighboring  cones.  Hence,  if  we  take  the 
centre  of  a  cone  as  the  centre  of  an  anatomical  retinal  area,  these  anatomical 
areas  correspond  very  fairly  to  the  physiological  visual  areas  as  determined 
above.  That  is  to  say,  if  two  points  of  the  retinal  image  are  less  than  4  /;. 
apart,  they  may  both  lie  within  the  area  of  a  single  cone;  and  it  is  just 
when  they  are  less  than  about  4  ;j.  apart  that  they  cease  to  give  rise  to  two 
distinct  sensations.  It  must  be  remembered,  however,  that  the  fusion  or 
distinction  of  the  sensations  is  ultimately  determined  by  the  brain  and  not 
by  the  retina.  Two  points  of  the  retinal  image  less  than  4  /j.  apart  might 
lie  both  within  the  area  of  a  single  cone ;  but  the  reason  why,  under  such 
circumstances,  they  give  rise  to  one  sensation  only  is  not  because  one  cone- 
fibre  only  is  stimulated.  Two  points  of  a  retinal  image  might  lie,  one  on 
the  area  of  one  cone  and  another  on  the  area  of  an  adjoining  cone,  and  still 
be  less  than  4  //  apart;  in  such  a  case  two  cone-fibres  would  be  stimulated, 

1  By  M  is  meant  the  micromillimeter,  one  one-thousandth  of  a  millimeter. 


VISUAL  SENSATIONS.  769 

and  yet  only  one  sensation  would  be  produced.  So  also  in  the  less  sensitive 
peripheral  parts  of  the  retina  two  points  of  the  retinal  image  might  stimu- 
late two  cones  a  considerable  distance  apart,  and  yet  give  ri£e  to  one  sensa- 
tion only. 

In  the  case  where  the  two  points  lie  entirely  within  the  area  of  a  single 
cone,  it  is  exceedingly  probable  that,  even  if  the  adjacent  cones  or  cone- 
fibres  in  the  retina  are  not  at  the  same  time  stimulated,  impulses  radiate 
from  the  cerebral  ending  of  the  excited  cone  into  the  neighboring  cerebral 
endings  of  the  neighboring  cones  ;  in  other  words,  the  sensation-area  in  the 
brain  does  not  exactly  correspond  to  and  is  not  sharply  defined  like  the 
retinal  area,  but  gradually  fades  away  into  neighboring  sensation-areas. 
We  may  imagine  two  points  of  the  retinal  image  so  far  apart  that  even  the 
extreme  margins  of  their  respective  cerebral  sensation-areas  do  not  touch 
each  other  in  the  least ;  in  such  a  case  there  can  be  no  doubt  about  the  two 
points  giving  rise  to  two  sensations.  We  might,  however,  imagine  a  second 
case  where  two  points  were  just  so  far  apart  that  their  respective  sensation- 
areas  should  coalesce  at  their  margins,  and  yet  that,  in  passing  from  the 
centre  of  one  sensation -area  to  the  centre  of  the  other,  we  should  find  on 
examination  a  considerable  fall  of  sensation  at  the  junction  of  the  two 
areas  ;  and  in  a  third  case  we  might  imagine  the  two  centres  to  be  so  close 
to  each  other  that  in  passing  from  one  to  the  other  no  appreciable  diminu- 
tion of  sensation  could  be  discovered.  In  the  last  case  there  would  be  but 
one  sensation,  in  the  second  there  might  still  be  two  sensations  if  the  mar- 
ginal fall  were  great  enough,  even  though  the  areas  partially  coalesced. 
Thus,  though  the  mosaic  of  rods  and  cones  is  the  basis  of  distinct  vision,  the 
distinction  or  fusion  of  two  vision  impulses  is  ultimately  determined  by  the 
disposition  and  condition  of  the  cerebral  centres.  Hence  the  possibility  of 
increasing  by  exercise  the  faculty  of  distinguishing  two  sensations,  since  by 
use  the  cerebral  sensation-areas  become  more  and  more  differentiated.  This, 
however,  is  even  more  strikingly  shown  in  touch  than  in  sight. 

Color  Sensations. 

§  661.  When  we  allow  sunlight  reflected  from  a  cloud  or  sheet  of  paper 
to  fall  into  the  eye,  we  have  a  sensation  which  we  call  a  sensation  of  white 
light.  When  we  look  at  the  same  light  through  a  prism,  and  allow  differ- 
ent parts  of  the  spectrum  to  fall  in  succession  into  the  eye,  we  have  sensa- 
tions which  we  call  respectively  sensations  of  red,  orange,  yellow,  green,  blue, 
violet,  etc.,  light.  In  other  words,  rays  of  light  falling  on  the  retina  give 
rise  to  different  sensations,  according  to  the  wave-lengths  of  the  rays. 
Though  we  speak  of  the  spectrum  as  consisting  of  a  few  colors,  such  as  red, 
orange,  etc.,  there  are  an  almost  infinite  number  of  intermediate  tints  in  the 
spectrum  itself;  and  we  perceive  in  external  nature  a  large  number  of  colors, 
such  as  purple,  brown,  gray,  etc.,  which  do  not  correspond  to  any  of  the 
color  sensations  gained  by  regarding  the  successive  parts  of  the  spectrum. 
We  find,  however,  on  examination,  that  certain  distinct  color  sensations,  not 
corresponding  to  any  of  the  colors  of  the  spectrum  may  be  obtained  by  the 
fusion  of  the  sensations  caused  by  two  or  more  of  the  prismatic  colors. 
Thus  purple,  which  is  not  present  in  the  spectrum,  may  be  at  once  produced 
by  fusing  the  sensations  of  blue  and  red  in  proper  proportions.  Moreover, 
many  of  the  various  tints  and  shades  of  nature  may  be  imitated  by  fusing  a 
particular  color  sensation  with  the  sensation  of  white,  or  by  allowing  a  cer- 
tain quantity  of  light  of  a  particular  color  to  fall  sparsely  over  the  area  of 
the  retina,  which  is  at  the  same  time  protected  from  the  access  of  any  other 
light,  i.  e.,  as  we  say,  by  mixing  the  color  with  black.  Thus  the  browns 

49 


770  SIGHT. 

of  nature  result  from  various  admixtures  of  yellow,  red,  white,  and  black  ; 
and  a  small  quantity  of  white  light,  scattered  over  a  large  area  of  the  retina, 
i.  e.,  white  largely  mixed  with  black,  forms  a  gray.  In  fact,  the  qualities  of 
a  color  depend  (1)  on  the  nature  of  the  prismatic  color  or  colors,  i.e.,  on  the 
wave-lengths  of  the  constituent  rays,  falling  on  a  given  area  of  the  retina; 
(2)  on  the  amount  of  this  colored  light  which  falls  on  the  area  of  the  retina 
in  a  given  time  ;  and  (3)  on  the  amount  of  white  light  falling  on  the  same 
area  at  the  same  time.  When  rays  corresponding  to  a  prismatic  color  fall 
upon  the  retina  unaccompanied  by  any  white  light,  the  color  is  said  to  be 
"  saturated  ;"  and  a  color  is  spoken  of  as  more  or  less  saturated  according 
as  it  is  mixed  with  less  or  more  white  light.  When  we  are  led  to  describe  a 
color  as  being  of  such  a  tint  or  hue,  we  are  guided  by  the  first  of  the  above 
conditions.  But  we  have  no  common  phrases  by  which  we  distinguish  the 
second  of  the  above  conditions  from  the  third.  The  word  "  pale,"  it  is  true 
is  most  frequently  used  to  express  a  color  very  slightly  saturated  ;  but  the 
words  "  rich  "  or  "  deep  "  are  used  sometimes  as  meaning  highly  saturated, 
sometimes  as  meaning  simply  that  a  large  quantity  of  light  of  the  particular 
hue  is  passing  into  the  eye.  So  also  with  the  phrase  "  bright ;"  this  we 
often  use  when  a  large  amount  of  colored  and  white  light  fall  at  the  same 
time  on  the  same  retinal  area,  but  we  sometimes  also  use  it  to  express  the 
mere  intensity  of  the  sensation. 

The  best  method  of  fusing  color  sensations  is  that  adopted  by  Maxwell,  of 
allowing  two  different  parts  of  the  spectrum  to  fall  on  the  same  part  of  the  retina 
at  the  same  time.  The  use  of  the  pure  prismatic  colors  eliminates  errors  which 
arise  when  pigments,  the  colors  of  which  are  not  pure,  but  mixed,  are  employed. 
And  where  pigments  are  used,  it  is  the  sensations  to  which  the  pigments  give  rise 
which  must  be  mixed  and  not  the  pigments  themselves.  Thus  while  the  sensations 
gained  by  looking  at  gamboge  yellow  and  indigo  respectively  when  fused  give  rise 
to  a  sensation  of  white,  gamboge  and  indigo  themselves  when  mixed  appear  green. 
The  color  of  the  mixed  pigment  is  due  to  the  fact  that  the  rays  which  reach  the  eye 
from  the  mixture  are  those  which  are  least  absorbed  by  the  two  pigments.  The 
gamboge  absorbs  the  blue  rays  very  largely,  but  the  green  to  a  much  less  extent ; 
while  the  indigo  absorbs  the  red  and  yellow  rays  very  largely,  but  also  absorbs  very 
little  of  the  green.  Hence  green  is  the  predominant  hue  of  the  mixture.  When 
pure  pigments,  i.  e. ,  pigments  corresponding  as  closely  as  possible  to  the  prismatic 
colors,  are  used,  satisfactory  results  may  be  gained,  either  by  using  the  reflected 
image  of  one  pigment,  and  arranging  so  that  it  falls  on  the  retina  at  the  same  spot 
as  the  direct  image  of  the  other  pigment,  or  by  allowing  the  image  of  one  pigment 
to  fall  on  the  retina  before  the  sensation  produced  by  the  other  has  passed  away. 
The  first  result  is  easily  reached  by  Helmholtz's  simple  method  of  placing  two 
pieces  of  colored  paper  a  little  distance  apart  on  a  table,  one  on  each  side  of  a  glass 
plate  inclined  at  an  angle.  By  looking  with  one  eye  down  on  the  glass  plate  the 
reflected  image  of  the  one  paper  may  be  made  to  coincide  with  the  direct  image  of 
the  other,  the  angle  which  the  glass  plate  makes  with  the  table  being  adjusted  to 
the  distance  between  the  two  pieces  of  paper.  In  the  second  method,  the  "  color 
top  "  is  used  ;  sectors  of  the  colors  to  be  investigated  are  placed  on  a  disc  made  to 
rotate  very  rapidly,  and  the  image  of  one  color  is  thus  brought  to  bear  on  the 
retina  so  soon  after  the  image  of  another  that  the  two  sensations  are  fused  into 
one. 

§  662.  When  the  sensation  corresponding  to  the  several  prismatic  colors 
are  fused  together  in  various  combinations,  the  following  remarkable  results 
are  brought  about : 

1.  When  red  and  yellow  in  certain  proportions  are  mixed  together  the 
result  is  a  sensation  of  orange,  quite  indistinguishable  from  the  orange  of  the 
spectrum  itself,  Now  the  latter  is  produced  by  rays  of  certain  wave-lengths, 
whereas  the  rays  of  red  and  of  yellow  are  respectively  of  quite  different 
wave-lengths.  The  orange  of  the  spectrum  cannot  be  made  up  by  any  mix- 


VISUAL  SENSATIONS.  771 

ture  of  the  red  and  the  yellow  of  the  spectrum  in  the  sense  that  the  red  and 
yellow  rays  can  unite  together  to  form  rays  of  the  same  wave-lengths  as  the 
orange  rays ;  the  three  things  are  absolutely  different.  It  is  simply  the 
mixed  sensation  of  the  red  and  yellow  which  is  so  like  the  sensation  of  orange  ; 
the  mixture  is  entirely  and  absolutely  a  physiological  one.  In  the  same 
way  we  may  by  appropriate  mixtures  produce  the  sensations  corresponding 
to  other  parts  of  the  spectrum.  Now  we  must  suppose  that  .rays  of  different 
wave-lengths  give  rise  to  different  sensory  impulses ;  that,  for  instance,  the 
sensory  impulses  generated  by  orange  rays  are  different  from  those  generated 
by  red  and  by  yellow  rays.  Hence  we  are  led  by  the  fact  of  mixed  sensa- 
tions being  identical  with  other  apparently  simple  sensations  to  infer  that 
the  sensory  impulses  which  any  ray  originates  are  either  themselves  of  a 
complex  character,  or  in  becoming  converted  into  sensations  give  rise  to 
complex  or  mixed  sensations ;  that,  for  instance,  the  impulse  or  sensation 
which  a  ray  in  the  middle  of  the  orange  gives  rise  to,  is  not  a  simple  impulse 
or  sensation  answering  exclusively  to  the  color  of  that  ray,  but  that  th£  ray 
gives  rise  either  to  a  complex  impulse  which  becomes  converted  into  a  com- 
plex sensation,  or  to  a  simple  impulse  which  eventually  develops  into  a 
mixed  or  complex  sensation,  into  the  composition  of  which  in  each  case 
other  orange  tints  and  shades  of  red  and  yellow  enter. 

§  663.  2.  When  certain  colors  are  mixed  together  in  pairs  in  certain 
definite  proportions,  the  result  is  white.  These  colors  are : 

Red  (near  a),1  and  Blue-Green  (near  F) ; 

Orange  (nearC),  and  Blue  (between  F  and  G) ; 

Yellow  (near  D),  and  Indigo-Blue  (near  G) ; 

Green- Yellow  (near  E),  and  Violet  (between  G  and  M), 

and  are  said  to  be  "  complementary "  to  each  other.  To  these  might  be 
added  the  peculiar  non-prismatic  color  purple,  which  with  green  also 
gives  white. 

3.  If  we  select  arbitrarily  any  three  colors  corresponding  to  any  three 
parts  of  the  spectrum  sufficiently  far  apart — say,  for  instance,  red,  green, 
and  blue — we  can,  by  a  proper  adjustment  of  the  proportions  of  each, 
produce  white.  Further,  these  three  colors  can  be  taken  in  such  pro- 
portions as  with  a  proper  addition,  if  necessary,  of  white  to  produce  the 
sensations  of  all  other  colors.2  That  is  to  say*  given  three  standard  sen- 
sations, all  the  other  sensations  may  be  gained  by  the  proper  mixture  of 
these. 

§  664.  It  is  obvious  from  the  foregoing  that  our  real  color  sensations  are 
much  fewer  in  number  than  those  which  we  appear  to  have  when  we  look  on 
the  colors  of  the  spectrum  or  of  nature ;  that  rays  of  light  awake  in  us  cer- 
tain simple  sensations,  which  mixed  in  various  proportions  reproduce  all  our 
sensations.  And  the  question  arises,  What  is  the  nature  or  what  are  the 
characters  of  these  simple  sensations  ? 

When  we  examine  our  own  sensations  of  light  we  find  that  certain  of 
these  seem  to  be  quite  distinct  in  nature  from  each  other,  so  that  each  is 
something  sui  generis,  whereas  we  easily  recognize  all  other  sensations  as 
various  mixtures  of  these.  Thus  red  and  yellow  are  to  us  quite  distinct ; 
we  do  not  recognize  anything  common  to  the  two ;  but  orange  is  obviously 
a  mixture  of  red  and  yellow.  The  sensations  caused  by  different  kinds  of 

1  These  letters  refer  to  Frauenhofer's  lines. 

2  A  few  highly  saturated  colors  cannot  be  so  reproduced,  but  a  mixture  of  any  one  of 
them  with  white  can.     We  may,  perhaps,  therefore  speak  of  these  saturated  colors  as 
being  reproduced  by  a  proper  combination  of  the  three  arbitrarily  selected  colors,  with 
the  subtraction  of  white. 


772  SIGHT. 

light,  which  thus  appear  to  us  distinct,  and  which  we  may  speak  of  as 
"  fundamental  sensations,"  are  white,  black,  red,  yellow,  green,  blue.  Each 
of  these  seems  to  us  to  have  nothing  in  common  with  any  of  the  others, 
whereas  in  all  other  colors  we  can  recognize  a  mixture  of  two  or  more  of 
these. 

This  result  of  common  experience  suggests  the  idea  that  these  fundamental 
sensations  are  the  primary  or  simple  sensations,  spoken  of  above  as  those  out 
of  which  all  other  sensations  may  be  supposed  to  be  compounded.  And  a 
theory  has  been  proposed  to  reconcile  the  various  facts  of  color  vision  with 
the  supposition  that  we  possess  these  six  fundamental  sensations.  This  theory, 
known  as  that  of  Hering,  is  somewhat  as  follows :  The  six  sensations  readily 
fall  into  three  pairs,  the  members  of  each  pair  having  analogous  relations 
to  each  other.  White  and  black  naturally  go  together,  the  one  being  the 
antagonistic  or  correlative  of  the  other.  There  is  a  similar  connection 
between  red  and  green,  the  one  being  the  complementary  of  the  other,  and 
between  yellow  and  blue,  which  are  similarly  complementary.  We  saw 
reason,  a  short  time  back  (p.  765),  for  believing  that  vision  originates  in 
the  changes  taking  place  in  certain  visual  substances  (or  a  visual  substance) 
in  the  retina.  And  the  theory  of  which  we  are  speaking  supposes  that  there 
exist  in  the  retina,  or  at  least  somewhere  in  the  visual  apparatus,  three  dis- 
tinct visual  substances  which  are  continually  undergoing  a  double  metab- 
olism, one  constructive,  of  assimilation  or  building  up,  and  the  other 
destructive,  of  dissimilation  or  breaking  down.  One  of  these  substances 
is  further  of  such  a  nature  that  when  dissimilation  is  in  excess  of  assimila- 
tion we  have  a  sensation  of  white,  and  when  assimilation  is  in  excess  a  sen- 
sation of  black.  With  a  second  substance  excess  of  dissimilation  provokes 
red,  of  assimilation  green  ;  and  with  the  third  substance,  yellow  and  blue 
respectively.  When  in  the  latter  two  substances  dissimilation  and  assimila- 
tion are  exactly  equal,  no  effect  is  produced  ;  but  with  the  first  substance 
this  condition  produces  in  us  the  effect  of  gray.  Further,  these  substances 
are  of  such  a  kind  that  while  the  first  or  white-black  substance  is  influenced 
by  rays  along  the  whole  range  of  the  spectrum,  the  two  other  substances  are 
differently  influenced  by  rays  of  different  wave-length.  Thus  in  the  part  of 
the  spectrum  which  we  call  red,  the  rays  promote  a  rapid  dissimilation  of 
the  red-green  substance  with  comparatively  slight  effect  in  either  direction 
on  the  yellow-blue  substance;  hence  our  sensation  of  red.  In  that  part  of 
the  spectrum  which  we  call  yellow  the  rays  effect  a  marked  dissimilation  of 
the  yellow-blue  substance,  but  their  action  on  the  red-green  substance  is 
equal  in  the  direction  of  both  assimilation  and  dissimilation  ;  hence  our  sen- 
sation of  yellow.  The  green  rays,  again,  promote  assimilation  of  the  red- 
green  substance,  leaving  the  assimilation  of  the  yellow-blue  substance  equal 
to  the  dissimilation,  and  similarly  blue  rays  cause  assimilation  of  the  yellow- 
blue  substance,  and  leave  the  red-green  substance  neutral.  Finally,  at  the 
extreme  blue  end  of  the  spectrum  the  rays  once  more  provoke  dissimila- 
tion of  the  red-green  substance.  When  orange  rays  fall  on  the  retina, 
there  is  an  excess  of  dissimilation  of  both  the  red-green  and  the  yellow- 
blue  substance  ;  when  greenish-blue  rays  are  perceived  there  is  an  excess 
of  assimilation  of  both  these  substances ;  and  other  intermediate  tints  cor- 
respond to  variable  amounts  of  dissimilation  or  assimilation  of  two  or  more 
of  these  substances. 

§  665.  When  all  the  rays  together  fall  on  the  retina,  the  red-green  and 
yellow-blue  substances  remain  in  equilibrium,  but  the  white-black  substance 
is  violently  dissimilated ;  and  we  say  the  light  is  white. 

Another  theory  (known  .as  the  Young-Helmholtz  theory,  because  it  was 
introduced  by  Young  and  more  fully  elaborated  by  Heimholtz)  strives  to 


VISUAL  SENSATIONS. 


773 


reduce  the  matter  to  still  further  simplicity.  Starting  from  the  fact  men- 
tioned a  short  time  since,  that  all  color  sensations,  including  the  sensation  of 
white,  may  be  obtained  by  the  appropriate  mixture  of  three  standard  sensa- 
tions, this  theory  teaches  that  our  visual  apparatus  is  so  constituted  as,  when 
excited,  to  give  rise  to  three  primary  sensations,  and  that  these  primary  sen- 
sations are  called  forth  in  different  degrees  by  different  rays  of  light,  so  that 
each  ray  gives  rise  to  a  different  mixture  of  the  three.  Several  sets  of  three 
such  primary  sensations  might  be  chosen,  which  would  satisfy  the  conditions 
of  giving  rise,  by  appropriate  mixture,  to  all  sensations  of  color,  including 
white ;  but  for  reasons  into  which  we  cannot  enter  fully  here,  the  sensations 
which  may  thus  be  taken  as  primary  sensations  appear  to  correspond  to  our 
sensations  of  red,  green,  and  blue  or  violet.  Such  a  view  of  three  primary 
color  sensations  is  represented  in  the  diagram  (Fig.  176).  Thus  the  red 


FIG.  176. 


ROY  Gr.  Bl.  V 

Diagram  of  Three  Primary  Color  Sensations,  1,  is  the  so-called  "  red ;"  2,  "green,'  and  3, 
"  violet "  primary  color  sensation.  R,  0,  Y,  etc.,  represent  the  red,  orange,  yellow,  etc.,  color  of 
the  spectrum,  and  the  diagram  shows  by  the  height  of  the  curve  in  each  case,  to  what  extent 
the  several  primary  color  sensations  are  respectively  excited  by  vibrations  of  different  wave- 
lengths. 

primary  sensation,  excited  to  a  certain  extent  by  the  rays  at  the  extreme  red 
end,  is  most  powerfully  affected  by  the  rays  at  a  little  distance  from  the  end, 
the  rays  from  this  point  onward  toward  the  blue  end  producing  less  and  less 
effect.  The  curve  of  the  green  primary  sensation  begins  later,  and  reaches 
its  maximum  in  the  green  of  the  spectrum,  while  the  blue  or  violet  primary 
sensation  is  still  later,  and  only  reaches  its  maximum  toward  the  blue  end 
of  the  spectrum.  Each  ray  calls  forth  each  sensation,  but  to  a  different 
degree,  and  the  total  result  of  each  ray,  or  of  each  group  of  rays,  is  deter- 
mined by  the  proportionate  amount  of  the  three  sensations.  Thus  the  sen- 
sation of  orange  (  0  in  the  figure)  is  brought  about  by  a  mixture  of  a  great 
deal  of  the  primary  red  with  much  less  of  the  primary  green,  and  hardly 
any  of  the  primary  blue ;  the  orange  sensation  is  converted  into  a  yellow 
sensation  by  diminishing  the  primary  red  and  largely  increasing  the  primary 
green,  the  primary  blue  undergoing  also  some  slight  increase.  And  similarly 
with  all  the  other  sensations.  When  each  of  the  primary  sensations  is  ex- 
cited to  a  maximum,  as  when  ordinary  light  falls  on  the  retina,  the  result  is 
a  sensation  of  white.  According  to  this  theory,  black  is  simply  the  absence 
of  sensation  from  the  visual  apparatus. 

In  the  view,  as  originally  put  forward  by  Young,  the  three  primary  sen- 
sations were  supposed  to  be  represented  by  three  sets  of  fibres,  each  set  of 
fibres  being  differently  affected  by  different  rays  of  light,  and  the  impulses 
passing  to  the  brain  along  each  set  awakening  a  distinct  sensation.  No  such 
distinction  of  fibres  can  be  found  in  the  retina ;  but  an  anatomical  basis  of 
this  kind  is  not  necessary  for  the  theory  ;  we  can  easily  conceive  of  the  same 


774  SIGHT. 

fibre  transmitting  three  distinct  kinds  of  impulses  ;  or  we  may  suppose  that 
the  visual  substances  are  three  in  number  instead  of  six,  the  changes  in  each 
substance  provoking  a  primary  sensation. 

§  666.  Such  are  the  two  main  theories  of  color  vision  ;  and  much  may  be 
said  in  favor  of  both  of  them  ;  at  the  same  time  both  of  them  present  many 
difficulties.  To  discuss  them  fully  is  a  task  beyond  the  limits  of  this  book, 
and  to  discuss  them  in  any  but  a  full  manner  would  be  unsatisfactory.  We 
must  be  satisfied,  therefore,  with  the  foregoing  simple  statement  of  the  two 
views.  Independently  of  any  theory,  however,  we  may  remember  (1) 
that  all  the  sensations  which  we  experience  under  the  action  of  light  of 
whatever  kind  may  be  reduced  to  six — white,  black,  red,  yellow,  green,  and 
blue  ;  and  (that  these  may  be  all  reproduced  by  various  mixtures  of  three 
standard  sensations,  if  black  be  allowed  to  indicate  the  absence  of  all  sen- 
sation. These  are  matters  of  fact ;  what  is  at  present  debated  is  whether 
the  six  fundamental  sensations  are  the  outcome  of  three  primary  sensa- 
tions or  whether  they  represent  six  distinct  conditions  of  the  visual 
apparatus. 

§  667.  Color-blindness.  Persons  vary  much  in  their  power  of  appre- 
ciating and  discriminating  color,  i.  e.,  in  the  intensity  and  accuracy  of  their 
color  sensations.  Some  people  regard  as  similar,  colors  which  to  most  peo- 
ple are  glaringly  distinct ;  the  former  are  said  to  be  "  color-blind."  The 
most  common  form  of  color-blindness  is  that  of  persons  unable  to  distinguish 
green  and  red  from  each  other.  As  in  the  case  of  Dalton,  they  tell  a  red 
gown  lying  on  a  green  grass  plot,  or  a  red  cherry  among  the  green  leaves, 
by  its  form,  and  not  by  its  color.  They  confound  not  only  red,  green,  and 
certain  forms  of  brown,  but  also  rose,  purple,  and  blue.  Such  persons  are 
often  spoken  of  as  "  red  blind."  On  the  Hering  theory  they  lack  the  red- 
green  visual  substance  ;  hence,  all  the  color  sensations  they  possess  must  be 
those  of  yellow  and  blue  free  from  all  mixture  of  red  or  green  ;  and  such 
accounts  as  have  been  given  of  their  sensations  by  those  persons  who  are 
"  red  blind  "  in  one  eye,  but  possess  normal  vision  with  the  other,  accord 
with  this  conclusion.  On  the  Young-Helmholtz  theory,  such  persons  lack 
the  primary  red  sensation  ;  and  hence  the  sensations  which  they  have  must 
be  mixtures  of  green  and  blue  alone,  our  yellow  appearing  to  them  a  bright 
green,  and  our  green-blue  a  kind  of  gray, 

All  such  red-blind  people  ought,  on  either  theory,  to  be  less  affected  than 
are  persons  with  normal  eyes,  by  the  red  end  of  the  spectrum  ;  this  ought 
with  them  to  be  shortened  and  obscure.  In  a  certain  number  of  persons 
who  confoun^  red  and  green,  this  is  the  case ;  but  in  some  instances  no  such 
lack  of  appreciation  of  the  red  end  of  the  spectrum  can  be  ascertained. 
Such  cases  have  been  supposed  to  be  green  blind,  that  is,  lacking  the  pri- 
mary sensation  of  green.  According  to  the  Hering  theory  green  blindness 
apart  from  red  blindness  is  impossible,  the  only  two  possible  color  defects 
being  red-green  and  blue-yellow  blindness.  And  the  existence  of  distinct 
green  blindness  has  been  held  to  contradict  that  theory.  On  the  other 
hand,  the  Hering  theory  admits  the  possibility  of  total  color-blindness,  i.  e.t 
the  inability  to  see  anything  but  white  and  black  ;  and  this  on  the  Young- 
Helmholtz  theory,  is  impossible,  since  for  vision  to  exist  at  all,  one  of  the 
three  primary  sensations  must  be  present ;  a  man  to  see  at  all  must  see 
things  in  various  shades  of  either  red,  or  of  green,  or  of  violet,  though  he 
may  confound  this  single-colored  vision  with  the  normal  vision  of  white  of 
different  intensities.  But,  indeed,  a  full  examination  of  color-blindness 
rather  increases  than  diminishes  the  difficulties  of  deciding  between  the  two 
rival  theories. 

§  668.   Influence  of  the  pigment  of  the  yellow  spot.     In  the  macula  lutea, 


VISUAL  SENSATIONS.  775 

which  part  of  the  retina  we  use  chiefly  for  vision,  images  falling  on  other 
parts  of  the  retina  being  said  to  give  rise  to  "  indirect  vision,"  the  yellow 
pigment  absorbs  some  of  the  greenish-blue  rays.  Hence,  the  sensation 
which  we  receive  from  objects  which  we  are  in  the  habit  of  calling  white 
is  that  which,  if  this  pigment  were  absent,  we  should  receive  from  objects 
more  or  less  yellow.  We  may  use  this  feature  of  the  yellow  spot  for  the 
purpose  of  making  the  spot,  so  to  speak,  visible  to  ourselves,  by  an  experi- 
ment suggested  by  Maxwell.  A  solution  of  chrome  alum,  which  only  trans- 
mits red  and  greenish-blue  rays,  is  held  up  between  the  eye  and  a  white 
cloud.  The  greenish-blue  rays  are  absorbed  by  the  yellow  spot,  and  here 
the  light  gives  rise  to  a  sensation  of  red  ;  whereas  in  the  rest  of  the  field  of 
vision  the  sensation  is  that  ordinarily  produced  by  the  purplish  solution. 
The  yellow  spot  is  consequently  marked  out  as  a  rosy  patch.  This  very  soon, 
however,  dies  away. 

In  speaking  of  sensation  as  a  function  of  the  stimulus  (p.  766),  we  re- 
ferred to  white  light  only  ;  but  the  different  colors  are  unequal  in  the  rela- 
tions borne  by  the  intensity  of  the  stimulus  to  the  amount  of  sensation 
produced.  Thus  the  more  refrangible  blue  rays  produce  a  sensation  more 
readily  than  the  yellow  or  red  rays.  Hence,  in  dim  lights,  as  those  of 
evening  and  moonlight,  the  blues  preponderate,  and  the  reds  and  yellows 
are  less  obvious.  So  also  when  a  landscape  is  viewed  through  a  yellow 
glass,  the  yellow  hue  suggests  to  the  mind  bright  sunlight  and  summer 
weather,  although  the  actual  illumination  which  reaches  the  eye  is  dimin- 
ished by  the  glass.  Conversely,  when  the  same  landscape  is  viewed  through 
a  blue  glass  the  idea  of  moonlight  or  winter  is  suggested. 

The  theory  of  three  primary  color  sensations  may  be  used  to  explain 
why  any  colored  light,  if  made  sufficiently  intense,  appears  white.  Thus  a 
violet  light  of  moderate  intensity  appears  violet  because  it  excites  the  pri- 
mary sensation  of  violet  much  more  than  those  of  green  and  red.  If  the 
stimulus  be  increased  the  maximum  of  violet  stimulation  will  be  reached, 
while  the  stimulation  of  green  will  continue  to  be  increased  and  even  that 
of  red  to  a  slight  degree.  The  result  will  be  that  the  light  appears  violet 
mixed  with  green,  that  is  blue.  If  the  stimulus  be  still  further  increased 
while  the  green  and  violet  are  both  excited  to  the  maximum,  the  red  stimu- 
lation may  be  increased  until  the  result  is  violet,  green,  and  red  in  the  pro- 
portions which  make  white  light.  And  so  with  light  of  other  colors. 

§  669.  After-images.  We  have  already  seen  that  in  vision  the  sensation 
lasts  much  longer  than  the  stimulus.  Under  certain  circumstances,  such  as 
particular  conditions  of  the  eye,  an  intense  stimulus,  etc.,  the  sensation  is  so 
prolonged  that  it  is  spoken  of  as  an  after-image.  Thus,  if  the  eye  be  directed 
to  the  sun,  the  image  of  that  body  is  present  for  a  long  while  after ;  and,  if, 
on  early  waking,  the  eye  be  directed  to  the  window  for  an  instant  and  then 
closed,  an  image  of  the  window  with  its  bright  panes  and  darker  sashes,  the 
various  parts  being  of  the  same  color  as  the  object,  will  remain  for  an  appre- 
ciable time.  These  images,  which  are  simply  continuations  of  the  sensation, 
are  spoken  of  as  positive  after-images.  They  are  best  seen  after  a  momentary 
exposure  of  the  eye  to  the  stimulus. 

When,  however,  the  eye  has  been  for  some  time  subject  to  a  stimulus,  the 
sensation  which  follows  the  withdrawal  of  the  stimulus  is  of  a  different  kind ; 
what  is  called  a  negative  after-image,  or  negative  image,  is  produced.  If,  after 
looking  steadfastly  at  a  white  patch  on  a  black  ground,  the  eye  be  turned  to 
a  white  ground,  a  gray  patch  is  seen  for  some  little  time.  A^black  patch  on 
a  white  ground  similarly  gives  rise  on  a  gray  ground  to  a  negative  image  in 
the  form  of  a  white  patch.  This  may  be  explained  as  the  result  of  exhaus- 
tion. When  the  white  patch  has  been  looked  at  steadily  for  some  time,  that 


776  SIGHT. 

part  of  the  retina  on  which  the  image  of  the  patch  fell  becomes  tired  ;  hence, 
the  white  light,  coming  from  the  white  ground  subsequently  looked  at,  which 
falls  on  this  part  of  the  retina,  does  not  produce  so  much  sensation  as  in 
other  parts  of  the  retina;  and  the  image,  consequently,  appears  gray.  And 
so,  in  the  other  instance,  the  whole  of  the  retina  is  tired,  except  at  the  patch  ; 
here  the  retina  is  for  a  while  most  sensitive,  and  hence  the  white  negative 
image. 

When  a  red  patch  is  looked  at,  the  negative  image  is  a  green-blue,  that 
is,  the  color  of  the  negative  image  is  complementary  to  that  of  the  object. 
Thus,  also,  orange  produces  a  blue,  green  a  pink,  yellow  an  indigo-blue,  neg- 
ative image,  and  so  on.  This,  too,  can  be  explained  as  a  result  of  exhaus- 
tion on  either  hypothesis  of  color  vision.  When  the  colored  patch  is  looked 
at,  one  of  the  three  primary  color  sensations  is  much  exhausted,  and  the 
other  two  less  so,  in  varying  proportions,  according  to  the  exact  nature  of 
the  color  of  the  patch  ;  and  the  less  exhausted  sensations  become  prominent 
in  the  after-image.  Thus,  the  red  patch  exhausts  the  red  sensation,  and  the 
negative  image  is  made  up  chiefly  of  green  and  blue  sensations,  that  is, 
appears  to  be  greenish-blue,  or  bluish-green,  according  to  the  tint  of  the  red. 
On  the  other  hypothesis,  we  may  suppose  that,  owing  to  the  continued  effect 
of  looking  at  the  red  patch,  dissimilation  of  the  red-green  substance 
becomes  less  and  less,  leading  to  a  prominence  and,  indeed,  to  an  actual  in- 
crease of  the  process  of  assimilation  of  the  same  substance  ;  hence,  the  sensa- 
tion of  green  dominating  in  the  negative  image. 

Similarly,  when  the  eye,  after  looking  at  a  colored  patch,  is  turned  to  a 
colored  ground,  the  effects  may  easily  be  explained  by  reference  to  the  com- 
parative exhaustion  of  the  color  sensations  excited  by  the  patch  and  the 
ground  respectively  ;  if  a  yellow  ground  be  chosen  after  looking  at  a  green 
object,  the  negative  image  will  appear  of  a  reddish-yellow,  and  so  on. 

§  670.  The  theory  of  three  primary  sensations  does  not  so  readily  ex- 
plain why  negative  images  should  make  their  appearance  without  any  sub- 
sequent stimulation  of  the  retina.  When  the  eyes  are  shut  and  all  access 
of  light,  even  through  the  eyelids,  carefully  avoided,  the  field  of  vision  is  not 
absolutely  dark ;  there  is  still  a  sensation  of  light,  the  so-called  "  proper 
light "  of  the  retina.  If  a  white  patch  on  a  black  ground  be  looked  at  for 
some  time,  and  the  eyes  then  shut,  a  negative  (black)  image  of  the  spot 
will  be  seen  on  the  ground  of  the  "  proper  light "  of  the  retina,  having  in 
its  immediate  neighborhood  a  specially  bright  corona.  So,  also,  if  a  window 
be  looked  at  and  the  eyes  then  closed,  the  positive  after-image  with  bright 
panes  and  dark  sashes  gives  rise  to  a  negative  after-image  with  bright  sashes 
and  dark  panes  ;  and  similar  effects  appear  with  colors.  These  and  similar 
facts  have  been  largely  used  in  support  of  the  Hering  theory.  When  the 
eye  has  been  looking  at  red,  and  so  has  caused  dissimilation  of  the  red- 
green  substance,  mere  rest,  as  on  shutting  the  eyes,  favors  assimilation  of  the 
same  substance  and  thus  leads  to  a  sensation  of  green.  And  the  rhythmic 
oscillations  from  one  color  to  its  correlative  and  back  again,  frequently 
observed  under  these  conditions  and  which  point  to  assimilation  and  dissimi- 
lation alternately  gaining  the  upper  hand,  are  not  without  analogies  in  other 
common  instances  of  protoplasmic  metabolism. 

VISUAL  PERCEPTIONS. 

§  671.  Hitherto  we  have  studied  sensations  only,  and  have  considered 
an  external  object,  such  as  a  tree,  as  simply  a  source  of  so  many  distinct 
sensations,  differing  from  each  other  in  intensity  and  kind  (color).  In  the 
mind  these  sensations  are  coordinated  into  a  perception.  We  are  not  only 


VISUAL  PERCEPTIONS.  777 

conscious  of  a  number  of  sensations  of  bright  and  dim  lights,  of  green, 
brown,  black,  etc.,  but  these  sensations  are  so  related  to  each  other  and  by 
virtue  of  cerebral  processes  so  fashioned  into  a  whole,  that  we  "  see  a  tree." 
We  sometimes,  in  illustration  of  such  an  effect,  speak  of  an  image  or  pic- 
ture in  the  mind  corresponding  to  the  physical  image  on  the  retina. 

When  we  look  upon  the  external  world,  a  variety  of  images  are  formed 
at  the  same  time  on  the  retina,  and  give  rise  to  a  number  of  contempora- 
neous visual  sensations.  The  sum  of  these  sensations  constitutes  "  the  field  of 
vision,"  which  varies,  of  course,  with  every  movement  of  the  eye.  This 
field  of  vision,  being  in  reality  an  aggregate  of  sensations,  is,  of  course,  a 
subjective  matter  ;  but  we  are  in  the  habit  of  using  the  same  phrase  to  denote 
the  sum  of  external  objects  which  give  rise  to  the  aggregate  of  visual  sensa- 
tions ;  in  common  language  the  field  of  vision  is  "  all  that  we  can  see  "  in 
any  position  of  the  eye,  and  we  have  a  field  of  vision  for  each  eye  sepa- 
rately and  for  the  two  eyes  combined. 

§  672.  Using  for  the  present  the  words  in  their  subjective  sense,  we  may 
remark  that  we  are  able  to  assign  to  each  constituent  sensation  its  place 
among  the  aggregate  of  sensations  constituting  the  field  of  vision  ;  we  can, 
as  we  say,  localize  the  sensation.  We  can  say  whether  it  belongs  to  (what 
we  regard  as)  the  right  hand  or  left  hand,  the  upper  or  the  lower  part  of 
the  field  of  vision.  We  are  able  to  distinguish  the  relative  positions  of 
any  two  distinct  sensations ;  and  the  relative  positions,  together  with  the  re- 
lative intensities  and  qualities  (color)  of  the  sensations  arising  from  any 
object  determine  our  perception  of  the  object.  It  need  hardly  be  remarked 
that  this  localization  is  purely  subjective.  We  simply  determine  the  posi- 
tion of  the  sensation  in  the  field  of  vision  (which  is  itself  a  wholly  subjective 
matter) ;  we  do  not  determine  the  position  of  the  object.  The  connection 
between  the  position  of  the  object  in  the  external  world  and  the  position  of 
the  sensation  in  the  field  of  vision  cannot  be  determined  by  visual  observa- 
tion alone.  All  the  information  which  can  be  gained  by  the  eye  is  limited 
to  the  field  of  vision,  and  provided  that  the  relative  position  of  the  sensa- 
tions in  the  field  of  vision  remained  the  same,  the  actual  position  of  exter- 
nal objects  might,  as  far  as  vision  is  concerned,  be  changed  without  our  being 
aware  of  it. 

As  a  matter  of  fact  the  field  of  vision  in  one  important  particular  does 
not  correspond  to  the  field  of  external  objects.  The  image  on  the  retina  is 
inverted ;  the  rays  of  light  proceeding  from  an  object  which  by  touch  we 
know  to  be  on  what  we  call  our  right  hand,  fall  on  the  left-hand  side  of  the 
retina.  If,  therefore,  the  field  of  vision  correspond  to  the  retinal  image, 
the  object  would  be  seen  on  the  left  hand.  We,  however,  see  it  on  the  right 
hand,  because  we  invariably  associate  right-hand  tactile  localization  with 
left-hand  visual  localization  ;  that  is  to  say,  our  field  of  vision,  when  inter- 
preted by  touch,  is  a  re-inversion  of  the  retinal  image. 

The  dimensions  of  the  field  of  vision  of  a  single  eye  are  about  145  degrees 
for  the  horizontal  and  100  degrees  for  the  vertical  meridian,  the  former 
being  distinctly  greater  than  the  latter.  The  horizontal  dimension  of  the 
field  of  vision  for  the  two  eyes  is  about  180  degrees.  By  movements  of  the 
eyes,  however,  even  apart  from  those  of  the  head,  the  extent  may  be  con- 
siderably increased. 

The  satisfactory  perception  of  external  objects  requires  distinct  vision ; 
and  of  this,  as  we  have  already  said,  the  formation  of  a  distinct  image  on 
the  retina  is  an  essential  condition.  We  can  receive  visual  sensations  of  all 
kinds  with  the  most  imperfect  dioptric  apparatus,  but  our  perception  of  an 
object  is  precise  in  proportion  to  the  clearness  of  the  image  on  the  retina. 

§  673.  Region  of  distinct  vision.    If  we  take  two  points,  such  as  two  black 


778  SIGHT. 

dots,  only  just  so  far  apart  that  they  can  be  seen  distinctly  as  two  when 
placed  near  the  axis  of  vision,  and  then,  keeping  the  axis  fixed,  move  the 
two  points  out  into  the  circumferential  parts  of  the  field  of  vision,  it  will  be 
found  that  the  two  soon  appear  as  one.  The  two  sensations  become  fused,  as 
they  would  do  if  brought  nearer  to  each  other  in  the  centre  of  the  field. 
The  further  away  from  the  centre  of  the  field,  the  further  apart  must  two 
points  be  in  order  that  they  may  be  seen  as  two.  In  other  words,  vision  is 
much  more  distinct  in  the  centre  of  the  field  than  toward  the  circumference. 
Practically  the  region  of  distinct  vision  may  be  said  to  be  limited  to  the 
macula  lutea,  or  even  to  the  fovea  centralis  ;  by  continual  movements  of  the 
eye  we  are  constantly  bringing  any  object  which  we  wish  to  see  in  such  a 
position  that  its  image  falls  on  this  region  of  the  retina. 

The  diminution  of  distinctness  does  not  take  place  equally  from  the 
centre  to  the  circumference  along  all  meridians.  The  outline  described  by 
a  line  uniting  the  points  where  two  spots  cease  to  be  seen  as  two  when  moved 
along  different  radii  from  the  centre  is  a  very  irregular  figure. 

The  sensations  of  color  are  much  more  distinct  in  the  centre  of  the  retina 
than  toward  the  circumference.  If  the  visual  axis  be  fixed  and  a  piece  of 
colored  paper  be  moved  toward  the  outside  of  the  field  of  vision,  the  color 
undergoes  changes  and  is  eventually  lost,  red  disappearing  first,  and  blue 
last,  the  object  remaining  visible,  though  with  very  indistinct  outlines,  when 
its  color  can  no  longer  be  recognized.  A  purple  color  becomes  blue,  and  a 
rose  color  a  bluish  white.  In  fact,  there  seems  to  be  a  certain  amount  of 
red-blindness  in  the  peripheral  parts  of  all  retinas. 

Modified  Perceptions. 

§  674.  Since  our  perception  of  external  objects  is  based  on  the  distinct- 
ness of  the  sensations  which  go  to  form  the  perception,  it  might  be  expected 
that  when  an  image  of  an  object  is  formed  on  the  retina  the  sensory  impulses 
would  correspond  to  the  retinal  image,  the  sensations  correspond  to  the  sen- 
sory impulses  and  the  perception  correspond  to  the  sensations,  and  that, 
therefore,  the  mental  condition  resulting  from  our  looking  at  any  object  or 
view  would  correspond  exactly  to  the  retinal  image.  We  find,  however,  that 
this  is  not  the  case.  The  sensations  and  probably  even  the  simple  sensory 
impulses  produced  by  an  image  react  upon  each  other,  and  these  reactions 
modify  our  perceptions,  independently  of  the  physical  conditions  of  the 
retinal  image.  There  arise  certain  discrepancies  between  the  retinal  image 
and  the  perception,  some  having  their  source  in  the  retina,  some  in  the  brain, 
and  others  being  of  such  a  nature  that  it  is  difficult  to  say  where  the 
irrelevancy  is  introduced. 

§  675.  Irradiation.  A  white  patch  on  a  dark  ground  appears  larger, 
and  a  dark  patch  on  a  white  ground  smaller,  than  it  really  is  (Fig.  177). 
This  is  especially  so  when  the  object  is  somewhat  out  of  focus,  and  may  in 
this  case  be  partly  explained  by  the  diffusion  circles  which,  in  each  case, 
encroach  from  the  white  upon  the  dark.  But  over  and  beyond  this,  any 
sensation  coming  from  a  given  retinal  area  occupies  a  larger  share  of  the 
field  of  vision,  when  the  rest  of  the  retina  and  central  visual  apparatus  are 
at  rest,  than  when  they  are  simultaneously  excited.  It  is  as  if  the  neighbor- 
ing, either  retinal  or  cerebral,  structures  were  sympathetically  thrown  into 
action  at  the  same  time. 

§  676.  Contrast.  If  a  white  strip  be  placed  between  two  black  strips, 
the  edges  of  the  white  strip  near  the  black  will  appear  whiter,  than  its 
median  portion  ;  and  if  a  white  cross  be  placed  on  a  black  background,  the 
centre  of  the  cross  will  appear  sometimes  so  dim,  compared  with  the  parts 


VISUAL  PERCEPTIONS. 


779 


close  to  the  black,  as  to  seem  shaded.  This  occurs  even  when  the  object  is 
well  in  focus ;  the  increased  sensation  of  light  which  causes  the  apparent 
greater  whiteness  of  the  borders  of  the  cross  is  the  result  of  the  "  con- 
trast "  with  the  black  placed  adjoining  it.  Still  more  curious  results  are 
seen  with  colored  objects.  If  a  small  piece  of  gray  paper  be  placed  on  a 
sheet  of  green  paper,  and  both  covered  with  a  sheet  of  thin  tissue  paper,  the 
gray  paper  will  appear  of  a  pink  color,  the  complementary  of  the  green. 

FIG.  177. 


This  effect  of  contrast  is  far  less  striking,  or  even  wholly  absent,  when  the 
small  piece  of  paper  is  white  instead  of  gray,  and  generally  disappears  when 
the  thin  covering  of  tissue  paper  is  removed.  It  also  vanishes  if  a  bold, 
broad  black  line  be  drawn  round  the  small  piece  of  paper,  so  as  to  isolate  it 
from  the  ground  color.  If  a  book  or  pencil  be  placed  vertically  on  a  sheet 
of  white  paper,  and  illuminated  on  one  side  by  the  sun  and  the  other  by  a 
.  candle,  two  shadows  will  be  produced,  one  from  the  sun,  which  will  be  illu- 
minated by  the  yellowish  light  of  the  candle,  and  the  other  from  the  candle, 
which  will  in  turn  be  illuminated  by  the  white  light  of  the  sun.  The  former 
naturally  appears  yellow ;  the  latter,  however,  appears  not  white  but  blue ; 
it  assumes,  by  contrast,  a  color  complementary  to  that  of  the  candle-light 
which  surrounds  it.  If  the  candle  be  removed,  or  its  light  shut  off  by  a 
screen,  the  blue  tint  disappears,  but  returns  when  the  candle  is  again  allowed 
to  produce  its  shadow.  If,  before  the  candle  is  brought  back,  and  vision  be 
directed  through  a  narrow  blackened  tube  at  some  part  falling  entirely 
within  the  area  of  what  will  be  the  candle's  shadow,  the  area,  which  in  the 
absence  of  the  candle  appears  white,  will  continue  to  appear  white  when  the 
candle  is  made  to  cast  its  shadow,  and  it  is  not  until  the  direction  of  the 
tube  is  changed  so  as  to  cover  part  of  the  ground  outside  the  shadow,  as  well 
as  part  of  the  shadow,  that  the  latter  assumes  its  blue  tint. 

§  677.  Filling  up  the  blind  spot.  Though,  as  we  have  seen,  that  part  of 
the  retina  which  corresponds  to  the  entrance  of  the  optic  nerve  is  quite  in- 
sensible to  light,  we  are  conscious  of  no  blank  in  the  field  of  vision.  When 
in  looking  at  a  page  of  print  we  fix  the  visual  axis  so  that  some  of  the  print 
must  fall  on  a  blind  spot,  no  gap  is  perceived.  We  could  not  expect  to  see 
a  black  patch,  because  what  we  call  black  is  the  absence  of  the  sensation  of 
light  from  structures  which  are  sensitive  to  light ;  we  must  have  visual 
organs  to  see  black.  But  there  are  no  visual  organs  in  the  blind  spot,  and 
consequently  we  are  in  no  way  at  all  affected  by  the  rays  of  light  which  fall 
on  it.  There  is  in  our  subjective  field  of  vision  no  gap  corresponding  to  the 
gap  in  the  retinal  image.  We  refer  the  sensations  coming  from  two  points 
of  the  retina  lying  on  opposite  margins  of  the  blind  spot  to  two  points  lying 
close  together,  since  we  have  no  indication  of  the  space  which  separates 
them.  Concerning  the  effects  which  are  produced  when  an  object  in  the 
field  of  view  passes  into  the  region  of  the  blind  spot  there  has  been  much 


780  SIGHT. 

discussion.  In  ordinary  vision,  of  course,  the  existence  of  the  blind  spot  is 
of  little  moment  since  it  is  outside  the  region  used  for  distinct  vision,  and 
besides,  the  image  of  an  object  does  not  fall  on  the  blind  spots  of  both  eyes 
at  the  same  time.  [See  Fig.  179.] 

§  678.  Ocular  spectra.  So  far  from  our  perceptions  exactly  correspond- 
ing to  the  arrangements  of  the  luminous  rays  which  fall  on  the  retina,  we 
may  have  visual  sensations  and  perceptions  in  the  entire  absence  of  light. 
Any  stimulation  of  the  retina  or  of  the  optic  nerve  sufficiently  intense  will 
give  rise  to  a  visual  sensation.  Gradual  pressure  on  the  eyeball  causes  a 
sensation  of  rings  of  colored  light,  the  so-called  phosphenes  ;  a  sudden  blow 
on  the  eye  causes  a  sensation  of  flashes  of  light,  and  the  seeming  identity  of 
the  visual  sensations  so  brought  about  with  visual  sensations  produced  by 
light  is  well  illustrated  by  the  statement  once  gravely  made  in  a  German 
court  of  law  by  a  witness  who  asserted  that  on  a  pitch-dark  night  he  recog- 
nized an  assailant  by  help  of  the  flash  of  light  caused  by  the  assailant's  hand 
coming  in  violent  contact  with  his  eye.  Electrical  stimulation  of  the  eye 
or  optic  nerve  will  also  give  rise  to  visual  sensations. 

The  sensations  which  may  arise  without  any  light  falling  on  the  retina 
need  not  necessarily  be  undefined  ;  on  the  contrary  they  may  be  most  clearly 
defined.  Complex  and  coherent  visual  images  or  perceptions  may  arise  in 
the  brain  without  any  corresponding  objective  luminous  cause.  These  so- 
called  ocular  spectra  or  phantoms,  which  are  the  result  of  an  intrinsic  stimu- 
lation of  some  (probably  cerebral)  part  of  the  visual  apparatus,  have  a  dis- 
tinctness which  gives  them  an  apparent  objective  reality  quite  as  striking  as 
that  of  ordinary  visual  perceptions.  They  may  occasionally  be  seen  with 
the  eyes  open  (and  therefore  while  ordinary  visual  perceptions  are  being 
generated)  as  well  as  when  the  eyes  are  closed.  They  sometimes  become  so 
frequent  and  obtrusive  as  to  be  distressing,  and  form  an  important  element 
in  some  kinds  of  delirium,  such  as  delirium  tremens. 

§  679.  Appreciation  of  apparent  size.  By  the  eye  alone  we  can  only  esti- 
mate the  apparent  size  of  an  object,  we  can  only  tell  what  space  it  takes  in 
the  field  of  vision,  we  can  only  perceive  the  dimensions  of  the  retinal  image, 
and  therefore  have  a  right  only  to  speak  of  the  angle  which  the  diameter  of 
the  object  subtends.  The  real  size  of  an  object  must  be  determined  by  other 
means.  But  our  perception  of  even  the  apparent  size  of  an  object  is  so 
modified  by  concurrent  circumstances  that  in  many  cases  it  cannot  be  relied 
on.  The  apparent  size  of  the  moon  must  be  the  same  to  every  eye,  and  yet 
while  some  persons  will  be  found  ready  to  compare  the  moon  in  the  mid- 
heavens  with  a  three-penny  piece,  others  will  liken  it  to  a  cart-wheel ;  that 

FIG.  178. 


is  to  say,  the  angle  subtended  by  the  moon  seems  to  the  one  to  be  about 
equal  to  that  subtended  by  a  three-penny  piece  held  at  the  distance  from  the 
eye  at  which  it  is  most  commonly  looked  at,  and  to  the  other  about  equal  to 
that  subtended  by  a  cart-wheel  similarly  viewed  at  the  distance  at  which  it 
is  most  commonly  looked  at.  If  a  line  such  as  A  C,  Fig.  178,  be  divided 
into  two  equal  parts,  A  B,  B  C\  and  A  B  be  divided  by  distinct  marks  into 
several  parts,  as  is  shown  in  the  figure,  while  B  C  is  left  entire,  the  distance 
A  B  will  always  appear  greater  than  CB.  So  also,  if  two  equal  squares  be 
marked,  one  with  horizontal  and  the  other  with  vertical  alternate  dark  and 
light  bands,  the  former  will  appear  higher,  and  the  latter  broader,  than  it 
really  is.  Hence  short  persons  affect  dresses  horizontally  striped  in  order  to 


BINOCULAK  VISION.  781 

increase  their  apparent  height,  and  very  stout  persons  avoid  longitudinal 
stripes.  Two  perfectly  parallel  lines  or  bands,  each  of  which  is  crossed  by 
slanting  parallel  short  lines,  will  appear  not  parallel,  but  diverging  or  con- 
verging according  to  the  direction  of  the  cross-lines. 

Again,  when  a  short  person  is  placed  side  by  side  with  a  tall  person,  the 
former  appears  shorter  and  the  latter  taller  than  each  really  is.  The  moon 
on  the  horizon  appears  larger  than  when  at  tho  zenith,  because  in  the  first 
position  it  can  be  most  easily  compared  with  terrestrial  objects.  The  absence 
of  comparison  may,  however,  contribute  to  an  opposite  effect,  as  when  a 
person  looks  larger  in  a  fog;  being  seen  indistinctly,  he  is  judged  to  be 
further  off  than  he  really  is,  and  so  appears  larger  than  he  naturally  would 
do  at  the  distance  at  which  he  is  supposed  to  be.  So,  conversely,  distant 
mountains,  when  seen  distinctly  in  a  clear  atmosphere  appear  small,  because, 
on  account  of  their  distinctness,  they  are  judged  to  be  nearer  than  they  really 
are.  Indeed,  our  daily  life  is  full  of  instances  in  which  our  direct  percep- 
tion is  modified  by  circumstances.  Among  those  circumstances  previous 
experience  is  one  of  the  most  potent,  and  thus  simple  perceptions  become 
mingled  with  what  are  in  reality  judgments,  though  frequently  made  uncon- 
sciously. But  this  intrusion  of  past  experience  into  present  perceptions  and 
sensations  is  most  obvious  in  binocular  vision,  to  which  we  now  turn. 

BINOCULAR  VISION. 

Corresponding  or  Identical  Points. 

§  680.  Though  we  have  two  eyes,  and  must  therefore  receive  from  every 
object  two  sets  of  sensations,  our  perception  of  an  object  is  under  ordinary 
circumstances  a  single  one ;  we  see  one  object,  not  two.  But  putting  either 
eye  into  an  unusual  position,  as  by  squinting,  we  can  render  the  perception 
double ;  we  see  two  objects  where  one  only  exists.  From  which  it  is  evi- 
dent that  singleness  of  perception  depends  on  the  image  of  the  object  fall- 
ing on  certain  parts  of  each  retina  at  the  same  time,  these  parts  being  so 
related  to  each  other  that  the  sensations  from  each  are  blended  into  one  per- 
ception ;  and  it  is  also  evident  that  the  movements  of  the  eyeballs  are 
adapted  to  bring  the  image  of  the  object  to  fall  on  these  "  corresponding  " 
or  "  identical "  parts,  as  they  are  called,  of  each  retina. 

When  we  look  at  an  object  with  one  eye  the  visual  axis  of  that  eye  is 
directed  to  the  object,  and  when  we  use  two  eyes  the  visual  axes  of  the  two 
eyes  converge  at  the  object,  the  eyeballs  moving  accordingly.  The  corre- 
sponding points  of  the  two  retinas  are  those  on  which  the  two  images  of  the 
object  fall  when  the  visual  axes  converge  at  the  object.  Thus  in  Fig.  179, 
if  xx,  xx'be  the  two  visual  axes,  xx'  being  the  centres  of  the  fovese  centrales 
of  the  two  eyes,  then,  the  object  y,  x,  z  being  seen  single,  the  point  y  on  the 
one  retina  will  "  correspond  "  to  or  be  "  identical "  with  the  point  y'  on  the 
other,  and  the  point  x  in  the  one  to  the  point  x'  in  the  other.  Hence  a  point 
lying  anywhere  on  the  right  side  of  one  retina  has  its  corresponding  point 
on  the  right  side  of  the  other  retina,  and  the  points  on  the  left  of  one  corre- 
spond with  those  on  the  left  of  the  other.  Thus,  while  the  upper  half  of  the 
retina  of  the  left  eye  corresponds  to  the  upper  half  of  the  retina  of  the  right 
eye  and  the  lower  to  the  lower,  the  nasal  side  of  the  left  eye  corresponds 
with  the  malar  side  of  the  right,  and  the  malar  of  the  left*  with  the  nasal 
side  of  the  right. 

The  blending  of  the  two  sensations  into  one  only  occurs  when  the  two 
images  of  an  object  fall  on  these  corresponding  points  of  the  two  retinas. 
Hence  it  is  obvious  that  in  single  vision  with  two  eyes  the  ordinary  move- 


782 


SIGHT. 


ments  of  the  eyeballs  must  be  such  as  to  bring  the  visual  axes  to  converge 
at  the  object  so  that  the  two  images  may  fall  on  corresponding  points. 
When  the  visual  axes  do  not  so  converge,  and  when,  therefore,  the  images  do 
not  fall  on  corresponding  points,  the  two  sensations  are  not  blended  into  one 
perception  and  vision  becomes  double. 


FIG.  179. 


Diagram  illustrating  Corresponding  or  Identical  Points:  L,  the  left,  R,  the  right  eye  ;  n,  nodal 
point ;  o,  optic  nerve  (blind  spot) ;  x,  fovea.  x'  y'  z'  in  the  right  eye  are  corresponding  points  to  x  y 
z  in  the  left  eye.  v  I,  visual  axis.  The  two  figures  below  are  projections  of  L  the  left  and  R  the 
right  retina.  /,  fovea ;  o,  blind  spot,  a  and  c  on  the  temporal  side  of  L  correspond  to  a'  and  c'  on  the 
nasal  side  of  R.  vmhm,  lines  separating  quadrants. 


Movements  of  the  Eyeballs. 

§  681.  The  eye  is  virtually  a  ball  placed  in  a  socket,  the  bulb  and  the 
orbit  forming  a  ball-and-socket  joint.  In  its  socket-joint  the  optic  ball  is 
capable  of  a  variety  of  movements,  but  it  cannot  by  any  voluntary  effort  be 
moved  out  of  its  socket.  It  is  stated  that  by  a  very  forcible  opening  of  the 
eyelids  the  eyeball  may  be  slightly  protruded  ;  but  this  trifling  locomotion 
may  be  neglected.  By  disease,  however,  the  position  of  the  eyeball  in  the 
socket  may  be  materially  changed. 

Each  eyeball  is  capable  of  rotating  round  an  immobile  centre  of  rotation, 
which  has  been  found  to  be  placed  a  little  (1.77  mm.)  behind  the  centre  of 
the  eye  ;  but  the  movements  of  the  eye  round  the  centre  are  limited  in  a  pecu- 


BINOCULAR   VISION. 


783 


FIG.  180. 


oblsup. 


liar  way.  The  shoulder-joint  is  also  a  ball-and-socket  joint ;  and  we  know  that 
we  can*  not  only  move  the  arm  up  and  down  round  a  horizontal  axis  passing 
through  the  centre  of  rotation  of  the  head  of  the  humerus,  and  from  side  to 
side  round  a  vertical  axis,  but  we  can 
also  rotate  it  round  its  own  longitudi- 
nal axis.  When,  however,  we  come  to 
examine  closely  the  movements  of  the 
eyeball  we  find  that  though  we  can 
move  it  up  and  down  round  a  horizon- 
tal axis,  as  when  with  fixed  head  we 
direct  our  vision  to  the  heavens  or  to 
the  ground,  and  from  side  to  side,  as 
when  we  look  to  left  or  right,  and 
though  by  combining  these  two  move- 
ments we  can  give  the  eyeball  a  variety 
of  inclinations,  we  cannot,  by  a  volun- 
tary effort,  rotate  the  eyeball  round 
its  longitudinal  visual  axis.  The  ar- 
rangement of  the  muscles  of  the  eye- 
ball will  permit  of  such  a  movement, 
but  we  cannot  by  any  direct  effort  of 
will  bring  it  about  by  itself.  In  cer- 
tain movements  of  the  eye  rotation  of 
the  eyeball  does  take  place  and  by 
bringing  about  these  movements  we 
can  indirectly  cause  rotation ;  but  we 
cannot  rotate  the  eyeball  except  thus 
indirectly  as  a  part  of  these  movements. 

If,  when  vision  is  directed  to  any 
object  the  head  be  moved  from  side  to 
side,  the  eyes  do  not  move  with  it ;  they 
appear  to  remain  stationary,  very 
much  as  the  needle  of  a  ship's  compass  remains  stationary  when  the  head 
of  the  ship  is  turned.  The  change  in  the  position  of  the  visual  axes  to  which 
the  movement  of  the  head  would  naturally  give  rise  is  met  by  compensating 
movements  of  the  eyeballs ;  were  it  not  so,  steadiness  of  vision  would  be 
impossible. 

§  682.  There  is  one  position  of  the  eyes  which  has  been  called  the  primary 
position.  It  corresponds  to  that  which  may  be  attained  by  looking  at  the 
distant  horizon  with  the  head  vertical  and  the  body  upright ;  but  its  exact 
determination  requires  special  precautions.  The  visual  axes  are  then  parallel 
to  each  other  and  to  the  median  plane  of  the  head.  All  other  positions  of 
the  eyes  are  called  secondary  positions. 

§  683.  Muscles  of  the  eyeball.  The  eyeball  is  moved  by  six  muscles,  the 
recti  inferior,  superior,  internus,  and  externus,  and  the  obliqui  inferior  and 
superior.  It  is  found  by  calculation  from  the  attachments  and  directions  of 
the  muscles,  and  confirmed  by  actual  observation,  that  the  six  muscles  may 
be  considered  as  three  pairs,  each  pair  rotating  the  eye  round  a  particular 
axis.  The  relative  attachments  and  the  axes  of  rotation  are  diagrammati- 
cally  shown  in  Fig.  180.  The  rectus  superior  and  the  rectus  inferior  rotate 
the  eye  round  a  horizontal  axis,  which  is  directed  from  the  upper  end  of  the 
nose  to  the  temple ;  the  obliquus  superior  and  obliquus  inferior  round  a 
horizontal  axis  directed  from  the  centre  of  the  eyeball  to  the  occiput ;  and 
the  rectus  internus  and  rectus  externus  round  a  vertical  axis  (which,  being 
at  right  angles  to  the  plane  of  the  paper,  cannot  be  shown  in  the  diagram), 


r.ext.  r.sup.  r.inf. 
r.inf. 

Diagram  of  the  Attachments  of  the  Mus- 
cles of  the  Eye  and  of  their  Axes  of  Rota- 
tion, the  latter  being  represented  by  dotted 
lines. 

The  axis  of  rotation  of  the  rectus  externus 
and  internus,  being  perpendicular  to  the 
plane  of  the  paper,  cannot  be  shown.  (After 
Fick.) 


784 


SIGHT. 


passing  through  the  centre  of  rotation  of  the  eyeball  parallel  to  the  median 
plane  of  the  head  when  the  head  is  vertical.  Thus  the  latter  pair  acting 
alone  would  turn  the  eye  from  side  to  side,  the  other  straight  pair  acting 
alone  would  move  the  eye  up  and  down,  while  the  oblique  muscles  acting 
alone  would  give  the  eye  an  oblique  movement.  The  rectus  externus  acting 
alone  would  turn  the  eye  to  the  malar  side,  the  internus  to  the  nasal  side, 
the  rectus  superior  upward,  the  rectus  inferior  downward,  the  oblique  superior 
downward  and  outward,  and  the  inferior  upward  and  outward.  The  recti 
superior  and  inferior  in  moving  the  eye  up  and  down  also  turn  it  somewhat 
inward  and  at  the  same  time  give  it  a  slight  amount  of  rotation  ;  but  this 
is  corrected  if  the  oblique  muscles  act  at  the  same  time  ;  and  it  is  found 
that  the  rectus  superior  acting  with  the  obliquus  inferior  moves  the  eye  up- 
ward, and  the  rectus  inferior  with  the  obliquus  superior  downward  in  a 
vertical  direction.  In  oblique  movements  also,  the  obliqui  are  always  asso- 
ciated with  the  recti.  Hence  the  various  movements  of  the  eyeball  may  be 
arranged  as  follows: 


f  I ,[ 

OQ  o 

s 


Elevation. 
Depression. 
Adduction  to 

nasal  side. 
Adduction  to 

malar  side. 
Elevation  with 

adduction. 

Depression 
with  adduction. 
Elevation  with 

abduction. 

Depression 
with  abduction. 


Rectus  superior  and  obliquus  inferior. 
Rectus  inferior  and  obliquus  superior. 

Rectus  internus. 

Rectus  externus. 

Rectus  superior  and  internus  with  obliquus 

inferior. 
Rectus  inferior  and  internus  with  obliquus 

superior. 
Rectus  superior  and  externus  with  obliquus 

inferior. 
Rectus  inferior  and  externus  with  obliquus 

superior. 


§  684.  Coordination  of  visual  movements.  Thus  even  in  the  movements 
of  a  single  eye  a  considerable  amount  of  coordination  takes  place.  When 
the  eye  is  moved  in  any  other  than  the  vertical  and  horizontal  meridians, 
impulses  must  descend  to  at  least  three  muscles,  and  in  such  relative  energy 
to  each  of  the  three  as  to  produce  the  required  inclination  of  the  visual 
axis.  But  the  coordination  observed  in  binocular  vision  is  more  striking 
still.  If  the  movements  of  any  person's  eyes  be  watched  it  will  be  seen  that 
the  two  eyes  move  alike.  If  the  right  eye  moves  to  the  right,  so  does  also 
the  left  ;  and,  if  the  object  looked  at  be  a  distant  one,  exactly  to  the  same 
extent  ;  if  the  right  eye  looks  up,  the  left  eye  looks  up  also,  and  so  in  every 
other  direction.  Very  few  persons  are  able  by  a  direct  effort  of  the  will  to 
move  one  eye  independently  of  the  other  ;  though  some,  and  among  them 
one  distinguished  both  as  a  physiologist  and  an  oculist,  have  acquired  this 
power.  In  fact,  the  movements  of  the  two  eyes  are  so  arranged  that  in  the 
various  movements  the  images  of  any  object  should  fall  on  the  corresponding 
points  of  the  two  retinas,  and  that  thus  single  vision  should  result.  We 
cannot  by  any  direct  effort  of  our  will  place  our  eyes  in  such  a  position  that 
the  rays  of  light  proceeding  from  any  object  shall  be  brought  to  a  focus  on 
parts  of  the  two  retinas  which  do  not  correspond,  and  thus  give  rise  to  two 
distinct  visual  images.  We  can  bring  the  visual  axes  of  the  two  eyes  from 
a  condition  of  parallelism  to  one  of  great  convergence,  but  we  cannot,  with- 
out special  assistance,  bring  them  from  a  condition  of  parallelism  to  one  of 
divergence.  The  stereoscope  will  enable  us  to  create  a  divergence.  If  in  a 
stereoscopic  picture  the  distance  between  the  pictures  be  increased  very 
gradually  so  as  carefully  to  maintain  the  impression  of  a  single  object,  the 


BINOCULAR  VISION.  785 

visual  axes  may  be  brought  to  diverge.  Similarly  if  a  distant  object  be 
looked  at  with  a  prism  before  one  eye,  and  the  image  of  the  object  be  kept 
carefully  single,  while  the  prism  is  turned  very  slowly  up  or  down,  then  on 
suddenly  removing  the  prism  a  double  image  is  for  a  moment  seen ;  show- 
ing that  the  eye  before  which  the  prism  was  placed  has  moved  in  disaccord- 
ance  with  the  other.  The  double  image,  however,  in  a  few  seconds  after  the 
removal  of  the  prism  becomes  single,  on  account  of  the  eyes  coming  into 
accordance. 

It  is  only  when  loss  of  coordination  occurs,  as  in  various  diseases  and  in 
alcoholic  or  other  poisoning,  that  the  movements  of  the  two  eyes  cease  to 
agree  with  each  other.  It  is  evident,  then,  that  when  we  look  at  an  object 
to  the  right,  since  we  thereby  abduct  the  right  eye  and  adduct  the  left,  we 
throw  into  action  the  rectus  externus  of  the  right  eye  and  the  rectus  intern  us 
of  the  left,  and  similarly  when  we  look  to  the  left  we  use  the  rectus  externus 
of  the  left  and  the  rectus  internus  of  the  right  eye.  On  the  other  hand, 
when  we  look  at  a  near  object,  and  therefore  converge  the  visual  axes,  we 
use  the  recti  interni  of  both  eyes  ;  and  when  we1  look  at  a  distant  object,  and 
bring  the  axes  from  converging  toward  parallelism,  we  use  the  recti  externi 
of  both  eyes.  In  the  various  movements  of  the  eye  there  is  therefore,  so  to 
speak,  the  most  delicate  picking  and  choosing  of  the  muscular  instruments. 
Bearing  this  in  mind,  it  cannot  be  wondered  at  that  the  various  movements 
of  the  eye  are  dependent  for  their  causation  on  visual  sensations.  In  order 
to  move  our  eyes  we  must  either  look  at  or  for  an  object ;  when  we  wish  to 
converge  our  axes  we  look  at  some  near  object,  real  or  imaginary,  and  the 
convergence  of  the  axes  is  usually  accompanied  by  all  the  conditions  of  near 
vision,  such  as  increased  accommodation  and  contraction  of  the  pupil.  And 
so  with  other  movements.  The  close  association  of  the  movements  of  the 
eye  may  be  illustrated  by  the  following  case:  Suppose  the  eyes,  to  start  with, 
accommodated  for  the  far  distance,  and  that  it  is  desired  to  direct  attention  to  a 
nearer  point  lying  in  the  visual  line  of  the  right  eye.  In  this  case  no  move- 
ment of  the  right  eye  is  required ;  all  that  is  necessary  is  for  the  left  eye  to 
be  turned  to  the  right,  that  is,  for  the  rectus  internus  of  the  left  eye  to  be 
thrown  into  action.  But  in  ordinary  movements  the  contraction  of  this 
muscle  is  always  associated  with  either  the  rectus  externus  of  the  right  eye, 
as  when  both  eyes  are  turned  to  the  right,  or  the  rectus  internus  of  that  eye, 
as  in  convergence  ;  the  muscle  is  quite  unaccustomed  to  act  alone.  This 
would  lead  us  to  suppose  that  in.  the  case  in  question  the  contraction  of  the 
rectus  internus  of  the  left  eye  is  accompanied  by  a  contraction  of  both  recti 
externus  and  internus  of  the  right  eye,  keeping  that  eye  in  lateral  equilib- 
rium. And  the  peculiar  oscillating  movements  seen  in  the  right  eye,  as  well 
as  the  sense  of  efforts  in  the  right  eye  which  is  felt  by  the  person,  show  this 
to  be  the  case. 

§  685.  Such  a  complex  coordination  requires  for  its  carrying  out  a  dis- 
tinct nervous  machinery,  and  we  have  reasons  for  thinking  that  such  a 
machinery  exists  in  certain  parts  of  the  corpora  quadrigemina  or  in  the 
underlying  structures.  In  the  nates  there  appears  to  be  a  common  centre 
for  both  eyes,  stimulation  of  the  right  side  producing  movements  of  both 
eyes  to  the  left,  of  the  left  side  movements  to  the  right;  while  stimula- 
tion in  the  middle  line  behind  causes  a  downward  movement  of  both 
eyes  with  convergence  of  the  axes  and  in  the  front  an  upward  move- 
ment with  return  to  parallelism,  both  accompanied  by  the  naturally  asso- 
ciated movements  of  the  pupil.  Stimulation  of  various  parts  of  the  nates 
causes  various  movements,  depending  on  the  position  of  the  spot  stimu- 
lated. After  an  incision  in  the  middle  line,  stimulation  of  the  nervous 
centre  on  one  side  produces  movements  in  the  eye  of  the  same  side  only. 

50 


786 


SIGHT. 


Diagram  illustrating  a  Simple  Horopter. 
When  the  visual  axes  converge  at  C,  the 
images  a,  a  of  any  point  A  on  the  circle 
drawn  through  C  and  the  optical  centres 
k,  k  will  fall  on  corresponding  points. 


The  Horopter. 

§  686.  When  we  look  at  any  object  we  direct  to  it  the  visual  axis,  so  that 
when  the  object  is  small,  the  "  corresponding  "  parts  of  the  two  retinas,  on 
which  the  two  images  of  the  object  fall,  lie  in  their  respective  fovese  cen- 
trales.  But  while  we  are  looking  at  the  particular  object,  the  images  of  other 

objects  surrounding  it  fall  on  the  retina 

FIG.  181.  surrounding  the  fovea,  and  thus  go  to 

form  what  is  called  indirect  vision.  And 
it  is  obviously  of  advantage  that  these 
images  also  should  fall  on  "  correspond- 
ing "  parts  in  the  two  eyes.  Now  for  any 
given  position  of  the  eyes  there  exists  in 
the  field  of  vision  a  certain  line  or  sur- 
face of  such  a  kind  that  the  images  of 
the  points  in  it  all  fall  on  corresponding 
points  of  the  retina.  A  line  or  surface 
having  this  property  is  called  a  horopter. 
The  horopter  is,  in  fact,  the  aggregate 
of  all  those  points  in  space  which  are 
projected  on  to  corresponding  points 
of  the  retina;  hence  its  determination 
in  any  particular  case  is  simply  a  mat- 
ter of  geometrical  calculation.  In  some 
instances  it  becomes  a  very  complicated 
figure.  The  case  whose  features  are 
most  easily  grasped  is  a  circle  drawn 
in  the  plane  of  the  two  visual  axes 
through  the  point  of  the  convergence  of  the  axes  and  the  optic  centres  of  the 
two  eyes.  It  is  obvious  from  geometrical  relations  that  in  Fig.  181  the 
images  of  any  point  in  the  circle  will  fall  on  corresponding  points  of  the 
two  retinas.  When  we  stand  upright  and  look  at  the  distant  horizon,  the 
horopter  is  (approximately,  for  normal  emmetropic  eyes)  a  plane  drawn 
through  our  feet,  that  is  to  say,  is  the  ground  on  which  we  stand ;  the 
advantage  of  this  is  obvious. 

VISUAL  JUDGMENTS. 
• 

§  687.  Binocular  vision  is  of  use  to  us,  inasmuch  as  the  one  eye  is  able 
to  fill  up  the  gaps  and  imperfections  of  the  other.  For  example,  over  and 
above  the  monocular  filling  up  of  the  blind  spot,  of  which  we  spoke  on  page 
779,  since  the  two  blind  spots  of  the  two  eyes,  being  each  on  the  nasal  side, 
are  not  "  corresponding  "  parts,  the  one  eye  supplies  that  part  of  the  field 
of  vision  which  is  lacking  in  the  other.  And  other  imperfections  are  sim- 
ilarly made  good.  But  the  great  use  of  binocular  vision  is  to  afford  us 
means  of  forming  visual  judgments  concerning  the  form,  size,  and  distance 
of  objects. 

§  688.  Judgment  of  distance  and  size.  The  perceptions  which  we  gain 
simply  and  solely  by  our  field  of  vision  concern  two  dimensions  only.  We 
can  become  aware  of  the  apparent  size  of  any  part  of  the  field  correspond- 
ing to  any  particular  object,  and  of  its  topographical  relations  to  the  rest  of 
the  field,  but  no  more.  Had  we  nothing  more  to  depend  on,  our  sight  would 
be  almost  valueless  as  far  as  any  exact  information  of  the  external  world 
was  concerned.  By  the  association  of  visual  sensations  with  sensations  of 
touch,  and  with  sensations  derived  from  the  movements  of  the  eyeballs  re- 
quired to  make  any  such  part  of  the  field  as  corresponds  to  a  particular 


VISUAL  JUDGMENTS.  787 

object  distinct,  we  are  led  to  form  judgments,  i.  e.,  to  draw  conclusions  con- 
cerning the  external  world  by  means  of  an  interpretation  of  our  visual  per- 
ceptions. Looking  before  us,  we  say  we  see  a  certain  object  of  a  certain 
color  nearly  in  front  of  us  or  much  on  our  right  hand  or  much  on  our 
left ;  that  is  to  say,  we  judge  such  an  object  to  be  in  such  a  position  because 
from  the  constitution  of  our  brain,  strengthened  by  all  our  experience,  we 
associate  such  a  part  of  our  field  of  vision  with  such  an  object.  The  sub- 
jective visual  complex  sensation  or  perception  is  to  us  a  symbol  of  the 
external  object. 

Even  with  one  eye  we  can,  to  a  certain  extent,  form  a  judgment,  not  only 
as  to  the  position  of  the  object  in  a  plane  at  right  angles  to  our  visual  axis, 
but  also  as  to  its  distance  from  us  along  the  visual  axis.  If  the  object  is 
near  to  us,  we  have  to  accommodate  for  near  vision  ;  if  far  from  us,  to  relax 
our  accommodation  mechanism  so  that  the  eye  becomes  adjusted  for  distance. 
The  muscular  sense  (of  which  we  shall  speak  presently)  of  this  effort  enables 
us  to  form  a  judgment  whether  the  object  is  far  or  near.  Seeing  the  narrow 
range  of  our  accommodation,  and  the  slight  muscular  effort  which  it  entails, 
all  molecular  judgments  of  distance  must  be  subject  to  much  error.  Every- 
one who  has  tried  to  thread  a  needle  without  using  both  eyes  knows  how 
great  these  errors  may  be.  When,  on  the  other  hand,  we  use  two  eyes,  we 
have  still  the  variations  in  accommodation,  and,  in  addition,  have  all  the 
assistance  which  arises  from  the  muscular  effort  of  so  directing  the  two 
eyes  on  the  object  that  single  vision  will  result.  When  the  object  is  near, 
we  converge  our  visual  axes  ;  when  distant,  we  bring  them  back  toward 
parallelism.  This  necessary  contraction  of  the  ocular  muscles  affords  a 
muscular  sense,  by  the  help  of  which  we  form  a  judgment  as  to  the  distance 
of  the  object.  Hence,  when  by  any  means  the  convergence  which  is  neces- 
sary to  bring  the  object  into  single  vision  is  lessened,  the  object  seems  to 
become  more  distinct,  when  increased,  to  move  toward  us — as  may  be  seen 
in  the  stereoscope. 

§  689.  The  judgment  of  size  is  closely  connected  with  that  of  distance. 
Our  perceptions,  gained  exclusively  from  the  field  of  vision,  go  no  further 
than  the  apparent  size  of  the  image,  i.  e.,  of  the  angle  subtended  by  the 
object.  The  real  size  of  the  object  can  only  be  gathered  from  the  apparent 
size  of  the  image  when  the  distance  of  the  object  from  the  eye  is  known. 
Thus  perceiving  directly  the  apparent  size  of  the  image,  we  judge  the  dis- 
tance of  the  object  giving  the  image,  and  upon  that  come  to  a  conclusion  as 
to  its  size.  And,  conversely,  when  we  see  an  object,  of  whose  real  size  we 
are  otherwise  aware,  or  are  led  to  think  we  are  aware,  our  judgment  of  its 
distance  is  influenced  by  its  apparent  size.  Thus  when  in  our  field  of  vision 
there  appears  the  image  of  a  man,  knowing  otherwise  the  ordinary  size  of  a 
man,  we  infer,  if  the  image  be  very  small,  that  the  man  is  far  off.  The 
reason  of  the  image  being  small  may  be  because  the  man  is  far  off,  in  which 
case  our  judgment  is  correct ;  it  may  be,  however,  because  the  image  has 
been  lessened  by  artificial  dioptric  means,  as  when  the  man  is  looked  at 
through  an  inverted  telescope,  in  which  case  our  judgment  becomes  a  delu- 
sion. So  also  an  image  on  a  screen  when  gradually  enlarged  seems  to  come 
forward,  when  gradually  diminished  seems  to  recede.  In  these  cases  the 
influence  on  our  judgment  of  the  muscular  sense  of  binocular  adjustment,  or 
monocular  accommodation,  is  thwarted  by  the  more  direct  influence  of  the 
association  between  size  and  distance. 

§  690.  Judgment  of  solidity.  When  we  look  at  a  small  circle,  all  parts 
of  the  circle  are  at  the  same  distance  from  us,  all  parts  are  equally  distinct 
at  the  same  time,  whether  we  look  at  it  with  one  eye  or  with  two  eyes. 
When,  on  the  other  hand,  we  look  at  a  sphere,  the  various  parts  of  which  are 


788 


SIGHT. 


at  different  distances  from  us,  a  sense  of  the  accommodation,  but  much  more 
a  sense  of  the  binocular  adjustment,  of  the  convergence  or  the  opposite  of  the 
two  eyes,  required  to  make  the  various  parts  successively  distinct,  makes  us 
aware  that  the  various  parts  of  the  sphere  are  unequally  distant ;  and  from 
that  we  form  a  judgment  of  its  solidity.  As  with  distance  of  objects,  so  with 
solidity,  which  is  at  bottom  a  matter  of  distance  of  the  parts  of  an  object,  we 
can  form  a  judgment  with  one  eye  alone ;  but  our  ideas  become  much  more 
exact  and  trustworthy  when  two  eyes  are  used.  And  we  are  much  assisted 
by  the  effects  produced  by  the  reflection  of  light  from  the  various  surfaces 
of  a  solid  object ;  so  much  so,  that  raised  surfaces  may  be  made  to  appear 
depressed,  or  vice  versa,  and  flat  surfaces  either  raised  or  depressed,  by 
appropriate  arrangements  of  shadings  and  shadow. 

FIG.  182. 


§  691.  Binocular  vision,  moreover,  affords  us  a  means  of  judging  of  the 
solidity  of  objects,  inasmuch  as  the  image  of  any  solid  object  which  falls  on 
the  right  eye  cannot  be  exactly  like  that  which  falls  on  the  left,  though 
both  are  combined  in  a  single  perception  of  the  two  eyes.  Thus,  when  we 
look  at  a  truncated  pyramid  placed  in  the  middle  line  before  us,  the  image 
which  falls  on  the  right  eye  is  of  the  kind  represented  in  Fig.  182,  R,  while 
that  which  falls  on  the  left  eye  has  the  form  of  Fig.  182,  L ;  yet  the  percep- 
tion gained  from  the  two  images  together  corresponds  to  the  form  of  which 
Fig.  182,  B,  is  the  projection.  Whenever  we  thus  combine  in  one  perception 
two  dissimilar  images,  one  of  the  one  and  the  other  of  the  other  eye,  we 
judge  that  the  object  giving  rise  to  the  images  is  solid. 

This  is  the  simple  principle  of  the  stereoscope,  in  which  two  slightly  dis- 
similar pictures,  such  as  would  correspond  to  the  vision  of  each  eye  sep- 
arately, are,  by  means  of  reflecting  mirrors,  as  in  Wheatstone's  original 
instrument,  or  by  prisms,  as  in  the  form  introduced  by  Brewster,  made  to 
cast  images  on  corresponding  parts  of  the  two  retinas,  so  as  to  produce  a 
single  perception.  Though  each  picture  is  a  surface  of  two  dimensions  only, 
the  resulting  perception  is  the  same  as  if  a  single  object,  or  group  of  objects, 
of  three  dimensions  had  been  looked  at. 

It  might  be  supposed  that  the  judgment  of  solidity  which  arises  when  two 
dissimilar  images  are  thus  combined  in  one  perception  was  due  to  the  fact 
that  all  parts  of  the  two  images  cannot  fall  on  corresponding  parts  of  the 
two  retinas  at  the  same  time,  and  that  therefore  the  combination  of  the  two 
needs  some  movement  of  the  eyes.  Thus,  if  we  superimpose  R  on  L  (Fig. 
182),  it  is  evident  that  when  the  bases  coincide  the  truncated  apices  will  not, 
and  vice  versa;  hence,  when  the  bases  fall  on  corresponding  parts,  the  apices 
will  not  be  combined  in  one  image,  and  vice  versa;  in  order  that  both  may 
be  combined,  there  must  be  a  slight,  rapid  movement  of  the  eyes  from  the 
one  to  the  other.  That,  however,  no  such  movement  is  necessary  for  each 
particular  case  is  shown  by  the  fact  that  solid  objects  appear  as  such  when 
illuminated  by  an  electric  spark,  the  duration  of  which  is  too  short  to  permit 
of  any  movement  of  the  eyes.  If  the  flash  occurred  at  the  moment  that 
the  eyes  were  binocularly  adjusted  for  the  bases  of  the  pyramids,  the  two 


THE  PROTECTED  MECHANISMS  OF  THE  EYE.  789 

apices  not  falling  on  exactly  corresponding  parts  would  give  rise  to  two 
perceptions,  and  the  whole  object  ought  to  appear  confused.  That  it  does 
not,  but,  on  the  contrary,  appears  a  single  solid,  must  be  the  result  of  cere- 
bral operations,  resulting  in  what  we  have  called  a  judgment. 

§  692.  Struggle  of  the  two  fields  of  vision.  If  the  images  of  two  surfaces, 
one  black  and  the  other  white,  are  made  to  fall  on  corresponding  parts  of  the 
eye,  so  as  to  be  united  into  a  single  perception,  the  result  is  not  always  a 
mixture  of  the  two  impressions,  that  is,  a  gray,  but,  in  many  cases,  a  sensa- 
tion similar  to  that  produced  when  a  polished  surface,  such  as  plumbago,  is 
looked  at :  the  surface  appears  brilliant.  The  reason,  probably,  is  because 
when  we  look  at  a  polished  surface,  the  amount  of  reflected  light  which  falls 
upon  the  retina  is  generally  different  in  the  two  eyes  ;  and  hence  we  associate 
an  unequal  stimulation  of  the  two  retinas  with  the  idea  of  a  polished  surface. 
So,  also,  when  the  impressions  of  two  colors  are  united  in  binocular  vision, 
the  result  is,  in  most  cases,  not  a  mixture  of  the  two  colors,  as  when  the  same 
two  impressions  are  brought  to  bear  together  at  the  same  time  on  a  single 
retina,  but  a  struggle  between  the  two  colors,  now  one  and  now  the  other, 
becoming  prominent,  intermediate  tints,  however,  being  frequently  passed 
through.  This  may  arise  from  the  difficulty  of  accommodating  at  the  same 
time  for  the  two  different  colors  (see  p.  758);  if  two  eyes,  one  of  which  is 
looking  at  red  and  the  other  at  blue,  be  both  accommodated  for  red  rays, 
the  red  sensation  will  overpower  the  blue,  and  vice  versa.  It  may  be,  how- 
ever, that  the  tendency  to  rhythmic  action,  so  manifest  in  other  simpler 
manifestations  of  protoplasmic  activity,  makes  its  appearance  also  in  the 
higher  cerebral  labors  of  binocular  vision. 


THE  PROTECTED  MECHANISMS  OF  THE  EYE. 

§  693.  The  eyeball  is  protected  by  the  eyelids,  which  are  capable  of  move- 
ments called  respectively  opening  and  shutting  the  eye.  The  eye  is  shut  by 
the  contraction  of  the  orbicularis  muscle,  carried  out  either  as  a  reflex  or 
voluntary  act  by  means  of  the  facial  nerve.  The  eye  is  opened  chiefly  by 
the  raising  of  the  upper  eyelid  through  the  contraction  of  the  levator  pal- 
pebrse  carried  out  by  means  of  the  third  nerve.  The  upper  eyelid  is  also 
raised  and  the  lower  depressed,  the  eye  being  thus  opened,  by  means  of 
plain  muscular  fibres  existing  in  the  two  eyelids  and  governed  by  the  cer- 
vical sympathetic.  The  shutting  of  the  eye,  as  in  winking,  is  in  general 
effected  more  rapidly  than  the  opening. 

The  eye  is  kept  continually  moist  partly  by  the  secretion  of  the  glands 
in  the  conjunctiva,  and  of  the  Meibomian  glands,  but  chiefly  by  the  secre- 
tion of  the  lachrymal  gland.  Under  ordinary  circumstances  the  fluid  thus 
formed  is  carried  away  by  the  lachrymal  canals  into  the  nasal  sac  and  thus 
into  the  cavity  of  the  nose.  When  the  secretion  becomes  too  abundant  to 
escape  in  this  way  it  overflows  on  to  the  cheeks  in  the  form  of  tears. 

If  a  quantity  of  tears  be  collected,  they  are  found  to  form  a  clear,  faintly 
alkaline  fluid,  in  many  respects  like  saliva,  containing  about  1  per  cent,  of 
solids,  of  which  a  small  part  is  proteid  in  nature.  Among  the  salts  present 
sodium  chloride  is  conspicuous. 

§  694.  The  nervous  mechanism  of  the  secretion  of  tears,  in  many  re- 
spects, resembles  that  of  the  secretion  of  saliva.  A  flow  is  usually  brought 
about  either  in  a  reflex  manner  by  stimuli  applied  to  the  conjunctiva,  the 
nasal  mucous  membrane,  tongue,  optic  nerve,  etc.,  or  more  directly  by 
emotions.  Venous  congestion  of  the  head  is  also  said  to  cause  a  flow.  The 
efferent  nerves  belong  either  to  the  cerebro-spinal  system  (the  lachrymal  and 


790  HEARING,  SMELL,   AND  TASTE. 

orbital  branches  of  the  fifth  nerve)  or  arise  from  the  cervical  sympathetic, 
the  afferent  nerves  varying  according  to  the  exciting  cause. 

The  act  of  blinking  undoubtedly  favors  the  passage  of  tears  through  the 
lachrymal  canals  into  the  nasal  sac,  and  hence  when  the  orbicularis  is  para- 
lyzed tears  do  not  pass  so  readily  as  usual  into  the  nose ;  but  the  exact 
mechanism  by  which  this  is  effected  has  been  much  disputed.  According 
to  some  authors,  the  contraction  of  the  orbicularis  presses  the  fluid  onward 
out  of  the  canals,  which  upon  the  relaxation  of  the  orbicularis  dilate  and 
receive  a  fresh  quantity.  Others  maintain  that  a  special  arrangement  of 
muscular  fibres  keeps  the  canals  open  even  during  the  closing  of  the  lids,  so 
that  the  pressure  of  the  contraction  of  the  orbicularis  is  able  to  have  full 
effect  in  driving  the  tears  through  the  canals. 


CHAPTER    IV. 
HEARING,  SMELL,  AND  TASTE. 

§  695.  As  in  the  eye,  so  in  the  ear,  we  have  to  deal  first  with  a  nerve 
of  special  sense,  the  stimulation  of  which  gives  rise  to  a  special  sensation  * 
secondly,  with  terminal  organs  through  which  the  physical  changes  proper 
to  the  special  sense  are  enabled  to  act  on  the  nerve ;  and  thirdly,  with  sub- 
sidiary apparatus,  by  which  the  usefulness  of  the  sense  is  increased.  The 
central  connections  of  the  auditory  nerve  are  such  that  whenever  the  audi- 
tory fibres  are  stimulated,  whether  by  means  of  the  terminal  organs  in  the 
usual  way  or  by  the  direct  application  of  stimuli,  electrical,  mechanical,  etc., 
the  result  is  always  a  sensation  of  sound.  Just  as  stimulation  of  the  optic 
fibres  produces  no  other  sensation  than  that  of  light,  so  stimulation  of  the 
auditory  fibres  produces  no  other  sensation  than  that  of  sound.1  The  ter- 
minal organs  of  the  auditory  nerve  are  of  two  kinds  :  the  complicated  organ 
of  Corti  in  the  cochlea,  and  the  epithelial  arrangements  of  the  maculae  and 
cristse  acusticse  in  other  parts  of  the  labyrinth.  Waves  of  sound  falling  on 
the  auditory  nerve  itself  produce  no  effect  whatever  ;  it  is  only  when  by  the 
medium  of  the  endolymph  they  are  brought  to  bear  on  the  delicate  and 
peculiar  epithelium  cells  which  constitute  the  peripheral  terminations  of  the 
nerve,  that  sensations  of  sound  arise.  Such  delicate  structures  are  for  the 
sake  of  protection  naturally  withdrawn  from  the  surface  of  the  body  where 
they  would  be  subject  to  injury.  Hence,  the  necessity  of  an  acoustic  appa- 
ratus, forming  the  middle  and  external  ear,  by  which  the  waves  of  sound 
are  most  advantageously  conveyed  to  the  terminal  organs. 

HEARING. 

§  696.  [The  ear,  or  organ  of  hearing,  is  composed  of  three  parts,  called 
the  external,  middle,  and  internal  ear. 

The  external  ear  consists  of  an  outer  projecting  portion,  called  the  pinna, 
and  the  auditory  canal,  or  meatus  auditorius  externus.  (Fig.  183.)  The 
pinna  is  a  somewhat  oblong  funnel-shaped  organ,  the  smaller  portion  of  the 
funnel  being  attached  to  the  skull  by  ligamentous  tissue,  the  larger  portion 
serving  to  collect  and  convey  the  sonorous  undulations  to  the  meatus.  It  is 
composed  of  cartilage  covered  by  integument.  Its  surface  is  irregularly 

1  It  will  be  seen  later  on  that  there  are  reasons  for  thinking  that  impulses  passing  along 
the  auditory  nerve  may  give  rise  to  other  effects  than  auditory  sensations. 


HEARING. 


'91 


curved  and  depressed.  The  outer  projecting  rim  is  the  helix;  anterior  to 
the  helix  is  a  second  elevation,  called  the  antihelix,  which  describes  a  curve 
partially  around  a  deep  depression  which  leads  to  the  meatus,  called  the 

FIG.  183. 


Vertical  Section  of  the  Meatus  Auditorius  and  Tympanum.  (Scarpa.)  a,  cartilaginous  part 
of  the  meatus ;  b,  osseous  portion  ;  c,  membrana  tympani ;  d,  cavity  of  the  tympanum ;  e,  Eusta- 
chian  tube. 


FIG.  185. 


FIG.  184. 


FIG.  184.— Inner  view  of  the  Membrana 
Tympani  in  the  Fretus,  with  the  Malleus  at- 
tached, a,  membrane  or  drum  of  the  tym- 
panum ;  6,  malleus ;  c,  band  of  circular  fibres 
at  the  circumference  ;  d,  inferior,  and  e,  supe- 
rior tympanic  artery  ;  /,  tympanic  bone. 


FIG.  185.— Plan  of  the  Ossicles  in  Position  in  the  Tympanum,  with  their  Muscles,  a,  cavity  of 
the  tympanum;  6,  membrana  tympani;  c,  Eustachian  tube;  d,  malleus;  e,  incus;  /,  stapes  • 
g,  laxator  tympani  muscle  ;  h,  tensor  tympani ;  i,  stapedius. 

concha.  Between  the  helix  and  antihelix  is  the  fossa  of  the  helix.  The 
antihelix  bifurcates  at  its  superior  portion,  and  encloses  the  fossa  of  the  anti- 
helix.  Projecting  posteriorly  from  the  anterior  portion  of  the  concha  is  a 


792  HEARING,   SMELL,   AND  TASTE. 

papillary  prominence  called  the  tragus ;  posterior  to  this,  separated  by  a 
fissure,  is  the  antitragus,  which  is  a  continuation  of  the  helix.  On  the 
inferior  portion  of  the  pinna  is  a  soft  pendulous  portion,  termed  the 
lobule.  The  meatus  leads  from  the  concha  to  the  middle  ear,  from  which  it 
is  separated  by  the  tympanic  membrane.  Its  direction  is  forward,  inward, 
and  slightly  upward  ;  its  lower  surface  being  longer  than  the  upper,  on  ac- 
count of  the  obliquity  of  the  position  of  the  tympanic  membrane.  The  canal 
consists  of  an  external  membrano-cartilaginous  portion,  which  is  continuous 
with  the  pinna,  and  an  internal  osseous  portion  formed  by  the  mastoid  bone. 
In  the  external  portion  of  the  canal  are  found  numerous  hairs  and  seba- 
ceous glands  ;  in  the  internal  portion  are  found  the  ceruminous  glands,  which 
secrete  a  peculiar  substance  commonly  known  as  the  earwax. 

§  697.  The  middle  ear  or  tympanum  is  an  irregular  flattened  cavity, 
situated  in  the  petrous  portion  of  the  temporal  bone,  and  lined  with  a  mu- 
cous membrane.  It  is  separated  from  the  meatus  by  a  membranous  dia- 
phragm, which  is  the  tympanic  membrane ;  and  from  the  internal  ear  by  an 
osseo-membranous  partition,  which  forms  a  common  wall  for  both.  Through 
the  Eustachian  tube  it  communicates  with  the  pharynx.  On  its  posterior 
wall  are  seen  orifices  of  the  mastoid  cells.  The  tympanic  membrane  is  a 
semi-transparent  oval  membrane,  concave  on  its  external  and  convex  on  its 

FIG.  186. 


FIG.  187. 


Natural  size. 


Interior  of  the  Osseous  Labyrinth.  (After  Sommerring.)  V,  vestibule ;  av,  aqueduct  of  the 
vestibule;  o,  fovea  hemielliptica ;  ?•,  fovea  hemispherica ;  S,  semicircular  canals;  s,  superior;  p, 
posterior;  i,  horizontal;  a,  a,  a,  the  ampullar  extremity  of  each;  C,  cochlea;  etc,  aqueduct  of  the 
cochlea  ;  sv,  osseous  zone  of  the  lamina  spiralis,  above  which  is  the  scala  vestibuli,  communicat- 
ing with  the  vestibule  ;  st,  scala  tympani  below  the  spiral  lamina. 

internal  surface,  where  it  has  attached  the  long  process  of  the  malleus,  one 
of  the  ossicles.  It  is  placed  in  an  oblique  position,  sloping  downward,  for- 
ward, and  inward  at  an  angle  of  about  45°.  Its  circumference  is  attached 
to  a  groove  in  the  temporal  bone.  In  the  fetus  this  portion  of  the  bone 
exists  as  a  separate  piece,  called  the  tympanic  bone  (Fig.  184),  but  it  after- 
ward becomes  ossified  to  the  temporal.  The  tympanic  membrane  consists 
of  three  layers — the  external,  middle,  and  internal.  The  external  is  a 
continuation  of  the  integument  covering  the  meatus ;  the  internal  is  a  con- 
tinuation of  the  mucous  membrane  lining  the  tympanum  ;  the  middle  layer, 


HEARING.  793 

which  is  the  most  important,  is  tense,  strong,  and  fibrous,  made  up  of  cir- 
cular and  radiating  fibres,  with  a  small  amount  of  elastic  tissue  intermixed. 
§  698,  In  the  internal  wall  of  the  tympanum  are  two  small  openings — 
the  fenestra  ovalis  and  fevestra  rotunda — which  communicate  with  the  laby- 
rinth. The  fenestra  rotunda  is  closed  by  a  membrane.  Extending  between 
the  tympanic  membrane  and  the  fenestra  ovalis  are  the  ossicles,  consisting 
of  three  small  bones,  which  form  a  system  of  levers.  These  ossicles  are 
termed,  from  their  resemblance  to  particular  objects,  the  malleus,  incus,  and 
stapes.  (Fig.  185.)  The  malleus  consists  of  a  head,  neck,  long  and  short 
process,  and  handle.  The  head  articulates  with  the  roof  of  the  tympanum 
and  in  a  depression  of  the  incus ;  the  handle  is  directed  downward  and 
attached  by  its  whole  length  to  the  tympanic  membrane ;  the  long  process 
( processus  gracilts)  is  directed  forward  and  has  attached  the  insertion  of  the 
laxator  tympani  muscle;  the  short  process,  which  is  at  the  base  of  the  long 
process,  has  attached  the  insertion  of  the  tensor  tympani  muscle.  The  incus 
consists  of  a  body,  a  long  and  short  process.  The  body  of  the  incus  has  a 
depression  in  which  articulates  the  head  of  the  malleus ;  the  short  process 
is  attached  to  the  posterior  wall  of  the  tympanum  ;  the  long  process  (len- 
ticular process)  is  placed  almost  vertically,  and  at  its  end  is  a  rounded 

FIG.  188. 


Representation  of  the  Semicircular  Canals  Enlarged.  (From  a  model  in  University  College 
Museum.)  a,  superior  vertical ;  6,  posterior  or  inferior  vertical ;  and  c,  horizontal  canal ;  d,  com- 
mon opening  of  the  two  vertical  canals  ;  e,  part  of  the  vestibular  cavity  ;  /,  opening  of  the  aque- 
duct of  the  vestibule. 

process  (the  os  orbiculare},  which  articulates  with  the  head  of  the  stapes. 
The  stapes  consists  of  a  head,  neck,  two  crura,  and  a  base.  The  head  arti- 
culates with  the  long  process  of  the  incus ;  the  neck  serves  as  a  point  of  in- 
sertion of  the  stapedius  muscle  ;  the  crura  diverge  from  the  neck  and  unite 
with  the  oval  base  at  its  greatest  diameter.  The  base  is  fixed  in  the  fe- 
nestra ovalis  by  attachments  formed  by  the  lining  membranes  of  both  the 
tympanum  and  internal  ear.  These  ossicles  are  connected  with  each  other 
and  to  the  walls  of  the  tympanum  by  ligaments,  and  at  their  articulations 
they  are  furnished  with  cartilages  and  synovial  membranes.  They  are  en- 
veloped by  prolongations  of  the  mucous  membrane  lining  the  tympanum. 

§  699.  The  internal  ear  or  labyrinth  is  the  most  essential  portion  of  the 
auditory  apparatus.     It  consists  of  three  portions — the  vestibule,  semicircular 


794  HEARING,  SMELL,  AND  TASTE. 

canals,  and  cochlea — and  is  situated  within  the  petrous  portion  of  the  tem- 
poral bone.  Within  the  osseous  labyrinth  is  a  membranous  labyrinth  to 
which  the  auditory  nerve  is  distributed.  The  vestibule  is  an  irregular 
chamber  which  serves  as  a  common  means  of  communication  between  the 
tympanum  and  the  semicircular  canals  and  cochlea.  On  its  external  wall 
is  the  fenestra  ovalis,  closed  by  the  base  of  the  stapes.  On  its  internal  wall 
is  a  depression  called  the  fovea  hemispherica,  which  is  perforated  by  minute 
openings  for  the  passage  of  auditory  nerve- filaments.  Above  and  posterior 
to  this  depression  is  another,  the  fovea  hemielliptica.  Posterior  to  the  fovea 
hemispherica  is  the  orifice  of  the  aqueductus  vestibuli.  In  the  posterior 
wall  are  five  openings  leading  to  the  semicircular  canals.  Anteriorly,  it 
communicates  with  the  cochlea  by  the  aperturce  scales  vestibuli  cochleae.  The 
semicircular  canals  are  three  in  number — superior,  posterior  or  inferior,  and 
horizontal.  They  form  the  greater  portion  of  a  circle,  and  communicate 
with  the  vestibule  by  five  openings,  one  of  which  is  common  to  the  superior 
and  horizontal  canals.  The  superior  canal  is  situated  vertically  and  at 
right  angles  with  the  posterior  surface  of  the  petrous  bone  ;  the  posterior 
canal  is  also  vertical  and  parallel  with  the  posterior  surface  of  the  petrous 
bone ;  the  inferior  canal  is  placed  horizontally  and  at  right  angles  to  the 
others.  At  the  commencement  of  each  of  these  canals  is  a  dilated  portion, 
called  the  ampulla. 

§  700.  The  cochlea  occupies  the  anterior  portion  of  the  labyrinth.  Its 
base,  which  corresponds  to  the  internal  auditory  meatus,  is  perforated  by 
many  minute  orifices  for  the  passage  of  filaments  of  the  cochlear  branch  of 
the  auditory  nerve.  The  cochlea  consists  of  a  central  axis,  or  modiolus, 
which  has  a  spiral  canal  wound  around  it.  This  canal  makes  two  and  a 
half  complete  turns,  and  terminates  in  the  apex  of  the  cochlea  in  an  ex- 
pansion termed  the  infundibulum.  (Fig.  189.)  The  modiolus  is  somewhat 

FIG.  189. 


Section  through  the  Cochlea.  (Breschet.)  a,  axis  with  its  canals;  6,  infundibulum  or  en- 
larged upper  end  of  the  axis  ;  c,  septum  of  the  cochlea;  d,  membrane  of  Corti ;  e,  membrane  of 
Reissner  ;  /,  hiatus  or  hellicotrema  ;  st,  scala  tympani ;  sv,  scala  vestibuli. 

cone-shaped,  and  forms  the  internal  wall  of  the  canal,  being  perforated  in  its 
centre  and  sides  by  apertures  for  the  passage  of  the  filaments  of  the  audi- 
tory nerve.  The  canal  is  divided  into  two  passages  or  scalce  by  a  septum 
called  the  lamina  spiralis,  which  is  partly  osseous  and  partly  membranous. 
The  osseous  portion  projects  from  the  modiolus,  midway  across  the  canal ; 
it  consists  of  two  laminae,  between  which  the  nerve-filaments  run.  The 
membranous  portion  extends  from  the  external  margin  of  the  osseous  lamina 


HEARING.  795 

to  the  external  wall  of  the  canal.  It  consists  of  two  layers;  the  superior  is 
the  membrane  of  Corti,  or  membrana  tectoria ;  the  inferior  the  membrana 
basilaris,  which  is  attached  externally  to  the  planum  semilunare.  These 
membranes  are  placed  parallel  with  each  other  and  contain  between  them 
the  organ  of  Corti,  which  rests  on  the  basilary  membrane.  (Fig.  190.) 

§  701.  The  scala  vestibuli 1  communicates  below  with  the  vestibule  by  the 
aperturce  scalw  vestibuli  cochleae ;  the  lower  passage,  or  scala  tympani,  com- 
municates with  the  tympanum  by  the  fenestra  rotunda.  These  scalse  com- 
municate at  the  apex  of  the  cochlea  by  an  opening  termed  the  hiatus  or 
helicotrema,  which  exists  in  consequence  of  a  deficiency  of  the  last  half  turn 
of  the  lamina  spiralis. 

The  osseous  portion  of  the  lamina  spiralis  has  on  its  superior  external 
portion  a  denticulated  cartilaginous  substance  called  the  lamina  denticulata. 
From  the  superior  surface  of  the  spiral  lamina,  and  internal  to  the  lamina 
denticulata,  is  a  delicate  membrane  extending  upward  and  outward  at  an 
angle  of  about  45  degrees  to  the  external  wall  of  the  scala.  This  is  called 
the  membrane  of  Reissner.  It  divides  the  scala  into  two  passages,  the  lower 
of  which  is  the  ductus  cochlearis.  This  duct  ends  in  the  apex  of  the  cochlea 
in  a  coeca,  and  communicates  at  the  base  with  the  saccule  by  the  ductus 
reuniens ;  it  contains  the  essential  portion  of  the  auditory  apparatus  of  the 
cochlea,  and  is  a  part  of  the  membranous  labyrinth. 

FIG.  190. 


A  Diagram  of  a  Section  of  the  Tube  of  the  Cochlea,  enlarged.  (Modified  from  Henle.):  SV, 
scala  vestibuli ;  ST,  scala  tympani ;  CC,  canal  of  the  cochlea ;  1,  membrane  of  Reissner;  2,  coch- 
lear  branch  of  the  auditory  nerve;  3,  lamina  spiralis  ossea;  4,  planum  semilunare;  a,  lamina 
denticulata ;  &,  sulcus  spiralis ;  c,  tympanic  lip  of  the  sulcus  spiralis ;  d,  inner  rods  of  Corti ;  e,  outer 
rods  of  Corti ;  /,  lamina  reticularis ;  i,  inner  hair-cells  ;  mb,  membrana  basilaris ;  me,  membrane  of 
Corti ;  p,  outer  hair-cells ;  sm,  central  space  between  the  rods. 

§  702.  The  organ  of  Corti  rests  upon  the  basilary  membrane.  It  consists 
of  the  inner  and  outer  hair-cells,  and  two  rows  of  elongated  cells,  placed 
parallel  with  each  other,  having  an  inclining  position  so  that  their  free  ex- 
tremities rest  against  each  other  and  thus  form  the  arch  of  Corti,  which  covers 
the  central  space.  (Fig.  190.)  These  rows  are  called'  the  inner  and  outer 
rods  or  pillars  of  Corti.  From  the  superior  extremity  of  both  the  inner  and 

1  The  upper  scala  is  divided  into  two  parts  by  a  membranous  partition,  the  upper  of 
which  is  called  the  scala  vestibuli ;  the  other,  ductus  cochlearis  (Fig.  190). 


796 


HEARING,  SMELL,  AND  TASTE. 


outer  rods,  finger-like  processes  project  externally.     At  their  bases  corre- 
sponding to  the  central  space  are  single  rows  of  nucleated  cells.     On  the 


FIG.  191. 


FIG.  192. 


FIG.  191.-Petrcms  Bone  partly  Removed  to  Show  the  Membranous  Labyrinth  in  Place. 
(Breschet.)  o,  small  sac ;  6,  its  otolith  ;  c,  ductus  reunions ;  d,  large  sac  or  utricle  ;  e,  its  otolith ; 
/,  ampullary  enlargements  on  a  semicircular  tube  ;  g,  semicircular  tube. 

FIG.  192.— Distribution  of  Nerves  to  the  Membranous  Labyrinth.  (Breschet.)  a,  nerve  to  the 
saccule  ;  b,  nerve  entering  the  ampullary  enlargement  on  a  semicircular  tube  ;  c,  branch  of  the 
nerve  entering  the  large  sac  or  utricle. 

FIG.  193. 


Diagram  of  the  mode  of  Termination  of  the  Auditory  Nerve  in  the  Ampullae  and  Sacculi.  1, 
wall  of  the  ampulla  ;  2,  structureless  basement-membrane;  3,  doubly  contoured  nerve-fibres;  4, 
axis-cylinder  traversing  the  basement-membrane  ;  5,  plexiform  union  of  fine  nerve-fibres  with 
interspersed  nuclei;  6,  fusiform  cells,  with  nucleus  and  dark  fibre  in  their  interior ;  7,  supporting 
cells  ;  8,  auditory  hairs. 


HEAKING.  797 

internal  side  of  the  inner  rod  is  a  single  row,  and  on  the  external  side  of  the 
outer  rods  are  three  rows  of  elongated  ciliated  cells.  Extending  across  the 
top  of  the  organ  of  Corti,  from  the  inner  hair-cells  to  the  external  wall  of 
the  canal,  is  a  very  delicate  structure  called  the  reticular  membrane.  The 
auditory  nerve-filaments  probably  terminate  in  the  ciliated  cells,  being 
intimately  connected  with  the  cilia. 

§  703.  The  osseous  labyrinth  is  lined  by  a  fibro-serous  membrane  which 
secretes  a  watery  fluid  called  the  perilymph.  The  perilymph  fills  the  scalse 
of  the  cochlea,  and  surrounds  the  duetus  cochlearis  and  the  membranous 
portions  of  the  labyrinth,  which  are  situated  in  the  vestibule  and  semicircu- 
lar canal. 

§  704.  The  membranous  labyrinth  is  a  closed  sac  consisting  of  the  semi- 
circular canals,  a  vestibular  portion,  and  the  duetus  cochlearis  of  the  cochlea. 
The  semicircular  canals  are  of  the  same  form  as  the  osseous  canals,  and  are 
contained  within  them.  The  vestibular  portion  consists  of  an  expanded 
body,  the  utricle,  and  a  smaller  body,  the  saceule.  The  utricle  is  situated  at 
the  fovea  hemielliptica  ;  the  semicircular  canals  open  on  its  internal  surface. 
The  saceule  lies  at  the  fovea  hemispherica ;  it  is  connected  with  the  duetus 
cochlearis  by  the  duetus  reuniens.  In  the  walls  of  the  saceule  and  utricle 
are  two  calcareous  bodies  called  the  otoliths.  The  walls  of  the  ampulla, 
according  to  Bowman,  also  contain  some  grains  of  a  similar  substance.  The 
walls  of  the  membranous  labyrinth  consist  of  a  fibrous  tissue,  lined  by  pave- 
ment nucleated  epithelial  cells,  having  a  structureless  basement-membrane. 
These  epithelial  cells  are  much  modified  at  the  place  of  entrance  of  the 
fibres  of  the  auditory  nerve.  The  vestibular  branches  of  the  auditory  nerve 
are  distributed  to  the  ampullae,  utricle,  and  saceule.  (Fig.  192.)  In  the 
utricle  and  the  saceule  the  fibres  terminate  in  oval  plates,  called  the  maculae 
acusticce,  which  are  more  or  less  colored  by  the  deposition  of  yellow  pigment. 
In  the  ampullae  the  fibres  terminate  in  elevations  called  the  cristce  acusticce. 
After  the  nerve-filament  pierces  the  membranous  wall  at  these  points,  the 
axis-cylinder  alone  penetrates  the  basement-membrane;  it  then  forms  a 
plexus  of  delicate  nerve-fibres  with  nuclei,  and  finally  terminates  in  fusiform 
epithelial  cells  which  have  terminal  cilia  called  the  auditory  hairs.  (Fig. 
193.)  These  ciliated  cells  are  supported  by  columnar  epithelium. 

The  membranous  labyrinth  is  lined  by  polygonal  nucleated  epithelium, 
which  secretes  the  endolymph  which  fills  the  sac.] 

The  Acoustic  Apparatus. 

§  705.  Waves  of  sound  can  and  do  reach  the  endolymph  of  the  laby- 
rinth by  direct  conduction  through  the  skull.  Since,  however,  sonorous 
vibrations  are  transmitted  with  great  difficulty  from  the  air  to  solids  and 
liquids,  and  most  sounds  come  to  us  through  the  air,  some  special  apparatus 
is  required  to  transfer  the  aerial  vibrations  to  the  liquids  of  the  internal  ear. 
This  apparatus  is  supplied  by  the  tympanum  and  its  appendages. 

§  706.  The  concha.  The  use'  of  this,  as  far  as  hearing  is  concerned,  is  to 
collect  the  waves  of  sound  coming  in  various  directions,  and  to  direct  them 
on  to  the  membrana  tympani.  In  ourselves  of  moderate  service  only,  in 
many  animals  it  is  of  great  importance. 

§  707.  The  membrana  tympani.  It  is  a  characteristic  property  of 
stretched  membranes  that  they  are  readily  thrown  into  vibration  by  aerial 
waves  of  sound.  The  membrana  tympani,  from  its  peculiar  conformation, 
being  funnel-shaped  with  a  depressed  centre  surrounded  by  sides  gently  con- 
vex outward,  is  peculiarly  susceptible  to  sonorous  vibrations,  and  is  most 
readily  thrown  into  corresponding  movements  when  waves  of  sound  reach  it 


798  HEARING,  SMELL,   AND  TASTE. 

by  the  meatus.  It  has,  moreover,  this  useful  feature,  that  unlike  other 
stretched  membranes,  it  has  no  marked  note  of  its  own.  It  is  not  thrown 
into  vibrations  by  waves  of  a  particular  length  more  readily  than  by  others. 
It  answers  equally  well  within  a  considerable  range  to  vibrations,  of  very 
different  wave-lengths.  Had  it  a  fundamental  tone  of  its  own,  we  should 
be  distracted  by  the  prominence  of  this  note  in  most  of  the  sounds  we  hear. 
When  sounds  impinge  on  the  solids  of  the  head,  as  when  a  watch  is  held 
between  the  teeth,  the  membrana  tympaui  is  still  functional.  Vibrations  are 
conveyed  from  the  temporal  bone  to  it  and  hence  pass  in  the  usual  way,  in 
addition  to  those  transmitted  directly  from  the  bone  to  the  perilymph. 

§  708.  The  auditory  ossicles.  The  malleus,  the  handle  of  which  de- 
scending forward  and  inward,  is  attached  to  the  membrana  tympani,  and  the 
incus,  whose  long  process  is  connected  by  means  of  its  os  orbiculare  or  len- 
ticular process  and  the  stapes  to  the  fenestra  ovalis,  form  together  a  body 
which  rotates  round  an  axis,  passing  through  the  short  process  of  the  incus, 
the  bodies  of  the  incus  and  malleus,  and  the  processus  gracilis  of  the  malleus. 
[Fig.  194.]  When  the  malleus  is  carried  inward,  the  incus  moves  inward 

[FlG.  194. 


M.- 


The  Ossicles  in  Position.  Magnified  four  times.  (After  Hensen.)  The  figure  represents  a  sec- 
tion through  tympanum  in  the  line  of  the  long  axis  of  the  malleus  and  incus ;  the  short  process 
of  the  incus,  p'b',  has  been  cut  through. 

T.C,  the  tympanic  cavity  ;  mbr,  handle  of  malleus ;  u,  umbo ;  p.b,  short  process  of  the  malleus 
shown  in  dotted  outline  as  pushing  outward  the  membrana  flaccida;  T.I,  the  attachment  of  the 
tendon  of  the  tensor  tympani ;  Ig,  the  attachment  of  the  external  ligament  of  the  malleus ;  Ig.s, 
the  superior  ligament  of  the  malleus ;  t.t,  the  teeth  of  the  incus  ;  p'l.  the  long  process  shaft  of  the 
incus ;  St,  the  stapes.] 

too,  and  when  the  malleus  returns  to  its  position,  the  incus  returns  with  it, 
the  peculiar  saddle-shaped  joint  with  its  catch-teeth  permitting  this  move- 
ment readily,  but  preventing  the  stapes  being  pulled  back  when  the  mem- 
brana tympani  with  the  malleus  is  for  any  reason  pushed  outward  more 
than  usual ;  the  joint  then  gapes,  so  as  to  permit  the  malleus  to  be  moved 
alone.  Various  ligaments,  the  superior  or  suspensory,  anterior,  and  external, 
also  serve  to  keep  the  malleus  in  place.  The  whole  series  of  ossicles  may 
be  regarded  as  a  single-armed  lever,  moving  on  the  ligamental  attachment 
of  the  short  process  of  the  incus  to  the  posterior  wall  of  the  tympanum,  the 
weight  being  brought  to  bear  at  the  end  of  the  long  process  of  the  incus, 
and  the  power  at  the  end  of  the  handle  of  the  malleus.  The  long,  malleal 


HEARING. 


799 


arm  of  this  lever  is  about  9J  mm.,  the  short,  stapedial,  6^  mm.  in  length  ; 
hence,  the  movements  of  the  stapes  are  less  than  those  of  the  tympanum  ; 
but  the  loss  in  amplitude  is  made  up  by  a  gain  of  force,  which  is  in  itself 
an  obvious  advantage. 

Thus  every  movement  of  the  tympanic  membrane  is  transmitted  through 
this  chain  of  ossicles  to  the  membrane  of  the  fenestra  ovalis,  and  so  to  the 
perilymph  of  the  labyrinth  ;  the  vibrations  of  the  tympanic  membrane  are 
conveyed  with  increased  intensity,  though  with  diminished  amplitude,  to  the 
latter.  That  the  bones  thus  move  en  masse  has  been  proved  by  recording 
their  movements  in  the  usual  graphic  method.  A  very  light  style  attached 
to  the  incus  or  stapes  is  made  to  write  on  a  travelling  surface;  when  the 
membrana  tympani  is  thrown  into  vibrations  by  a  sound  the  curves  described 
by  the  style  indicate  that  the  chain  of  bones  moves  with  every  vibration  of 
the  tympanum.  On  the  other  hand,  the  comparatively  loose  attachments  of 
the  several  bones  is  an  obstacle  to  the  molecular  transmission  of  sonorous 
vibrations  through  them.  Moreover,  sonorous  vibrations  can  only  be  trans- 
mitted to  or  pass  along  such  bodies  as  either  are  very  long  compared  to  the 
length  of  the  sound-waves,  or,  as  in  the  case  of  membranes  and  strings,  have 
one  dimension  very  much  smaller  than  the  others.  Now  the  bones  in  ques- 
tion are  not  especially  thin  in  any  one  dimension,  but  are  in  all  their  dimen- 
sions exceedingly  small  compared  with  the  length  of  the  vibrations  of  even 
the  shrillest  sounds  we  are  capable  of  hearing;  hence,  they  must  be  useless 
for  the  molecular  propagation  of  vibrations. 


FIG.  195.— Diagram  of  the  Outer  Wall  of  the  Tympanum  (Right  Ear)  as  Seen  from  the  Mesial 
Side,  showing  Insertion  of  Tensor  Tympani.  Magnified  twice.  (After  Schwalbe.)  m.t,  mem- 
brana tympani;  m.b,  handle  of  M,  the  malleus;  7,  the  incus;  E.t,  Eustachian  tube;  T.T,  tensor 
tympani,  the  tendon  of  which  is  attached  to  the  handle  of  the  malleus;  Ig.a,  the  anterior,  and 
Ig.s,  the  superior,  ligament  of  the  malleus ;  ch.t,  the  chorda  tympani  nerve  passing  through  the 
tympanic  cavity. 

FIG.  196.— The  Stapes  in  Position.  Much  magnified.  (Schwalbe.)  1,  the  end  of  the  shaft  of  the 
incus ;  2,  its  expansion  or  os  orbiculare  ;  2',  the  articular  cartilage  of  the  same  ;  3,  the  capitulum 
of  the  stapes  ;  3',  its  articular  cartilage  ;  4,  the  hoops  of  the  stapes ;  5,  the  foot-plate  of  the  stapes ; 
5',  its  articular  cartilage;  6,  the  membrane  of  the  fenestra  ovalis;  ST,  the  tendon  of  the  stape- 
dius  muscle  attached  to  the  capitulum  of  the  stapes.] 

§  709.  The  tensor  tympani  muscle  even  in  a  quiescent  state  is  of  use  in 
preventing  the  membrana  tympani  being  pushed  out  far.  (Fig.  195.)  When 
it  contracts  it  renders  the  membrana  tympani  more  tense,  and  hence  has 
been  supposed  to  act  as  a  damper,  lessening  the  amount  of  vibration  of  the 
membrane  in  the  case  of  too  powerful  sounds  ;  it  is  said  to  be  readily  thrown 


800 


HEARING,   SMELL,  AND  TASTE. 


into  contraction  at  the  commencement  of  a  sound  or  noise,  but  to  return  to 
rest  during  the  continuance  of  a  musical  note.  Efferent  impulses  reach  it 
through  fibres  of  the  fifth  nerve,  and  its  activity  is  regulated  by  a  reflex 
action.  In  some  persons  the  muscle  seems  to  be  partly  under  the  dominion 
of  the  will,  since  a  peculiar  crackling  noise  which  these  persons  can  produce 
at  pleasure  appears  to  be  caused  by  a  contraction  of  the  tensor  tympani. 

The  so-called  laxator  tympani  is  considered  to  be  not  a  muscle  at  all,  but 
a  part  of  the  ligamentous  supports  of  the  malleus. 

§  710.  The  stapedius  muscle  by  pulling  upon  the  head  of  the  bone  (Fig. 
196)  is  supposed  to  regulate  the  movements  of  the  stapes,  and  especially  to 
prevent  its  base  being  driven  too  far  into  the  fenestra  ovalis  during  large  or 
sudden  movements  of  the  membrana  tympani.  It  is  governed  by  fibres 
from  the  facial  nerve. 

§  711.  The  Eustachian  tube.  This  serves  to  maintain  an  equilibrium  of 
pressure  betwen  the  external  air  and  that  within  the  tympanum,  and  to 
serve  as  an  exit  for  the  secretions  of  that  cavity.  Were  the  tympanum  per- 
manently closed  the  vibrations  of  the  membrana  tympani  would  be  inju- 
riously affected  by  variations  of  pressure  occurring  either  inside  or  outside. 
The  Eustachian  tube  is  undoubtedly  open  during  swallowing,  but  it  is  still 
disputed  whether  it  remains  permanently  open  or  is  opened  only  at  inter- 
vals ;  probably  it  is,  at  most  times,  neither  widely  open  nor  closely  shut. 

Auditory  Sensations. 

§  712.  Each  vibration  communicated  by  the  stapes  to  the  perilymph 
travels  as  a  wave  over  the  vestibule,  the  semicircular  canals,  and  other  parts 
of  the  labyrinth ;  and  from  the  perilymph  is  transmitted  through  the  mem- 

FIG.  197. 


i.v 


n.aud. 


t.spn. 


tg.sp. 


Diagram  of  the  Organ  of  Corti.  (After  Retzius.)  i.r,  inner  rod  of  Corti ;  o.r,  outer  rod  of  Corti. 
i.h.c,  inner  hair-cells ;  n.c,  the  group  of  nuclei  beneath  it ;  o.h.c,  outer  hair-cell,  or  cell  of  Corti, 
of  the  first  row  ;  c.D,  its  twin  cell  of  Deiters— four  rows  of  these  twin  cells  are  shown. 

n.aud,  the  auditory  nerve  perforating  the  tympanic  lip,  l.t,  and  lost  to  view  among  the  nuclei 
beneath  the  inner  hair-cell ;  i.sp.n,  the  inner  spiral  strand  of  nerve-fibrillse ;  t.sp.n,  the  spiral 
strand  of  the  tunnel;  o.sp.n,  the  outer  spiral  strand  belonging  to  the  first  row  of  outer  hair-cells; 
the  three  succeeding  spiral  strands  belonging  to  the  three  other  rows  are  also  shown.  Nerve- 
fibrillse  are  shown  stretching  radially  across  the  tunnel. 

H.c,  Hensen's  cells ;  Cl.c,  Claudius's  cells;  m.b,  basilar  membrane;  t.l,  lymphatic  epithelioid 
lining  of  the  basilar  membrane  on  the  side  toward  the  scala  tympani ;  Ig.sp,  spiral  ligament ;  c't 
cells  lining  the  spiral  groove,  overhung  by  I.v,  the  vestibular  lip ;  m.t,  the  tectorial  membrane— a 
fragment  of  it  is  seen  torn  from  the  rest  and  adherent  to  the  organ  of  Corti  just  outside  the  outer- 
most row  of  outer  hair-cells. 

branous  wall  to  the  endolymph.  From  the  vestibule  it  passes  on  into  the 
scala  vestibuli  of  the  cochlea,  and  descending  the  scala  tympani,  ends  as  an 
impulse  against  the  membrane  of  the  fenestra  rotunda.  In  the  regions  of 
the  maculae  and  cristse  the  vibrations  of  the  endolymph  are  suppose^  to  throw 


HEAKING.  801 

into  corresponding  vibrations  the  so-called  auditory  hairs.  In  the  cochlea 
the  vibrations  of  the  perilymph  are  supposed  to  throw  into  vibrations  the 
basilar  membrane  with  the  superimposed  organ  of  Corti,  consisting  of  the 
rods  of  Corti  with  the  inner  and  outer  hair-cells.  (Fig.  197.)  The  vibra- 
tions thus  transmitted  to  these  structures  give  rise  to  nervous  impulses  in 
the  terminations  of  the  auditory  nerves,  and  these  impulses  reaching  certain 
parts  of  the  brain  produce  what  we  call  auditory  sensations.  We  are  accus- 
tomed to  divide  our  auditory  sensations  into  those  caused  by  noises  and  those 
caused  by  musical  sounds.  It  is  the  characteristic  of  the  latter  that  the 
vibrations  which  constitute  them  are  periodical ;  they  occur  and  recur  at 
regular  intervals.  When  no  marked  periodicity  is  present  in  the  vibrations, 
when  the  repetition  of  the  several  vibrations  is  irregular,  or  the  period  so 
complex  as  not  to  be  readily  appreciated,  the  sensation  produced  is  that  of 
a  noise.  There  is,  however,  no  abrupt  line  between  the  two.  Between  a 
pure  and  simple  musical  sound  produced  by  a  series  of  vibrations,  each  of 
which  has  exactly  the  same  wave-length,  and  a  harsh  noise  in  which  no 
consecutive  vibrations  may  be  alike,  there  are  numerous  intermediate  stages. 

§  713.  In  both  noises  and  musical  sounds  we  recognize  a  character 
which  we  call  loudness.  This  is  determined  by  the  amplitude  of  the  vi- 
brations ;  the  greater  the  disturbance  of  the  air  (or  other  medium)  the 
louder  the  sound.  In  a  musical  sound  we  recognize  also  a  character 
which  we  call  pitch.  This  is  determined  by  the  wave-length  of  the  vi- 
brations ;  the  shorter  the  wave-length,  the  larger  the  number  of  consec- 
utive vibrations  which  fall  upon  the  ear  in  a  second,  the  higher  the  pitch. 
We  are  able  to  speak  of  a  whole  series  of  tones  or  musical  sounds  of  differ- 
ent pitch,  from  the  lowest  to  the  highest  audible  tone.  And  even  in  many 
noises  we  can,  to  a  certain  extent,  recognize  a  pitch,  indicating  that  among 
the  multifarious  vibrations  there  is  a  periodicity  of  certain  groups  of  vibra- 
tions. 

§  714.  Lastly,  we  distinguish  musical  sounds  by  their  quality  ;  the  same 
note  sounded  on  a  piano  and  on  a  violin  produce  very  different  sensations, 
even  when  a  series  of  vibrations  having  in  each  case  the  same  period  of 
repetition  is  set  going.  This  arises  from  the  fact  that  the  musical  sounds 
generated  by  most  musical  instruments  are  not  sftaple  but  compound  vibra- 
tions. When  the  note  C  in  the  treble,  for  instance,  is  struck  on  the  piano, 
and  we  analyze  the  total  sound,  we  find  that  it  can  be  resolved  partly  into 
a  series  of  vibrations  with  a  period  characteristic  of  the  pure  tone  of  the 
treble  C,  and  partly  into  other  series  of  vibrations  with  periods  character- 
istic of  the  C  in  the  octave  above,  of  the  G  above  that,  of  the  C  in  the  next 
octave,  and  of  the  E  above  that.  And  the  sensation  which  we  associate  with 
the  sound  of  the  treble  C  on  the  piano  is  determined  by  the  characters  of 
the  complex  vibration  arising  out  of  these  several  constituent  simple  vibra- 
tions. Almost  all  musical  sounds  are  thus  composed  of  what  is  called  a 
"  fundamental  tone  "  accompanied  by  a  number  of  "  overtones."  And  the 
overtones  varying  in  number  and  relative  prominence  in  different  instruments, 
give  rise  to  a  difference  in  the  sensation  caused  by  the  whole  tone.  So  that 
while  the  fundamental  tone  determines  the  pitch  of  the  sound,  the  quality 
of  the  sound  is  determined  by  the  number  and  relative  prominence  of  the 
overtones.  In  a  somewhat  similar  way  we  distinguish  the  quality  of 
noises,  such  as  a  banging,  crackling,  or  rustling  noise,  by  an  appreciation 
of  sudden  or  irregular  changes  in  the  amplitude  and  period  of  the  con- 
stituent vibrations. 

§  715.  Since  we  have  a  very  considerable  appreciation,  capable  by 
exercise  of  astonishing  enlargement,  of  the  loudness,  pitch,  and  quality 
of  a  wide  range  of  noises  and  musical  sounds,  it  is  clear  that,  within  the 

51 


802  HEARING,   SMELL,   AND  TASTE. 

limits  of  hearing,  each  vibration  or  series  of  vibrations  must  produce  its 
effect  on  the  auditory  nerves,  according  to  the  measure  of  its  intensity  and 
period.  Out  of  those  effects,  out  of  the  sensory  impulses  to  which  the  several 
vibrations  thus  give  rise,  are  generated  our  sensations  of  the  noise  or  of  the 
sound. 

The  vibrations  of  a  musical  sound  (and  since  noises  are  so  imperfectly 
understood,  we  may,  with  benefit,  chiefly  confine  ourselves  to  musical 
sounds),  as  they  pass  through  the  air  (or  other  medium)  are  not  discrete ; 
the  vibrations  corresponding  to  the  fundamental  tone  and  overtones  do 
not  travel  as  so  many  separate  waves  ;  they  all  together  form  one  com- 
plex disturbance  of  the  medium  ;  and  it  is  as  one  composite  wave  that 
the  sound  falls  on  the  membrana  tympaui,  and  passing  through  the  audi- 
tory apparatus,  breaks  on  the  terminations  of  the  auditory  nerve.  And 
when  two  or  more  musical  sounds  are  heard  at  the  same  time,  the  same 
fusion  of  the  waves  occurs.  Since  we  can  distinguish  several  tones  reach- 
ing our  ear  at  the  same  time,  it  is  clear  that  we  must  possess  in  our 
minds  or  in  our  ears  some  means  of  analyzing  these  composite  waves  of 
sound  which  fall  on  our  acoustic  organs,  and  of  sorting  out  their  con- 
stituent vibrations. 

§  716.  There  is  at  hand  a  simple  and  easy  physical  method  of  analyzing 
composite  sounds.  If  a  person  standing  before  an  open  piano  sings  out  any 
note,  it  will  be  observed  that  a  number  of  the  strings  of  the  piano  will  be 
thrown  into  vibration,  and  on  examination  it  will  be  found  that  those  strings 
which  are  thus  set  going  correspond  in  pitch  to  the  fundamental  tone  and  to 
the  several  overtones  of  the  note  sung.  The  note  sung  reaches  the  strings 
as  a  complex  wave,  but  these  strings  are  able  to  analyze  the  wave  into  its 
constituent  vibrations,  each  string  taking  up  those  vibrations  and  those  vibra- 
tions only  which  belong  to  the  tone  given  forth  by  itself  when  struck.  If  we 
suppose  that  each  terminal  fibril  of  the  auditory  nerve  is  connected  with  an 
organ  so  far  like  a  piano-string  that  it  will  readily  vibrate  in  response  to  a 
series  of  vibrating  impulses  of  a  given  period  and  to  none  other,  and  that 
we  possess  a  number  of  such  terminal  organs  sufficient  for  the  analysis  of  all 
the  sounds  which  we  can  analyze,  and  that  each  terminal  organ  so  affected 
by  particular  vibrations  gflves  rise  to  a  sensory  impulse  and  thus  to  a  sensa- 
tion of  a  distinct  character — if  we  suppose  these  organs  to  exist,  our  appre- 
ciation of  sounds  is  in  a  large  measure  explained.  Tn  the  organ  of  Corti 
we  find  structures,  the  arrangement  of  which  irresistibly  suggests  to  us  that 
these  are  the  organs  we  are  seeking.  We  have  only  to  suppose  that  of  the 
long  series  of  rods  of  Corti,  varying  regularly  as  these  do  from  the  bottom 
to  the  top  of  the  spiral,  in  length  and  in  the  span  of  their  arch,  each  pair 
will  vibrate  in  response  to  a  particular  tone,  and  the  whole  matter  seems 
explained.  But  the  more  the  subject  is  inquired  into,  the  more  complex  and 
difficult  it  appears ;  and  we  are  obliged  to  conclude  that  the  part  played  by 
the  rods  of  Corti  is  only  a  subordinate  part  of  the  function  of  the  whole 
organ  of  Corti. 

In  the  first  place,  it  is  difficult  to  see  how  the  rods  of  Corti,  even  if  they 
are  thrown  into  vibration,  can  originate  sensory  impulses,  for  the  fibrils  of 
the  auditory  nerve  terminate  in  the  inner  and  outer  hair-cells,  and  it  is  in 
these  cells,  and  not  along  the  course  of  these  fibrils  as  they  pass  under  and 
between  the  rods  of  Corti,  that  the  sensory  impulses  must  begin.  In  the 
second  place,  the  variation  in  length  of  the  fibres  along  the  series  is  insuffi- 
cient for  the  work  assigned  to  them.  Moreover,  they  appear  not  to  be  clastic. 
Lastly,  they  are  wholly  absent  in  birds,  who  very  clearly  can  appreciate 
musical  sounds.  This  last  fact  proves  indubitably  that  the  rods  in  question 
are  not  absolutely  essential  for  the  recognition  of  tones.  In  the  face  of  these 


HEARING.  803 

difficulties  it  has  been  suggested  that  the  basilar  membrane,  which  is  present 
in  birds  as  well  as  in  mammals,  and  which,  being  tense  radially  but  loose 
longitudinally,  i.  e.,  along  the  spiral  of  the  cochlea,  may  be  considered  as 
consisting  of  a  number  of  parallel  radial  strings,  each  capable  of  inde- 
pendent vibrations,  is  the  sought-for  organ  of  analysis  ;  for  it  may  be  shown 
mathematically  that  a  membrane  so  stretched  in  one  direction  only  is  capa- 
ble of  vibrating  in  such  a  manner.  And  the  radial  dimensions  of  the 
basilar  membrane  give  a  much  greater  range  of  difference  than  do  the  rods 
of  Corti,  diminishing  in  man  downward  from  0.495  mm.  at  the  top  to 
0.04125  mm.  near  the  bottom  of  the  spiral,  whereas  the  difference  in  length 
of  the  latter  is  simply  that  between  0.048  and  0.085  mm.  for  the  inner  and 
between  0.019  and  0.085  mm.  for  the  outer  fibres.  According  to  this  view, 
a  particular  simple  vibration  reaching  the  scala  tympani  of  the  cochlea 
throws  into  sympathetic  vibrations  a  small  portion  of  the  basilar  membrane, 
the  vibrations  of  which  in  turn  so  affect  the  structures  overlying  it  that  sen- 
sory impulses  are  generated.  The  sensory  impulses  reaching  the  brain  give 
rise  to  a  corresponding  sensation  of  a  particular  tone. 

The  remarkable  reticular  membrane  which  has  such  peculiar  relations 
with  the  hair-cells,  and  through  them  with  the  basilar  membrane,  must,  one 
might  imagine,  have  some  special  function  ;  but  it  is  impossible  at  present 
to  assign  to  it  any  satisfactory  duty.  The  structural  arrangements  seem,  if 
anything,  to  indicate,  that  when  a  segment  of  the  basilar  membrane  is 
thrown  into  vibrations,  the  overlying  hair-cells,  reticular  membrane,  and 
rods  of  Corti  vibrate  en  masse  with  it.  But  this  renders  the  whole  matter 
still  more  difficult.  Indeed  the  whole  subject  is  in  the  highest  degree  ob- 
scure, and  the  most  we  can  say  is  that  the  organ  of  Corti  as  a  whole  seems 
to  be  in  some  way  connected  with  the  appreciation  of  tones,  but  that  at 
present  it  is  very  hazardous  to  attempt  to  explain  how  it  acts,  or  to  assign 
particular  functions  to  particular  parts.  The  distinction  between  the  inner 
and  outer  hair-cells  seems  to  be  very  parallel  to  that  between  the  rods  and 
cones  of  the  retina ;  but  even  this  analogy  may  be  a  fallacious  one. 

It  has  been  observed  that  among  the  auditory  hairs  of  the  Crustacea, 
some  will  vibrate  to  particular  notes ;  but  the  auditory  hairs  of  the  mammal 
are  far  too  much  of  the  same  length  to  permit  the  supposition  that  they  can 
act  as  organs  of  analysis. 

If  the  organ  of  Corti  is  the  means  by  which  we  appreciate  tones,  it  is 
evident  that  by  it  also  we  must  be  able  to  estimate  loudness,  for  the  quality 
of  a  musical  sound  is  dependent  on  the  relative  intensity,  as  well  as  on  the 
nature,  of  the  overtones.  And  since  noise  is  at  best  but  confused  music,  the 
cochlea  must  be  a  means  of  appreciating  noises  as  well  as  sounds.  But  this 
would  leave  nothing  whatever  for  the  rest  of  the  labyrinth  to  do  in  respect 
to  the  appreciation  of  sound  save  so  far  as  the  difference  in  structure  between 
the  hair-cells  of  Corti,  with  their  short,  thick  rods,  and  the  hair-bearing 
structures  in  the  maculae  and  cristse,  with  their  thin,  delicate  hairs,  may 
possibly  indicate  a  difference  of  function,  the  latter  being  more  susceptible 
to  the  irregular  vibrations  of  noises.  That  the  vestibule  and  semicircular 
canals  are,  however,  concerned  in  hearing  is  shown  by  its  being  the  only 
auditory  organ  in  the  ichthyopsida,  unless  we  suppose  that  in  the  higher 
vertebrates  its  function  has  been  wholly  transferred  to  the  cochlea.  That 
the  semicircular  canals  may  have  duties  apart  from  hearing  we  shall  show 
later  on. 

§  717.  Concerning  the  function  of  the  other  parts  of  the  internal  ear  we 
know  very  little.  The  otoliths  have  been  supposed  to  intensify  the  vibrations 
of  the  endolymph  ;  but  since  apparently  they  are  lodged  in  a  quantity  of 
mucus  it  is  probable  that  they  really  act  as  dampers.  A  similar  damping 


804  HEARING,  SMELL,  AND  TASTE. 

action  has  been  suggested  for  the  membrane  of  Corti  (membrana  tectoria) 
overhanging  the  fibres  and  hair-cells ;  and  some  writers  have  supposed  that 
muscular  fibres  present  in  the  planum  semilunare  may  by  tightening  the 
basilar  membrane  serve  as  a  sort  of  accommodation  mechanism. 

It  must,  however,  be  borne  in  mind  that  even  making  the  fullest  allow- 
ance for  the  assistance  afforded  us  by  the  organ  of  Corti,  the  appreciation 
of  any  sound  is  ultimately  a  mental  act.  The  analysis  of  the  vibrations  by 
the  fibres  of  Corti  or  the  basilar  membrane  is  simply  preliminary  to  a 
synthesis  of  the  sensory  impulses  so  generated  into  a  complex  sensation. 
We  do  not  receive  a  distinct  series  of  specific  auditory  impulses  resulting  in 
a  specific  sensation  for  every  possible  variation  in  the  wave-length  of  sonor- 
ous vibrations  any  more  than  we  receive  a  distinct  series  of  specific  visual 
impulses  for  every  possible  wave-length  of  luminous  vibrations.  In  each 
case  we  have  probably  a  number  of  primary  sensations,  from  the  various 
mingling  of  which,  in  different  proportions,  our  varied  complex  sensations 
arise ;  the  difference  between  the  eye  and  the  ear  being  that  whereas  in  the 
former  the  number  of  primary  sensations  appears  to  be  limited  to  three  or 
at  most  to  six,  in  the  latter,  thanks  to  the  organ  of  Corti,  the  number  is 
very  large ;  what  the  exact  number  is  we  cannot  at  present  tell.  Our  ap- 
preciation for  a  sound  is  at  bottom  an  appreciation  of  the  combined  effect 
produced  by  the  relative  intensities  to  which  the  primary  auditory  sensations 
are,  with  the  help  of  the  organ  of  Corti,  excited  by  the  sound. 

§  718.  Whatever  be  the  explanation  of  the  manner  in  which  our  distinct 
auditory  sensations  arise,  the  range  and  precision  of  our  appreciation  of 
musical  sounds  is  very  great.  Vibrations  with  a  recurrence  below  30  a  sec- 
ond l  are  unable  to  produce  a  sensation  of  sound  ;  if  the  waves  are  powerful 
enough  we  may  feel  them,  but  we  do  not  hear  them  if  the  vibrations  are  sim- 
ple, and  such  as  would  give  rise  to  a  pure  tone  ;  if  the  fundamental  tone  is 
accompanied  by  overtones  we  may  hear  these,  and  are  thus  apt  to  say  we 
hear  the  former  when  in  reality  we  only  hear  the  latter.  The  note  of  the 
16-feet  organ  pipe,  33  vibrations  a  second,  gives  us  the  sensation  of  a  droning 
sound.  A  tone  of  forty  vibrations  is,  however,  quite  distinct.  In  the  other 
direction  it  is  possible  to  hear  a  note  caused  by  38,000  vibrations  a  second, 
though  the  limit  for  most  persons  is  far  lower — about  16,000.  Some  persons 
hear  grave  sounds  more  easily  than  high  ones  and  vice  versa.  This  may  be 
so  pronounced  as  to  justify  the  subjects  being  spoken  of  as  deaf  to  grave  or 
high  tones  respectively.  The  range  in  different  animals  is  very  different. 

The  power  of  distinguishing  one  note  from  another  varies,  as  is  well 
known,  in  different  individuals,  according  as  they  have  or  have  not  a  "  mu- 
sical ear."  A  well-trained  ear  can  distinguish  the  difference  of  a  single  or 
even  of  a  half  vibration  a  second,  and  that  through  a  long  range  of  notes. 
The  range  of  an  ordinary  appreciation  of  tones  lies  between  40  and  4000 
vibrations  a  second,  i.  e.,  between  the  lowest  bass  C  (Cj  32  vibrations)  and 
the  highest  treble  C  (C5  4224  vibrations)  of  the  piano ;  tones  above  and 
below  these,  even  when  audible,  being  distinguished  from  each  other  with 
great  difficulty. 

§  719.  When  two  consecutive  sounds  follow  each  other  at  a  sufficiently 
short  interval  the  sensations  are  fused  into  one.  In  this  respect  auditory 
sensations  are  of  shorter  duration  than  ocular  sensations.  When  ocular 
sensations  are  repeated  ten  times  in  a  second  they  become  fused  (p.  766), 
whereas  the  ticks  of  a  pendulum  beating  100  in  a  second  are  readily  audible 
as  distinct  sounds.  When  two  tuning-forks  not  quite  in  tune  are  struck  to- 
gether the  interference  of  the  vibrations  gives  rise  to  an  alternating  rise  and 

1  By  some  authors  the  limit  is  placed  as  low  as  24  or  even  15  a  second. 


HEARING.  805 

fall  of  the  sound,  known  as  "  beats."  When  the  beats  follow  each  other  as 
rapidly  as  132  in  a  second  they  cease  to  be  recognized,  that  is  to  §ay,  the 
sensations  which  they  cause  become  fused.  Before  they  disappear  they  give 
a  peculiar  disagreeable  roughness  to  the  sound.  The  pleasure  given  by 
musical  sounds  depends  largely  on  the  absence  of  this  incomplete  fusion  of 
sensations. 

§  720.  Corresponding  to  entoptic  phenomena  there  are  various  entotic 
phenomena,  sensations  or  modifications  of  sensations  originating  in  the 
tympanum  or  in  the  labyrinth  ;  moreover,  sensations  of  sound  may  rise  in  the 
auditory  nerve  or  in  the  brain  itself,  without  any  vibration  whatever  falling 
on  the  labyrinth. 

Auditory  Judgments. 

§  721.  In  seeking  for  the  cause  of  our  visual  sensations  we  invariably 
refer  to  the  external  world.  The  sensation  caused  by  direct  stimulation  of 
the  optic  nerve  or  retina  by  a  blow  or  a  galvanic  current,  we  identify  with 
that  caused  by  a  flash  of  light.  A  sensation  arising  from  any  stimulation 
of  the  left  side  of  our  retina  we  regard  as  caused  by  some  object  on  the 
right-hand  side  of  our  external  visible  world.  In  a  similar  way,  but  to  a 
less  extent,  we  project  our  auditory  sensations  into  the  world  outside  us,  and 
when  the  auditory  nerve  is  affected  we  seek  the  cause  in  vibrations  starting 
at  a  greater  or  less  distance  from  us.  We  do  not  think  of  the  sound  as 
originating  in  the  ear  itself. 

This  mental  projection  of  the  sound  is  much  more  complete  when  the  ear 
is  stimulated  by  vibrations  reaching  it  through  the  membrana  tympani  than 
when  the  vibrations  are  conducted  by  the  solids  of  the  head  directly  to  the 
perilymph  of  the  labyrinth.  When  the  meatus  externus  is  filled  with  fluid 
and  the  vibrations  of  the  membrana  tympani  are  in  consequence  interfered 
with  the  apparent  outwardness  of  sounds  is  to  a  very  large  extent  lost ; 
sounds,  however  caused,  seem  under  these  circumstances  to  arise  in  the  ear. 
Hence  it  would  seem  that  the  vibrations  of  the  membrana  tympani,  or  pos- 
sibly the  action  of  the  muscles  attached  to  the  ossicula,  give  rise  to  obscure 
sensations  of  which,  by  themselves,  we  are  not  distinctly  conscious,  but  which 
nevertheless  lead  us  to  judge  that  the  sounds  heard  by  means  of  the  tym- 
panum come  from  outside  the  ear. 

§  722.  Our  judgment  of  the  distance  of  sounds  is  very  limited.  A  sound 
whose  characters  we  know  appears  to  us  near  when  it  is  loud  and  far  off 
when  it  is  faint.  A  blindfold  person  will  be  unable  to  distinguish  between 
the  difference  of  intensity  produced  on  the  one  hand  by  a  tuning-fork  being 
held  before  him,  first  with  the  broad  edge  of  the  fork  toward  him  and  then 
with  the  narrow  edge,  and  the  difference  on  the  other  hand  caused  by  the 
removal  of  the  tuning-fork  to  a  distance.  We  can,  on  the  whole,  better 
appreciate  the  distance  of  noises  than  of  musical  sounds. 

§  723.  Our  judgment  of  the  direction  of  sounds  is  also  very  limited. 
Our  chief  aid  in  this  is  the  position  in  which  we  have  to  place  the  head  in 
order  that  we  may  hear  the  sound  to  the  best  advantage.  If  a  tuning-fork 
be  held  in  the  median  vertical  plane  over  the  head,  though  it  is  easy  to  rec- 
ognize it  as  being  in  the  median  plane,  it  becomes  very  difficult  when  the 
eyes  are  shut  to  say  what  is  its  position  in  that  plane,  i.  e.,  whether  it  is  more 
toward  the  front  or  back  of  the  head.  In  this  respect,  too,  our  appreciation 
is  more  accurate  in  the  case  of  noises  than  of  musical  sounds,  with  the  excep- 
tion of  those  given  out  by  the  human  voice,  the  direction  of  which  can  be 
judged  better  than  even  that  of  a  noise. 


806 


HEARING,   SMELL,   AND  TASTE. 


SMELL. 

§  724.  [The  nasal  fossae  are  two  irregular  cavities  which  communicate 
anteriorly  with  the  air  through  the  anterior  nares  and  posteriorly  with  the 
pharynx  through  the  posterior  nares.  The  fossae  are  partially  divided  into 
upper,  middle,  and  lower  air-passages  or  chambers  by  the  superior,  middle, 
and  inferior  turbinated  bones.  They  are  lined  by  the  Schneiderian  or 
pituitary  mucous  membrane,  which  is  continuous  anteriorly  with  the  integu- 
ment and  posteriorly  with  the  mucous  membrane  of  the  pharynx ;  and  with 
the  membrane  lining  the  ducts  and  sinuses  connected  with  the  fossae.  At  the 
position  of  the  distribution  of  the  olfactory  nerve-filaments  it  is  much 
thicker,  more  vascular,  pigmented,  and  lined  by  columnar  nucleated  epithe- 
lial cells ;  the  remaining  portion  of  the  membrane  covering  the  fossae,  ex- 
cepting near  the  anterior  nares,  is  lined  by  columnar  ciliated  epithelium. 
This  membrane  contains  racemose  mucous  glands,  which  secrete  mucus  for 
the  purpose  of  keeping  the  membrane  constantly  moist,  which  is  a  condition 
essential  to  perfect  olfaction. 

§  725.  The  olfactory  tract  is  a  prolongation  of  the  cerebrum,  which  ter- 
minates anteriorly  in  a  bulbous  expansion,  the  olfactory  ganglion.  It  consists 
principally  of  gray  matter.  This  ganglion  rests  upon  the  cribriform  plate 


FIG.  198. 


FIG.  199. 

d  e  f 


FIG.  198.— Vertical  Section  of  Right  Nasal  Fossa, 
showing  Outer  Side  of  Fossa.  1,  olfactory  tract;  2,  ol- 
factory nerves  ;  3,  middle  turbinated  bone  ;  4,  lower 
turbinated  bone ;  5,  branches  from  the  fifth  nerve. 
Branches  of  the  fifth  are  also  shown  in  the  anterior 
portion.  (After  Arnold.) 

FIG.  199.— Cells  of  the  Olfactory  Mucous  Membrane, 
hart  Clarke.) 


(a,  6,  c,  after  Schultze ;   d,  e,f,  after  Lock- 


of  the  ethmoid  bone,  and  in  this  position  sends  about  twenty  filaments,  which 
consist  of  gray  matter  alone,  through  the  cribriform  plate  to  be  distributed  to 
the  pituitary  membrane  of  the  upper  third  of  the  septum  nasi,  the  upper 
portion  of  the  root  of  the  nose,  the  superior,  and  a  portion  of  the  middle 
turbinated  bones.  (Fig.  198.)  The  whole  surface  corresponding  to  the  dis- 
tribution of  the  olfactory  nerve  is  colored  brownish  by  the  pigment  in  the 
epithelial  cells  of  the  mucous  glands  and  membrane.  This  pigmented  region 
is  called  the  regio  olfactoria,  and  is  the  essential  portion  of  the  nasal  fossae 
concerned  in  olfaction. 

§  726.  According  to  Schultze,  the  epithelium  of  the  regio  olfartoria  is 
of  two  kinds:  The  first  (Fig.  199,  a)  consists  of  yellow  nucleated   proto- 


SMELL.  807 

plasmic  cells,  which  have  a  cylindrical  body  terminating  at  its  free  extremity 
as  a  squared  truncated  surface  ;  the  other  extremity  of  the  body  is  stretched 
out  as  a  filamentous  prolongation,  which  expands  into  a  triangular  plate  as 
it  approaches  the  submucous  tissue.  From  the  base  of  this  plate  a  number 
of  filaments  are  given  off,  which  are  prolonged  into  the  submucous  tissue.  The 
second  variety  of  epithelial  cells  (c)  is  found  at  the  borders  of  the  regio 
otfactoria.  They  are  similar  to  those  just  described,  excepting  that  their 
free  surface  is  covered  with  cilia.  Between  the  epithelial  cells  the  olfac- 
tory nerves  terminate.  These  terminal  filaments  (&,/)  are  long,  delicate 
structures,  which  have  a  number  of  fusiform  expansions  along  their  course; 
in  the  largest  expansion  is  found  an  oval  nucleus.  The  terminal  filaments 
are  called  the  olfactory  cells.  As  yet  no  connection  between  the  subepi- 
thelial  and  interepithelial  nerve-filaments  has  been  demonstrated.  The 
epithelial  cells  (d  and  e)  in  the  above  figure  are  shown  connected  with 
the  subepithelial  tissue.  The  fifth  nerve  supplies  the  fossae  with  sensory 
filaments.] 

§  727.  Odorous  particles  present  in  the  inspired  air  passing  through 
the  lower  nasal  chambers  diffuse  into  the  upper  nasal  chambers,  and  falling 
on  the  olfactory  epithelium  produce  sensory  impulses  which,  ascending  to 
the  brain,  give  rise  to  sensations  of  smell.  We  may  presume  that  the  sen- 
sory impulses  are  originated  by  the  contact  of  the  odorous  particles  with 
the  peculiar  rod-shaped  olfactory  cells  described  by  Max  Schultze  ;  but  we 
are  as  much  in  the  dark  about  this  matter  as  about  the  development  of 
visual  sensory  impulses  in  the  rods  and  cones  or  of  auditory  sensory  im- 
pulses in  the  organ  of  Corti. 

The  susidiary  apparatus  of  smell  is  exceedingly  meagre.  By  the  forced 
nasal  inspiration,  called  sniffing,  we  draw  air  so  forcibly  through  the  nostrils 
that  currents  pass  up  into  the  upper  as  well  as  the  lower  nasal  chambers; 
and  thus  a  more  complete  contact  of  the  odorous  particles  with  the  olfactory 
membrane  than  that  supplied  by  mere  diffusion  is  provided  for. 

We  have  every  reason  to  think  that  any  stimulus  applied  to  the  olfactory 
nerve  will  produce  the  sensation  of  smell ;  but  the  proof  of  this  is  not  so 
clear  as  in  the  case  of  the  optic  and  auditory  nerves.  We  are,  however,  sub- 
ject to  sensations  of  smell  not  caused  by  objective  odors.  The  olfactory 
membrane  is  the  only  part  of  the  body  in  which  odors  as  such  can  give  rise 
to  any  sensations ;  and  the  sensations  to  which  they  give  rise  are  always 
those  of  smell.  The  mucous  membrane  of  the  nose  is,  however,  also  an  in- 
strument for  the  development  of  afferent  impulses  other  than  the  specific 
olfactory  ones.  Chemical  stimulation  of  the  olfactory  membrane  by  pungent 
substances  such  as  ammonia  gives  rise  to  a  sensation  distinct  from  that  of 
smell,  a  sensation  which  affords  us  no  information  concerning  the  chemical 
nature  of  the  stimulus,  and  which  is  indistinguishable  from  the  sensations 
produced  by  chemical  stimulation  of  other  parts  of  the  nasal  membrane  as 
well  as  of  other  surfaces  equally  sensitive  to  chemical  action.  It  is  probable 
that  these  two  kinds  of  sensations  thus  arising  in  the  olfactory  membrane 
are  conveyed  by  different  nerves,  the  former  by  the  olfactory,  the  latter  by 
the  fifth  nerve. 

§  728.  For  the  development  of  smell  it  appears  necessary  that  the  odor- 
ous particles  should  be  conveyed  to  the  nasal  membrane  in  a  gaseous  medium, 
or,  at  least,  that  the  surface  of  the  membrane  should  not  be  exposed  at  the 
same  time  to  the  action  of  fluids.  Thus,  when  the  nostril  is  filled  with  rose- 
water,  the  odor  of  roses  is  not  perceived  ;  and  simply  filling  the  nostrils  with 
distilled  water  suspends  for  a  time  all  smell,  the  sense  returning  gradually 
after  the  water  has  been  removed  ;  the  water  apparently  acts  injuriously  on 
fche  delicate  olfactorv  cells. 


808  HEARING,  SMELL,  AND  TASTE. 


Each  substance  that  we  smell  causes  a  specific  sensation,  and  we  are  not 
only  able  to  recognize  a  multitude  of  distinct  odors,  but  also  to  distinguish 
individual  odors  in  a  mixed  smell. 

As  in  the  previous  senses,  we  project  our  sensation  into  the  external 
world ;  the  smell  appears  to  be  not  in  our  nose,  but  somewhere  outside  us. 
We  can  judge  of  the  position  of  the  odor,  however,  even  less  definitely  that 
we  can  of  that  of  a  sound. 

The  sensation  takes  some  time  to  develop  after  the  contact  of  the  stim- 
ulus with  the  olfactory  membrane,  and  may  last  very  long.  When  the 
stimulus  is  repeated  the  sensation  very  soon  dies  out ;  the  sensory  terminal 
organs  speedily  become  exhausted.  Mental  associations  cluster  more 
strongly  around  sensations  of  smell  than  around  any  other  impressions 
we  receive  from  without.  And  reflex  effects  are  very  frequent,  many 
people  fainting  in  consequence  of  the  contact  of  a  few  odorous  particles 
with  their  olfactory  cells. 

Apparently  the  larger  the  surface  the  more  intense  the  sensation ; 
animals  with  acute  scent  having  a  proportionately  large  area  of  olfactory 
membrane.  The  quantity  of  material  required  to  produce  an  olfactory 
sensation  may  be,  as  in  the  case  of  musk,  almost  immeasurably  small. 

When  two  different  odors  are  presented  to  the  two  nostrils,  an  oscilla- 
tion of  sensation  similar  to  that  spoken  of  in  binocular  vision  (p.  788)  takes 
place. 

§  729.  The  assertion  that  the  olfactory  nerve  is  the  nerve  of  smell  has 
been  disputed.  Cases  have  been  recorded  of  persons  who  appeared  to  have 
possessed  the  sense  of  smell,  and  yet  in  whom  the  olfactory  lobes  were  found 
after  death  to  be  absent.  Direct  experiments  on  animals,  however,  show 
that  loss  of  the  olfactory  lobes  entails  loss  of  smell.  On  the  other  hand,  it 
is  stated  that  section  or  injury  of  the  fifth  nerve  causes  a  loss  of  smell 
though  the  olfactory  nerve  remains  intact ;  but  in  these  cases  it  has  not 
been  shown  that  the  olfactory  membrane  remains  intact,  and  it  is  quite  pos- 
sible that,  as  in  the  case  of  the  eye,  changes  may  take  place  in  the  nasal  mem- 
brane as  the  result  of  the  injury  to  the  fifth  nerve,  sufficient  to  prevent  its 
performing  its  usual  functions. 

TASTE. 

§  730.  [The  peripheral  organs  concerned  in  the  sense  of  taste  are  local- 
ized in  the  mucous  membrane  covering  the  dorsumof  the  tongue,  the  fauces, 
soft  palate,  and  uvula,  and  possibly  a  portion  of  the  upper  part  of  the 
pharynx.  This  membrane  is  analogous  in  structure  to  other  membranes  of 
its  type,  except  on  the  dorsum  of  the  tongue,  where  its  structure  is  similar 
to  that  of  the  integument.  At  this  position  it  consists  of  a  corium,  with  a 
papillary  and  a  superficial  epithelial  layer. 

The  structure  of  the  corium  is  similar  to  that  of  the  skin,  but  is  thinner 
and  less  compact.  It  serves  as  a  point  of  insertion  of  the  muscular  fibres  of 
the  tongue. 

§  731.  The papillce  are  thickly  distributed  over  the  whole  dorsal  surface, but 
more  particularly  marked  in  the  anterior  two-thirds.  They  project  as  minute 
prominences,  which  give  the  tongue  a  roughened,  characteristic  appearance. 
The  papillae  are  of  two  kinds,  the  simple  and  compound.  The  simple  papillae 
are  similar  to  those  found  in  the  skin ;  they  are  found  scattered  over  the 
whole  dorsal  surface  between  the  compound  papillae.  They  are  most  nu- 
merous in  the  posterior  portion  of  the  organ.  The  compound  papillae  are 
of  three  varieties  :  the  papillae  maximae  or  circumvallatse,  the  papillae  mediae 
or  fungiformes,  and  the  papillae  minimse  or  filiformes. 


TASTE. 


809 


The  papillae  circumvallatce  (Fig.  200),  which  are  the  largest,  are  about 
eight  or  ten  in  number  and  form  a  V-shaped  row  at  the  junction  of  the 
middle  and  posterior  two-thirds  of  the  tongue.  They  consist  of  a  central, 
broad  papilla,  surrounded  by  an  annular  ring  or  wall  of  about  the  same 
elevation  and  separated  from  the  central  papilla  by  a  circular  fissure.  The 
central  papilla,  as  well  as  the  surrounding  wall,  is  covered  by  simple  papillae. 
Each  of  them  receives  one  or  more  capillary  loops  arid  nerve-filaments. 

FIG.  200. 


Vertical  Section  of  the  Circumvallate  Papillae.  (From  Kolliker.)  A,  the  papillae ;  B,  the  sur- 
rounding wall ;  a,  the  epithelial  covering;  6,  the  nerves  of  the  papilla  and  wall  spreading  toward 
the  surface ;  c,  the  secondary  papillae.  10/i. 

Thepapillce  fungiformes  (Fig.  201)  are  found  principally  on  the  tip  and 
sides  of  the  tongue,  although  scattered  sparsely  over  the  whole  of  the  anterior 
two-thirds.  These  are  so  named  from  their  fungiform  shape,  being  expanded 
at  their  free  extremity  and  projecting  on  a  short,  thick  pedicle.  They  are 
covered  by  simple  papillae,  and  contain  plexuses  of  vessels  and  nerves. 

FIG.  201. 


Surface  and  Section  of  the  Fungiform  Papillae.  (From  Kolliker,  and  after  Todd  and  Bowman.) 
A,  the  surface  of  a  fungiform  papilla,  partially  denuded  of  its  epithelium,  &h ;  a,  epithelium.  B, 
section  of  a  fungiform  papilla  with  the  bloodvessels  injected  ;  a,  artery ;  v,  vein ;  c,  capillary  loops 
of  simple  papillae  in  the  neighboring  structure  of  the  tongue. 

The  papillce  filiformes  (Fig.  202)  are  by  far  the  most  numerous,  and  are 
found  thickly  distributed  over  the  entire  surface  of  the  anterior  two-thirds 
of  the  tongue.  They  are  minute,  conical  in  shape,  and  generally  arranged 
in  bipenniform  rows,  which  are  more  or  less  parallel  with  the  two  rows  of 
papillae  circumvallatae.  Their  free  surface  is  covered  with  simple  papillae. 
The  epithelium  covering  them  is  greatly  modified  and  appears  in  the  form 
of  hair-like  processes.  (Fig.  202.)  These  processes  are  bathed  in  mucus, 
are  movable,  and  have  a  general  inclination  pointing  backward.  The  exist- 
ence of  these  hair-like  processes  on  the  filiform  papillae  suggests  that  this 
variety  of  papillae  is  intimately  connected  with  the  tactile  sensibility  of  the 
tongue,  and  not  with  gustation.  In  carnivora  and  herbivora  these  processes 
are  of  a  horny  structure,  and  perform  an  active  function  in  the  attrition 
and  prehension  of  food.  In  man  their  special  function  appears,  through 


810 


HEARING,   SMELL,   AND  TASTE. 


their  intimate  connection  with  the  tactile  sense,  to  guide  the  tongue  in  its 
variable  and  complicated  movements. 


FIG.  202. 


a     b     A     c         a 

A,  vertical  section  near  the  middle  of  the  dorsal  surface  of  the  tongue ;  a,  a,  fungiform  papillae ; 
b,  filiform  papillae,  with  their  hair-like  processes;  c,  similar  ones  deprived  of  their  epithelium, 
magnified  2  diameters;  B,  filiform  compound  papillae  ;  a,  artery;  v,  vein  ;  c,  capillary  loops  of  the 
secondary  papillae ;  6,  line  of  basement-membrane  ;  d,  secondary  papillae,  deprived  of  e,  e,  the 
epithelium ;  /,  hair-like  processes  of  epithelium  capping  the  simple  papillae,  magnified  25  diam- 
eters; g,  separated  nucleated  particles  of  epithelium,  magnified  300  diameters.  1,  2,  hairs  found 
on  the  surface  of  the  tongue;  3,  4,  5,  ends  of  hair-iike  epithelial  processes,  showing  varieties  in 
the  imbricated  arrangement  of  the  particles,  but  in  all  a  coalescence  of  the  particles  toward  the 
point;  5  encloses  a  soft  hair,  magnified  160  diameters.  (After  Todd  and  Bowman.) 

FIG.  203. 


Gustatory  Bulbs  from  the  Lateral  Gustatory  Organ  of  the  Rabbit.    (Magnified  450  diameters.) 

§  732.  The  ultimate  terminations  of  the  gustatory  nerves  are  yet  envel- 
oped in  obscurity.     According  to  Engelmann,  the  glosso-pharyngeal  nerves 


TASTE.  811 

terminate  in  flask-shaped  organs  which  are  termed  the  gustatory  bulbs  or 
taste  buds.  (Fig.  203.)  These  bulbs  are  found  principally  in  the  papillary 
surface  of  the  wall  of  the  circumvallate  papillae.  They  are  also  found  in 
the  fungiform  papillae,  but  are  less  numerous.  They  consist  of  a  flask-shaped 
fundus,  which  rests  upon  the  subepithelial  tissue,  and  a  mouth  which  opens 
upon  the  surface  of  the  mucous  membrane.  The  mouth  is  known  as  the 
gustatory  pore.  The  fundus  of  the  flask  is  composed  of  two  varieties  of  cells  ; 
the  outer  or  investing  cells  are  fusiform,  nucleated,  and  granular,  placed 
parallel  and  arranged  concentrically  in  a  direction  from  the  base  to  the 
neck  ;  they  thus  form  a  wall  which  encloses  elongated  nucleated  cells  with 
filamentous  processes,  which  extend  through  the  gustatory  pore  and  project 
as  very  finely  pointed  or  truncated  extremities.  These  inner  cells  are  called 
the  gustatory  cells,  and  are  supposed  to  be  the  essential  terminal  elements 
concerned  in  gustation.  Their  relation  to  the  gustatory  nerves  has  not  as 
yet  been  clearly  demonstrated,  but  they  are  evidently  connected  with  the 
ganglionic  plexuses  of  nerve-fibres  at  the  papillary  bases.  The  gustatory 
nerves  are  also  supposed  to  terminate  in  the  epithelium  of  the  papillae.] 

§  733.  The  word  taste  is  frequently  used  when  the  word  smell  ought  to 
be  employed.  We  speak  of  "  tasting  "  odoriferous  substances,  such  as  an 
onion,  wines,  etc.,  when  in  reality  we  only  smell  them  as  we  hold  them  in  our 
mouth ;  this  is  proved  by  the  fact  that  the  so-called  taste  of  these  things 
is  lost  when  the  nose  is  held,  or  the  nasal  membrane  rendered  inert  by 
a  catarrh. 

The  terminal  organs  of  the  sense  of  taste  thus  more  strictly  defined  are 
the  endings  of  the  glosso-pharyngeal  and  lingual  nerves  in  the  mucous  mem- 
brane of  the  tongue  and  palate,  those  nerves  serving  as  the  special  nerves 
of  taste.  Whether  the  so-called  gustatory  buds  can  be  regarded  as  specific 
organs  of  taste  appears  doubtful.  The  subsidiary  apparatus  is  confined  to 
the  tongue  and  lips,  which  by  their  movements  assist  in  bringing  the  sapid 
substances  into  contact  with  the  mucous  membrane  of  the  mouth. 

Though  we  can  hardly  be  said  to  project  our  sensation  of  taste  into  the 
external  world,  we  assign  to  it  no  subjective  localization.  When  we  place 
quinine  in  our  mouth,  the  resulting  sensation  of  taste  gives  us  no  informa- 
tion as  to  where  the  quinine  is,  though  we  may  learn  that  by  concomitant 
general  sensations  arising  in  the  buccal  mucous  membrane. 

§  734.  We  recognize  a  multitude  of  distinct  tastes,  which  may  be  broadly 
classified  into  acid,  saline,  bitter,  and  sweet  tastes.  Sapid  substances  have 
the  power  of  producing  these  sensations  by  virtue  of  their  chemical  nature. 
But  other  stimuli  will  also  give  rise  to  sensations  of  taste.  When  the  tongue 
is  tapped,  a  taste  is  felt ;  and  when  a  constant  current  is  passed  through  the 
mouth,  an  alkaline  or,  in  some  persons,  a  bitter  metallic  taste  is  developed 
when  the  anode,  and  an  acid  taste  when  the  kathode,  is  placed  on  the  tongue. 
It  is  probable  that  in  these  cases  the  terminal  organs  are  indirectly  affected 
by  the  current.  When  hot  or  pungent  substances  are  introduced  into  the 
mouth,  sensations  of  general  feeling  are  excited,  which  obscure  any  strictly 
gustatory  sensations  which  may  be  present  at  the  same  time. 

Though  analogy  would  lead  us  to  suppose  that  a  stimulus  applied  to  any 
part  of  the  course  of  the  real  gustatory  fibres  of  either  the  glosso-pharyngeal 
or  lingual  nerves  would  give  rise  to  a  sensation  of  taste  and  nothing  else,  the 
proof  is  not  forthcoming,  since  both  these  nerves  are  mixed  nerves  contain- 
ing other  afferent  fibres  as  well  as  those  of  taste. 

When  the  constant  current  is  used  as  a  means  of  exciting  taste,  gustatory 
sensations  are  found  to  be  developed  in  the  back,  edges,  and  tip  of  the 
tongue,  the  soft  palate,  the  anterior  pillar  of  the  fauces,  and  a  small  tract 
of  the  posterior  part  of  the  hard  palate.  They  are  absent  from  the  anterior 


812  FEELING  AND  TOUCH. 

and  middle  dorsal  and  under  surface  of  the  tongue,  the  front  portion  of  the 
hard  palate,  the  posterior  pillars  of  the  fauces,  the  gums,  and  the  lips. 
Sapid  substances  are  unsuitable  as  a  test  for  this  purpose,  on  account  of  their 
rapid  diffusion.  Bitter  substances  produce  most  effect  when  placed  on  the 
back,  and  sweet  substances  when  placed  on  the  tip,  of  the  tongue ;  but  the 
tasting  power  of  the  tip  of  the  tongue  varies  very  much  in  different  indi- 
viduals, and  in  many  seems  almost  entirely  absent.  It  is  said  that  acids  are 
best  appreciated  by  the  edge  of  the  tongue. 

§  735.  It  is  essential  for  the  development  of  taste  that  the  substance  to 
be  tasted  should  be  dissolved,  and  the  effect  is  increased  by  friction.  The 
larger  the  surface  the  more  intense  the  sensation.  The  sensation  takes  some 
time  to  develop,  and  endures  for  a  long  time,  though  this  may  be  in  part 
due  to  the  stimulus  remaining  in  contact  with  the  terminal  organs.  A  tem- 
perature of  about  40°  is  the  one  most  favorable  for  the  production  of  the 
sensation.  At  temperatures  much  above  or  below  this,  taste  is  much  im- 
paired. The  nerves  of  taste  are,  as  we  have  said,  the  glosso-pharyngeal 
and  the  lingual  or  gustatory.  The  former  supplies  the  back  of  the  tongue, 
and  section  of  it  destroys  taste  in  that  region.  The  latter  is  distributed  to 
the  front  of  the  tongue,  and  section  of  it  similarly  deprives  the  tip  of  the 
tongue  of  taste.  There  is  no  reason  for  doubting  that  the  gustatory  fibres 
in  the  glosso-pharyngeal  are  proper  fibres  of  that  nerve  ;  but  it  has  been 
urged  by  many  that  the  gustatory  fibres  of  the  lingual  are  derived  from  the 
chorda  tympani,  and  that  those  fibres  of  the  lingual  which  come  from  the 
fifth  are  employed  exclusively  in  the  sensations  of  touch  and  feeling ;  the 
evidence  in  favor  of  this  view  is,  however,  inconclusive. 


CHAPTER  Y. 

FEELING  AND  TOUCH. 

GENERAL  SENSIBILITY  AND  TACTILE  PERCEPTIONS. 

§  736.  WE  have  taken  the  foregoing  senses  first  in  the  order  of  discussion 
on  account  of  their  being  eminently  specific.  The  eye  gives  us  only  visual 
sensations,  the  ear  only  auditory  snsations.  The  sensations  are  produced  in 
each  case  by  specific  stimuli ;  the  eye  is  only  affected  by  light  and  the  ear 
only  by  sound.  Moreover,  the  information  they  afford  us  is  confined  to  the 
external  world  ;  they  tell  us  nothing  about  ourselves.  The  various  visual 
sensations  which  arise  in  our  retina  are  referred  by  us  not  to  the  retina  itself, 
but  to  some  real  or  imaginary  object  in  the  world  without  (including  as  part 
of  the  external  world  such  portions  of  our  own  bodies  as  are  visible  to  our- 
selves). Such  also,  with  diminishing  precision,  is  the  information  gained  by 
hearing,  taste,  and  smell. 

All  the  other  afferent  nerves  of  the  body,  centripetal  impulses  along  which 
are  able  to  affect  our  consciousness,  are  the  means  of  conveying  to  us  in- 
formation concerning  ourselves.  The  sensations,  arising  in  them  from  the 
action  of  various  stimuli,  are  referred  by  us  to  appropriate  parts  of  our  own 
body.  When  any  body  comes  in  contact  with  our  finger,  we  know  that  it  is 
our  finger  which  has  been  touched  ;  from  the  resultant  sensations  we  not  only 
learn  the  existence  of  certain  qualities  in  the  object  touched,  but  we  also  are 


GENERAL  SENSIBILITY  AND  TACTILE  PERCEPTIONS.         813 

led  to  connect  the  cognizance  of  these  qualities  with  a  particular  part  of  our 
own  body. 

§  737.  Like  the  more  specific  senses  previously  studied,  the  sensations  of 
which  we  are  now  speaking,  and  which  may  be  referred  to  under  the  name  of 
touch,  using  that  word  for  the  present  in  a  wide  meaning,  require  for  their 
production  terminal  organs  ;  and  the  chief  but  not  exclusive  organ  of  touch 
is  to  be  found  in  the  epidermis  of  the  skin  and  certain  underlying  nervous 
structures.  For  the  development  of  specific  tactile  sensations  these  terminal 
organs  are  as  essential  as  are  the  terminal  organs  of  the  eye  for  sight  or  of 
the  ear  for  hearing.  Contact  of  the  skin  with  a  hard  or  with  a  hot  body 
gives  rise  to  a  distinct  sensation,  whereby  we  recognize  that  we  have  touched 
a  hard  or  a  hot  body.  But  the  application  of  either  body  or  of  any  other 
stimulus  to  a  nerve-trunk  gives  rise  to  a  sensation  of  general  feeling  only, 
corresponding  to  the  simple  sensation  of  light  which  is  produced  by  direct 
stimulation  of  the  optic  nerve.  We  have  no  more  tactile  perception  of  a 
body  which  is  in  contact  with  a  nerve-trunk  than  we  could  have  visual 
perception  of  any  luminous  object,  the  rays  proceeding  from  which  were 
strong  enough  to  excite  sensory  impulses  when  directed  on  to  the  optic 
nerve  instead  of  on  to  the  retina,  supposing  such  a  thing  to  be  possible.  It  is 
further  characteristic  of  these  ordinary  nerves  of  general  feeling,  that  the 
sensations  caused  by  any  stimulation  of  them  beyond  a  certain  degree  de- 
velop that  state  of  consciousness  which  we  are  in  the  habit  of  speaking  of 
as  "  pain."  Putting  aside  the  general  feeling  which  many  parts  of  the  eye 
possess,  a  very  strong  luminous  stimulation  of  the  retina  is  required  to  pro- 
duce a  sensation  of  pain,  if  indeed  it  can  be  at  all  brought  about ;  whereas 
a  very  moderate  stimulation  of  the  skin,  and  almost  every  stimulation  of  an 
ordinary  nerve-trunk,  is  said  by  us  to  be  painful. 

Though  the  skin  is  the  chief  organ  of  touch,  the  mucous  membrane 
lining  the  various  passages  of  the  body  also  serves  as  an  instrument  for 
the  same  sense,  but  only  for  a  short  distance  from  the  respective  orifices. 
We  can  recognize  hard  or  hot  bodies  with  our  lips  or  mouth,  but  a  hot 
liquid,  when  it  has  reached  the  oesophagus  or  stomach,  simply  gives  rise  to 
a  sensation  of  pain  ;  we  cannot  distinguish  the  sensation  caused  by  it  from 
the  sensation  caused  by  a  draught  of  a  too  acid  fluid. 

From  parts  and  tissues  of  the  body  other  than  the  skin  and  the  portions 
of  mucous  membrane  just  mentioned  we  have  obscure  sensations  of  general 
feeling,  by  which  we  are  made  vaguely  aware  of  the  general  condition  of  our 
body,  though  our -judgments  in  this  matter  are  chiefly  influenced  by  what 
we  shall  have  to  speak  of  directly  as  a  muscular  sense."  In  all  parts  of  the 
body,  however,  on  occasions  all  too  frequent,  this  general  feeling  may  become 
prominent  as  pain. 

§  738.  The  stimuli  which,  when  applied  to  the  skin,  give  rise  to  tactile 
perceptions  are  of  two  kinds  only  :  (1)  mechanical,  that  is,  the  contact  of 
bodies  exerting  varying  degrees  of  pressure ;  and  (2)  thermal,  i.  e.,  the  rais- 
ing or  lowering  of  the  temperature  of  the  skin  by  the  approach  or  contact 
of  hot  or  cold  bodies.  We  can  judge  of  the  weight  and  of  the  temperature 
of  a  body,  because  we  can,  through  touch,  perceive  how  much  it  presses  when 
allowed  to  rest  on  our  skin  or  how  hot  it  is.  But  we  can  through  touch  de- 
rive no  other  perceptions  and  form  no  other  judgments.  An  electric  shock 
sent  through  the  skin  will  give  rise  to  a  sensation,  but  the  sensation  is  an 
indefinite  one,  because  the  electric  current  acts  not  on  the  terminal  organs 
of  touch,  but  on  the  fine  nerve-branches  of  the  skin.  We  cannot  distinguish 
the  sensation  so  caused  from  a  mechanical  prick  of  similar  intensity ;  we 
cannot  perceive  that  the  sensation  is  caused  by  an  electric  current.  Sim- 
ilarly, certain  chemical  substances,  such  as  a  strong  acid,  will  give  rise  to  a 


814  FEELING  AND  TOUCH. 

sensation,  but  we  cannot  perceive  the  acid,  we  can  form  no  judgment  of 
its  nature  such  as  we  could  if  we  tasted  it ;  and  if  the  acid  does  not  per- 
meate the  skin  so  as  to  act  directly  and  chemically  on  the  fine  nerve-fibres, 
we  cannot  distinguish  the  acid  from  any  other  liquid  giving  rise  to  the 
same  simple  contact  impressions.  The  terminal  organs  of  the  skin  are 
such  as  are  only  affected  by  pressure  or  by  temperature.  Conversely, 
pressure  or  a  variation  in  temperature  brought  to  bear  on  a  nerve-trunk, 
instead  of  on  the  terminal  organs,  produces  no  specific  tactile  sensations  of 
pressure  or  temperature,  but  merely  general  sensations  of  feeling  rapidly 
rising  into  pain. 

TACTILE  SENSATIONS. 
Sensations  of  Pressure. 

§  739.  As  with  visual,  so  with  tactile,  and,  indeed,  with  all  other  sensa- 
tions, the  intensity  of  the  sensation  maintains  that  general  relation  to  the 
intensity  of  the  stimulus  which  we  spoke  of  at  p.  767  as  being  formulated 
under  Weber's  law.  We  can  distinguish  the  difference  of  pressure  between 
one  and  two  grammes  as  readily  as  we  can  that  between  ten  and  twenty  or 
one  hundred  and  two  hundred. 

When  two  sensations  follow  each  other  in  the  same  spot  at  a  sufficiently 
short  interval,  they  are  fused  into  one ;  thus,  if  the  finger  be  brought  to  bear 
lightly  on  a  rotating  card  having  a  series  of  holes  in  it,  the  holes  cease  to  be 
felt  as  such  when  they  follow  each  other  at  a  rapidity  of  about  1500  in  a 
second.  The  vibrations  of  a  cord  cease  to  be  appreciable  by  touch  when 
they  reach  the  same  rapidity.  When  sensations  are  generated  at  points  of 
the  skin  too  close  together  they  become  fused  into  one ;  but  to  this  point  we 
shall  return  presently. 

§  740.  The  sensation  caused  by  pressure  is  at  its  maximum  soon  after 
its  beginning,  and  thenceforward  diminishes.  The  more  suddenly  the  pressure 
is  increased,  the  greater  the  sensation ;  and  if  the  increase  be  sufficiently 
gradual,  even  very  great  pressure  may  be  applied  without  giving  rise  to  any 
sensation.  A  sensation  in  any  spot  is  increased  by  contrast  when  the  sur- 
rounding areas  are  not  subject  to  pressure.  Thus,  if  the  finger  be  dipped 
into  mercury,  the  pressure  will  be  felt  most  at  the  surface  of  the  fluid  ;  and 
if  the  finger  be  drawn  up  and  down,  the  sensation  caused  will  be  that  of  a 
ring  moving  along  the  finger. 

All  parts  of  the  skin  are  not  equally  sensitive  to  pressure ;  small  differ- 
ences of  simple  pressure  are  more  readily  appreciated  when  brought  to  bear 
on  the  palmar  surface  of  the  finger,  or  on  the  forehead,  than  on  the  arm  or 
on  the  sole  of  the  foot.  In  making  these  determinations,  all  muscular  move- 
ments should  be  avoided  in  order  to  eliminate  the  muscular  sense,  of  which 
we  shall  speak  presently ;  and  the  area  stimulated  should  be  as  small  and 
the  surfaces  in  contact  as  uniform  as  possible.  In  a  similar  manner,  small, 
consecutive  variations  of  pressure,  as  in  counting  a  pulse,  are  more  readily 
appreciated  by  certain  parts  of  the  skin  than  by  others ;  and  the  minimum 
of  pressure  which  can  be  felt  differs  in  different  parts.  In  all  cases,  varia- 
tions of  pressure  are  more  easily  distinguished  when  they  are  successive  than 
when  they  are  simultaneous. 

Sensations  of  Temperature. 

§  741.  When  the  temperature  of  the  skin  is  raised  or  lowered  in  any 
spot,  we  receive  sensations  of  heat  and  cold  respectively ;  and  by  these  sen- 
sations of  the  temperature  of  our  skin,  we  form  judgments  of  the  temper- 


TACTILE  SENSATIONS.  815 

ature  of  bodies  in  contact  with  it.  Bodies  of  exactly  the  same  temperature 
as  the  region  of  the  skin  to  which  they  are  applied  produce  no  such  thermal 
sensations,  though  we  can,  from  the  very  absence  of  sensation,  form  a  judg- 
ment as  to  their  temperature ;  and  good  conductors  of  heat  appear  respec- 
tively hotter  and  colder  than  bad  conductors  raised  to  the  same  temperature. 

§  742.  We  may  consider  the  skin  as  having  at  any  given  time  and  in 
any  given  spot  a  normal  temperature  at  which  the  sensation  of  temperature 
is  at  zero ;  for  under  ordinary  circumstances  we  are  not  directly  conscious 
of  the  temperature  of  our  skin  ;  it  is  only  when  the  normal  temperature  at 
the  spot  is  raised  or  lowered  that  we  have  a  sensation  of  heat  or  cold  re- 
spectively. This  normal  temperature  may  be  at  the  same  time  different  in 
different  parts  of  the  body ;  thus,  at  a  time  when  neither  the  forehead  nor 
the  hand  are  giving  rise  to  any  sensation  of  temperature,  we  may,  by  putting 
the  hand  to  the  forehead,  frequently  feel  the  former  hot  or  cold  because  the 
normal  temperatures  of  the  two  parts  differ.  The  normal  temperature  in 
any  spot  may  also  vary  from  time  to  time.  Thus,  when  the  hand  is  placed 
in  a  warm  medium  for  some  time,  the  sensation  of  warmth  ceases;  a  new 
normal  temperature  is  established  with  the  zero  of  sensation  at  a  higher  level, 
a  depression  or  elevation  of  this  new  temperature  giving  rise,  however,  as 
before,  to  sensations  of  heat  and  cold  respectively.  That  it  is  the  changed 
condition,  and  not  the  change  itself,  of  which  we  are  conscious,  is  shown  by 
the  fact  that  when  a  portion  of  the  skin  is  cooled,  by  brief  contact  with  a 
cold  metal,  for  instance,  we  are  still  conscious  of  the  spot  being  cold  after  the 
cooling  agent  has  been  removed — that  is,  at  a  time  when  a  cooled  spot  is  in 
reality  being  heated  by  the  surrounding  warmer  tissues. 

§  743.  The  change  in  temperature  of  the  skin  necessary  to  produce  a  sen- 
sation must  have  a  certain  rapidity ;  and  the  more  gradual  the  change  the 
less  intense  the  sensation.  The  repeated  dipping  of  the  hand  into  hot  water 
produces  a  greater  sensation  than  when  the  hand  is  allowed  to  remain  all 
the  time  in  the  water,  though  in  the  latter  case  the  temperature  of  the  skin 
is  most  affected.  The  effects  of  contrast  are  also  seen  in  these  sensations  as 
in  those  of  pressure. 

We  can  with  some  accuracy  distinguish  variations  of  temperature,  espe- 
cially those  lying  near  the  normal  temperature  of  the  skin.  These  sensations, 
in  fact,  follow  Weber's  law,  though  apparently  sensations  of  slight  cold  are 
more  vivid  than  those  of  slight  heat,  the  range  of  most  accurate  sensation 
seeming  to  lie  between  27°  and  33°. 

The  regions  of  the  skin  most  sensitive  to  variations  in  temperature  are  not 
identical  with  those  most  sensitive  to  variations  in  pressure.  Thus  the 
cheeks,  eyelids,  temples,  and  lips  are  more  sensitive  than  the  hands.  The 
least  sensitive  parts  are  the  legs,  and  front  and  back  of  the  trunk. 

§  744.  The  simplest  view  which  can  be  taken  with  regard  to  the  dis- 
tinction between  pressure  sensations  and  temperature  sensations,  and  which  is 
suggested  by  the  facts  just  mentioned,  is  to  suppose  that  two  distinct  kinds 
of  terminal  organs  exist  in  the  skin,  one  of  which  is  affected  only  by  pressure, 
and  the  other  only  by  variations  in  temperature  ;  and  that  the  two  kinds  of 
peripheral  organs  are  connected  with  different  parts  of  the  central  sensory 
organs  by  separate  nerve-fibres.  Certain  pathological  cases  have  been 
quoted  as  showing  not  only  that  this  is  the  case,  but  that  the  two  sets  of 
fibres  pursue  different  courses  in  the  spinal  cord.  Thus  in  certain  diseases 
or  injuries  to  the  brain  or  spinal  cord,  hypera3sthesia  as  regards  temperature 
has  been  observed  unaccompanied  by  an  augmentation  of  sensitiveness  to 
pressure  ;  and,  conversely,  instances  have  been  seen  where  the  patient  could 
tell  when  he  was  touched,  but  could  not  distinguish  between  hot  and  cold. 
On  the  other  hand,  there  are  facts  which  show  a  close  dependence  between 


816  FEELING  AND  TOUCH. 

the  sensations  of  pressure  and  temperature.  When  each  stimulus  is  brought 
to  bear  on  a  very  limited  area,  the  two  sensations  are  frequently  confounded, 
especially  in  those  regions  of  the  body  where  sensations  are  not  acute.  So, 
also,  a  penny  cooled  down  nearly  to  zero,  and  placed  on  the  forehead  will 
be  judged  by  most  people  to  be  as  heavy  or  even  heavier  than  two  pennies 
of  the  temperature  of  the  forehead  itself;  and,  conversely,  a  body  warmer 
than  the  skin  will  often  appear  heavier  than  a  body  of  the  same  weight,  but 
of  the  same  temperature  as  the  skin.  Moreover,  cases  have  been  recorded 
where  a  hot  body,  such  as  a  heated  spoon,  was  felt,  though  the  application  of 
the  same  spoon  at  the  temperature  of  the  body  produced  no  sensations,  and 
yet  the  heated  spoon  was  not  recognized  as  a  hot  body,  but  appeared  to  be 
simply  something  touching  the  skin.  It  may  be  argued  that  these  instances 
show  nothing  more  than  the  changes  in  the  skin,  whatever  they  be,  which 
give  rise  to  sensations  of  pressure,  are  modified  by  the  temperature  of  the 
skin  for  the  time  being,  whereby  the  judgment  as  to  the  pressure  which  is 
being  exerted  is  rendered  faulty  ;  but  they  may  also  be  taken  to  indicate 
that  variations  in  pressure  and  temperature  affect  the  same  terminal  organs, 
and  the  same  nerve-fibres,  though  affecting  them  in  a  different  way,  and 
generating  nervous  impulses  so  far  different  that  they  give  rise  to  different 
sensations.  And  we  may  here  note  that  we  certainly  cannot  speak  of  nerves 
of  warmth  in  the  same  sense  in  which  we  speak  of  nerves  of  sight  or  of 
hearing.  A  stimulus  (of  whatever  kind)  applied  to  an  optic  or  auditory 
nerve,  if  adequate,  gives  rise,  as  we  have  seen,  to  a  sensation  of  light  or  of 
sound ;  a  stimulus,  on  the  other  hand,  applied  to  the  trunk  of  a  cutaneous 
nerve,  gives  rise  only  to  general  feeling  or  pain  ;  though  the  nerve  certainly 
contains  fibres  by  which  sensations  of  pressure  and  of  temperature  reach  the 
brain,  the  general  feeling  which  stimulation  of  the  trunk  causes  is  akin 
neither  to  sensations  of  pressure  nor  to  those  of  warmth. 

§  745.  The  rapidity  with  which  hot  or  cold  bodies  brought  into  contact 
with  the  skin  give  rise  to  sensations  of  temperature,  suggests  that  the  ter- 
minal apparatus  for  generating  these  sensations,  whatever  be  its  nature,  is 
placed  in  the  epidermis,  and  indeed  as  near  as  possible  to  the  surface.  Pres- 
sure, on  the  other  hand,  can  be  readily  transmitted  through  even  a  thick 
layer  of  skin.  And  those  who  maintain  the  existence  of  different  terminal 
organs  for  pressure  and  temperature,  regard  the  nerve-endings  in  the  epider- 
mis as  the  latter,  and  the  corpuscular  tactus,  end-bulbs,  and  allied  organs  as 
the  former.  But  the  evidence  we  possess  concerning  this  matter  is  at  present 
inconclusive. 

TACTILE  PERCEPTIONS  AND  JUDGMENTS. 

§  746.  When  a  body  presses  on  any  part  of  our  skin,  or  when  the 
temperature  of  the  skin  at  that  point  is  raised,  we  are  not  only  conscious  of 
pressure  or  of  heat,  but  perceive  that  a  particular  part  of  our  body  has  been 
touched  or  heated.  We  refer  the  sensations  to  their  place  of  origin,  and  we 
thus  by  touch  perceive  the  relations  to  ourselves  of  the  body  which  gives 
rise  to  the  tactile  sensations,  in  the  same  way  as  in  our  visual  perception  of 
external  objects  we  refer  to  external  nature  the  sensations  originating  in 
certain  parts  of  the  retina.  When  we  are  touched  on  the  finger  and  on  the 
back  we  refer  the  sensations  to  the  finger  and  to  the  back  respectively,  and 
when  we  are  touched  at  two  places  on  the  same  finger  at  the  same  time  we 
refer  the  sensations  to  two  points  of  the  finger.  In  this  way  we  can  localize 
our  sensations,  and  are  thus  assisted  in  perceiving  the  space  relations  of  ob- 
jects with  which  we  come  in  contact. 

§  747.  This  power  of  localizing  pressure  sensations  varies  in  different 


TACTILE  PEKCEPTIONS  AND  JUDGMENTS.  817 

parts  of  the  body.  The  following  table,  from  Weber,  gives  the  distance  at 
which  two  points  of  a  pair  of  compasses  must  be  held  apart,  so  that  when 
the  two  points  are  in  contact  with  the  skin  the  two  consequent  sensations 
can  be  localized  with  sufficient  accuracy  to  be  referred  to  two  points  of  the 
body,  and  not  confounded  as  one : 

Tip  of  tongue     l.lmm. 

Palm  of  last  phalanx  of  finger 2.2 

Palm  of  second   "                        4.4 

Tip  of  nose 6.6 

White  part  of  lips 8.8 

Back  of  second  phalanx  of  finger 11.1 

Skin  over  malar  bone 15.4 

Back  of  hand 29.8 

Forearm • 39.6 

Sternum 44.0 

Back 66.0 

And  an  analogous  distribution  has  been  observed  in  reference  to  the  locali- 
zation of  sensations  of  temperature.  As  a  general  rule,  it  may  be  said  that 
the  more  mobile  parts  are  those  by  which  we  can  thus  discriminate  sensations 
most  readily.  The  lighter  the  pressure  used  to  give  rise  to  the  sensations, 
the  more  easily  are  two  sensations  distinguished  ;  thus  two  points  which,  when 
touching  the  skin  lightly,  appear  as  two,  may,  when  firmly  pressed,  give  rise 
to  one  sensation  only.  The  distinction  between  the  sensations  is  obscured  by 
neighboring  sensations  arising  at  the  same  time.  Thus,  two  points  brought 
to  bear  within  a  ring  of  heavy  metal  pressing  on  the  skin,  are  readily  con- 
fused into  one.  And  it  need  hardly  be  said  that  these  tactile  perceptions, 
like  all  other  perceptions,  are  immensely  increased  by  exercise. 

§  748.  Our  *'  field  of  touch,"  if  we  may  be  allowed  the  expression,  is 
composed  of  tactile  areas  or  units,  in  the  same  way  that  our  field  of  vision 
is  composed  of  visual  areas  or  units.  The  tactile  sensation  is,  like  the  visual 
sensation,  a  symbol  to  us  of  some  external  event,  and  we  refer  the  sensation 
to  its  appropriate  place  in  the  field  of  touch.  All  that  has  been  said  (p.  768) 
concerning  the  subjective  nature  of  the  limits  of  visual  areas,  applies  equally 
well,  mutatis  mutandis,  to  tactile  areas.  When  two  points  of  the  compasses 
are  felt  as  two  distinct  sensations,  it  is  not  necessary  that  two,  and  only  two, 
nerve-fibres  should  be  stimulated  ;  all  that  is  necessary  is  that  the  two  cere- 
bral sensation-areas  should  not  be  too  completely  fused  together.  The  im- 
provement by  exercise  of  the  sense  of  touch  must  be  explained  not  by  an 
increased  development  of  the  terminal  organs,  not  by  a  growth  of  new  nerve- 
fibres  in  the  skin,  but  by  a  more  exact  limitation  of  the  sensational  areas  in 
the  brain,  by  the  development  of  a  resistance  which  limits  the  radiation 
taking  place  from  the  centres  of  the  several  areas. 

§  749.  By  a  multitude  of  simultaneous  and  consecutive  tactile  sensations 
thus  converted  into  perceptions  we  are  able  to  make  ourselves  acquainted 
with  the  form  of  external  objects.  We  can  tell  by  variations  of  pressure 
whether  a  surface  is  rough  or  smooth,  plane  or  curved,  what  variations  of 
surface  a  body  presents,  and  how  far  it  is  heavy  or  light ;  and  from  the  in- 
formation thus  gained  we  build  up  judgments  as  to  the  form  and  nature  of 
objects,  judgments,  however,  which  are  most  intimately  bound  up  with  visual 
judgments,  the  knowledge  derived  by  one  sense  correcting  and  completing 
that  obtained  by  the  other.  As  in  other  senses,  so  in  this,  our  sensations 
may  mislead  us  and  cause  us  to  form  erroneous  judgments.  This  is  well 
illustrated  by  the  so-called  experiment  of  Aristotle.  It  is  impossible  in  an 
ordinary  position  of  the  fingers  to  bring  the  radial  side  of  the  middle  finger 

52 


818  FEELING   AND  TOUCH. 

and  the  ulriar  side  of  the  ring  finger  to  bear  at  the  same  time  on  a  small 
object,  such  as  a  marble.  Hence,  when  with  the  eyes  shut  we  cross  one 
finger  over  the  other,  and  place  a  marble  between  them  so  that  it  touches 
the  radial  side  of  the  one  and  the  ulnar  side  of  the  other,  we  recognize  that 
the  object  is  such  as  could  not  under  ordinary  conditions  be  touched  at  the 
same  time  by  these  two  portions  of  our  skin,  and  therefore  judge  that  we 
are  touching  not  one  but  two  marbles.  Upon  repetition,  however,  we  are 
able  to  correct  our  judgment,  and  the  illusion  disappears. 

§  750.  Distinct  tactile  sensations  are,  as  we  have  seen,  produced  only 
when  a  stimulus  is  applied  to  a  terminal  organ.  When  sensations  or  affect- 
ions of  general  sensibility  other  than  the  distinct  tactile  sensations  are 
developed  in  the  termination  of  a  nerve,  we  are  still  able,  though  with  less 
exactitude,  to  refer  the  sensation  to  a  particular  part  of  the  body.  Thus, 
when  we  are  pricked  or  burned,  we  can  feel  where  the  prick  or  burn  is. 
When  a  sensory  nerve-trunk  is  stimulated,  the  sensation  is  always  referred 
to  the  peripheral  terminations  of  the  nerve.  Thus  a  blow  on  the  ulnar  nerve 
at  the  elbow  is  felt  as  a  tingling  in  the  little  and  ring  fingers  corresponding 
to  the  distribution  of  the  nerve,  and  sensations  started  in  the  stump  of  an 
amputated  limb  are  referred  to  the  absent  member.  When  cold  is  applied 
to  the  elbow  it  is  felt  as  cold  in  the  skin  of  the  elbow ;  but  a  cooling  of  the 
ulnar  nerve  at  this  spot,  since  stimulation  of  a  nerve-trunk  gives  rise  to 
general  sensations  only,  simply  gives  rise  to  pain  which  is  referred  to  the 
ulnar  side  of  the  hand  and  arm. 

THE  MUSCULAR  SENSE. 

§  751.  When  we  come  into  contact  with  external  bodies  we  are  con- 
scious not  only  of  the  pressure  exerted  by  the  object  on  our  skin,  but  also  of 
the  pressure  which  we  exert  on  the  object.  If  we  place  the  hand  and  arm 
flat  on  a  table,  we  can  estimate  the  pressure  exerted  by  bodies  resting  on  the 
palm  of  the  hand,  and  so  come  to  a  conclusion  as  to  their  weights ;  in  this 
case  we  are  conscious  only  of  the  pressure  exerted  by  the  body  on  our  skin. 
If,  however,  we  hold  the  body  in  the  hand,  we  not  only  feel  the  pressure  of 
the  body,  but  we  are  also  aware  of  the  muscular  exertion  required  to  sup- 
port and  lift  it.  We  possess  a  muscular  sense  ;  and  we  find  by  experience 
that  when  we  trust  to  this  muscular  sense  as  well  as  to  sensations  of  pres- 
sure, we  can  form  much  more  accurate  judgments  concerning  the  weight  of 
bodies  than  when  we  rely  on  sensations  of  pressure  alone.  When  we  want 
to  tell  how  heavy  a  body  is,  we  are  not  in  the  habit  of  allowing  it  simply  to 
press  on  the  hand  laid  flat  on  a  table ;  we  hold  it  in  our  hand  and  lift  it  up 
and  down.  We  appeal  to  our  muscular  sense  to  inform  us  of  the  amount 
of  exertion  necessary  to  move  it,  and  by  help  of  that,  judge  of  its  weight. 
And  in  all  the  movements  of  our  body  we  are  guided,  even  to  an  astonish- 
ing degree  of  accuracy,  as  is  well  seen  in  the  discussions  concerning  vision, 
by  an  appreciation,  more  or  less  distinctly  conscious,  of  the  amount  of  the 
contraction  to  which  we  are  putting  our  muscles.  In  some  way  or  other  we 
are  made  aware  of  what  particular  muscles  or  groups  of  muscles  are  being 
thrown  into  action,  and  to  what  extent  that  action  is  being  carried.  We  are 
also  conscious  of  the  varying  condition  of  our  muscles,  even  when  they  are 
at  rest ;  the  tired  and  especially  the  paralyzed  limb  is  said  to  "  feel"  heavy. 
In  this  way  the  state  of  our  muscles  largely  determines  our  general  feeling 
of  health  and  vigor,  of  weariness,  ill  health  and  feebleness. 

It  has  been  suggested  that  since  muscle  possesses  little  or  no  general 
sensibility,  comparatively  little  pain  being  felt  for  instance  when  muscles  are 
cut,  our  muscular  sense  is  chiefly  derived  from  the  traction  of  the  contract- 


THE  VOICE.  819 

ing  muscle  on  its  attachments  ;  and  undoubtedly  in  many  instances  of  cramp, 
the  pain  is  chiefly  felt  at  the  joints ;  and,  as  we  know,  Pacinian  bodies  are 
abundant  around  the  joints.  Afferent  nerves,  however,  having  a  different 
disposition  from  the  ordinary  motor  nerves  which  terminate  in  end-plates, 
have  been  described  as  present  in  muscle ;  and  analogy  would  lead  us  to  sup- 
pose that  these  afferent  fibres,  though  possessing  a  low  general  sensibility, 
might  be  easily  excited  in  a  specific  manner  by  a  muscular  contraction  ;  but 
further  investigations  are  necessary  before  these  can  be  accepted  as  the  true 
nerves  of  the  muscular  sense. 

§  752.  In  favor  of  the  view  that  the  muscular  sense  is  peripheral  and 
not  central  in  origin  may  be  urged  the  fact  that  the  sense  is  felt  when  the 
muscles  are  thrown  into*  contraction  by  direct  galvanic  stimulation  instead 
of  by  the  agency  of  the  will.  Many  authors,  while  admitting  the  existence 
of  a  muscular  sense  of  peripheral  origin,  contend  that  we  also  possess  and 
are  very  largely  guided  in  our  movements  by  what  might  be  called  a 
"  neural  "  sense  of  central  origin.  That  is  to  say,  the  changes  in  the  central 
nervous  system  involved  in  initiating  and  carrying  out  a  movement  of  the 
body  so  affect  our  consciousness  that  we  have  a  sense  of  the  effort  itself. 

It  has  been  observed  that  when  the  posterior  roots  are  divided,  move- 
ments become  less  orderly,  as  if  they  lacked  the  guidance  of  a  muscular 
sense ;  and  although  the  impairment  of  the  movements  may  be  due  in  part 
to  the  coincident  loss  of  tactile  sensations,  it  is  probable  that  it  is  increased 
by  the  loss  of  the  muscular  sense.  There  is  a  malady  or  rather  a  condition 
attending  various  diseased  states  of  the  central  nervous  system  called  loco- 
motor  ataxia,  the  characteristic  feature  of  which  is  that,  though  there  is  no 
loss  of  direct  power  over  the  muscles,  the  various  bodily  movements  are 
effected  imperfectly  and  with  difficulty,  from  want  of  proper  coordination, 
In  such  diseases  the  pathological  mischief  is  frequently  found  in  the  posterior 
columns  of  the  spinal  cord  and  the  posterior  roots  of  the  spinal  nerves,  that 
is,  in  distinctly  afferent  structures  ;  and  the  phenomena  seem  in  certain  cases 
at  least  to  be  due  to  inefficient  coordination  caused  by  the  loss  both  of  the 
muscular  sense  and  of  ordinary  tactile  sensations.  The  patients  walk  with 
difficulty,  because  they  have  imperfect  sensations  both  of  the  condition  of 
their  muscles  and  of  the  contact  of  their  feet  with  the  ground.  In  many 
of  their  movements  they  have  to  depend  largely  on  visual  sensations  ;  hence 
when  their  eyes  are  shut,  they  become  singularly  helpless.  In  other  cases 
again  ataxia  may  be  present  without  any  impairment  of  touch  ;  but  a  dis- 
cussion of  the  varied  phenomena  of  this  class  of  maladies  cannot  be  entered 
into  here. 


CHAPTER  VI. 

SPECIAL  MUSCULAR  MECHANISMS. 
THE  VOICE. 

§  753.  [THE  larynx  is  a  membrane-cartilaginous  chamber,  broader  above 
than  below,  and  situated  in  the  anterior  median  portion  of  the  neck.  It 
consists  of  a  number  of  cartilages,  which  are  articulate  with  each  other, 
connected  by  ligaments,  moved  by  a  number  of  muscles,  and  lined  by  a 
mucous  membrane. 


S20 


SPECIAL  MUSCULAK  MECHANISMS. 


The  principal  cartilages  are  the  thyroid,  cricoid,  the  two  arytenoid,  and 
the  epiglottis. 

The  thyroid  cartilage  is  the  largest  and  consists  of  two  quadrilateral 
plates  or  alee,  which  are  continuous  with  each  other  in  front,  where  they 
form  the  prominence  called  the  pomum  Adami.  The  posterior  borders  of 
the  thyroid  cartilage  serve  as  a  point  of  attachment  of  the  stylo-pharyngeus 
and  palato-pharyngeus  muscles.  The  upper  part  of  each  of  these  borders 
terminates  in  a  superior  cornu,  which  articulates  with  the  hyoid  bone  ;  the 
lower  portion  terminates  in  the  inferior  cornu,  which  articulates  with  the 
cricoid  cartilage.  The  upper  border  between  the  cornua  is  connected  with 

FIG.  204. 


Median  Section  of  Mouih,  Nose,  Pharynx,  and  Larynx,  a,  septum  of  nose,  below  it  section 
of  hard  palate ;  b,  tongue ;  c,  section  of  velum  pendulum  palati ;  d,  d,  lips ;  u,  uvula ;  r,  anterior 
arch  or  pillar  of  fauces :  i,  posterior  arch  ;  t,  tonsil ;  p,  pharynx :  h,  hyoid  bone ;  k,  thyroid  carti- 
lage ;  n,  cricoid  cartilage ;  s,  epiglottis  ;  v,  glottis  ;  1,  posterior  opening  of  the  nares;  3,  isthmus 
faucium  ;  4,  superior  opening  of  larynx  ;  5,  passage  into  oesophagus ;  6,  mouth  of  right  Eustachian 
tube. 

the  hyoid  bone  by  the  thyro-hyoid  membrane.  The  lower  border  is  con- 
nected with  the  cricoid  cartilage  by  the  thyro-cricoid  membrane  at  the 
median  line,  and  at  the  sides  by  the  crico-thyroid  muscles. 

The  cricoid  cartilage  is  situated  below  the  thyroid  cartilage  with  its  broad 
portion  posteriorly.  At  the  upper  part  of  its  broad  portion  are  two  smooth 
surfaces  on  which  the  arytenoid  cartilages  articulate. 

The  arytenoid  cartilages  are  pyramidal  in  form  and  articulate  on  the 
upper  surface  of  the  cricoid.  Each  cartilage  has  an  external,  posterior,  and 
internal  (median)  surface,  an  apex  and  a  base.  The  apex  is  pointed  back- 
ward and  inward,  and  is  surmounted  by  a  small  cartilaginous  tubercle,  called 
the  cartilage  of  Santonin  (Fig.  207).  The  base,  which  articulates  with  the 
cricoid  cartilage,  presents  at  its  external  internal  angle  a  projection  called 
the  processus  vocalis.  At  the  posterior  internal  angle  is  a  second  projection, 
called  the  processus  muscularis. 

§  754.  The  superior  opening  of  the  larynx  is  formed  anteriorly  by  the 
epiglottis,  posteriorly  by  the  apices  of  the  arytenoid  cartilages,  and  laterally 
by  the  aryteno-epiglottidean  folds  stretching  between  these  points.  The  in- 


THE  VOICE. 


821 


ferior  opening  corresponds  to  the  inferior  border  of  the  cricoid  cartilage. 
Between  these  points  is  the  cavity  of  the  larynx,  which  has  stretching  across 
its  sides  the  vocal  cords.  The  vocal  cords  consist  of  two  pairs  ;  the  supe- 
rior or  false  vocal  cords  are  membrano-ligamentous  bands,  which  extend 
from  the  receding  angle  of  the  thyroid  to  the  external  surfaces  of  the  ary- 
tenoid  cartilages  ;  the  inferior  or  true  vocal  cords  (chorda  vocales)  are  mem- 
brano-ligamentous bands  which  stretch  across  the  cavity  of  the  larynx  from 
the  receding  angle  of  the  thyroid  to  the  processus  vocales  of  the  arytenoid 
cartilages.  &Between  the  borders  of  the  true  and  false  vocal  cords  is  an 
elliptical  opening,  the  ventricle,  which  leads  to  a  space  running  upward  and 
behind  the  false  vocal  cords,  called  the  sacculus  laryngis.  The  mucous 


FIG.  205. 


FIG.  206. 


FIG.  207. 


FIG.  205.— View  of  the  Larynx  and  part  of  the  Trachea  from  Behind  with  the  Muscles  Dissected 
h,  the  body  of  the  hyoid  bone ;  e,  epiglottis  ;  t,  the  posterior  borders  of  the  thyroid  cartilage ;  c,  the 
median  ridge  of  the  cricoid ;  a,  upper  part  of  the  arytenoid ;  s,  placed  on  one  of  the  oblique  fasci- 
culi of  the  arytenoid  muscle  ;  b,  left  posterior  crico-arytenoid  muscle ;  ends  of  the  incomplete  car- 
tilaginous rings  of  the  trachea;  I,  fibrous  membrane  crossing  the  back  of  the  trachea;  n,  muscular 
fibres  exposed  in  a  part.  (From  Quain's  Anatomy.) 

FIG.  206. — View  of  the  Larynx  from  Above.  1,  aperture  of  glottis  ;  2,  arytenoid  cartilages  ;  3, 
vocal  cords ;  4,  posterior  crico-arytenoid  muscles ;  5,  lateral  crico-arytenoid  muscle  of  right  side, 
that  of  left  side  removed ;  6,  arytenoid  muscle  ;  7,  thyro-arytenoid  muscle  of  left  side,  that  of  right 
side  removed  ;  8,  thyroid  cartilage  ;  9,  cricoid  cartilage ;  13,  posterior  crico-arytenoid  ligament.  With 
the  exception  of  the  arytenoid  muscle,  this  diagram  is  a  copy  from  Mr.  Willis's  figure. 

FIG.  207. — View  of  the  Upper  Part  of  the  Larynx  as  seen  by  means  of  the  Laryngoscope  during 
the  Utterance  of  a  Grave  Note,  c,  epiglottis ;  s,  cartilages  of  Santorini ;  a,  arytenoid  cartilages; 
z,  base  of  the  tongue ;  ph,  posterior  wall  of  the  pharynx. 

membrane  lining  this  sac  contains  a  great  number  of  follicular  glands 
which  discharge  a  mucous  secretion  for  the  purpose  of  lubricating  the  true 
vocal  cords. 

Between  the  true  vocal  cords  is  an  opening  which  is  called  the  rima  glot- 
tidis.  The  form  of  the  glottis  varies  very  much  both  in  the  inspiratory  and 
expiratory  acts,  and  in  the  act  of  phonation. 

§  755.  The  muscles  of  the  larynx  are  divided  anatomically  into  the  in- 
trinsic and  extrinsic.  The  former  are  nine  in  number,  four  of  them  being 


822  SPECIAL  MUSCULAR  MECHANISMS. 

in  pairs.  They  are  the  essential  muscles  concerned  in  the  movements  of  the 
arytenoid  cartilages  and  chordae  vocales.  The  extrinsic  muscles  connect  the 
larynx  with  adjacent  parts,  and  are  for  the  most  part  concerned  in  the 
elevation  and  depression  of  the  organ. 

The  larynx  is  lined  with  a  mucous  membrane  which  is  continuous  above 
with  that  lining  the  pharynx  and  mouth,  and  below  with  that  lining  the 
trachea.  Above  the  chorda  vocales  it  is  lined  with  pavement  epithelium, 
excepting  at  the  lower  anterior  portion,  where  it  is  ciliated ;  below  the 
chordae  vocales  the  epithelium  is  of  a  ciliated  columnar  variety.  The  mu- 
cous membrane  contains  many  mucous  glands,  which  are  pretty  uniformly 
distributed ;  they  are,  however,  very  abundant  in  the  part  of  the  membrane 
lining  the  sacculus  laryngis.] 

§  756.  A  blast  of  air,  driven  by  a  more  or  less  prolonged  expiratory 
movement,  throws  into  vibrations  two  elastic  membranes — the  chordce 
vocales.  These  impart  their  vibrations  to  the  column  of  air  above  them, 
and  so  give  rise  to  the  sound  which  we  call  the  voice.  Since  the  sound  is 
generated  in  the  vocal  cords,  we  may  speak  of  them  and  of  those  parts  of 
the  larynx  which  decidedly  affect  their  condition  as  constituting  the  essential 
vocal  apparatus  ;  while  the  chamber  above  the  vocal  cords,  comprising  the 
ventricles  of  the  larynx  with  the  false  vocal  cords,  the  pharynx  and  the 
cavity  of  the  mouth,  the  latter  varying  much  in  form,  constitutes  a  sub- 
sidiary apparatus  of  the  nature  of  a  resonance  tube,  modifying  the  sound 
originating  in  the  vocal  cords.  In  the  voice,  as  in  other  sounds,  we  distin- 
guish :  1,  Loudness.  This  depends  on  the  strength  of  the  expiratory  blast. 
2,  Pitch.  This  depends  on  the  length  and  tension  of  the  vocal  cords. 
Their  length  may  be  regarded  as  constant,  or  varying  only  with  age.  It 
consequently  determines  the  range  only  of  the  voice,  and  not  the  particular 
note  given  out  at  any  one  time.  The  shrill  voice  of  the  child  is  deter- 
mined by  the  shortness  of  the  cords  in  infancy,  and  the  voices  of  a  soprano, 
tenor,  and  baritone  are  all  dependent  on  the  respective  length  of  their 
vocal  cords.  Their  tension  is,  on  the  contrary,  variable  ;  and  the  chief 
problems  connected  with  the  voice  refer  to  variations  in  the  tension  of  the 
vocal  cords.  3,  Quality.  This  depends  on  the  number  and  character  of 
the  overtones  accompanying  any  fundamental  note  sounded,  and  is  deter- 
mined by  a  variety  of  circumstances,  chief  among  which  is  the  physical 
quality  of  the  cords. 

§  757.  The  vocal  cords,  attached  in  front  to  the  thyroid  cartilage,  end 
behind  in  the  processus  vocales  of  the  arytenoid  cartilages.  Hence  a  dis- 
tinction has  been  drawn  between  the  rirna  vocalis,  i.  e.,  the  opening  bounded 
laterally  by  the  vocal  cords,  and  the  rima  respiratoria,  or  space  between  the 
arytenoid  cartilages  behind  the  processus  vocales ;  these  names,  however, 
are  not  free  from  objections.  In  quiet  breathing  (Fig.  208,  B)  the  two 
form  together  a  VsnaPe(i  space,  which,  as  we  have  seen  (p.  343),  in  deep 
inspiration  is  widened  into  a  rhomboidal  opening  by  the  divergence  of  the 
processus  vocales  (Fig.  208,  C ).  When  a  note  is  about  to  be  uttered,  the 
vocal  cords  are  by  the  approximation  of  the  processus  vocales  brought  into 
a  position  parallel  to  each  other,  and  the  whole  rima  is  narrowed  (Fig. 
208,  A).  By  their  parallelism  and  by  the  narrowness  of  the  interval  be- 
tween them  the  cords  are  rendered  more  susceptible  of  being  thrown  into 
vibration  by  a  moderate  blast  of  air.  The  problems  we  have  to  consider 
are,  first,  by  what  means  are  the  cords  brought  near  to  each  other  or  drawn 
asunder  as  occasion  demands  ;  and,  secondly,  by  what  means  is  the  tension 
of  the  cords  made  to  vary.  We  may  speak  of  these  two  actions  as  narrow- 
ing or  widening  of  the  glottis,  and  tightening  or  relaxation  of  the  vocal 
cords. 


THE  VOICE. 


823 


$  758  Narrowing  of  the  glottis.  The  change  of  form  of  the  glottis  is 
best  understood  when  it  is  borne  in  mind  that  each  arytenoid  cartilage  is, 
when  seen  in  horizontal  section  (Fig.  208),  somewhat  of  the  form  of  a  tri- 
angle with  an  internal  or  median,  an  external,  and  a  posterior  side,  the  pro- 
cessus  vocalis  being  placed  in  the  anterior  angle  at  the  j  unction  of  the 
median  and  external  sides.  When  the  cartilages  are  so  placed  that  the  pro- 
cessus  vocales  are  approximated  to  each  other,  and  the  internal  surfaces 


FIG.  208. 


A' 


The  Larynx  as  Seen  by  Means  of  the  Laryngoscope  in  Different  Conditions  of  the  Glottis. 
(From  Quain's  Anatomy,  after  Czermak.)  A,  while  singing  a  high  note  ;  B,  in  quiet  breathing; 
C,  during  a  deep  inspiration.  The  corresponding  diagrammatic  figures  A',  B',  C',  illustrate  the 
changes  in  position  of  the  arytenoid  cartilages  and  the  form  of  the  rima  vocalis  and  rima  re- 
spiratoria  in  the  above  three  conditions.  I,  the  base  of  the  tongue ;  e,  the  upper  free  part  of  the 
epiglottis ;  e',  the  tubercle  or  cushion  of  the  epiglottis ;  ph,  part  of  the  anterior  wall  of  the 
pharynx  behind  the  larynx  ;  w,  swelling  in  the  aryteno-epiglottidean  fold  caused  by  the  carti- 
lage of  Wrisberg;  s,  swelling  caused  by  the  cartilage  of  Santorini ;  a,  the  summit  of  the  ary- 
tenoid cartilage;  cv,  the  true  vocal  cords;  cvs,  the  false  vocal  cords;  tr,  the  trachea  with  its 
rings  ;  6,  the  two  bronchi  at  their  commencement. 

the  cartilages  nearly  parallel,  the  glottis  is  narrowed.  When  on  the  con- 
trary the  cartilages  are  wheeled  round  on  the  pivots  of  their  articulations,  so 
that  the  processus  vocales  diverge,  and  the  internal  surfaces  of  the  cartilages 
form  an  angle  with  each  other,  the  glottis  is  widened. 

§  759.  There  are  several  muscles  forming  together  a  group  which  has 
been  called  by  Henle  the  sphincter  of  the  larynx.  These  are  :  1,  the  thyro- 
ary-epiglotticus,  proceeding  from  the  inner  surface  of  the  thyroid  cartilage 
and  from  the  arytenoid  epiglottidean  ligament,  and  sweeping  round  the  outer 
ridge  of  the  arytenoid  cartilage  of  its  own  side  to  be  inserted  into  the  pro- 
cessus muscularis  of  the  arytenoid  cartilage  of  the  other  side ;  2,  the  thyro- 
arytenoideus  externus,  passing  from  the  reentrant  angle  of  the  thyroid  carti- 
lage to  be  inserted  into  the  outer  edge  of  the  arytenoid  cartilage  of  the  same 


824  SPECIAL  MUSCULAR  MECHANISMS. 

side  ;  3,  the  thyro-arytenoideus  interims,  passing  from  the  angle  of  the  thyroid 
cartilage  to  the  processus  vocalis  and  outer  side  of  the  arytenoid  cartilage ; 
4,  the  arytenoideus  (posticus),  passing  transversely  from  one  arytenoid  carti- 
lage to  another.  All  these  muscles,  when  they  act  together,  grasp  round  the 
glottis  and  tend  to  close  it  up  ;  and  each  of  them,  acting  alone,  has,  with  the 
exception  of  the  last  named  (arytenoideus),  the  same  effect.  In  addition  to 
these,  the  crico-arytenoideus  lateralis,  which  passes  from  the  lateral  border 
of  the  cricoid  cartilage  upward  and  backward  to  the  outer  angle  of  the  ary- 
tenoid, by  pulling  this  outer  angle  forward  throws  the  processus  vocalis 
inward,  and  so  also  narrows  the  glottis. 

§  760.  Widening  of  the  glottis.  The  crico-arytenoideus  posticus,  passing 
from  the  posterior  surface  of  the  cricoid  cartilage  to  the  outer  angle  of  the 
arytenoid  cartilage  behind  the  attachment  of  the  lateral  crico-arytenoideus, 
pulls  back  this  outer  angle,  and  so  causing  the  processus  vocalis  to  move 
outward,  widens  the  glottis.  The  arytenoideus  postieus,  acting  alone,  has  a 
similar  effect. 

§  761.  Tightening  of  the  vocal  cords.  The  crico-thyroideus  pulls  the 
thyroid  downward  and  forward,  and  so  increases  the  distance  between  that 
cartilage  and  the  arytenoids  when  the  latter  are  fixed.  Supposing,  then,  the 
arytenoideus  and  crico-arytenoideus  posticus  to  fix  the  arytenoids,  the  effect 
of  the  contraction  of  the  crico-thyroideus  would  be  to  tighten  the  vocal 
cords. 

§  762.  Slackening  of  the  vocal  cords.  This  is  effected  by  the  whole 
sphincter  group  just  mentioned,  but  more  especially  by  the  thyro-arytenoidei 
externus  and  internus ;  these,  acting  alone,  supposing  the  arytenoid  cartilages 
to  be  fixed,  would  pull  the  thyroid  cartilage  upward  and  backward,  and  so 
shorten  the  distance  between  the  processus  vocalis  and  that  body. 

§  763.  Thus  almost  every  movement  of  the  larynx  is  effected  not  by  one 
muscle  only,  but  by  several,  or  at  least  by  more  than  one,  acting  in  concert. 
The  movements  which  give  rise  to  the  voice  are  preeminently  combined  and 
coordinate  movements.  When  we  remember  how  a  very  slight  variation  in 
the  tension  of  the  vocal  cords  must  give  rise  to  a  marked  difference  in  the 
pitch  of  the  note  uttered,  and  yet  what  a  multitude  of  fine  differences  of 
pitch  are  at  the  command  of  a  singer  of  even  moderate  ability,  it  appears 
exceedingly  probable  that  the  various  muscular  combinations  required  to 
produce  the  possible  variations  in  pitch  are  of  such  a  kind  that  frequently  a 
part  only,  possibly  a  few  fibres  only,  of  a  particular  muscle  may  be  thrown 
into  contraction,  while  all  the  rest  of  the  muscle  remains  quiet.  Taking 
into  view,  moreover,  the  great  range  of  pitch  possessed  by  even  common 
voices,  as  compared  with  the  possible  variations  of  tension  of  which  the 
vocal  cords  in  their  natural  length  are  capable,  it  has  been  suggested  that 
some  of  the  fibres  of  the  thyro-arytenoideus  internus,  which,  passing  either 
from  the  thyroid  or  from  the  arytenoid,  appear  to  end  in  the  vocal  cords 
themselves,  may,  by  fixing  particular  points  of  the  cords,  so  to  speak,  "stop" 
them ;  and  by  thus  artificially  shortening  the  length  actually  thrown  into 
vibration,  produce  higher  notes  than  the  cords  in  their  natural  length  are 
capable  of  producing.  It  has  also  been  suggested  that  the  processus  vocales 
may  overlap  each  other,  and  thereby  shorten  the  length  of  cord  available 
for  vibration. 

§  764.  These  various  muscles  are  supplied  by  the  vagus  nerve,  or  rather 
by  spinal  accessory  fibres  running  in  the  vagus  trunk.  The  superior  laryn- 
geal  is  the  afferent  nerve  supplying  the  mucous  membrane,  but  it  also  con- 
tains the  motor  fibres  distributed  to  the  crico-thyroid  muscle  ;  hence,  when 
this  nerve  is  divided  on  one  side  the  corresponding  vocal  cord  is  relaxed  and 
high  notes  become  impossible.  It  is  worthy  of  notice  that  this,  the  chief 


THE  VOICE.  825 

tensor,  and  therefore  the  most  important,  muscle  of  the  larynx,  has  a  sepa- 
rate and  distinct  nervous  supply.  According  to  some  authors  the  aryte- 
noideus  posticus  also  receives  its  nervous  supply  from  this  nerve  ;  but  this 
is  denied  by  others. 

The  inferior  laryngeal  or  recurrent  branch  supplies  all  the  other  muscles. 
When  this  nerve  is  divided  the  voice  is  lost,  since  the  approximation  and 
parallelism  of  the  vocal  cords  can  no  longer  be  effected.  When  in  a  living 
animal  both  recurrent  nerves  are  divided,  the  glottis  is  seen  to  become  im- 
mobile and  partially  dilated,  the  vocal  cords  assuming  the  position  in  which 
they  are  found  in  the  body  after  death,  and  which  may  be  considered  as  the 
condition  of  equilibrium  between  the  dilating  and  constricting  muscles. 
During  forcible  inspiration  the  glottis  passes  from  this  condition  in  the 
direction  of  more  complete  dilatation  ;  during  forcible  expiration,  the  change 
is  one  of  constriction.  When  the  peripheral  portion  of  one  recurrent  nerve 
is  stimulated,  the  vocal  cord  of  the  same  side  is  approximated  to  the  middle 
line  ;  when  both  nerves  are  stimulated,  the  vocal  cords  are  brought  together 
and  the  glottis  is  narrowed.  Though  the  nerve  is  distributed  to  both  dilating 
and  constricting  muscles,  the  latter  overcome  the  former  when  the  nerve  is 
artificially  stimulated.  In  the  complete  closure  of  the  glottis,  which  is  so 
important  a  part  of  the  act  of  coughing  (p.  403),  the  group  of  muscles 
which  we  have  spoken  of  as  constituting  a  sphincter  is  thrown  into  forcible 
contractions  by  the  recurrent  laryngeal  nerve. 

§  765.  Though  fundamentally  a  voluntary  act,  the  utterance  of  a  given 
note  is  not  affected  by  the  direct  passage  of  simple  volitional  impulses  down 
to  the  laryngeal  muscles.  So  complex  and  coordinate  a  movement  as  that 
of  sounding  even  a  simple  and  natural  note  requires  a  coordinating  nervous 
mechanism  in  which,  as  in  other  complex  muscular  actions,  afferent  impulses 
play  an  important  part.  Auditory  sensations,  if  not  as  important  for  an 
accurate  management  of  the  voice  as  are  visual  sensations  for  the  movements 
of  the  eye,  are  yet  of  prime  importance.  This  is  recognized  when  we  say 
that  such  and  such  a  one  whose  power  over  his  laryngeal  muscles  is  imperfect 
"  has  no  ear." 

A  person  may  speak  or  sing  in  two  kinds  of  voice.  In  the  one  the 
sounds  are  full  and  strong,  and  the  resonance  chamber  which  is  supplied  by 
the  trachea,  bronchi,  and  indeed  by  the  whole  chest,  is  thrown  into  power- 
ful and  palpable  vibrations  ;  hence  this  voice  is  spoken  of  as  the  chest- 
voice. The  other  kind  of  voice,  called  the  falsetto,  is  thin  and  poor,  deals 
chiefly  with  high  notes,  and  is  not  accompanied  by  the  same  conspicuous 
vibrations  of  the  chest.  Much  controversy  has  taken  place  as  to  the  exact 
manner  in  which  these  two  voices  are  respectively  produced.  The  prevailing 
opinion  teaches  that  in  the  chest-voice  the  vocal  cords  are  somewhat  thick, 
their  substance  being  thrust  inward  toward  the  median  line  by  the  contrac- 
tion of  the  thyro-arytenoidei  externi  muscles,  and  the  opening  between  them, 
sometimes  so  narrow  as  to  be  almost  linear,  extends  along  their  whole  length. 
In  the  falsetto  voice,  on  the  other  hand,  the  vocal  cords  are  said  to  be  thin 
and  membranous,  and  the  note  to  be  given  forth  by  a  vibration,  not  of  the 
whole  width  of  the  cords,  as  in  the  chest-voice,  but  of  the  extreme  edges 
only,  the  lateral  parts,  though  not  absolutely  at  rest,  vibrating  with  a  differ- 
ent rhythm.  Though  the  whole  larynx  in  the  falsetto  voice  is  stretched  in 
the  antero-posterior  direction,  and  the  vocal  cords  correspondingly  elongated, 
the  rima  vocalis  does  not  extend  along  their  whole  length  ;  at  their  posterior 
part  the  cords  are  in  contact,  and  indeed,  according  to  some  authors,  the 
high  falsetto  notes  are  produced  by  a  sort  of  "  stopping  "  of  the  cords.  The 
sense  of  effort  which  accompanies  the  falsetto  voice  indicates  that  the  changes 
in  the  larynx  which  bring  it  about  are  effected  by  some  special  muscular 


826  SPECIAL  MUSCULAR  MECHANISMS. 

manoeuvres,  as  is  also  suggested  by  the  fact  that  the  ease  with  which  falsetto 
notes  can  be  uttered  is  readily  increased  by  practice.  The  change  from  the 
chest  to  the  falsetto  voice  is  an  abrupt  one,  and  the  combined  range  may  be 
very  extensive,  as  in  the  case  of  persons  who  can  carry  on  a  duet,  singing 
alternately,  for  instance,  in  a  tenor  (chest)  and  a  soprano  (falsetto)  voice. 

§  766.  The  ventricles  of  Morgagni  are  apparently  of  use  in  giving  the 
vocal  cords  sufficient  room  for  their  vibrations,  and  perhaps  supply  a  secre- 
tion by  which  the  vocal  cords  are  kept  adequately  moist.  The  purpose  of 
the  false  vocal  cords  is  not  exactly  known.  Some  authors  think  that  in  the 
falsetto  voice  they  are  brought  down  into  contact  with,  and  thus  serve  to 
stop,  the  true  vocal  cords. 

At  the  age  of  puberty  a  rapid  development  of  the  larynx  takes  place, 
leading  to  a  change  in  the  range  of  the  voice.  The  peculiar  harshness  of 
the  voice  when  it  is  thus  "  breaking  "  seems  to  be  due  to  a  temporary  con- 
gested and  swollen  condition  of  the  mucous  membrane  of  the  vocal  cords 
accompanying  the  active  growth  of  the  whole  larynx.  The  change  in  the 
mucous  membrane  may  come  on  quite  suddenly,  the  voice  "  breaking,"  for 
instance,  in  the  course  of  a  night. 

SPEECH. 

Vowels. 

§  767.  Every  sound,  every  note  (for  all  vocal  sounds  when  considered 
by  themselves  are  musical  sounds),  caused  by  the  vibrations  of  the  vocal 
cords,  besides  its  loudness  due  to  the  force  of  the  expiratory  blast,  and  its 
pitch  due  to  the  tension  of  the  cords,  has  a  quality  of  its  own,  due  to  the 
number  and  relative  prominence  of  the  overtones  which  accompany  the 
fundamental  tone.  Some  of  these  features  which  make  up  the  quality  are  im- 
posed on  the  note  by  the  nature  of  the  vocal  cords,  but  still  more  arise  from 
various  modifications  which  the  relative  intensities  of  the  overtones  undergo 
through  the  resonance  of  the  cavity  of  the  mouth  and  throat.  Whenever 
we  hear  a  note  sounded  by  the  larynx  we  are  able  to  recognize  in  it  features 
which  enable  us  to  state  that  one  or  other  of  the  "  vowels  "  is  being  uttered. 
Vowel  sounds  are  in  fact  only  extreme  cases  of  quality,  extreme  prominence 
of  certain  overtones  brought  about  by  the  shape  assumed  by  the  buccal  arid 
pharyngeal  passages  and  orifices,  as  the  vibrations  pass  through  them.  Each 
vowel  has  its  appropriate  and  causative  disposition  of  these  parts.  When  i 
(ee  in  feet)  is  sounded,  the  sounding-tube  of  the  upper  air-passages  is  made 
as  short  as  possible,  the  larynx  is  raised  and  the  lips  are  retracted,  the  whole 
cavity  of  the  mouth  taking  on  the  form  of  a  broad  flask  with  a  narrow  neck. 
During  the  giving  out  of  e  (a  in  fat)  the  shape  of  the  mouth  is  similar,  but 
somewhat  longer.  For  the  production  of  a  (as  in  father)  the  mouth  is  widely 
open,  so  that  the  buccal  cavity  is  of  the  shape  of  a  funnel  with  the  apex  at 
the  pharynx.  With  o,  the  buccal  cavity  is  again  flask-shaped,  with  the 
mouth  more  closed  than  in  a,  but  the  lips,  instead  of  being  retracted  as  in  i 
and  e,  are  somewhat  protruded,  so  that  the  sounding-tube  is  prolonged.  The 
greatest  length  of  the  tube  is  reached  in  u  (oo),  in  which  the  larynx  is 
depressed  and  the  lips  protruded  as  much  as  possible.  While  the  two  latter 
vowels  are  being  uttered,  the  general  form  of  the  buccal  cavity  is  that  of  a 
flask  with  a  short  neck  and  a  small  opening,  the  orifice  being  smaller  for  u 
than  for  o. 

§  768.  Each  of  these  various  "  vowel "  forms  of  the  mouth  possesses  a 
note  of  its  own,  one  toward  which  it  acts  as  a  resonance  chamber.  Thus,  if 
several  tuning-forks  of  various  pitch  be  held  while  sounding  before  a  mouth 
which  has  assumed  the  particular  form  necessary  for  sounding  U,  it  will  be 


SPEECH.  827 

found  that  the  resonance  will  be  particularly  great  with  the  fork  having  the 
pitch  of  the  bass  6-flat.  Similarly,  other  and  higher  notes  will  be  intensi- 
fied when  the  mouth  is  moulded  to  utter  the  other  vowels.  And  it  is  the 
experience  of  singers  that  each  vowel  is  sung  with  peculiar  ease  on  a  note 
having  a  prominent  overtone  corresponding  to  the  tone  proper  to  the  mouth 
when  moulded  to  utter  the  vowel.  The  precise  nature  of  the  vowel  sounds 
is,  however,  still  disputed. 

As  the  vibrations  are  travelling  through  the  pharyngeal  and  buccal  cavi- 
ties, the  posterior  nares  are  closed  by  the  soft  palate ;  and  it  may  be  shown, 
by  holding  a  flame  before  the  nostril,  that  no  current  of  air  issues  from  the 
nose  when  a  vowel  is  properly  said  or  sung.  When  the  posterior  nares  are 
not  effectually  closed  the  sound  acquires  a  nasal  character.  The  same  hap- 
pens when  the  anterior  nares  are  closed,  as  when  the  nose  is  held  between 
the  fingers,  the  nasal  chamber  then  forming  a  cavity  of  resonance. 


Consonants. 

§  769.  Vowels  are,  as  their  name  implies,  the  only  real  vocal  sounds ;  it 
is  only  on  a  vowel  that  a  note  can  be  said  or  sung.  Our  speech,  however,  is 
made  up  not  only  of  vowels  but  also  of  consonants,  i.  e.,  of  sounds  which  are 
produced  not  by  the  vibrations  of  the  vocal  cords  but  by  the  expiratory  blast 
being  in  various  ways  interrupted  or  otherwise  modified  in  its  course  through 
the  throat  and  mouth. 

The  distinction  between  the  two  is,  however,  not  an  absolute  one,  since, 
as  we  have  seen,  the  characters  of  the  several  vowels  depend  on  the  form  of 
the  mouth,  and  in  the  production  of  some  consonants  (B,  D,  M,  N,  etc.) 
vibrations  of  the  vocal  cords  form  a  necessary  though  adjuvant  factor. 

Consonants  have  been  classified  according  to  the  place  at  which  the 
characteristic  interruption  or  modification  takes  place.  Thus  it  may  occur: 

1.  At  the  lips,  by  the  movement  or  position  of  the  lips  in  reference  to 
each  other  or  to  the  teeth,  giving  rise  to  labial  consonants. 

2.  At  the  teeth,  by  the  movement  or  position  of  the  front  part  of  the 
tongue  in  reference  to  the  teeth  or  the  hard  palate,  giving  rise  to  dental 
consonants. 

3.  In  the  throat,  by  the  movement  or  position  of  the  root  of  the  tongue 
in  reference  to  the  soft  palate  or  pharynx,  giving  rise  to  guttural  consonants. 

Among  the  dentals  again  may  be  distinguished  the  dentals  commonly  so 
called,  such  as  T,  the  sibilants  such  as  S,  and  the  lingual  L,  all  differing  in 
the  relative  position  of  the  tongue,  teeth,  and  palate. 

Consonants  may  also  be  classified  according  to  the  character  of  the  move- 
ments which  give  rise  to  them.  Thus  they  may  be  either  explosive  or  con- 
tinuous. 

1.  Explosives.  In  these  the  characters  are  given  to  the  sound  by  the  sud- 
den establishment  or  removal  of  the  appropriate  interruption.  Thus,  in 
uttering  the  labial  P,  the  lips  are  first  closed,  then  an  expiratory  current  of 
air  is  driven  against  them,  and  upon  their  being  suddenly  opened,  the  sound 
is  generated.  Similarly,  the  dental  T  is  generated  by  the  sudden  removal 
of  the  interruption  caused  by  the  approximation  of  the  tip  of  the  tongue  to 
the  front  of  the  hard  palate,  and  the  guttural  K  by  the  sudden  removal  of 
the  interruption  caused  by  the  approximation  of  the  root  of  the  tongue  to 
the  soft  palate. 

The  labial  B  differs  from  P,  inasmuch  as  it  is  accompanied  by  vibrations 
of  the  vocal  cords  (that  is,  a  vowel  sound  is  uttered  at  the  same  time),  and 
these  vibrations  continue  after  the  removal  of  the  interruption.  Hence,  B  is 


828  SPECIAL  MUSCULAR  MECHANISMS. 

often  spoken  of  as  being  uttered  with  voice  and  P  without  voice  ;  and  D  and 
G  (hard)  with  voice  bear  the  same  relation  to  T  and  K  without  voice. 

The  continuous  consonants  may  further  be  divided  into — 

2.  Aspirates.  In  these  the  sound  is  generated  by  a  rush  of  air  through  a 
constriction  formed  by  the  partial  closure  of  the  lips,  or  by  the  raising  of 
the  tongue  against  the  hard  or  soft  palate,  etc.  Thus,  F  is  sounded  when 
the  lips  are  brought  into  partial,  and  not  as  in  P  and  B  into  complete  approxi- 
mation, and  a  current  of  air  is  driven  through  the  narrowed  opening.  F  is 
uttered  without  any  accompanying  vibration  of  the  vocal  cords,  i.  e.,  without 
voice.  With  voice  it  becomes  V. 

The  sibilant  S  is  formed  by  a  rush  of  air  past  an  obstruction  caused  by 
the  partial  closure  of  the  teeth,  the  front  of  the  tongue  being  depressed  at 
the  same  time ;  and  S  accompanied  with  vibrations  of  the  vocal  cords 
becomes  Z. 

In  Sh  the  dorsal  surface  of  the  tongue  is  raised  so  as  to  narrow  the 
passage  between  that  organ  and  the  palate  for  a  considerable  portion  of  its 
length. 

Th  is  formed  by  placing  the  tongue  between  the  two  partially  open  rows 
of  teeth ;  and  the  hard  and  soft  Th  bear  to  each  other  the  same  relation  as 
do  P  and  B. 

L  is  produced  when  the  passage  is  closed  in  the  middle  by  pressing  the 
tip  of  the  tongue  against  the  hard  palate  and  the  air  is  allowed  to  escape  at 
the  sides  of  the  tongue. 

When  the  constriction  in  an  aspirate  is  formed  by  the  approximation  of 
the  root  of  the  tongue  to  the  soft  palate,  we  have  the  guttural  CH  (as  in 
loch)  without  voice  and  GH  (as  in  lough)  with  voice. 

3.  Resonants  or  nasals.  In  these,  all  of  which  must  have  vibrations  of 
the  vocal  cords  as  a  basis,  the  usual  passage  through  the  mouth  is  closed 
either  in  a  labial,  dental,  or  guttural  fashion,  and  the  peculiar  character  is 
given  to  the  sound  by  the  nasal  chambers  acting  as  a  resonance  cavity.  Thus 
in  M,  the  passage  is  closed  by  the  approximation  of  the  lips,  in  N  by  the 
approximation  of  the  tongue  to  the  hard  palate,  and  in  NG  by  the  approxi- 
mation of  the  root  of  the  tongue  to  the  soft  palate. 

4.  The  various  forms  of  R  are  often  spoken  of  vibratory,  the  charac- 
teristic sounds  being  caused  by  the  vibration  of  some  or  other  of  the  parts 
forming  a  constriction  in  the  vocal  passage.  Thus  the  ordinary  II  is  pro- 
duced by  vibrations  of  the  point  of  the  tongue  elevated  against  the  hard 
palate,  the  guttural  R  by  the  vibrations  of  the  uvula  or  other  parts  of  the 
walls  of  the  pharynx  ;  and  in  some  languages  there  seems  to  be  an  R  pro- 
duced by  the  vibrations  of  the  lips. 

H  is  caused  by  the  rush  of  air  through  the  widely  opened  glottis.  When, 
in  sounding  a  vowel,  the  sound  coincides  with  a  sudden  change  in  the  posi- 
tion of  the  vocal  cords  from  one  of  divergence  to  one  of  approximation,  the 
vowel  is  pronounced  with  the  spiritus  asper.  When  the  vocal  cords  are 
brought  together  before  the  blast  of  air  begins,  the  vowel  is  pronounced  with 
the  spiritus  lenis.  The  Arabic  H  is  produced  by  closing  the  rima  vocalis, 
the  epiglottis  and  false  vocal  cords  being  depressed,  and  sending  a  blast  of  air 
through  the  rima  respiratoria. 

On  many  of  the  above  points,  however,  there  are  great  differences  of 
opinion,  the  discussion  of  which  as  well  as  of  other  more  rare  consonantal 
sounds  would  lead  us  too  far  away  from  the  purpose  of  this  book.  The 
following  tabulated  statement  must,  therefore,  be  regarded  as  introduced  for 
convenience  only. 


LOCOMOTOR  MECHANISMS.  829 

EXPLOSIVES.  Labials,    without  voice P. 

with  voice B. 

Dentals,     without  voice T. 

with  voice D. 

Gutturals,  without  voice K  (hard  C). 

with  voice G-  (hard). 

ASPIRATES.     Labials,     without  voice F. 

with  voice V. 

Dentals,     without  voice L,  S  (soft  C),  Sh,  Th  (hard). 

with  voice Z,  Zh  (in  azure,  the  French  j), 

Th  (soft). 

Gutturals,  without  voice CH  (as  in  loch}. 

with  voice GrH  as  in  lough), 

RESONANTS.    Labial,      M. 

Dental,      N. 

Guttural, NG-. 

VIBRATORY.    Labial,      not  known  in  European  speech. 
Dental,       R  (common). 
Guttural,  R  (guttural). 

§  770.  Whispering  is  speech  without  any  employment  of  the  vocal  cords, 
and  is  effected  chiefly  by  the  lips  and  tongue.  Hence,  in  whispering  the 
distinction  between  consonants  needing  and  those  not  needing  voice,  such  as 
B  and  P,  becomes  for  the  most  part  lost. 

LOCOMOTOR  MECHANISMS. 

§  771.  The  skeletaj  muscles  are  for  the  most  part  arranged  to  act  on  the 
bones  and  cartilages  as  on  levers,  examples  of  the  first  kind  of  lever  being 
rare,  and  those  of  the  third  kind,  where  the  power  is  applied  nearer  to  the 
fulcrum  than  is  the  weight,  being  more  common  than  the  second.  This 
arises  from  the  fact  that  the  movements  of  the  body  are  chiefly  directed  to 
moving  comparatively  light  weights  through  a  great  distance,  or  through  a 
certain  distance  with  great  precision,  rather  than  to  moving  heavy  weights 
through  a  short  distance.  The  fulcrum  is  generally  supplied  by  a  (perfect 
or  imperfect)  joint,  and  one  end  of  the  acting  muscle  is  made  fast  by  being 
attached  either  to  a  fixed  point,  or  to  some  point  rendered  fixed  for  the  time 
being  by  the  contraction  of  other  muscles.  There  are  few  movements  of  the 
body  in  which  one  muscle  only  is  required ;  in  the  majority  of  cases  several 
muscles  act  together  in  concert ;  nearly  all  our  movements  are  coordinate 
movements.  Where  gravity  or  the  elastic  reaction  of  the  parts  acted  on  does 
not  afford  a  sufficient  antagonism  to  the  contraction  of  a  muscle  or  group  of 
muscles,  the  return  to  the  condition  of  equilibrium  is  provided  for  by  the 
action  either  elastic  or  contractile  of  a  set  of  antagonistic  muscles ;  this  is 
seen  in  the  case  of  the  face. 

§  772.  The  erect  posture,  in  which  the  weight  of  the  body  is  borne  by 
the  plantar  arches,  is  the  result  of  a  series  of  contractions  of  the  muscles  of 
the  trunk  and  legs,  having  for  their  object  the  keeping  the  body  in  such  a 
position  that  the  line  of  gravity  falls  within  the  area  of  the  feet.  That  this 
does  require  muscular  action  is  shown  by  the  facts,  that  a  person  when  stand- 
ing perfectly  at  rest  in  a  completely  balanced  position  falls  when  he  becomes 
unconscious,  and  that  a  dead  body  cannot  be  set  on  its  feet.  The  line  of 
gravity  of  the  head  falls  in  front  of  the  occipital  articulation,  as  is  shown 
by  the  nodding  of  the  head  in  sleep.  The  centre  of  gravity  of  the  combined 
head  and  trunk  lies  at  about  the  level  of  the  ensiform  cartilage,  in  front  of 
the  tenth  dorsal  vertebra,  and  the  line  of  gravity  drawn  from  it  passes 
behind  a  line  joining  the  centres  of  the  two  hip-joints,  so  that  the  erect  body 


830  SPECIAL  MUSCULAR  MECHANISMS. 

would  fall  backward  were  it  not  for  the  action  of  the  muscles  passing  from 
the  thighs  to  the  pelvis,  assisted  by  the  anterior  ligaments  of  the  hip-joints. 
The  line  of  gravity  of  the  combined  head,  trunk  and  thighs  falls,  moreover, 
a  little  behind  the  knee-joints,  so  that  some,  though  little,  muscular  exertion 
is  required  to  prevent  the  knees  from  being  bent.  Lastly,  the  line  of  gravity 
of  the  whole  body  passes  in  front  of  the  line  drawn  between  the  two  ankle- 
joints,  the  centre  of  gravity  of  the  whole  body  being  placed  at  the  end  of 
the  sacrum ;  hence  some  exertion  of  the  muscles  of  the  calves  is  required  to 
prevent  the  body  falling  forward. 

§  773.  In  walking,  there  is  in  each  step  a  moment  at  which  the  body 
rests  vertically  on  the  foot  of  one,  say  the  right,  leg,  while  the  other,  the  left 
leg,  is  inclined  obliquely  behind  with  the  heel  raised  and  the  toe  resting  on 
the  ground.  The  left  leg,  slightly  flexed  to  avoid  contact  with  the  ground, 
is  then  swung  forward  like  a  pendulum,  the  length  of  the  swing  or  step  being 
determined  by  the  length  of  the  leg;  and  the  left  toe1  is  brought  to  the 
ground.  On  this  left  toe  as  a  fulcrum,  the  body  is  moved  forward,  the  centre 
of  gravity  of  the  body  describing  a  curve  the  convexity  of  which  is  upward 
and  the  left  leg  necessarily  becoming  straight  and  rigid.  As  the  body  moves 
forward,  a  point  will  be  reached  similar  to  that  with  which  we  supposed  the 
step  to  be  started,  the  body  resting  vertically  on  the  left  foot,  and  the  right 
leg  being  directed  behind  in  an  oblique  position.  The  movement  on  the  left 
foot,  however,  carries  the  body  beyond  this  point,  and  in  doing  so  swings  the 
right  leg  forward  until  it  is  the  length  of  a  step  in  advance  of  its  previous 
position,  and  its  toe  in  turn  forms  a  fulcrum  on  which  the  body,  and  with  it 
the  left  leg,  is  again  swung  forward.  Hence  in  successive  steps  the  centre 
of  gravity,  and  with  it  the  top  of  the  head,  describes^  series  of  consecutive 
curves,  with  their  convexities  upward,  very  similar  to  the  line  of  flight  of 
many  birds. 

Since  in  standing  on  both  feet  the  line  of  gravity  falls  between  the  two 
feet,  a  lateral  displacement  of  the  centre  of  gravity  is  necessary  in  order  to 
balance  the  body  on  one  foot.  Hence  in  walking  the  centre  of  gravity  de- 
scribes not  only  a  series  of  vertical,  but  also  a  series  of  horizontal  curves, 
inasmuch  as  at  each  step  the  line  of  gravity  is  made  to  fall  alternately  on 
each  standing  foot.  While  the  left  leg  is  swinging,  the  line  of  gravity  falls 
within  the  area  of  the  right  foot,  and  the  centre  of  gravity  is  on  the  right 
side  of  the  pelvis.  As  the  left  foot  becomes  the  standing  foot,  the  centre  of 
gravity  is  shifted  to  the  left  side  of  the  pelvis.  The  actual  curve  described 
by  the  centre  of  gravity  is,  therefore,  a  somewhat  complicated  one,  being 
composed  of  vertical  and  horizontal  factors.  The  natural  step  is  the  one 
which  is  determined  by  the  length  of  the  swinging  leg,  since  this  acts  as  a 
pendulum  ;  and  hence  the  step  of  a  long-legged  person  is  naturally  longer 
than  that  of  a  person  with  short  legs.  The  length  of  the  step,  however,  may 
be  diminished  or  increased  by  a  direct  muscular  effort,  as  when  a  line  of 
soldiers  keep  step  in  spite  of  their  having  legs  of  different  lengths.  Such  a 
mode  of  marching  must  obviously  be  fatiguing,  inasmuch  as  it  involves  an 
unnecessary  expenditure  of  energy. 

In  slow  walking  there  is  an  appreciable  time,  during  which,  while  one 
foot  is  already  in  position  to  serve  as  a  fulcrum,  the  other,  swinging,  foot  has 
not  yet  left  the  ground.  In  fast  walking  this  period  is  so  much  reduced  that 
one  foot  leaves  the  ground  the  moment  the  other  touches  it ;  hence  there  is 
practically  no  period  during  which  both  feet  are  on  the  ground  together. 

When  the  body  is  swung  forward  on  the  one  foot  acting  as  a  fulcrum 
with  such  energy  that  this  foot  leaves  the  ground  before  the  other,  swinging, 

1  This  indicates  perhaps  what  should  he  done  rather  than  the  actual  practice ;  most 
people  put  the  heel  to  the  ground  first,  the  contact  with  the  toe  coming  later. 


LOCOMOTOK   MECHANISMS.  831 

foot  has  reached  the  ground,  there  being  an  interval  during  which  neither 
foot  is  on  the  ground,  the  person  is  said  to  be  running,  not  walking. 

In  jumping,  this  propulsion  of  the  body  takes  place  on  both  feet  at  the 
same  time ;  in  hopping,  it  is  effected  on  one  foot  only. 

§  774.  The  locomotion  of  four-footed  animals  is  necessarily  more  com- 
plicated than  that  of  man.  The  simple  walk,  such  as  that  of  the  horse,  is 
executed  in  four  times,  with  a  diagonal  succession ;  thus,  right  fore-leg,  left 
hind-leg,  left  fore-leg,  right  hind-leg.  In  the  amble,  such  as  that  of  the 
camel,  the  two  feet  of  the  same  side  are  put  down  at  one  and  the  same  time, 
this  movement  being  followed  by  a  similar  movement  of  the  other  two  legs ; 
it  corresponds,  therefore,  very  closely  to  human  walking.  In  the  trot,  which 
corresponds  to  human  running,  the  two  diagonally  opposite  feet  are  brought 
to  the  ground  at  the  same  time,  and  the  body  is  propelled  forward  on  them. 
Concerning  this,  however,  as  well  as  concerning  the  still  more  complicated 
gallop  and  canter,  observers  are  not  agreed,  and  much  discussion  has  arisen. 

The  other  problems  connected  with  the  action  of  the  various  skeletal 
muscles  of  the  body  are  too  special  to  be  considered  here. 


BOOK  IV. 

THE  TISSUES  AND  MECHANISMS  OF  REPRODUCTION, 


CHAPTER    I. 

ORGANS  OF  REPRODUCTION. 

§  775.  MANY  of  the  individual  constituent  parts  of  the  body  are  capa- 
ble of  reproduction — i.  e.,  they  can  give  rise  to  parts  like  themselves ;  or 
they  are  capable  of  regeneration — i.  e.,  their  places  can  be  taken  by  new 
parts  more  or  less  closely  resembling  themselves.  The  elementary  tissues 
undergo  during  life  a  very  large  amount  of  regeneration.  Thus,  the  old 
epithelial  scales  which  fall  away  from  the  surface  of  the  body  are  suc- 
ceeded by  new  scales  from  the  underlying  layers  of  the  epidermis;  old 
blood-corpuscles  give  place  to  new  ones  ;  worn-out  muscles,  or  those  which 
have  failed  from  disease,  are  renewed  by  the  accession  of  fresh  fibres ; 
divided  nerves  grow  again  ;  broken  bones  are  united  ;  connective  tissue 
seems  to  disappear  and  reappear  almost  without  limit;  new  secreting  cells 
take  the  place  of  the  old  ones  which  are  cast  off;  in  fact,  with  the  excep- 
tion of  some  cases,  such  as  cartilage,  and  these  doubtful  exceptions,  all 
those  fundamental  tissues  of  the  body,  which  do  not  form  part  of  highly 
differentiated  organs,  are,  within  limits  fixed  more  by  bulk  than  by  any- 
thing else,  capable  of  regeneration.  That  regeneration  by  substitution  of 
molecules,  which  is  the  basis  of  all  life,  is  accompanied  by  a  regeneration 
by  substitution  of  mass. 

In  the  higher  animals  regeneration  of  whole  organs  and  members,  even 
of  those  whose  continued  functional  activity  is  not  essential  to  the  well- 
being  of  the  body,  is  never  witnessed,  though  it  may  be  seen  in  the  lower 
animals  ;  the  digits  of  a  newt  may  be  restored  by  growth,  but  not  those  of 
a  man.  And  the  repair  which  follows  even  partial  destruction  of  highly 
differentiated  organs,  such  as  the  retina,  is  in  the  higher  animals  very  im- 
perfect. 

In  the  higher  animals  the  reproduction  of  the  whole  individual  can  be 
effected  in  no  other  way  than  by  the  process  of  sexual  generation,  through 
which  the  female  representative  element  or  ovum  is,  under  the  influence 
of  the  male  representative  or  spermatozoon,  developed  into  an  adult  indi- 
vidual. 

We  do  not  purpose  to  enter  here  into  any  of  the  morphological  problems 
connected  with  the  series  of  changes  through  which  the  ovum  becomes  the 
adult  being ;  or  into  the  obscure  biological  inquiry  as  to  how  the  simple, 
all-but-structureless  ovum  contains  within  itself,  in  potentiality,  all  its  future 
developments,  and  as  to  what  is  the  essential  nature  of  the  male  action. 
These  problems  and  questions  are  fully  discussed  elsewhere  ;  they  do  not 

53  833 


834  ORGANS  OF  REPRODUCTION. 

properly  enter  into  a  work  on  physiology,  except  under  the  view  that  all 
biological  problems  are,  when  pushed  far  enough,  physiological  problems. 
We  shall  limit  ourselves  to  a  brief  survey  of  the  more  important  physio- 
logical phenomena  attendant  on  the  impregnation  of  the  ovum  and  on  the 
nutrition  and  birth  of  the  embryo. 

§  776.  [  The  female  organs  of  generation  are  anatomically  divided  into  the 
internal  and  external  organs.  The  latter  comprise  the  labia  majora  and 
minora,  the  clitoris,  the  hymen,  the  meatus  urinarius,  the  vulvo-vaginal 
glands,  and  the  mucous  and  sebaceous  glands  which  are  distributed  in 
the  mucous  membrane  covering  the  parts.  The  external  organs  play  a 
very  subsidiary  part  in  the  function  of  reproduction,  and  they  will  be  passed 
by  with  this  brief  notice. 

The  internal  organs  comprise  the  vagina,  uterus,  Fallopian  tubes,  and 
ovaries. 

The  vagina  is  a  musculo-membranous  canal,  about  four  to  six  inches 
long,  directed  obliquely  upward  and  backward,  and  extending  from  the 
hymen  to  the  cervix  uteri,  where  it  is  attached  at  a  point  a  short  distance 

FIG.  209. 


Diagrammatic  View  of  the  Uterus  and  its  Appendages,  as  seen  from  Behind.  (From  Quain.) 
Half  natural  size.  The  uterus  and  upper  part  of  the  vagina  have  been  laid  open  by  removing 
the  posterior  wall ;  the  Fallopian  tube,  round  ligament,  and  ovarian  ligament  have  been  cut 
short,  and  the  broad  ligament  removed  on  the  left  side  :  u,  the  upper  part  of  the  uterus ;  c,  the 
cervix  opposite  the  os  internum  ;  the  triangular  shape  of  the  uterine  cavity  is  shown,  and  the 
dilatation  of  the  cervical  cavity  with  the  rugse,  termed  arbor  vitse  ;  v,  upper  part  of  the  vagina  ; 
od,  Fallopian  tube  or  oviduct ;  the  narrow  communication  of  its  cavity  with  that  of  the  cornu  of 
the  uterus  on  each  side  is  seen ;  I,  round  ligament ;  lo,  ligament  of  the  ovary  ;  o,  ovary  ;  i,  wide 
outer  part  of  the  right  Fallopian  tube  ;  ft,  its  fimbriated  extremity  ;  po,  parovarium  ;  h,  one  of  the 
hydatids  frequently  found  connected  with  the  broad  ligament. 

above  the  os  uteri.  Its  walls  consist  of  an  external  coat  of  longitudinal 
muscular  fibres,  a  middle  erectile  coat,  and  an  internal  mucous  coat.  The 
mucous  membrane  is  continuous  below  with  that  covering  the  external  geni- 
tals, and  above  with  the  mucous  membrane  lining  the  uterus.  The  anterior 
and  posterior  surfaces  are  marked  by  longitudinal  folds  or  raphe,  from 
which  a  number  of  transverse  folds  are  given  off.  This  membrane  is  pro- 
vided with  mucous  glands,  and  is  thickly  covered  with  sensitive  papillae. 

The  uterus  is  a  flattened,  pyriform,  muscular  organ  (Fig.  209).  Ana- 
tomically it  is  divided  into  the  fundus,  neck,  and  cervix.  The  neck  indicates 
the  point  of  division  between  the  lower  constricted  portion,  which  is  the 
cervix,  and  the  upper  expanded  portion,  the  fundus.  The  cervix,  which 
extends  from  the  neck  to  the  end  of  the  organ,  projects  into  the  vagina,  at 
which  point  it  is  marked  by  a  transverse  fissure,  called  the  os  uteri. 


ORGANS  OF  REPRODUCTION. 


835 


FIG.  210. 


The  cavity  of  the  uterus  is  somewhat  triangular  in  shape,  and  very  much 
flattened  antero-posteriorly.  The  inferior  angle  of  the  cavity  is  continuous 
with  the  canal  running  through  the  cervix  to  the 
vagina.  The  superior  angles  are  called  the  cornua ; 
at  the  bottom  of  each  is  an  orifice  of  a  Fallopian 
tube.  The  uterus  is  composed  of  three  coats;  a 
serous  (formed  by  the  peritoneum),  a  muscular,  and 
a  mucous  coat.  The  mucous  coat  is  continuous  with 
that  lining  the  Fallopian  tubes  and  vagina.  It  is 
covered  with  columnar  ciliated  epithelium,  and,  if 
examined  with  a  lens,  the  openings  of  the  mucous 
follicles  will  be  seen  to  be  very  profusely  distributed 
over  the  surface.  If  a  vertical  section  is  made,  as 
in  Fig.  210,  the  tubules  will  be  seen  to  be  arranged 
perpendicularly  to  the  surface,  having  a  wavy 
course.  In  the  impregnated  uterus  they  become 
much  swollen  and  enlarged.  The  mucous  mem- 
brane lining  the  cervix,  on  account  of  its  peculiar 
appearance,  is  called  the  arbor  vitce  uterinus. 

The  Fallopian  tubes  (Fig.  209)  are  about  four 
inches  in  length,  and  extend  from  the  cornua  of  the 
uterus  to  the  ovaries,  where  they  end  in  enlarged 
expanded  extremities,  the  margins  of  which  are 
covered  by  long,  slender  processes,  one  of  them 
being  connected  to  the  ovary.  This  portion  of  the 
tube  is  called  the  fimbriated  extremity.  The  tubes 
are  composed  of  a  serous,  muscular,  and  mucous 
layer.  The  mucous  membrane  is  covered  with 
ciliated  columnar  epithelium. 

The  ovaries  (Fig.  209)  are  flattened,  ovoidal 
bodies,  which  are  situated  one  on  each  side  of  the 
uterus,  and  enclosed  in  the  folds  of  the  broad  liga- 
ments. They  are  each  connected  with  the  uterus 
by  a  ligament,  and  with  the  Fallopian  tube  by  one 
of  its  fimbrise.  They  each  consist  of  a  fibrous  coat 
(tunica  albuginea~]  which  encloses  the  stroma  of  the  organ.  (Fig.  211.)  The 
stroma  is  composed  of  a  soft,  vascular  fibrous  tissue,  having  imbedded  in  it  a 
number  of  small  bodies,  called  Graafian  vesicles,  which  are  in  divers  stages 
of  development.  These  vesicles  commence  their  development  in  the  deeper 
portions  of  the  ovary,  and  as  they  approach  maturity  gradually  make  their 
way  to  the  surface,  where  they  project  as  prominences,  and,  their  capsule 
finally  rupturing,  discharge  their  contents  into  the  Fallopian  tube.  Each 
vesicle  consists  of  an  external  coat  formed  by  the  ovary,  an  internal  coat  or 
capsule,  and  within  this  a  layer  of  cells,  which  constitutes  the  membrana 
granulosa.  The  interior  of  the  vesicle  consists  of  an  albuminous  fluid,  in 
which  is  suspended  the  ovule. 

§  777.  The  male  generative  organs.  The  same  physiological  interest  is 
not  centred  in  the  male  organs  of  generation  as  in  those  of  the  female, 
the  principal  interest  being  concentrated  upon  the  organs  which  secrete  the 
male  fluid  by  which  the  ovule  is  impregnated.  Our  remarks  will,  therefore, 
be  almost  entirely  confined  to  the  organs  concerned  in  the  secretion  of  this 
fluid. 

The  male  organs  comprise  the  penis,  or  organ  of  copulation,  the  pros- 
tate and  Cowper's  glands,  the  testicles,  and  vasa  deferentia  and  vesiculse 
seminales. 


Section  of  the  Lining 
Membrane  of  a  Human 
Uterus  at  the  Period  of  Com- 
mencing Pregnancy,  show- 
ing the  arrangements  and 
other  peculiarities  of  the 
glands,  d,  d,  d,  with  their  ori- 
fices, a,  a,  a,  on  the  internal 
surface  of  the  organ.  Twice 
the  natural  size. 


836  ORGANS  OF  REPRODUCTION. 

The  prostate  gland  surrounds  the  neck  of  the  bladder  and  commence- 
ment of  the  urethra  (Fig.  211).  It  secretes  a  milky  fluid,  which  is  con- 
veyed by  the  prostatic  ducts  to  the  floor  of  the  urethra.  Cowper's  glands 
are  two  small  glands  which  are  situated  between  the  layers  of  the  deep 
perineal  fascia  at  the  anterior  part  of  the  membranous  urethra.  They 
secrete  a  viscid  fluid,  which  is  conveyed  by  ducts  to  the  floor  of  the 
urethra. 

The  testes  or  testicles  are  two  small,  flattened,  ovoidal  glands,  which  are 
situated  in  a  musculo-inembranous  pouch,  called  the  scrotum,  and  suspended 
by  the  spermatic  cords.  Each  testicle  consists  of  two  parts :  the  gland 
proper  and  the  epididymis.  The  gland  (Fig.  213)  is  composed  of  an  outer 

FIG.  211. 


i 

View  of  a  Section  of  Prepared  Ovary  of  the  Cat.  (After  Schron.)  X  6.  1,  outer  covering  and 
free  border  of  the  ovary ;  1',  attached  border ;  2,  the  ovarian  stroma,  presenting  a  fibrous  and 
vascular  structure  ;  3,  granular  substance  lying  external  to  the  fibrous  stroma ;  4,  bloodvessels  ; 
5,  ovigerms  in  their  earliest  stages  occupying  a  part  of  the  granular  layer  near  the  surface ;  6, 
ovigerms  which  have  begun  to  enlarge  and  to  pass  more  deeply  into  the  ovary ;  7,  ovigerms 
round  which  the  Graafian  follicle  and  tunica  granulosa  are  now  formed,  and  which  have  passed 
somewhat  deeper  into  the  ovary  and  are  surrounded  by  the  fibrous  stroma ;  8,  more  advanced 
Graafian  follicle  with  the  ovum  imbedded  in  the  layer  of  cells  constituting  the  proligerous  disc ; 
9,  the  most  advanced  follicle  containing  the  ovum  ;  9',  a  follicle  from  which  the  ovum  has  acci- 
dentally escaped ;  10,  corpus  luteum. 

fibrous  coat,  the  tunica  albuginea,  this  being  covered  by  a  serous  membrane, 
the  tunica  vaginalis.  The  substance  of  the  gland  consists  of  a  number  of 
pyramidal  lobular  divisions,  which  are  situated  with  their  bases  toward  the 
surface.  Each  lobule  is  composed  of  several  convoluted  tubuli  seminiferi, 
and  are  separated  from  adjoining  lobules  by  prolongation  of  fibrous  tissue 
from  the  tunica  albuginea.  The  tubules  are  composed  of  a  homogeneous 
basement  membrane,  which  is  lined  by  granular  nucleated  epithelium.  In 
the  apices  of  the  lobules  they  have  a  straight  course,  and  form  the  vasa  recta. 
They  then  enter  the  fibrous  tissue  of  the  mediastinum  (Fig.  213),  and  form 
a  plexus  of  tubes  called  the  rete  testis,  which  end  in  the  upper  part  of  the 
mediastinum  as  the  vasa  efferentia,  and  these  becoming  very  much  convoluted 
from  the  globus  major  or  head  of  the  epididymis.  The  tubules  of  the  globus 
major  unite  to  form  a  single  tube,  which  is  Very  much  convoluted,  and  con- 
stitutes the  body  and  globus  minor  of  the  epididymis,  and  is  then  continued 
from  the  globus  minor  to  the  base  of  the  bladder  as  the  excretory  duct  or 
vas  deferens. 

The  vas  deferens,  commencing  at  the  globus  minor,  ascends  in  the  pos- 
terior part  of  the  spermatic  cord  through  the  spermatic  canal  into  the  pelvis, 
where  it  runs  to  the  base  of  the  bladder  and  becomes  enlarged,  sacculated, 


ORGANS  OF  REPRODUCTION 


837 


and  narrowed,  and  joins  with  the  duct  of  the  vesicula  seminalis  to  form  a 
common  ejaculatory  duct.  The  walls  of  the  vas  deferens  are  composed  of 
fibrous  and  muscular  tissue,  which  is  lined  by  a  mucous  membrane  with 
columnar  epithelium. 

The  vesiculce  seminales  are  two  elongated  sacculated  bodies,  placed  ex- 


FIG.  212. 


FIG.  213. 


FIG.  212.— The  Base  of  the  Male  Bladder,  with  the  Vesicuise  Seminales  and  Prostat  Jland. 
(After  Haller.)  1,  the  urinary  bladder;  2,  the  longitudinal  layer  of  muscular  fibres;  3,  >pii«  pros- 
tate gland ;  4,  membranous  portion  of  the  urethra ;  5,  the  ureters ;  6,  bloodvessels  ;  7,  lef^.S,  right 
vas  deferens  ;  9,  left  seminal  vesicle  in  its  natural  position ;  10,  ductus  ejaculatorius  of  the  left 
side  traversing  the  prostate  gland;  11,  right  seminal  vesicle  injected  and  unravelled;  12,13, 
blind  pouches  of  vesiculse  ;  14,  right  ductus  ejaculatorius  traversing  the  prostate. 

FIG.  213.— a,  lobules  ;  b,  vasa  recta ;  c,  mediastinum  ;  d,  vasa  efferentia  ;  e,  body  of  epMidymis ; 
/,  rete  testes  ;  g,  globus  minor  ;  h,  vas  deferens  ;  i,  tunica  albuginea  and  its  interlobu;ar  reflec- 
tions ;  I,  globus  major. 

B 


\ 

A,  Spermatozoa  from  the  Human  Vas  Deferens.  (After  Kolliker.)  1,  magnified  350  diameters  ; 
•2,  magnified  800  diameters ;  a,  from  the  side  ;  b,  from  above.  B,  Spermatic  Cells  and  Spermatozoa 
of  the  Bull  undergoing  Development.  (After  Kolliker.)  450;  1.  1,  spermatic  cells,  with  one  or 
two  nuclei,  one  of  them  clear;  2,  3,  free  nuclei,  with  spermatic  filaments  forming;  4,  the  fila- 
ments enlongated  and  the  body  widened ;  5,  filaments  nearly  developed.  . 

ternal  to  the  vasa  deferentia.  The  structure  of  the  seminal  vesicles  is  simi- 
lar to  that  of  the  vasa  deferentia,  consisting  of  a  fibre-muscular  wall  lined 
with  a  mucous  membrane,  which  is  covered  by  granular,  nucleated,  polygonal 
epithelium  cells.  These  organs  serve  as  receptacles  for  the  seminal  fluid 
secreted  by  the  testes,  and  at  the  same  time  produce  a  secretion  of  their  own 


838  MENSTRUATION. 

which  is  added  to  it.  The  ejaculatory  ducts,  which  are  formed  by  the 
union  of  the  ducts  of  the  vasa  deferentia  and  vesiculse  seminales,  open  into 
the  prostatic  portion  of  the  urethra.  Their  coats  are  thinner,  but  have 
essentially  the  same  structure  as  the  vasa  deferentia,  with  which  they  are 
continuous. 

§  778.  The  seminal  fluid  is  a  complex  secretion,  being  composed  of  the 
anatomical  elements  of  spermatozoa,  which  are  formed  in  the  testes,  and  of 
the  secretions  of  the  vasa  deferentia,  vesiculse  seminales,  the  prostrate  and 
Cowper's  glands,  and  the  mucous  glands  of  the  urethra.  The  seminal  fluid 
is  of  a  thick,  whitish  striated  appearance,  and,  if  examined  microscopically, 
is  seen  to  contain  innumerable  bodies  which  are  in  active  motion.  These 
are  the  spermatozoids,  and  are  the  essential  male  elements  concerned  in  the 
fecundation  of  the  ovule.  Each  of  these  bodies  (Fig.  214)  consists  of  a 
flattened,  ovoidal  head,  having  at  its  base  a  tapering  caudate  appendage  in 
active  vibratile  motion.  These  anatomical  elements  were  at  first  considered 
animalcula,  but  they  are  now  looked  upon  as  free  mases  of  protoplasm  with 
ciliary  appendages,  which  endow  them  with  the  power  of  migration. 

The  spermatozoa  are  developed  from  the  nuclei  of  vesicles  which  are 
formed  in  the  tubules  of  the  testes.  The  nuclei  are  metamorphosed  into 
the  heads  of  the  spermatozoa,  the  ciliary  appendages  being  afterward  devel- 
oped as  a  sort  of  outgrowth.  Different  stages  of  the  development  and  other 
interesting  features  are  shown  in  the  above  figure.] 


CHAPTER    II. 

MENSTRUATION. 


§  779.  FROM  puberty,  which  occurs  at  from  13  to  17  years  of  age,  to 
the  climacteric,  which  arrives  at  from  45  to  50  years  of  age,  the  human 
female  is  subject  to  a  monthly  discharge  of  ova  from  the  ovaries,  accompanied 
by  special  changes,  not  only  in  those  organs  but  also  in  the  Fallopian  tubes 
and  uterus,  as  well  as  by  general  changes  in  the  body  at  large,  the  whole 


[FiG.  216. 


5      v    '  fi     5  4  3 

FIG.  215.— Section  of  Graafian  Follicle  of  a  Mammal.  (After  Von  Barr.)  1,  stroma  of  the  ovary 
with  bloodvessels ;  2,  peritoneum  ;  3  and  4,  layers  of  the  external  coat  of  the  Graafian  follicle ;  5, 
membrana  granulosa :  6,  fluid  of  the  Graafian  follicle ;  7,  granular  zone,  or  discus  proligerus,  con- 
taining the  ovule  (8).] 

FIG.  216.-0vula  of  the  Sow.  (After  Barry.)  1,  germinal  spot ;  2,  germinal  vesicle ;  3,  yolk  ;  4, 
zona  pellucida;  5,  discus  proligerus;  6,  adherent  granules  or  cells.] 

constituting  "menstruation."  The  essential  event  in  menstruation  is  the 
escape  of  an  ovum  from  its  Graafian  follicle  (Fig.  215).  The  whole  ovary 
at  this  time  becomes  congested,  and  the  ripe  follicle  bulges  from  its  surface. 
The  most  projecting  portion  of  the  wall  of  the  follicle,  which  has  previously 


MENSTRUATION. 


839 


become  excessively  thin,  is  now  ruptured,  and  the  ovum,  which  having  left 
its  earlier  position,  is  lying  close  under  the  projecting  surface  of  the  follicle, 
escapes,  together  with  the  cells  of  the  discus  proligerus  (Fig.  216),  into  the 
Fallopian  tube.  How  the  entrance  of  the  ovum  into  the  Fallopian  tube  is 
secured  is  not  exactly  known.  Some  maintain  that  the  ovary  is  grasped  by 
the  trumpet-shaped  fimbriated  mouth  of  the  Fallopian  tube,  itself  turgid 
and  congested,  the  movement  necessary  to  bring  this  about  being  effected 
by  the  plain  muscular  fibres  present  in  the  mouth  of  the  tube.  Others,  re- 
jecting this  view,  and  asserting  that  the  turgescence  of  the  tube  does  not 
occur  until  after  the  ovum  has  become  safely  lodged  in  the  tube,  suggest 

FIG.  217. 


e  f  g  h 

Successive  Stages  of  the  Formation  of  the  Corpus  Luteum  in  the  Graafian  Follicle  of  the  Sow 
(as  seen  in  Vertical  Section).  At  a  is  shown  the  state  of  the  follicle  immediately  after  the  expul- 
sion of  the  ovule,  its  cavity  being  filled  with  blood,  and  no  ostensible  increase  of  its  epithelial 
lining  having  yet  taken  place;  at  b  a  thickening  of  this  lining  has  become  apparent;  at  c  it 
begins  to  present  folds,  which  are  deepened  at  d,  and  the  clot  of  blood  is  absorbed  pari  passu,  and 
at  the  same  time  decolorized ;  a  continuance  of  the  same  process,  as  shown  at  e,f,  g,  h,  forms  the 
corpus  luteum,  with  its  delicate  cicatrix. 

that  the  ovum  is  carried  in  the  proper  direction  by  currents  in  the  peritoneal 
cavity  set  up  by  the  action  of  the  ciliated  epithelium  lining  the  tube,  cur- 
rents whose  direction  and  strength  seem,  as  shown  by  experiment,  to  be 
adequate  to  carry  into  the  uterus  particles  present  in  the  peritoneal  fluid. 
Arrived  in  the  tube,  the  ovum  travels  downward,  very  slowly,  by  the  action 
probably  of  the  cilia  lining  the  tube,  though  possibly  its  progress  may 
occasionally  be  assisted  by  the  peristaltic  contractions  of  the  muscular  walls. 
The  stay  of  the  ovum  in  the  Fallopian  tube  may  extend  to  several  days. 
There  is  an  effusion  of  blood  into  the  ruptured  follicle,  which  is  subsequently 
followed  by  histological  changes  in  the  coats  of  the  follicle  resulting  in  a 
corpus  luteum1  (Fig.  217).  The  discharge  of  the  ovum  is  accompanied  not 

1  [The  following  tabular  statement  by  Dalton  expresses  the  principal  differences  be- 
tween the  corpus  luteum  of  the  non-pregnant  and  pregnant  female: 

CORPUS  LUTEUM  OF  MENSTRUATION.  ;      CORPUS  LUTEUM  OF  PREGNANCY. 


At  the  end   of  three 
weeks. 

One  Month. 
Two  Months. 

Six  Months. 
Nine  Months. 


Three-quarters  of  an  inch  in  diam- 
eter; central  clot  reddish;  convo- 
luted wall  pale. 

Smaller;  convoluted  wall  bright  yel- 
low ;  clot  still  reddish. 

Reduced  to  the  condition  of  an  insig- 
nificant cicatrix. 

Absent. 
Absent. 


Larger;  convoluted  with  bright  yel- 
low; clot  still  reddish. 

Seven-eighths  of  an  inch  in  diameter ; 
convoluted  wall  bright  yellow  ;  clot 
perfectly  decolorized. 

Still  as  large  as  at  end  of  second 
month  ;  clot  fibrinous ;  convoluted 
wall  paler. 

One-half  an  inch  in  diameter ;  central 
clot  converted  into  radiating  cica- 
trix ;  the  external  wall  tolerably 
thick  and  convoluted,  but  without 
any  bright  yellow  color.] 


840  MENSTRUATION. 

only  by  a  congestion  or  erection  of  the  ovary  and  Fallopian  tube,  but  also 
by  marked  changes  in  the  uterus,  especially  in  the  uterine  mucous  mem- 
brane. While  the  whole  organ  becomes  congested  and  enlarged,  the  mucous 
membrane,  and  especially  the  uterine  glands,  are  distinctly  hypertrophied. 
The  swollen  internal  surface  is  thrown  into  folds  which  almost  obliterate  the 
cavity ;  and  a  hemorrhagic  discharge,  often  considerable  in  extent,  constitut- 
ing the  menstrual  or  catamenial  flow,  takes  place  from  the  greater  part  of 
its  surface.  The  blood  as  it  passes  through  the  vagina  becomes  somewhat 
altered  by  the  acid  secretions  of  that  passage,  and  when  scanty  coagulates 
but  slightly ;  when  the  flow,  however,  is  considerable,  distinct  clots  may 
make  their  appearance.  The  swollen  and  hypertrophied  mucous  membrane 
then  undergoes  a  rapid  degeneration,  and  is  shed,  passing  away  sometimes  in 
distinct  masses,  forming  the  latter  part  of  the  menstrual  flow.  The  loss  of 
the  mucous  membrane  is  so  complete,  that  the  bases  only  of  the  uterine 
glands  are  left,  and  from  the  epithelial  cells  lining  these  the  regeneration  of 
the  new  membrane  is  said  to  take  place.  It  is  not  certain  that  menstrua- 
tion, in  the  human  subject  at  all  events,  is  always  accompanied  by  a  dis- 
charge of  an  ovum ;  indeed  cases  have  been  recorded  in  which  menstruation 
continued  after  what  appeared  to  be  complete  removal  of  both  ovaries.  And 
it  seems  probable  also  that  under  certain  circumstances,  e.  g.,  coitus,  a  dis- 
charge of  an  ovum  may  take  place  at  other  times  than  at  the  menstrual 
period.  Since,  however,  the  time  during  which  both  the  ovum  and  the 
spermatozoon  may  remain  in  the  female  passages  alive  and  functionally 
capable  is  considerable,  probably  extending  to  some  days,  coitus  effected 
either  some  time  after  or  some  time  before  the  menstrual  escape  of  an  ovum 
might  lead  to  impregnation  and  subsequent  development  of  an  embryo ; 
hence  the  fact  that  impregnation  may  follow  upon  coitus  at  some  time  after 
or  before  menstruation  is  no  very  cogent  argument  in  favor  of  the  view  that 
such  a  coitus  has  caused  an  independent  escape  of  an  ovum.  The  escape  of 
the  ovum  is  said  to  precede,  rather  than  coincide  with  or  follow,  the  cata- 
menial flow.  If  no  spermatozoa  come  in  contact  with  the  ovum  it  dies,  the 
uterine  membrane  returns  to  its  normal  condition,  and  no  trace  of  the  dis- 
charge of  an  ovum  is  left,  except  the  corpus  luteum  in  the  ovary. 

§  780.  It  is  obvious  that  in  these  phenomena  of  menstruation  we  have 
to  deal  with  complicated  reflex  actions  affecting  not  only  the  vascular 
supply,  but,  apparently  in  a  direct  manner,  the  nutritive  changes  of  the 
6rgans  concerned.  Our  studies  on  the  nervous  action  of  secretion  render  it 
easy  for  us  to  conceive  in  a  general  way  how  the  several  events  are  brought 
about.  It  is  no  more  difficult  to  suppose  that  the  stimulus  of  the  enlarge- 
ment of  a  Graafian  follicle  causes  nutritive  as  well  as  vascular  changes  in 
the  uterine  mucous  membrane,  than  it  is  to  suppose  that  the  stimulus  of  food 
in  the  alimentary  canal  causes  those  nutritive  changes  in  the  salivary  glands 
or  pancreas  which  constitute  secretion.  In  the  latter  case  we  can  to  some 
extent  trace  out  the  chain  of  events ;  in  the  former  case  we  hardly  know 
more  than  that  the  maintenance  of  the  lumbar  cord  is  sufficient,  as  far  as 
the  central  nervous  system  is  concerned,  for  the  carrying  on  of  the  work. 
In  the  case  of  a  dog  in  which  the  spinal  cord  had  been  completely  divided 
in  the  dorsal  region  while  the  animal  was  as  yet  a  mere  puppy,  "  heat  "  or 
menstruation  took  place  as  usual. 


IMPREGNATION.  841 

CHAPTER    III. 

IMPREGNATION. 

§  781.  IN  coitus  the  discharge  of  the  semen  containing  the  spermatozoa 
is  most  probably  effected  by  means  of  the  peristaltic  contractions  of  the  vesi- 
cu\2d  semiiiales  and  vasa  deferentia,  assisted  by  rhythmical  contractions  of 
the  bulbo-cavernosus  muscle,  the  whole  being  a  reflex  act,  the  centre  of 
which  appears  to  be  in  the  lumbar  spinal  cord.  In  the  dog  emissions  of 
semen  can  be  brought  about  by  stimulation  of  the  glans  penis  after  complete 
division  of  the  spinal  cord  in  the  dorsal  region.  The  emission  of  semen  is 
preceded  by  an  erection  of  the  penis.  This  we  have  already  seen  (p.  274)  is 
in  part  at  least  due  to  an  increased  vascular  supply  brought  about  by  means 
of  the  nervi  erigentes ;  it  is  probable,  however,  that  the  condition  is  further 
secured  by  a  compression  of  the  efferent  veins  of  the  corpora  cavernosa  by 
means  of  smooth  muscular  fibres  present  in  those  bodies.  The  semen  being 
received  into  the  female  organs,  which  are  at  the  time  in  a  state  of  turges- 
cence  resembling  the  erection  of  the  penis,  but  less  marked,  the  spermatozoa 
find  their  way  into  the  Fallopian  tubes,  and  here  (probably  in  its  upper 
*part)  come  into  contact  with  the  ovum.  In  the  case  of  some  animals  im- 
pregnation may  take  place  at  the  ovary  itself.  The  passage  of  the  sperma- 
tozoa is  most  probably  effected  mainly  by  their  own  vibratile  activity  ;  but 
in  some  animals  a  retrograde  peristaltic  movement  travelling  from  the  uterus 
along  the  Fallopian  tubes  has  been  observed  ;  this  might  assist  in  bringing 
the  semen  to  the  ovum,  but  inasmuch  as  these  movements  are  probably 
parts  of  the  act  of  coitus,  and  impregnaton  may  be  deferred  till  some  time 
after  that  event,  no  great  stress  can  be  laid  upon  them. 

As  the  result  of  the  action  of  the  spermatozoa  on  the  ovum,  the  latter, 
instead  of  dying  as  when  impregnation  fails,  awakes  to  great  nutritive 
activity  accompanied  by  remarkable  morphological  changes ;  it  enlarges 
and  develops  into  an  embryo. 

§  782.  [Preceding  the  time  of  the  occurrence  of  the  entrance  of  the 
spermatozoon  into  the  egg,  certain  histological  changes  have  been  observed 
to  occur,  and  in  order  thoroughly  to  understand  these,  as  well  as  the  changes 
which  follow  in  the  ovum,  it  will  first  be  necessary  to  review  the  histology 
of  the  egg. 

The  ovule  is  a  minute  cell,  the  wall  being  formed  by  a  structureless, 
transparent  membrane,  called  the  zona  pellucida,  or  vitelline  membrane. 
Within  this  is  the  yolk,  or  vitellus,  which  consists  of  a  granular  semi-fluid 
mass,  having  suspended  in  it  a  nucleus  or  germinal  vesicle  which  contains  a 
nucleolus  or  germinal  spot  The  germinal  vesicle  consists  of  a  very  delicate, 
transparent,  homogeneous  membrane  which  encloses  a  fluid  with  granules, 
suspended  in  which  is  an  eccentric  nucleolus  of  a  granular  and  fibrillated 
structure. 

§  783.  Previous  to  the  occurrence  of  the  impregnation  of  the  ovule  a 
very  interesting  series  of  changes  have  been  observed  to  take  place.  Accord- 
ing to  Balfour,  the  first  interesting  point  to  be  noticed  is  the  migration  of 
the  germinal  vesicle  toward  the  cell  wall.  The  vesicular  wall  then  becomes 
wavy  and  gradually  disappears,  while  at  the  same  time  the  nucleolus  or 
germinal  spot  has  undergone  metamorphosis,  so  that  what  remains  of  these 
structures  is  a  spindle-shaped  mass.  One  extremity  of  this  mass  gradually 
projects  through  the  cell  wall  and  is  thrown  off  as  a  polar  vesicle.  From  the 
other  remaining  portion  a  second  polar  vesicle  is  formed,  the  part  of  the 


842 


IMPEEGNATION. 


mass  then  remaining  in  the  ovule  being  permanent,  and  is  called  the  female 
pronudeus.  The  next  change  observed  is  the  appearance  of  a  zone,  of 
radial  striae  around  the  pronucleus  and  its  migration  to  the  centre  of  the  egg. 
The  spermatozoon  then  penetrates  the  wall  of  the  ovule,  probably  at  the 
point  of  the  formation  of  the  polar  vesicles.  The  tail  of  the  spermatozoon 
becomes  absorbed,  and  the  head  is  metamorphosed  into  the  male  pronucleus. 
From  the  male  pronucleus  a  number  of  radiating  striae  are  given  off  in  all 
directions,  and  it  then  migrates  toward  the  female  pronucleus,  and  afterward 
fuses  with  it,  forming  a  single  or  cleavage  nucleus. 

§  784.  Cleavage  or  segmentation  of  the  vitellus  then  begins  (Fig.  218), 
by  which  process  the  nucleus  thus  formed  divides  into  two  parts,  each  taking 

FIG.  218. 


FIG.  219. 


Diagrams  of  the  Various  Stages  of  Cleavage  of  the  Yolk.    (After  Dalton.) 

with  it  half  of  the  vitelline  mass.  These  two  divide  into  four,  and  these  four 
into  eight,  and  so  on  indefinitely  until  an  agglomerate  mass  of  nucleated 
cells  results,  each  of  which  contains  a  part  of  the  cleavage  nucleus.  This 
mass  of  cells  is  called  the  mulberry  mass,  and  the 
cells  constituting  it  arrange  themselves  about  the  in- 
terior of  the  zona  pellucida  and  form  the  blastodermic 
vesicle  or  membrane.  This  membrane  then  splits  up 
into  two  layers,  the  external  and  internal,  a  third 
or  middle  layer  being  afterward  formed  between 
them. 

Immediately  after  the  formation  of  the  two  layers 
of  blastoderm,  an  opaque  rounded  collection  of  small 
cells  occurs,  called  the  area  germinativa  or  embryonic 
spot.  (Fig.  219.)  This  spot  then  becomes  elongated, 
and  in  its  longitudinal  axis  the  first  trace  of  the  em- 
bryo appears  as  a  faint  line,  termed  the  primitive  trace, 
this  being  in  the  midst  of  a  clear  elongated  mass  of 
cells,  the  area  pellucida,  which  is  itself  surrounded  by 
a  more  opaque  zone. 
§  785.  In  front  of  the  primitive  trace  two  folds  are  formed  from  which 
a  groove  is  prolonged  backward  in  a  line  with  the  primitive  trace.  These 
folds  gradually  extend  along  the  entire  length  of  the  groove,  and  from  the 
laminae  dorsales,  which,  by  growing,  project  more  and  more  above  the 
groove,  and,  gradually  approaching  each  other,  coalesce  and  enclose  the 
neural  canal,  which  will  afterward  contain  the  cerebro-spinal  axis.  At 
about  the  same  period,  corresponding  to  the  development  of  the  dorsal 
laminse  similar  laminae  are  given  off  from  the  under  surface  of  the  blasto- 
derm. These  are  the  lamince  ventrales,  which,  by  gradually  enlarging  and 
finally  coalescing,  enclose  the  abdominal  cavity.  Beneath  the  floor  of  the 
groove  above  described  a  delicate,  whitish  collection  of  cells  appears.  This 
is  the  chorda  dorsalis  or  notochord,  around  which  are  afterward  developed, 
the  bodies  and  processes  of  the  vertebra. 


Impregnated  Egg,  with 
Commencement  of  For- 
mation of  Embryo.  (Af- 
ter Dalton.)  Showing  the 
area  germinativa  or  em- 
bryonic spot,  the  area 
pellucida,  and  the  prim- 
itive groove  or  trace. 


IMPKEGNATION. 


843 


During  this  period  other  changes  have  also  taken  place.  The  cephalic 
and  caudal  extremities  have  become  fixed  and  form  the  cephalic  and  caudal 
flexures;  and  the  embryo  also  being  curved  upon  itself  laterally,  the  vitelline 
mass  appears  separated  from  it  by  a  constriction.  This  constriction  gradu- 
ally increasing,  finally  separates  the  vitelline  mass  as  a  vesicular  body,  it 
being  connected  with  the  body  of  the  embryo  by  the  vitelline  duct.  (Fig. 
220.)  The  vesicular  body  thus  formed  is  called  the  umbilical  vesicle.  This 
at  first  communicates  with  the  intestinal  cavity,  but  as  development  proceeds 
the  duct  of  communication  becomes  closed,  and  the  vesicle  is  merely  attached 
by  a  pedicle,  and  finally  disappears  altogether.  At  the  time  of  the  develop- 
ment of  the  bloodvessels,  vessels  appear  on  the  surface  of  the  umbilical 
vesicle,  constituting  the  vascular  area,  the  chief  vessels  being  the  omphalo- 
mesenteric  arteries  and  veins.  The  vessels  of  the  vascular  area  absorb  the 
nutritive  material  contained  within  the  vesicle  and  convey  it  to  the  embryo 
for  its  sustenance. 

FIG.  220. 


Diagrammatic  Section  showing  the  Relation  in  a  Mammal  and  in  a  Man  between  the  Primi- 
tive Alimentary  Canal  and  the  Membranes  of  the  Ovum.  The  stage  represented  in  this  diagram 
corresponds  to  that  of  the  fifteenth  or  seventeenth  day  in  the  human  embryo,  previous  to  the  ex- 
pansion of  the  allantois  ;  c,  the  villpus  chorion  ;  a,  the  amnion ;  a',  the  place  of  convergence  of 
the  amnion  and  reflection  of  the  false  amnion,  a"  a",  or  outer  or  corneous  layer ;  e,  the  head  and 
trunk  of  the  embryo,  comprising  the  primitive  vertebrae  and  cerebro-spinal  axis  ;  i,  i,  the  simple 
alimentary  canal  in  its  upper  and  lower  portions ;  the  yolk-sac  or  umbilical  vesicle ;  vi,  the  vitel- 
line duct :  u,  the  allantois  connected  by  a  pedicle  with  the  anal  portion  of  the  alimentary  canal. 

§  786.  Shortly  after  the  occurrence  of  the  commencement  of  the  forma- 
tion of  the  umbilical  vesicle,  double  folds,  formed  of  the  external  layer  of 
the  blastoderm,  are  given  off  from  the  cephalic  and  caudal  extremities,  and 
laterally,  which  curve  around  over  the  dorsal  surface  of  the  embryo,  where 
they  meet  and  coalesce,  and,  their  point  of  junction  becoming  absorbed,  form 
the  amniotic  cavity.  (Figs.  220,  221,  222,  and  223.)  The  outer  layer  of 
the  fold,  or  false  amnion,  gradually  expands  and  covers  the  whole  of  the  in- 
ternal surface  of  the  vitelline  membrane,  which  it  ultimately  replaces ;  the 


844 


IMPREGNATION. 


inner  layer,  or  true  amnion,  is  continuous  with  the  skin  of  the  embryo  at  the 
umbilicus,  and  closely  envelops  it.  The  amniotic  cavity  or  sac  thus  formed 
becomes  filled  with  the  liquor  amnii,  which  gradually  increases  in  quantity 
as  pregnancy  advances,  up  to  about  the  fifth  or  sixth  month,  when  the 
quantity  gradually  decreases  up  to  the  time  of  labor. 


FIG.  221. 


FIG.  222. 


a,  chorion  with  villi.  The  villi  are  shown  to  be  best  developed  in  the  part  of  the  chorion  to 
which  the  allantois  is  extending ;  this  portion  ultimately  becomes  the  placenta,  b,  space  between 
the  two  layers  of  the  amnion  ;  c,  amniotic  cavity ;  d,  situation  of  the  intestine,  showing  its  con- 
nection with  the  umbilical  vesicle ;  e,  umbilical  vesicle  ;/,  situation  of  heart  and  vessels ;  g,  allan- 
tois. (After  Todd  and  Bowman.) 

§  787.  At  about  the  time  of  the  commencement  of  the  development  of 
the  amnion  a  new  organ,  the  allantois,  appears  as  a  pyriform  mass  of  cells  at 
a  point  immediately  posterior  to  the  vitelliue  duct  and  projecting  through 
the  same  opening.  (Fig.  223.)  This  mass  of  cells  undergoes  rapid  growth, 
spreading  itself  between  the  true  and  false  amniotic  folds,  finally  completely 


FIG.  223. 


FIG.  224. 


FIG.  223.— Diagram  of  Fecundated  Egg.  (After  Dalton.)  a,  umbilical  vesicle  ;  6,  amniotic 
cavity ;  c,  allantois. 

FIG.  224. — Fecundated  Egg  with  Allantois  nearly  Complete,  a,  inner  layer  of  amniotic  fold ; 
6.  outer  layer  of  ditto  ;  c,  point  where  the  amniotic  folds  come  in  contact.  The  allantois  is  seen 
penetrating  between  the  outer  and  inner  layers  of  the  amniotic  folds.  This  figure,  which  repre- 
sents only  the  amniotic  folds  and  the  parts  within  them,  should  be  compared  with  Figs.  221  and 
222,  in  which  will  be  found  the  structures  external  to  these  folds. 

enclosing  the  embryo  and  amnion  (Fig.  224),  becoming  at  the  same  time 
adjoined  to  the  false  amnion,  when  it  is  developed  into  the  true  chorion.  Dur- 
ing the  process  of  the  development  of  the  allantois,  it  has  become  very  vas- 
cular ;  at  first  there  are  two  arteries  and  two  veins,  afterward  one  of  the 
veins  disappears.  These  vessels  constitute  the  umbilical  vessels,  forming  part 
of  the  umbilical  cord  which  connects  the  allantois  with  the  embryo.  During 


IMPREGNATION. 


845 


the  development  of  the  allantois  it  presents  three  distinct  anatomical  por- 
tions :  a  portion  which  becomes  constricted  off,  as  it  were,  from  the  rest  and 
forms  the  urinary  bladder ;  the  outer  portion  forms  the  chorion,  the  inter- 
mediate portion  forming  the  umbilical  cord. 


FIG.  225. 


FIG.  225.— Entire  Human  Ovum  of  Eighth  Week ;  sixteen  lines  in  length  (not  reckoning  the 
tufts) ;  the  surface  of  the  chorion  partly  smooth  and  partly  rendered  shaggy  by  the  growth  of  tufts. 

FIG.  226— Portion  of  One  of  the  Foetal  Villi,  about  to  form  part  of  the  Placenta;  highly  magni- 
fied, a,  a,  its  cellular  covering ;  b,  b,  b,  its  looped  vessels ;  c,  c,  its  basis  of  connective  tissue. 

During  the  development  of  the  embryo  up  to  this  time,  the  first  chorion 
was  formed  by  villosities  formed  on  the  vitelline  membrane ;  and  following 
that  by  villosities  developed  upon  the  false  amnion.  The  allantois  then 
becoming  developed,  completely  covers  the  internal  surface  of  the  false 
amnion,  which  then  gradually  disappears  as  a  distinct  structure.  The  true 
chorion  is  then  formed  by  the  allantois,  which  becomes  covered  by  a  growth 
of  a  multitude  of  vascular  shaggy  tufts  or  villi  (Fig.  225).  These  villi,  at 
first,  are  distributed  over  the  entire  surface  of  the  organ,  but  they  soon  com- 
mence disappearing,  except  at  a  small  area  corresponding  to  the  attachment 
of  the  pedicle  which  connects  the  allantois  with  the  embryo.  At  this  point 
they  become  greatly  increased  in  number,  and  also  in  size  and  vascularity. 
These  villi  are  composed  of  a  fibro-granular  matrix,  in  which  are  numerous 
capillary  loops,  and  are  covered  with  a  layer  of  epithelial  cells  (Fig.  226). 
This  portion  of  the  chorion  forms  the  foetal  portion  of  the  placenta. 

No  sooner,  however,  have  these  changes  begun  in  the  ovum  than  correla- 
tive changes,  brought  about  probably  by  reflex  action,  but  at  present  most 
obscure  in  their  causation,  take  place  in  the  uterus.  The  mucous  membrane 
of  this  organ,  whether  the  coitus  resulting  in  impregnation  be  coincident 
with  a  menstrual  period  or  not,  becomes  congested,  and  a  rapid  growth  takes 
place,  characterized  by  a  rapid  proliferation  of  the  epithelial  and  subepithe- 
lial  tissues.  Unlike  the  case  of  menstruation,  however,  this  new  growth  does 
not  give  way  to  immediate  decay  and  hemorrhage,  but  remains,  and  may  be 


846 


IMPREGNATION. 


distinguished  as  a  new  temporary  lining  to  the  uterus,  the  so-called  decidua. 
Into  this  decidua  the  ovum,  on  its  descent  from  the  Fallopian  tube,  in  which 
it  has  undergone  developmental  changes,  extending  most  probably  as  far,  at 
least,  as  the  formation  of  the  blastoderm,  if  not  further,  is  received  ;  and  in 
this  it  becomes  imbedded,  the  new  growth  closing  in  over  it.  (Figs.  227,  228.) 
Meanwhile  the  rest  of  the  uterine  structures,  especially  the  muscular  tissue, 


FIG.  227. 


FIG.  228. 


FIG.  227.— First  Stage  of  the  Formation  of  the  Decidua  Reflexa  around  the  Ovum. 
FIG.  228.— More  Advanced  Stage  of  Decidua  Reflexa. 

become  also  much  enlarged ;  as  pregnancy  advances  a  large  number  of  new 
muscular  fibres  are  formed.  As  the  ovum  continues  to  increase  in  size,  it 
bulges  into  the  cavity  of  the  uterus,  carrying  with  it  the  portion  of  the 
decidua  which  has  closed  over  it.  Henceforward,  accordingly,  a  distinction 
is  made  in  the  now  well-developed  decidua  between  the  decidua  reflexa,  or 

FIG.  229. 


Section  of  a  Portion  of  a  Fully  Formed  Placenta,  with  the  part  of  the  Uterus  to  which  it  is 
attached,  a,  umbilical  cord  ;  b,  b,  section  of  uterus,  showing  the  venous  sinuses  ;  c,  c,  c,  branches, 
of  the  umbilical  vessels  ;  d,  d,  curling  arteries  of  the  uterus. 

that  part  of  the  membrane  which  covers  the  projecting  ovum,  and  the 
decidua  vera,  or  the  rest  of  the  membrane  lining  the  cavity  of  the  uterus, 
the  two  being  continuous  around  the  base  of  the  projecting  ovum.  That  part 
of  the  decidua  which  intervenes  between  the  ovum  and  the  nearest  uterine 


THE  NUTRITION  OF  THE  EMBRYO.  847 

wall  is  frequently  spoken  of  as  the  decidua  serotina.  As  the  ovum  develops 
into  the  foetus  with  its  membranes,  the  decidua  reflexa  becomes  pushed 
against  the  decidua  vera ;  about  the  end  of  the  third  month,  in  the  human 
subject,  the  two  come  into  complete  contact  all  over,  and  ultimately  the  dis- 
tinction between  them  is  lost.  In  the  region  of  the  decidua  serotina,  the 
allautoic  vessels  of  the  fcetus  develop  a  placenta. 

§  788.  In  the  earliest  stages  of  the  development  of  the  placenta,  the 
delicate  villous  processes  of  the  chorion  insinuate  themselves  into  the  hyper- 
trophied  follicles  of  the  decidua  serotina.  The  villi  then  undergo  a  rapid 
increase  in  size  and  vascularity,  becoming  branched  into  secondary  and 
tertiary  ramifications ;  while,  at  the  same  time,  corresponding  changes  are 
taking  place  in  the  follicles,  by  which  they  become  greatly  increased  in  size 
and  vascularity,  and  at  the  same  time  forming  diverticula  in  which  are  im- 
bedded the  ramifications  of  the  villi.  The  villi  and  follicles  thus  grow 
simultaneously,  and  finally  become  blended  with  each  other  and  are  no 
longer  separate  structures.  The  follicular  bloodvessels  first  form  capillary 
plexuses ;  these  vessels,  however,  become  enlarged,  forming  frequent  anasto- 
moses, and  finally  coalescing  to  form  venous  sinuses  (Fig.  229),  in  which  are 
bathed  the  fcetal  villi.  There  is  no  continuity  established  between  the 
maternal  and  fcetal  blood ;  the  interchange  of  nutritive  material  necessary 
for  the  growth  and  development  of  the  foetus  takes  place  through  the  delicate 
walls  of  the  villi.] 

For  further  account  of  the  various  changes  by  which  these  events  are 
brought  about,  as  well  as  of  the  history  of  the  embryo  itself,  we  must  refer 
the  reader  to  anatomical  treatises. 


CHAPTER  IV.       . 

THE  NUTRITION  OF  THE  EMBRYO. 

§  789.  During  the  development  of  the  chick  within  the  hen's  egg  the 
nutritive  material  needed  for  the  growth' first  of  the  blastoderm,  and  subse- 
quently of  the  embryo,  is  supplied  by  the  yolk,  while  the  oxygen  of  the  air, 
passing  freely  through  the  porous  shell,  gains  access  to  all  the  tissues  both  of 
the  embryo  and  the  yolk,  either  directly  or  by  the  intervention  of  the  allan- 
toic  vessels.  The  mammalian  embryo,  during  the  period  which  precedes  the 
extension  of  the  allantoic  vessels  into  the  cavities  of  the  uterine  wall  to 
form  the  placenta,  must  be  nourished  by  direct  diffusion,  first  from  the  con- 
tents of  the  Fallopian  tube  and  subsequently  from  the  decidua ;  and  its 
supply  of  oxygen  must  come  from  the  same  sources.  All  analogy  would 
lead  us  to  suppose  that,  from  the  very  first,  oxidation  is  going  on  in  the 
blastodermic  and  embryonic  structures ;  but  the  amount  of  oxygen  actually 
withdrawn  from  without  is  probably  exceedingly  small  in  the  early  stages, 
seeing  that  nearly  the  whole  energy  of  the  metabolism  going  on  is  directed 
to  the  building  up  of  structures,  the  expenditure  of  energy  in  the  form  of 
either  heat  or  external  work  being  extremely  small.  The  marked  increase 
of  bulk  which  takes  place  during  the  conversion  of  the  mulberry  mass  into 
the  blastodermic  vesicle  shows  that  at  this  epoch  a,  relatively  speaking,  large 
quantity  of  water  at  least,  and  probably  of  nutritive  matter,  must  pass  from 
without  into  the  ovum ;  and  subsequently,  though  the  blastoderm  and  em- 


848  THE  NUTRITION  OF  THE  EMBRYO. 

V 

bryo  may  for  some  time  draw  the  material  for  their  continued  construction 
at  first  hand  from  the  yolk-sac  or  umbilical  vesicle,  both  this  and  they  con- 
tinue probably  until  the  allantois  is  formed  to  receive  fresh  material  from 
the  mother  by  direct  diffusion. 

§  790.  As  the  thin-walled  allantoic  vessels  come  into  closer  and  fuller 
connection  with  the  maternal  uterine  sinuses,  until  at  last  in  the  fully 
formed  placenta  the  former  are  freely  bathed  in  the  blood  streaming 
through  the  latter,  the  nutrition  of  the  embryo  becomes  more  and  more 
confined  to  this  special  channel.  The  blood  of  the  foetus  flowing  along 
the  umbilical  arteries  effects  exchanges  with  the  venous  blood  of  the 
mother,  and  leaves  the  placenta  by  the  umbilical  vein  richer  in  oxygen 
and  nutritive  material  and  poorer  in  carbonic  acid  and  excretory  prod- 
ucts than  when  it  issued  from  the  foetus. 

As  far  as  the  gain  of  oxygen  and  the  loss  of  carbonic  acid  are  concerned 
these  are  the  results  of  simple  diffusion.  Venous  blood,  as  we  have  already 
seen,  always  contains  a  quantity  of  oxy-hsemoglobin,and  the  quantity  of  this 
substance  present  in  the  blood  of  the'uterine  veins  is  sufficient  to  supply  all 
the  oxygen  that  the  embryo  needs ;  the  blood  of  the  foetus,  containing  less 
oxygen  than  even  the  venous  blood  of  the  mother,  will  take  up  a  certain 
though  small  quantity.  The  foetal  blood  travelling  in  the  umbilical  artery 
must,  in  proportion  to  the  extent  of  the  nutritive  changes  going  on  in  the 
embryo,  possess  a  higher  carbonic  tension  than  that  in  the  umbilical  vein  or 
uterine  sinus ;  and  by  diffusion  gets  rid  of  this  surplus  during  its  stay  in 
the  placenta.  The  blood  in  the  umbilical  arteries  and  veins  is,  therefore, 
relatively  speaking,  venous  and  arterial  respectively,  though  the  small  ex- 
cess of  oxy-hsemoglobin  in  the  blood  of  the  umbilical  vein  is  insufficient  to 
give  it  a  distinctly  arterial  color,  or  to  distinguish  it  as  sharply  from  the 
more  venous  blood  of  the  umbilical  artery,  as  in  ordinary  arterial  from  or- 
dinary venous  blood.  Thus,  the  foetus  breathes  by  means  of  the  maternal 
blood,  in  the  same  way  that  a  fish  breathes  by  means  of  the  water  in  which 
it  dwells. 

The  blood  of  the  foetus  is  very  poor  in  haemoglobin,  corresponding  to  its 
low  oxygen  consumption.  When  the  mother  is  asphyxiated  the  foetus  is 
asphyxiated  too,  the  oxygen  of  the  latter  passing  back  again  into  the  blood 
of  the  former ;  and  the  asphyxia  thus  produced  in  the  foetus  is  much  more 
rapid  than  that  which  results  when  the  oxygen  is  used  up  by  the  tissues  of 
the  foetus  alone,  as  when  the  umbilicus  is  ligatured  and  the  foetus  not  allowed 
to  breathe. 

If  oxygen  and  carbonic  acid  thus  pass  by  diffusion  to  and  from  the 
mother  and  the  foetus,  one  might  fairly  expect  that  diffusible  salts,  proteids, 
and  carbohydrates  would  be  conveyed  to  the  latter,  and  diffusible  excretions 
carried  away  to  the  former,  in  the  same  way ;  and  if  fats  can  pass  directly 
into  the  portal  blood  during  ordinary  digestion,  there  can  be  no  reason  for 
doubting  that  this  class  of  food-stuffs  also  would  find  its  way  to  the  foetus 
through  the  placental  structures.  We  do  know  from  experiment  that  dif- 
fusible substances  will  pass  both  from  the  mother  to  the  foetus,  and  from 
the  foetus  to  the  mother ;  but  we  have  no  definite  knowledge  as  to  the  exact 
form  and  manner  in  which,  during  normal  intra-uterine  life,  nutritive  mate- 
rials are  conveyed  to  or  excretions  conveyed  from  the  growing  young.  The 
placenta  is  remarkable  for  the  great  development  of  cellular  structures, 
apparently  of  an  epithelial  nature,  on  the  border-land  between  maternal  and 
foetal  elements ;  and  it  has  been  suggested  that  these  form  a  temporary  di- 
gestive and  secretory  (excretory)  organ.  But  we  have  no  exact  knowledge 
of  what  actually  does  take  place  in  these  structures.  From  the  cotyledons 
of  ruminants  may  be  obtained  a  white  creamy-looking  fluid,  which  from 


THE  NUTRITION  OF  THE  EMBRYO.  849 

many  features  of  its  chemical  composition  might  almost  be  spoken  of  as  a 
"  uterine  milk." 

Speaking  broadly,  the  foetus  lives  on  the  blood  of  its  mother,  very  much 
in  the  same  way  as  all  the  tissues  of  any  animal  live  on  the  blood  of  the  body 
of  which  they  are  the  parts. 

§  791.  For  a  long  time  all  the  embryonic  tissues  are  "  protoplasmic"  in 
character ;  that  is,  the  gradually  differentiating  elements  of  the  several  tis- 
sues remain  still  imbedded,  so  to  speak,  in  undifferentiated  protoplasm  ; 
and  during  this  period  there  must  be  a  general  similarity  in  the  metabolism 
going  on  in  various  parts  of  the  body.  As  differentiation  becomes  more  and 
more  marked,  it  obviously  would  be  an  economical  advantage  for  partially 
elaborated  material  to  be  stored  up  in  various  foetal  tissues,  so  as  to  be  ready 
for  immediate  use  when  a  demand  arose  for  it,  rather  than  for  a  special  call 
to  be  made  at  each  occasion  upon  the  mother  for  comparatively  raw  mate- 
rial needing  subsequent  preparatory  changes.  Accordingly,  we  find  the 
tissues  of  the  foetus  at  a  very  early  period  loaded  with  glycogen.  The 
muscles  are  especially  rich  in  this  substance,  but  it  occurs  in  other  tissues 
as  well.  The  abundance  of  it  in  the  former  may  be  explained  partly  by  the 
fact  that  they  form  a  very  large  proportion  of  the  total  mass  of  the  foetal 
body,  and  partly  by  the  fact  that,  while  during  the  presence  of  the  gly- 
cogen they  contain  much  undifferentiated  protoplasm,  they  are  exactly  the 
organs  which  will  ultimately  undergo  a  large  amount  of  differentiation, 
and,  therefore,  need  a  large  amount  of  material  for  the  metabolism  which 
the  differentiation  entails.  It  is  not  until  the  later  stages  of  intra-uterine 
life,  at  about  the  fifth  month,  when  it  is  largely  disappearing  from  the 
muscles,  that  the  glycogen  begins  to  be  deposited  in  the  liver.  By  this 
time  histological  differentiation  has  advanced  largely,  and  the  use  of  the 
glycogen  to  the  economy  has  become  that  to  which  it  is  put  in  the  ordinary 
life  of  the  animal ;  hence  we  find  it  deposited  in  the  usual  place.  Besides 
being  present  in  the  foetal,  glycogen  is  found  also  in  the  placental,  structures  ; 
but  here  probably  it  is  of  use,  not  for  the  foetus,  but  for  the  nutrition  and 
growth  of  the  placental  structures  themselves.  We  do  not  know  how  much 
carbohydrate  material  finds  it  way  into  the  umbilical  vein ;  and  we  cannot, 
therefore,  state  what  is  the  source  of  the  fcetal  glycogen  ;  but  it  is  at  least 
possible,  not  to  say  probable,  that  it  arises,  in  part  at  all  events,  from  a 
splitting  up  of  proteid  material. 

§  792.  Concerning  the  rise  and  development  of  the  functional  activities 
of  the  embryo  our  knowledge  is  almost  a  blank.  We  know  scarcely  any- 
thing about  the  various  steps  by  which  the  primary  fundamental  qualities 
of  the  protoplasm  of  the  ovum  are  differentiated  into  the  complex  phenom- 
ena which  we  have  attempted  in  this  book  to  expound.  We  can  hardly 
state  more  than  that  while  muscular  contractility  becomes  early  developed, 
and  the  heart  probably,  as  in  the  chick,  beats  even  before  the  blood-corpus- 
cles are  formed,  movements  of  the  foetus  do  not,  in  the  human  subject, 
become  pronounced  until  after  the  fifth  month  ;  from  that  time  forward  they 
increase  and  subsequently  become  very  marked.  They  are  often  spoken  of 
as  reflex  in  character ;  but  only  a  preconceived  bias  would  prevent  them 
from  being  regarded  as  largely  automatic.  The  digestive  functions  are 
naturally,  in  the  absence  of  all  food  from  the  alimentary  canal,  in  abeyance. 
Though  pepsin  may  be  found  in  the  gastric  membrane  at  about  the  fourth 
month,  it  is  doubtful  whether  a  truly  peptic  gastric  juice  is  secreted  during 
intra-uterine  life ;  trypsin  appears  in  the  pancreas  somewhat  later,  but  an 
amylolytic  ferment  cannot  be  obtained  from  that  organ  till  after  birth.  The 
date,  however,  at  which  these  several  ferments  make  their  appearance  in  the 
embryo  appears  to  differ  in  different  animals.  The  excretory  functions  of 

54 


850 


THE  NUTRITION  OF  THE  EMBKYO. 


the  liver  are  developed  early,  and  about  the  third  month  bile  pigment  and 
bile  salts  find  their  way  into  the  intestines.  The  quantity  of  bile  secreted 
during  mtra-uterine  life  accumulates  in  the  intestine,  and  especially  in  the 
rectum,  forming,  together  with  the  smaller  secretion  of  the  rest  of  the  canal, 
and  some  desquamated  epithelium,  the  so-called  meconium.  Bile  salts,  both 
unaltered  and  variously  changed,  the  usual  bile  pigments,  and  cholesterin, 
are  all  present  in  the  meconium.  The  distinct  formation  of  bile  is  an  indi- 
cation that  the  products  of  foetal  metabolism  are  no  longer  wholly  carried 
off  by  the  maternal  circulation  ;  and  to  the  excretory  function  of  the  liver 
there  are  now  added  those  of  the  skin  and  kidney.  The  substances  escaping 
by  these  organs  find  their  way  into  the  allantois  or  into  the  amnion,  accord- 
ing to  the  arrangement  of  the  foetal  mem- 
[FIG.  230.  branes  in  different  classes  of  animals ;  in 

both  these  fluids  urea  or  allied  bodies  have 
been  found  as  well  as  the  ordinary  saline 
constituents ;  the  latter  may  or  may  not 
have  been  actually  secreted.  From  the 
allantoic  fluid  of  ruminants  the  body  allan- 
toin  has  been  obtained,  and  human  and 
other  amniotic  fluids  have  been  found  to 

Diagram  of  the  Fcetal  Circulation.  1,  the  umbilical 
cord,  consisting  of  the  umbilical  vein  and  two  umbilical 
arteries,  proceeding  from  the  placenta  (2) ;  3,  the  umbi- 
lical vein  dividing  into  three  branches,  two  (4,  4)  to  be 
distributed  to  the  liver,  and  one  (5),  the  ductus  venosus, 
which  enters  the  inferior  vena  cava  (6) ;  7,  the  portal 
vein,  returning  the  blood  from  the  intestines,  and  unit- 
ing with  the  right  hepatic  branch  ;  8,  the  right  auricle; 
the  course  of  the  blood  is  denoted  by  the  arrow  proceed- 
ing from  8  to  9,  the  left  auricle;  10,  the  left  ventricle ; 
the  blood  following  the  arrow  to  the  arch  of  the  aorta 
(11),  to  be  distributed  through  the  branches  given  off  by 
the  arch  to  the  head  and  upper  extremities;  the  arrows 
12  and  13  represent  the  return  of  the  blood  from  the  head 
and  upper  extremities  through  the  jugular  and  sub- 
clavian  veins  to  the  superior  vena  cava  (14),  to  the  right 
auricle  (8),  and  in  the  course  of  the  arrow  through  the 
right  ventricle  (15),  to  the  pulmonary  artery  (16) ;  17,  the 
ductus  arteriosus,  which  appears  to  be  a  proper  con- 
tinuation of  the  pulmonary  artery  :  the  offsets  at  each 
side  are  the  right  and  left  pulmonary  arteries  cut  off. 
The  ductus  arteriosus  joins  the  descending  aorta  (18,18), 
which  divides  into  the  common  iliacs;  and  these  into 
the  internal  iliacs,  which  become  the  umbilical  arteries 
(19),  and  return  the  blood  along  the  umbilical  cord  to 
the  placenta,  and  the  external  iliacs  (20),  which  are  con- 
tinued into  the  lower  extremities.  The  arrows  at  the 
termination  of  these  vessels  mark  the  return  of  the 
venous  blood  by  the  veins  to  the  inferior  vena  cava.] 

contain  urea.  It  is  maintained  by  some,  however,  that  the  fluid  in  the 
amnion  is  secreted  by  the  mother,  and  that  hence  the  substances  present  in 
it  are  of  maternal  origin. 

§  793.  About  the  middle  of  intra-uterine  life,  when  the  foetal  circula- 
tion [Fig.  230]  is  in  full  development,  the  blood  flowing  along  the  umbilical 
vein  is  carried  chiefly  by  the  ductus  venosus  into  the  inferior  vena  cava  and 
so  into  the  right  auricle.  Thence  it  is  directed  by  the  valve  of  Eustachius 
through  the  foramen  ovale  into  the  left  auricle,  passing  from  which  into  the 
left  ventricle  it  is  driven  into  the  aorta.  Part  of  the  umbilical  blood,  how- 
ever, instead  of  passing  directly  to  the  inferior  cava,  enters  by  the  portal 
vein  into  the  hepatic  circulation,  from  which  it  returns  to  the  inferior  cava 


THE  NUTRITION  OF  THE  EMBRYO.  851 

by  the  hepatic  veins.  The  inferior  cava  also  contains  blood  coming  from 
the  lower  limbs  and  lower  trunk.  Hence  the  blood  which  passing  from  the 
right  auricle  into  the  left  auricle  through  the  foramen  ovale  is  distributed 
by  the  left  ventricle  through  the  aortic  arch,  though  chiefly  blood  coming 
direct  from  the  placenta,  is  also  blood  which  on  its  way  from  the  placenta 
has  passed  through  the  liver,  and  blood  derived  from  the  tissues  of  the  lower 
part  of  the  body  of  the  foetus.  The  blood  descending  as  foetal  venous  blood 
from  the  head  and  limbs  by  the  superior  vena  cava  does  not  mingle  with 
that  of  the  inferior  vena  cava,  but  falls  into  the  right  ventricle,  from  which 
it  is  discharged  through  the  ductus  arteriosus  (Botalli)  into  the  aorta,  below 
the  arch,  whence  it  flows  partly  to  the  lower  trunk  and  limbs,  but  chiefly  by 
the  umbilical  arteries  to  the  placenta.  A  small  quantity  only  of  the  con- 
tents of  the  right  ventricle  finds  its  way  into  the  lungs.  Now  the  blood 
which  comes  from  the  placenta  by  the  umbilical  vein  direct  into  the  right 
auricle  is,  as  far  as  the  foetus  is  concerned,  arterial  blood ;  and  the  portion 
of  umbilical  blood  which  traverses  the  liver  probably  loses  at  this  epoch 
very  little  oxygen  during  its  transit  through  that  gland,  the  liver  being  at 
this  period  a  simple  excretory  rather  than  an  actively  metabolic  organ. 
Hence  the  blood  of  the  inferior  vena  cava,  though  mixed,  is  on  the  whole 
arterial  blood  ;  and  it  is  this  blood  which  is  sent  by  the  left  ventricle  through 
the  arch  of  the  aorta  into  the  carotid  and  subclavian  arteries.  Thus  the 
head  of  the  foetus  is  provided  with  blood  comparatively  rich  in  oxygen. 
The  blood  descending  from  the  head  and  upper  limbs  by  the  superior  vena 
cava  is  distinctly  venous  ;  and  this  passing  from  the  right  ventricle  by  the 
ductus  arteriosus  is  driven  along  the  descending  aorta,  and  together  with 
some  of  the  blood  passing  from  the  left  ventricle  around  the  aortic  arch  falls 
into  the  umbilical  arteries  and  so  reaches  the  placenta.  The  foetal  circula- 
tion then  is  so  arranged  that  while  the  most  distinctly  venous  blood  is  driven 
by  the  right  ventricle  back  to  the  placenta  to  be  oxygenated,  the  most  dis- 
tinctly arterial  (but  still  mixed)  blood  is  driven  by  the  left  ventricle  to  the 
cerebral  structures,  which  have  more  need  of  oxygen  than  have  the  other 
tissues.  Contrary  to  what  takes  place  afterward,  the  work  of  the  right  ven- 
tricle is  in  the  foetus  greater  than  that  of  the  left ;  and,  accordingly,  that 
greater  thickness  of  the  left  ventricular  walls,  so  characteristic  of  the  adult, 
does  not  become  marked  until  close  upon  birth. 

In  the  later  stages  of  pregnancy  the  mixture  of  the  various  kinds  of 
blood  in  the  right  auricle  increases  preparatory  to  the  changes  taking  place 
at  birth.  But  during  the  whole  time  of  intra-uterine  life  the  amount  of 
oxygen  in  the  blood  passing  from  the  aortic  arch  to  the  medulla  oblongata 
is  sufficient  to  prevent  any  inspiratory  impulses  being  originated  in  the 
medullary  respiratory  centre.  This,  during  the  whole  period  elapsing  be- 
tween the  date  of  its  structural  establishment,  or  rather  the  consequent  full 
development  of  its  irritability,  and  the  epoch  of  birth,  remains  dormant; 
the  oxygen-supply  to  the  protoplasm  of  its  nerve-cells  is  never  brought  so 
low  as  to  set  going  the  respiratory  molecular  explosions.  As  soon,  however, 
as  the  intercourse  between  the  maternal  and  umbilical  blood  is  interrupted 
by  separation  of  the  placenta  or  by  ligature  of  the  umbilical  cord,  or  when, 
as  by  the  death  of  the  mother,  the  umbilical  blood  ceases  to  be  replenished 
with  oxygen  by  the  maternal  blood,  or  when  in  any  other  way  blood  of 
sufficiently  arterial  quality  ceases  to  find  its  way  by  the  left  ventricle  to  the 
medulla  oblongata,  the  supply  of  oxygen  in  the  respiratory  centre  sinks, 
and  when  the  fall  has  reached  a  certain  point  an  impulse  of  inspiration  is 
generated  and  the  foetus  for  the  first  time  breathes.  This  action  of  the 
respiratory  centre  may  be  assisted  by  adjuvant  impulses  reaching  the  centre 
along  various  afferent  nerves,  such  as  those  started  by  exposure  of  the  body 


852  THE  NUTRITION  OF  THE  EMBRYO. 

to  the  air,  or  to  cold  ;  but  these  are  subordinate,  not  essential.  A  retarded 
first  breath  may  be  hurried  on  by  dashing  water  on  the  face  of  the  new- 
born infant ;  but,  on  the  other  hand,  the  foetus,  upon  the  cessation  of  the 
placental  circulation,  will  make  its  first  respiratory  movements  while  it  is 
still  invested  with  the  intact  membranes  and  thus  sheltered  from  the  air  and 
indeed  from  all  external  stimuli. 

§  794.  Before  this  first  breath  is  taken  the  pulmonary  alveoli  contain 
no  air,  and  the  lungs  when  thrown  into  water  sink  at  once  ;  they  are  then 
said  to  be  "  atelectatic."  After  the  first  breath,  the  alveoli  contain  air  and 
the  lungs  float  when  thrown  into  water.  A  striking  difference,  however, 
exists  between  the  lungs  of  a  newborn  infant  and  those  of  an  older  person. 
When  the  pleural  cavity  of  the  former  is  opened,  the  lungs  do  not  collapse, 
no  air  is  driven  out  by  the  trachea  ;  that  partial  distention  of  the  lungs,  and 
negative  thoracic  pressure,  appears  not  to  be  established  immediately  upon 
birth.  That  portion  of  the  residual  air  in  the  lungs  of  the  adult  which, 
remaining  after  the  most  forcible  expiration,  is  still  driven  from  the  lungs 
upon  the  pleural  cavity  being  laid  open,  and  which  might  be  called  "  col- 
lapse air,"  is  wanting  in  the  newborn  infant.  When  the  change  from  one 
condition  to  the  other  is  effected  is  not  at  present  known  ;  it  may  possibly 
arise  from  the  growth  of  the  chest  outstripping  that  of  the  lungs. 

When  the  first  breath  is  taken,  as  under  normal  circumstances  it  is,  with 
free  access  to  the  atmosphere,  the  lungs  become  filled  with  air,  the  scanty 
supply  of  blood  which  at  the  moment  was  passing  from  the  right  ventricle 
along  the  pulmonary  artery  returns  to  the  left  auricle  brighter  and  richer 
in  oxygen  than  ever  was  the  foetal  blood  before.  With  the  diminution  of 
resistance  in  the  pulmonary  circulation  caused  by  the  expansion  of  the 
thorax,  a  larger  supply  of  blood  passes  into  the  pulmonary  artery  instead 
of  into  the  ductus  arteriosus,  and  this  derivation  of  the  contents  of  the 
right  ventricle  increasing  with  the  continued  respiratory  movements,  the 
current  through  the  latter  canal  at  last  ceases  altogether,  and  its  channel 
shortly  after  birth  becomes  obliterated.  Corresponding  to  the  greater  flow 
into  the  pulmonary  artery,  a  larger  and  larger  quantity  of  blood  returns 
from  the  pulmonary  veins  into  the  left  auricle.  At  the  same  time  the  cur- 
rent through  the  ductus  venosus  from  the  umbilical  vein  having  ceased,  the 
flow  from  the  inferior  cava  has  diminished  ;  and  the  blood  of  the  right 
auricle  finding  little  resistance  in  the  direction  of  the  ventricle,  which  now 
readily  discharges  its  contents  into  the  pulmonary  artery,  but  finding  in  the 
left  auricle,  which  is  continually  being  filled  from  the  lungs,  an  obstacle  to 
its  passage  through  the  foramen  ovale,  ceases  to  take  that  course.  Any  re- 
turn of  blood  from  the  now  vigorous  and  active  left  auricle  into  the  right 
auricle  is  prevented  by  the  valve  which,  during  the  later  stages  of  intra- 
uterine  life,  has  been  growing  up  in  the  left  auricle  over  the  foramen  ovale. 
At  birth  the  edge  of  this  valve  is  to  a  certain  extent  free,  so  that,  in  case  of 
an  emergency,  as  when  the  pulmonary  circulation  is  obstructed,  a  direct 
escape  of  blood  into  the  left  auricle  from,  the  over-burdened  right  auricle 
can  take  place.  Eventually,  in  the  course  of  the  first  year,  adhesion  takes 
place,  and  the  separation  of  the  two  auricles  becomes  complete.  With  its 
larger  supply  of  blood  and  greater  work  the  left  ventricle  acquires  the 
greater  thickness  characteristic  of  it  during  life.  Thus  the  fcetal  circula- 
tion, in  consequence  of  the  respiratory  movements  to  which  its  interruption 
gives  rise,  changes  its  course  into  that  characteristic  of  the  adult. 


PARTURITION.  853 

CHAPTER  V. 

PARTURITION. 

§  795.  IN  spite  of  the  increasing  distention  of  its  cavity,  the  uterus 
remains  quiescent,  as  far  as  any  marked  muscular  contractions  are  concerned, 
until  a  certain  time  has  been  run.  In  the  human  subject  the  period  of 
gestation  generally  lasts  from  275  to  280  days,  i.  e.,  about  40  weeks,  the 
general  custom  being  to  expect  parturition  at  about  280  days  from  the  last 
menstruation.  Seeing  that  in  many  cases  it  is  uncertain  whether  the  ovum 
which  develops  into  the  embryo  left  the  ovary  at  the  menstruation  preceding 
or  succeeding  coitus,  or,  as  some  have  urged,  independent  of  menstruation, 
by  reason  of  the  coitus  itself,  an  exact  determination  of  the  duration  of 
pregnancy  is  impossible. 

In  the  cow  the  period  of  gestation  is  aoout  280  days,  in  the  mare  about  350, 
sheep  about  150  days,  dog  about  60  days,  rabbit  about  30  days. 

§  796.  The  extrusion  of  the  foetus  is  brought  about  partly  by  rhythmical 
contractions  of  the  uterus  itself  and  partly  by  a  pressure  exerted  by  the 
contraction  of  the  abdominal  muscles,  similar  to  that  described  in  defeca- 
tion. The  contractions  of  the  uterus  are  the  first  to  appear,  and  their  first 
effect  is  to  bring  about  a  dilatation  of  the  os  uteri ;  it  is  not  until  the  later 
stages  of  labor,  while  the  foatus  is  passing  into  the  vagina,  that  the  ab- 
dominal muscles  are  brought  into  play. 

§  797.  The  whole  process  of  parturition  may  be  broadly  considered  a 
reflex  act,  the  nervous  centre  being  placed  in  the  lumbar  cord.  In  a  dog, 
whose  dorsal  cord  had  been  completely  severed,  parturition  took  place  as 
usual ;  and  the  fact  that,  in  the  human  subject,  labor  will  progress  quite 
naturally  while  the  patient  is  unconscious  from  the  administration  of  chloro- 
form, shows  that  in  woman  also  the  whole  matter  is  an  involuntary  action, 
however  much  it  may  be  assisted  by  direct  volitional  efforts.  That  the  uterus 
is  capable  of  being  thrown  into  contractions  through  reflex  action,  excited 
by  stimuli  applied  to  various  afferent  nerves,  is  well  known.  The  contraction 
of  the  uterus,  which  is  so  necessary  for  the  prevention  of  hemorrhage  after 
delivery,  may  frequently  be  brought  about  by  exerting  pressure  or  by  dash- 
ing cold  water  on  the  abdomen,  by  the  introduction  of  foreign  bodies  into 
the  vagina,  and  especially  by  putting  the  child  to  the  nipple.  An'd  we  learn 
from  experiments  on  animals  that  rhythmic  contractions  of  the  uterus  re- 
sembling at  least  those  of  parturition,  may  be  brought  about  in  a  reflex 
manner  by  stimulating  various  afferent  nerves.  Similar  movements  may  be 
induced  by  direct  stimulation  of  the  spinal  cord  along  its  whole  length,  as 
well  as  of  various  parts  of  the  brain  ;  but  there  are  reasons  for  thinking  that 
in  these  cases  the  impulses  started  in  the  brain  and  upper  part  of  the  spinal 
cord  produce  their  effects  by  working  upon  what  may  be  called  a  "  parturi- 
tion "  centre  in  the  upper  lumbar  regions  of  the  cord.  And  it  would  appear 
that  the  uterine  contractions  which  are  induced  by  such  drugs  as  ergot,  as 
well  as  those  caused  by  asphyxia,  are,  at  all  events  in  part,  brought  about 
by  the  agency  of  the  same  lumbar  centre.  From  this  centre  the  paths  for 
the  efferent  impulses  appear  (in  the  dog)  to  be  two-fold ;  one  along  sympa- 
thetic tracts,  by  nerves  passing  from  the  inferior  mesenteric  ganglion  to  the 
hypogastric  plexus,  and  the  other  along  spinal  tracts  by  branches  of  the 
sacral  nerves  to  the  same  plexus.  It  is  stated  that  the  characters  of  the 


854  PARTURITION. 

movements  induced  by  stimulating  these  two  tracts  are  somewhat  different, 
and  moreover  that  the  sympathetic  tract  is  vaso-constrictor  and  the  spinal 
tract  vaso-motor  in  nature ;  but  the  matter  has  not  yet  been  fully  worked  out. 

§  798.  We  are,  however,  hardly  justified  in  considering  the  rhythmical 
contractions  of  the  uterus  during  parturition  as  simple  reflex  acts  excited  by 
the  presence  of  the  foetus.  We  are  utterly  in  the  dark  as  to  why  the  uterus, 
after  remaining  apparently  perfectly  quiescent  (or  with  contractions  so  slight 
as  to  be  with  difficulty  appreciated)  for  months,  is  suddenly  thrown  into 
action,  and  within,  it  may  be,  a  few  hours  or  even  less  get  rid  of  the  burden 
it  has  borne  with  such  tolerance  for  so  long  a  time ;  none  of  the  various 
hypotheses  which  have  been  put  forward  can  be  considered  as  satisfactory. 
And  until  we  know  what  starts  the  active  phase,  we  shall  remain  in  ignor- 
ance of  the  exact  manner  in  which  the  activity  is  brought  about.  The 
peculiar  rhythmic  character  of  the  contractions,  each  "  pain  "  beginning 
feebly,  rising  to  a  maximum,  then  declining,  and  finally  dying  away  alto- 
gether, to  be  succeeded  after  a  pause  by  a  similar  pain  just  like  itself,  pain 
following  pain  like  the  tardy  long-drawn  beats  of  a  slowly  beating  heart, 
suggests  that  the  cause  of  the  rhythmic  contraction  is  seated,  like  that  of  the 
rhythmic  beat  of  the  heart,  in  the  organ  itself.  And  this  view  is  supported 
by  the  fact  that  contractions  of  the  uterus  similar  to  those  of  parturition 
have  been  observed  in  animals  even  after  complete  destruction  of  the  spinal 
cord ;  and  the  movements  induced  by  asphyxia  seem  in  part,  and  those 
caused  by  some  drugs  such  as  ammonia  seem  to  be  wholly,  due  to  an  intrin- 
sic action  of  the  uterus  itself.  Nevertheless,  general  evidence  supports  the 
conclusion  that,  in  a  normal  state  of  things  at  all  events,  the  contractions  of 
the  uterus,  like  those  of  the  lymph-hearts,  are  largely  dependent  on  the 
spinal  cord. 

The  occurrence  of  contractions  in  conseqence  of  an  asphyxiated  condition 
of  the  blood  explains  why,  when  pregnant  animals  are  asphyxiated,  an  ex- 
trusion of  the  foetus  frequently  takes  place.  There  is  no  evidence,  however, 
that  the  onset  of  labor  is  caused  by  a  gradual  diminution  of  oxygen  in  the 
blood,  reaching  at  last  to  a  climax.  Nor  are  there  sufficient  facts  to  connect 
parturition  with  any  condition  of  the  ovary  resembling  that  of  menstruation. 

The  action  of  the  abdominal  muscles  in  parturition  is,  on  the  other  hand, 
obviously  a  reflex  act  carried  out  by  means  of  the  spinal  cord,  the  necessary 
stimulus  being  supplied  by  the  pressure  of  the  foetus  in  the  vagina  or  by  the 
contraction  of  the  uterus.  Hence  the  whole  act  of  parturition  may  with 
reason  be  considered  as  a  reflex  one. 

Whether  it  be  wholly  a  reflex  or  partly  an  automatic  one,  the  act  can 
readily  be  inhibited  by  the  action  of  the  central  nervous  system.  Thus 
emotions  are  a  very  frequent  cause  of  the  progress  of  parturition  being  sud- 
denly stopped  ;  as  is  well  known,  the  entrance  into  the  bedroom  of  a  stranger 
often  causes  for  a  time  the  sudden  and  absolute  cessation  of  "labor"  pains, 
which  previously  may  have  been  even  violent.  Judging  from  the  analogy 
of  micturition,  between  which  and  parturition  there  are  many  points  of 
resemblance,  we  may  suppose  that  this  inhibition  of  uterine  contractions  is 
brought  about  by  an  inhibition  of  the  centre  in  the  lumbar  cord. 

§  799.  After  the  expulsion  of  the  foetus,  the  foetal  placenta  separates 
from  the  uterine  walls,  and  is,  together  with  the  remnants  of  the  membranes, 
expelled  after  it.  The  uterus  then  falls  into  a  firm  tonic  contraction  similar 
to  that  of  the  emptied  bladder,  by  which  means  hemorrhage  from  the  vessels 
torn  by  the  separation  of  the  placenta  is  avoided.  The  lining  membrane  of 
the  uterus  is  gradually  restored,  the  muscular  elements  are  reduced  by  a 
rapid  fatty  degeneration-  and  in  a  short  time  the  whole  organ  has  returned 
to  its  normal  condition. 


THE  PHASES  OF  LIFE.  855 

CHAPTER  VI. 

THE  PHASES  OF  LIFE. 

§  800.  The  child  has  at  birth,  on  an  average,  rather  less  than  one-third 
the  maximum  length,  and  about  one-twentieth  the  maximum  weight,  to 
which  in  future  years  it  will  attain. 

The  composition  of  the  body  of  the  newborn  babe,  as  compared  with  that 
of  the  adult,  will  be  seen  from  the  following  table,  in  which  the  details  are 
more  full  than  those  given  on  p.  482. 

/ 

Weight  of  organ  in  percentage  Weight  of  organ  in 

of  body-weight.  adult,  as  compared 

, • with  that  of  newborn 

Newborn  babe.               Adult.  babe  taken  as  1. 

Eye 0.28                       0.028  1.7 

Brain 14.34                       2.37  3.7 

Kidneys 0.88                       0.48  12 

Skin 11.3                          6.3  12 

Liver 4.39                       2.77  13.6 

Heart 0.89                       0.52  15 

Stomach  and  intestine    2.53                       2.34  20 

Lungs 2.16                       2.01  20 

Skeleton 16.7                       15.35  26 

Muscles,  etc 23.4                       43.1  28 

Testicle 0.037                    0.8  60 

It  will  be  observed  that  the  brain  and  eyes  are,  relatively  to  the  whole 
body-weight,  very  much  larger  in  the  babe  than  in  the  adult,  as  is  also, 
though  to  a  less  extent,  the  liver.  This  disproportion  is  a  very  marked  em- 
bryonic feature,  and,  as  far  as  the  brain  and  eye  are  concerned  at  least,  has 
a  morphological  or  phylogenic,  as  well  as  a  physiological  or  teleological,  sig- 
nificance. Inasmuch  as  the  smaller  body  has  relatively  the  larger  surface, 
the  skin  is  naturally  proportionately  greater  in  the  babe.  It  is  chiefly  by 
the  accumulation  of  muscle  or  flesh,  properly  so  called,  that  the  child  ac- 
quires the  bulk  and  weight  of  man,  the  skeletal  framework,  in  spite  of  its 
being  specifically  lighter  in  its  earlier  cartilaginous  condition,  maintaining 
throughout  life  about  the  same  relative  weight. 

§  801.  The  increase  in  stature  is  very  rapid  in  early  infancy,  proceed- 
ing, however,  by  decreasing  increments.  During  or  shortly  before  puberty, 
there  is  again  a  somewhat  sudden  rise,  with  a  subsequent  more  steady  but 
diminishing  increase  up  to  about  the  twenty-fifth  year.  From  thence  to 
about  fifty  years  of  age  the  height  remains  stationary,  after  which  there  may 
be  a  decrease,  especially  in  extreme  old  age. 

§  802.  The  increase  in  weight  is  also  very  rapid  at  first,  and  proceeding, 
like  the  height,  with  diminishing  increments,  may  continue  till  about  the 
fortieth  year.  After  the  sixtieth  year  a  decline  of  variable  extent  is  gen- 
erally witnessed.  It  is  a  remarkable  fact,  however,  that  in  the  first  few  days 
of  life,  so  far  from  there  being  an  increase,  there  is  an  actual  decrease  of 
weight,  so  that,  even  on  the  seventh  day  the  weight  still  continues  to  be  less 
than  at  birth. 

§  803.  The  saliva  of  the  babe  is  active  on  starch,  and  its  gastric  juice, 
unlike  that  of  many  newborn  animals,  has  good  peptic  powers,  from  which 
we  may  infer  that  its  digestive  processes  in  general  are  identical  with  those 
of  the  adult ;  but  the  feces  of  the  infant  contain,  besides  considerable  quan- 


856  THE   PHASES  OF   LIFE. 

tity  of  undigested  food  (fat,  casein,  etc.),  unaltered  bile-pigment,  and  unde- 
composed  bile-salts. 

§  804.  The  heart  of  the  babe  (see  Table,  p.  855)  is,  relatively  to  its 
body-weight,  larger  than  the  adult,  and  the  frequency  of  the  heart-beat 
much  greater,  viz.,  about  130  or  140  per  minute,  falling  to  about  110  in  the 
second  year,  and  about  90  in  the  tenth  year.  Corresponding  to  the  smaller 
bulk  of  the  body,  the  whole  circuit  of  the  blood  system  is  traversed  in  a 
shorter  time  than  in  the  adult  (12  seconds  as  against  22)  ;  and,  consequently, 
the  renewal  of  the  blood  in  the  tissues  is  exceedingly  rapid.  The  respiration 
of  the  babe  is  quicker  than  that  of  the  adult,  being  at  first  about  35  per 
minute,  falling  to  28  in  the  second  year,  to  26  in  the  fifth  year,  and  so  on- 
ward. The  respiratory  work,  while  it  increases  absolutely  as  the  body 
grows,  is,  relatively  to  the  body-weight,  greatest  in  the  earlier  years.  It 
fs  worthy  of  notice  that  the  absorption  of  oxygen  is  said  to  be  relatively 
more  active  than  the  production  of  carbonic  acid  ;  that  is  to  say,  there 
is  a  continued  accumulation  of  capital  in  the  form  of  a  store  of  oxygen- 
holding  explosive  compounds.  This,  indeed,  is  the  striking  feature  of 
infant  metabolism.  It  is  a  metabolism  directed  largely  to  constructive  ends. 
The  food  taken  represents,  undoubtedly,  so  much  potential  energy ;  but 
before  that  energy  can  assume  a  vital  mode,  the  food  must  be  converted  into 
tissue ;  and,  in  such  a  conversion,  morphological  and  molecular,  a  large 
amount  of  energy  must  be  expended.  The  metabolic  activities  of  the  infant 
are  more  pronounced  than  those  of  the  adult,  for  the  sake,  not  so  much  of 
energies  which  are  spent  on  the  world  without,  as  of  energies  which  are  for 
a  while  buried  in  the  rapidly  increasing  mass  of  flesh.  Thus,  the  infant  re- 
quires, over  and  above  the  wants  of  the  man,  not  only  an  income  of  energy 
corresponding  to  the  energy  of  the  flesh  actually  laid  on,  but  also  an  income 
corresponding  to  the  energy  used  up  in  making  that  living  sculptured  flesh 
out  of  the  dead  amorphous  proteids,  fats,  carbohydrates,  and  salts,  which 
serve  as  food.  Over  and  above  this,  the  infant  needs  a  more  rapid  metab- 
olism to  keep  up  the  normal  bodily  temperature.  This,  which  is  no  less, 
indeed,  slightly  (0.3°)  higher,  than  that  of  the  adult,  requires  a  greater  ex- 
penditure, inasmuch  as  the  infant  with  its  relatively  far  larger  surface,  and 
its  extremely  vascular  skin,  loses  heat  to  a  proportionately  much  greater 
degree  than  does  the  grown-up  man.  It  is  a  matter  of  common  experience 
that  children  are  more  affected  by  cold  than  are  adults. 

This  rapid  metabolism  is,  however,  not  manifest  immediately  upon  birth. 
During  the  first  few  days,  corresponding  to  the  loss  of  weight  mentioned 
above,  the  respiratory  activities  of  the  tissues  are  feeble  ;  the  embryonic 
habits  seem  as  yet  not  to  have  been  completely  thrown  off,  and  as  was  stated 
on  p.  305,  newborn  animals  bear  with  impunity  a  deprivation  of  oxygen 
which  would  be  fatal  to  them  later  on  in  life. 

§  805.  The  quantity  of  urine  passed,  though  scanty  in  the  first  two  days, 
rises  rapidly  at  the  end  of  the  first  week,  and  in  youth  the  quantity  of  urine 
passed  is,  relatively  to  the  body-weight,  larger  than  in  adult  life.  This  may 
be,  at  least  in  quite  early  life,  partly  due  to  the  more  liquid  nature  of  the 
food,  but  is  also  in  part  the  result  of  the  more  active  metabolism.  For  not 
only  is  the  quantity  of  urine  passed,  but  also  the  amount  of  urea  and  some 
other  urinary  constituents  secreted,  relatively  to  the  body-weight,  greater  in 
the  child  than  in  the  adult.  The  presence  of  uric,  of  oxalic,  and,  according 
to  some,  of  hippuric  acids  in  unusual  quantities  is  a  frequent  characteristic 
of  the  urine  of  children.  It  is  stated  that  calcic  phosphates,  and  indeed 
the  phosphates  generally,  are  deficient,  being  retained  in  the  body  for  the 
building  up  of  the  osseous  skeleton. 

§  806.  Associated  probably  with  these  constructive  labors  of  the  growing 


THE  PHASES  OF  LIFE.  857 

frame  is  the  prominence  of  the  lymphatic  system.  Not  only  are  the  lym- 
phatic glands  largely  developed  and  more  active  (as  is  probably  shown  by 
their  tendency  to  disease  in  youth),  but  the  quantity  of  lymph  circulation 
is  greater  than  in  later  years.  Characteristic  of  youth  is  the  size  of  the 
thymus  body,  which  increases  up  to  the  second  year,  and  may  then  remain 
for  a  while  stationary,  but  generally  before  puberty,  has  suffered  a  retro- 
gressive metamorphosis,  and  frequently  hardly  a  vestige  of  it  remains  behind. 
The  thyroid  body  is  also  relatively  greater  in  the  babe  than  in  the  adult ; 
the  spleen,  on  the  other  hand,  which  grows  rapidly  in  early  infancy,  is  not 
only  absolutely,  but  also  relatively,  greater  in  the  adult.  It  need  hardly 
be  said  that  the  recuperative  power  of  infancy  and  early  youth  is  very 
marked. 

§  807.  It  would  be  beyond  the  scope  of  this  work  to  enter  into  the 
psychical  condition  of  the  babe  or  the  child,  and  our  knowledge  of  the  de- 
tails of  the  working  of  the  nervous  system  in  infancy  is  too  meagre  to  per- 
mit of  any  profitable  discussion.  It  is  hardly  of  use  to  say  that  in  the 
young  the  whole  nervous  system  is  more  irritable  or  more  excitable  than  in 
later  years ;  by  which  we  probably  to  a  great  extent  mean  that  it  is  less 
rigid,  less  marked  out  into  what,  in  preceding  portions  of  this  work,  we 
have  spoken  of  as  nervous  mechanisms.  It  may  be  mentioned  that  stimula- 
tion of  the  various  cerebral  areas,  in  newborn  animals,  does  not  give  rise  to 
the  usual  localized  movements.  The  sense  of  touch,  both  as  regards  pres- 
sure and  temperature,  appears  well  developed  in  the  infant,  as  does  also  the 
sense  of  taste,  and,  possibly,  though  this  is  disputed,  that  of  smell.  The 
pupil  (larger  in  the  infant  than  in  the  man)  acts  fully,  and  Donders  observed 
normal  binocular  movements  of  the  eyes  in  an  infant  less  than  an  hour  old. 
The  eye  is  (in  man)  from  the  outset  fully  sensitive  to  light,  though  of  course 
visual  perceptions  are  imperfect.  As  regards  hearing,  on  the  other  hand, 
very  little  reaction  follows  upon  sounds — L  e.,  auditory  sensations  seem  to  be 
dull  during  the  first  few  days  of  life  ;  this  may  be  partly,  at  least,  due  to 
absence  of  air  from  the  tympanum  and  a  tumid  condition  of  the  tympanic 
mucous  membrane.  As  the  child  grows  up  his  senses  rapidly  culminate,  and 
in  his  early  years  he  possesses  a  general  acuteness  of  sight,  hearing,  and  touch 
which  frequently  becomes  blunted  as  his  psychical  life  becomes  fuller. 
Children,  however,  are  said  to  be  less  apt  at  distinguishing  colors  than  in 
sighting  objects ;  but  it  does  not  appear  whether  this  arises  from  a  want  of 
perceptive  discrimination  or  from  their  being  actually  less  sensitive  to  vari- 
ations in  hue.  A  characteristic  of  the  nervous  system  in  childhood,  the 
result,  probably,  of  the  more  active  metabolism  of  the  body,  is  the  necessity 
for  long  or  frequent  and  deep  slumber. 

§  808.  Dentition  marks  the  first  epoch  of  the  new  life.  At  about  seven 
months  the  two  central  incisors  of  the  lower  jaw  make  their  way  through 
the  gum,  followed  immediately  by  the  corresponding  teeth  in  the  upper  jaw. 
The  lateral  incisors,  first  of  the  lower  and  then  of  the  upper  jaw,  appear  at 
about  the  ninth  month,  the  first  molars  at  about  the  twelfth  month,  the 
canines  at  about  a  year  and  a  half,  and  the  temporary  dentition  is  completed 
by  the  appearance  of  the  second  molars  usually  before  the  end  of  the  second 
year. 

§  809.  Ahout  the  sixth  year  the  permanent  dentition  commences  by  the 
appearance  of  the  first  permanent  molar  beyond  the  second  temporary 
molar  ;  in  the  seventh  year  the  central  permanent  incisors  replace  their  tem- 
porary representatives,  followed  in  the  next  year  by  the  lateral  incisors.  In 
the  ninth  year  the  temporary  first  molars  are  replaced  by  the  first  bicuspids, 
and  in  the  tenth  year  the  second  temporary  molars  are  similarly  replaced 
by  the  second  bicuspids.  The  canines  are  exchanged  about  the  eleventh  or 


858  THE  PHASES  OF  LIFE. 

twelfth  year,  and  the  second  permanent  molars  are  cut  about  the  twelfth  or 
thirteenth  year.  There  is  then  a  long  pause,  the  third  or  wisdom  tooth  not 
making  its  appearance  till  the  seventeenth,  or  even  twenty-fifth  year,  or  in 
some  cases  not  appearing  at  all. 

§  810.  Shortly  after  the  conclusion  of  the  permanent  dentition  (the 
wisdom  teeth  excepted)  the  occurrence  of  puberty  marks  the  beginning  of  a 
new  phase  of  life ;  and  the  difference  between  the  sexes,  hitherto  merely 
potential,  now  becomes  functional.  In  both  sexes  the  maturation  of  the 
generative  organs  is  accompanied  by  the  well-known  changes  in  the  body  at 
large ;  but  the  events  are  much  more  characteristic  in  the  typical  female 
than  in  the  aberrant  male.  Though  in  the  boy,  the  breaking  of  the  voice 
and  the  rapid  growth  of  the  beard  which  accompany  the  appearance  of  act- 
ive spermatozoa,  are  striking  features,  yet  they  are,  after  all,  superficial. 
The  curves  of  his  increasing  weight  and  height,  and  of  the  other  events  of 
his  economy,  pursue  for  a  while  longer  an  unchanged  course ;  the  boy  does 
not  become  a  man  till  some  years  after  puberty ;  and  the  decline  of  his 
functional  manhood  is  so  gradual  that  frequently  it  ceases  only  when  disease 
puts  an  end  to  a  ripe  old  age.  With  the  occurrence  of  menstruation,  on  the 
other  hand,  at  from  thirteen  to  seventeen  years  of  age,  the  girl  almost  at 
once  becomes  a  woman,  and  her  functional  womanhood  ceases  suddenly  at 
the  climacteric  in  the  fifth  decennium.  During  the  whole  of  the  childbear- 
ing  period  her  organism  is  in  a  comparatively  stationary  condition.  While 
before  the  age  of  puberty,  up  to  about  the  eleventh  or  twelfth  year,  the  girl 
is  lighter  and  shorter  than  the  boy  of  the  same  age,  in  the  next  few  years 
her  rate  of  growth  exceeds  his ;  but  she  has  then  nearly  reached  her  maxi- 
mum, while  he  continues  to  grow.  Her  curve  of  weight  from  the  nineteenth 
year  onward  to  the  climacteric  remains  stationary,  being  followed  subse- 
quently by  a  late  increase,  so  that  while  the  man  reaches  his  maximum  of 
weight  at  about  forty,  the  woman  is  at  her  greatest  weight  about  fifty. 

§  811.  Of  the  statical  differences  of  sex,  some,  such  as  the  formation 
of  the  pelvis  and  the  costal  mechanism  of  respiration,  are  directly  connected 
with  the  act  of  childbearing,  while  others  have  only  an  indirect  relation  to 
that  duty ;  and  indications,  at  least,  of  nearly  all  the  characteristic  differ- 
ences are  seen  at  birth.  The  baby  boy  is  heavier  and  taller  than  the  baby 
girl,  and  the  maiden  of  five  breathes  with  her  ribs  in  the  same  way  as  does 
the  matron  of  forty.  The  woman  is  lighter  and  shorter  than  the  man,  the 
limits  in  the  case  of  the  former  being  from  1.444  to  1.740  metres  of  height, 
and  from  39.8  to  93.8  kilos  of  weight,  in  the  latter  from  1.467  to  1.890  of 
height,  and  from  49.1  to  98.5  kilos  of  weight.  The  muscular  system  and 
skeleton  are  both  absolutely  and  relatively  less  in  woman,  and  her  brain  is 
lighter  and  smaller  than  that  of  man,  being  about  1272  grammes  to  1424. 
Her  metabolism,  as  measured  by  the  respiratory  and  urinary  excreta,  is  also 
not  only  absolutely  but  relatively  to  the  body-weight  less,  and  her  blood  is 
not  only  less  in  quantity,  but  also  of  lighter  specific  gravity,  and  contains  a 
smaller  proportion  of  red  corpuscles.  Her  strength  is  to  that  of  man  as 
about  5  to  9,  and  the  relative  length  of  her  step  as  1000  to  1157. 

§  812.  From  birth  onward  (and  indeed  from  early  intra-uterine  life)  the 
increment  of  growth  progressively  diminishes.  At  last  a  point  is  reached  at 
which  the  curve  cuts  the  abscissa  line,  and  the  increment  becomes  a  decre- 
ment. After  the  culmination  of  manhood  at  forty  and  of  womanhood  at  the 
climacteric,  the  prime  of  life  declines  into  old  age.  The  metabolic  activity 
of  the  body,  which  at  first  was  sufficient  not  only  to  cover  the  daily  waste, 
but  to  add  new  material,  later  on  is  able  only  to  meet  the  daily  wants,  and 
at  last  is  too  imperfect  even  to  sustain  in  its  entirety  the  existing  frame. 
Neither  as  regards  vigor  and  functional  capacity,  nor  as  regards  weight  and 


THE  PHASES  OF  LIFE.  859 

bulk,  do  the  turning-points  of  the  several  tissues  and  organs  coincide  either 
with  each  other  or  with  that  of  the  body  at  large.  We  have  already  seen 
that  the  life  of  such  an  organ  as  the  thymus  is  far  shorter  than  that  of  its 
possessor.  The  eye  is  in  its  dioptric  prime  in  childhood,  when  its  media  are 
clearest  and  its  muscular  mechanisms  most  mobile,  and  then  it  for  the  most 
part  serves  as  a  toy ;  in  later  years,  when  it  could  be  of  the  greatest  service 
to  a  still  active  brain,  it  has  already  fallen  into  a  clouded  and  rigid  old  age. 
The  skeleton  reaches  its  limit  very  nearly  at  the  same  time  as  the  whole 
frame  reaches  its  maximum  of  height,  the  coalescence  of  the  various  epiphyses 
being  pretty  well  completed  by  about  the  twenty-fifth  year.  Similarly  the 
muscular  system  in  its  increase  tallies  with  the  weight  of  the  whole  body. 
The  brain,  in  spite  of  the  increasing  complexity  of  structure  and  function  to 
which  it  continues  to  attain  even  in  middle  life,  early  reaches  its  limit  of 
bulk  and  weight.  ,  At  about  seven  years  of  age  it  attains  what  may  be  con- 
sidered as  its  first  limit,  for  though  it  may  increase  somewhat  up  to  twenty, 
thirty,  or  even  later  years,  its  progress  is  much  more  slow  after  than  before 
seven.  The  vascular  and  digestive  organs  as  a  whole  may  continue  to 
increase  even  to  a  very  late  period.  From  these  facts  it  is  obvious  that 
though  the  phenomena  of  old  age  are,  at  bottom,  the  result  of  the  individual 
decline  of  the  several  tissues,  they  owe  many  of  their  features  to  the  dis- 
arrangement of  the  whole  organism  produced  by  the  premature  decay  or  dis- 
appearance of  one  or  other  of  the  constituent  bodily  factors.  Thus,  for 
instance,  it  is  clear  that  were  there  no  natural  intrinsic  limit  to  the  life  of 
the  muscular  and  nervous  systems,  they  would  nevertheless  come  to  an  end 
in  consequence  of  the  nutritive  disturbances  caused  by  the  loss  of  the  teeth. 
And  what  is  true  of  the  teeth  is  probably  true  of  many  other  organs,  with 
the  addition  that  these  cannot,  like  the  teeth,  be  replaced  by  mechanical 
contrivances.  Thus  the  term  of  life  which  is  allotted  to  a  muscle  by  virtue 
of  its  molecular  constitution,  and  which  it  could  not  exceed  were  it  always 
placed  under  the  most  favorable  nutritive  conditions,  is,  in  the  organism, 
determined  by  the  similar  life-terms  of  other  tissues ;  the  future  decline  of 
the  brain  is  probably  involved  in  the  early  decay  of  the  thymus. 

§  813.  Two  changes  characteristic  of  old  age  are  the  so-called  calcare- 
ous and  fatty  degenerations.  These  are  seen  in  a  completely  typical  form 
in  cartilage,  as  for  instance,  in  the  ribs ;  here  the  protoplasm  of  the  carti- 
lage-corpuscle becomes  hardly  more  than  an  envelope  of  fat-globules,  and 
the  supple  matrix  is  rendered  rigid  with  amorphous  deposits  of  calcic  phos- 
phates and  carbonates,  which  are  at  the  same  time  the  signs  of  past  and  the 
cause  of  future  nutritive  decline.  And  what  is  obvious  in  the  case  of  carti- 
lage is  more  or  less  evident  in  other  tissues.  Everywhere  we  see  a  disposi- 
tion on  the  part  of  protoplasm  to  fall  back  upon  the  easier  task  of  forming 
fat  rather  than  to  carry  on  the  more  arduous  duty  of  manufacturing  new 
material  like  itself;  everywhere  almost  we  see  a  tendency  to  the  replacement 
of  a  structured  matrix  by  a  deposit  of  amorphous  material.  In  no  part  of 
the  system  is  this  more  evident  than  in  the  arteries ;  one  common  feature  of 
old  age  is  the  conversion  by  such  a  change  of  the  supple  elastic  tubes  into 
rigid  channels,  whereby  the  supply  to  the  various  tissues  of  nutritive  material 
is  rendered  increasingly  more  difficult,  and  their  intrinsic  decay  proportion- 
ately hurried. 

§  814.  Of  the  various  tissues  of  the  body  the  muscular  and  nervous  are, 
however,  those  in  which  the  functional  decline,  if  not  structural  decay,  be- 
comes soonest  apparent.  The  dynamic  coefficient  of  the  skeletal  muscles 
diminishes  rapidly  after  thirty  or  forty  years  of  life,  and  a  similar  want  of 
power  comes  over  the  plain  muscular  fibres  also ;  the  heart,  though  it  may 
not  diminish,  or  even  may  still  increase  in  weight,  possesses  less  and  less 


860  THE  PHASES  OF  LIFE. 

force,  and  the  movements  of  the  intestine,  bladder,  and  other  organs,  dimin- 
ish in  vigor.  In  the  nervous  system,  the  lines  of  resistance,  which,  as  we 
have  seen,  help  to  map  out  the  central  organs  into  mechanisms,  and  so  to 
produce  its  multifarious  actions,  become  at  last  hindrances  to  the  passage  of 
nervous  impulses  in  any  direction,  while  at  the  same  time  the  molecular 
energy  of  the  impulses  themselves  becomes  less.  The  eye  becomes  feeble, 
not  only  from  cloudiness  of  the  medja  and  presbyopic  muscular  inability, 
but  also  from  the  very  bluntness  of  the  retina ;  the  sensory  and  motor  im- 
pulses pass  with  increasing  slowness  to  and  from  the  central  nervous  system, 
and  the  brain  becomes  a  more  and  more  rigid  mass  of  protoplasm,  the  molec- 
ular lines  of  which  rather  mark  the  history  of  past  actions  than  serve  as 
indications  of  present  potency.  The  epithelial  glandular  elements  seem  to 
be  those  whose  powers  are  the  longest  preserved ;  and  hence  the  man  who  in 
the  prime  of  his  manhood  was  a  "  martyr  to  dyspepsia  "  by  reason  of  the 
sensitiveness  of  gastric  nerves  and  the  reflex  inhibitory  and  other  results  of 
their  irritation,  in  his  later  years,  when  his  nerves  are  blunted,  and  when, 
therefore,  his  peptic  cells  are  able  to  pursue  their  chemical  work  undisturbed 
by  extrinsic  nervous  worries,  eats  and  drinks  with  the  courage  and  success 
of  a  boy. 

§  815.  Within  the  range  of  a  lifetime  are  comprised  many  periods  of  a 
more  or  less  frequent  recurrence.  In  spite  of  the  aids  of  a  complex  civili- 
zation, all  tending  to  render  the  conditions  of  his  life  more  and  more  equable, 
man  still  shows  in  his  economy  the  effects  of  the  seasons.  Some  of  these  are 
the  direct  results  of  varying  temperature,  but  some  probably,  such  as  the 
gain  of  weight  in  winter  and  the  loss  in  summer,  are  habits  acquired  by 
descent.  Within  the  year,  an  approximately  monthly  period  is  manifested 
in  the  female  by  menstruation,  though  there  is  no  exact  evidence  of  even  a 
latent  similar  cycle  in  the  male.  The  phenomena  of  recurrent  diseases,  and 
the  marked  critical  days  of  many  other  maladies,  may  be  regarded  as  point- 
ing to  cycles  of  smaller  duration  than  that  of  the  moon's  revolution,  unless 
we  admit  the  view  urged  by  some  authors  that  in  these  cases  the  recurrence 
is  to  be  attributed  rather  to  periodical  phases  in  the  disease-producing  germ 
itself,  than  to  variations  in  the  medium  of  the  disease. 

§  816.  Prominent  among  all  other  cyclical  events  is  the  fact  that  most 
animals  possessing  a  well-developed  nervous  system,  must,  night  after  mirht, 
or  day  after  day,  or  at  least  time  after  time,  lay  them  down  to  sleep.  The 
salient  feature  of  sleep  is  the  cessation  of  the  automatic  activity  of  the  brain  ; 
it  is  the  diastole  of  the  cerebral  beat.  But  the  condition  is  not  confined  to 
the  cerebral  hemispheres ;  all  parts  of  the  body  either  directly  or  indirectly 
take  share  in  it.  The  phenomena  of  sleep  are  perhaps  seen  in  their  simplest 
form  in  the  winter  sleep  of  hibernation,  to  which  especially  cold-blooded 
animals,  but  also  to  some  extent  warm-blooded  animals,  are  subject.  In 
these  cases  the  cold  of  winter  slackens  the  vibrations  and  lessens  the  explo- 
sions of  the  protoplasm,  not  only  of  nervous  but  also  of  muscular  and  glandu- 
lar structures ;  indeed  the  activity  of  the  whole  body  is  lowered,  in  some 
respects  almost  to  actual  arrest.  At  the  same  time  that  the  labor  of  the  cere- 
bral molecules  becomes  insufficient  to  develop  consciousness,  the  respiratory 
centre  is  either  wholly  quiescent  or  discharges  feeble  impulses  at  rare  inter- 
vals, and  the  heart  beats  with  a  slow,  infrequent  stroke,  not  by  reason  of  any 
inhibitory  restraint,  but  because  its  very  substance  in  its  slow  molecular 
travail  can  gather  head  for  explosions  only  after  long  pauses  of  rest.  And 
such  few  and  distant  beats  as  do  occur  are  amply  sufficient  to  meet  the  needs 
of  the  feeble  metabolism  of  the  several  tissues.  The  sleep  of  every  day 
differs  from  the  sleep  of  winter  cold  chiefly  because  the  slackening  of  molec- 
ular activities  is  clue  in  the  former  not  to  extrinsic  but  to  intrinsic  causes, 


THE   PHASES   OF   LIFE.  861 

Dot  to  changes  in  the  medium,  but  to  exhaustion  of  the  subject,  and  because 
the  phenomena  are  largely  confined  to  the  cerebral  hemispheres.  It  is  true 
that  the  whole  body  shares  in  the  condition.  The  pulse  and  breathing  are 
slower,  the  intestine  and  other  internal  muscular  mechanisms  are  more  or  less 
at  rest,  the  secreting  organs  are  less  active,  some  apparently  being  wholly 
quiescent,  and  the  sleeper  on  waking  rubs  his  eyes  to  bring  back  to  his  con- 
junctiva its  needed  moisture.  Indeed  the  whole  metabolism  and  the  depend- 
ent temperature  of  the  body  are  lowered  ;  but  we  cannot  say  at  present  how 
far  these  are  the  indirect  results  of  the  condition  of  the  nervous  system,  or 
how  far  they  indicate  a  partial  slumbering  of  the  several  tissues. 

§  817.  Thoracic  respiration  is  said  to  become  more  prominent  than  dia- 
phragmatic respiration  during  sleep,  and  the  Cheyne-Stokes  rhythm  of 
respiration  (see  p.  385)  is  frequently  observed.  During  sleep  the  pupil  is 
contracted,  during,  deep  sleep  exceedingly  so ;  and  dilatation,  often  unaccom- 
panied by  any  visible  movements  of  the  limbs  or  body,  takes  place  when 
any  sensitive  surface  is  stimulated  ;  on  awaking  also  the  pupils  dilate.  The 
eyeballs  have  been  generally  described  as  being  during  sleep  directed  upward 
and  converging  or,  according  to  some  authors,  diverging ;  but  others  main- 
tain that  in  true  sleep  the  visual  axes  are  parallel  and  directed  to  the  far 
distance.  The  eyes  of  children  have  been  described  as  continally  executing 
during  sleep  movements,  often  irregular  and  unsymmetrical  and  unaccom- 
panied by  changes  in  the  pupils. 

§  818.  We  are  not  at  present  in  a  position  to  trace  out  the  events  which 
culminate  in  this  inactivity  of  the  cerebral  structures.  It  has  been  urged 
that  during  sleep  the  brain  is.  anaemic ;  but  even  if  this  anaemia  is  a  constant 
accompaniment  of  sleep,  it  must,  like  the  vascular  condition  of  a  gland  or 
any  other  active  organ,  be  regarded  as  an  effect,  or  at  least  as  a  subsidiary 
event,  rather  than  as  a  primary  cause.  Nor  can  the  view  which  regards 
sleep  as  the  result  of  a  shifting  of  the  mechanical  arrangements  of  the  cranial 
circulation  be  considered  as  satisfactory.  The  explanation  of  the  condition 
is  rather  to  be  sought  in  purely  molecular  changes ;  and  the  analogy  between 
the  systole  and  the  diastole  of  the  heart,  and  the  waking  and  sleeping  of  the 
brain,  may  be  profitably  pushed  to  a  very  considerable  extent.  The  sleep- 
ing brain  in  many  respects  closely  resembles  a  quiescent  but  still  living  ven- 
tricle. Both  are,  as  far  as  outward  manifestations  are  concerned,  at  rest,  but 
both  may  be  awakened  to  activity  by  an  adequately  powerful  stimulus. 
Both,  though  quiescent,  are  irritable,  in  both  the  quiescence  will  ultimately 
give  place  to  activity,  and  in  both  an  appropriate  stimulus  applied  at  the 
right  time  will  determine  the  change  from  rest  to  action.  Just  as  a  single 
prick  will  under  certain  circumstances  awaken  a  ventricle,  which  for  some 
seconds  has  been  motionless,  into  a  rhythmic  activity  of  many  beats,  so  a 
loud  noise  will  start  a  man  from  sleep  into  a  long  day's  wakefulness.  And 
just  as  in  the  heart  the  cardiac  irritability  is  lowest  at  the  beginning  of  the 
diastole  and  increases  onward  till  a  beat  bursts  out,  so  is  sleep  deepest  at  its 
commencement  after  the  day's  labor  ;  thence  onward  slighter  and  slighter 
stimuli  are  needed  to  wake  the  sleeper.  For,  judging  of  the  depth  of  ordi- 
nary nocturnal  sleep  by  the  intensity  of  the  noise  required  to  wake  the 
sleeper,  it  may  be  concluded  that,  increasing  very  rapidly  at  first,  it  reaches 
its  maximum  within  the  first  hour ;  from  thence  it  diminishes,  at  first  rapidly, 
but  afterwards  more  slowly. 

§  819.  We  cannot,  however,  at  present  make  any  definite  statements 
concerning  the  nature  of  trie  molecular  changes  which  determine  this  rhythmic 
rise  and  fall  of  cerebral  irritability.  The  fact  that  the  products  of  proto- 
plasmic activity  when  they  accumulate  within  the  protoplasm  appear  to 
become  in  the  end  an  obstruction  to  that  activity,  has  suggested  the  idea  that 


862  THE  PHASES  OF  LIFE. 

the  presence  in  the  cerebral  tissue  of  an  excess  of  the  products  of  nervous 
metabolism  is  the  cause  of  sleep.  Indeed  lactic  acid,  the  increase  of  which 
was  supposed  to  be  the  cause  of  the  acid  reaction  of  muscular  and  nervous 
tissues  after  exercise,  has  been  especially  pointed  to  in  this  connection ;  but, 
as  we  have  seen,  the  acid  reaction  in  question  appears  not  to  be  due  to  any 
increased  production  of  lactic  acid.  Besides,  if  the  accumulation  of  meta- 
bolic products  of  any  kind  were  the  cause  of  sleep,  it  is  not  clear  why  we 
should  ever  have  any  hope  of  waking.  More  may  be  said  in  favor  of  the 
conception  that  during  the  waking  hours  the  expenditure  of  oxygen  exceeds 
the  income,  and  that  the  quiescence,  which  we  call  sleep,  comes  from  the 
exhaustion  of  the  body's  store  of  oxygen,  more  especially  of  that  "  intra- 
molecular "  oxygen  of  which  we  spoke  in  dealing  with  the  respiration  of  the 
tissues.  But  to  this  view  must  be  added  some  hypothesis,  such  as  the  byplay 
of  some  inhibitory  mechanism,  whereby  the  respiratory  centre  is  not  roused 
to  increased  activity  by  this  lack  of  oxygen,  for,  as  we  have  seen,  the  breath- 
ing shares  in  the  slumber  of  the  body,  though  continuing  to  play  with  an 
amount  of  energy  .which  permits  a  gradual  restoration  of  the  lost  store  of 
oxygen  and  so  finally  brings  on  the  awakening  which  ends  the  sleep.  And 
the  necessity  for  such  a  complication  indicates  that  the  explanation  is,  at 
present  at  least,  inadequate. 

The  phenomena  of  sleep  show  very  clearly  to  how  large  an  extent  an 
apparent  automatism  is  the  ultimate  outcome  of  the  effects  of  antecedent 
stimulation.  When  we  wish  to  go  to  sleep  we  withdraw  our  automatic  brain 
as  much  as  possible  from  the  influence  of  all  extrinsic  stimuli ;  and  an  in- 
teresting case  is  recorded  of  a  lad  whose  connection  with  the  external  world 
was,  from  a  complicated  anaesthesia,  limited  to  that  afforded  by  a  single  eye 
and  a  single  ear,  and  who  could  be  sent  to  sleep  at  will  by  closing  the  eye 
and  stopping  the  ear. 

§  820.  The  cycle  of  the  day  is,  however,  manifested  in  many  other  ways 
than  by  the  alternation  of  sleeping  and  waking,  with  all  the  indirect  effects 
of  these  two  conditions.  There  is  a  diurnal  curve  of  temperature  (see  p.  537), 
apparently  independent  of  all  immediate  circumstances,  the  hereditary  im- 
press of  a  long  and  ancient  sequence  of  days  and  nights.  Even  the  pulse,  so 
sensitive  to  all  bodily  changes,  shows,  running  through  all  the  immediate 
effects  of  the  changes  of  the  minute  and  the  hour,  the  working  of  a  diurnal 
influence  which  cannot  be  accounted  for  by  waking  and  sleeping,  by  work- 
ing and  resting,  by  meals  and  abstinence*  between  meals.  And  the  same 
may  be  said  concerning  the  rhythm  of  respiration,  and  the  products  of  pul- 
monary, cutaneous,  and  urinary  excretion.  There  seems  to  be  a  daily  curve 
of  bodily  metabolism,  which  is  not  the  product  of  the  day's  events.  Within 
the  day  we  have  the  narrower  rhythm  of  the  respiratory  centre  with  the 
accompanying  rise  and  fall  of  activity  in  the  vasomotor  centres.  And 
lastly,  there  stands  out  the  fundamental  fact  of  all  bodily  periodicity,  that 
alternation  of  the  heart's  systole  and  diastole  which  ceases  only  at  death. 
Though,  as  we  have  seen,  the  intermittent  flow  in  the  arteries  is  toned  down 
in  the  capillaries  to  an  apparently  continuous  flow,  still  the  constantly  re- 
peated cycle  of  the  cardiac  shuttle  must  leave  its  mark  throughout  the 
whole  web  of  the  body's  life. 


DEATH.  863 

CHAPTER  VII. 
DEATH. 

§  821.  WHEN  the  animal  kingdom  is  surveyed  from  a  broad  stand- 
point, it  becomes  obvious  that  the  ovum, or  its  correlative  the  spermatozoon, 
is  the  goal  of  an  individual  existence;  that  life  is  a  cycle  beginning  in  an 
ovum  and  coming  round  to  an  ovum  again.  The  greater  part  of  the  actions 
which,  looking  from  a  near  point  of  view  at  the  higher  animals  alone,  we 
are  apt  to  consider  as  eminently  the  purposes  for  which  animals  come  into 
existence,  when  viewed  from  the  distant  outlook  whence  the  whole  living 
world  is  surveyed,  fade  away  into  the  likeness  of  the  mere  byplay  of  ovum- 
bearing  organisms.  The  animal  body  is  in  reality  a  vehicle  for  ova ;  and 
after  the  life  of  the  parent  has  become  potentially  renewed  in  the  offspring, 
the  body  remains -as  a  cast-off  envelope  whose  future  is  but  to  die. 

Were  the  animal  frame  not  the  complicated  machine  we  have  seen  it  to 
be,  death  might  come  as  a  simple  and  gradual  dissolution,  the  "sans  every- 
thing "  being  the  last  stage  of  the  successive  loss  of  fundamental  powers. 
As  it  is,  however,  death  is  always  more  or  less  violent ;  the  machine  comes 
to  an  end  by  reason  of  the  disorder  caused  by  the  breaking  down  of  one  of 
its  parts.  Life  ceases  not  because  the  molecular  powers  of  the  whole  body 
slacken  and  are  lost,  but  because  a  weakness  in  one  or  other  part  of  the 
machinery  throws  its  whole  working  out  of  gear. 

§  822.  We  have  seen  that  the  central  factor  of  life  is  the  circulation  of 
the  blood,  but  we  have  also  seen  that  blood  is  not  only  useless,  but  injurious, 
unless  it  is  duly  oxygenated  ;  and  we  have  further  seen  that  in  the  higher 
animals  the  oxygenation  of  the  blood  can  only  be  duly  effected  by  means 
of  the  respiratory  muscular  mechanism,  presided  over  by  the  medulla  oblon- 
gata.  Thus  the  life  of  a  complex  animal  is,  when  reduced  to  a  simple  form, 
composed  of  three  factors:  the  maintenance  of  the  circulation,  the  access  of 
air  to  the  haemoglobin  of  the  blood,  and  the  functional  activity  of  the  respi- 
ratory centre  ;  and  death  may  come  from  the  arrest  of  either  of  these.  As 
Bichat  put  it,  death  takes  place  by  the  heart,  or  by  the  lungs,  or  by  the  brain. 
In  reality,  however,  when  we  push  the  analysis  further,  the  central  fact  of 
death  is  the  stoppage  of  the  heart,  and  the  consequent  arrest  of  the  circula- 
tion ;  the  tissues  then  all  die,  because  they  lose  their  internal  medium.  The 
failure  of  the  heart  may  arise  in  itself,  on  account  of  some  failure  in  its 
nervous  or  muscular  elements,  or  by  reason  of  some  mischief  affecting  its 
mechanical  working.  Or  its  stoppage  may  be  due  to  some  fault  in  its  inter- 
nal medium,  such  for  instance  as  a  want  of  oxygenation  of  the  blood,  which 
in  turn  may  be  caused  by  either  a  change  in  the  blood  itself,  as  in  carbonic- 
oxide  poisoning,  or  by  a  failure  in  the  mechanical  conditions  of  respiration, 
or  by  a  cessation  of  the  action  of  the  respiratory  centre.  The  failure  of 
this  centre,  and  indeed  that  of  the  heart  itself,  may  be  caused  by  nervous 
influences  proceeding  from  the  brain,  or  brought  into  operation  by  means  of 
the  central  nervous  system  ;  it  may,  on  the  other  hand,  be  due  to  an  imper- 
fect state  of  blood,  and  this  in  turn  may  arise  from  the  imperfect  or  perverse 
action  of  various  secretory  or  other  tissues.  The  modes  of  death  are  in 
reality  as  numerous  as  are  the  possible  modifications  of  the  various  factors 
of  life ;  but  they  all  end  in  a  stoppage  of  the  circulation,  and  the  with- 
drawal from  the  tissues  of  their  internal  medium.  Hence  we  come  to  con- 
sider the  death  of  the  body  as  marked  by  the  cessation  of  the  heart's  beat, 
a  cessation  from  which  no  recovery  is  possible  ;  and  by  this  we  are  enabled 


864  DEATH. 

to  fix  an  exact  time  at  which  we  say  the  body  is  dead.  We  can,  however, 
fix  no  such  exact  time  to  the  death  of  the  individual  tissues.  They  are  not 
mechanisms,  and  their  death  is  a  gradual  loss  of  power.  In  the  case  of  the 
contractile  tissues,  we  have  apparently  in  rigor  mortis  a  fixed  term  by 
which  we  can  mark  the  exact  time  of  their  death.  If  we  admit  that  after 
onset  of  rigor  mortis  recovery  of  irritability  is  impossible,  then  a  rigid 
muscle  is  one  permanently  dead.  In  the  case  of  the  other  tissues  we  have 
no  such  objective  sign,  since  the  rigor  mortis  of  simple  protoplasm  manifests 
itself  chiefly  by  obscure  chemical  signs.  And  in  all  cases  it  is  obvious  that 
the  possibility  of  recovery,  depending  as  it  does  on  the  skill  and  knowledge 
of  the  experimenter,  is  a  wholly  artificial  sign  of  death.  Yet  we  can  draw 
no  other  sharp  line  between  the  seemingly  dead  tissue  whose  life  has  flickered 
down  into  a  smouldering  ember  which  can  still  be  fanned  back  again  into 
flame,  and  the  handful  of  dust,  the  aggregate  of  chemical  substances  into 
which  the  decomposing  tissue  finally  crumbles. 

Moreover,  the  failure  of  the  heart  itself  is  at  bottom  loss  of  irritability, 
and  the  possibility  of  recovery  here  also  rests,  as  far  as  is  known  at  present, 
on  the  skill  and  knowledge  of  those  who  attempt  to  recover.  So  that,  after 
all,  the  signs  of  the  death  of  the  whole  body  are  as  artificial  as  those  of  the 
death  of  the  constituent  tissues. 


APPENDIX. 

ON  THE  CHEMICAL  BASIS  OF  THE  ANIMAL  BODY. 


THE  animal  body,  /from  a  chemical  point  of  view,  may  be  regarded  as  a  mixture 
of  various  representatives  of  three  large  classes  of  chemical  substances,  viz.,  pro- 
teids,  carbohydrates,  and  fats,  in  association  with  smaller  quantities  of  various  saline 
and  other  crystalline  bodies.  By  proteids  are  meant  bodies  containing  carbon,  oxy- 
gen, hydrogen,  and  nitrogen  in  a  certain  proportion,  varying  within  narrow  limits, 
and  having  certain  general  features ;  they  are  frequently  spoken  of  as  albuminoids. 
By  carbohydrates  are  meant  starches  and  sugars  and  their  allies.  We  have  also 
seen  that  the  animal  body  may  be  considered  as  an  assemblage  of  protoplasm  under 
various  modifications  and  of  numerous  products  of  protoplasmic  activity.  We  do 
not  at  present  know  anything  definite  about  the  molecular  composition  of  active 
living  protoplasm ;  but  when  we  submit  protoplasm  to  chemical  analysis,  in  which 
act  it  is  killed,  we  always  obtain  from  it  a  considerable  quantity  of  the  material 
spoken  of  as  proteid.  And  many  authors  go  so  far  as  to  speak  of  protoplasm  as 
being  purely  proteid  in  nature ;  they  regard  the  living  protoplasm  as  proteid  mate- 
rial, which,  in  passing  from  death  to  life,  has  assumed  certain  characters  and  pre-" 
sumably  has  been  changed  in  construction,  but  still  is  proteid  matter  ;  they  some- 
times speak  of  protoplasm  as  "living  proteid  "  or  "living  albumin."  It  is  worthy 
of  notice,  however,  that  even  simple  forms  of  protoplasm,  like  that  constituting 
the  body  of  a  white  corpuscle,  forms  of  protoplasm  which  we  may  fairly  consider 
as  native  protoplasm,  when  they  can  be  obtained  in  sufficient  quantity  for  chemical 
analysis,  are  found  to  contain  some  representatives  of  carbohydrates  and  fats  as 
well  as  of  proteids.  We  might,  perhaps,  even  go  so  far  as  to  say,  that  in  all 
forms  of  living  protoplasm,  the  proteid  basis  is  found  upon  analysis  to  have  some 
carbohydrates  and  some  kind  of  fat  associated  with  it.  Further,  not  only  does  the 
normal  food,  which  is  eventually  built  up  into  protoplasm,  consist  of  all  three  classes, 
but  as  we  have  seen  in  the  sections  on  nutrition,  protoplasm  gives  rise  by  metab- 
olism to  members  of  the  same  three  classes ;  and,  as  far  as  we  know  at  present, 
carbohydrates  and  fats,  when  found  in  the  body  out  of  proteid  food,  are  so  formed 
by  the  agency  of  living  protoplasm,  by  some  living  tissue.  Hence  there  is  at  least 
some  reason  for  thinking  it  probable  that  the  molecule  of  protoplasm,  if  we  may  use 
such  a  phrase,  is  far  more  complex  than  a  molecule  of  proteid  matter,  that  it  con- 
tains in  itself  residues,  so  to  speak,  not  only  of  proteid,  but  also  of  carbohydrate  and 
fatty  material. 

Be  this  as  it  may,  for  no  dogmatic  statement  can  at  present  be  made,  when  we 
examine  the  various  tissues  and  fluids  of  the  animal  body  from  a  chemical  point  of 
view  we  find  present  in  different  places,  or  at  different  times,  several  varieties  and 
derivatives  of  the  three  chief  classes ;  we  find  many  forms  of  proteids,  and  bodies 
closely  allied  to  proteids,  in  the  forms  of  mucin,  gelatin,  etc.  ;  many  varieties  of 
fats  ;  and  several  kinds  of  carbohydrates. 

We  find,  moreover,  many  other  bodies  which  we  may  regard  as  stages  in  the 
constructive  or  destructive  metabolism  of  both  native  and  differentiated  protoplasm, 
and  which  are  important  not  so  much  from  the  quantity  in  which  they  occur  in  the 
animal  body  at  any  one  time  as  from  their  throwing  light  on  the  nature  of  animal 
metabolism ;  these  are  such  bodies  as  urea,  other  organic  crystalline  bodies,  and  the 
extractives  in  general. 

In  the  following  pages  the  chemical  features  of  the  more  important  of  these 
various  substances  which  are  known  to  occur  in  the  animal  body  will  be  briefly 
considered,  such  characters  only  being  described  as  possess  or  promise  to  possess 
55  865 


APPENDIX. 

jical  interest.  The  physiological  function  of  any  substance  must  depend 
ultimately  on  its  molecular  (including  its  chemical)  nature  ;  and  though  at  present 
our  chemical  knowledge  of  the  constituents  of  an  animal  body  gives  us  but  little 
insight  into  their  physiological  properties,  it  cannot  be  doubted  that  such  chemi- 
cal information  as  is  attainable  is  a  necessary  preliminary  to  all  physiological 
study. 

PROTEIDS. 

These  form  the  principal  solids  of  the  muscular,  nervous,  and  glandular  tissues, 
of  the  serum  of  blood,  of  serous  fluids,  and  of  lymph.  In  a  healthy  condition, 
sweat,  tears,  bile,  and  urine  contain  mere  traces,  if  any,  of  proteids.  Their  general 
percentage  composition  may  be  taken  as 

0.  H.  N.  C.  S. 

From      20.9  6.9  15.2  51.5  0.3 

to  23.5        to  7.3         to  17.0         to  54.5         to  2.0 

From      50.0  6.8  15.4  22.8  0.4 

to  55.0         to  7.3         to  18.2         to  24.1         to  5.0 


(Hoppe-Seyler.1) 
(Drechsel.) 


These  figures  are  obtained  from  a  consideration  of  numerous  analyses,  slight  differences  in  the 
various  results  being  immaterial,  where  the  purity  of  the  substance  operated  upon  cannot  be  def- 
initely determined. 

In  addition  to  the  above  constituents,  proteids  leave  on  ignition  a  variable  quantity  of  ash.  In 
the  case  of  egg-albumin  the  principal  constituents  of  the  ash  are  chlorides  of  sodium  and  potas- 
sium, the  latter  greatly  exceeding  the  former  in  amount.  The  remainder  consists  of  sodium  and 
potassium,  in  combination  with  phosphoric,  sulphuric,  and  carbonic  acids,  and  very  small  quan- 
tities of  calcium,  magnesium,  and  iron,  in  union  with  the  same  acids.  There  is  also  a  trace  of 
silica.2  The  ash  of  serum-albumin  contains  an  excess  of  sodium  chloride,  but  the  ash  of  the  pro- 
teids of  muscle  contains  an  excess  of  potash  salts  and  phosphates.  The  nature  of  the  connection 
of  the  ash  with  the  proteid  is  still  a  matter  of  obscurity.  Globin  from  haemoglobin  is  said  to  leave 
no  ash  on  ignition. 

Proteids  as  met  with  in  the  animal  body  are  all  amorphous ;  some  are  soluble, 
some  insoluble  in  water,  and  all  are,  for  the  most  part,  insoluble  in  alcohol  and  ether; 
they  are  all  soluble  in  strong  acids  and  alkalies,  but  in  becoming  dissolved  mostly 
undergo  decomposition.  Their  solutions  possess  a  left-handed  rotatory  action  on  the 
plane  of  polarization,  the  amount  depending  on  various  circumstances,  and  being, 
with  one  exception,  viz. ,  peptones,  changed  by  heating. 

Crystals  into  whose  composition  certain  proteid  (especially  globulin)  3  elements  enter  were 
long  since  observed  in  the  seeds  of  many  plants  ;  as  yet  they  have  not  been  obtained  sufficiently 
isolated  or  in  quantities  large  enough  to  permit  any  "accurate  analysis  to  be  made.  A  method  of 
isolating  in  quantity  and  recrystallizing  these  s'ubstances  has,  however,4  been  indicated,  and  it 
seems  probable  that  analysis  of  these  may  lead  to  interesting  information  on  the  subject  of  the 
constitution  and  combinations  of  proteids. 

The  presence  of  proteids  may  be  determined  by  the  following  tests : 

1.  Heated  with  strong  nitric  acid,  they  or  their  solutions  turn  yellow,  and  this 
color  is,  on  the  addition  of  ammonia,  or  caustic  soda  or  potash,  changed  to  a  deep 
orange  hue.     (Xanthoproteic  reaction.) 

2.  With  Millon's  reagent  they  give,  when  present  in  sufficient  quantity,  a  pre- 
cipitate, which  turns  red  on  heating.     If  they  are  only  present  in  traces,  no  precip- 
itate is  obtained,  but  merely  a  red  coloration  of  the  solution. 

3.  If  mixed  with  some  concentrated  solution  of  sodic  hydrate,  and  one  or  two 
drops  of  a  solution  of  cupric  sulphate,  a  violet  color  is  obtained,  which  deepens  in 
tint  on  boiling. 

The  above  serve  to  detect  the  smallest  traces  of  all  proteids.  The  two  following 
tests  may  be  used  when  there  is  more  than  a  trace  present,  but  do  not  hold  for 
every  kind  of  proteid. 

4.  Render  the  fluid  strongly  acid  with  acetic  or  other  acid,  and  add  a  few  drops 
of  a  solution  of  ferrocyanide  of  potassium  ;   a  precipitate  shows  the  presence  of 
proteids. 

5.  Render  the  fluid,  as  before,  strongly  acid  with  acetic  acid,  add  an  equal  volume 
of  a  concentrated  solution  of  sodic  sulphate,  and  boil.     A  precipitate  is  formed  if 
proteids  are  present. 

1  Hdb.  Phys.  Path.  Chem.  Anal.,  Ed.  iv.  (1875),  S.  223. 

2  See  Gmelin,  Hdb.  Org.  Chem.,  Bd.  viii.  S.  285. 

3  Vines,  Journ.  of  Physiol.,  vol.  iii.  (1880),  p.  93. 

4  Drechsel,  Journ.  f.  prakt  Chem.,  N.  F.,  Bd.  xix  (1879),  S.  331. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  867 

This  last  reaction  is  useful,  not  only  on  account  of  its  exactness,  but  also  because  the  reagents 
used  produce  no  decomposition  of  other  bodies  which  may  be  present ;  and  hence  after  filtration 
the  same  fluid  may  be  further  analyzed  for  other  substances.  Additional  methods  of  freeing  a 
solution  from  proteids  are  :  acidulating  with  acetic  acid  and  boiling,  avoiding  any  excess  of  the 
acid  ;  precipitation  by  excess  of  alcohol ;  in  the  latter  case  the  solution  must  be  neutral  or  faintly 
acid.  Hoppe-Seyler1  recommends  the  employment  of  a  saturated  solution  of  freshly  precipitated 
ferric  hydrate,  in  acetic  acid  ;  this  is  added  to  the  solution,  and  on  boiling  the  whole  of  the  pro- 
teids are  precipitated  as  well  as  the  ferric  salt,  the  latter  as  a  basic  acetate.  Briicke's  method  of 
removing  the  last  traces  of  proteids  from  glycogen  solution  is  also  of  use  (see  Glycogen).  Precip- 
itation of  the  last  traces  of  proteids  by  means  of  hydrated  oxide  of  lead  at  a  boiling  tempera- 
ture2 may  be  also  employed. 

[Solutions  may  entirely  be  freed  from  proteids  by  one  of  the  following  methods : 
(1)  Add  acetic  acid  to  faint  acidity,  then  tannic  acid;  (2)  render  acid  with  hydro- 
chloric acid,  then  add  iodide  of  mercury  and  potassium  ;  (3)  render  decidedly  acid 
with  hydrochloric  acid,  then  precipitate  any  proteids  in  solution  with  phospho- 
tungstic  acid.  ] 

Proteids  may  be  conveniently  divided  into  classes. 

CLASS  I.     Native  Albumins. 

Members  of  this  class,  as  their  name  implies,  occur  in  a  natural  condition  in 
animal  tissues  and  fluids.  They  are  soluble  in  water,  are  not  precipitated  by  very 
dilute  acids,  by  carbonates  of  the  alkalies,  or  by  sodium  chloride.  They  are  coagu- 
lated by  heating  in  solution  to  a  temperature  of  about  70°  C.  If  dried  at  40°  C. , 
the  resulting  mass  is  of  a  pale  yellow  color,  easily  friable,  tasteless,  inodorous,  and 
soluble. 

1.  Egg-albumin. 

Forms  in  aqueous  solution  a  neutral,  transparent,  yellowish  fluid.  From  this 
it  is  precipitated  by  excess  of  strong  alcohol.  If  the  alcohol  be  rapidly  removed 
the  precipitate  may  be  readily  redissolved  in  water ;  if  subjected  to  longer  action 
a  coagulation  occurs,  and  the  albumin  is  then  no  longer  thus  soluble.  Strong 
acids,  especially  nitric  acid,  cause  a  coagulation  similar  to  that  produced  by  heat 
or  by  the  prolonged  action  of  alcohol ;  the  albumin  becomes  profoundly  changed 
by  the  action  of  the  acid  and  does  not  dissolve  upon  removal  of  the  acid.  Mer- 
curic chloride,  argentic  nitrate,  and  lead  acetate,  precipitate  the  albumin,  forming 
insoluble  compounds  of  variable  composition  with  it :  the  precipitants  may  be  re- 
moved by  means  of  sulphuretted  hydrogen  and  the  albumin  again  obtained,  appar- 
ently unaltered,  in  solution. 

Strong  acetic  acid  in  excess  gives  no  precipitate,  but  when  the  solution  is  con- 
centrated the  albumin  is  transformed  into  a  transparent  jelly.  A  similar  jelly  is 
produced  when  a  strong  caustic  potash  is  added  to  a  concentrated  solution  of  egg- 
albumin.  In  both  these  cases  the  substance  is  profoundly  altered,  becoming,  in  the 
one  case,  acid  ;  in  the  other,  alkali-albumin. 

The  specific  rotatory  power  of  egg-albumin  in  aqueous  solution  is,  for  yellow 
light — 35.5°.  Hydrochloric  acid,  added  until  the  reaction  is  strongly  acid,  in- 
creases this  rotation  to— 37.7°.  The  formation  of  the  gelatinous  compound  with 
caustic  potash  is  at  first  accompanied  by  an  increase,  but  this  is  followed  by  a 
decrease  of  rotation. 

Preparation.  White  of  hen's  egg  is  broken  up  with  scissors  into  small  pieces, 
diluted  with  an  equal  bulk  of  water,  and  the  mixture  shaken  strongly  in  a  flask 
till  quite  frothy ;  on  standing,  the  foam  rises  to  the  top,  and  carries  all  the  fibres 
in  whose  meshwork  the  albumin  was  contained.  The  fluid  from  which  the  foam 
has  been  removed,  is  strained,  and  treated  carefully  with  dilute  acetic  acid  as  long 
as  any  precipitate  is  formed ;  the  precipitate  is  then  filtered  off,  and  the  filtrate 
after  neutralization  purified  by  dialysis  and  then  concentrated  at  40°  to  its  original 
bulk. 

2.  Serum-albumin. 

This  form  of  albumin  resembles,  to  a  great  extent,  the  one  previously  described. 
The  following  may  suffice  as  distinguishing  features : 

1.  The  specific  rotation  of  serum-albumin  is — 56°;  that  of  egg-albumin  is  — 
35.5°,  both  measured  for  yellow  light, 

i  Op.  cit.,  8.  227.  2  Hofmeister,  Zeitschr.  f.  Physiol.  Chem.,  Bd.  ii.  (1878),  S.  288. 


868  APPENDIX. 

2.  Serum-albumin  is  not  coagulated  by  being  shaken  up  with  ether ;  egg-albu- 
min is. 

3.  Serum-albumin  is  not  very  readily  precipitated  by  strong  hydrochloric  acid, 
and  such  precipitate  as  does  occur  is  readily  redissolved  on  further  addition  of  the 
acid ;  the  exact  reverse  of  these  two  features  holds  good  for  egg-albumin. 

4.  Precipitated  or  coagulated  serum-albumin  is  readily  soluble,  egg- albumin  is 
with  difficulty  soluble,  in  strong  nitric  acid. 

5.  Egg-albumin,  if  injected  subcutaneously  or  into  a  vein,  appears  unaltered  in 
the  urine  j1  serum-albumin  similarly  injected  does  not  thus  normally  pass  out  by  the 
kidney. 

[6.  Gautier  states  that  10  c.c.  of  the  following  solution,  added  to  2  c.c.  of  the 
solution  to  be  tested,  will  precipitate  egg-albumin  but  not  serum-albumin :  Caustic 
soda,  sp.  gr.  0.7,  250  c.c.  ;  1  per  cent,  sulphate  of  copper,  50  c.c.  ;  glacial  acetic 
acid,  TOOc.c.J 

Serum-albumin  is  found  not  only  in  blood-serum,  but  also  in  lymph,  both  that 
contained  in  the  proper  lymphatic  channels  and  that  diffused  in  the  tissues ;  in 
chyle,  milk,  transudations,  and  many  pathological  fluids. 

It  is  this  form  in  which  albumin  generally  appears  in  the  urine. 

In  addition  to  the  above,  Scherer2  has  described  two  closely  related  bodies,  to  which  he  gives 
the  names  paralbumin  and  metalbumin.  The  first  he  obtained  from  ovarian  cysts ;  its  alkaline 
solutions  are  remarkable  for  being  very  ropy.  It  seems  doubtful  whether  this  body  is  a  proteid  ; 
it  differs  sensibly  in  composition  from  these.  Haerlin8  gives  as  its  composition,  O.  26.8,  H.  6.9. 
N.  12.8,  C.  51.8.  S.  1.7  per  cent.  It  seems  to  be  associated  with  some  body  like  glycogen,  capable  of 
being  converted  into  a  substance  giving  the  reactions  of  dextrose.  Metalbumin,  found  in  a  drop- 
sical fluid,  resembles  the  preceding,  but  is  not  precipitated  by  hydrochloric  acid,  or  by  acetic  acid 
and  ferrocyanide  of  potassium ;  it  is  precipitated,  but  not  coagulated,  by  alcohol ;  its  solution  Is 
scarcely  coagulated  on  boiling. 

Albumins  are  generally  found  associated  with  small  but  definite  amounts  of 
saline  matter.  A.  Schmidt*  says  that  they  may  be  freed  from  these  by  dialysis, 
and  that  they  are  then  not  coagulated  on  boiling.  From  this  it  might  be  inferred 
that  the  albumin  and  the  saline  matters  were  peculiarly  related,  and  that  the 
latter  played  some  special  part  during  the  coagulation  of  the  former  by  heat. 
Schmidt's  observations,  however,  have  not  been  conclusively  corroborated  by  sub- 
sequent observers. 

CLASS  II.     Derived  Albumins  [Albuminates]. 
1.  Acid -albumin. 

When  a  native  albumin  in  solution,  such  as  serum-albumin,  is  treated  for  some 
little  time  with  a  dilute  acid,  such  as  hydrochloric,  the  properties  become  entirely 
changed.  The  most  marked  changes  are:  (1)  that  the  solution  is  no  longer 
coagulated  by  heat ;  (2)  that  when  the  solution  is  carefully  neutralized  the  whole 
of  the  proteid  is  thrown  down  as  a  precipitate ;  in  other  words,  the  serum-albumin 
which  was  soluble  in  water,  or  at  least  in  a  neutral  fluid  containing  only  a  small 
quantity  of  neutral  salts,  has  become  converted  into  a  substance  insoluble  in  water 
or  in  similar  neutral  fluids.  The  body  into  which  serum-albumin  thus  becomes 
converted  by  the  action  of  an  acid  is  spoken  of  as  acid-albumin.  Its  characteristic 
features  are  that  it  is  insoluble  in  distilled  water,  and  in  neutral  saline  solutions, 
such  as  those  of  spdic  chloride,  that  it  is  readily  soluble  in  dilute  acids  or  dilute 
alkalies,  and  that  its  solutions  in  acids  or  alkalies  are  not  coagulated  by  boiling. 
When  suspended,  in  the  undissolved  state,  in  water,  and  heated  to  70°  C. ,  it  be- 
comes coagulated,  and  is  then  undistinguishable  from  coagulated  serum-albumin, 
or  indeed  from  any  other  form  of  coagulated  proteid.  It  is  evident  that  the  sub- 
stance when  in  solution  in  a  dilute  acid  is  in  a  different  condition  from  that  in 
which  it  is  when  precipitated  by  neutralization.  If  a  quantity  of  serum-  or  egg- 
albumin  be  treated  with  dilute  hydrochloric  acid,  it  will  be  found  that  the  con- 
version of  the  native  albumin  into  acid-albumin  is  gradual ;  a  specimen  heated 
to  70°  C.  immediately  after  the  addition  of  the  dilute  acid,  will  coagulate  almost 
as  usual ;  and  another  specimen  taken  at  the  same  time  will  give  hardly  any 

1  Stokvis,  Rech.  exp.  sur  les  Condit.  pathol.  de  1'Albuminurie,  Bruxelles,  1867;  also  Lehmann, 
Arch.  f.  Pathol.  Anat.,  Bd.  xxx.  (1864),  S.  593. 

2  Ann  der  Chem.  und  Pharm.,  Bd.  Ixxxii.,  S.  135. 

8  Chem.  Centralblatt,  1862,  No.  56.  *  pfliiger's  Archiv,  xi.  (1875),  S.  1. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  869 

precipitate  on  neutralization.  Some  time  later,  the  interval  depending  on  the  pro- 
portion of  the  acid  to  the  albumin,  on  temperature,  and  on  other  circumstances, 
the  coagulation  will  be  less,  and  the  neutralization  precipitate  will  be  considerable. 
Still  later,  the  coagulation  will  be  absent,  and  the  whole  of  the  proteid  will  be 
thrown  down  on  neutralization. 

If  finely-chopped  muscle,  from  which  the  soluble  albumins  have  been  removed 
by  repeated  washing,  be  treated  for  some  time  with  dilute  (0.2  per  cent.)  hydro- 
chloric acid,  the  greater  part  of  the  muscle  is  dissolved.  The  transparent  acid 
filtrate  contains  a  large  quantity  of  proteid  material  in  a  form  which,  in  its  gen- 
eral characters  at  least,  agrees  with  acid-albumin.  The  acid  solution  of  the  pro- 
teid is  not  coagulated  by  boiling,  but  the  whole  of  the  proteid  is  precipitated  on 
neutralization  ;  and  the  precipitate,  insoluble  in  neutral  sodic  chloride  solutions, 
is  readily  dissolved  by  even  dilute  acids  or  alkalies.  The  proteid  thus  obtained  from 
muscle  has  been  called  syntonm,  but  we  have  at  present  no  satisfactory  test  to  dis- 
tinguish the  acid-albumin  (or  syntonin)  prepared  from  muscle  from  that  prepared 
from  egg-  or  serum-albumin.  When  coagulated  albumin  or  other  coagulated  pro- 
teid or  fibrin  is  dissolved  in  strong  acids,  acid-albumin  is  formed  ;  and  when  fibrin 
or  any  other  proteid  is  acted  upon  by  gastric  juice,  acid-albumin  is  one  of  the  first 
products ;  and  these  acid-albumins  cannot  be  distinguished  from  acid-albumin  pre- 
pared from  muscle  or  native  albumin.  Though  hydrochloric  acid  is  perhaps  the 
most  convenient  acid  for  forming  acid-albumin,  other  acids  may  also  be  used  for 
the  purpose  of  preparing  it.  Acid-albumin  is  soluble  not  only  in  dilute  alkalies, 
but  also  in  dilute  solutions  of  alkaline  carbonates ;  its  solutions  in  these  are  not 
coagulated  by  boiling. 

If  sodic  phosphate  in  excess  is  added  to  an  acid  solution  of  acid-albumin,  the 
acid-albumin  is  precipitated  ;  this  also  occurs  on  adding  sodic  acetate  or  phos- 
phate. 

As  special  tests  of  acid- albumin  may  be  given :  1.  Partial  coagulation  of  its 
solution  in  lime-water  on  boiling.  2.  Further  precipitation  of  the  same  solution 
after  boiling,  on  the  addition  of  calcic  chloride,  magnesic  sulphate,  or  sodic 
chloride. 

Dissolved  in  very  dilute  hydrochloric  acid,  acid-albumin  (syntonin)  prepared  from 
muscle  possesses  a  specific  laevp-rotatory  power  of — 72°  for  yellow  light,  this  being 
independent  of  the  concentration. l  On  heating  the  solution  in  a  closed  vessel  in  a 
water-bath  the  rotatory  power  rises  to  — 84. 8°. 

The  body  known  as  parapeptone,  which  makes  its  appearance  during  the  peptic 
digestion  of  proteids,  is  closely  allied  to  the  substances  just  described. 

2.  Alkali-albumin. 

If  serum-  or  egg-albumin  or  washed  muscle  be  treated  with  dilute  alkali  instead 
of  with  dilute  acid,  the  proteid  undergoes  a  change  quite  similar  to  that  which  was 
brought  about  by  the  acid.  The  alkaline  solution,  when  the  change  has  become 
complete,  is  no  longer  coagulated  by  heat,  the  proteid  is  wholly  precipitated  on  neu- 
tralization, and  the  precipitate,  insoluble  in  water  and  in  neutral  sodic  chlorine  solu- 
tion, is  readily  soluble  in  dilute  acids  or  alkalies.  Indeed  in  a  general  way  it  may 
be  said  that  acid-albumin  and  alkali-albumin  are  nothing  more  than  solutions  of  the 
same  substance  in  dilute  acids  and  alkalies  respectively.  When  the  precipitate 
obtained  by  the  neutralization  of  a  solution  of  acid-albumin  in  dilute  acid  is  dis- 
solved in  a  dilute  alkali,  it  may  be  considered  to  become  alkali-albumin ;  and  con- 
versely when  the  precipitate  obtained  from  an  alkali-albumin  solution  is  dissolved  in 
dilute  acid,  it  may  be  regarded  as  acid-albumin. 

It  is  stated  '2  as  a  characteristic  reaction  of  this  modified  or  derived  albumin  that 
it  is  not  precipitated  when  its  alkaline  solutions  are  neutralized  in  the  presence  of 
alkaline  phosphates ;  solutions  of  acid-albumin,  on  the  contrary,  are  said  to  be  pre- 
cipitated on  neutralization  in  the  presence  of  alkaline  phosphates,  and  this  difference 
is  considered  to  be  a  distinguishing  feature  of  the  two  proteids.  But  doubt  has  been 
cast  on  this  statement,3 

Alkali-albumin  may  be  prepared  by  the  action  not  only  of  dilute  alkalies,  but 
also  of  strong  caustic  alkalies  on  native  albumins  as  well  as  on  coagulated  albu- 

1  Hoppe-Seyler,  Hdb.  Phys.  Path.  Chem.  Anal.,  Ed.  iv.  (1875),  S.  246. 

2Hoppe-Seyler,  loc.  cit,  S.  245. 

3Soyka.    Ptluger's  Arch.,  Bd.  xii.  (1876),  S.  347. 


870  APPENDIX. 

min  and  other  proteids.  The  jelly  produced  by  the  action  of  caustic  potash  on  white 
of  egg,  spoken  of  in  Class  I. ,  1 ,  is  alkali-albumin ;  the  similar  jelly  produced  by 
strong  acetic  acid  is  acid-albumin.  One  of  the  most  productive  methods  of  obtain- 
ing alkali-albumin  is  that  introduced  by  Lieberkiihn,1  and  consists  in  adding  a  strong 
solution  of  caustic  potash  to  purified  white  of  egg  until  the  above-mentioned  jelly 
is  obtained.  This  is  then  cut  into  small  pieces,  and  dialyzed  until  quite  white.  The 
lumps  are  then  dissolved  by  heating  on  the  water-bath,  and  the  alkali-albumin  pre- 
cipitated by  the  careful  addition  of  acetic  acid. 

Both  alkali- and  acid- albumin  are  with  difficulty  precipitated  by  alcohol  from 
their  alkaline  or  acid  solutions.  The  neutralization  precipitates,  however,  become 
coagulated  under  the  prolonged  action  of  alcohol. 

The  body  "protein,"  described  by  Mulder,  appears,  if  it  exists  at  all,  to  be  closely  connected 
with  this  body.  All  subsequent  observers  have,  however,  failed  to  confirm  his  views. 

The  rotatory  power  of  alkali-albumin  varies  according  to  its  source ;  thus 
when  prepared  by  strong  caustic  potash  from  serum-albumin,  the  rotation  rises 
from  — 56°  (that  of  serum-albumin)  to  — 86° ;  for  yellow  light.  Similarly  prepared 
from  egg-albumin,  it  rises  from  — 38. 5°  to  — 47° ;  and  if  from  coagulated  white  of 
egg,  it  rises  to — 58.8°.  Hence  the  existence  of  various  forms  of  alkali-albumin  is 
probable. 

In  addition  to  the  methods  given  above,  alkali-albumin  may  be  also  readily  obtained  by  shak- 
ing milk  with  strong  caustic  soda  solution  and  ether,  removing  the  ethereal  solution,  precipitat- 
ing the  remaining  fluid  with  acetic  acid  and  washing  the  precipitate  with  water,  cold  alcohol  and 
ether. 

The  most  satisfactory  method  of  regarding  acid-  and  alkali-albumin  is  to  con- 
sider them  as  respectively  acid  and  alkali  compounds  of  the  neutralization  precip- 
itate. We  have  reason  to  think  that  when  the  precipitate  is  dissolved  in  either  an 
acid  or  an  alkali,  it  does  enter  into  combination  with  them.  The  neutralization  pre- 
cipitate is  in  itself  neither  acid-  nor  alkali-albumin,  but  may  become  either,  upon 
solution  in  the  respective  reagent. 

It  is  probable  that  several  derived  albumins  exist,2  differing  according  to  the  proteid  from 
which  they  are  formed  or  possibly  according  to  the  mode  of  their  preparation,  and  that  each  of 
these  may  exist  in  its  correlative  forms  of  acid- and  alkali-albumin ;  but  the  whole  subject  re- 
quires further  investigation. 

Acid-albumin,  prepared  by  the  direct  action  of  dilute  acids  on  native  albumin,  or 
on  muscle-substance,  contains  sulphur,  as  shown  by  the  brown  coloration  which  ap- 
pears when  the  precipitate  is  heated  with  caustic  potash  in  the  presence  of  basic 
lead  acetate.  Alkali-albumin,  at  all  events  as  prepared  by  the  action  of  strong  caus- 
tic potash  or  soda,  does  not  contain  any  sulphur ;  and  the  acid-albumin,  prepared 
by  the  solution  in  an  acid  of  the  neutralization  precipitate  from  such  an  alkali-albu- 
min solution,  is  similarly  free  from  sulphur. 

3.  Casein. 

This  is  the  well-known  proteid  existing  in  milk.  When  freed  from  fat,  and  in 
the  moist  condition,  it  is  a  white,  friable,  opaque  body.  In  most  of  its  reactions 
it  corresponds  closely  with  alkali-albumin ;  thus  it  is  readily  soluble  in  dilute  acids 
and  alkalies,  and  is  re-precipitated  on  neutralization ;  if,  however,  potassic  phos- 
phate is  present,  as  is  the  case  in  milk,  the  solution  must  be  strongly  acid  before 
any  precipitate  is  obtained. 

Various  reactions  have  at  different  times  been  assigned  to  casein  as  distinguishing  it  from  the 
closely  allied  body  alkali-albumin.  Later  researches  have,  however,  in  most  cases  cast  so  much 
doubt  on  these  differences  that  the  identity  or  non-identity  of  casein  and  alkali-albumin  must 
still  be  left  an  open  question,  the  discussion  of  which  would  be  out  of  place  here. 

Casein,  as  occurring  in  milk,  has  had  several  reactions  ascribed  to  it,  as  characteristic ;  but 
these  lose  their  importance  on  considering  that  milk  contains,  in  addition  to  casein,  other  sub- 
stances, such  as  potassic  phosphate,  and  a  number  of  bodies  which  yield  acids  by  fermentation. 
The  presence  of  potassic  phosphate  has  an  especial  influence  on  the  reactions  of  casein.  In  the 
entire  absence  of  this  salt,  acetic  acid  in  the  smallest  quantities,  as  also  carbonic  anhydride,  gives 
a  precipitate  ;  but  if  this  salt  is  present,  carbonic  anhydride  gives  no  precipitate,  and  acetic  acid 
only  one  when  the  solution  is  acid  from  the  presence  of  free  acid,  and  not  from  that  of  acid 
potassic  phosphate.8 

1  PoggendorflPs  Annalen,  Bd.  Ixxxvi.,  S.  118. 

2  Morner.    Pttviger's  Arch.,  Bd.  xvii.  (1878),  S.  468. 

a  See  Kiihne,  Lehrb.  d.  Physiol.  Chem.,  1868,  S.  565. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  871 

When  prepared  from  milk  by  magnesic  sulphate  (see  below),  freed  by  ether 
from  fats,  and  dissolved  in  water,  casein  possesses  a  specific  rotatory  power  of  — 80° 
for  yellow  light ;  in  dilute  alkaline  solutions,  of  — 76° ;  in  strong  alkaline  solutions, 
of  — 91° ;  in  dilute  hydrochloric  acid,  of  —87°. 

Casein  has  been  asserted  to  occur  in  muscle,  in  serous  fluids,  and  in  blood-serum 
(serum-casein).  In  many  cases  it  has  probably  been  confounded  with  globulins 
(see  Class  III. ) ;  but  blood-serum  and  muscle-plasma  undoubtedly  contain  an 
alkali-albumin  in  addition  to  whatever  globulin  may  be  present,  and  the  usual 
doubt  exists  as  to  the  identity  of  this  with  true  casein.  Its  presence  maybe  shown 
by  adding  dilute  acetic  acid  to  blood-serum  which  has  been  freed  from  globulin  by 
a  current  of  carbonic  anhydride ;  a  distinct  precipitate  is  thrown  down.  A  sub- 
stance similar  to  casein  has  also  been  described  as  existing  in  unstriated  muscle  and 
in  the  protoplasm  of  nerve-cells. 

Preparation.  Dilute  milk  with  several  (10  to  15)  times  its  bulk  of  water,  add 
dilute  acetic  acid  till  a  precipitate  begins  to  appear,  then  pass  a  current  of  carbonic 
anhydride,  filter,  and  wash  the  precipitate  with  water,  alcohol,  and  ether ;  the  com- 
plete removal  of  the  fat  carried  down  with  the  casein  presents  some  difficulties. 
Magnesic  sulphate  added  to  saturation  also  precipitates  casein  from  milk ;  the  pre- 
cipitate as  thus  formed  is  readily  soluble  on  the  addition  of  water. 

CLASS  III.     Globulins. 

Besides  the  native  albumins  there  are  a  number  of  native  proteids  which  differ 
from  the  albumins  in  not  being  soluble  in  distilled  water ;  they  need  for  their  solu- 
tion the  presence  of  an  appreciable,  though  it  may  be  a  small,  quantity  of  a  neutral 
saline  body,  such  as  sodic  chloride.  Thus  they  resemble  the  albuminates  in  not 
being  soluble  in  distilled  water,  but  differ  from  them  in  being  soluble  in  dilute  sodic 
chloride  or  other  neutral  saline  solutions.  Their  general  characters  may  be  stated 
as  follows : 

They  are  insoluble  in  water,  soluble  in  dilute  ( 1  per  cent. )  solutions  of  sodic  chlo- 
ride ;  they  are  also  soluble  in  dilute  acids  and  alkalies,  being  changed  on  solution 
into  acid-  and  alkali-albumin  respectively,  unless  the  acids  and  alkalies  are  exceed- 
ingly dilute.  The  saturation  with  solid  sodic  chloride  of  their  solutions  in  dilute 
sodic  chloride,  precipitates  most  members  of  this  class. 

1.  Globulin  (CrystalUn). 

If  the  crystalline  lens  be  rubbed  up  with  fine  sand,  extracted  with  water  and 
filtered,  the  filtrate  will  be  found  to  contain  at  least  three  proteids.  On  passing  a 
current  of  carbonic  anhydride  a  copious  precipitate  occurs;  this  is  globulin. 

The  addition  of  dilute  acetic  acid  to  the  filtrate  from  the  globulin  gives  a  precipitate  of 
alkali-albumin;1  and  the  filtrate  from  this,  if  heated,  gives  a  further  precipitate,  due  to  serum- 
albumin. 

In  its  general  reactions  globulin  corresponds  almost  exactly  with  the  next  mem- 
bers of  this  class  (paraglobulin  and  fibrinogen),  but  has  no  power  to  form  or  pro- 
mote the  formation  of  fibrin  in  fluids  containing  the  above-mentioned  bodies,  and 
possesses  the  following  special  features :  1.  According  to  Lehmann,  its  oxygenated 
neutral  solutions  become  cloudy  on  heating  to  73°  C.,  and  are  coagulated  at  93°  C. 
2.  It  is  readily  precipitated  on  the  addition  of  alcohol.  According  to  Hoppe- 
Seyler,  it  is  not  precipitated  on  saturation  with  sodic  chloride,  resembling  vitellin 
in  this  respect. 

According  to  Kiihne2  and  Eichwald3  a  globulin  with  properties  identical  with  those  just 
given  may  be  precipitated  from  dilute  serum  by  the  cautious  addition  of  acetic  acid.  This  body 
is  stated  by  Weyl*  to  be  the  same  as  paraglobulin  (fibrinoplastin),the  latter  differing  from  it  only 
by  a  small  admixture  of  fibrin-ferment. 

2.  Paraglobulin  (Fibrmoplastm}. 

Preparation.  Blood-serum  is  diluted  tenfold  with  water,  and  a  brisk  current  of 
carbonic  anhydride  is  passed  through  it.  The  first-formed  cloudiness  soon  becomes 

1  But  see  also  Pfiiiger's  Arch.,  Bd.  xiii.  (1876),  S.  631. 
2Lehrb.  d.  Physiol.  Chem.,  1868,  S.  175. 

3Beitrnge  zur  Chem.  d.  gewebebild.  Subst.,  Berlin,  1873,  H.  1. 
4  Zeitschr.  f.  Physiol.  Chem.,  Bd.  i.  (1878),  S.  79. 


872  APPENDIX. 

a  flocculent  precipitate,  which  is  finally  quite  granular,  and  may  easily  be  separated 
by  decantation  and  filtration  ;  it  should  be  washed  on  the  filter  with  water  contain- 
ing carbonic  acid. 

It  has  usually  been  stated  that  paraglobulin  may  be  separated  from  serum  by 
saturation  with  sodic  chloride.  But  Hammarsten1  has  shown  that  this  is  only  in 
part  true,  a  considerable  portion  of  the  globulin  remaining  unprecipitated.  The 
separation  may,  however,  be  completely  effected  by  saturation  with  magnesic  sul- 
phate. When  determined  by  this  method  the  amount  of  paraglobulin  in  serum  is 
very  considerable,  amounting  in  some  cases,  according  to  Hammarsten,  to  as  much 
as  4.565  per  cent,  (reckoned  on  100  c.c.  of  serum).  The  quantity  seems  to  vary  in 
different  animals,  the  precipitation  being  much  more  complete  in  serum  from  ox- 
blood  than  m  that  from  the  blood  of  horses. 

From  its  solution  in  dilute  sodic  chloride,  paraglobulin  may  be  precipitated  by  a 
current  of  carbonic  anhydride  or  the  addition  of  exce.edi.ugly  dilute  (less  than  1  pro 
mille)  acetic  acid.  If  the  acid  is  strong,  the  precipitated  proteid  becomes  immedi- 
ately changed  into  acid-albumin  (Class  II. ,  1).  In  pure  water,  free  from  oxygen, 
paraglobulin  is  insoluble,  but  on  shaking  with  air  or  passing  a  current  of  oxygen, 
solution  readily  takes  place ;  from  this  it  may  be  reprecipitated  by  a  current  of 
carbonic  anhydride.  Very  dilute  alkalies  dissolve  this  body  without  change ;  if, 
however,  the  strength  of  the  alkali  be  raised  even  to  1  per  cent,  the  paraglobulin  is 
changed  into  alkali-albumin  (Class  II.,  2). 

According  to  Kuhne  and  A.  Schmidt  the  solutions  of  this  body  in  water  con- 
taining oxygen  or  in  very  dilute  alkalies  are  not  coagulated  on  heating.  The 
sodic  chloride  solutions  do,  however,  coagulate  when  heated  to  68°-70°  C.,2  and 
if  the  substance  itself  be  suspended  in  water  and  heated  to  70°  C.  it  is  coagulated. 
Although  insoluble  in  alcohol,  its  solutions  are  with  difficulty  precipitated  by  this 
reagent. 

Paraglobulin  occurs  not  only  in  blood-serum,  but  it  is  also  found  in  white  cor- 
puscles, in  the  stroma  of  red  corpuscles  (to  some  extent  at  least),  in  connective 
tissue,  the  cornea,  aqueous  humor,  lymph,  chyle,  and  serous  fluids. 

For  the  occurrence  of  globulin  in  urine  see  Edlefsen3  and  Senator.4 

3.  Fibrinogen. 

The  general  reactions  of  this  body  are  identical  with  those  of  paraglobulin.  The 
most  marked  difference  between  the  two  is  the  point  at  which  coagulation  of  their 
solutions  takes  place.  Hammarsten5  has  shown  that  fibrinogen  in  a  1-5  per  cent, 
solution  of  sodic  chloride  coagulates  at  from  52°-55°  C. ,  whereas,  as  stated  above, 
paraglobulin  (fibrinoplastin)  coagulates  first  at  from  68°-70°  C.  /Ihis,  however, 
is  disputed  by  A.  Schmidt,  who  holds  that  the  substance  coagulating  at  52°-55°  is 
not  fibrinogen,  but  a  sort  of  nascent  fibrin.  There  is  also  a  marked  difference  in 
the  precipitability  of  the  bodies  by  sodic  chloride.  (See  below. )  Other  differences 
between  the  two  may  be  thus  enumerated :  In  precipitating  fibrinogen  by  a  cur- 
rent of  carbonic  anhydride  the  containing  fluid  must  be  much  more  strongly  diluted, 
and  the  gas  must  pass  for  a  much  longer  time.  The  precipitate  thus  obtained 
differs  from  that  of  paraglobulin  in  that  it  forms  a  viscous  deposit,  adhering  more 
closely  to  the  sides  and  bottom  of  the  containing  vessel ;  there  is  also  no  flocculent 
stage  previous  to  the  viscous  precipitate. 

Fibrinogen  occurs  in  blood,  chyle,  serous  fluids,  and  in  various  transudations. 
The  relations  of  fibrinogen  and  paraglobulin  to  the  formation  of  fibrin  have  been 
discussed  in  the  text,  p.  33. 

Preparation.6  Salted  plasma,  obtained  by  centrifugalizing  blood  whose  coagula- 
tion is  prevented  by  the  addition  of  a  certain  proportion  of  magnesic  sulphate,  is 
mixed  with  an  equal  volume  of  a  saturated  (35.87  per  cent,  at  14°  C.)7  solution  of 
sodic  chloride ;  the  fibrinogen  is  thus  precipitated,  while  the  paraglobulin  remains 
in  solution.  The  adhering  plasma  may  be  removed  by  washing  with  a  solution  of 

i  Pflviger's  Archiv,  Bd.  xvii.  (1878),  S.  446 ;  Bd.  xviii.  (1878),  S.  38. 
2Hammarsten.  op.  cit. 

aCentralblatt  f.  med.  Wiss.,  Jahrg.  1870,  S.  367.    Also  Arch.  f.  klin.  Med.,  Bd.  vii.  S.  69. 
4  Virchow's  Archiv,  Bd.  Ix.  S.  476.        5  Upsala  Liikareforenings  forhandlingar.    Bd.  xi.,  1876. 
e See  Hammarsten,  Nov.  Act.  Reg.  See.  Sci..  Upsala,  Ser.  iii.  vol.  x.  (1875),  p.  31.    Also  Pfluger's 
Archiv,  Bd.  xix.  (1879),  S.  563,  and  Bd.  xxii.  (1880),  S.  431. 
i  Poggiale,  Ann.  Chim.  Phys.  (3),  vol.  viii.  p.  469. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  873 

sodic  chloride,  and  the  fibrinogen  finally  purified  by  being  several  times  dissolved  in 
and  reprecipitated  by  sodic  chloride. 

There  is  no  proof  that  the  whole  of  the  substance  thrown  down  by  carbonic 
anhydride  from  diluted  blood-serum  is  fibrinoplastic ;  indeed  we  know  that  a  true 
globulin  devoid  of  fibrinoplastic  properties  may  be  prepared  from  serum.1  Weyle2 
considers  that  there  is  only  one  globulin  in  serum,  which  he  characterizes  by  the 
name  of  " serum-globulin,"  and  regards  h'brinoplastin  as  a  mixture  of  this  body 
with  a  portion  of  fibrin-ferment.  We  know  for  certain  (see  p.  28)  that  the  whole 
of  the  n'brinoplastic  precipitate,  used  to  cause  the  coagulation  of  a  fibrinogenous 
fluid,  does  not  enter  into  the  composition  of  the  fibrin  produced  ;  we  also  know  that 
such  a  precipitate  may  lose  its  fibrinoplastic  powers  without  any  marked  change  in 
its  general  reactions.  It  would  seem  advisable,  therefore,  to  speak  of  the  deposit 
produced  by  carbonic  anhydride  in  dilute  serum,  or  by  saturation  with  sodic  chloride 
in  undiluted  serum,  as  globulin,  and  to  distinguish  it  as  fibrinoplastic  globulin  when 
it  is  able  to  give  rise  to  fibrin.  Fibrinogen  similarly  might  be  spoken  of  as  fibrin- 
ogenous globulin.  Tl>e  name  crystallin,  rather  than  globulin,  might  then  be  given 
to  the  substance  obtained  from  the  crystalline  lens. 

4.  Myosin. 

This  is  the  substance  which  forms  the  chief  proteid  constituent  of  dead,  rigid 
muscle ;  its  general  properties  and  mode  of  preparation  have  been  already 
described  at  p.  84.  In  the  moist  condition  it  forms  a  gelatinous,  elastic,  clotted 
mass ;  dried,  it  is  very  brittle,  slightly  transparent,  and  elastic.  From  its  solution 
in  sodic  chloride  it  is  precipitated,  either  by  extreme  dilution  or  by  saturation 
with  the  solid  salt.  When  precipitated  by  dilution  and  submitted  to  the  pro- 
longed action  of  water,  myosin  loses  its  property  of  being  soluble  in  solutions  of 
sodic  chloride.3  The  sodic  chloride  solution,  if  exposed  to  a  rising  temperature, 
becomes  milky  at  55°  C. ,  and  gives  a  flocculent  precipitate  at  60°  C.  This  precip- 
itate is,  however,  no  longer  myosin,  for  it  is  insoluble  in  a  10  per  cent,  sodic  chloride 
solution,  and  does  riot,  until  after  many  days'  digestion,  yield  syntonin  on  treat- 
ment with  hydrochloric  acid  (0.1  per  cent.).  It  is.  in  fact,  coagulated  proteid  (see 
Class  V.). 

Myosin  is  excessively  soluble  in  dilute  acids  and  'alkalies.  Advantage  may  be 
taken  of  its  solubility  in  the  former  to  extract  it  from  muscles.4  But  if  the 
reagents  are  at  all  concentrated,  myosin  undergoes  in  the  act  of  solution  a  radical 
change,  becoming  in  the  one  case  acid-albumin  or  syntonin,  in  the  other  alkali- 
albumin  (Class  II.). 

Like  fibrin,  it  can  in  some  cases  decompose  hydrogen  dioxide,  and  oxidize  guaiacum  with  for- 
mation of  a  blue  color. 

5.  Vitellin. 

As  obtained  from  the  yolk  of  egg,  of  which  it  is  the  chief  proteid  constituent, 
vitellin  is  a  white  granular  body,  insoluble  in  water,  but  very  soluble  in  dilute  sodic 
chloride  solutions ;  it  surpasses  myosin  in  this  respect,  for  the  solution  may  be  easily 
filtered.  Its  coagulation  temperature  is  higher  than  that  of  myosin,  lying,  accord- 
ing to  Weyl,5  between  70°  C.  and  80°  C.  Saturation  with  sodic  chloride  gives 
no  precipitate ;  in  this  respect  it  differs  from  most  other  members  of  this  class.  In 
yolk  of  egg  vitellin  is  always  associated  with,  and  probably  exists  in  combination 
with,  the  peculiar  complex  body  lecithin. 

Denis,  and  after  him  Hoppe-Seyler,  have  shown  that  vitellin  before  the  treatment  requisite  to 
free  it  from  lecithin  possesses  properties  quite  different  from  other  proteids. 

A  theory  has  been  advanced  that  vitellin  is  really  a  complex  body  like  haemo- 
globin, and  on  treatment  with  alcohol  splits  up  into  coagulated  proteid  and  leci- 
thin. When  well  purified  it  contains  0.75  percent,  sulphur,  but  no  phosphorus. 
Dilute  acids  or  alkalies  readily  convert  it  in  its  uncoagulated  form  into  a  member 
of  Class  II. 

1  Kiihne  and  Eichwald,  loc.  cit.  2  LOC.  cit. 

»  Weyl,  Zeitschr.  f.  Physiol.  Chem.,  Bd.  i.  (1878),- S.  77. 

«  Danilewsky,  Zeitschr.  f.  Physiol.  Chem.,  Bd.  v.  (1881),  S.  158.  5  Qp.  cit. 


874  APPENDIX. 

Fremy  and  Valenciennes1  have  described  a  series  of  proteids,  viz.,  ichthin,  ichthidin,  etc. 
derived  from  fish  and  amphibia.  They  appear  to  be  either  identical  with,  or  closely  allied  to, 
vitellin. 

Preparation.  Yolk  of  egg  is  treated  with  successive  quantities  of  ether  as  long 
as  this  extracts  any  yellow  coloring  matter ;  the  residue  is  dissolved  in  moderately 
strong  (10  per  cent. )  sodic  chloride  solution,  and  filtered.  The  filtrate  on  falling 
into  a  large  excess  of  water  is  precipitated.  In  this  state  it  is  mixed  with  lecithin 
and  nuclein,  and  in  order  to  free  it  from  these  it  was  usually  treated  with  alcohol.'2 
This,  as  above  stated,  entirely  changes  the  vitellin  into  a  coagulated  form.  It 
seems  probable  that  the  separation  of  vitellin  from  the  other  bodies  with  which  it 
is  mixed  in  the  yolk  of  egg  may  be  effected  by  precipitating  the  sodic  chloride 
solution  by  the  addition  of  excess  of  water ;  the  precipitate  is  then  redissolved  in 
10  per  cent,  solution  of  sodic  chloride,  and  the  process  repeated  as  rapidly  as  pos- 
sible. 

6.  Globin. 

Globin,  stated  by  Preyer3  to  be  the  proteid  residue  of  the  complex  body  haemoglobin  (see  p. 
351),  ought  probably  to  be  considered  as  an  outlying  member  of  this  class.  It  is,  however,  not 
readily  soluble  either  in  dilute  acids  or  sodic  chloride  solutions.  It  is  said  to  be  absolutely  free 
from  ash. 

CLASS  IV.     Fibrin. 

Insoluble  in  water  and  dilute  sodic  chloride  solutions ;  soluble,  with  difficulty,  in 
dilute  acids  and  alkalies,  and  more  concentrated  neutral  saline  solutions. 

Fibrin,  as  ordinarily  obtained,  exhibits  a  filamentous  structure,  the  component 
threads  possessing  an  elasticity  much  greater  than  that  of  any  other  known  solid 
proteid. 

If  allowed  to  form  gradually  in  large  masses,  the  filamentous  structure  is  not  so  noticeable,  and 
it  resembles  in  this  form  pure  India-rubber.  Such  lumps  of  fibrin  are  capable  of  being  split  in 
any  direction,  and  no  definite  arrangement  of  parallel  bundles  of  fibres  can  be  made  out. 

At  ordinary  temperature  fibrin  is  insoluble  in  water,  being  dissolved  only  at  very 
high  temperatures,  and  then  undergoing  a  complete  change  in  its  characters.  In 
hydrochloric  solutions  of  1-5  per  cent,  fibrin  swells  up  and  becomes  transparent, 
but  is  not  dissolved.4  In  this  condition  the  mere  removal  of  the  acid  by  an  excess 
of  water,  neutralization,  or  the  addition  of  some  salt,  causes  a  return  to  the 
original  state.  If,  however,  the  acid  be  allowed  to  act  for  many  days  at  ordinary 
temperatures,  or  for  a  few  hours  at  40°-60°  C.,  solution  takes  place,  and  the  result- 
ing proteid  is  syntonin.  In  dilute  alkalies  and  ammonia,  fibrin  is  much  more 
readily  soluble,  though  in  this  case  also  the  solution  is  greatly  aided  by  warming ; 
the  resulting  fluid  contains  no  longer  fibrin,  but  alkali-albumin.  This  property  is 
not  distinctly  characteristic  of  fibrin,  although  it  dissolves  perhaps  more  readily  in 
both  dilute  acids  and  alkalies  than  do  coagulated  proteids.  None  of  these  solu- 
tions can  be  coagulated  on  heating,  which  is  intelligible  when  it  is  remembered 
that  they  no  longer  contain  fibrin,  but  either  acid-  or  alkali-albumin.  In  addition 
to  the  above,  fibrin  is  soluble,  though  with  difficulty  and  only  after  a  considerable 
time,  in  10  per  cent,  solutions  of  sodic  chloride,  potassic  nitrate,  or  sodic  sulphate, 
the  solution  being  often  accompanied  by  putrefactive  changes.  These  solutions 
may  be  coagulated  by  a  temperature  of  60°  C. ,  and  are  precipitated  by  dilution  with 
water  or  saturation  with  solid  sodic  chloride ;  in  fact,  by  the  action  of  the  neutral 
saline  solutions  the  fibrin  has  become  converted  into  a  body  exceedingly  like  myosin 
or  globulin.5 

On  ignition  of  fibrin  a  residue  of  inorganic  matter  is  always  obtained  ;  it  is,  how- 
ever, considered  that  sulphur  is  the  onty  one  of  these  elements  which  enters  essen- 
tially into  its  composition.  In  other  respects  fibrin  corresponds  entirely  in  general 
composition  with  the  other  proteids. 

Suspended  in  water  and  heated  to  70°  C.,  it  loses  its  elasticity  and  becomes 
opaque  ;  it  is  then  indistinguishable  from  other  coagulated  proteids. 

1  Compt.  Rend.,  T.  xxxviii.  pp.  469,  525. 

2  Weyf,  op.  cit.,  S.  74.  3  Die  Blutkrystalle  (1871),  S.  166. 

4  Complete  solution  may,  however,  take  place  if  the  fibrin,  as  is  frequently  the  case,  contains 


any  adherent  pepsin. 
6  Gautier,  Co: 


rapt.  Rend.,  T.  Ixxix.  (1874),  p.  227. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  875 

A  peculiar  property  of  this  body  remains  yet  to  be  mentioned,  viz.,  its  power  of  decomposing 
hydrogen  dioxide.  Pieces  of  fibrin  placed  in  this  fluid,  though  themselves  undergoing  no  change, 
soon  become  covered  with  bubbles  of  oxygen ;  and  guaiacum  is  turned  blue  by  fibrin  in  presence 
of  hydrogen  dioxide  or  ozonized  turpentine. 

Preparation.  By  vigorously  stirring  blood  with  a  bundle  of  twigs  and  then 
washing  with  water  until  it  is  quite  white.  If  required  perfectly  pure  and  colorless 
it  should  be  prepared  from  plasma  free  from  corpuscles.  If  the  blood,  before  stir- 
ring, be  diluted  with  an  equal  bulk  of  water,  the  subsequent  washing  of  the  fibrin 
is  much  facilitated,  and  it  may  readily  be  obtained  quite  white.  Any  adherent  fats 
may  be  removed  by  ether. 

When  globulin,  myosin,  and  fibrin  are  compared  each  with  the  other,  it  will  be 
seen  that  they  form  a  series  in  which  myosin  is  intermediate  between  globulin  and 
fibrin.  Globulin  is  excessively  soluble  in  even  the  most  dilute  acids  and  alkalies ; 
fibrin  is  almost  insoluble  in  these ;  while  myosin,  though  more  soluble  than  fibrin, 
is  less  soluble  than  globulin.  Globulin  again  dissolves  with  the  greatest  ease  in  a 
very  dilute  solution  of  'sodic  chloride.  Myosin,  on  the  other  hand,  dissolves  with 
difficulty ;  it  is  much  more  soluble  in  a  10  per  cent,  than  in  a  1  per  cent,  solution 
of  sodic  chloride ;  and  even  in  a  1 0  per  cent,  solution  the  myosin  can  hardly  be  said 
to  be  dissolved,  so  viscid  is  the  resulting  fluid  and  with  such  difficulty  does  it  filter. 
Fibrin  again  dissolves  with  great  difficulty  and  very  slowly  in  even  a  10  per  cent, 
solution  of  sodic  chloride,  and  in  a  1  per  cent,  solution  it  is  practically  insoluble. 
When  it  is  remembered  that  fibrin  and  myosin  are,  both  of  them,  the  results  of 
coagulation,  their  similarity  is  intelligible.  Myosin  is,  in  fact,  a  somewhat  more 
soluble  form  of  fibrin,  deposited  not  in  threads  or  filaments  but  in  clumps  and 
masses. 

CLASS  V.     Coagulated  Proteids. 

These  are  insoluble  in  water,  dilute  acids  and  alkalies,  and  neutral  saline  solu- 
tions of  all  strengths.  In  fact,  they  are  really  soluble  only  in  strong  acids  and  strong 
alkalies,  though  prolonged  action  of  even  dilute  acids  and  alkalies  will  effect  some 
solution,  especially  at  high  temperatures.  During  solution  in  strong  acids  and 
alkalies  a  destructive  decomposition  takes  place,  but  some  amount  of  acid-  or  alkali- 
albumin  is  always  produced. 

Very  little  is  known  of  the  chemical  characteristics  of  this  class.  They  are  pro- 
duced by  heating  to  70°  C.  solutions  of  egg-  or  serum-albumin,  globulins,  suspended 
in  water  or  dissolved  in  saline  solutions;  by  boiling  for  a  short  time  fibrin  suspended 
in  water  or  dissolved  in  saline  solutions,  or  precipitated  acid-  and  alkali-albumin  sus- 
pended in  water.  They  are  readily  converted  at  the  temperature  of  the  body  into 
peptones  by  the  action  of  gastric  juice  in  an  acid,  or  of  pancreatic  juice  in  an  alka- 
line medium. 

All  proteids  in  solutions  are  precipitated  by  an  excess  of  strong  alcohol.  If  the 
precipitant  be  rapidly  removed  they  are  again  soluble  in  water,  but  if  the  precip- 
itated proteids  are  subjected  for  some  time  to  the  action  of  the  alcohol  they  are, 
with  the  exception  of  peptones,  coagulated  and  lose  their  solubility.  It  appears, 
however,  that  the  proteids  contained  in  the  aleurone-grains  of  plants  are  exceed- 
ingly resistant  to  this  coagulating  action  of  alcohol.1 


It  seems  scarcely  necessary  to  point  out  the  distinction  in  the  use  of  the  word  "  coagulation  " 
as  applied  to  blood-  or  muscle- plasma  on  the  one  hand  and  to  the  action  of  heat  and  alcohol  upon 
proteids  on  the  other.  The  difference  is  obvious  when  it  is  remembered  that  in  the  first  case  the 
coagulation  leads  to  the  formation  of  fibrin  (Class  iv.),or  myosin  (Class  in.),  and  that  these  bodies 
may  then  further  be  coagulated  by  heat  or  alcohol  as  described  above. 

CLASS  VI.     Peptones.'2' 

Very  soluble  in  water,  and  not  precipitated  from  their  aqueous  solutions  by  the 
addition  of  acids  or  alkalies,  or  by  boiling.  Insoluble  in  alcohol,  they  are  precip- 
itated with  difficulty  by  this  reagent,  and  are  unchanged  m  the  process ;  they  differ 
from  all  other  proteids  in  not  being  coagulated  by  prolonged  exposure  to  alcohol. 
They  are  not  precipitated  by  cupric  sulphate,  ferric  chloride,  or,  except  in  the  in- 
stances to  be  mentioned  presently,  by  potassic  ferrocyanide,  and  acetic  acid.  In 
these  points  they  differ  from  most  other  proteids.  On  the  other  hand,  precipita- 

i  See  Vines,  Journ.  of  Phvsiol.,  vol.  iii.  p.  108. 

[2  The  albumoses  are  so  closely  related  to  peptones  that  they  are  commonly  considered  together 
with  peptones  as  a  class.] 


876  APPENDIX. 

tion  is  caused  by  chlorine,  iodine,  tannin,  mercuric  chloride,  nitrates  of  mercury  and 
silver,  and  both  acetates  of  lead ;  also  by  bile-acids  in  an  acid  solution.  In  common 
with  all  proteids,  these  bodies  possess  a  specific  laevo-rotatory  power  over  polarized 
light ;  but  they  differ  from  all  other  proteids  in  the  fact  that  boiling  produces  no 
change  in  the  amount  of  rotation. 

A  solution  of  peptones,  mixed  with  a  strong  solution  of  caustic  potash,  gives, 
on  the  addition  of  a  mere  trace  of  cupric  sulphate,  a  pink  color.  An  excess  of  the 
cupric  salt  gives  a  violet  color,  which  deepens  in  tint  on  boiling,  in  fact  the  ordinary 
proteid  reaction.  Other  proteids  simply  give  the  violet  color.  But  the  most  cha- 
racteristic feature  of  peptones  is  their  relatively  great  diffusibility,  a  property  which 
they  alone,  of  all  the  proteids,  may  be  said  to  possess,  since  all  other  forms  of  pro- 
teids pass  through  membranes  with  the  greatest  difficulty,  if  at  all. 

The  diffusibility  of  peptones  is,  however,  absolutely  small  as  compared  with  that  of  crystalline 
bodies  such  as  sodic  chloride;  in  fact,  solutions  of  peptones  may  be  freed  from  salts  by  dialysis, a 
process  employed  in  their  preparation. 

Notwithstanding  their  probable  formation  in  large  quantities  in  the  stomach  and 
intestine,  to  judge  from  the  results  of  artificial  digestion,  a  very  small  quantity  only 
can  be  found  in  the  contents  of  these  organs.  They  are  probably  absorbed  as  soon 
as  formed.  Another  point  of  interest  is  their  reconversion  into  other  forms  of  pro- 
teids, since  this  must  occur  to  a  great  extent  in  the  body.  We  are,  however,  as  yet 
ignorant  of  the  manner  in  which  this  reverse  change  is  effected. 

Preparation.  All  proteids  with  the  exception  of  lardacein,  yield  peptones 
(and  other  products)  on  treatment  with  acid  gastric  or  alkaline  pancreatic  juice 
most  readily  at  the  temperature  of  the  human  body.  Peptones  are  likewise 
produced  in  the  absence  of  pepsin  and  trypsin,  by  the  action  of  dilute  and  mode- 
rately strong  acids  at  medium  temperatures,  also  by  the  action  of  distilled  water  at 
high  temperatures  under  pressure.  For  various  methods  of  preparing  peptones,  see 
Maly,1  Adarnkiewicz,2  Henninger,3  and  Pekelharing.4 

It  appears  possible  to  reobtain  ordinary  coagulable  proteids  from  peptones  by  the  action  of 
either  prolonged  heating  to  140°-170°  C.  or  of  dehydrating  agents.6 

No  difference  in  percentage  composition  between  peptones  and  the  proteid  from 
which  they  are  formed  has,  at  present,  been  definitely  established. 

We  have  used  the  word  "peptones"  in  the  plural  number  because  we  have 
reason  to  think  that  more  than  one  kind  of  peptone  exists.  Meissner6  described 
three  peptones,  naming  them  respectively  A-  B-  and  C-peptone.  He  distinguished 
them  as  follows :  A- peptone  is  precipitated  from  its  aqueous  solutions  by  concen- 
trated nitric  acid,  and  also  by  potassic  ferrocyariide  in  the  presence  of  even  weak 
acetic  acid.  B-peptone  is  not  precipitated  by  concentrated  nitric  acid,  nor  will 
potassic  ^ferrocyanide  give -a  precipitate  unless  a  considerable  quantity  of  strong 
acetic  acid  be  added  at  the  same  time.  C-peptone  is  precipitated  neither  by  nitric 
acid  nor  by  potassic  ferrocyanide  and  acetic  acid,  whatever  be  the  strength  of  the 
acetic  acid.  In  place,  however,  of  speaking  of  all  these  as  peptones,  it  is  better  to 
consider  C-peptone  as  the  only  real  peptone,  and  the  A-  and  B-peptones  as  not 
peptones  at  all.  Nevertheless  we  have  reason,  from  the  researches  of  Kiihne,  to 
speak  of  more  than  one  peptone,  viz. ,  of  a  hemipeptone  which  is  capable  under  the 
action  of  trypsin  of  being  converted  into  leucin  and  tyrosin,  and  of  an  antipeptone 
which  resists  such  a  decomposition.  The  name  antipeptone  is  given  to  the  latter 
on  account  of  this  resistance  which  it  offers  toward  trypsin ;  the  name  hemipep- 
tone, given  to  the  former,  signifies  that  this  peptone  is  the  twin  or  correlative  half 
of  antipeptone. 

We  have  seen  (p.  255)  that  when  any  proteid  is  digested  with  pepsin,  what  we 
may  preliminarily  call  a  by-product  makes  its  appearance.  This  by-product  which 
has  many  resemblances  to  acid-albumin  or  syntonin,  appearing  as  a  neutralization 
precipitate  soluble  in  dilute  acids  and  alkalies  but  insoluble  in  distilled  water,  is 

i  Piluger's  Arch.,  Bd.  ix.  (1874),  S.  585. 

*  Die  Natur  u.  Nahwerth  d.  Peptons  (1877),  S.  33. 

3  De  la  Nature  et  du  R61e  physiologiques  des  Peptones,  Paris,  1878. 
4Pfiuger's  Arch.,  Bd.  xxii.  (1882),  S.  185. 

*  Heiminger,  loc.  cit. ;  Hofmeister,  Zeitsch.  f.  phj^siol.  Chem.,  Bd.  ii.  (1878),  S.  206 ;  Pekelharing, 
loc.  cit. 

«  Zeitsch.  f.  rat.  Med.,  Bde.  vii.,  vili.,  x.,  xii.  u.  xiv. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  877 

generally  spoken  of  as  parapeptpne.  According  to  Finkler1  this  neutralization 
precipitate  is  especially  abundant  if  the  pepsin  be  previously  modified  by  exposure 
to  a  temperature  of  40°  to  60°  C.  The  pepsin  thus  modified  is  spoken  of  by  Fink- 
ler as  "isopepsin."  Many  authors  regard  parapeptone,  syntonin,  and  acid-albu- 
min as  being  the  same  thing.  Meissner,  however,  gave  the  name  parapeptone  to  a 
body  which  need  not  and  probably  does  not  make  its  appearance  during  normal 
natural  digestion  or  during  artificial  digestion  with  a  thoroughly  active  pepsin,  but 
which  is  formed  when  proteids  are  subjected  to  the  action  of  weak  hydrochloric 
acid,  either  alone  or  in  company  with  an  imperfectly  acting  pepsin,  and  which  in 
certain  characters  is  quite  distinct  from  ordinary  syntonin  or  acid-albumin.  Its 
distinguishing  feature  is  that  it  cannot  be  changed  into  peptone  by  the  action  of 
even  the  most  energetic  pepsin,  though  it  is  really  so  converted  under  the  influ- 
ence of  trypsin ;  otherwise  it  very  closely  resembles  syntonin.  We  have  here  an 
indication  that  the  simple  characters  by  which  we  have  described  acid-albumin 
may  be  borne  by  bodies  having  marked  differences  from  each  other.  The  re- 
searches of  Kiihne2  have  thrown  an  important  light  on  these  differences.  The 
fundamental  notion  of  Kiihne' s  view  is  that  an  ordinary  native  albumin  or  fibrin 
contains  within  itself  two  residues,  which  he  calls  respectively  an  anti-residue  and 
a  hemi-residue.  The  result  of  either  peptic  or  tryptic  digestion  is  to  split  up  the 
albumin  or  fibrin,  and  to  produce  on  the  part  of  the  anti-residue  an ti peptone,  and 
on  the  part  of  the  hemi-residue  hemipeptone,  the  latter  being  distinguished  from 
the  former  by  its  being  susceptible  of  further  change  by  tryptic  digestion  into 
leucin,  tyrosin,  etc.  Antipeptone  remains  as  antipeptone  even  when  placed  under 
the  action  of  the  most  powerful  trypsin,  provided  putrefactive  changes  do  not 
intervene. 

Before  the  stage  of  peptone  (whether  anti-  or  hemi-)  is  reached,  there  is  an  in- 
termediate stage  corresponding  to  the  formation  of  syntonin.  In  both  normal 
peptic  and  tryptic  digestion  antipeptone  is  preceded  by  an  anti-albumose  and 
hemipeptone  by  a  hemi-albumose.  Of  these  the  anti-albumose  is  closely  related 
to  syntonin,  and  has  hitherto  been  regarded  as  syntonin.  The  hemi-albumose 
has  not  been  so  frequently  observed ;  it  was,  however,  isolated  by  Meissner ;  it  is 
apparently  the  body  called  by  him  A-peptone.  It  possesses  several  peculiar  feat- 
ures. If  its  solutions  are  heated  they  partially  coagulate  at  about  60°-63°  C.  :  the 
precipitate  is  soluble  at  about  70°  C.  and  is  reprecipitated  as  the  temperature 
again  falls.  It  also  yields  a  precipitate  with  nitric  acid  and  potassic  ferrocyanide, 
and  this  also  is  soluble  at  the  higher  temperature,  reprecipitating  on  cooling.  In 
these  respects  it  closely  resembles  a  proteid  body  observed  by  Bence-Jones  in  the 
urine  of  osteomalacia.  It  approaches  myosin  in  being  readily  soluble  in  a  10  per 
cent,  solution  of  sodic  chloride. 

If,  however,  albumin  be  digested  with  insufficient  or  with  imperfectly  active 
pepsin,  or  simply  with  dilute  hydrochloric  acid  at  40° C.,  anti-albumose  is  not 
formed,  but  in  its  place  a  body  makes  its  appearance  which  Kiihne  calls  anti-albu- 
mate.3  Its  characteristic  property  is  that  it  cannot  be  converted  by  peptic  digestion 
into  peptone,  though  it  can  be  so  changed  by  tryptic  digestion.  It  is  in  fact  the 
parapeptone  of  Meissner. 

It  may  perhaps  be  advisable,  now  that  Meissner' s  parapeptone  is  cleared  up,  to 
reserve  the  name  parapeptone  for  the  initial  products  of  both  peptic  and  tryptic 
digestion,  and  to  speak  of  anti-albumose  and  hemi-albumose  as  being  both  para- 
peptones.  But  in  this  sense  parapeptone  will  be  an  intermediate  and  not  a  collat- 
eral product  of  digestion. 

Meissner  also  described  a  particularly  insoluble  form  of  his  parapeptone  as  dys- 
peptone,  and  another  intermediate  product  as  a  metapeptone  ;  but  further  investi- 
gation of  both  these  bodies,  as  well  as  his  B-peptone,  is  necessary.  Under  the 
influence  of  dilute  hydrochloric  acid,  anti-albumate  becomes  changed  into  a  body 
which  Kiihne  calls  anti-albumid,  and  which  seems  identical  with  the  very  insoluble 
proteid  described  by  Schiitzenberger  as  "hemiprotein,"  and  probably  with  Meiss- 
ner'a  dyspeptone.  The  same  body  is  produced  at  once  in  company  with  products 
belonging  to  the  hemi-group  by  the  action  of  3  to  5  per  cent,  sulphuric  acid  on 
native  albumin  or  fibrin.  The  following  tables  show  the  relations  and  genesis  of 

*  Pfliiger's  Archiv,  Bd.  xiv.  (1887),  S.  128. 

2  Only  a  short  account  of  these  has  as  yet  been  published.    Verhandl.  d.  Naturhist.-med. 
Verein,  Heidelberg,  Bd.  i.  Heft  4,  1876. 

3  An  albumate  must  not  be  confounded  with  an  albuminate. 


878  APPENDIX. 

the  bodies  we  have  just  described.  The  several  products  (antipeptone,  etc.)  are 
given  in  duplicate,  on  the  hypothesis  (which,  though  not  proved,  is  probable)  that 
the  changes  of  digestion  are  essentially  hydrolytic  changes,1  accompanied  by  a  re- 
duplication ;  that,  just  as  a  molecule  of  starch  splits  up  into  at  least  two  molecules 
of  dextrose,  or  as  a  molecule  of  cane-sugar  splits  up  into  a  molecule  of  dextrose 
and  a  molecule  of  levulose,  so  a  molecule  of  anti-albumose,  for  instance,  splits  up 
into  two  molecules  of  antipeptone,  and  so  on.  But  the  whole  scheme  is,  of  course, 
only  provisional. 

DECOMPOSITION  OF  PROTEIDS  BY  DIGESTION. 

*  si 

Albumin. 


Anti-albumose.  Hemi-albuuiose. 


Antipeptone.  Antipeptone.          Hemipeptone.  Hemipeptone. 


Leucin.       Tyrosin,         Leucin.       Tyrosin, 
etc.  etc. 


DECOMPOSITION  BY  ACIDS. 


By  0.25  per  cent.  HC1  at  40°  C. 
Albumin. 

Anti-albumate.  Hemi-albumose. 


Anti-albumid.  Hemipeptone.  Hemipeptone. 


By  3-5  per  cent.  H2SO4  at  100°  C. 
Albumin. 


Anti-albumid.  Hemi-albumose. 


Hemipeptone.  Hemipeptone. 

I  I 

Leucin,  Tyrosin,  etc.  Leucin,  Tyrosin,  etc. 

CLASS  VII.     Lardacein,  or  the  so-called  Amyloid  Substance. 

The  substance  to  which  the  above  name  is  applied  is  found  as  a  pathological 
deposit  in  the  spleen  and  liver,  also  in  numerous  other  organs,  such  as  the  blood- 
vessels, kidneys,  lungs,  etc. 

It  is  insoluble  in  water,  dilute  acids  and  alkalies,  and  neutral  saline  solutions. 

In  centesimal  composition  it  is  almost  identical  with  other  proteids,2  viz. : 

OandS.  H.  N.  C. 

24.4  7.0  15.0  53.6 

The  sulphur  in  this  body  exists  in  the  oxidized  state,  for  boiling  with  caustic 
potash  gives  no  sulphide  of  the  alkali.  The  above  results  of  analysis  would  lead 
at  once  to  the  ranking  of  lardacein  as  a  proteid,  and  this  is  strongly  supported  by 
other  facts.  Strong  hydrochloric  acid  converts  it  into  acid-albumin,  and  caustic 
alkalies  into  alkali-albumin.  On  the  other  hand  it  exhibits  the  following  marked 
differences  from  other  proteids.  It  wholly  resists  the  action  of  ordinary  digestive 
fluids;  it  is  colored  red,  not  yellow,  by  iodine,  and  violet  or  pure  blue  by  the  joint 
action  of  iodine  and  sulphuric  acid.  From  these  last  reactions  it  has  derived  one 
of  its  names,  "  amyloid,"  though  this  is  evidently  badly  chosen ;  for  not  only  does 
it  differ  from  the  starch  group  in  composition,  but  by  no  means  can  it  be  converted 

1  Henninger,  loc.  cit.,  p.  49. 

2  C.  Schmidt,  Ann.  d.  Chem.  u.  Pharm.,  Bd.  ex.  S.  250,  and  Friedreich  u.  Kekuel,  Virchow's 
Archiv,  Bd.  xvi.  S.  50. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  879 

into  sugar :  this  latter  is  one  of  the  crucial  tests  for  a  true  member  of  the  carbo- 
hydrate group.  According  to  Heschl1  and  Cornil,2  anilin-violet  (methyl-anilin) 
colors  lardaceous  tissue  rosy  red,  but  sound  tissue  blue. 

The  colors  mentioned  above  as  being  produced  by  iodine  and  sulphuric  acid  are  much  clearer 
and  brighter  when  the  reagents  are  applied  to  the  purified  lardacein.  When  the  reagents  are 
applied  to  the  crude  substance  in  its  normal  position  in  the  tissues,  the  colors  obtained  are  always 
dark  and  dirty  looking. 

Purified  lardacein  is  readily  soluble  in  moderately  dilute  ammonia,  and  can.  by 
evaporation,  be  obtained  from  this  solution  in  the  form  of  tough,  gelatinous  fiakes 
and  lumps ;  in  this  form  it  gives  feeble  reactions  only  with  iodine.  If  the  excess 
of  ammonia  is  expelled,  the  solution  becomes  neutral,  and  is  precipitated  by  dilute 
acids. 

Preparation.  The  gland  or  other  tissue  containing  this  body  is  cut  up  into  small 
pieces,  and  as  much  as  possible  of  the  surrounding  tissue  removed.  The  pieces  are 
then  extracted  several  times  with  water  and  dilute  alcohol,  and  if  not  thus  rendered 
colorless  are  repeatedly  boiled  with  alcohol  containing  hydrochloric  acid.  The 
residue  after  this  operation  is  digested  at  40°  C.,  with  good  artificial  gastric  juice  in 
excess.  Everything,  except  lardacein  and  small  quantities  of  mucin,  nuclein, 
keratin,  together  with  some  portion  of  the  elastic  tissue,  will  thus  be  dissolved  and 
removed.3  From  the  latter  impurities  it  may  be  separated  by  decantation  of  the 
finely-powdered  substance. 


The  chief  products  of  the  decomposition  of  proteids  are  ammonia,  carbonic 
anhydride,  leucin,  and  tyrosin.  Several  other  bodies,  for  the  most  part,  like  leucin, 
amidated  acids,  such  as  aspartic  acid,  glutamic  acid,  etc. ,  have  also  been  obtained ; 
also  by  tryptic  digestion,  hypoxanthin,  and  perhaps  xanthin.  But  urea  has  never 
yet  been  derived  by  direct  decomposition  from  proteid  material,  the  statements  to 
this  effect  having  been  based  on  errors.  In  spite  of  numerous  researches,  we  cannot 
at  present  state  definitely  what  is  the  real  constitution  of  a  proteid,  or  in  what 
manner  these  several  residues  are  contained  in  the  undecomposed  substance.  It  is 
unnecessary  to  give  here  any  of  the  formulae,  nearly  all  empirical,  which  have  been 
made  to  represent  a  proteid ;  they  all  give  with  equal  exactitude  the  percentage 
composition,  but  beyond  this  they  are  untrustworthy.  Of  the  various  attempts 
which  have  been  made  to  assign  to  proteids  some  definite  molecular  structure,  none 
appear,  at  the  present  stage  of  information,  sufficiently  reliable  for  general  accept- 
ance. 

Among  the  most  elaborate  labors  in  this  direction  may  be  mentioned  those  of  Hlasiwetz  and 
Haberman.  In  their  first  publication,4  starting  from  the  general  similarity  of  the  products  of 
decomposition  of  the  proteids  and  carbohydrates,  they  tried  to  establish  a  definite  relation  between 
the  two  classes  of  bodies.  In  this  they  were  not  successful,  and  in  their  second  research 5  they 
came  to  the  conclusion  that  the  carbohydrates  take  no  part  in  the  formation  of  the  proteids. 

Other  experiments  in  the  same  direction  have  been  made  by  Schiitzenberger.e  He  shows  that 
albumin  can  be  decomposed  into  carbonic  anhydride  and  ammonia,  and  thai  the  ratio  of  these  two 
is  the  same  as  though  urea  had  been  the  body  on  which  he  operated.  From  this  he  concludes  that 
"  the  molecule  of  albumin  contains  the  grouping  of  urea  and  represents  a  complex  ureide."  In 
his  second  publication 7  he  confirms  his  previous  results,  stating  that  the  ammonia,  carbonic  anhy- 
dride, and  oxalic  acid,  produced  by  the  decomposition  of  proteids,  are  so  connected  quantitatively 
as  to  be  capable  of  derivation  from  varying  proportions  of  urea  and  oxamide.  He  also  obtained 
from  the  decomposition  of  proteids  a  nitrogenous  residue  which  could  be  formulated  as  giving 
rise  to  all  the  amidated  acids  and  other  bodies  spoken  of  above.  Thus,  according  to  him,  albumin, 
built  up  as  a  complex  xireide,  decomposes  into  ammonia,  carbonic,  oxalic,  and  acetic  acids,  and 
this  nitrogenous  body  ;  this  last  then  gives  rise  to  the  other  products  of  decomposition.8 

It  will  be  noticed  that  in  the  general  description  of  the  various  proteids  distinc- 
tive reactions  for  each  could  not  be  given,  but  that  varying  solubilities  were  the 
chief  means  at  our  disposal  for  distinguishing  them.  They  may  be  arranged 
according  to  their  solubilities  in  the  following  tabular  form: 

1  Wien.  med.  Wochenschr.,  No.  32,  S.  714. 

2  Compt.  Rend.,  T.  Ixxx.  (1875),  p.  1288. 

3  Kiihne  und  Rudneff,  Virchow's  Archiv,  Bd.  xxxiii.  (1865),  S.  66. 

4  Ann.  d.  Chem.  u.  Pharm.,  Bd.  clix.  S.  304.  5  ibid.,  Bd.  clxix.  S.  150. 

e  Comptes  Reridus,  T.  Ixxx.  (1875),  p.  232.  Bull,  de  la  Soc.  Chim.  xxiii.,  161. 193,  216,  242,  385, 
483,  xxiv.2et!45. 

7  Comptes  Rendus,  T.  Ixxxi.  p.  1108.    Bull,  de  la  Soc.  Chim.  xxv.  147. 

8  See  also  Schutzenberger,  Ann.  de  Chim.  et  de  Phys.,  T.  xvi.  (1879),  p.  280. 


880  APPENDIX. 

Soluble  in  distilled  water — 

Aqueous  solution  not  coagulated  on  boiling Peptones. 

Aqueous  solution  coagulated  on  boiling      ......        Albumins. 

Insoluble  in  distilled  water  : 

Soluble  in  NaCl  solution  1  per  cent Globulins. 

Soluble  in  HC1  0. 1  per  cent,  in  the  cold |  Acid-  and  Alkali- 

Insoluble  in  NaCl  solution  1  per  cent.     .  j      albumin. 

Insoluble  in  HC1  0. 1  per  cent,  in  the  cold,  but  soluble    )   77»-7    • 

at60°q \Fllrm- 

Insoluble  in  HC1  0. 1  per  cent,  at  60°  C.  ;  soluble  in  strong  acids. 

Soluble  in  gastric  juice Coagulated  albumin. 

Insoluble  Lardacein. 

Such  a  classification  is,  however,  obviously  a  wholly  artificial  one,  useful  for  tem- 
porary purposes,  but  in  no  way  illustrating  the  natural  relations  of  the  several 
members.  Nor  is  a  division  into  "native"  and  "derived"  proteids  much  more 
satisfactory.  It  is  true  that  we  may  thus  put  together  serum-  and  egg-albumin, 
with  vitellin,  myosin,  and  fibrin,  on  the  one  hand ;  and  peptones,  coagulated  pro- 
teids, and  acid-  with  alkali-albumin,  on  the  other.  But  in  what  light  are  we  to 
consider  casein,  seeing  that,  though  a  natural  product,  it  has  so  many  resemblances 
to  alkali-albumin  ?  Moreover,  the  system  of  classification  must  be  useless  which 
would  place  fibrinoplastic  globulin  and  fibrinogen  in  the  same  class  as  fibrin,  and 
yet  we  can  hardly  speak  of  either  of  the  two  former  bodies  as  derived  proteids.  If 
the  view  be  true  that  when  fibrin  is  converted  into  peptone  the  large  molecule  of 
the  former  is  split  up,  with  assumption  of  water,  into  two  smaller  molecules  of  the 
latter,  one  belonging  to  the  "anti "  and  the  other  to  the  "  hemi "  group,  we  might 
speculate  on  a  possible  classification  of  all  proteids  into  hemi-proteids,  anti- 
proteids,  and  holo-proteids.  Thus  serum-  and  egg-albumin,  myosin.  and  fibrin 
would  be  undoubtedly  holo-proteids,  peptones  either  anti-  or  hemi-proteids,  and 
we  should  have  to  distinguish  probably  in  the  heterogeneous  group  of  derived 
albumins  both  anti-,  hemi-,  and  holo-proteid  members.  It  is  possible,  moreover, 
that  fibrinoplastic  and  fibrinogenous  globulin  and  casein  may  be  natural  hemi-  or 
anti-proteids,  and  not  holo-proteids.  But  we  have  at  present  no  positive  knowledge 
on  these  points. 

[ENZYMES. 

Enzymes  are  unorganized  soluble  ferments,  such  as  ptyalin,  p'epsin,  trypsin, 
amylopsin,  rerinin,  fibrin-ferment,  etc.,  and  have  been  considered  in  various  parts 
of  this  work.] 

NITROGENOUS  NON-CRYSTALLINE  BODIES  ALLIED  TO  PROTEIDS. 

These  resemble  the  proteids  in  many  general  points,  but  exhibit  among  them- 
selves much  greater  differences  than  do  the  proteids.  As  regards  their  molecular 
structure  nothing  satisfactory  is  known.  Their  percentage  composition  approaches 
that  of  the  proteids,  and  like  these  they  yield,  under  hydrotytic  treatment,  large 
quantities  of  leucin  and  in  some  cases  tyrosin.  They  are  all  amorphous. 

Mucin.     (0,35.75.     H,  6.81.     N,  8.50.     C,  48.94. )' 

The  characteristic  component  of  mucus.  Its  exact  composition  is  not  yet  known, 
the  figures  given  above  being  merely  an  approximation. 

As  occurring  in  the  normal  condition  it  gives  to  the  fluids  which  contain  it  the 
well-known  ropy  consistency,  and  can  be  precipitated  from  these  by  acetic  acid, 
alcohol,  alum,  and  mineral  acids ;  the  latter,  if  in  excess,  redissolve  the  pre- 
cipitate, but  this  is  not  the  case  with  acetic  acid.  In  its  precipitated  form  it  is 
insoluble  in  water,  but  swells  up  strongly  in  it,  and  this  effect  is  increased  by  the 
presence  of  many  alkali  salts.  Alkalies  and  alkaline  earths  dissolve  it  readily. 
Its  solutions  do  not  dialyse ;  they  give  the  proteid  reactions  with  Millon's 
reagent  and  nitric  acid,  but  not  that  with  sulphate  of  copper,  and  are  precipitated 
by  basic  lead  acetate  only  when  neutral  or  faintly  alkaline.  According  to  Eichwald,2 

i  Eichwald,  Ann.  d.  Chem.  u.  Pharm.,  Bd.  cxxxiv.  S.  198.  *Op.  cit. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  881 

when  heated  with  dilute  mineral  acids,  mucin  yields  acid-albumin,  and  another  body 
which  in  many  of  its  properties  closely  resembles  a  sugar,  inasmuch  as  it  reduces 
solutions  of  cupric  sulphate.  Prolonged  boiling  with  sulphuric  acid  gives  leucin 
and  about  7  per  cent,  of  tyrosin. 

Preparation.1  Ox-gall  or  an  aqueous  extract  of  finely  chopped  submaxillary 
gland  is  acidulated  with  acetic  acid ;  the  precipitated  mucin  is  then  washed  with 
water,  dissolved  in  dilute  sodic  carbonate  and  finally  precipitated  with  acetic  acid. 
It  may  also  be  obtained  from  snails.2 

Chondrin.     (0,31.04.     H,  6.76.     N,  13.87.     0,47.74.     S,  0.60  per  cent.)  3 

This  is  usually  regarded  as  forming  the  essential  part  of  the  matrix  of  hyaline 
cartilage,  and  is  contained  in  the  interstices  of  the  fibres  in  elastic  cartilage.  A 
similar  substance  can  be  prepared  from  the  cornea.  Boiled  with  water,  it  dissolves 
slowly,  forming  an  opalescent  solution,  which  is  precipitated  by  acetic  acid,  lead 
acetate,  dilute  mineral  acids,  alum,  and  salts  of  silver  and  copper;  an  excess  of  the 
last  four  reagents  redissolves  the  precipitate.  Solutions  of  this  body  gelatinize  on 
standing,  even  if  very  dilute ;  the  solid  mass  is  insoluble  in  cold  water,  readily 
soluble  in  hot  water,  alkalies,  and  ammonia. 

The  aqueous  and  alkaline  solutions  of  chondrin  possess  a  left-handed  rotatory 
power  on  polarized  light  of — 213.5°;  in  presence  of  excess  of  alkali  this  be- 
comes— 552.0°,  both  measured  for  yellow  light.4 

It  seems,  according  to  the  observations  of  many,  that  chondrin  can,  by  heating 
with  hydrochloric  acid,  be  converted  into  a  body  whose  reactions  resemble  those  of 
syntonin,  and  another  substance,  which  like  the  similar  product  from  mucin,  so  far 
resembles  grape-sugar  that  it  reduces  cupric  salts  in  alkaline  solution  ;5  it  appears, 
however,  to  contain  nitrogen.  The  existence  of  chondrin  as  a  distinct  substance 
has,  however,  been  denied6  on  the  supposition  that  it  is  in  all  cases  a  mere  mixture 
of  other  bodies.  It  is  stated  that  a  substance  having  all  the  reactions  of  the  so- 
called  chondrin,  may  at  any  time  be  produced  by  a  mixture  of  mucin,  glutin,  and 
inorganic  salts.  The  extreme  similarity  in  the  reactions  of  chondrin  and  mucin 
point  to  a  close  relationship  between  the  two.  The  whole  subject,  however,  requires 
more  complete  investigation.  With  alkalies  or  dilute  sulphuric  acid  chondrin  gives 
leucin,  but  no  tyrosin  or  glycin.  Whether  chondrin  exists  as  such  in  cartilage  is 
uncertain  ;  it  seems  probable  that  it  does  not,  since  its  extraction  from  cartilage  re- 
quires an  amount  of  boiling  with  water  much  greater  than  that  requisite  to  dissolve 
dried  chondrin. 

Preparation.  From  cartilage  by  extracting  with  water  and  precipitating  with 
acetic  acid. 

[Chondrin  is  not  probably  a  distinct  substance,  but  a  mixture  of  mucin  and 
gelatin.  ] 

Gelatin  or  Glutin.7  (0,  23.21.  H,  7.15.  N,  18.32.  C,  50.76.  S,  0.56 
per  cent, ) 

This  is  the  substance  which  is  yielded  when  connective- tissue  fibres  are  heated 
for  several  days  with  very  dilute  acetic  acid,  at  a  temperature  of  about  15°  C.,  or 
by  the  prolonged  action  of  water  in  a  Papin's  digester.  The  elastic  elements  of 
connective  tissue  are  unaffected  by  the  above  treatment. 

As  obtained  in  this  way  glutin  is  when  heated  a  thin  fluid,  solidifying  on  cooling 
to  the  well-known  gelatinous  form.  When  dried  it  is  a  colorless,  transparent,  brittle 
body,  swelling  up,  but  remaining  undissolved  in  cold  water ;  heating,  or  the  addition 
of  traces  of  acids  or  alkalies,  readily  effects  its  solution.  When  dissolved  in  water 
it  possesses  a  Isevo-rotatory  power  of — 130°,  at  30°  C.  ;  the  addition  of  strong  alkali 
or  acetic  acid  reduces  this  to  —112° or —114°,  both  measured  for  yellow  light.8  Its 
solutions  will  not  dialyse. 

Mercuric  chloride  and  tannic  acid  are  the  only  two  reagents  which  yield  insoluble 

1  Eichwald,  op.  cit.  and  Chem.  Centralb.,  1866,  No.  14.    Staedeler,  Ann.  d.  Chem.  u.  Pharm.,  Bd. 
cxi.  S.  14.    Landwehr,  Zeitschr.  f.  physiol.  Chem.,  Bd.  v.  (1881),  S.  371. 

2  Landwehr,  Zeitschr.  f.  physiol.  Chem.,  Bd.  vi.  (1882),  S.  75. 
3 1.  v.  Mering,  Beitrag  zur  Chemie  des  Knorpels,  1873. 

4  Hoppe-Seyler,  Hdb.  phys.  path.  chem.  Anal..  Aufl.  1875,  S.  262. 
6  De  Bary,  Hoppe-Seyler's  Untersuch.,  Hft.  1,  S.  71. 

6  Morochowetz,  Yerh'and.  Naturhist.-med.  Ver.,  Heidelberg,  Bd.  i.  (1876),  Hft.  5. 

7  Not  to  be  confounded  with  the  vegetable  proteid  "gluten." 

8  Hoppe-Seyler,  Hdb.  d.  phys.  path.  chem.  Anal.,  4  Aufl.  1875,  S.  222. 

56 


882  APPENDIX. 

precipitates  with  this  body.  Its  presence  prevents  the  action  of  Trommer's  sugar- 
test,  since  it  readily  dissolves  the  precipitated  cuprous  oxide.  The  prpteid  reactions 
of  glutiri  are  so  feeble  that  they  are  probably  due  merely  to  impurities.  Heated 
with  sulphuric  acid  it  yields  ammonia,  leucin,  and.  glycin,  but  no  tyrosin. 

[When  gelatin  is  digested  it  undergoes  alterations  similar  to  those  of  fibrin  and 
albumin,  forming  gelatoses  and  gelatin- peptones,  but  the  results  of  recent  investi- 
gation indicate  that  while  gelatin  is  valuable  as  a  food-stuff  for  furnishing  energy, 
it  is  not  of  value  for  the  growth  of  the  nitrogenous  tissues. 

It  appears  improbable^that  glutin  exists  ready-formed  in  connective-tissue  fibres, 
since  these  do  not  swell  up  in  water,  and  only  yield  glutin  after  prolonged  treat- 
ment with  boiling  water  ;  to  which  it  may  be  added  that  while  glutin  is  acted  upon 
by  trypsin,  the  connective-tissue  fibres  in  their  natural  condition  resist  its  action 
(see  p.  284).  When  glutin  is  submitted  for  some  time  to  the  action  of  dilute  hydro- 
chloric acid  at  38°  C.,  and  the  change  is  brought  about  even  more  readily  by  the 
action  of  pepsin,  it  loses  its  power  of  gelatinizing  and  is  now  diffusible  through 
porous  membranes :  the  name  of  gelatin-peptone  has  been  given  to  the  product 
thus  obtained.1 

Elastin.     (0,  20.5.     H,  7.4.     N,  16.7.     C,  55.5  percent.) 

This  characteristic  component  of  elastic  fibres  is  left  on  the  removal  of  all  the 
glutin,  mucin,  etc.,  from  such  tissues  as  "ligamentum  nuchae,"  advantage  being 
taken  of  its  not  being  altered  when  it  is  heated  with  water,  even  under  pressure, 
with  strong  acetic  acid,  or  with  dilute  alkalies.  When  moist  it  is  yellow  and  elastic, 
but  on  drying  becomes  brittle.  It  is  soluble  in  strong  alkalies  at  boiling  tempera- 
tures, and  concentrated  sulphuric  and  nitric  acids  dissolve  it  even  in  the  cold  ;  it  is 
also  dissolved  by  the  action  of  papaya  juice.  It  is  precipitated  from  solutions  by 
tannic  acid,  but  not  by  the  addition  of  ordinary  acids.  Notwithstanding  that  it 
closely  approaches  the  proteids  in  its  percentage  composition,  and  gives  distinct 
although  feeble  proteid  reactions,  any  very  close  relationship  between  the  two  ap- 
pears improbable,  since  elastin  when  treated  with  sulphuric  acid,  yields  leucin  ( 30-40 
per  cent, )  only  and  no  tyrosin. 

Hilger2  has  obtained  a  similar  body  from  the  shell  membrane  of  snakes'  eggs. 

Keratin.3  (0,  20.7-25.0.  H,  6.4-7.0.  N,  16.2-17.7.  C,  50.3-52.5.  S, 
0.7-5.0  percent.) 

This  body,  though  somewhat  resembling  the  proteids  in  general  composition, 
differs  from  them  and  also  from  the  preceding  bodies  so  widely  in  other  properties, 
that  its  description  is  placed  here  for  convenience  rather  than  anything  else.  Hair, 
nails,  feathers,  horn,  and  epidermic  scales  consist  for  the  most  part  of  keratin. 
Heated  with  water  in  a  digester  at  150°  C.  keratin  is  partially  dissolved  with  evo- 
lution of  sulphuretted  hydrogen  ;  the  solution  then  gives  with  acetic  acid  and  fer- 
rocyanide  of  potassium  a  precipitate  soluble  in  excess  of  the  acid.  Prolonged  boiling 
with  alkalies  and  acids,  even  acetic,  dissolves  keratin ;  the  alkaline  solutions  evolve 
sulphuretted  hydrogen  on  treatment  with  acids.  The  sulphur  in  keratin  is  evi- 
dently very  loosely  united  to  the  substance,  and  in  all  its  reactions  there  appears  to 
be  a  want  of  similarity  between  keratin  and  either  proteids,  mucin  or  gelatin.  The 
most  common  of  its  products  of  decomposition  are  leucin  (10  per  cent.)  and  tyrosin 
(3.6  percent),  and  some  aspartic  acid;  no  glycin  is  formed.  What  is  generally 
known  as  keratin  is  probably  a  compound  body,  which  has  not  yet  been  resolved 
into  its  components. 

Ewald  and  Kuhne4  have  described  a  new  body  to  which,  since  it  occurs  as  a  constituent  of 
nervous  tissue  (both  of  nerves  and  of  the  central  nervous  system),  and  is  yet  closely  identical  with 
ordinary  horny  tissue,  they  give  the  name  of  neuro-keratin.  It  is  prepared  in  quantity  from  the 
brain  by  extracting  this  tissue  with  alcohol  and  ether,  and  subjecting  the  residue  to  the  action  of 
pepsin  and  trypsin.  The  final  residue  is  neuro-keratin,  and  amounts  to  15  to  20  per  cent,  of  the 
original  tissue. 

Nuclein.    C29H49N9P3022. 

Discovered  by  Miescher5  in  the  nuclei  of  pus  corpuscles  and  in  the  yellow  cor- 
puscles of  yolk  of  egg.  Other  observers  have  subsequently  obtained  it  from  yeast, 

1  Hofmeister,  Zeitschr.  f.  physiol.  Chem.,  Bd.  ii.  (1878),  S.  299. 

2  Ber.  d.  Deutsch.  Chem.  Gesellsch.,  1873,  S.  166.    But  see  also  next  reference. 

8  Linclwall,  "  Nagra  bidrag  till  kann.  om.  Ker.,  Upsala  Lakarefs.  forh.  xvi.  (1881),  p.  546. 
*  Verhand.  Naturhist.-med.  Ver.,  Heidelberg.  Bd.  i.  (1876),  Heft  5. 
5  Med.-chem.  Untersuch.,  Hoppe-Seyler,  Heft  4,  1872,  S.  441  u.  502. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  883 

from  semen,  from  the  nuclei  of  the  red  blood-corpuscles  of  birds  and  amphibia, 
from  hepatic  cells,  and  it  is  probably  present  in  all  nuclei. 

When  newly  prepared  it  is  a  colorless  amorphous  body,  soluble  to  a  slight  ex- 
tent in  water,  readily  soluble  in  many  alkaline  solutions ;  but  its  solubilities  alter  on 
keeping.  If  added  gradually  in  sufficient  quantity  to  a  solution  of  caustic  alkali 
it  first  neutralizes  the  solution  and  then  renders  it  acid.  It  seems  to  possess  an 
indistinct  xantho-prpteic  reaction,  but  gives  no  reaction  with  Millon's  fluid.  It 
yields  precipitates  with  several  salts,  e.  g. ,  zinc  chloride,  argentic  nitrate,  and  cupric 
sulphate. 

Preparation.1  Since  nuclein  is  very  resistant  to  the  action  of  pepsin,  it  may  be 
obtained  from  the  granular  residue  consisting  chiefly  of  nuclei,  which  occurs  after 
digesting  pus  with  pepsin.  The  most  remarkable  feature  of  this  body  is  its  large 
percentage  of  phosphorus,  9.59  per  cent.  This  phosphorus  is  readily  separated 
by  boiling  \yith  strong  hydrochloric  acid  or  caustic  alkalies ;  the  same  occurs  when 
solutions  of  nuclein  are  acidulated  and  allowed  to  stand. 


Chitin.    C13H26N20 


10- 

Although  not  found  as  a  constituent  of  any  mammalian  tissue,  this  substance 
composes  the  chief  part  of  the  exo-skeleton  of  many  invertebrates.  It  may 
probably  be  regarded  as  the  animal  analogue  of  the  cellulose  of  plants,  and  from 
this  point  of  view  it  possesses  considerable  morphological  interest.  Both  cellulose 
and  chitin  appear  to  yield  some  form  of  sugar  when  treated  with  strong  acids. 

When  purified,  chitin  is  a  white  amorphous  body,  often  retaining  the  shape  of 
the  tissue  from  which  it  has  been  prepared.  It  is  insoluble  in  all  reagents  except 
strong  mineral  acids,  the  best  solvents  being  sulphuric  or  hydrochloric  acids.  The 
immediate  addition  of  water  to  these  solutions  reprecipitates  the  chitin  in  an  un- 
altered form ;  but  the  prolonged  action  of  sulphuric  acid  causes  a  decomposition 
resulting,  according  to  some  observers,  in  the  formation  of  an  amorphous  ferment- 
able carbohydrate ;  and  when  hydrochloric  acid  is  used  an  amidated  carbohydrate 
is  obtained  to  which  the  name  of  glycosamin3  (C6H]3N05)  has  been  given. 

Preparation.*  The  cleansed  exo-skeleton  of  a  lobster  is  thoroughly  extracted 
with  dilute  hydrochloric  acid  and  then  with  caustic  soda.  To  purify  it  finally  it  is 
submitted  to  prolonged  boiling  with  a  solution  of  potassic  permanganate. 

[Nucleo-albumin. 

This  term  is  applied  to  a  class  of  substances  which  seem  to  be  composed  of 
nuclein  and  an  albuminous  proteid.  Casein  is  probably  a  nucleo-albumin,  for  when 
subjected  to  peptic  digestion  a  residue  of  nuclein  is  obtained.] 

CARBOHYDRATES. 

Certain  members  only  of  this  class  occur  in  the  human  body ;  of  these  the  most 
important  and  widespread  are  those  known  as  glycogen  and  the  two  sugars,  grape- 
sugar  or  dextrose  (glucose),  with  which  diabetic  sugar  seems  to  be  identical,5  and 
maltose.  Next  to  these  comes  milk-sugar.  Inosit  is  another  body  of  this  class, 
although  it  differs  in  many  important  points  from  the  preceding  two. 

Sugars  are  often  considered  to  be  polyatomic  alcohol.  Several  of  them  stand  in  peculiar  rela- 
tion to  mannit,  and  may  be  converted  into  that  substance  by  the  action  of  sodium  amalgam.6 

[When  solutions  of  sugars  are  warmed  in  the  presence  of  phenyl-hydrazin  and 
dilute  acetic  acid,  amorphous  or  crystalline  substances  separate,  which  are  termed 
osazones.  These  differ  according  to  the  different  sugars.] 

1.  Dextrose  (grape-sugar).     C6H1206+H,0. 

Occurs  in  the  contents  of  the  alimentary  canal  to  a  variable  extent  dependent 
on  the  nature  of  the  food  taken.  It  is  also  a  normal  constituent  of  blood,  chyle, 
and  lymph.  Concerning  its  presence  in  the  liver,  see  p.  538.  The  amniotic  fluid 

1  See  Kossel,  Zeitschr.  f.  physiol.  Chem.,  Bd.  iii.  (1879),  S.  284 ;  iv.  (1880),  S.  290  ;  vii.  (1883),  S.  7. 
"  Untersuch.  iiber  d.  Nuclein  u.  ihre  Spaltungsprod,  Strassb.,  1881. 

2  Ledderhose.  Zeitschr.  f.  physiol.  Chem.,  Bd.  ii.  (1878),  S.  213. 

3  Ledderhose,  loc.  cit.,  Bd.  iv.  (1880),  S.  139. 

4  Biitschli,  Arch.  f.  Anat.  u.  Physiol.,  Jahrg.,  1874,  S.  362. 

6  The  question,  however,  whether  several  varieties  of  sugar  occurring  in  the  animal  body  have 
not  been  confounded  together  under  the  common  name  of  dextrose  or  glucose  maybe  consid- 
ered at  present  an  open  one. 

6  Linnemann,  Ann.  d.  Chem.  u.  Pharm.,  Bd.  cxxiii.,  S.  136. 


884  APPENDIX. 

also  contains  this  body.  Bile  in  the  normal  condition  is  free  from  sugar,  so  also  is 
urine,  though  this  point  has  given  rise  to  great  dispute.1  The  disease  diabetes  is 
characterized  by  an  excess  of  dextrose  in  the  fluids  and  tissues  of  the  body  (see  p. 
448). 

When  pure,  dextrose  is  colorless  and  crystallizes  from  its  aqueous  solution  in 
six-sided  tables  or  prisms,  often  agglomerated  into  warty  lumps.  The  crystals  will 
dissolve  in  their  own  weight  of  cold  water,  requiring,  however,  some  time  for  the 
process;  they  are  very  readily  soluble  in  hot  water.  Dextrose  is  somewhat  spar- 
ingly soluble  in  alcohol,  and  crystallizes  from  anhydrous  alcohol  in  prisms  free  from 
water  of  crystallization ;  it  is,  moreover,  insoluble  in  ether. 

The  freshly  prepared  cold  aqueous  solution  of  the  crystals  possesses  a  dextro-rotatory  power  of 
+104°  for  yellow  light.  This,  quickly  on  heating,  more  slowly  on  standing,  falls  to  +56°,  at 
which  point  it  remains  constant. 

Dextrose  readily  forms  compounds  with  acids  and  many  salts  ;  the  latter  are  very  unstable, 
decomposition  rapidly  ensuing  on  heating  them.  When  its  metallic  compounds  are  decom- 
posed, the  decomposition  is,  in  many  cases,  accompanied  by  the  precipitation  of  the  metals,  e.  g., 
silver,  gold,  mercury,  bismuth.  Caustic  alkalies  readily  decompose  them,  as  also  does  ammonia. 

Dextrose  is  readily  and  completely  precipitated  by  lead  acetate  and  ammonia. 

An  important  property  of  this  body  is  its  power  of  undergoing  fermentations. 
Of  these  the  two  principal  are:  1.  Alcoholic.  This  is  produced  in  aqueous  solu- 
tions of  dextrose,  under  the  influence  of  yeast.  The  decomposition  is  the  follow- 
ing :  C6H1206  =  2C2H60  -f  2C02,  yielding  (ethyl)  alcohol  and  carbonic  anhydride. 
Other  alcohols  of  the  acetic  series  are  found  in  traces,  as  also  are  glycerin,  succinic 
acid,  and  probably  many  other  bodies.  The  fermentation  is  most  active  at  about 
25°  C.  Below  5°  C.  or  above  45°  C.  it  almost  entirely  ceases.  If  the  saccharine 
solution  contains  more  than  15  per  cent,  sugar  it  will  not  all  be  decomposed,  as 
excess  of  alcohol  stops  the  reaction.  2.  Lactic.  This  occurs  in  the  presence  of 
decomposing  nitrogenous  matter,  especially  of  casein,  and  is  probably  the  result 
of  the  action  of  a  specific  ferment.2  The  first  stage  is  the  production  of  lactic  acid, 
C6H1206  —  2C3H603.  In  the  second  butyric  acid  is  formed  with  evolution  of  hydro- 
gen and  carbonic  anhydride :  2C3H603  =  C4H802+2C02-|-4H.  The  above  changes, 
the  first  of  which  is  probably  undergone  by  sugar  to  a  considerable  extent  in  the 
intestine,  are  most  active  at  35°  C.  ;  the  presence  of  alkaline  carbonates  is  also 
favorable.  It  is,  moreover,  essential  that  the  lactic  acid  should  be  neutralized  as 
fast  as  it  is  formed,  otherwise  the  presence  of  the  free  acid  stops  the  process. 

The  preparation,  detection,  and  estimation  of  dextrose  are  so  fully  given  in 
various  books  that  they  need  not  be  detailed  here. 

2.  Maltose.     C12H220U+H20. 

This  form  of  sugar  was  first  described  by  Dubrunfaut 3  as  a  product  of  the  action 
of  malt  extract  on  starch.  Its  existence  was  for  a  long  time  doubted  until  0' Sul- 
livan 4  repeated  and  confirmed  the  previous  experiments.  According  to  him  it 
crystallizes  in  fine  acicular  crystals,  possesses  a  specific  rotatory  power  of +150°  and 
a  reducing  power  which  is  only  one-third  as  great  as  that  of  dextrose.  It  seems 
probable  that  this  is  the  chief  sugar  obtained  by  the  action  not  only  of  diastase 
but  of  ptyalin  and  pancreatic  ferment  upon  starch  and  perhaps  also  upon  glycogen ; 5 
although  some  dextrose  may  at  the  same  time  be  formed.  Musculus  and  Gruber6 
have  shown  that  maltose  may  also  be  formed  by  the  action  of  dilute  sulphuric  acid 
on  starch,  and  that  it  is  capable  of  undergoing  alcoholic  fermentation. 

Preparation.     See  Musculus  and  Gruber  (loc.  cit.}. 

3.  Milk-sugar.     C12H22On+H20. 

Also  known  as  lactose.  It  is  found  in  milk,  and  is  characteristic  of  this  secre- 
tion. It  is  said,  however,  to  occur  abnormally  in  the  urine  of  lying-in  women.7 

It  yields,  when  pure,  hard,  colorless  crystals,  belonging  to  the  rhombic  system 
(four-sided  prisms).  It  is  less  soluble  in  water  than  dextrose,  requiring  for  solu- 

1  See  Seegen,  Der  Diabetes  Mellitus,  2  ed.,  S.  196. 

2  Lister,  Path.  Soc.  Trans.,  vol.  for  1878,  p,  425,  also  Quart.  Journ.  of  Micros.  Sci.,  vol   xviii. 
(1878),  p.  177. 

3  Ann.  Chim.  Phys.  (3)  xxi.  (1847),  p.  178. 

4  Journ.  Chem.  Soc  ,  Ser.  2,  vol.  x.  (1872),  p.  579. 

5  Musculus  u.  v.  Mering,  Zeitschr.  f.  physiol.  Chem.,  Bd.  ii.  (1878),  S.  403. 

«  Zeitschr.  f.  physiol.  Chem.,  Bd.  ii.  (1878),  S.  177.  1  Hofmeister,  Ibid.,  Bd.  i.  (1877),  S.  101. 


CHEMICAL  BASIS  OF  THE   ANIMAL  BODY.  885 

tion  six  times  its  weight  of  cold,  but  only  two  parts  of  boiling,  water ;  it  is  entirely 
insoluble  in  alcohol  and  ether.  It  is  fully  precipitated  from  its  solutions  by  the 
addition  of  lead  acetate  and  ammonia. 

When  freshly  dissolved,  its  aqueous  solution  possesses  a  specific  dextro-rotatory  power  of  +93.1° 
for  sodium  light";  this  diminishes  slowly  on  standing,  rapidly  on  boiling,  until  it  finally  remains 
constant  at  -(-52.5°.  The  amount  of  rotation  is  independent  of  the  concentration  of  the  solution. 

Lactose  unites  readily  with  bases,  forming  unstable  compounds;  from  its  metallic  compounds 
the  metal  is  precipitated  in  the  reduced  state  on  boiling;  it  reduces  copper  salts  as  readily  as 
dextrose,  but  to  a  less  extent,  viz.,  in  the  ratio  of  70  :  100. 

Lactose  is  generally  stated  to  admit  of  no  direct. alcoholic  fermentation;  this 
may,  however,  sometimes  be  induced  by  the  prolonged  action  of  yeast.  By  boiling 
with  dilute  mineral  acids  lactose  is  converted  into  gal'actose,  which  readily  undergoes 
alcholic  fermentation  and  possesses  a  greater  rotatory  power  than  lactose. 

It  may  be  remarked  here  that  though  isolated  lactose  is  incapable  of  direct  alcoholic  fermenta- 
tion, milk  itself  may  be  fermented.  Berthelot  was  unable  in  this  direct  alcoholic  fermentation 
to  detect  any  intermediate  change  of  the  lactose  into  any  other  fermentable  sugar. 

Lactose  is,  however,  directly  capable  of  undergoing  the  lactic  and  butyric  fer- 
mentation ;  the  circumstances  and  products  are  the  same  as  in  the  case  of  dextrose 
(see  above).  The  action  is  generally  productive  of  a  collateral  small  quantity  of 
alcohol.  . 

Lactose  is  thus  distinguished  from  dextrose  by  its  smaller  solubility  in  water, 
insolubility  in  alcohol,  crystalline  form,  lower  cupric  oxide  reducing  power,  and  its 
incapability  of  undergoing  direct  alcoholic  fermentation. 

Preparation.  After  the  removal  of  the  casein  and  other  proteids  of  the  milk, 
the  mother-liquor  is  evaporated  to  the  crystallizing  point ;  the  crystals  are  purified 
by  repeated  crystallization  from  warm  water. 

4.  Inosit.    C6H1206  +  2H20. 

This  substance  occurs  but  sparingly  in  the  human  body ;  it  was  found  originally 
by  Scherer  *  in  the  muscles  of  the  heart.  Cloetta  showed  its  presence  in  the  lungs, 
kidneys,  spleen,  and  liver,2  and  Miiller  in  the  brain.3  It  occurs,  also,  in  diabetic 
urine  and  in  that  of  u  Bright' s  disease,"  and  is  found  in  abundance  in  the  vegetable 
kingdom. 

Pure  inosit  forms  large  efflorescent  crystals  (rhombic  tables) ;  in  microscopic 
preparations  it  is  usually  obtained  in  tufted  lumps  of  fine  crystals.  Easily  soluble 
in  water,  it  is  insoluble  in  alcohol  and  ether.  It  possesses  no  action  on  polarized 
light,  and  does  not  reduce  solutions  of  metallic  salts. 

It  admits  of  no  direct  alcoholic,  but  is  capable  of  undergoing  the  lactic,  fermen- 
tation;  according  to  Hilger,4  the  acid  formed  is  sarcolactic.  It  is  unaltered  by 
heating  with  dilute  mineral  acids. 

Preparation.  It  may  be  precipitated  from  its  solutions  by  the  action  of  basic 
lead  acetate  and  ammonia;  the  lead  is  then  removed  by  sulphuretted  hydrogen,  and 
the  inosit  precipitated  with  excess  of  alcohol. 

As  a  special  test  (Scherer's)  may  be  mentioned  the  production  of  a  bright  violet 
color  by  careful  evaporation  to  dryness  on  platinum  foil,  with  a  little  ammonia  and 
calcium  chloride. 

5.  Dextrin.     C6H1005. 

By  boiling  starch-paste  with  dilute  acids,  or  by  the  action  of  ferments,  the  starch 
is  converted  into  an  isomeric  body,  to  which,  from  its  action  on  polarized  light,  the 
name  dextrin  has  been  given.  It  is  soluble  in  water,  but  is  precipitated  by  alcohol. 
It  does  not  undergo  alcoholic  fermentation  until  after  it  has  been  changed  into 
dextrose,  nor  can  it  reduce  metallic  salts.  It  yields  a  reddish  port-wine  color  with 
iodine,  which  disappears  on  warming  and  does  not  return  on  cooling.  Further 
action  of  acids  or  of  ferments  converts  dextrin  into  dextrose.  Dextrin  is  present 
in  the  contents  of  the  alimentary  canal  after  a  meal  containing  starch,  and  has  also 
been  found  in  the  blood. 

There  is  not  the  least  doubt  that  several  modifications  of  dextrin  exist,  and  may 
be  obtained  by  the  action  of  acids  and  ferments  on  starch.  Of  these  two  of  the 

i  Ann.  d.  Chem.  u.  Pharm.,  Bd.  Ixxiii.,  S.  322.  2 ibid.,  Bd.  xcix.,  S.  289. 

s  Ibid.,  Bd.  ciii.,  S.  140.  4  ibid.,  Bd.  clx.,  S.  333. 


886  APPENDIX. 

best  known  are  those  described  by  Briicke l  under  the  name  of  erythrodextrin  and 
achrob'dextrin,  the  former  giving  a  red  color  with  iodine,  the  latter  not  yielding  any 
color  at  all.  Erythrodextrin  may  be  readily  converted  into  a  sugar  by  the  action  of 
ferments,  and  thus  is  not  found  as  a  product  of  the  complete  action  of  ptyalin  on 
starch.  Achropdextrin,  on  the  other  hand,  is  not  thus  converted  by  ferments,  and 
therefore  remains  in  solution,  together  with  the  sugar  formed  by  the  action  of 
ptyalin  on  starch.  Achroodextrin  may  be  converted  into  dextrose  by  boiling  with 
dilute  hydrochloric  acid. 

6.  Glycogen.     C6H1005. 

Belongs  to  the  starch  division  of  carbohydrates.  Discovered  by  Bernard  in  the 
liver  and  other  organs  (see  p.  437). 

Glycogen  is,  when  pure,  an  amorphous  powder,  colorless  and  tasteless,  readily 
soluble  in  water,  insoluble  in  alcohol  and  ether.  Its  aqueous  solution  is  generally, 
though  not  always,  strongly  opalescent,  but  contains  no  particles  visible  microscopi- 
cally ;  the  opalescence  is  much  reduced  by  the  presence  of  free  alkalies.  The  same 
solution  possesses,  according  to  Hoppe-Seyler,  a  very  strong  dextro-rotatory  power, 
about  three  times  as  great  as  that  of  dextrose  ;2  it  dissolves  hydrated  cupric  oxide ; 
but  this  is  not  reduced  on  boiling. 

By  the  action  of  dilute  mineral  acids  (except  nitric)  it  is  partially  converted  into 
a  form  of  sugar  very  closely  resembling,  though  probably  differing  somewhat  from, 
true  dextrose,  and  the  same  conversion  is  also  readily  effected  by  the  action  of  amy- 
lolytic  ferments.  The  sugar  into  which  the  glycogen  of  the  liver  is  naturally  con- 
verted after  death  (see  p.  438)  appears  to  be  true  dextrose;3  so  also  the  sugar  of 
diabetes.  The  result  of  the  action  of  diastase,  or  salivary  or  pancreatic  ferment, 
upon  glycogen  is,  however,  according  to  Musculus  and  v.  Mering,*  a  mixture  of 
achroodextrin  and  maltose ;  the  quantity  of  dextrose  making  its  appearance  at  the 
same  time  being  very  small. 

Opalescent  solutions  of  glycogen  usually  become  clear  on  the  addition  of  caustic 
alkali;  Vintschgau  and  Dietl5  have  shown  that  this  is  accompanied  on  boiling  by 
a  change  which  converts  a  portion  of  the  glycogen  into  a  substance  to  which  they 
gave  the  name  of /3-glycogen- dextrin.  (Kuhne6  had  previously  described  a  body 
to  which  he  gave  the  name  glycogen-dextrin.  That  described  by  Vintschgau  and 
Dietl  differs  slightly  from  Kuhne' s  body,  hence  the  name.  According  to  these 
authors  one-fifth  of  the  glycogen  is  at  the  same  time  changed  into  some  other,  at 
present  undetermined,  substance.  Normal  lead  acetate  gives  a  cloudiness,  the  basic 
salt  a  precipitate,  in  solutions  of  glycogen. 

As  tests  for  this  body  may  be  used  the  formation  of  a  port-wine  color  with 
iodine ;  this  disappears  on  warming,  but  returns  on  cooling.  The  same  color  is 
produced  by  the  action  of  iodine  on  dextrin,  but  this  does  not  reappear  on  cooling 
after  its  disappearance  by  warming. 

Preparation  of  glycogen.  The  following  is  Brii eke' s7  method  :  The  filtered  or 
simply  strained  decoction  of  perfectly  fresh  liver  or  other  glycogenic  tissue  is,  when 
cold,  treated  alternately  with  dilute  hydrochloric  acid  and  a  solution  of  the 
double  iodide  of  potassium  and  mercury 8  as  long  as  any  precipitate  occurs.  In 
the  presence  of  free  hydrochloric  acid  the  double  iodide  precipitates  proteid  mat- 
ters so  completely  as  to  render  their  separation  by  filtration  easy.  The  proteids 
being  thus  got  rid  of,  the  glycogen  is  precipitated  from  the  filtrate  by  adding 
alcohol  to  the  extent  of  between  60  and  70  per  cent.  Too  much  alcohol  is  to  be 
avoided,  since  other  substances  as  well  as  glycogen  are  thereby  precipitated.  The 
glycogen  is  now  washed  with  alcohol,  first  of  60  and  then  of  95  per  cent. ,  after- 
ward with  ether,  and  finally  with  absolute  alcohol.  It  is  then  dried  over  sulphuric 
acid. 

i  Sitzber.  d.  Wien.  Akad.,  1872,  iii.  Abth.    Also,  Vorlesungen,  2  Aufl.,  1875,  Bd.  i.,  S.  224. 

2 See  Kulz,  Pfliiger's  Arch.,  Bd.  xxiv.  (1881),  S.  85. 

3Pniiger's  Arch.,  Bd.  xix.  (1879),  S.  106,  and  xxii.  (1880),  S.  206.  Also,  Kulz,  Ibid.,  Bd.  xxiv. 
(1881),  S.  52. 

^Zeitschr.  f.  physiol.  Chem.,  Bd.  ii.  (1878),  S.  403. 

5  Pfluger's  Arch'.,  Bd.  xvii.  (1878),  S.  154. 

SLehrb.  d.  physiol.  Chem.  (1868).  S.  63. 

*  Sitzungsber.  d.  Wiener  Akad.,  Bd.  Ixiii.  (1871),  ii.  Abth. 

8  This  may  be  prepared  by  precipitating  potassic  iodide  with  mercuric  chloride  and  dissolving1 
the  washed  precipitate  in  a  hot  solution  of  potassic  iodide  as  long  as  it  continues  to  be  taken  up. 
On  cooling,  some  amount  of  precipitate  occurs,  which  must  be  filtered  off;  the  nitrate  is  then 
ready  for  use. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  887 

[7.  Animal  Gum. 

This  carbohydrate  is  found  in  milk  and  urine,  and  is  prepared  by  the  action  of 
superheated  water  on  the  salivary  glands.  J 

8.  Tunicin.     (C6H1005)». 

This  body  is  regarded  by  many  observers  as  identical  with  the  true  cellulose  of 
plants,  while  others  have  ascribed  to  it  properties  differing  from  those  of  cellulose 
sufficiently  to  justify  its  receivings  distinct  name.  It  appears  to  be  more  resistant 
to  the  action  of  chemical  reagents  than  plant  cellulose. 

It  constitutes  the  chief  part  of  the  integument  of  the  ascidia  or  tunicata.  As 
prepared  from  this  source  it  is,  when  pure,  quite  white,  and  usually  retains  the  shape 
of  the  tissue.  It  is  unacted  upon  by  any  reagent  except  strong  acids  and  alkalies, 
and  by  the  action  of  the  former  it  yields  some  form  of  sugar. 

FATS,   THEIR  DERIVATIVES   AND   ALLIES. 

THE  ACETIC  ACID  SERIES. 

General  formula,  CBH2n02  (monobasic). 

This,  which  is  one  of  the  most  complete  homologous  series  of  organic  chemistry, 
runs  parallel  to  the  series  of  monatomic  alcohols.  Thus  formic  acid  corresponds  to 
methyl  alcohol,  acetic  acid  to  ethyl  (ordinary)  alcohol,  and  so  on.  The  several  acids 
may  be  regarded  as  being  derived  from  their  respective  alcohols  by  simple  oxida- 
tion ;  thus  ethyl  alcohol  yields  by  oxidation  acetic  acid  C2H6O  +  02  =  C2H402  + 
H2O.  The  various  members  differ  in  composition  by  CH2,  and  the  boiling-points 
rise  successively  by  about  19°  C.  Similar  relations  hold  good  with  regard  to  their 
melting-points  and  specific  gravities.  The  acid  properties  are  strongest  in  those 
where  n  has  the  least  value.  The  lowest  members  of  the  series  are  volatile  liquids, 
acting  as  powerful  acids;  these  successively  become  less  and  less  fluid,  and  the 
highest  members  are  colorless  solids,  closely  resembling  the  neutral  fats  in  outward 
appearance.  Consecutive  acids  of  the  series  present  but  very  small  differences  of 
chemical  and  physical  properties,  hence  the  difficulty  of  separating  them  ;  this  is 
further  increased  in  the  animal  body  by  the  fact  that  exactly  those  acids  which 
present  the  greatest  similarities  usually  occur  together. 

The  free  acids  are  found  only  in  small  and  very  variable  quantities  in  various 
parts  of  the  body ;  their  derivatives,  on  the  other  hand,  form  most  important  con- 
stituents of  the  human  frame,  and  will  be  considered  further  on. 

Formic  Acid.     CHO.OH. 

When  pure  is  a  strongly  corrosive,  fuming  fluid,  with  powerful  irritating  odor, 
solidifying  at  0°  C.,  boiling  at  100°  C.,  and  capable  of  being  mixed  in  all  propor- 
tions with  water  and  alcohol.  It  has  been  obtained  from  various  parts  of  the  body, 
such  as  the  spleen,  thyrnus,  pancreas,  muscles,  brain,  and  blood ;  in  the  latter  its 
presence  may  be  due  to  the  action  of  acids  on  the  haemoglobin.  According  to  some 
authors,1  it  occurs  only  in  urine. 

Heated  with  sulphuric  acid  it  yields  carbonic  oxide  and  water ;  with  caustic 
potash  it  gives  hydrogen  and  oxalic  acid. 

Acetic  Acid.     C2H3O.OH. 

Is  distinguished  by  its  characteristic  odor;  its  boiling-point  is  117°  C. ;  it  solid- 
ifies at  5°,  and  is  fluid  at  all  temperatures  above  15°  C.  It  is  soluble  in  all  propor- 
tions in  alcohol  and  water. 

It  occurs  in  the  stomach  as  the  result  of  fermentative  changes  in  the  food,  and 
is  frequently  present  in  diabetic  urine.  In  other  organs  and  fluids  it  exists  only  in 
minute  traces. 

With  ferric  chloride  it  yields  a  blood-red  solution,  decolorized  by  hydrochloric  acid.  (It  differs 
in  this  last  reaction  from  sulphocyanide  of  iron.)  Heated  with  alcohol  and  sulphuric  acid,  the 
characteristic  odor  of  acetic  ether  is  obtained.  It  does  not  reduce  silver  nitrate. 

1  Buliginsky,  Hoppe-Seyler's  Med.  chem.  Mittheilung,  Heft  2,  S.  240.  Thudichum,  Journ. 
Chem.  Soc.,  vol.  viii.  p.  400. 


APPENDIX. 

Propionic  Acid.     C3H5O.OH. 

This  acid  closely  resembles  the  preceding  one.  It  possesses  a  very  sour  taste  and 
pungent  odor;  it  is  soluble  in  water,  boils  at  141°  C.,  and  may  be  separated  from 
its  aqueous  solution  by  excess  of  calcic  chloride. 

It  occurs  in  small  quantities  in  sweat,  in  the  contents  of  the  stomach,  and  in 
diabetic  urine  when  undergoing  fermentation.  It  is  similarly  produced,  mixed, 
however,  with  other  products,  during  alcoholic  fermentation,  or  by  the  decompo- 
sition of  glycerin.  It  partially  reduces  silver  nitrate  solution  on  boiling. 

Butyric  Acid.    C4H7O.OH. 

An  oily,  colorless  liquid,  with  an  odor  of  rancid  butter,  soluble  in  water,  alcohol, 
and  ether,  boiling  at  162°  C.  Calcic  chloride  separates  it  from  its  aqueous  solution. 

Found  in  sweat,  the  contents  of  the  large  intestine,  feces,  and  in  urine.  It  oc- 
curs in  traces  in  many  other  fluids,  and  is  plentifully  obtained  when  diabetic  urine  is 
mixed  with  powdered  chalk  and  kept  at  a  temperature  of  35°  C.  It  exists  as  a 
neutral  fat  in  small  quantities  in  milk. 

Valerianic  Acid.     C5H9O.OH. 

An  oily  liquid,  of  penetrating  odor  and  burning  taste ;  soluble  in  30  parts  of 
water  at  12°  C.  ;  readily  soluble  in  alcohol  and  ether.  Boils  at  175°  C. ;  possesses, 
in  free  and  combined  form,  a  feeble  right-handed  rotation  of  the  plane  of  polariza- 
tion. 

It  is  found  in  the  solid  excrements,  and  is  formed  readily  by  the  decomposition, 
through  putrefaction,  of  impure  leucin,  ammonia  being  at  the  same  time  evolved ; 
hence  its  occurrence  in  urine  when  that  fluid  contains  leucin,  as  in  cases  of  acute 
atrophy  of  the  liver. 

Caproic  Acid.  C6HnO.OH.. 

Caprylic  Acid.  C8H15O.OH. 

Capric  (Rutic)  Acid.     C10H19O.OH. 

These  three  occur  together  (as  fats)  in  butter  and  are  contained  in  varying  pro- 
portions in  the  feces  from  a  meat  diet.  The  first  is  an  oily  fluid,  slightly  soluble  in 
water ;  the  others  are  solids  and  scarcely  soluble  in  water ;  they  are  soluble  in  all 
proportions  in  alcohol  and  ether.  They  may  be  prepared  from  butter,  and  sepa- 
rated by  the  varying  solubilities  of  their  barium  salts. 

Laurostearic  Acid.     C12H.,3O.OH. 
Myristic  Acid.  CUH27O.OH. 

These  occur  as  neutral  fat  in  spermaceti,  in  butter,  and  other  fats.  They  pre- 
sent no  points  of  interest. 

Palmitic  Acid.    C16H310  OH. 
Stearic  Acid.       C]8H35O.OH. 

These  are  solid,  colorless  when  pure,  tasteless,  odorless  crystalline  bodies,  the 
former  melting  at  62°  C.,  the  latter  at  69.2°  C.  In  water  they  are  quite  insoluble  ; 
palmitic  acid  is  more  readily  soluble  in  cold  alcohol  than  stearic ;  both  are  readily 
dissolved  by  hot  alcohol,  ether,  or  chloroform.  Glacial  acetic  acid  dissolves  them 
in  large  quantity,  the  solution  being  assisted  by  warming.  They  readily  form  soaps 
with  the  alkalies,  also  with  many  other  metals.  The  varying  solubilities  of  their 
barium  salts  aiford  the  means  of  separating  them  when  mixed  ; l  this  may  also  be 
applied  to  many  others  of  the  higher  members  of  this  series. 

These  acids  in  combination  with  glycerin  (see  below),  together  with  the  analo- 
gous compound  of  oleic  acid,  form  the  principal  constituents  of  human  fat.  As 
salts  of  calcium  they  occur  in  the  feces  and  in  "  adipocere."  and  probably  in  chyle, 
blood,  and  serous  fluids,  as  salts  of  sodium.  They  are  found  in  the  free  state  in 
decomposing  pus,  and  in  caseous  deposits  of  tuberculosis. 

The  existence  of  margaric  acid  intermediate  to  the  above  two  is  not  now  admitted,  since 
Heintz2  has  shown  that  it  is  really  a  mixture  of  palmitic  and  stearic  acids.  Margaric  acid  pos- 
sesses die  anomalous  melting-point  of  59.9°  C  A  mixture  of  60  parts  stearic  and  40  of  palmitic 
acid  melts  at  60.3° 

1  Heintz,  Annal.  d.  Phys.  u.  Chem.,  Bd.  xclii.,  S.  588.  8  Op.  cit. 


CHEMICAL  BASIS  OF  THE  ANIMAL   BODY.  889 

ACIDS  OF  THE  OLEIC  (ACRYLIC)  SERIES.     H(CnH27l-3)02  (monobasic). 

Many  acids  of  this  series  occur  as  glycerin  compounds  in  various  fats.  They  are 
very  unstable  and  readily  absorb  oxygen  when  exposed  to  the  air.  The  higher 
members  are  decomposed  on  attempting  to  distil  them.  Their  most  peculiar  prop- 
erty is  that  of  being  converted  by  traces  of  N02  into  solid,  stable  metameric  acids 
capable  of  being  distilled.  They  bear  an  interesting  relation  to  the  acids  of  the 
acetic  series,  breaking  up  when  heated  with  caustic  potash  into  acetic  acid  and 
some  other  member  of  the  same  series,  thus: 

Oleic  acid.  Potassic  acetate.        Potassic  palmitate 

HC18H3302  +  2KHO  =  KC2H302  +  KC16H3102  +  H2. 
Oleic  Acid.     C18H330.  OH. 

This  is  the  only  acid  of  the  series  which  is  phj^siologically  important.  It  is  found 
united  with  glycerin  in  all  the  fats  of  the  human  body. 

When  pure  it  is,  at  ordinary  temperatures,  a  colorless,  odorless,  tasteless,  oily 
liquid,  solidifying  at  4°  C.  to  a  crystalline  mass.  Insoluble  in  water,  it  is  soluble 
in  alcohol  and  ether.  It  cannot  be  distilled  without  decomposition.  It  readily 
forms  soaps  with  potassium  and  sodium,  which  are  soluble  in  water;  its  com- 
pounds with  most  other  bases  are  insoluble.  It  may  be  distinguished  from  the 
acids  of  the  acetic  series  by  its  reaction  with  N02  and  by  the  changes  it  under- 
goes when  exposed  to  the  air. 

THE  NEUTRAL  FATS. 

These  may  be  considered  as  ethers  formed  by  replacing  the  exchangeable  atoms 
of  hydrogen  in  the  triatomic  alcohol  glycerin  (see  below)  by  the  acid  radicles  of  the 
acetic  and  oleic  series.  Since  there  are  three  such  exchangeable  atoms  of  hydrogen 
in  glycerin,  it  is  possible  to  form  three  classes  of  these  ethers;  only  those,  however, 
which  belong  to  the  third  .class  occur  as  natural  constituents  of  the  human  body  : 
those  of  the  first  and  second  are  of  theoretical  importance  only. 

They  possess  certain  general  characteristics.  Insoluble  in  water  and  cold  alcohol, 
they,  are  readily  soluble  in  hot  alcohol,  ether,  chloroform,  etc.  ;  they  also  dissolve 
one  another.  They  are  neutral  bodies,  colorless  and  tasteless  when  pure,  are  not 
capable  of  being  distilled  without  undergoing  decomposition,  and  yield  as  a  result 
of  this  decomposition  solid  and  liquid  hydrocarbons,  water,  fatty  acids,  and  a  pecu- 
liar body,  acrolein.  (Glycerin  contains  the  elements  of  one  molecule  of  acrolein  and 
two  molecules  of  water.) 

They  possess  no  action  on  polarized  light, 

They  may  readily  be  decomposed  into  glycerin  and  their  respective  fatty  acids  by 
the  action  of  caustic  alkalies  or  of  superheated  steam. 

Palmitin   (Tri-palmitin).       JCAH^    j  03. 

V^16n36Uj3  j 

The  following  reaction  for  the  formation  of  this  fat  is  typical  for  all  the  others  : 
Glycerin.  Palmitic  acid.  Palmitin. 

0,  +  3°''°     0  =,<  02  +  3        0. 


Palmitin  is  slightly  soluble  in  cold  alcohol,  readily  so  in  hot  alcohol,  or  in  ether  ; 
when  pure  it  crystallizes  in  fine  needles  ;  if  mixed  with  stearin,  it  generally  forms 
shapeless  lumps,  although  the  mixture  may  at  times  assume  a  crystalline  form,  and 
was  then  regarded  as  a  distinct  body,  namely,  margarin.  It  possesses  three  differ- 
ent melting-points,  according  to  the  previous  temperatures  to  which  it  has  been 
subjected.  It  solidifies  in  all  cases  at  45°  C. 

Preparation.  From  palm  oil,  by  removing  the  free  palmitic  acid  with  alcohol 
and  crystallizing  repeatedly  from  ether. 


Stearin  (Tri-stearin).     CCi^W))a  I  ^ 


This  is  the  hardest  and  least  fusible  of  the  ordinary  fats  of  the  body,  is  also  the 
least  soluble,  and  hence  is  the  first  to  crystallize  out  from  solutions  of  the  mixed 


890  APPENDIX. 

fats.     It  crystallizes  usually  in  square  tables.     It  presents  peculiarities  in  its  fusing- 
points  similar  to  those  of  palmitin. 

Preparation.  From  mutton  suet,  its  separation  from  palmitin  and  olein  being 
effected  by  repeated  crystallization  from  ether,  stearin  being  the  least  soluble. 

Olein  (Tri-olein).     ^ffj}^1  J  03. 

It  is  obtained  with  difficulty  in  the  pure  state,  and  is  then  fluid  at  ordinary 
temperatures.  It  is  more  soluble  than  the  two  preceding  ones.  It  readily  under- 
goes oxidation  when  exposed  to  the  air,  and  is  converted  by  mere  traces  of  N02 
into  a  solid  isomeric  fat.  Olein  yields,  on  dry  distillation,  a  characteristic  acid, 
the  sebacic,  and  is  saponified  with  much  greater  difficulty  than  are  palmitin  and 
stearin. 

Preparation.  From  olive  oil,  either  by  cooling  to  0°  C.  and  pressing  out  the 
olein  that  remains  fluid,  or  by  dissolving  in  alcohol  and  cooling,  when  the  olein 
remains  in  solution  while  the  other  fats  crystallize  out. 

Glycerin.     C|j*5 1  03. 

This  principal  constituent  of  the  neutral  fats  may,  as  above  stated,  be  looked 
upon  as  a  triatomic  alcohol. 

When  pure,  glycerin  is  a  viscid,  colorless  liquid,  of  a  well-known  sweet  taste.  It 
is  soluble  in  water  and  alcohol  in  all  proportions,  insoluble  in  ether.  Exposed  to 
very  low  temperature  it  becomes  almost  solid ;  it  may  be  distilled  in  close  vessels 
without  decomposition,  between  275°-280°  C. 

It  dissolves  the  alkalies  and  alkaline  earths,  also  many  oxides,  such  as  those  of 
lead  and  copper ;  many  of  the  fatty  acids  are  also  soluble  in  glycerin. 

It  possesses  no  rotatory  power  on  polarized  light. 

It  is  easily  recognized  by  its  ready  solubility  in  water  and  alcohol,  its  insolubility 
in  ether,  its  sweet  taste,  and  its  reaction  with  bases.  The  production  of  acrolein  is 
also  characteristic  of  glycerin. 

C3H8O3  —  2H20  --=  C3H4O  (Acrolein). 

Preparation.  By  saponification  of  the  various  oils  and  fats.  It  is  also  formed 
in  small  quantities  during  the  alcoholic  fermentation  of  sugar. 1 

Soaps.  These  may  be  formed  by  the  action  of  caustic  alkalies  on  fats.  The 
process  consists  in  a  substitution  of  the  alkali  for  the  radicle  of  glycerin,  the  latter 
combining  with  the  elements  of  water  to  form  glycerin.  Thus : 

Tristearin.                       Potassic  stearate.  Glycerin. 

/r\    TT    r\\    •»                  T7"  )  r\    TT    f\  )  f^  TT    i 

\v-'18ll35^-' /3   [    i~\      |     o-1-*-   [     f\  Q^18n35'~'   I  C\  _L  ^3AA5   (    f\ 

C H       I     3       H  I      —        K  "   H          3< 

Pancreatic  juice  can  split  up  fats  into  glycerin  and  free  fatty  acids,  and  the  bile  is  known  to 
be  capable  of  saponifying  these  fatty  acids/  The  amount  of  soaps  formed  in  the  alimentary 
canal  is,  however,  small  and  unimportant. 


ACIDS  OF  THE  GLYCOLIC  SERIES. 

Running  parallel  to  the  monatomic  alcohols  (GJBL»+*0)  is  the  series  of  diatomic 
alcohols  or  glycols  (CnHw+202).  Thus  corresponding  to  ethyl  alcohol  is  the  dia- 
tomic alcohol,  ethyl-glycol.  As  from  the  monatomic  alcohols,  so  from  the  glycols, 
acids  may  be  derived  by  oxidation  ;  from  the  latter  (glycols),  however,  two  series 
of  acids  can  be  obtained,  known  respectively  as  the  glycolic  and  oxalic  series. 
The  first  stage  of  oxidation  of  the  glycol  gives  a  member  of  the  glycolic  series. 
Thus: 

Ethyl-glycol.  Glycolic  acid. 

C2H602  +  02  =  C2H4O3  +  H.,0,  or  more  generally 
CnH2M+  ,0,  +  O.2  -  CnH2n08  +  H20. 

1  Pasteur,  Ann.  d.  Chem.  u.  Pharm.,  Bd.  cvi.,  S.  338. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY. 


891 


By  further  oxidation  a  member  of  the  glycolic  series  can  be  converted  into  a 
member  of  the  oxalic  series.  Thus : 

.    Glycolic  acid.  Oxalic  acid. 

C2H4O3  +  02  =  C2H204  +  H20,  or  more  generally 
CnH2w03  +  02  =  CnH2n-204  +  H20. 

The  acids  of  the  glycolic  series  are  diatomic  but  monobasic;  but  those  of  the 
oxalic  series  are  diatomic  and  dibasic. 

The  following  table  may  be  given  to  show  the  general  rejationship  of  alcohols 
and  acids : 


Radicle. 

Alcohol. 

Acid. 

Glycols. 

Acid  I. 

Acid  II. 

Formic. 

Carbonic. 

Methyl  (CH3) 

CH3(OH) 

HCH02 

H  CO, 

Ethyl   (C2H5) 

C2H5(OH) 

Acetic. 
HC2H302 

Ethyl-glycol. 
C2H4(OH)2 

Glycolic. 
HC2HS0, 

Oxalic. 
H2C404 

Propyl  (C3H7) 

C3H7(OH) 

Propionic. 
HCSH502 

Propyl-glycol. 
C3H6(OH)2 

Lactic. 
HC8H5QS 

Malonic. 
H2C3H204 

Butyric. 

Butyl-glycol. 

Oxybutyric. 

Succinic. 

Butyl   (C4H9) 

cjyoH) 

HC4H702 

I  C4H8(OH)2 

CH5H703 

H2C4H404 

GLYCOLIC  ACID  SERIES. 
Lactic  Acid.     C3H603. 

Next  to  carbonic  acid,  the  most  important  member  of  this  series,  as  far  as  physi- 
ology is  concerned,  is  lactic  acid. 

Lactic  acid  exists  in  four  isomeric  modifications,  but  of  these  only  three  have 
been  found  in  the  human  body.  These  three  all  form  syrupy,  colorless  fluids,  solu- 
ble in  all  proportions  in  water,  alcohol,  and  ether.  They  possess  an  intensely  sour 
taste,  and  a  strong  acid  reaction.  When  heated  in  solution  they  are  partially  dis- 
tilled over  in  the  escaping  vapor.  They  form  salts  with  metals,  of  which  those 
with  the  alkalies  are  very  soluble  and  crystallize  with  difficulty.  The  calcium  and 
zinc  salts  are  of  the  greatest  importance,  as  will  be  seen  later  on. 

1.  Ethyli dene-lactic  acid.    This  is  the  ordinary  form  of  the  acid,  obtained  as 
the  characteristic  product  of  the  well-known  kt  lactic  fermentation."     It  occurs  in 
the  contents  of  the  stomach  and  intestines.     According  to  Heintz.1  it  is  found  also 
in  muscles,  and  according  to  Gscheidlen 2  in  the  ganglionic  cells  of  the  gray  sub- 
stance of  the  brain.     In  many  diseases  it  is  found  in  urine,  and  exists  in  a  large 
amount  in  this  excretion  after  poisoning  by  phosphorus.3 

It  may  be  prepared  by  the  general  methods  of  slowly  oxidizing  the  corresponding  glycol  or  by 
acting  on  monochlorinated  propionic  acid  with  moist  silver  oxide.  In  obtaining  it  from  the  pro- 
ducts 9f  lactic  fermentation,  the  crusts  of  zinc  lactate  are  purified  by  several  crystallizations,  and 
the  acid  liberated  from  the  compounds  by  the  action  of  sulphuretted  hydrogen. 

2.  Ethylene-lactic  acid.    This  acid  is  found  accompanying  the  next  to  be 
described,  in  the  watery  extract  of  muscles.4    From  this  it  is  separated  by  taking 
advantage  of  the  different  solubilities  in  alcohol  of  the  zinc  salts  of  the  two  acids. 
It  seems  probable,  however,  that  it  has  not  yet  been  prepared  in  the  pure  state  by 
this  method. 

Wislicenus  first  obtained  this  acid  by  heating  hydroxycvanide  of  ethylene  with  aqueous  solu- 
tions of  the  alkalies. 

The  same  observer  found  it  also  in  many  pathological  fluids. 

3.  Sarcolactic  acid.     This  acid  has  not  yet  been  procured  synthetically.     As 
its  name  implies,  it  is  that  form  of  the  acid  which  chiefly  occurs  in  muscles,  and 


1  Ann.  d.  Chem.  u.  Pharm.,  Bd.  clvii.,  S.  320. 

'•>  Pflxiger's  Archiv.  Bd.  viii.  (1873-74),  S.  171. 

3  Schultzen  and  Riess,  Ueber  acute  Phosphorvergiftung. 

*  Ann.  d.  Chem.  u.  Pharm..  Bd.  cxxviii.,  S.  6. 


Chem.  Centralb.,  1869,  8.  681. 


892 


APPENDIX. 


hence  exists  in  large  quantities  in  Liebig's  "extract  of  meat."  It  is  often  found 
also  in  pathological"  fluids.  This  is  the  only  acid  of  the  series  which  possesses  any 
power  of  rotating  the  plane  of  polarized  light ;  it  is  otherwise  indistinguishable 
from  the  preceding  ethylidene-lactic  acid,  and  is  generally  represented  by  the  same 
formula.  The  free  acid  has  dextro-,  the  anhydride  Isevo-rotatory  action.  The 
specific  rotation  for  the  zinc  salt  in  solution  is  — 7. 65°  for  yellow  light. 


[FIG.  231. 


[FiG.  232. 


Zinc  Sarcolactate.    (After  Kiihne.)] 


Calcium  Sarcolactate.    (After  Kiihne.)] 


The  zinc  and  calcium  salts  [Figs.  231 ,  232]  for  sarcolactic  acid  are  more  soluble, 
both  in  water  and  alcohol,  than  those  of  ethylidene-lactic  acid,  but  less  so  than 
those  of  ethylene  lactic  acid,  and  the  same  salts  of  ethylene-lactic  acid  contain 
more  water  of  crystallization  than  those  of  the  other  two. 

Heintz1  has  compared  the  above  acids  to  the  modifications  capable  of  existing  in  tartaric 
acid.2 

Hydracrylic  acid,  the  fourth  in  this  series  of  lactic  acids,  is  distinguished  by  the  nature  of  its 
decomposition  on  heating.  It  is  never  found  as  a  constituent  of  animal  bodies. 


OXALIC  ACID  SERIES. 


Oxalic  Acid.     H2C204. 


In  ^he  free  state  this  acid  does  not  occur  in  the  human  body.     Calcic  oxalate, 
however,  is  a  not  unfrequent  constituent  of  urine,  and  enters  into  the  composition 

[FIG.  233. 


Calcium  Oxalate.] 

of  many  urinary  calculi,  the  so-called  mulberry  calculus  consisting  almost  entirely 
of  it.    It  may  occur  in  feces,  and  in  the  gall-bladder,  though  this  is  rarely  observed. 

1  Op.  cit. 

-  See,  further,  Wislicenus,  op.  cit.    Also  Ann.  d.  Chem.  u.  Pharm.,  Bd.  clxvi.,  S.  3;  Bd.  clxvii., 
S.  302,  and  Zeitschr.  f.  Chem.,  Bd.  xiii.,  S.  159. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  893 

As  ordinarily  precipitated  from  solutions  of  calcic  salts  by  ammonic  oxalate, 
calcic  oxalate  is  quite  amorphous,  but  in  urinary  deposits  it  assumes  a  strong  cha- 
racteristic crystalline  form,  viz.,  that  of  rectangular  octohedra  [Fig.  233].  In  some 
cases  it  presents  the  anomalous  forms  of  rounded  lumps,  dumb-bells,  or  square 
columns  with  pyramidal  ends.  It  is  insoluble  in  water,  alcohol,  and  ether,  also  in 
ammonia  and  acetic  acid.  Mineral  acids  dissolve  this  salt  readily,  as  also  to  a  smaller 
extent  do  solutions  of  sodic  phosphate  or  urate.  All  the  above  characteristics  serve 
to  detect  this  salt ;  its  microscopical  appearance,  however,  is  generally  of  most  use 
for  this  purpose. 

The  pure  acid  is  prepared  either  by  oxidizing  sugar  with  nitric  acid,  or  decom- 
posing ligneous  tissue  with  caustic  alkalies. 

Succinic  Acid.    H2C4H404. 

This  is  the  third  acid  of  the  oxalic  series,  being  separated  from  oxalic  acid  by 
the  intermediate  malonic  acid,  H.2C3H204.  It  occurs  in  the  spleen,  the  thymus, 
and  thyroid  bodies,  hydrocephalic  and  hydrocele  fluids. 

According  to  Meissner  and  Shepard l  it  is  found  as  a  normal  constituent  of  urine.  This  is  con- 
tested by  Salkowski,2  and  also  by  v.  Speyer.  It  seems  probable,  however,  that  since  wines  and 
fermented  liquors  contain  succinic  acid,  and  this  latter  passes  unchanged  into  the  urine,  that  it 
may  thus  be  occasionally  present  in  this  excretion. 

Succinic  acid  crystallizes  in  large  rhombic  tables,  also  at  times  in  the  form  of 
large  prisms;  they  are  soluble  in  5  parts  of  cold  water,  and  2.2  of  boiling,  slightly 
soluble  in  alcohol,  and  almost  insoluble  in  ether.  The  crystals  melt  at  180°  C., 
and  boil  at  236°  C. ,  being  at  the  same  time  decomposed  into  the  anhydride  and 
water.  The  alkali  salts  of  this  acid  are  soluble  in  water,  insoluble  in  alcohol  and 
ether.  • 

Preparation.  Apart  from  the  synthetic  methods,  it  may  readily  be  obtained  by 
the  fermentation  of  calcic  malate,  acetic  acid  being  produced  simultaneously. 

Its  presence  is  recognized  by  the  microscopic  examination  of  its  crystals,  and 
its  characteristic  reaction  with  normal  lead  acetate.  With  this  it  gives  a  precip- 
itate, easily  soluble  in  excess  of  the  precipitant,  but  coming,  down  again  on  warm- 
ing and  shaking.3 

CHOLESTERIN.     (C26H440.) 

This  is  the  only  alcohol  which  occurs  in  the  human  body  in  the  free  state. 
(The  triatomic  alcohol  glycerin  is  almost  always  found  combined  as  in  the  fats ; 
and  cetyl-alcohol,  or  sethal,  is  obtained  only  from  spermaceti. )  It  is  a  white  crys- 
talline body,  crystallizing  in  fine  needles  from  its  solution  in  ether,  chloroform,  or 
benzol ;  from  its  hot  alcoholic  solutions  it  is  deposited  on  cooling  in  rhombic 
tables  [Fig.  234].  When  dried  it  melts  at  145°  C.,  and  distils  in  closed  vessels  at 
360°  C.  It  is  quite  insoluble  in  water  and  cold  alcohol :  soluble  in  solutions  of  bile 
salts- 
Solutions  of  cholesterin  possess  a  left-handed  rotatory  action  on  polarized  light, 
of —32°  for  yellow  light,  this  being  independent  of  concentration  and  of  the  nature 
of  the  solvent. 

Heated  with  strong  sulphuric  acid  it  yields  a  hydrocarbon ;  with  concentrated 
nitric  it  gives  cholesteric  acid  and  other  products.  It  is  capable  of  uniting  with 
acids  and  forming  compound  ethers. 

Cholesterin  occurs  in  small  quantities  in  the  blood  and  many  tissues,  and  is  pres- 
ent in  abundance  in  the  white  matter  of  the  cerebro-spinal  axis  and  in  nerves. 

It  is  a  constant  constituent  .of  bile,  forming  frequently  nearly  the  whole  mass  of 
some  gall-stones.  It  is  found  in  many  pathological  fluids,  hydrocele,  the  fluid  of 
ovarian  cysts,  etc. 

Preparation.  From  gall-stones  by  simple  extraction  with  boiling  alcohol,  and 
treatment  with  alcoholic  potash  to  free  from  extraneous  matter. 

As  tests  for  this  substance  may  be  given :  With  concentrated  sulphuric  acid  and 
a  little  iodine  a  violet  color  is  obtained,  changing  through  green  to  red  or  blue. 
This  is  applicable  to  the  microscopic  crystals.  After  dissolving  in  chloroform  a 
blood-red  solution  is  formed  on  the  addition  of  an  equal  volume  of  concentrated 

1  Untersuch,  viber  d.  Entsteh.  d.  Hippursaiire.    Hanover,  1866. 

2  Pfliiger's  Archiv,  Bd.  ii.  (1869),  S.  367,  and  Bd.  iv.  (1871),  S.  95. 

8  For  further  particulars  see  Meissner,  op.  cit.,  and  Meissner  and  Solly,  Zeitschr  f.  rat.  (Med.  3). 
B.  xxiv.,  S.  97. 


894 


APPENDIX. 


sulphuric  acid ;   this  solution  if  exposed  to  the  air  in  an  open  dish  turns  blue, 
green,  and  finally  yellow ;   the  sulphuric  acid  under  the  chloroform  has  a  green 

[FiG.  234. 


Cholesterin  Crystals  and  Fatty  Aggregations  and  Molecules  Spontaneously 
Deposited  in  the  Urine.] 

fluorescence.     After  evaporation  to  dryness  with  nitric  acid,  the  residue  turns  red 
on  treating  with  ammonia. 

This  body  is  described  here  rather  for  the  sake  of  convenience  than  from  its  possessing  any 
close  relationship  to  the  substances  immediately  preceding. 


COMPLEX  NITROGENOUS  FATS. 
Lecithin.     C44H90NP09. 

Occurs  widely  spread  throughout  the  body.  Blood,  bile,  and  serous  fluid  con- 
tain it  in  small  quantities,  while  it  is  a  conspicuous  component  of  the  brain,  nerves, 
yolk  of  egg,  semen,  pus,  white  blood-corpuscles,  and  the  electrical  organs  of  the  ray. 

When  pure  it  is  a  colorless,  slightly  crystalline  substance,  which  can  be  kneaded, 
but  often  crumbles  during  the  process.  It  is  readily  soluble  in  cold,  exceedingly  so 
in  hot  alcohol  ;  ether  dissolves  it  freely  though  in  less  quantities,  as  also  do  chloro- 
form, fats,  benzol,  carbon  disulphide,  etc.  It  is  often  obtained  from  its  alcoholic 
solution  by  evaporation,  in  the  form  of  oily  drops.  It  swells  up  in  water  and  in 
this  state  yields  a  flocculent  precipitate  with  sodium  chloride. 

Lecithin  is  easily  decomposed  ;  not  only  does  this  decomposition  set  in  at  70°  C., 
but  the  solutions,  if  merely  allowed  to  stand  at  the  ordinary  temperature,  acquire 
an  acid  reaction,  and  the  substance  is  decomposed.  Acids  and  alkalies,  of  course, 
effect  this  much  more  rapidly.  If  heated  with  baryta  water  it  is  completely  decom- 
posed, the  products  being  neurin,  glycerin-phosphoric  acid,  and  baric  stearate.  This 
may  be  thus  represented  : 


Lecithin. 


Stearicacid. 


Glycerin^phosphoric 

C3H9P06    + 


Neurin> 

C5H15N02. 


When  treated  in  an  ethereal  solution  with  dilute  sulphuric  acid,  it  is  merely  split 
up  into  neurin  and  distearyl-glycerin-phosphoric  acid.  Hence,  Diakonow  1  regards 
lecithin  as  the  distearyl-glycerin-phosphate  of  neurin,  two  atoms  of  hydrogen  in  the 


1  Hoppe-Seyler's  Med.-Chem.  Untersuch.,  Heft  ii.  (1867),  S.  221 ;  Heft  iii. 
f.  d.  med.  Wiss.  (1868),  Nr.  1,  7,  u.  28. 


S.  405.    Centralbl. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  895 

glycerin-phosphoric  acid  being  replaced  by  the  radicle  of  stearic  acid.  It  appears 
also  that  there  probably  exist  other  analogous  compounds  in  which  the  radicles  of 
oleic  and  palmitic  acids  take  part. 

Preparation.  Usually  from  the  yolk  of  egg,  where  it  occurs  in  union  with 
vitellin.  Its  isolation  is  complicated,  and  the  reader  is  referred  to  Hoppe-Seyler. l 

Glycerin -phosphoric  Acid.     C3H9P06. 

Occurs  as  a  product  of  the  decomposition  of  lecithin,  and  hence  is  found  in  those 
tissues  and  fluids  in  which  this  latter  is  present ;  in  leukaemia  the  urine  is  said  to 
contain  this  substance.  It  has  not  been  obtained  in  the  solid  form.  It  has  been 
produced  synthetically  by  heating  glycerin  and  glacial  phosphoric  acid ;  it  may  be 
regarded  as  formed  by  the  union  of  one  molecule  of  glycerin  with  one  of  phosphoric 
acid,  with  elimination  of  one  molecule  of  water.  It  is  a  dibasic  acid  ;  its  salts  with 
barium  and  calcium  are  insoluble  in  alcohol,  soluble  in  cold  water.  Solutions  of  its 
salts  are  precipitated  by  lead  acetate. 

Protagon.    (C160H308N5P035?) 

A  crystalline  body  containing  nitrogen  and  phosphorus,  obtained  by  Liebreich 2 
from  the  brain  substance  and  regarded  by  him  as  its  principal  constituent.  The 
researches  of  Hoppe-Seyler  and  Diakonow  tended  to  show  that  protagon  was 
merely  a  mixture  of  lecithin  and  cerebrin.  A  repetition  of  Liebreich's  experiments 
has,  liowever,  led  Gamgee  and  Blankenhorn 3  to  confirm  the  truth  of  his  results. 
Protagon  appears  to  separate  out  from  warm  alcohol  on  gradual  cooling  in  the  form 
of  very  small  needles,  often  arranged  in  groups ;  it  is  slightly  soluble  in  cold,  more 
soluble  in  hot  alcohol,  and  ether.  It  is  insoluble  in  water,  but  swells  up  and  forms 
a  gelatinous  mass.  It  melts  at  200°  C. ,  and  forms  a  brown  syrupy  fluid. 

Preparation.  Finely  divided  brain  substance,  freed  from  blood  and  connective 
tissue,  is  digested  at  45°  C.  with  alcohol  (85  percent.)  as  long  as  the  alcohol  extracts 
anything  from  it.  The  protagon  which  separates  out  from  the  filtrate  is  well 
washed  with  ether  to  get  rid  of  all  cholesterin  and  other  bodies  soluble  in  ether, 
and  finally  purified  by  repeated  crystallization  from  warm  alcohol. 

Neurin  (Cholin).     C3H15N02. 

Discovered  by  Strecker4  in  pig's  gall,  then  in  ox-gall.  It  does  not  occur  in  the 
free  state  except  as  a  product  of  the  decomposition  of  lecithin.  It  is  a  colorless 
fluid,  of  oily  consistence,  possesses  a  strong  alkaline  reaction,  and  forms  with  acids 
very  deliquescent  salts.  The  salts  with  hydrochloric  acid  and  the  chlorides  of 
platinum  and  gold  are  the  most  important. 

Neurin  is  a  most  unstable  body,  mere  heating  of  its  aqueous  solution  sufficing  to 
split  it  up  into  glycol,  trimethylamin  and  ethylene  oxide. 

Preparation.     From  yoke  of  egg.     For  this  see  Diakonow.5 

Wurtze  has  obtained  it  synthetically,  first  by  the  action  of  glycol  hydrochloride  on  trimethyl- 
amin, and  then  by  that  of  ethylene  oxide  and  water  on  the  same  substance.  The  above,  together 
with  the  mode  of  its  decomposition,  point  to  the  idea  that  neurin  may  be  regarded  as  trimethyl- 
oxyethyl-ammonium  hydrate,  N(CH3)3(C2H5O)OH. 

Cerebrin.     C17H33N03  (?). 

Is  found  in  the  axis-cylinder  of  nerves,  in  pus-corpuscles,  and  largely  in  the 
brain.  In  former  times  many  names  were  given  to  the  substance  when  in  an 
impure  state,  e.  </.,  cerebric  acid,  cerebrote,  etc.  W.  Miiller7  first  prepared  it  in 
the  pure  form,  and  constructed  the  above  formula  from  his  analysis ;  the  mean  of 
these  is  0,  15.85  ;  H,  11.2  ;  N,  4.5  ;  C,  68.45.  Great  doubts  are,  however,  thrown 
upon  its  purity  by  the  researches  of  later  observers.  According  to  Liebreich 8  and 
Diakonow,9  it  is  a  glucoside.10 

1  Med.-Chem.  Untersuch.,  Heft  ii.  (1867),  S.  215. 

2  Ann.  d.  Chem.  \i.  Pharm.,  Bd.  cxxxiv.,  S.  29. 

3Zeitschr.  f.  physiol.  Chem.,  Bd.  iii.  (1879),  S.  260,  and  Journ.  of  Physiol.,  vol.  ii.  (1879),  p.  113. 
*  Ann.  d.  Chem.  u.  Pharm.,  Bd.  cxxiii.,  S.  353 ;  Bd.  cxlviii.,  S.  76.    ' 

6  Op.  cit.  (sub.  Lecithin).  «  Ann.  d.  Chem.  u.  Pharm.,  Sup.  Bd.  vi.,  S.  116  u.  127. 

7  Ann.  d.  Chem.  u.  Pharm.,  Bd.  cv.,  S.  361. 

8  Arch.  f.  pathoi.  Anat.,  Bd.  xxxix.  (1867). 
"Centrallb.  f.  d.  med.  Wiss.,  1868,  No.  7. 

10 See  also  Geogheghan,  Zeitschr.  f.  physiol.  Chem.,  Bd.  iii.  (1879),  S.  332. 


896 


APPENDIX. 


Cerebrin  is  a  light,  colorless,  exceedingly  hygroscopic  powder,  which  swells  up 
strongly  in  water,  slowly  in  the  cold,  rapidly  on  heating.  When  heated  to  80°  0. 
it  turns  brown,  and  at  a  somewhat  higher  temperature  melts,  bubbles  up,  and 
finally  burns  away.  It  is  insoluble  in  cold  alcohol  or  ether  ;  warm  alcohol  dissolves 
it  easily.  Heated  with  dilute  mineral  acids,  cerebrin  yields  a  sugar-like  body,  pos- 
sessing left-handed  rotation,  but  incapable  of  fermentation. 

Preparation.     For  this  see  W.  Miiller. l 

[CHARCOT'S  CRYSTALS.  These  crystals  [Fig.  235],  first  discovered  by  Charcot, 
have  been  obtained  from  the  semen,  the  blood  of  leukaemics,  the  expectoration  of 

[FiG.  235. 


Charcot's  Crystals.] 


asthmatics,  and  the  various  tissues, 
azine,  as  was  supposed.  J 


It  is  not  identical  with  ethylinimme  or  piper- 


NITROGENOUS  METABOLITES. 
THE  UREA  GROUP,  AMIDES,  AND  SIMILAR  BODIES. 
Urea.     (NH2)2CO. 

The  chief  constituent  of  normal  urine  in  mammalia  and  some  other  animals ; 
the  urine  of  birds  also  contains  a  small  amount.  Normal  blood,  serous  fluids, 
lymph,  and  the  liver  all  contain  the  same  body  in  traces.  It  is  not  found  in  the 
muscles  as  a  normal  constituent,  but  may  make  its  appearance  there  under  certain 
pathological  conditions. 

[FIG.  236. 


Urea  Crystals  separated  by  slow  evaporation  from  Aqueous  Solution.    (After  Funke.)] 

When  pure  it  crystallizes  from  a  concentrated  solution  in  the  form  of  long,  thin, 
glittering  needles.  [Fig.  236.]  If  deposited  slowly  from  dilute  solutions,  the  form 
is  that  of  four-sided  prisms  with  pyramidal  ends ;  these  are  always  anhydrous.  It 
possesses  a  somewhat  bitter  cooling  taste,  like  saltpetre.  It  is  readily  soluble  in 
water  and  alcohol,  the  solutions  being  neutral.  In  anhydrous  ether  it  is  insoluble. 

i  Op.  cit. 


CHEMICAL   BASIS   OF  THE  ANIMAL   BODY. 


897 


The  crystals  may  be  heated  to  120°  C.  without  being  decomposed;  at  a  higher  tem- 
perature they  are  first  liquefied  and  then  decomposed,  leaving  no  residue.  Heated 
with  strong  acids  or  alkalies,  decomposition  ensues,  the  final  products  being  car- 
bonic anhydride  and  ammonia.  The  same  decomposition  may  also  occur  as  the 
result  of  the  action  of  a  specific  ferment  on  urea  in  an  aqueous  solution. l  Nitrous 
acid  at  once  decomposes  it  into  carbonic  anhydride  and  free  nitrogen.  It  readily 
forms  compounds  with  acids  and  bases ;  of  these  the  following  are  of  importance : 

Nitrate  of  Urea.     (NH2)2CO.HN03. 

Crystallizes  in  six-sided  or  rhombic  tables  [Fig.  237].     Insoluble  in  ether  and 
nitric  acid,  soluble  in  water,  slightly  soluble  in  alcohol. 

[FIG.  237. 


Crystals  of  Nitrate  of  Urea.    (Krukenberg,  after  Kiihne.}] 

Oxalate  of  Urea.     [(NH2)2CO]2,H2CA+H20. 

Often  crystallizes  in  long  thin  prisms  [Fig.  238],  but  under  the  microscope  is 
obtained  in  a  form  closely  resembling  the  nitrate ;  it  is  slightly  soluble  in  water, 
less  so  in  alcohol. 

.  238. 


Crystals  of  Oxalate  of  Urea.    (Krukenberg,  after  Kiihne.)] 

With  mercuric  nitrate  urea  yields  three  salts,  containing  respectively,  four, 
three,  and  two  equivalents  of  mercuric  oxide  to  one  of  urea.     The  first  is  the  pre- 


nsfii 
(,lool), 


'  pfltiSer's  Archiv,  Bd.  xii.  (1876),  S.  214.    Jaksch,  Zeitsch.  f.  physiol.  Chem.,  Bd.  v. 


57 


898  APPENDIX. 

cipitate  formed  in  Liebig's  quantitative  determination  of  urea,  and  may  be  repre- 
sented by  the  formula:  2N2H4CO.Hg(N03)23HgO.  The  exact  constitution  of 
these  salts  has  not  yet  been  determined. 

Preparation.  Ammonic  sulphate  and  potassic  cyanate  are  mixed  together  in 
aqueous  solution,  and  the  mixture  is  evaporated  to  dryness.  The  residue,  when 
extracted  with  absolute  alcohol,  yields  urea.  From  urine,  either  by  evaporating  to 
dryness.  having  previously  precipitated  the  urine  with  normal  and  basic  lead  acetate 
in  succession  and  removed  the  lead  by  sulphuretted  hydrogen,  and  then  extracting 
with  alcohol ;  or  concentrating  only  to  a  syrup,  and  then  forming  the  nitrate  of 
urea ;  this  is  washed  with  pure  nitric  acid  and  decomposed  with  baric  compound. 

Detection  in  solution.  In  addition  to  the  microscopic  appearance  of  the  crys- 
tals obtained  on  evaporation,  the  nitrate  and  oxalate  should  be  formed  and  exam- 
ined. Another  part  should  give  precipitate  with  mercuric  nitrate,  in  the  absence 
of  sodic  chloride,  but  not  in  the  presence  of  this  last  salt  in  excess.  A  third  por- 
tion is  treated  with  nitric  acid  containing  nitrous  fumes ;  if  urea  is  present,  nitrogen 
and  carbonic  anhydride  will  be  obtained.  To  a  fourth  part  nitric  acid  in  excess 
and  a  little  mercury  are  added,  and  the  mixture  is  warmed.  In  presence  of  urea 
a  colorless  mixture  of  gases  (N  and  CO2)  is  given  off.  A  fifth  portion  is  kept 
melted  for  some  time,  dissolved  in  water,  and  cupric  sulphate  and  caustic  soda  are 
added  ;  a  red  or  violet  color,  due  to  biuret,  is  developed. 

Quantitative  determination.  For  this  some  special  manual  must  be  con- 
sulted.1 It  will  suffice  here  to  point  out  that  the  determination  is  made  either  with 
a  solution  of  mercuric  nitrate  of  known  strength  (Liebig)  ;  by  decomposing  the 
urea  by  means  of  sodic  hypobromite  into  nitrogen,  carbonic  anhydride,  and  water, 
and  measuring  the  nitrogen  (Knop)  [N2H4CO'+  SNaBrO  =  3NaBr  +  C02+  2H20 
H~N2],  or  by  heating  the  urea  with  caustic  baryta  in  a  sealed  tube,  the  urea  being 
determined  by  the  weight  of  baric  carbonate  formed  (Bunsen). 

Urea  is  generally  considered  to  be  an  amide  of  carbonic  acid,  i.  e. ,  carbamide. 
The  amide  of  an  acid  is  formed  when  water  is  removed  from  the  ammonium  salt 
of  the  acid;  if  the  acid  be  dibasic  and  two  molecules  of  water  be  removed,  the 
result  is  often  spoken  of  as  a  diamide.  Thus  if  from  ammonic  carbonate  (NHJ2 
C03,  two  molecules  of  water,  2H20,  be  removed,  carbonic  acid  being  a  dibasic 
acid,  the  result  is  urea ;  thus : 

(NH4)2C03  -  2H20  =  (NH2)2CO, 
which  may  be  written  either  according  to  the  ammonia  type  as 

CO  ) 

H,    N,          or  as          CO 

X12  ) 

two  atoms  of  amidogen  (NH2)  being  substituted  for  two  atoms  of  hydroxyl 
(HO). 

This  connection  between  carbonic  acid  and  urea  is  shown  by  the  fact  that 
ammonic  carbonate  may  be  formed  out  of  urea  by  hydration,  as  when  urea  is 
subjected  to  the1  specific  ferment  mentioned  above.  Regarded,  then,  as  a  diamide 
of  carbonic  acid,  urea  may  be  spoken  of  as  carbamide.  But  the  theoretical  deri- 
vation of  urea  from  ammonic  carbonate  by  dehydration  cannot  be  realized  in 
practice,  whereas  urea  can  readily  be  formed  from  ammonic  carbamate,  and 
Kolbe  is  inclined  to  regard  it,  not  as  the  diamide  of  carbonic  acid,  but  as  the 
amide  of  carbamic  acid.  Ammonium  carbamate,  C02N2H6  minus  H20,  gives 
urea,  CO,  N«,  H4 — which,  if  carbamic  acid  be  written  as  CO,  OH,  NH2,  may  be 
written  as  CO,  NH2,  NH2,  one  atom  of  amidogen  being  substituted  for  one  atom 
of  hydroxyl,  and  not  two,  as  when  the  substance  is  regarded  as  derived  from  car- 
bonic acid.  Drechsel's  experiments  indicate  a  ready  derivation  of  urea  from 
ammonic  carbamate.  He  has  obtained  urea  by  the  electrolysis  of  a  solution  of 
this  salt  with  rapidly  alternating  currents,  thus  removing  the  elements  of  water 

i  Neubauer  and  Vogel,  Analyse  des  Harns,  viii.  Aufl.,  1881,  S.  264. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  899 

from  the  carbamate  by  such  alternating  processes  of  oxidation  and  reduction  as 
may  be  supposed  to  take  place  in  the  body.  The  reaction  is  expressed  as  fol- 
lows: 


2.  NH2.CO.ONH2  +  H2  =  NH2.CO.NH2  +  H20. 

Wanklyn1  and  Gamgee,2  however,  since  urea,  when  heated  with  a  large  excess 
of  pptassic  permanganate,  gives  off  all  its  nitrogen  in  a  free  state,  and  not  in  the 
oxidized  form  of  nitric  acid,  as  do  all  other  amides,  conclude  that  it  is  not  an  amide 
at  all,  that  it  is  isomeric  only  and  not  identical  with  carbamide. 

It  is  important  to  remember  that  urea  is  also  isomeric  with  ammonic  cyanate, 

C  I  QNET  an(*'  in(*eed'  was  first  f°rme(l  artificially  by  Wbhler  (1823)  from  this 
body.  We  thus  have  three  isomeric  compounds,  ammonium  cyanate,  urea,  and 
carbamide,  related  to  each  other  in  such  a  way  that  urea  may  be  obtained  readily 
either  from  ammonium  cyanate  or  from  ammonic  carbamate,  and  may  with  the 
greatest  ease  be  converted  into  ammonic  carbonate.3  Now  urea  is  a  much  more 
stable  body  than  ammonic  cyanate,  and  in  the  transformation  of  the  latter  into  the 
former  energy  is  set  free  ;  and  it  is  worthy  of  notice  that,  though  the  presence  of 
sulphocyanides  in  the  saliva  probably  indicates  the  presence  of  cyanic  residues  in 
the  body,  the  nitrogenous  products  of  the  decomposition  of  proteids  belong  chiefly 
to  the  class  of  amides,  cyanogen  compounds  being  rare  among  them.  Pfliiger4  has 
called  attention  to  the  great  molecular  energy  of  the  cyanogen  compounds,  and 
has  suggested  that  the  functional  metabolism  of  protoplasm  by  which  energy  is  set 
free  may  be  compared  to  the  conversion  of  the  energetic  unstable  cyanogen  com- 
pounds into  the  less  energetic  and  more  stable  amides.  In  other  words,  ammonium 
cyanate  is  a  type  of  living,  and  urea  of  dead  nitrogen,  and  the  conversion  of  the 
former  into  the  latter  is  an  image  of  the  essential  change  which  takes  place  when 
a  living  proteid  dies. 

Compound  ureas.  The  hydrogen  atoms  of  urea  can  be  replaced  by  alcohol  and  acid  radicles. 
The  results  are  compound  ureas  or  ureides  when  the  hydrogen  is  replaced  by  an  acid  radicle. 
Many  of  them  are  called  acids,  since  the  hydrogen  from  the  amide  group,  if  not  all  replaced  as 
above,  can  be  replaced  by  metal.  Thus,  the  substitution  of  oxalyl  (oxalic  acid)  gives  parabanic 
acid, 

(CO 
N  J  H2  or  CO,  NH2,  N.C2O2  ; 


of  tartronyl  (tartronic  acid),  dialuric  acid,  CO,  NH2,  N.C3H2O3;  of  mesoxalyl  (mesoxalic  acid), 
alloxan,  CO,  NH2,  N.C3O3.  These  bodies  are  interesting  as  being  also  obtained  by  the  artificial 
oxidation  of  uric  acid.  (See  below.) 

Uric  Acid.    C5H4N403. 

The  chief  constituent  of  the  urine  in  birds  and  reptiles  ;  it  occurs  only  sparingly  in 
this  excretion  in  man  and  most  mammalia.  It  is  normally  present  in  tne  spleen, 
and  traces  of  it  have  been  found  in  the  lungs,  muscles  of  the  heart,  pancreas,  brain, 
and  liver.  Urinary  and  renal  calculi  often  consist  largely  of  this  body  or  its  salts. 
In  gout,  accumulations  of  uric  acid  salts  may  occur  in  various  parts  of  the  body, 
forming  the  so-called  gouty  concretions. 

It  is  when  pure  a  colorless,  crystalline  powder,  tasteless,  and  without  odor. 
The  crystalline  form  is  very  variable,  but  usually  tends  toward  that  of  rhombic 
tables.5  When  impure  it  crystallizes  readily,  but  then  possesses  a  yellowish  or 
brownish  color.  In  water  it  is  very  insoluble  (1  in  14,000  or  15,000  of  cold  water) ; 
ether  and  alcohol  do  not  dissolve  it  appreciably.  On  the  other  hand,  sulphuric 
acid  takes  it  up  without  decomposition,  and  it  is  also  readily  soluble  in  many 
salts  of  the  alkalies,  as  in  the  alkalies  themselves.  Ammonia,  however,  scarcely 
dissolves  it. 

1  Arch.  f.  Physiol.,  1880,  S.  550. 

2  Journ.  Chem.  Soc.,  2,  vol.  vi.  p.  25. 

3  The  following  literature  is  interesting  in  connection  with  the  question  of  the  cyanic  or  amide 
origin  of  urea-Drechsel :  Ber.  d.  k.  s.  Gesell.  d.  Wiss.,  Leipzig,  Sitz.  25  Juli,  1875 ;  Arch.  f.  Pbysiol., 
1880,  S.  505.     v.  Knieriem  :  Zt.  f.  Biol.,  Bd.  x.  (1874),  S.  263.      Munk  :  Zt.  f.  physiol.  Chem.,  Bd.  ii. 
(1878),  S.  29.     E.  Salkowski :  Centralbl.  f.  d.  med.  Wiss.,  1875,  No.  58;  Ber.  d.  Deutsch.  Chem.  Ge- 
sell., 1875,  S.  116.    Zeitsch.  f.  physiol.  Chem.,  Bd.  i.  (1877),  Sn.  1  u.  374  ;  Bd.  iv.  (1880),  Sn.  54  u.  103. 
Schmiedeberg :  Arch.  f.  exp.  Pathol.,  Bd.  viii.  (1877),  S.  1. 

*  Pfliiger's  Archiv.,  Bd.  x.  (1875).  S.  337. 

6  See  Ultzmann  and  K.  B.  Hoffmann,  Atlas  der  Harnsedimente,  Wien,  1872. 


900 


APPENDIX. 


Salts  of  uric  acid.  Of  these  the  most  important  are  the  acid  urates  of  sodium, 
potassium  and  ammonium.  The  sodium  salt  crystallizes  in  many  different  forms- 
[Fig.  239J,  these  not  being  characteristic,  since  they  are  almost  the  same  for  the 
corresponding  compounds  of  the  other  two  bases.  It  is  very  insoluble  in  cold  water 
(1  in  1100  or  1200),  more  soluble  in  hot  (1  in  125).  It  is  the  principal  constituent 
of  several  forms  of  urinary  sediment,  and  constitutes  a  large  part  of  many  calculi ; 
the  excrement  of  snakes  contains  it  largely.  The  potassium  resembles  tlie  sodium 
salt  very  closely,  as  also  does  the  compound  with  ammonium  ;  the  latter  occurs  gen- 
erally in  the  sediment  from  alkaline  urine.  [Fig.  240.] 


[FIG.  239. 


[FiG.  240. 


FIG.  239.— Urate  of  Soda,  a  a,  from  a  gouty  concretion  ;  6  b,  artificially  prepared  by  adding 
liq.  sodse  to  the  amorphous  urate  deposit.] 

FIG.  240.— The  Normal  Deposit  from  Ammoniacal  Urine,  showing  Crystals  of  Ammoniaco- 
Magnesian  Phosphate,  Amorphous  Phosphate  of  Lime,  and  Spheres  of  Urate  of  Ammonia.] 

Preparation.  •  Usually  from  guano  or  snake's  excrement.  From  guano  by  boil- 
ing with  caustic  potash  (1  part  alkali  to  20  of  water)  as  long  as  ammonia  is  evolved. 
In  the  filtrate  a  precipitate  of  acid  urate  of  potassium  is  formed  by  passing  a, 
current  of  carbonic  anhydride;  this  salt  is  then  washed,  dissolved  in  a  caustic 
potash,  and  decomposed  by  carefully  pouring  its  solution  into  an  excess  of  hydro- 
chloric acid. 

The  presence  of  uric  acid  is  recognized  by  the  following  tests :  The  substance 
having  been  examined  microscopically,  a  portion  is  evaporated  carefully  to  dryness 
with  one  or  two  drops  of  nitric  acid.  The  residue  will,  if  uric  acid  is  present,  be 
of  a  red  color,  which  on  the  addition  of  ammonia  turns  to  purple.  This  is  the  mu- 
rexide  test,  and  depends  on  the  presence  of  alloxan  and  alloxantin  in  the  residue. 
Schiff1  has  given  a  delicate  reaction  for  uric  acid.  The  substance  is  dissolved  in 
sodic  carbonate  and  dropped  on  paper  moistened  with  a  silver  salt.  If  uric  acid 
be  present  a  brown  stain  is  formed,  due  to  the  reduction  of  the  carbonate  of  silver. 
An  alkaline  solution  of  uric  acid  can,  like  dextrose,  reduce  cupric  sulphate,  with 
precipitation  of  the  cuprous  oxide. 

Uric  acid  resists  very  largely  the  action  of  even  strong  acids  and  alkalies,  exhibit- 
ing in  this  respect  a  marked  difference  from  urea.  It  might  therefore  perhaps  be 
supposed  that  urea  residues  do  not  pre-exist  in  uric  acid ;  nevertheless  by  oxida- 
tion uric  acid  does  give  rise  not  only  to  ordinary  urea,  but  also,  and  at  the  same 
time,  to  the  compound  ureas  (ureides)  spoken  of  above.  Thus,  by  oxidation  with 
acids, 

Uric  acid.  Alloxan.  Urea. 

C5H4N403  +  H20  +  0  -  C4N2H A  +  CN2H40. 

Now  alloxan,  as  was  stated  above,  is  a  compound  urea,  viz.,  mesoxalyl-urea,  and 
by  hydration  can  be  converted  into  mesoxalic  acid  and  urea,  thus : 

1  Ann.  d.  Chern.  u.  Pharm.,  Bd.  cix.  S.  65. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  901 

Alloxan.  Mesoxalic,         Urea. 

C4N2H204  -f  2H20  -  C2H205  4-  CN2H40  ; 

and  by  the  action  of  chlorine  uric  acid  can  be  split  up  directly  into  a  molecule  of 
mesoxalic  acid  and  two  molecules  of  urea  : 

Uric  acid.  Mesoxalic  acid.         Urea. 

C5H4N403  +  Cl,  +  4H20  =  C3H205  +  2CN2H4OC  +  2HCL 

By  oxidation  with   alkalies,  uric  acid  is  converted  into  allantoin  and  carbonic 

acid, 

Uric  acid.  Allantoin. 

C5H4N403  +  H20  4-  0  =  C4H6N403  +  C02; 

and  allantoin,  by  hydration,  becomes  allanturic  or  lantanuric  acid  and  urea, 
Allantoin.  Urea.       Allanturic  acid. 

C4H6N403  +  H20  -  CH4N20  +  CSH4N80. 

Now  allanturic  acid  is  a  compound  urea,  with  a  residue  of  glyoxylic  acid.  By 
other  oxidations  of  uric  acid,  parabanic  acid  (oxalyl-urea),  oxaluric  acid  (which  is 
hydrated  parabanic  acid),  and  dialuric  acid  (tartrpnyl-urea)  are  obtained.  In  fact, 
all  these  decompositions  of  a  molecule  of  uric  acid  lead  to  the  production  of  urea 
and  of  a  carbon  acid  of  some  kind  or  other.  The  relation  of  uric  acid  to  urea,  as 
illustrated  by  the  above  reactions,  is  brought  very  prominently  into  view  by  the 
synthesis  of  uric  acid  which  has  recently  been  performed.  '  It  is  obtained  by  simply 
fusing  together  glycocine  (amido-acetic  acid)  and  urea  at  a  temperature  of  200- 
230°  C.  The  converse  formation  of  glycocine  from  uric  acid  with  the  simultaneous 
production  of  ammonia  and  carbonic  anhydride  has  been  known  for  some  time. 
Since  in  this  latter  reaction  the  ammonia  and  carbonic  anhydride  are  in  the  propor- 
tions in  which  they  would  be  obtained  from  cyanic  or  cyanuric  acid,  uric  acid  has 
been  regarded  as  built  up  from  residues  of  cyanuric  acid  and  glycin,  just  as  hippuric 
acid  is  formed  from  glycin  and  benzoic  acid.  It  was  also  at  one  time  supposed  that 
uric  acid  might  be  regarded  as  tartronyl  cyanamide. 


N(CN)2 
H2 

If  the  existence  of  some  cyanogen  residue  is  thus  assumed  in  the  molecule  of 
uric  acid,  then  it  must  be  supposed  that  before  urea  can  be  obtained  from  it  a 
molecular  change  takes  place  by  which  a  portion  at  least  of  the  nitrogen  of  the  uric 
acid  is  converted  into  the  same  condition  as  the  rest  of  the  nitrogen,  viz.,  into  the 
amide  state. 

If  this  be  so,  since  the  metabolism  of  the  animals  in  which  uric  acid  replaces 
urea  cannot  be  supposed  to  be  fundamentally  different  from  that  of  the  urea-pro- 
ducing animals,  we  may  infer  that  the  antecedent  of  both  uric  acid  and  urea  in  the 
regressive  metabolism  of  proteids  is,  as  we  suggested  above,  a  body  containing  some 
at  least  of  its  nitrogen  in  the  form  of  cyanogen.2 

Kreatin.     C4H9N302. 

Occurs  as  a  constant  constituent  of  the  juices  of  muscles,  though  possibly  it 
may  be  formed  during  the  process  of  extraction  by  the  hydration  of  kreatinin. 
Kreatin  is  not  a  normal  constituent  of  urine,  but  it  is  said  to  occur  in  traces  in 
several  fluids  of  the  body.  When  found  in  urine  its  presence  is  probably  due  to 
the  conversion  of  kreatinin,  a  constant  constituent  of  urine,  into  kreatin  during 
its  extraction,  since  Dessaignes  3  has  shown  that  the  more  rapidly  the  separation  is 
-effected,  the  less  is  the  quantity  of  kreatin  obtained,  and  the  greater  the  amount  of 
kreatinin. 

In  the  anhydrous  form  it  is  white  and  opaque,  but  crystallizes  with  one  molecule 
of  water  in  colorless,  transparent  rhombic  prisms  [Fig.  241,  a].  It  possesses  a  some- 

1  Horbaczewski  ;  Ber.  d.  Deutseh.  Chem.  Gesell.,  Jahrg.  1882,  S.  2678. 

2  See  v.  Knieriem,  Zeitschr.  f.  Biol.,  Bd.  xiii.  (1877),  S.  36.    Schroder,  Zeitschr.  f.  physiol.  Chem., 
Bd.  ii.  (1878),  S.  228. 

3  Jahrb.  Pharm.  (3),  Bd.  xxxii.  S.  41. 


902 


APPENDIX. 


what  bitter  taste,  is  soluble  in  cold,  extremely ^  soluble  in  hot  water,  is  less  soluble  in 
absolute  than  in  dilute  alcohol,  and  is  soluble  in  ether. 

It  is  a  very  weak  base,  scarcely  neutralizing  the  weakest  acids.     It  forms  crystal- 
line compounds  with  sulphuric,  hydrochloric,  and  nitric  acids. 

[FIG.  241. 


Crystals  of  Kreatin  and  Kreatinin.    a,  crystals  of  kreatin;  b,  crystals  of  kreatinin  ;  c,  crystals  of 
chloride  of  zinc  and  kreatinin.] 

Preparation.  From  extract  of  muscle  by  precipitating  completely  with  basic 
lead  acetate,  and  crystallizing  out  the  kreatin,  mixed  with  kreatinin.  From  this 
latter  it  is  separated  by  the  formation  of  the  zinc-salt  of  kreatinin,  kreatin  not 
readily  yielding  a  similar  compound. 

Kreatin  may  be  converted  into  kreatinin  under  the  influence  of  acids,  the  transformation 
being  one  of  simple  dehydration. 

Kreatin  may  be  decomposed  into  sarcosin  (methyl-glycin)  and  urea: 

C4H9N302  +  H20  =  C3H7N02  +  CH4N20  ; 

it  may  be  formed  synthetically l  by  the  action  of  sarcosin  and  cyanamide : 
C3H7N02  +  CH2N2  =  C4H9N30. 

Sarcosin  is  glycin  in  which  one  atom  of  hydrogen  has  been  replaced  by  the  alcohol 
radicle  methyl,  thus : 

Glycin  g2^0  1  0  becomes 
like  glycin,  sarcosin  has  not  been  found  in  free  state  in  the  body. 
Kreatinin.    C4H7N30. 

This,  which  is  simply  a  dehydrated  form  of  kreatin,  occurs  normally  as  a  con- 
stant constituent  of  urine  and  of  muscle  extract.  It  crystallizes  in  colorless  shining 
prisms  [Fig.  241,  6],  possessing  a  strong  alkaline  taste  and  reaction.  It  is  readily 
soluble  in  cold  water  (1  in  11.5),  also  in  alcohol,  but  is  scarcely  soluble  in  ether.  It 
acts  as  a  powerful  base,  forming  with  acids  and  salts  compounds  with  crystallize 
well.  Of  these  the  most  important  is  the  salt  with  zinc  chloride  (C4H7N30)2 
ZnCl2.  It  is  formed  when  a  concentrated  solution  of  the  chloride  is  added  to  a  not 
too  dilute  solution  of  kreatinin.  Since  the  compound  is  very  little  soluble  in  alcohol 
it  is  better  to  use  alcoholic  rather  than  aqueous  solutions.  It  crystallizes  in  warty 
lumps  composed  of  aggregated  masses  of  prisms  or  fine  needles.  [Fig.  241,  c. } 

Preparation.  Either  by  the  action  of  acids  on  kreatin,  or  from  human  urine  by 
concentrating  and  precipitating  with  lead  acetate  ;  in  the  filtrate  from  this  a  second 

1  Sitzungsber.  d.  Bayerisch.  Akad.,  1868,  Heft  3,  S.  472. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY. 


903 


com- 
ro- 


precipitate  is  caused  by  the  addition  of  mercuric  chloride,  and  consists  of  a  co 
pound  of  this  salt  with  kreatinin.     The  mercury  is  removed  by  sulphuretted  hyd 
gen,  and  the  kreatin  purified  by  the  formation  of  the  zinc  salt  and  washing  with 
alcohol. 

Kreatinin-zinc  chloride  may  be  converted  into  kreatin,  by  the  action  of  hydrated  oxide  of  lead 
on  its  boiling  aqueous  solution. 

Allantoin.     C4H6N403. 

The  characteristic  constituent  of  the  allantoic  fluid  of  the  foetus ;  it  occurs  also 
in  the  urine  of  animals  for  a  short  period  after  their  birth.  Traces  of  it  are  some- 
times detected  in  this  excretion  at  a  later  date. 

It  crystallizes  in  small,  shining,  colorless  prisms  [Fig.  242] ,  which  are  tasteless 


[FIG.  242. 


[FiG.  243. 


FIG.  242.— Crystals  from  Concentrated  Urine  of  Calf,  showing  in  Centre  a  Large  Bundle  of 
Prisms  of  Allantoin.    (After  Ktthne.)] 

FIG.  243.— Hypoxanthin-silver-nitrate,  C6H4N4O.AgNO3.    (Krukenberg,  after  Kuhne.)] 

and  odorless.  They  are  soluble  in  160  parts  of  cold,  more  soluble  in  hot  water, 
insoluble  in  cold  alcohol  and  ether,  soluble  in  hot  alcohol.  Carbonates  of  the  alka- 
lies dissolve  them,  and  compounds  may  be  formed  of  allantoin  with  metals  but  not 
with  acids. 

Allantoin,  as  already  stated,  p.  901,  is  one  of  the  products  of  the  oxidation  of 
uric  acid,  and  by  further  oxidation  gives  rise  to  urea. 

Preparation.  This  is  best  carried  out  by  the  careful  oxidation  of  uric  acid,  either 
by  means  of  potassic  permanganate  or  ferrocyanide,  or  by  plumbic  oxide. 

Hypoxanthin  or  Sarkin.     C5H4N4O. 

Is  a  normal  constituent  of  muscle,  occurring  also  in  the  spleen,  liver,  and  medulla 
of  bones.  In  lukaemia  it  appears  in  the  blood  and  urine.  It  crystallizes  in  fine 

[FIG.  244. 


Hypoxanthin-nitrate,  C6H4N4O.HNO3.    (Kuhne).] 

needles  which  are  soluble  in  300  parts  of  cold,  more  soluble  in  hot  water,  insoluble 
in  alcohol,  soluble  in  acids  and  alkalies.     It  forms  crystalline  compounds  with  acids 


904 


APPENDIX. 


and  bases  [Figs.  243,  244,  ~ 
cipitate  being  soluble  in  a 


245].     It  is  precipitated  by  basic  acetate  of  lead,  the  pre- 
a  solution  of  the  normal  acetate.     Its  preparation  from 


[FiG.  245. 


Hypoxanthin-hydrochloride,  C5H4N4O.HC1.    (Kiihne.)] 
[FiG.  246.  [FiG.  247. 


Xanthin-hydrochloride,  C5H4N4O2.HC1. 
(Kuhne.)] 


Xanthin-nitrate,  C5H4N4O2.HNO3. 
(Kiihne.)] 


muscle-extract  depends  on  its  precipitation  first  by  basic  acetate  of  lead,  and  then 
by  an  ammoniacal  solution  of  silver  nitrate  after  the  removal  of  kreatin. 

Both  hypoxanthin  and  the  next  body,  xanthin,  can  also  be  obtained  from  proteids  by  the 
action  of  putrefactive  changes,  of  water  at  boiling  temperature,  of  dilute  hydrochloric  acid  (0.2 
per  cent.)  at  40°  C  ,  and  by  the  action  of  gastric  and  pancreatic  ferments.1  Chittenden  lias  noticed 
a  peculiar  difference  between  fibrin  and  egg-albumin  when  submitted  to  the  above  processes  ;  he 
finds  that  the  latter  does  not  yield  hypoxanthin  when  treated  with  boiling  water,  with  dilute 
hydrochloric  acid,  or  gastric  ferment,  while  the  former  does.  Egg-albumin,  on  the  other  hand, 
yields  hypoxanthin  by  the  action  of  pancreatic  ferment  in  alkaline  solution,  but  not  so  readily  as 
fibrin  does. 

Xanthin.    C5H4N402. 

First  discovered  in  a  urinary  calculus,  and  called  xanthic  oxide.  More  recently 
it  has  been  found  as  a  normal,  though  scanty,  constituent  of  urine,  muscles,  and 
several  organs,  such  as  the  liver,  spleen,  thymus,  etc. 

[FiG.  248. 


Crystals  of  Xanthin-silver-nitrate,  C5H4N4O2.AgNO3.    (Krukenberg,  after  Kiihne.)] 

When  precipitated  by  cooling  from  its  hot,  saturated,  aqueous  solution  it  falls 
in  white  flocks,  but  if  the  solution  be  allowed  to  precipitate  slowly  it  is  obtained  in 

*  Salomon,  Zeitschr.  f.  physiol.  Chem.,  Bd.  ii.  (1878-1879),  S.  60.  Krause,  Inaug.  Diss.,  Berlin, 
1878.  Chittenden,  Journ.  of  Physiol.,  vol.  ii.  (1879),  p.  28.  See  also  Drechsel,  Ber.  d.  Deutsch.  Chem. 
Gesell..  Jahrg.  xiii.  (1880),  S.  240.  Salomon,  Ibid.,  S.  1160.  Kossel,  Zeitsch.  f.  physiol.  Chem.,  Bd.  v. 
(1881),  Sn.  152  u.  267. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY. 


905 


small  scales.  When  pure  it  is  a  colorless  powder,  very  insoluble  in  water,  requir- 
ing 1 500  times  its  bulk  for  solution  at  100°  C.  Insoluble  in  alcohol  and  ether,  it 
readily  dissolves  in  dilute  acids  and  alkalies,  forming  crystallizable  compounds. 
[Figs.  246,  247,  248.] 

Hypoxanthin  by  oxidation  becomes  xanthin.  Both  these  bodies  as  well  as  the 
following,  guanin  and  carnin,  are  evidently  closely  allied  to  uric  acid ;  indeed,  uric 
acid  by  the  action  of  sodium-amalgam  may  be  converted  into  a  mixture  of  xanthin 
and  hypoxanthin. 

Preparation.  It  is  obtained  from  urine  and  the  aqueous  extract  of  muscles  by 
a  process  similar  to  that  for  hypoxanthin,  and  is  then  separated  from  the  latter  by 
the  action  of  dilute  hydrochloric  acid  ;  this  separation  depends  on  the  different 
solubilities  of  the  hydrochlorides  of  the  two  bodies.  For  further  information  see 
Neubauer  and  Vogel.1 

Carnin.     C7H8N403. 

Discovered  by  Weidel 2  in  extract  of  meat,  of  which  it  constitutes  about  1  per 
cent, 

It  crystallizes  in  white  masses  composed  of  very  small,  irregular  crystals ;  it  is 
soluble  with  difficulty  in  cold,  more  easily  soluble  in  hot  water,  insoluble  in  alcohol 
and  ether.  Its  aqueous  solution  is  not  precipitated  by  normal  lead  acetate,  but  is 
by  the  basic  acetate  of  this  metal.  It  unites  with  acids  and  salts,  forming  crystal- 
line compounds. 

Preparation.  Is  found  in  the  precipitate  caused  in  extract  of  meat  by  basic 
acetate  of  lead.3 

This  body  possesses  an  interesting  relation  to  hypoxanthin,  into  which  it  maybe  converted  by 
the  action  either  of  nitric  acid  or,  still  better,  of  bromine. 

Guanin.    C5H5N50. 

First  obtained  from  guano,  but  recently  observed  as  occurring  in  small  quanti- 
ties in  the  pancreas,  liver,  and  muscle  extract. 


[FiG.  249. 


[Fio.  250. 


Guanin  Hydrochloride.    C5H5N5O.HC1  +  H2O. 
(After  Kvihne.)] 


Guanin  Nitrate. 

(After  Kiihne.)] 


It  is  a  white  amorphous  powder,  insoluble  in  water,  alcohol  ether,  and  ammo- 
nia. It  unites  with  acids,  alkalies,  and  salts  to  form  crystallizable  compounds. 
[Figs.  249,  250.] 

Preparation.  From  guano  by  boiling  successively  with  milk  of  lime  and  caustic 
soda,  precipitating  with  acetic  acid,  and  purifying  by  solution  in  hydrochloric  acid 
and  precipitation  by  ammonia. 


1  Harn- Analyse,  ed.  viii.  (1881),  S.  26.    Also  the  literature  quoted  above  on  hypoxanthin. 

2  Ann.  d.  Chem.  u.  Pharm.,  Bd.  clviii.  S.  365.  3  see  Weidel,  op.  cit. 


906  APPENDIX. 

Guanin  may,  by  the  action  of  nitrous  acid,  be  converted  into  xanthin.  By 
oxidation  it  can  be  made  to  yield  principally  guanidine  and  parabanic  acid,  accom- 
panied, however,  by  small  quantities  of  urea,  xanthin,  and  oxalic  acid.  Capranica 
has  given  several  reactions  characteristic  of  this  body.1 

Its  separation  from  hypoxanthin  and  xanthin  depends  on  its  insolubility  in 
water  and  behavior  with  hydrochloric  acid. 

Kynurenic  acid.     C20H14N206+2H20. 

Found  in  the  urine  of  dogs,  and  first  described  by  Liebig.2  When  pure  it  crys- 
tallizes in  brilliant  white  needles,  insoluble  in  cold,  soluble  in  hot  alcohol.  The  only 
salt  of  this  body  which  crystallizes  well  is  that  formed  with  barium.  t\>r  prepara- 
tion and  other  particulars  see  Liebig,3  Schultzen,  and  Schmiedeberg.4 

Glycin.    C2H2(NH2)0(OH).     Also  called  Glycocol  and  Glycocin. 

Does  not  occur  in  a  free  state  in  the  human  body,  but  enters  into  the  composi- 
tion of  many  important  substances,  e.  g. ,  hippuric  and  bile  acids.  It  crystallizes 
into  large,  colorless,  hard  rhombohedra,  which  are  easily  soluble  in  water,  insoluble 
in  cold,  slightly  soluble  in  hot  alcohol,  insoluble  in  ether.  It  possesses  an  acid  reac- 
tion, but  a  sweet  taste.  It  has  also  the  property  of  uniting  with  both  acids  and 
bases  to  form  crystallizable  compounds.  In  this  it  exhibits  its  amide  nature,  and 
that  it  is  an  amide  is  rendered  evident  from  the  methods  of  its  synthetic  prepara- 
tion ;  thus  mono-chloracetic  acid  and  ammonia  give  glycin  and  anmionic  chloride ; 
C3H3C1.02  +  2NH3  =  C2H2(NH2)0(OH)  +  NH4C1.  It  is  amido-acetic  acid. 
Heated  with  caustic  baryta  it  yields  ammonia  and  methylamine. 

Preparation.  From  glutin  by  the  action  of  acids  or  alkalies ;  from  hippuric 
acid  by  decomposing  it  with  hydrochloric  acid  at  a  boiling  temperature  and  remov- 
ing by  precipitation  the  simultaneously  formed  benzoic  acid. 

Taurin.     C2H7N03S. 

In  addition  to  entering  into  the  composition  of  taurocholic  acid  taurin  is  found 
in  traces  in  the  juices  of  muscle  and  in  the  lungs. 

It  crystallizes  in  colorless,  regular  six-sided  prisms  [Fig.  251] ;  these  are  readily 
soluble  in  water,  less  so  in  alcohol.  The  solutions  are  neutral,  It  is  a  very  stable 

r'Fio.  251. 


Taurin  Crystals.] 

compound,  resisting  temperatures  of  less  than  240°  C.  ;  it  is  not  acted  on  by  dilute 
alkalies  and  acids,  even  when  boiled  with  them.     It  is  not  precipitated  by  metallic 


Taurin  is  amido-isethionic  acid;  and  may  be  synthetically  prepared  from  isethi- 
onic  (ethyl-sulphuric)  acid  by  the  action  of  ammonia  ;  thus  : 

04  +  NH3  =  S03+H20. 


1  Zeitschr.  f.  phys.  Chem.,  Bd.  iv.  (1880),  S.  240. 

2  Ann.  d.  Chem.  u.  Pharm.,  Bd.  Ixxxvi.  S.  125,  and  Bd.  cviii.,  S.  354. 

3  Op.  cit.  *  Ami.  d.  Chem.  u.  Pharm.,  Bd.  clxiv.  S.  155. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  907 

Preparation.  As  a  product  of  the  decomposition  of  bile,  and  is  purified  by 
removing  any  traces  of  bile  acids  by  means  of  lead  acetate,  and  then  successively 
crystallizing  from  water. 

Leucin.    C6H13N02. 

Is  one  of  the  principal  products  of  the  decomposition  of  nitrogenous  matter, 
either  under  the  influence  of  putrefaction  or  of  strong  acids  and  alkalies.  It  occurs, 
however,  normally  in  the  pancreas,  spleen,  thymus,  thyroid,  salivary  glands,  liver, 
etc.,  and  is  one  of  the  products  of  the  tryptic  (pancreatic)  digestion  of  proteids; 
in  acute  atrophy  of  the  liver  it  is  present  in  the  urine  in  large  quantity  in  company 
with  tyrosin. 

As  usually  obtained  in  an  impure  form  it  crystallizes  in  rounded  lumps  [Fig.  252], 
which  are  often  collected  together  and  sometimes  exhibit  radiating  striation.  When 

[FiG.  252. 


Leucin  Crystals.] 

pure,  it  forms  very  thin,  white,  glittering,  flat  crystals.  These  are  easily  soluble  in 
hot  water,.  less  so  in  cold  water  and  alcohol,  insoluble  in  ether.  They  feel  oily  to 
to  the  touch,  and  are  without  smell  and  taste.  Acids  and  alkalies  dissolve  them 
readily,  and  crystallizable  compounds  are  formed. 

Carefully  heated  to  170°  C.  it  sublimes,  but  at  a  higher  temperature  is  decomposed,  yielding 
amylamin,  carbonic  anhydride,  and  ammonia.  In  the  presence  of  putrefying  animal  matter  it 
splits  up  into  valeric  acid  and  ammonia. 

Leucin  is  amido-caproic  acid,  and  may  be  represented  thus  : 


Preparation.  From  horn  shavings  by  boiling  with  sulphuric  acid,  neutralizing 
with  baryta  and  separating  from  tyrosin  by  successive  crystallization.  See  also 
Kiihne,1  who  prepares  it  by  the  action  of  pancreatic  ferment  (trypsin)  on  proteids. 

Scherer  has  given  the  following  test  lor  leucin.  The  suspected  substance  is 
evaporated  carefully  to  dryness  with  nitric  acid  ;  the  residue,  if  it  is  leucin,  will  be 
almost  transparent  and  turn  yellow  or  brown  on  the  addition  of  caustic  soda.  If 
this  be  again  very  carefully  concentrated  with  the  alkali,  an  oily  drop  is  obtained, 
which  is  quite  characteristic  of  this  substance.  Leucin,  if  not  too  impure,  may  be 
easily  recognized  by  its  subliming  on  being  heated  ;  a  characteristic  color  of  amyl- 
amin is  at  the  same  time  evolved. 

Asparagin.     C4H8N203 

Is  not  found  as  a  constituent  of  the  animal  body,  but  appears  to  be  formed  by 
the  decomposition  of  proteids,  notably  during  the  germinative  changes  of  the  pro- 
teids in  leguminous  seeds.2  It  is  a  crystalline  body,  and  when  boiled  with  acids  or 
alkalies  is  readily  converted  into  aspartic  acid. 

Aspartic  (or  asparaginic]  Acid.     C4H7NC4. 

This  acid  has  been  obtained  in  small  quantities  among  the  products  of  the  pan- 
creatic digestion  of  fibrin  3  and  vegetable  glutin,4  although  not  occurring  as  a  con- 

1  Virchow's  Archiv,  Bd.  xxxix.,  S.  130. 

2  Landwirthski  u.  Versuchs  Statioiien,  Bd.  xvii.  1. 

3  Radziejewski  u.  Salkowski.  Ber.  d.  Deutsch.  chem.  Gesell.,  Jahrg.  vii.  (1874),  S.  1050. 
*  V.  Knieriem,  Zeitschr.  f.  Biol..  Bd.  xi.  (1875),  S.  198. 


908 


APPENDIX. 


stituent  of  any  animal  tissue  or  secretion.  It  is,  on  the  other  hand,  found  normally 
in  plants,  notably  in  beet-sugar  molasses.  It  arises  also  as  a  constant  product  of 
the  action  of  alkalies  and  other  reagents  on  both  vegetable  and  animal  proteids,  and 
of  acids  on  gelatin.1  It  thus  possesses  considerable  interest  in  respect  of  its  re- 
lation to  the  proteids.  It  crystallizes  in  rhombic  prisms  which  are  but  sparingly 
soluble  in  cold  water  or  alcohol,  readily  soluble  in  boiling  water.  Its  acid  solutions 
are  dextro-rotatory,  its  alkaline  Isevo-rotatory  and  reduce  Fehling's  fluid.  It  forms 
a  characteristic  readily  crystallizable  compound  with  copper.  Nitrous  acid  converts 
it  into  malic  acid. 

Glutaminic  Acid.    C5H9N04. 

The  circumstances  and  conditions  under  which  this  body  occurs  are  in  general 
the  same  as  for  the  aspartic  acid,  and  hence  as  a  product  of  proteid  decomposition 
it  acquires  some  importance.  It  has  not,  however,  as  yet  been  obtained  by  the 
action  of  pancreatic  ferments  on  proteids,  and  in  this  it  differs  from  the  preceding 
body. 

It  crystallizes  in  rhombic  tetrahedra  or  octahedra ;  is  not  very  soluble  in  cold,  but 
readily  soluble  in  hot  water ;  insoluble  in  alcohol  and  ether.  Its  acid  solutions 
possess  a  strong  dextro-rotatory  power,  and  it  reduces  Fehling's  fluid. 

Cystin.    C3H7NS02. 

Is  the  chief  constituent  of  a  rarely  occurring  urinary  calculus  in  men  and  dogs. 
It  may  also  occur  in  renal  concretions  and  in  gravel,  and  is  occasionally  found  in 
urine. 

From  calculi  it  is  obtained,  by  extraction  with  ammonia,  as  colorless  six-sided 
tables  or  rhombohedra  [Fig.  253],  which  are  neutral  and  tasteless.  It  is  insoluble 


in  water,  alcohol,  and  ether,  soluble  in  ammonia  and  the  other  alkalies,  and  also  in 
mineral  acids.  The  fact  that  this  body  is  one  of  the  few  oystalline  substances, 
occurring  physiologically,  which  contain  sulphur,  renders  its  detection  very  easy. 
Apart  from  its  insolubility  in  water,  etc. ,  it  yields,  with  caustic  potash  and  salts  of 
either  silver  or  lead,  a  brown  coloration  due  to  the  presence  of  the  sulphides  of 
these  metals. 

According  to  Dewar  and  Gamgee,2  cystin  is  amido-sulpho-pyruvic  acid,  and  its  formula  is 
C3H5NSO2— pyruvic  being  lactic  acid  minus  two  atoms  of  hydrogen. 

1  Horbaczewski,  Sitzb  d.  k.  Akad.  d.  Wiss.,  Wien,  1880.    2  Abth.,  Juni  Heft, 

2  Journ.  of  Anat.  and  Physiol.,  Nov.,  1870,  p.  143. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  909 

THE  AROMATIC  SERIES. 
Benzole  Acid.     HC7H502. 

This  is  not  found  as  a  normal  constituent  of  the  body,  but  owes  its  presence  in 
urine  to  the  fermentative  decomposition  of  hippuric  acid,  whereby  glycin  and  ben- 
zoic  acid  are  formed : 

Hippuric  acid.  Glycin.       Benzoic  acid. 

C2H4(C7H50)N02  +  H20  -  C2H5N02  +  C7H602. 

The  sublimed  acid  is  generally  crystallized  in  fine  needles,  which  are  light  and 
glistening ;  any  odor  they  possess  is  not  due  to  the  acid,  but  to  an  essential  oil.  with 
which  they  are  mixed.  \Vhen  precipitated  from  solution  the  crystalline  form  is 
always  indistinct.  This  acid  is  soluble  in  200  parts  cold  or  25  parts  of  boiling  water, 
but  is  easily  soluble  in  alcohol  or  ether.  It  sublimes  readily  at  145°  C.  ;  it  also 
passes  off  in  the  vapors  arising  from  its  heated  solutions. 

Preparation.  Either  as  above  from  hippuric  acid  by  fermentation,  by  boiling 
the  hippuric  acid  with  acids  or  alkalies,  or  by  sublimation  from  gum-benzoin. 

Tyrosin.    C9HUN03. 

Generally  accompanies  leucin,  and  is  perhaps  found  normally  in  small  quan- 
tities in  the  pancreas  and  spleen.  It  is  also  usually  obtained  in  large  quantities 
by  the  decomposition  of  proteid  matter,  either  by  putrefaction  or  the  action  of 
acids. 

The  researches  of  Radziejewski*  render  it  probable  that  tyrqsin  does  not  occur  normally  in 
any  part  of  the  human  organism,  except  as  a  product  of  pancreatic  digestion. 

All  attempts  to  synthetize  tyrosin  were  for  some  time  fruitless,  although  evi- 
dence was  obtained  sufficient  to  indicate  the  probable  existence  in  its  molecule  of 
some  aromatic  (phenyl)  radicle.2  More  recently  the  synthesis  has  been  performed,3 
and  we  now  have  every  reason  for  regarding  tyrosin  as  para-hydroxy-phenyl  a 
alanine.  This  synthesis,  as  well  as  that  of  uric  acid,  referred  to  above,  is  of  con- 
siderable importance,  since  the  more  definite  the  knowledge  which  is  possessed  of 
the  true  molecular  structure  of  the  products  of  proteid  decomposition,  the  more 
reason  is  there  for  expecting  that  the  synthesis  of  a  proteid  itself  may  be  realizable 
in  the  not  very  remote  future. 

[Fie.  254. 


Tyrosin  Crystals.] 


Tyrosin  crystallizes  in  exceedingly  fine  needles  which  are  usually  collected  into 
feathery  masses  [Fig.  254].  The  crystals  are  snow-white,  tasteless  and  odorless, 
almost  insoluble  in  cold  water,  readily  soluble  in  hot  water,  acids,  and  alkalies, 

1  Archiv.  f.  path.  Anat.,  Bd.  xxxvi.,  S.  1.    Zeitschr.  f.  anal.  Chem.,  Bd.  v.,  S.  466. 

2  Earth.,  Chem.  Centralbl.,  1865,  S.  1029;  1869,  S.  761;  1872,  S.  830.    Hufner,  Ibid.,  1869,  S.  139. 
Beilstein  u.  Kiihlberg,  Ibid.,  1872,  S.  830. 

3Erlenmeyer  u.  Lipp.,  Ber.  d.  Deutsch.  Chem.  Gesell.,  Jahrg.  xv.  (1882),  S.  1544. 


910 


APPENDIX. 


insoluble  in  alcohol  and  ether.  If  crystallized  from  an  alkaline  solution  tyrosin  often 
assumes  the  ibrm  of  rosettes  composed  of  fine  needles  arranged  radiately. 

Tyrosin  does  not  sublime  by  heating,  but  is  decomposed  with  an  odor  of  phenol 
and  nitrobenzol.  On  boiling  with  Millon's  reagent  it  gives  a  reaction  almost  identi- 
cal with,  but  much  more  marked  than,  that  for  proteids  (Hoffman's  test).  If 
tyrosin  is  treated  on  a  watch-glass  with  one  or  two  drops  of  strong  sulphuric  acid, 
then  diluted  with  a  little  water,  neutralized  with  calcic  carbonate,  and  the  solution 
filtered,  a  characteristic  violet  color  is  obtained  on  the  addition  of  a  drop  of  acid-free 
ferric  chloride  (Piria's  test). 

Preparation.  By  means  similar  to  those  employed  for  leucin,  the  separation  of 
the  two  depending  on  their  widely  differing  solubilities.  According  to  Kiihne's 
method,1  large  quantities  are  easily  obtained  as  the  result  of  pancreatic  digestion. 

Hippuric  Acid.    C9H9N03.     Or  Benzoyl-glycin.     C2H4(C7H50)N02. 

Is  found  in  considerable  quantities  in  the  urine  of  herbivora,  and  also,  though 
to  a  much  smaller  amount,  in  the  urine  of  man.  It  is  formed  in  the  body  by  the 
union  with  dehydration  of  glycin  and  benzoic  acid. 

Crystallized  from  a  saturated  aqueous  solution  it  assumes  the  form  of  fine 
needles ;  if  from  a  more  dilute  solution,  white,  semitransparent  four-sided  prisms 
are  obtained  [Fig.  255].  These  when  pure  are  odorless,  with  a  somewhat  bitter 

[Fio.  255. 


Hippuric  Acid  Crystals.] 


taste.  They  are  soluble  in  600  parts  of  cold  water,  readily  soluble  in  boiling  water, 
readily  soluble  in  alcohol,  less  so  in  ether.  All  the  solutions  redden  litmus. 

Hippuric  acid  is  monobasic,  and  forms  salts  which  are  readily  soluble  in  water 
(except  the  iron  salts) ;  from  these,  if  in  sufficiently  concentrated  solutions,  excess 
of  hydrochloric  acid  precipitates  the  acid  in  fine  needles.  When  heated  with 
concentrated  mineral  acids  it  is  resolved  into  benzoic  acid  and  glycin.  The  same 
decomposition  occurs  in  presence  of  putrefying  bodies.  Strong  nitric  acid  pro- 
duces an  odor  of  nitrobenzol. 

Preparation.  Fresh  urine  of  horses  or  cows  is  treated  with  milk  of  lime  in 
order  to  form  calcic  hippurate  and  thus  prevent  the  decomposition  of  the  hippuric 
acid,  filtered,  and  the  filtrate  evaporated  to  a  small  bulk ;  the  hippuric  acid  is  then 
precipitated  by  adding  an  excess  of  hydrochloric  acid  ;  the  acid  is  then  purified  by 
several  crystallizations  from  boiling  water. 

When  heated  in  a  small  tube,  hippuric  acid  gives  a  sublimate  of  benzoic  acid  and 
ammonic  benzoate,  accompanied  by  an  odor  like  that  of  new  hay,  while  oily,  red 
drops  are  observed  in  the  tube.  This  is  very  characteristic  and  distinguishes  it 
from  benzoic  acid. 

1  Op.  cit.  (sub.  Leucin). 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY. 


911 


Phenylic  (carlo!  ic)  Acid,  or  Ph 


C«H60. 


This  body  is  undoubtedly  obtained  as  the  result  of  the  putrefactive  decomposi- 
tion of  proteids,  notably  in  putrefactive  pancreatic  digestions.1  It  may  be  obtained 
from  the  distillate  of  such  digestive  mixtures.  It  is  also  found  in  the  contents  of 
the  alimentary  canal  under  the  same  conditions  which  give  rise  to  indol.  When  so 
occurring  a  portion  of  it  may  be  obtained  from  the  feces,  while  the  rest  reappears 
in  the  urine.'2 

Buliginsky3  says  the  urine  of  many  animals,  of  cows  and  horses  always,  contains  a  substance 
insoluble  in  alcohol,  and  not  precipated.by  lead  acetate  and  ammonia,  which  by  the  action  of 
dilute  mineral  acids  gives  carbolic  acid.  The  same  acid  applied  to  the  body  externally  or  inter- 
nally also  passes  into  the  urine.4  Similarly  benzol  (CgHe),  when  taken  into  the  stomach,  appears 
as  a  carbolic  acid  in  the  urine.s 

[Fir*.  250. 


Crystals  of  Kynurenic  Acid.    (After  Kiihne.)] 
[FiG.  257. 


Crystals  of  Barium  Kynurenate.    (After  Kiihne.)] 

The  pure  acid  crystallizes  in  long,  colorless  prismatic  needles ;  they  melt  at  35°  C. , 
and  boil  at  180°  C.  It  is  readily  soluble  in  alcohol  and  ether,  slightly  soluble  in 
water  (1  part  in  20).  In  most  cases  it  acts  as  a  weak  acid,  forming  crystalline  salts 
with  the  alkalies.  With  nitric  acid  it  yields  picric  acid.  Its  solutions  reduce  silver 
and  mercury  salts. 

1  Baumann,  Zeitsch.  f.  physiol.  Chem.,  Bd.  i.  (1877),  S.  70. 

2  Salkowski,  Ber.  d.  Deutsch.  Chem.  Gesell.,  Bd.  ix.  (1876),  S.  1595.    Centralb.  f.  d.  med.  Wiss. 
(1876\  S.  818.    Ber.  d.  Deutsch.  Chem.  Gesell.,  Bd.  x.  (1877),  S.  842.    Virchow's  Arch.,  Bd.  Ixxii. 
(1878),  S.  409.    See  also  Centralbl.  f.  d.  med.  Wiss.  (1878),  Nos.  30,  31,  34,  42,  and  Zeitschr.  f.  physiol. 
Chem.,  Bd.  ii.  (1878),  S.  241. 

3  Hoppe-Seyler,  Med.  chem.  Untersuch.,  Heft  ii.  (1867),  S.  234. 

*  Almen,  Neues  Jahrb.  d.  Pharm.,  Bd.  xxxiv.,  S.  111.    Salkowski,  Pfluger's  Archiv,  Bd.  v. 
(1871-72),  S.  335. 

6  Schultzen  u.  Naunyn,  Reichert  u.  DuBois  Reymond's  Archiv  (1867),  Heft  iii.,S.  349. 


912  APPENDIX. 

Preparation.  By  the  dry  distillation  of  salicylic  acid,  also  from  the  acid  pro- 
ducts of  the  distillation  of  coal.  It  is  obtained  in  the  last  portions  of  the  distillate 
when  preparing  indol,  and  is  separated  by  forming  a  compound  with  bromine, 


3. 

[KYNURENIC  ACID.  This  acid  is  obtained  from  the  urine  of  dogs.  It  crystal- 
lizes in  long  needles  or  four-sided  prisms  [Fig.  256],  and  with  barium  forms  peculiar 
triangular  crystals  [Fig.  257]. 

THE  BILE  SERIES. 

Cholalic  (or  Cholic)  Acid.    H.CH24H3905  +  H20. 

Occurs  in  traces  in  the  small  intestine,  in  large  quantities  in  the  contents  of  the 
large  intestine,  and  the  feces  of  men,  cows,  and  dogs.  In  icterus,  the  urine  often 
contains  traces  of  this  acid.  But  its  principal  interest  lies  in  its  being  the  starting- 
point  for  the  various  bile  acids  (see  below).  The  pure  acid  may  be  amorphous  or 
crystalline,  in  the  latter  case  crystallizing  from  hot  alcoholic  solutions  in  tetrahedra. 
These  crystals  are  insoluble  in  water  and  ether.  In  the  amorphous  form  it  is  some- 
what soluble  in  water  and  ether.  Heated  to  200°  C.  ,  it  is  converted  into  water  and 
dyslysin  (C24H4303). 

This  acid  possesses,  in  the  anhydrous  condition,  a  specific  rotatory  power  of 
+50°  for  the  yellow  light;  when  it  crystallizes  with  H20,  the  rotation  is  +35°. 
The  rotatory  power  of  the  alkali  salts  is  always  less  than  the  above,  and  when  in 
solution  in  alcohol  the  rotation  is  independent  of  the  concentration.  For  the 
alcoholic  solution  of  the  sodium  salt  the  rotation  is  +31.4°. 

Preparation.  By  the  decomposition  of  bile  acids  by  means  of  acids,  alkalies,  or 
fermentative  changes. 

Bayer  *  has  examined  the  bile  acids  obtained  from  human  bile,  and  has  prepared  from  them 
cholalic  acid.  To  this  he  assigns  the  formula  Ci8H28O4.  If  this  be  so,  then  Cholalic  acid  of  human 
bile  would  seem  to  be  a  body  entirely  different  from  that  obtained  from  ox-bile,  and  analyzed  by 
Strecker.  Bayer's  results,  however,  require  further  confirmation. 

Pettenkofer'  s  test* 

This  well-known  test  for  bile  acids  depends  on  the  reaction  of  cholalic  acid  in 
presence  of  sugar  and  sulphuric  acid.  If  to  a  solution  of  the  acid  a  little  sugar  be 
added  and  then  sulphuric  acid,  keeping  the  temperature  below,  but  not  much  below, 
70°  C.,  a  beautiful  reddish  purple  is  obtained.  If  diluted  with  alcohol  this  solution 
gives  a  characteristic  spectrum  with  two  absorption  bands,  one  between  D  and  E, 
nearest  to  E,  the  other  close  to  F  on  the  red  side  of  F. 

The  reaction  is  much  impeded  by  the  presence  of  coloring  matters  ;  moreover, 
proteids  and  other  bodies  easily  decomposed  by  sulphuric  acid,  such  as  amyl-alcohol 
and  oleic,  give  a  similar  result  ;  the  coloring  matter  produced  from  these  bodies 
does  not,  however,  give  the  absorption  bands  described  above.3 

Glycocholic  Acid.    C26H43N06. 

This  body  was  first  obtained  in  the  crystalline  form  and  described  by  Gmelin 
(1826),  who  gave  it  the  name  of  "  cholic  "  acid. 

To  avoid  confusion  it  is  now  best  to  use  the  term  "  cholic  "  as  a  synonym  for  "  cholalic," 
Demarcay,  who  first  (1838)  described  the  cholalic  acid  as  a  product  of  th'e  decomposition  of  bile 
acids,  having  given  it  the  name  of  cholic  acid.  The  name  cholalic  acid  is  perhaps  the  best,  since 
it  indicates  the  method  by  which  the  bile  acids  are  split  up,  viz.,  by  treatment  with  alkali. 

This  is  the  principal  bile  acid  of  ox-gall  :  it  is  also  present  in  the  bile  of  man,  but 
has  so  far  not  been  observed  in  that  of  carnivora.  In  icterus  the  urine  may  contain 
traces  of  this  acid. 

It  crystallizes  in  fine,  glistening  needles.  These  are  slightly  soluble  in  cold  water, 
readily  so  in  hot  water  and  alcohol,  but  insoluble  in  ether.  They  possess  a  bitter 
and  yet  sweet  taste,  and  a  strong  acid  reaction. 

The  salts  of  this  acid  are  readily  soluble  in  water  and  crystallize  well.  The  salts, 
as  well  as  the  free  acid,  exert  right-handed  polarization  amounting  to  +29.0°  for 
the  acid,  and  +  25.7°  for  the  sodium  salt,  both  measured  for  yellow  light. 

1  Zeitschr.  f.  physiol.  Chem.,  Bd.  ii.  H878-79),  S.  358. 

2  Pettenkofer.  Annalen  d.  Chem.  u.  Pharm.,  Bd.  lii:  (1844),  S.  90. 

8  For  further  information  on  this  subject  see  :  Bischoff,  Zeitschr.  f.  rat.  Med.,  Ser.  3,  Bd.  xxi.,  S. 
126.  Schulze,  Ann.  d.  Chem.  u.  Pharm.,  Bd.  Ixxi.  (1849),  S.  266.  Pchenk,  Anatom.  -physiol.  Unter- 
such.,  Wien,  1872,  S.  47.  Adamkiewicz,  Pfliiger's  Arch.,  Bd.  ix.  (1874),  S.  156. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  91& 

Glycocholic  acid  is  a  compound  of  glycin  and  cholalic  acid  ;  thus : 
Cholalic  acid.      Glycin.  Glycocholic  acid. 

C24H4005  +  C2NH50  -  H20  =  C26H43N06. 

Prolonged  boiling  with  dilute  mineral  acids  or  caustic  alkalies  decomposes  glycocholic  acid 
into  glycin  and  cholalic  acid ;  if  dissolved  in  concentrated  sulphuric  acid  and  then  warmed, 
glycocholic  acid  by  the  removal  of  one  molecule  of  water  yields  cholonic  acid,  CsoH^NOs.  The 
barium  salt  of  this  last  acid  is  insoluble  in  water,  which  fact  is  of  importance,  since  cholonic 
acid  possesses  nearly  the  same  specific  rotatory  power  as  glycocholic  acid. 

Preparation.  From  ox-gall  by  evaporation  to  a  syrup,  decolorizing  with  animal 
charcoal,  extracting  with  strong  alcohol,  and  precipitating  by  a  large  excess  of  ether. 
Its  separation  from  taurocholic  acid  depends  on  its  precipitation  by  normal  lead 
acetate,  taurocholic  acid  not  being  precipitated  by  this  reagent. 

Taurocholic  Acid.     C26H45NS07. 

Occurs  also  in  ox-gall,  but  is  found  especially  plentiful  in  human  bile  and  that 
of  carnivora,  notably  of  the  dog. 

It  crystallizes  with  difficulty  in  very  fine  needles,  which  are  exceedingly  deliques- 
cent. When  dried  it  is  an  amorphous  powder,  with  pure  bitter  taste,  easily  sol- 
uble in  water  and  alcohol,  insoluble  in  ether.  All  its  salts  are  soluble  in  water,  and 
are  precipitated  by  basic  lead  acetate  in  the  presence  of  free  ammonia.  The  sodium 
salt  dissolved  in  alcohol  has  a  specific  rotatory  power  of +24.5°  ;  if  dissolved  in 
water  this  rotation  is  less,  and  in  this  respect  it  resembles  glycocholic  acid. 

This  acid  is  far  more  unstable  than  the  preceding  one,  being  decomposed  if 
boiled  with  water.  The  products  of  decomposition  are  taurin  and  cholalic  acid. 

Taurocholic  acid  is  a  compound  of  taurin  and  cholalic  acid  ;  thus  : 

Cholalic  acid.       Taurin.  Taurocholic  acid. 

C24H4005  +  C2H7N03S  -H20  =  C26H45N07S. 

Preparation.  From  the  bile  of  dogs  by  a  process  similar  to  that  for  glycocholic 
acid.  It  is  separated  from  traces  of  this  latter  and  from  cholalic  acid  by  precipita- 
tion with  basic  lead  acetate  and  ammonia.1 


BILE  PIGMENTS. 
Bilirubin.    C16H18N203. 

It  is  found  chiefly  in  the  fresh  bile  of  man  and  carnivora,  to  which  it  gives  the 
characteristic  dark  golden-red  color.  It  frequently  constitutes  a  considerable  part 
of  some  kind  of  gall-stones,  not,  however,  as  free  bilirubin,  but  as  a  compound  with 
earthy  matter,  chiefly  chalk  ;  the  gall-stones  of  oxen  and  pigs  often  contain  40  per 
cent,  of  this  compound. 2  These  are,  therefore,  the  best  material  from  which  to 
prepare  bilirubin. 

Preparation.  The  gall-stones  are  treated  with  strong  acetic  or  dilute  hydro- 
chloric acid,  to  separate  the  earthy  matter,  and  the  residue  is  thoroughly  washed 
with  water  and  alcohol  and  dried.  From  this  residue  the  prolonged  action  of  hot 
chloroform  extracts  the  bilirubin,  which  may  either  be  obtained  in  the  amorphous 
form  by  precipitation  with  alcohol  of  its  solution  in  chloroform,  or  as  well-defined 
crystals  by  the  slow  evaporation  of  the  chloroform  solution. 

The  most  usual  form  of  the  crystals  is  that  of  rhombic  prisms  ;  they  are  readily 
soluble  in  chloroform  and  alkaline  solutions  only. 

By  treatment  with  oxidizing  agents,  such  as  nitrous  acid,  bilirubin  takes  up 
oxygen  and  becomes  biliverdin,  the  color  at  the  same  time  changing  to  green.  The 
possible  oxidation  does  not  end  here,  and  if  continued  a  series  of  products  are  ob- 
tained, each  with  a  characteristic  color,  as  in  the  well-known  Gmelin's  test.3  Of 
these  only  the  final  product  of  the  oxidation  has  been  obtained  in  a  state  of  suffi- 
cient purity  to  enable  any  definite  statements  to  be  made  of  its  characteristics.* 

This  is  the  body  known  as  Choletelin  (see  below). 

1  Parke,  Tubing.  Med.-chem.  Unters.,  Bd.  i.,  S.  160. 

2  Maly,  Sitzber.  d.  Wien.  Akad.,  Bd.  Ivii.  (1868),  ii.  Abth.,  Febr.  Hft. 

3  Tiedmann  u.  Gmelin,  Die  Verdauung,  1826,  S.  79. 

4  Heynsius  u.  Campbell,  Pfliiger's  Arch.,  Bd.  iv.  (1871),  S. 497. 

58 


914  APPENDIX. 

Biliverdin.    C16H18N204. l 

This  product  of  the  oxidation  of  bilirubin  gives  the  characteristic  color  to  the  bile 
of  herbivora  and  to  biliary  vomits.  It  occurs  also  probably  at  times  in  the  urine  of 
jaundice  and  in  the  pigmentary  matter  of  the  placenta.  It  is  found,  or  occurs  in 
traces  only,  in  gall-stones. 

Preparation.  An  impure  product  is  obtained  by  precipitating  ordinary  herbiv- 
orous bile  with  baric  chloride,  washing  the  precipitate  with  water  and  alcohol,  and 
decomposing  it  with  hydrochloric  acid.  The  biliyerdin  thus  obtained  is  washed  with 
ether  and  dissolved  in  alcohol.  From  its  solution  in  the  latter  it  is  obtained  as  an 
amorphous  green  powder  by  slow  evaporation.  Pure  biliverdin  is  best  prepared  by 
the  slow  oxidation  in  the  air  of  bilirubin,  dissolved  in  dilute  caustic  soda. 

It  does  not  crystallize,  and  is  insoluble  in  ether  or  chloroform  ;  readily  soluble  in 
alcohol.  When  oxidized  it  gives  the  same  play  of  colors  as  does  bilirubin,  with  the 
formation  of  the  same  final  and  intermediate  products. 

Neither  this  body  nor  bilirubin  gives  any  characteristic  absorption  bands. 

There  seems  now  no  reason  for  doubting  that  the  bile  pigments  are  derived  ulti- 
mately from  the  coloring  matter  of  the  blood. 

Virchow  has  described 2  the  gradual  changes  in  old  blood-clots,  as  of  cerebral 
hemorrhage,  which  lead  to  the  presence  of  the  so-called  hsematoidin-crystals. 
Though  these  have  not  been  obtained  in  sufficient  quantities  to  enable  their 
composition  to  be  finally  fixed  by  a  chemical  analysis,3  still  the  identity  of  their 
crystalline  form  with  that  of  bilirubin,  and  the  fact  that  they  both  give  the  same 
play  of  colors  when  oxidized,  as  in  Gmelin's  test,  justify  the  assumption  that 
haematoidin  and  bilirubin  are  identical.4  Moreover,  the  balance  of  experimental 
evidence  distinctly  supports  the  view  that  a  liberation  from  the  corpuscles  of  the 
coloring  matter  of  the  blood  in  the  bloodvessels  by  an  injection  of  chloroform, 
water,  etc.,  leads  generally  to  the  appearance  of  bile-pigments  in  the  urine.5  The 
occurrence  of  bilirubin  crystals  in  the  urine  has  frequently  been  observed  after 
the  operation  of  transfusion  of  blood  in  man.  The  chemical  possibility  of  the  con- 
version of  haemoglobin  into  biliverdin  is  readily  seen  by  a  comparison  of  the  formulae 
of  haematin  (see  p.  464)  and  bilirubin.  The  former  has,  according  to  Hoppe-Sey- 
ler,6  the  composition  indicated  in  the  formula  2  (Cg^gN^FeCy,  while  that  of  bili- 
rubin is  Ci6H18N203.  Although  the  conversion  has  not  as  yet  been  directly  effected, 
the  following  facts  are  significant :  If  bilirubin  is  treated  with  sodium  amalgam  the 
substance  known  as  hydrobilirubin  (see  below)  is  obtained.  If  haematin  is  dissolved 
in  caustic  soda  and  treated  with  sodium  amalgam  or  in  hydrochloric  acid  solution 
with  zinc  dust,  a  substance  is  obtained  which  is  now  recognized  as  identical  with 
hydrobilirubin.7  This  is  the  most  direct  chemical  evidence  of  the  relation  of  the 
coloring  matters  of  the  blood  and  bile. 

Choletelin.    C16H18N206(?).8 

This  substance  is  obtained  as  the  final  product  of  the  oxidation  of  either  bilirubin 
or  biliverdin.  It  is  best  prepared  by  acting  upon  bilirubin  with  nitrous  acid  in 
presence  of  alcohol ;  the  various  colors  of  Gmelin's  reaction  are  observed  and  the 
final  red  dish-yellow  solution,  if  poured  into  water,  yields  a  precipitate  of  choletelin. 
It  is  not  crystalline  and  is  soluble  in  alcohol,  ether,  and  chloroform.  When  freshly 
prepared  it  seems  to  give  an  uncertain  absorption  band  if  examined  in  an  acid  solu- 
tion. On  this  account  some  observers 9  have  been  led  to  regard  it  as  identical  with 
hydrobilirubin  (urobilin).  There  is,  however,  no  doubt  that  they  are  quite  distinct 
bodies.10 

1  Maly,  Sitzb.  d.  Wien.  Akad.,  Ed.  Ixx.  (1874).  iii.  Abth. 

2  Arch.  f.  path.  Anat.,  Bd.  i.,  S.  383. 

3  Robin,  Ann.  d.  Chem.  u.  Pharm.,Bd.cxvi.,S.  89. 
*  But  see  also  Preyer,  Die  Blutkrystalle,  1871,  S.  187. 

5  Tarchanoff,  Pfliiger's  Arch.,  Bd.  ix.  (1874),  S.  53.    See  also  Bd.  x.  (1875),  S.  208. 
e  Physiologische  Chemie,  1879,  S.  395. 

T  Hoppe-Seyler,  Med.-chem.  Untersuch.,  Heft  iv.,  1871,  S.  523.  Ber.  d.  Deutsch.  chem.  Gesell., 
Vii.  (1874),  S.  1065. 


7  Hoppe-S 
i.  (1874),  S. 

8  Maly,  Si 


aly,  Sitzb.  d.  Wien.  Akad.,  Bd.  Ivii.  (1868) ;    2  Abth.  Febr.  und  Bd.  lix.,  1869 ;    2  Abth.  April. 
See  also  Heynsius  and  Campbell,  loc.  cit. 

9  Heynsius  and  Campbell,  loc.  cit.    Stokvis,  Centralbl.  f.  d.  med.  Wiss.,  No.  14  (1873),  S.  211. 

10  Maly,  Centralbl.  f.  d.  med.  Wiss.,  No.  21  (1875),  S.  321.    Liebermann,  Pfliiger's  Arch.,  Bd.  xi. 
(1875),  S.  181. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  915 

Hydrobilirubin.     C32H40N407. 

This  body  was  first  described  by  Maly1  as  resulting  from  the  action  of  sodium 
amalgam  on  an  alkaline  solution  of  bilirubin.  When  the  reaction  is  complete  the 
solution  is  precipitated  with  hydrochloric  acid,  the  precipitate  dissolved  in  ammo- 
nia, again  precipitated  by  acid,  and  the  substance  thus  finally  obtained  is  washed 
with  water.  It  is  readily  soluble  in  alcohol,  less  so  in  ether.  Its  alkaline  solutions 
are  yellow,  and  these  turn  pink  on  the  addition  of  acid.  Both  its  acid  and  alka- 
line solutions,  the  latter  especially  on  the  addition  of  a  few  drops  of  chloride  of 
zinc,  give  a  characteristic  absorption  band  between  b  and  F.2  In  the  colors  of  its 
alkaline  and  acid  solutions  and  the  greenish  fluorescence  of  its  ammoniacal  solution 
on  the  addition  of  chloride  of  zinc,  and  in  its  absorption  spectrum,  hydrobilirubin 
shows  its  close  relation  to  the  urobilin  (see  below),  with  which  indeed  it  is  now  con- 
sidered to  be  identical.  It  is  also  identical  with  a  body  named  stercobilin,3  which 
had  previously  been  described  as  a  product  of  the  alteration  of  the  bile-pigments  in 
the  alimentary  canal  occurring  in  feces.  There  is  no  difficulty  in  seeing  how  this 
change  (hydrogenation)  can  be  brought  about  in  the  intestine  since  it  is  known  that 
a  considerable  quantity  of  hydrogen  may  make  its  appearance  by  fermentative  pro- 
cesses in  the  intestine,  and  in  its  nascent  state  might  readily  produce  the  simple 
change  which  is  known  to  occur  when  bilirubin  is  converted  into  hydrobilirubin. 

PIGMENTS  OF  URINE. 

Our  knowledge  of  these  bodies  is  at  present  limited  and  imperfect.  Most  prob- 
ably4 they  are  numerous,  but  only  two  appear  sufficiently  well  characterized  to 
deserve  mention  here. 

Urobilin.    C32H40N407. 

As  stated  above,  this  is  now  regarded  as  identical  with  hydrobilirubin.  It  was 
first  described  by  Jaffe5  as  a  well-characterized  normal  urinary  pigment  and  its 
identity  with  hydrobilirubin  subsequently  determined.6 

Normal  urine  contains  only  small  quantities  of  urobilin,  but  there  is  present  a 
substance  (chromogen)  which  under  the  influence  of  acids,  with  absorption  of 
oxygen,  yields  urobilin.  The  urine  of  fever  frequently  contains  a  considerable 
amount  of  actual  urobilin  as  such. 

The  properties  described  above  for  hydrobilirubin  are  identical  with  those  of 
urobilin.  Its  preparation  from  urine  is  somewhat  difficult,  and  for  this  some  spe- 
cial manual  must  be  consulted.7 

Uroerythrin. 

Is  considered  to  be  the  substance  which  gives  to  the  urine  of  rheumatism  its 
characteristic  color.  Very  little  is  known  of  its  chemical  properties.8  It  appears 
to  be  an  amorphous  reddish  body  with  an  acid  reaction,  slowly  soluble  in  water, 
alcohol,  and  ether.  When  treated  with  caustic  alkali  it  turns  green.  Urine  con- 
taining this  body  takes  on  a  characteristic  reddish-yellow  color  on  the  addition  of 
concentrated  hydrochloric  acid. 

Thudichum  considers  that  normal  urine  contains  only  one  pigment,  which  he  calls  uro- 
chrorne.9  Maly  is  inclined  to  regard  this  as  the  same  as  urobilin.10  More  recently  Thudichum 
has  upheld  his  former  views.11 

THE  INDIGO  SERIES. 
Indican.    C8H7NS04. 

A  body  was  long  ago  described 12  as  occurring  in  the  urine  and  sweat  of  men 
and  other  animals  which  yielded  by  the  action  of  acids  the  blue  coloring  matter 

i  Centralb.  f.  d.  med.  Wiss.,  No.  54, 1871.    Annal.  d.  Chem.,  Bd.  clxiii.  (1872),  S.  77. 
Vierordt,  Zeitschr.  f.  Biol.,  Bd.  ix.  (1873),  S.  160. 
Vanlair  and  Masius,  Centralbl.  f.  d.  med.  Wiss.,  No.  24,  1871. 
Vierordt,  Die  quantitativ  Spectralanalyse,  etc.,  Tubingen,  1876,  S.  81. 
Centralb.  f.  d.  med.  Wiss.,  1868,  S.  243.  "Virchow's  Arch.,  Bd.  xlvii.  (1869),  S.  405. 
Maly,  Ann.  d.  Chem.  u.  Pharm.,  Bd.  clxiii.  (1872),  S.  77. 
Vide  Neubauer  and  Vogel,  Harnanalyse,  ed.  viii.  (1881),  S.  81. 
Heller's  Archiv  (2)  Bd.  iii.  (1854),  S.  361. 
Brit.  Med  Journ.,  N.  S.,  No.  201,  1864,  p.  509. 

10  Maly,  Ann.  d.  Chem.  u.  Pharm.,  loc.  cit.,  1872,  S.  90. 

11  Journ.  Chem.  Soc.,  Ser.  2,  vol.  xiii  (1875),  pp.  397,  401. 

12  Schunk,  Phil.  Mag.,  vol.  x.  p.  73 ;  xiv.  p.  228;  xv.  pp.  29,  117,  183.    Chem.  Centralbl.,  1856,  S. 
50 ;  1857,  S.  957  ;  1858,  S.  225.    Hoppe-Seyler,  Arch.  f.  path.  Anat.,  Bd.  xxvii.,  S.  388.  Jaffe,  Pfltiger's 
Arch.,  Bd.  iii.  (1870),  S.  448. 


916  APPENDIX. 

indigo  as  one  of  the  products  of  its  decomposition.  Schunk  considered  this  sub- 
stance to  be  identical  with  the  indicari  known  to  occur  in  several  plants  (Indigofera, 
Isatis).  Hoppe-Seyler,1  on  the  other  hand,  having  regard  to  the  greater  ease  with 
which  the  indican  from  plants  undergoes  decomposition,  regarded  them  as  most 
probably  different  substances.  Baumann  has  shown 2  that  the  two  are  really  differ- 
ent, and  has  confirmed  his  earlier  statements  in  a  more  recent  publication.3 
According  to  him,  the  indican  obtained  from  urine  is  not  a  glucoside  (so  also 
Hoppe-Seyler)  and  yields  sulphuric  acid  by  the  action  of  hydrochloric  acid.  He 
assigns  to  it  the  formula  C8H6N.O.S02.OH,  and  regards  it  as  indoxy-sulphuric  acid. 
The  acid  itself  is  not  yet  known  in  the  free  state,  but  it  yields  stable  salts  such  as 
that  of  potassium.  CgHeN.SCXK.  It  occurs  largely  in  the  urine  as  the  result  of 
the  presence  of  indol  in  the  alimentary  canal.  In  this  way  Baumann  and  Brieger  * 
were  enabled  to  obtain  large  quantities  by  giving  indol  to  a  dog.  For  its  prepara- 
tion their  original  paper  must  be  consulted. 

When  treated  in  aqueous  solution  with  hydrochloric  acid  in  presence  of  oxygen 
it  yields  indigo-blue. 

2C8H6NS04K  +  02  -  2C8H5NO  +  2KHS04. 

It  is  always  estimated  in  urine  by  conversion  into  indigo-blue. 

Indigo.    C8H5NO. 

It  is  formed,  as  stated  above,  from  indican,  and  gives  rise  to  the  bluish  color 
sometimes  observed  in  sweat  and  urine. 

It  may,  by  slow  formation  from  indican,  be  obtained  in  fine  crystals;  these  are 
insoluble  in  water,  slightly  soluble,  with  a  faint  violet  color,  in  alcohol  and  ether. 
Chloroform  also  dissolves  them  to  a  slight  extent.  Indigo  is  soluble  in  strong  sul- 
phuric acid,  forming  at  the  same  time  two  compounds  with  this  acid ;  these  are 
soluble  in  water.  It  possesses  a  pure  blue  color ;  when  pressed  with  a  hard  body 
a  reddish  copper-colored  mark  is  left,  and  the  crystals  exhibit  the  same  color  if  seen 
in  reflected  light. 

The  soluble  compounds  with  sulphuric  acid  give  an  absorption  band  in  the  spec- 
trum which  lies  close  to  the  D  line  and  to  the  red  side  of  it.  This  may  be  used  to 
detect  indigo. 

Treated  with  reducing  agents,  indigo  is  decolorized,  being  reduced  to  indigo- 
white.  The  latter  contains  two  atoms  more  hydrogen  than  indigo. 

Indol.    C8H7N. 

To  this  body  the  specific  odor  of  the  feces  is  partly  due.  It  is  obtained  as  the 
final  product  of  the  reduction  of  indigo,  and  also  by  the  distillation  of  proteid 
matter  with  caustic  alkalies.5 

It  often  occurs  among  the  products  of  the  action  of  pancreatic  ferment  on  pro- 
teids ;  its  presence  in  such  cases  appears,  however,  to  be  due,  not  to  the  action  of 
the  trypsin,  but  to  a  simultaneous  putrefaction  under  the  influence  of  bacteria, 
etc.6  If  the  pancreatic  digestion  be  carried  on  in  the  presence  of  salicylic  acid, 
indol  does  not  make  its  appearance.  Indol  is  a  crystalline  body,  soluble  in  boiling 
water,  alcohol,  and  ether.  It  passes  over  in  the  steam  when  its  aqueous  solution 
is  boiled.  It  is  characterized  by  the  following  reactions :  A  strip  of  pine-wood 
moistened  with  hydrochloric  acid  is  colored  bright  crimson  when  dipped  into  a 
solution  of  indol.  Its  alcoholic  solution  turns  red  when  treated  with  nitrous  acid, 
and  its  aqueous  solution  gives  a  copious  red  precipitate  with  the  same  reagent.  It 
also  yields  a  characteristic  crystalline  compound  with  picric  acid. 

i  Handb.  d.  path.  chem.  Anal.,  ed.  iv.  (1875),  S.  191. 

8  Pfliiger's  Arch  ,  Bd.  xiii.  (1876),  S.  301.    Zeitschr.  f.  physiol.  Chem.,  Bd.  i.  (1877-78),  S.  60. 

3  Zeitschr.  f.  physiol.  Chem.,  Bd.  iii.  (1879),  S.  254. 

*  Zeitschr.  f.  physiol.  Chem.,  Bd.  iii.  (1879),  S.  254.  See  also  Ber.  d.  Deutsch.  chem.  Gesell.,  xii. 
(1879),  Sn.  1098,  1192,  2166;  and  xiii.  (1880),  S.  408. 

5  Kuhne.  Ber.  d.  deutsch.  chem.  Gesell.,  viii.  (1875),  S.  206. 

e  Kuhne,  Verhand.  Heidelb.  naturhist.-med.  Ver.,  N.S.,  Bd.  i.,  Hft.  3.  Ber.  d.  Deutsch.  chem. 
Gesell.  (1875),  S.  206. 


CHEMICAL  BASIS  OF  THE  ANIMAL  BODY.  917 

Skatol.    C9H9N  (?). 

Noticed  by  Brieger1  as  one  of  the  products  of  putrefactive  changes  in  the  small 
intestine.  Secretan2  had  previously  described  a  similar  substance  as  arising  from 
the  putrefaction  of  albumin. 

Skatol  is  crystalline  and  contains  nitrogen ;  it  is  more  soluble  in  water  than  indol, 
and  does  not  give  rise  to  any  red  coloration  with  nitrous  acid. 

Skatol  readily  passes  into  the  urine  when  it  occurs  in  the  alimentary  canal,  and 
then  gives  a  violet-red  reaction  with  strong  hydrochloric  acid. 

v.  Nencki 3  prepares  this  substance  by  the  putrefaction  of  a  mixture  of  finely 
divided  pancreas  and  muscle  substance.  After  the  addition  of  acetic  acid  the 
mass  is  distilled,  when  the  skatol  readily  passes  over.  From  the  distillate  it  is 
precipitated  by  picric  acid,  and  the  precipitate  when  again  distilled  with  ammonia 
gives  off  pure  skatol,  which  may  be  finally  purified  by  crystallization. 

[PTOMAINES  AND  LEUKOMAINES. 

These  substances  comprise  the  so-called  animal  alkalies,  and  belong  to  a  class 
of  amines.  They  are  called  alkaloids  because  of  their  close  resemblance  in  toxic 
properties  to  the  same  class  of  substances  obtained  from  plants.  The  difference 
between  ptomaines  and  leukomaines  as  classes  is  that  the  former  are  products  of 
abnormal  metabolism  and  the  latter  of  normal  metabolism.  Ptomaines  are  usually 
highly  toxic,  while  leukomaines  are  of  feeble  toxicity.  It  seems  probable  that  in 
specific  diseases  the  disease  germs  form  ptomaines  and  that  the  peculiar  patholog- 
ical states  are  due  in  a  large  measure  to  these  substances  acting  as  peculiar  poisons.] 

1  Ber.  d.  Deutsch.  chem.  Gesell.,  Jahrg.  x.  (1877),  S.  1027. 

2  Recherches  sur  putrefaction  de  1'albumine.    Geneva.  1876. 

3  Centralbl.  f.  d.  med  Wiss.,  1878,  S.  849. 


INDEX. 


A  BEKRATION,  chronic,  758 

ll_         spherical,  757 

Absorption,  from  alimentary  canal,  325 

of  diffusible  substances  and  water,  331 

of  fats,  329 
Accommodation,  745,  751 

mechanism  of,  749 
Acetic  acid  series,  887 
Acid-albumin,  84 

formation  of,  255 
Acid,  acetic,  887 

aspartic,  907 

benzoic,  909 

butyric,  888 

capric,  888 

caproic,  888 

caprylic,  888 

cholic,  912 

ethylene-lactic,  891 

ethylidene-lactic,  891 

formic,  887 

glutaminic,  908 

glycerin-phosphoric,  895 

glycocholic,  912 

hippuric,  910 

kynurenic,  906 

lactic,  891 

laurostearic,  888 

myristic,  888 

oleic,  889 

oxalic,  892 

palmitic,  888 

phenylic,  911 

propionic,  888 

sarcolactic,  891 

stearic,  888 

succinic,  892 

taurocholic,  913 

uric,  899 

salts  of,  900 

valerianic,  888 
Acoustic  apparatus,  798 
Adipose  tissue,  470 
Afferent  impulses,  526,  706 
After-images,  775 

negative,  775 

positive,  775 
Albumin,  acid,  868 

alkali,  869 

derived,  868 

egg,  867 

native,  867 

nucleo-,  884 

serum,  865 


Albuminous    glands,   changes    in,  during 

secretion,  269 
Alkali  albumin,  85 
Allantoin,  903 
Allantois,  842 
Alvergniat's  pump,  349 
Amblyopia,  695 
Amides,  896 

Amoeba,  characteristics  of,  17 
Amoeboid  movements,  121 
Anelectrotonus,  103 
Animal  body,  chemical  basis  of,  863 
Animal  gum,  888 
heat,  496 

distribution  of,  496 

production  of,  500 

regulation  by  variations  in  loss, 
499 

sources  of,  496 

temperature  of  body,  498 
Aphasia,  676 

complete,  679 
partial,  679 
Apncea,  384 
Aqueous  humor,  744 
Area  for  mastication,  666 

production  of  voice,  666 

speech,  676 

swallowing,  666 
Aromatic  series,  909 
Arteries,  changes  in  calibre  of,  211 
Ascites,  322 
Asparagine,  907 
Aspartic  acid,  907 
Asphyxia,  386 
Astigmatism,  757 
Atropine,  action  of,  on  heart,  208 

on  pupil,  756 
Auditory  judgments,  805 

sensations,  800 
Automatic  actions,  131 
Automatism,  irregular,  721 
regular,  721 


DENZOIC  acid,  909 
D     Bile,  278 

acids,  280 

formation  of,  456 

action  of,  on  food,  281 

antiseptic  qualities  of,  281 

characters  of,  278 

composition  of,  278 

formation  of  constituents  of,  453 
919 


920 


INDEX. 


Bile,  influence  on  peptic  digestion,  309 
pigments  of,  279,  453,  913 
Gmelin's  test  for,  280 
resorption  of,  291 
salts,  280 

tests  for,  280 

Pettenkofer's,  280 
secretion  of,  288 
series,  912 
Bilirubin,  279,  912 
Biliverdin,  280,  913 
Binocular  vision,  781 
Bladder,  muscles  of,  427 
Blindness,  695 
color,  774 

Blind  spot,  filling  up  of,  779 
Blood,  24 

carbonic  acid  in,  360 
changes  in  quantity  of,  236 
chemical  composition  of,  50 
clotting  of,  26 

buffy  coat  in,  27 
causes  of,  38 
corpuscles  in,  37 
crassamentum  in,  26 
effect  of  neutral  salts  on,  32 
effect  of  sodium  chloride  on,  28 
effect  of  temperature  on,  31 
fibrin  in,  28 
fibrin  ferment  in,  34 
fibrinogen  in,  33 
in  arteries,  37 
in  veins,  36 
length  of  time  of,  27 
paraglobulin  in,  30 
plasmine  in,  32 
rapidity  of,  27 
serum  in,  29 
serum-albumin  in,  30 
color  of,  37 

brightness  of,  due  to  reflection  of 

light  through  corpuscles,  39 
venous  and  arterial,  356 
composition  of,  50 
gases,  349 

methods  of  obtaining,  346 
hsematin,  359 

features  of,  359 
method  of  obtaining,  359 
haemin,  360 

crystals  of,  360 
haemoglobin,  351 

characters  of,  351 
method  of  obtaining,  352 
spectroscopic  features  of,  353 
varieties  of  crystals.  352 
composition  of,  methsemoglobin,  360 
corpuscles,  37 

average    number    of,    in    human 

blood,  40 

characteristics  of,  37 
red,  structure  of,  40 

haemoglobin  in,  40 
white,  43 

effect  of  deficient  aeration  on,  382 
platelets,  49 


Blood  plaques,  49 
pressure,  137 

curves  from  carotid  of  rabbit,  140 
quantity  of,  its  distribution  in  body,  54 
rate  of  circulation,  150 
relations  of  oxygen  in,  350 
respiratory  changes  in,  346 
supply,  influence  of,  on  contractile  tis- 
sues, 113 
of  liver,  289 
Brain,  584 

absence  of  signs  of  volition  and  intelli- 
gence, 637 

and   spinal   cord,  lymphatic  arrange- 
ments of,  728 
membranes  of,  728 
vascular  arrangements  of,  732     . 
arteries  of,  distribution  and  characters 

732 
blood-supply  of,  734 

methods  of  investigating,  735 
bulb  of,  589 

fibres  of,  598 

central  gray  matter  of,  601 
cerebro-spinal  fluid,  731 
changes  in  gray  matter  of,  595 
commissural  fibres  of,  634 
connections  of  grny  and  white  matter 

of,  601 

corpora  geniculata,  625 
quadrigemina,  625 

gray  matter  of,  625 
corpus  striatum,  625 
cortical  fibres,  631 
cranial  nerves,  589 
embryonic,  584 
fibres  of,  627 
functions  of,  716 

gracile  and  cuneate  nuclei  of,  597 
gray  matter  of,  601 
histological  features  of,  653 
intermediate  gray  matter  of,  615 
longitudinal  fibres  of,  628 
nature  and   relations  of   the  several 

nuclei  of,  613 
olivary  nucleus  of,  596 
optic  thalamus,  615 
posterior  bundles  of,  633 

cortical  of,  630 
pyramidal  tract  of,  628 
splanchnic  functions  of,  720 
structure  of,  584 
superficial  gray  matter  of,  615 
superior  or  sensory  decussation,  592 
Brain,  tracts  from  corpora  quadrigemina  of, 

633 

venous  arrangement  of,  733 
white  matter  of,  530 
Broca's  convolution,  676 
Burdach.  columns  of,  540 
Butyric  acid,  889 


CALABAR  bean,  action  of,  on  pupil,  756 
U     Calorimeters,  494 
Capillaries,  circulation  in,  143,  231 


INDEX. 


921 


Capillaries,  pressure  of  blood  in,  142 

stagnation  or  stasis,  235 

vessels  of  muscle,  77 
Capric  acid,  889 
Caproic  acid,  889 
Caprylic  acid,  889 
Carbohydrates,  88,  884 
Carbonic  acid  exhaled,  344,  364 
Cardiac  contraction,  features  of,  196 

curves,  discussion  of,  168 

impulse,  162 

phases,  duration  of,  173 
Carnin,  899 
Casein,  259,  868 
Cell  body,  126 
Cells,  pyramidal,  656 
Central  nervous  system,  vasomotor  functions 

of,  222 

Cerebellar  tract,  course  of,  556 
Cerebellum,  corpus  dentatum  of,  625 

functions  of,  716,  719 

gray  matter  of,  653 

histology  of,  654 

superior  pedicle  of,  632 
Cerebral  convolutions,  areas  of,  662 
of  the  dog,  662 

cortex,  655 

hemispheres,  phenomena  exhibited  by 
animals  deprived  of,  637-643 

operations,  time  taken  up  by,  724 
Cerebrin,  98,  895 
Cerebro-spinal  fluid,  730 

nerves,  124 

Cerebrum,  under  surface  or  base  of,  123 
Changes  of  living  tissue,  46 
Charcot's  crystals,  896 
Chauveau   and   Lortet's  hsematachometer, 

153 

Chemical  changes,  89 
Cheyne-Stokes  respiration,  385 
Chitin,  883 
Cholalic  acid,  912 
Cholesterin,  97,  279,  893 
Choletelin,  915 
Chondrin,  882 
Chorda  saliva,  721 

tympani  nerve,  nature  and  action  of, 

264 

Chordae  vocales,  821 
Choroid  coat,  740 
Chromatic  aberration,  757 
Chyle,  characters  of,  316 
Chyme,  307 
Ciliary  movement,  119 
Circles,  diffusion,  746 

Circulation,  causes  of  irregular  heart-beat, 
238 

circumstances  determining  character  of 
flow,  144 

effects  of  alcohol  on,  241,  242 
of  food  on,  244 

foetal,  851 

hydraulic  principles  of,  144 

influence  of  exercise  on,  242 
Clarke,  columns  of,  538 
Cochlea,  794 


Cold,  sensations  of,  703 
Color-blindness,  774 

sensations,  769 
Colostrum,  480 
Composition  of  bile,  279 

of  blood.  50 

of  gastric  juice,  252 

of  milk,  480 

of  perspiration,  432 

of  saliva,  246 

of  starving  body,  484 

of  the  animal  body,  483 

of  urine,  409 
Concha,  798 
Connective  tissue,  134 
Constant  current,  action  of,  100 
Contractile  muscles,  85 

tissues,  54,  116 
Contraction,  breaking,  101 

idio-muscular,  112 

making,  101 

tetanic,  57 

wave  of,  78,  117 

Contrast  of  visual  perceptions,  778 
Coordinated  movements,  machinery  of,  643- 

652 
Cord,  spinal,  529 

volitional  impulses  in,  585 
Cornea,  738 
Cornu  ammonis,  660 
Corpora  geniculata,  625 

quadrigemina,  functions  of,  719 
Corpus  luteum,  840 
Corpuscles,  connective  tissue,  134 

number  of,  in  human  blood,  40 

of  blood,  37 
Cortex,  cerebral,  655,  721 

histology  of,  655-661 
psychical  processes  in,  722 
Corti,  organ  of,  795 

pillars  of,  795 

rods  of,  795 
Cortical  areas,  removal  of,  674 

motor  region,  662-668 
Cramps,  muscular,  118 
Cristse  acusticse,  798 
Crystalline  lens,  744 
Crystals,  Charcot's,  896 
Currents,  constant,  58 

electrotonic,  104 

induced,  58 

of  action,  95 

of  rest,  95 

Curves  of  endocardiac  pressure,  168 
Cutaneous  sensations,  703 
Cystin,  908 


DANIELL'S  battery,  57 
Death,  863 

Decomposition  of  proteids  by  digestion,  879 
Defecation,  301 
Deglutition,  294 
Depressor  nerve,  225 
Detection  in  solutions,  808 
Development  of  embryo,  841 


922 


INDEX. 


Development  of  placenta,  846 

Dextrin,  248,  886 

Dextrose,  248,  884 

Diabetes,  448 

Diagrammatic  eye,  745 

Diaphragm  as  a  respiratory  muscle,  339 

Dicrotism,  185 

Diet,  normal,  513 

modifications  of,  519 
Difference  in  respiration  of  sexes,  339 
Digestion,  245 

decomposition  of  proteids  by,  879 
Dilatation  of  arteries,  134 
Dioptric  apparatus,  imperfections  in,  757 

mechanisms,  744 
Discharge  of  energy,  722 
Diuretics,  425 
Divisions  of  ear,  790 

of  spinal  cord,  530 
Dubois-Reymond  key,  60 
Ducts  of  mammary  glands,  477 
Dyspnoea,  339,  380 


EAR,  790 
cochlea  of,  794 

division  of,  790 

Eustachian  tube,  798 

external,  790 

internal,  or  labyrinth,  793 

meatus  auditorius  externus,  790 

membrane  of  Reissner,  796 

membrani  tympani,  792 

membranous  labyrinth,  798 

organ  of  Cord,  796 

osseous  labyrinth  of,  797 

ossicles  of,  798 

perilymph,  797 

physiological  anatomy  of,  790 

stapedius  muscle  of,  800 

tensor  tympani  muscle  of,  799 
Efferent  impulses.  525 
Elastin,  883 
Electrical  changes,  92 

stimuli,  57 

Electrodes,  non-polarizable,  92 
Electrotonic  currents,  104 
Electrotonus,  101 

variations  of  irritability  during,  103 
Eleventh  nerve,  603 
Embryo,  nutrition  of,  847 
Emmetropic  eye,  752 
Energy,  discharge  of,  722 

expenditure  of,  493 

income  of,  492 
of  mechanical  work,  495 
Entoptic  phenomena,  759 
Enzymes,  880 
Epiglottis,  820 

Epileptiforrn  convulsions,  671,  675 
Epithelium  of  the  ducts,  functions  of,  276 
Ethylene-lactic  acid,  892 
Ethylidene-lactic  acid,  892 
Eustachian  tube,  800 
Eupnoea,  380 
Exhalation  of  aqueous  vapor,  344 


Exhalation  of  carbonic  acid,  344,  364 

of  organic  matters,  346 
Exhaustion,  causes  of,  115 
Expiration,  342 
Eye,  738 

apparent  size,  780 

ciliary  region  of,  740 

diagrammatic,  745 

emmetropic,  752 

horizontal  section  of,  739 

hypermetropic,  748 

images  reflected  from,  750 

mechanisms  of,  789 

movements  of,  781 

muscles  of,  781 

myopic,  748 

physiological  anatomy  of,  738-744 

pigment  cells  of,  741 

presbyopic,  748 

FACIAL  nerve,  607 
course  of,  607 
^allopian  tubes.  832 
faradization,  63 
Fats,  complex  nitrogenous,  892 
formation  of,  474 
nature  of,  in  adipose  tissue,  472 
neutral,  890 

their  derivatives  and  allies,  888 
Feces,  313 

Female  generative  apparatus,  834 
Fibres,  afferent,  124 
efferent,  124 
sensory,  124 
vaso-constrictor,  216 
vaso-dilator,  216 

course  of,  218 
Fibrin,  875 

in  clotting  of  blood,  28 
Fibrinogen,  872 
Fick's  spring  manometer,  176 
Fifth  nerve,  608 

branches  of,  609 
Flatulence,  308 
Foetal  circulation,  850 
Food,  action  of  bile  and  pancreatic  juice  in 

small  intestine,  308 
changes  of,  in  large  intestine,  312 
in  mouth,  306 
in  small  intestine,  308 
in  stomach,  306 

effects  of  fatty  and  carbohydrate,  489 
of  gelatin  as,  490 
salt  as,  491 
peptone  as,  491 
-stuffs,  245 

classes  of,  245 
Formic  acid,  888 
Fourth  nerve,  612 
Fovea  hemispherica,  794 
Functional  activity,  influence  of,  on  con- 
tractile tissues,  114 


VI 


N,  veins  of,  734 
Gall-stones,  286 


INDEX. 


923 


Gases  in  blood,  50 

Gastric  and  intestinal  movements,  nervous 

mechanisms  of,  302 
digestion,  circumstances  affecting,  257 
acidity,  258 
temperature,  258 
juice,  251 

action  of,  on  milk,  259 

on  proteids,  252 
chemical  characters  of,  251 
formation  of  free  acid  of,  276 
nature  of  the  action  of,  258 
secretion  of,  266 
Gelatin,  134,  882 
Generative  organs,  female,  834 
male,  833 

physical  anatomy  of,  834 
Globin,  874 
Globulins,  871 
Glottis,  narrowing  of,  823 

widening  of,  824 
Glutamidic  acid,  908 
Glutin,  882 
Glycerin,  891 

-phosphoric  acid,  895 
Glycin,  906 
Glycogen,  88,  437,  887 
characters  of,  438 
conversion    of,  into    sugar    by    liver, 

438 

in  muscle,  447 
in  placenta,  448 
uses  of,  in  liver,  446 
Glycolic  acid  series,  892 
Gmelin's  test  for  bile,  280 
Goll,  columns  of,  540 
Graafian  follicle,  938 
Gray  matter,  128 
Grove's  battery,  57 
Guanin,  905 

Gudden's  commissure,  693 
Gum,  animal,  888 
Gustatory  bulbs,  811 

mucous  membrane,  physiological  anat- 
omy of,  808 
pore,  811 

H^MACYTOMETEE  of  Gowers,  42 
Hsemadrornometer,  Volkmann's,  151 
Haematachometer,  Chauveau  and  Lortet's, 

153 

Hjfimatin,  359 
Hsemin,  360 
Haemoglobin,  351 

in  red  blood-corpuscles,  40 
Hearing,  sensations  of,  701 
Heart,  157 

-beat,  augmentation  of,  203 

government  of,  by  the  nervous  sys- 
tem, 199 

reflex  inhibition  of,  203 
cardiac  cycle  of,  158 

sound  and  tambour,  166 
change  of  form  of,  160 
duration  of  the  several  phases  of  the 
cardiac  cycle,  173 


Heart,  endocardiac  pressure,  164 

methods  of  determining,  165 
first  sound  of,  163 
impulse  of,  162 

influences  regulating  beat  of,  209 
main  events  occurring  in  ventricle  dur- 
ing a  beat,  171 
negative  pressure,  nature  and  cause  of, 

172 
normal  beat  of,  157 

analysis  of,  193 
development  of,  192 
regulation  of  beat  of,  191 
second  sound  of,  163 
sounds  of,  162 
summary  of  events  constituting  a  beat, 

174 

valves  of,  158 
ventricular  systoles,  159 
visible  movements  of,  157 
work  done  by,  175 
Heat,  sensations  of,  703 
Helmholtz's  arrangement  for  equalizing  the 

make  and  break  shocks,  63 
phakoscope,  750 
Hemiamblyopia,  696 
Hemianopia,  696 
Hemianopsia,  696 
Hemiopia,  696 
Hippocampal  lobule,  700 
Hippuric  acid,  910 

formation  of,  422 
Histohsematin,  86 
Horopter,  786 
Human  brain,  nomenclature  of  the  surface 

of,  676, 
ovum,  836 

Hyaloid  membrane  of  eye,  741 
Hydrobilirubin,  915 
Hypernoea,  380 
Hypnotic  phases,  684 
Hypoglossal  nerve,  602 
course  of,  602 
Hypoxanthin,  903 


IMAGE,  formation  of,  744 
Imperfect  speech,  679 
Impregnation,  841 
Impulse,  nervous,  98 
Indican,  915 
Indigo,  916 

series,  915 
Indol,  284,  916 
luduction  coil,  59,  71 
Inflammation,  phenomena  of,  232 
Inhibition,  features  of,  200 

of  frog  s  heart  by  stimulation  of  vagus 

nerve,  200 

Inhibitory  nerves,  132 
Inogen,  116 
Inosit,  886 
Inspiration,  340 

Intercostal  muscles  in  respiration,  341 
Interfibrillar  substance,  80 
Intestinal  digestion,  308 


924 


INDEX. 


Iris,  741 
Irradiation,  778 


JACOB'S  membrane,  743 
eJ     Jaundice,  456 
Juice,  gastric,  251 

action  on  proteids,  253 

artificial,  253 

composition  of,  252 
pancreatic,  281 

action  on  food-stuffs,  282 


T7ATELECTEOTONUS,  103 

IV     Keratin,  883 

Kidneys,  vaso-constrictor  nerves  of,  415 
vaso-dilator  nerves  of,  416 
vasomotor  mechanisms  of,  411 

Kinesodic,  714 

Kreatin,  88,  901 

Kreatinin,  902 

Kymograph,  142 
Ludwig's,  142 

Kynurenic  acid,  906 


T  ACTEALS,  313 

JJ     Lactic  acid,  892 

Laminae  in  a  hardened  lens,  744 

Lardacein,  879 

Large  intestine,  movements  of,  300 

Larynx,  819 

cartilages  of,  820 

cavity  of,  821 

muscles  of,  821 

vocal  cords,  821 
Latent  period,  70 
Laurostearic  acid,  889 
Lecithin,  97,  894 
Leclanche  battery,  57 
Leucin,  907 
Leucocythaemia,  characterized  by  increase 

of  white  corpuscles,  49 
Leukomaines,  917 
Life,  phases  of,  855,  862 
Ligamentum  nuchse,  134 
Limb,  movements  of,  682 
Liver,  nerves  of,  449 
Living  body,  study  of,  17 
Load,  influence  of,  110 
Lobes  of  brain,  585 
Ludwig's  kymograph,  142 

strom  uhr,  152 
Lungs,  332 

atelectatic,  334 

capacity  of,  336 

pressure  exerted  in  breathing,  335 
Lymph,  characters  of,  315 

chemical  composition  of,  315 

-hearts,  structure  and  functions,  324 

microscopic  characters  of,  215 

movements  of,  317 


M 


ACUL^E  acusticse,  798 
Magnetic  interrupter,  62 


Male  generative  apparatus,  835 
glands,  835 
prostate,  836 
semen,  838 
spermatozoa,  838 
testicles,  835 
tunica  albuginea,  836 
vas  deferens,  836 
vesiculae  seminales,  837 
Maltose,  248,  885 
Mammary  glands,  477 

changes  occurring  in,  478 
relation  to  nervous  system,  482 
secretion  of  milk,  481 
structure,  477 
Manometer,  mercury,  139 

venous,  146,  147 

Marey's  tambour  with  cardiac  sound,  167 
Mastication,  294 
Maximum  manometer  of  Goltz  and  Gaule 

165 
Maxwell's  method  of  fusing  color  sensations, 

770,  775 

Mechanisms,  dioptric,  744 
of  accommodation,  749 
locomotor,  829 
Membrana  tympani,  798 
Membrane  of  Keissner,  795 
Menstruation,  836 
Mercurial  gas  pump,  Ludwig's,  346 
Metabolism,  general  features  of,  506 

influence  of  nerves  on,  510 
Metabolites,  nitrogenous,  896 
Metamere,  neural.  123 
Methsemoglobin,  360 
Meynert's  commissure,  693 
Microorganisms,  action  of,  in  alimentary 

canal,  311 
Micturition,  427 

involuntary,  429 
nervous  mechanism  of,  428 
voluntary,  429 
Middle  ear,  792 
Milk,  composition  of,  480 
constituents  of,  478 
human,  478 
quantity  secreted,  480 
secretion  of,  481 
sugar,  885 

Mil  Ion's  reagent,  29 
Morse  key,  60 
Motor  area,  characteristics  of,  659 

for  leg,  arm,  and  face  in  man,  posi- 
tion and  relative  extent  of,  676 
Movements  of  heart,  157 
of  limbs,  682 
of  pupil,  751 
skilled,  673 
Mucin,  246,  881 
Mucous  gland,  changes  in  during  secretion, 

Muscae  volitantes,  759 

Muscle  and  nerve,  degree  of  irritability  of, 

experiments  with,  60 
phenomena  of,  55 


INDEX. 


925 


Muscle  case,  80 

chemistry  of,  83 
contractions,  simple,  65 
currents,  92 
curve,  65 

double,  73 

from  the  gastrocnemius  of  the  frog, 

65 
single   induction   shock   repeated 

rapidly,  73,  74,  75 
slowly,  73 
dead,  84 
during    contraction,    changes     taking 

place  in,  77 
energy  of,  115 

Muscle  fibres,  microscopic  changes,  81 
-nerve  preparation,  57,  102 
as  a  machine,  107 
plasma,  85 
sartorius  of  frog,  78 
size  and  form  of,  111 
stapedius,  800 
under  polarized  light,  81 
Muscular  and  nervous  action,  nature  of,  115 

irritability,  55 
contraction,  single,  57 

simple,  57 
irritability,  57 
sense,  808* 
Myoglobulin,  86 
Myograph,  pendulum,  69 

spring,  69 
Myosin,  84,  873 
Myosinogen,  86 
Myristic  acid,  889 


NASAL  fossa,  right,  806 
fossae,  806 

Nerve  and  muscle,  electric  currents  of,  92 
cells,  grouping  of,  535 
of  spinal  cord,  129 
variations  in,  552 
centre,  130 
chemistry  of,  97 
eighth  or  auditory,  605 
eleventh  or  spinal  accessory,  603 
fibres  in  retina,  connections  of,  742 
fifth  or  trigeminal,  608 
fourth  or  trochlear,  612 
ninth  or  glosso-pharyngeal,  603 
roots,  connections  of,  544 
seventh  or  facial,  607 
sixth  or  abducens,  608 
tenth  or  vagus,  603 
tetanization  of,  99 
third  or  oculo- motor,  612 
twelfth  or  hypoglossal,  602 
Nerves,  electric  currents  in,  99 
Nervous  impulse,  98 

changes  in  nerve  during  passage 

of,  97 

measurement  of  velocity  of,  70 
system,  central,  128 
tissues,  general  features  of,  122 
Neurin,  98,  895 


Neurokeratin,  98 
Neutral  fats,  890 
Nicol  prism,  81 
Nitrate  of  urea,  897 
Nitrogenous  metabolism,  487 

metabolites,  896 

non-crystalline  bodies  allied  to  proteids, 

Nuclein,  883 
Nucleo-albumin,  884 


OCCIPITAL  region,  prominence  of  nu- 
clear cells  in,  659 
I  Ocular  spectra,  780 
'  (Edema,  322 

(Esophagus,  movements  of,  296 
Oleic  series,  acids  of,  890 
Olein,  891 
Olfactory  mucous  membrane,  cells  of,  807 

sensations,  700 
Oncograph,  413 
Oncometer,  412 
|  Optic  radiation,  694 

thalamus,  615 

Organic  matter,  exhalation  of,  345 
Organs  of  Corti,  795 

of  reproduction,  832 
Ovaries,  835 
Ovum,  838 
Oxalate  of  urea,  897 
Oxalic  acid  series,  892 


PALMITIC  acid,  889 

1      Palmitin,  890 

Pancreas,  changes  in,  during  secretion,  269 

of  rabbit,  269 
Pancreatic  juice,  282 

action  of,  on  fats  and  starch,  284 

on  food-stuffs,  282 
characters  of,  282 
secretion  of,  286 
Papillae,  circumvallate,  809 

filiform,  809 

fungiform,  809 
Paraglobulin,  871 

in  the  clotting  of  blood,  30 
Paralysis,  crossed,  685 
Parapeptone,  256 

Parotid  gland,  nervous  mechanism  of,  265 
Parturition,  853 
Pendulum  myograph,  68 
Pepsin,  258 
Peptogenous  food,  277 
Peptones,  256,  875 

Perceptions,  general  sensibility  and  tactile, 
812 

modified,  778 
Peristaltic  movements,  292 

influences  bearing  on,  305 
Perspiration,  431 

Pettenkofer's  test  for  bile  salts,  280,  912 
Phenylic  acid,  911 

Physostigmine,  action  of,  on  pupil,  756 
Pituitary  body,  468 


926 


INDEX. 


Plasmine,  in  clotting  of  blood,  32 
Pons  Varolii,  584 

gray  matter  of,  623 
Posterior  column  of  spinal  cord,  531 
Presbyopic  eye,  748 
Pressure,  arterial,  138 
in  capillaries,  143 
in  veins,  138 
Products  of  digestion,  course  taken  by  fats, 

325 

by  proteids,  327 
by  sugar,  326 
by  water  and  salts,  326 
Proprionic  acid,  889 
Prostate  gland,  834 
Protagon,  98,  895 

Proteid  bodies,  characters  of  the  more  im- 
portant, 253 
Proteids,  866 

classification  of,  257 
coagulated,  875 

dec6mposition  of,  by  digestion,  878 
general  composition  of,  28 
xanthoproteic  test  for,  28 
Protoplasm,  20 
Pseudopodia,  121 

Psychical  processes,  duration  of,  725 
Ptomaines,  313,  345,  896 
Pulse,  176 

anacrotic,  185,  190 
katacrotic,  185 
methods  of  recording,  176 
venous,  190 

Pulse-curve,  characters  of,  181 
Pulse-curves  described  by  sphygmographic 

levers,  177 
Pulse-tracing    from    an    artificial    model, 

179 
from  carotid  artery  of  healthy  man, 

185 
Pulse-wave,  anacrotic,  causes  of,  190 

changes  of,  along  the  arterial  tract, 

182 
diacrotic,  185 

cause  of,  185 
features  of,  182 
length  of,  185 
predicrotic,  189 
velocity  of,  184 
Pupil,  movements  of,  751 

nerves  governing,  753 
Purkinje',  cells  of,  653 
Purkmje's  figures,  760,  762 
Pyramidal  cells,  656 
Pyrexia,  504 
Pyriform  lobe,  700 


fvUADRIGEMINAL  bodies,  625 
y    Quality  of  sound,  801 

Quantitative  determination,  898 
Quantity  of  aqueous  vapor  exhaled,  344 

of  blood,  53 

mode  of  estimating,  54 

of  carbonic  acid  exhaled,  344 

of  organic  matters  exhaled,  345 


RAPIDITY  of  circulation,  150 
in  capillaries,  154 
in  veins,  154 
of  clotting  of  blood,  27 
Reaction  of  urine,  409 
Bed-blind,  774 

blood-corpuscles,  37 

counting  of,  42 

apparatus  for  the,  42 
diameter  of,  37 
disintegration  of,  42 
number  of,  41 
shape  of,  37 
structure  of,  37 
study  of,  37 

marrow  in  bone,  a  source  of  red  cor- 
puscles, 43 
Reflex  actions,  130 

nature  of,  130 
of  spinal  cord,  564 
inhibition,  203 
Refraction,  745 
Regulation  of  heat,  499 
Reissner,  membrane  of,  795 
Rennin,  250 
Reproduction,  832 

corpus  luteum,  840 

of  menstruation,  840 
of  pregnancy,  840 
development  of  embryo,  842 
allantois,  844 
placenta,  847 
segmentation  of  ovum  in, 

842 

vascular  area,  843 
Graafian  follicle  in,  838 
impregnation  of  ovule,  841 
menstruation,  838 
organs  of,  832 

Fallopian  tubes,  835 
male,  835 
ovaries,  835 
uterus,  834 
vagina,  834 
ovule  in,  841 
ovum  in,  838 
spermatozoa  in,  841 
Respiration,  332 

absorption  of  oxygen  in,  361 
apncea  in,  384 

apparatus  for  taking  tracings,  337 
asphyxia  in,  336,  386 

phenomena  of,  386 

carbonic  acid  exhaled,  amount  of,  344 
changes  of  air  in,  344 
Cheyne-Stokes,  385 
complemental  air  in,  334 
dyspncea  in,  336,  380 
effect  of  breathing  foreign  gases,  388 
of  changes  in  atmospheric  pres- 
sure, 389 

of  muscular  exercise  on,  383 
exhalation  of  aqueous  vapor,  344 
of  carbonic  acid,  344,  364 
of  organic  matters,  345 
expiration,  342 


INDEX. 


927 


Respiration,  expired  air,  impurities  of,  345 
nature  of,  345 
temperature  of,  344 
facial  and  laryngeal,  343 
graphic  records  of  movements  of,  336 
influence  of  vagus  nerves  on,  372 
inspiration,  340 
labored  inspiration,  342 
modified  movements  of,  403 
coughing,  403 
crying,  404 
hiccough,  403 
laughing,  404 
sighing,  403 
sneezing,  403 
sobbing,  403 
yawning,  403 

movements  of  diaphragm,  339 
of  expiration,  342 
of  inspiration,  339 
muscles  of,  340 
nervous  mechanism  of,  369 
number  of,  339 

relations  of  respiratory  system  to  vas- 
cular and  other  systems,  390 
residual  air  in,  334 
ribs  in,  action  of,  341 

function  of,  341 
stationary  air  in,  833 
tidal  air  in,  333 
visible  movements  in,  339 
Respiratory  centre,  370 
changes  in  blood,  346 
in  the  tissues,  365 
undulations,  391 
Ketina,  741 

connective  tissues  of,  741 
inner  surface  of,  743 
of  man,  rod  and  cone  from,  742 
photo-chemistry  of,  762 
Ribs,  action  of,  in  respiration,  341 

function  of,  in  respiration,  340 
Rigor  mortis,  83 
Ritter-Valli  law,  112 
Rods  of  Corti,  795 

Rolando,  substantia  gelatinosa  of,  534 
tubercle  of,  598 


O  A  ORAL  nerves  of  dog,  125 
O     Saline  matters  in  blood,  51 
.Saliva,  246 

action  of,  on  starch,  247 
characters  of  mixed,  250 
of  parotid,  251 
of  sublingual,  251 
of  submaxillary,  251 
chemical  characters  of,  246 
composition  of,  246 
nature  of  amylolytic  action  of,  249 
secretion  of,  by  means  of  chorda  tym- 

pani  nerve,  261 
Salts  of  bile,  280 

of  uric  acid,  900 
Santorini,  cartilage  of,  820 
Sarcolactic  acid,  892 


Sarkin,  903 
I  Scala  tympani,  794 

vestibnli,  795 

i  Schemer's  experiment,  diagram  of,  747 
|  Schlemm,  canal  of,  739 
Sclerotic  coat,  739 
Secretion,  general  nature  of,  273 

nature  of  the  act  of,  275 
Segmentation  of  ovum,  842 
Semicircular  canals,  794 
Seminal  fluid,  838 
Seminiferous  tubules,  836 
Sensation,  759 
Sensations,  auditory,  800 

cutaneous,  703 

distinction  and  fusion  of,  766 

of  cold,  703 

of  color,  769 

primary,  773 

of  hearing,  701 

of  heat,  703 

of  pressure,  814 

of  smell,  700 

of  taste,  701 

of  temperature,  703,  814 

of  touch,  703 

relation  of,  to  stimulus,  766 

simple,  766 

tactile,  814 

visual,  689,  759 
Sense,  muscular,  814 
Sensory  impulses,  759 
Serous  fluid,  chemical  character  of,  316 
Serum-albumin  in  clotting  of  blood,  30 
Serum  in  clotting  of  blood,  29 
Seventh  nerve,  607 
Sexes,  difference  in  respiration,  339 
Sight,  738 
Sixth  nerve,  608 
Skatol,  917 
Skin,  absorption  by,  433 

cutaneous  respiration,  432 

perspiration,  amount  of,  431 
composition  of,  432 

secretion  of  sweat,  mechanism  of,  434 
Small  intestine,  contents  of,  308 
movements  of,  292,  300 
Smell,  sensations  of,  700 
Soaps,  891 

Somatic  and  splanchnic  nerves.  123 
Sommerring,  yellow  spot  of,  743 
Sound,  801 

pitch  of,  801 

quality  of,  801 
Sounds  of  the  heart,  162 
Specific  gravity  of  urine,  405 
Spectra,  ocular,  780 
Spectroscopic     analysis     of    hemoglobin, 

353 
Speech,  826 

areas  for,  676 

imperfect,  679 

caused  by  bulbar  disease,  680 

thick,  679 
Spermatozoa,  838 
Spherical  aberration,  757 


928 


INDEX. 


Sphygmograph,  Dudgeon's,  177 

Marey's  177 

Spinal  accessory  nerve,  603 
cord,  525 

antero-lateral  ascending  tract  of, 

559 

ascending  tracts  of,  543 
automatic  actions  of,  578 
central  canal  of,  530,  534 
cerebellar  tract,  556 
columns  of  Burdach,  540 
of  Clark,  539 
of  Goll,  540 
complexity  of  reflex  movements 

of,  570,  574 
cornu  of,  530 
descending  tracts  of,  541 
features  of^  548 
fissures  of,  529 
gelatinous  substance  of  Rolando, 

534 
gray  matter,  nature  of,  560-563 

structure  of,  523 
longitudinal,    commissural    tracts 

of,  563 
loss  of  tone  of  skeletal  muscles  in, 

579 
reflex  actions  of,  564 

inhibitions  of,  574 
time  required  for,  577 
relative  size  and  form  of,  549 
reticular  formation,  538 
rigidity  of  muscles  through  action 

of,  583 

structure  of,  529 
tendon  phenomena  of,  582 
white  matter,  structure  of,  530 

tracts  of,  540 

disease,  sensations  of  pain  in,  711 
ganglia,  125 
nerves,  525 

Spleen,  chemical  constituents  of,  453 
movements  of,  451 
red  corpuscles  in,  43 
Spring  manometer,  Pick's,  176 
Stapedius  muscle,  800 
Starch,  247 
Stearic  acid,  889 
Stearin,  890 
Stellate  nerve  cells,  127 
Stimuli,  characters  of,  107 
Stimulus,  nature  and  mode  of  application 

of,  107 
Stomach,  gastric  juice  in,  266 

movements  of,  297 
Submaxillary  gland,  261 

nerves  of,  261-263 
Succinic  acid,  893 
Succus  entericus,  285 

nature  and  action  of,  285 
Supra-renal  bodies,  467 

chemical  constituents  of,  468 
functions  of,  469 
structure  of,  468 
Sylvius,  aqueduct  of,  585 
Syntonin,  85 


rpACTILE  perceptions  and  judgments,  816 
1     Tambour,  Marey's  167 
Taste,  808 

buds,  811 

circumvallate  papillae,  809 

filiform  papillae,  809 

fungi  form  papillae,  809 

gustatory  bulbs,  811 

sensations  of,  701 
Taurin,  906 
Taurocholic  acid,  913 
Temperature,  498 

effects  of  great  cold  on,  505 
of  great  heat  on,  504 

influence  of  food  on,  501 

on  contractile  tissues,  113 
of  muscular  action  on,  501 
of  time  of  day  on,  503 

of  body,  497,  498 

of  various  animals,  497 

pyrexia,  504 

sensations  of,  703 

Tensor  tympani  muscle  of  ear,  799 
Tenth  nerve,  603 
Testicles,  836 
Tests  for  bile,  280 
Tetanic  contractions,  57,  73 
Tetanus,  73 

contractions  in,  109 
Thalami  optici,  623 
Thermal  changes,  90 
Thermopile,  90 
Third  nerve,  612 
Thoracic  duct,  314 
Thymus,  functions  of,  469 

*  structure  of,  470 
Thyroid  body,  465 

functions  of,  466 
structure  of,  465 
Tidal  air,  333 
Tissues,  contractile,  54 
Tongue,  808 

Tonic  contraction,  118,  669 
Tracing  from  heart  of  cat,  161 

of  respiration,  338 
Transudation,  phenomena  of,  319 
Traube-Hering  variations,  736 
Trigeminal  nerve,  608 
Trochlear  nerve,  612 
Trypsin,  274 
Trypsinogen,  275 
Tunica  albuginea,  836 
Tunicin,  885 
Twelfth  nerve,  602 
Ty rosin,  909 


TTMBILICAL  vesicle,  843 
U     Urari,  poisoning  by,  56 

rise  of  blood-pressure  in  animals 

under,  708 
Urea,  896 

formation  of,  in  liver,  459,  460 
group,  896 
nitrate  of,  897 
oxalate  of,  897 


INDEX. 


929 


Urea,  relations  to  cyanogen  compounds,  464 

synthesis  of,  406,  462 
Uric  acid,  405,  462,  899 

salts  of,  900 
Urine,  404 

abnormal  constituents  of,  409 
albumin,  410 
sugar,  410 

acidity  of,  409 

amount  of,  408 

composition  of,  409 

ferments  in,  408 

general  characters  of,  404 

hippuric  acid  in,  406 

inorganic  salts  in,  406 

non-nitrogenous  constituents  of,  407 

normal  organic  constituents  of,  405 

pigments,  481,  915 

reaction  of,  409 

relations   of  secretion  of,  to  food  and 
drink,  425 

secretion  of,  410 

glornerular,  420 

specific  gravity  of,  405 

urea  in,  405 

uric  acids  in,  405 

amount  of,  in,  405 
Urobilin,  915 
Uroerythrin.  915 
Uterus,  834 

os  uteri,  835 

structure  of,  833 
Utricles  of  ear,  797 


T7AGINA,  834 
V      Vagus  nerve,  603 
Valerianic  acid,  889 
Various  forms  of  stimuli,  58 
Vascular  mechanism,  133 
Vas  deferens,  836 

Vaso-constrietor  fibres,  course  of,  218 
nerves,  124 

-dilator  fibres,  course  of,  220 

-motor  actions.  2 1 1 

effects  of,  220 
Veins,  blood- pressure  in,  137 

circulation  in,  rapidity  of,  154 

vasomotor  nerves  of,  23 1 
.Velocity  of  blood  in  arteries,  150 
in  capillaries,  154 

of  the  pulse  wave,  154 
Venous  pulse,  190 

sinuses,  734 

Vertigo,  phenomena  and  causation  of,  647 
Vesicuke  seminales,  837 
Vibrating  tuning-fork  with  Despretz  signal, 

70 

Vierordt  heematachometer,  153 
Vision,  759 


Vision,  binocular,  781 

in  man,  apparatus  of,  691 
region  of  distinct,  777 
Visual  field,  689 

impulses,  origin  of,  760 
judgments,  786 

of  distance,  786 
of  size,  786 
of  solidity,  787 

movements,  coordination  of,  784 
perceptions,  776 

contrast  of,  778 
purple,  764 
sensations,  689,  759 
white,  764 
yellow,  764 
Vitellin,  873 
Vitreous  body,  744 
Vocal  cords,  821 

movements  of,  823 
slackening  of,  824 
tightening  of,  824 

Volitional  impulses  in  the  cord,  684 
Volkmann's  hsernadromometer,  151 
Voluntary  movements,  661,  679 

action  of,  on  the  lower  animals,  662 
Vomiting,  299 


WATERY  vapor,  exhalation  of,  344 
Wave,  contraction,  79 
White  blood-corpuscles,  43 

chemical  examination  of,  45 

migration  of,  48 

movements  of,  47 

nuclei  of,  44 

number  of,  44 

origin  of,  47 

proportion  of,  to  red,  44 

size  of,  44 

structure  of,  44 

transformation  into  red,  49 

work  of,  47 

White  matter,  97,  128 
Willis,  circle  of,  733 
Work  done  by  heart,  175 


VANTHIN,  904 

xV     Xanthoproteic  test  for  proteids,  28 


YOUNG-HELMHOLTZ  theory 
sensations,  772-775 


of  color 


LA 


,  zone  of,  744 
Zona  pellucida,  841 
spongiosa,  535 
Zymogen,  275 


59 


v& 


